electroless copper deposition: a sustainable … electroless copper deposition: a sustainable...
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Electroless Copper Deposition: A Sustainable Approach
by
Marika Renée Kutnahorsky
A thesis submitted in conformity with the requirements
for the degree of Masters of Applied Science
Department of Materials Science and Engineering
University of Toronto
© Copyright by Marika Renée Kutnahorsky (2009)
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Electroless Copper Deposition: A Sustainable Approach Masters of Applied Science (2009)
Marika Renée Kutnahorsky
Department of Materials Science and Engineering
University of Toronto
Abstract
A sustainable electroless copper coating process was developed for plating automotive
fasteners shaped from AISI 9255 low carbon, high silicon steel. The objective was to
minimize the ionic and organic species present in each step of the plating process. A
sulfuric acid solution inhibited with quinine was defined to clean the steel prior to
plating. The corrosivity of the solution was examined through electrochemical and
weight loss measurements to evaluate the efficiency of the cleaning process at high
temperatures and high acid concentrations. An electroless copper coating process was
then developed using a simple copper sulfate chemistry inhibited with quinine to extend
the possible operating window. Finally, benzotriazole was evaluated as a possible anti-
oxidant coating. Accelerated thioacetamide corrosion tests were used to evaluate the
corrosion inhibition of benzotriazole on copper coatings.
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Acknowledgments
I would like to thank Professors Donald Kirk and Steven Thorpe, without whose
advice and guidance this project would not have been possible. This process was a very
valuable learning experience.
Thanks to all of the group members in the Surface Engineering and
Electrochemistry Group. Your support and encouragement kept me going through the
rough patches.
Thank you to the MSE department for all of their aid and advice.
To Matt Olmstead – this thesis wouldn’t have been possible without your love
and support. You always had my back, and provided me with strength, encouragement,
coffee, backrubs and occasionally poutine. Thank you.
Wolf and Francine Kutnahorsky, thanks for the encouragement, love, support and
patience. G.I.R. Unit and Darwin, thanks for the snorgles.
To all of my friends who supported me during my writing phase, your love and
support is appreciated more than you could know.
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Table of Contents
ABSTRACT .................................................................................................................................. II
ACKNOWLEDGMENTS ........................................................................................................... III
TABLE OF CONTENTS ............................................................................................................ IV
LIST OF TABLES ..................................................................................................................... VII
LIST OF FIGURES ..................................................................................................................... IX
LIST OF APPENDICES ............................................................................................................. XI
1. INTRODUCTION .................................................................................................................. 1
1.1. OBJECTIVES .......................................................................................................................... 3
2. BACKGROUND ..................................................................................................................... 4
2.1. METHODS OF CLEANING IRON ............................................................................................. 4 2.1.1. ACID CLEANING ................................................................................................................. 4 2.1.1.1. HIGH-CONCENTRATION ACID CLEANING (PICKLING) .................................................... 4 2.1.1.2. LOW-CONCENTRATION ACID CLEANING ........................................................................ 5 2.1.1.3. ORGANIC ACID CLEANING .............................................................................................. 6 2.1.2. ELECTROLYTIC CLEANING ................................................................................................. 7 2.1.3. ALKALINE CLEANING ......................................................................................................... 7 2.2. DISSOLUTION OF IRON IN ACIDIC MEDIA ............................................................................ 8 2.2.1. MECHANISM OF IRON DISSOLUTION ................................................................................... 8 2.2.1.1. ELECTRODE POTENTIAL .................................................................................................. 8 2.2.1.2. KINETIC DISSOLUTION PARAMETERS ............................................................................. 10 2.2.1.3. IRON DISSOLUTION PATHWAY ....................................................................................... 12 2.2.1.4. CORROSION RATES ........................................................................................................ 15 2.2.1.5. POLARIZATION RESISTANCE .......................................................................................... 15 2.2.1.6. TAFEL EXTRAPOLATION ................................................................................................ 17 2.2.1.7. COMPARISON OF WEIGHT LOSS VERSUS ELECTROCHEMICAL METHODS ...................... 18 2.2.2. EFFECT OF TEMPERATURE ON CORROSION RATE .............................................................. 19 2.2.3. EFFECT OF ANION TYPE .................................................................................................... 20 2.2.3.1. HYDROCHLORIC ACID ................................................................................................... 20 2.2.3.2. SULFURIC ACID ............................................................................................................. 21 2.2.4. EFFECT OF ACID CONCENTRATION ................................................................................... 21 2.2.5. EFFECT OF FLUID VELOCITY ............................................................................................. 22 2.3. SMUT FORMATION ............................................................................................................. 22 2.3.1. DEFINITION OF SMUT ........................................................................................................ 22
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2.3.2. CHEMISTRY ....................................................................................................................... 23 2.3.3. FACTORS AFFECTING SMUT FORMATION .......................................................................... 23 2.3.4. METHODS OF CONTROLLING SMUT FORMATION .............................................................. 24 2.4. INHIBITORS ......................................................................................................................... 24 2.4.1. DEFINITION AND CLASSIFICATION OF INHIBITOR TYPES FOR FE ...................................... 25 2.4.2. ANODIC INHIBITORS ......................................................................................................... 25 2.4.3. CATHODIC INHIBITORS ..................................................................................................... 27 2.4.4. MIXED INHIBITORS ........................................................................................................... 28 2.4.5. ORGANIC INHIBITORS ....................................................................................................... 28 2.4.6. QUININE ............................................................................................................................ 29 2.4.6.1. MECHANISMS OF QUININE INHIBITION .......................................................................... 30 2.4.6.2. TEMPERATURE STABILITY OF QUININE .......................................................................... 32 2.4.6.3. MEASUREMENT OF QUININE IN PROCESS CONTROL ....................................................... 33 2.4.7. OTHER POSSIBLE INHIBITORS ........................................................................................... 33 2.4.8. BENZOTRIAZOLE AS A COPPER INHIBITOR ........................................................................ 35 2.5. COPPER COATING TECHNOLOGIES .................................................................................. 37 2.5.1. FUNDAMENTALS OF ELECTROLESS COPPER PLATING ....................................................... 37 2.5.2. FUNDAMENTALS OF IMMERSION PLATING ........................................................................ 41 2.5.3. FUNDAMENTALS OF MECHANICAL PLATING ..................................................................... 41 2.5.4. FUNDAMENTALS OF BARREL PLATING ............................................................................. 42 2.5.5. CURRENT SYSTEMS USED IN INDUSTRY ............................................................................ 43 2.5.6. LIMITATIONS AND CHALLENGES ...................................................................................... 44 2.5.7. PROPOSED SYSTEM ........................................................................................................... 44
3. EXPERIMENTAL METHODOLOGY ............................................................................. 47
3.1. FASTENERS USED ............................................................................................................... 47 3.2. CLEANING CIRCUIT ........................................................................................................... 47 3.2.1. BEAKER TESTS .................................................................................................................. 47 3.2.2. SMUT RATING ................................................................................................................... 49 3.2.3. BARREL PLATING TESTS ................................................................................................... 50 3.2.4. DIGITAL IMAGING ............................................................................................................. 50 3.2.5. ADHESION TESTS .............................................................................................................. 51 3.2.6. WEIGHT LOSS TESTS ........................................................................................................ 51 3.2.7. ELECTROCHEMICAL TESTS ............................................................................................... 51 3.3. COPPER PLATING ............................................................................................................... 52 3.3.1. BEAKER TESTS .................................................................................................................. 52 3.3.2. BARREL PLATING TESTS ................................................................................................... 53 3.3.3. ADHESION TESTS .............................................................................................................. 54 3.4. ANTI-OXIDIZING AGENT .................................................................................................... 54 3.4.1. BEAKER TESTS .................................................................................................................. 54 3.4.2. BATCH TESTS .................................................................................................................... 55 3.4.3. COPPER TARNISHING TESTS ............................................................................................. 56
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4. RESULTS AND DISCUSSION ........................................................................................... 57
4.1. HYDROCHLORIC ACID CLEANING SOLUTIONS ................................................................ 57 4.1.1. UNINHIBITED HYDROCHLORIC ACID SOLUTIONS ............................................................ 58 4.1.1.1. ACID CONCENTRATION VARIATION .............................................................................. 58 4.1.1.2. TEMPERATURE VARIATION ........................................................................................... 60 4.1.2. INHIBITED HYDROCHLORIC ACID CLEANING SOLUTION ................................................. 61 4.1.2.1. TEMPERATURE VARIATION ........................................................................................... 61 4.2. SULFURIC ACID CLEANING SOLUTION ............................................................................. 63 4.2.1. UNINHIBITED SULFURIC ACID SOLUTION ........................................................................ 63 4.2.1.1. TEMPERATURE VARIATIONS .......................................................................................... 63 4.2.1.2. ACID CONCENTRATION VARIATIONS ............................................................................ 68 4.2.2. INHIBITED SULFURIC ACID SOLUTION .......................................................................... 71 4.2.2.1. INHIBITOR CONCENTRATION VARIATION ...................................................................... 71 4.2.2.2. TEMPERATURE VARIATIONS .......................................................................................... 75 4.2.2.3. ACID CONCENTRATION VARIATIONS ............................................................................ 77 4.3. COPPER FLASH ................................................................................................................... 77 4.3.1. PRELIMINARY CLEANING SOLUTION ADHESION TESTS ..................................................... 77 4.3.2. BEAKER TESTS .................................................................................................................. 80 4.3.2.1. ACID CONCENTRATION VARIATION .............................................................................. 80 4.3.2.2. COPPER SULFATE CONCENTRATION VARIATION .......................................................... 81 4.3.3. INHIBITED COPPER FLASH TESTS ..................................................................................... 82 4.3.4. BARREL PLATING TESTS ................................................................................................... 83 4.3.4.1. COPPER SULFATE CONCENTRATION VARIATIONS .......................................................... 83 4.3.4.2. INHIBITOR CONCENTRATION VARIATIONS ..................................................................... 85 4.4. ANTI-OXIDIZING AGENT .................................................................................................... 88 4.4.1. BEAKER TESTS .................................................................................................................. 88 4.4.2. BATCH TESTS .................................................................................................................... 91 4.5. DEVELOPED SYSTEM.......................................................................................................... 92
5. CONCLUSIONS ................................................................................................................... 94
6. REFERENCES ..................................................................................................................... 95
APPENDICES ............................................................................................................................. 99
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List of Tables Table 1 Criteria for improving the sustainability of electroless copper plating processes ............................................................................................................. 2 Table 2 Calculated anodic Tafel slopes for the proposed reaction mechanisms in steady-
state and nonsteady-state reactions. .................................................................. 14 Table 3 Tafel data for iron in various inhibited and uninhibited acidic media .............. 14 Table 4 Corrosion data for low carbon steel (wt%: 0.1 C, 0.29 Mn, 0.07 Si, 0.021P) in a 1 M uninhibited hydrochloric acid solution at various temperatures ......... 20 Table 5 Electrochemical data for iron dissolution in hydrochloric acid solutions of
varying concentration, performed at room temperature on Armco iron [33] ... 22 Table 6 Inhibitors commonly used for industrial cleaning applications ........................ 25 Table 7 Electrochemical parameters for quinine at various temperatures and
concentrations in 1 M hydrochloric acid [31]. .................................................. 30 Table 8 Environmentally friendly inhibitors researched as quinine alternates .............. 33 Table 9 Potentiodynamic polarization parameters for the corrosion of iron in acidic
media with varying inhibitor concentration and type. ...................................... 34 Table 10 Corrosion rate and inhibition efficiency data obtained by Kahled [58] from
weight loss measurements for copper in 0.5 M HCl solutions in the absence and presence of various concentrations of BTA at 30°C .................................. 36 Table 11 Process flow for electroless copper deposition with outline of environmental
concerns associate with each step ..................................................................... 40 Table 12 Process outline for MacDermid SC-G Copper Flash solution .......................... 43 Table 13 Comparison of main coating options (from [41]) ............................................. 45 Table 14 Parameters for initial development of cleaning solution inhibited with quinine ............................................................................................................. 48 Table 15 Evaluation of smut by appearance .................................................................... 49 Table 16 Summary of solutions subjected to electrochemical testing ............................. 52 Table 17 Copper sulfate solution parameters, uninhibited solutions and solutions inhibited with 0.003 g/L quinine ....................................................................... 53 Table 18 Summary of test parameters for barrel plating tests of uninhibited and inhibited
with 0.003g/L quinine copper flash solutions ................................................. 54 Table 19 Process outline for beaker testing of benzotriazole anti-oxidant coating ......... 55 Table 20 Smut formation in varying acid concentration in an uninhibited hydrochloric
acid solution .................................................................................................... 58 Table 21 Timeframe determinations for hydrochloric acid cleaning solution at room
temperature ...................................................................................................... 59 Table 22 Temperature variations for uninhibited hydrochloric acid cleaning solutions. ......................................................................................................... 60 Table 23 Hydrochloric acid (10 vol%) cleaning solutions inhibited with 0.03 g/L
quinine ............................................................................................................. 62 Table 24 Summary of inhibited and uninhibited sulfuric acid cleaning solutions tested ............................................................................................................... 63 Table 25 Uninhibited sulfuric acid cleaning solutions at room temperature and elevated temperatures ...................................................................................... 64
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Table 26 Electrochemical data for uninhibited 1vol% sulfuric acid cleaning solution ............................................................................................................ 65 Table 27 Variation of acid concentration in uninhibited sulfuric acid cleaning
solutions at 25°C ............................................................................................. 69 Table 28 Low acid cleaning using 1 vol% sulfuric acid and various quinine
concentrations, cleaned samples (top row) and copper coated samples (bottom row) ................................................................................................................. 72
Table 29 Electrochemical data for variations in quinine concentration at 25°C ........... 74 Table 30 Electrochemical data for temperature variations in uninhibited and 0.03 g/L
quinine inhibited sulfuric acid solution .......................................................... 76 Table 31 Initial copper flash trials used to prove suitability of inhibited sulfuric acid
cleaning solution. ............................................................................................ 79 Table 32 Sulfuric Acid concentration variations in a beaker set-up with 15g/L copper
sulfate copper flash solution ............................................................................ 80 Table 33 Copper sulfate concentration variations in a 0.1 vol% sulfuric acid copper
flash solution prior to adhesion tests with adhesion test results ...................... 82 Table 34 Samples and adhesion results for variations in copper sulfate concentrations of copper flash solutions inhibited with quinine ............................................. 83 Table 35 Batch tests of copper sulfate concentration variations in an uninhibited copper sulfate flash solution ............................................................................ 84 Table 36 Quinine concentration variations in 5 g/L copper sulfate copper flash batch
tests .................................................................................................................. 85 Table 37 Copper sulfate concentration variations and rpm variations in inhibited copper
flash solution ................................................................................................... 86 Table 38 Summary of corrosion test results for 0.01M and 0.1M benzotriazole anti-
oxidant solutions without preliminary alkaline dip ......................................... 89 Table 39 Summary of corrosion test results for 0.01M and 0.1M benzotriazole anti-
oxidant solutions with preliminary alkaline dip .............................................. 90 Table 40 Corrosion test results for benzotriazole dip batch tests. .................................. 92 Table 41 Summary of electroless copper flash system developed for a room
temperature barrel plating system ................................................................... 93 Table 42 Raw electrochemical data ............................................................................ 102 Table 43 Complete uninhibited hydrochloric acid test results ..................................... 106 Table 44 Temperature and acid variations in uninhibited sulfuric acid cleaning
solutions ........................................................................................................ 107 Table 45 Inhibited sulfuric acid solution samples ........................................................ 108 Table 46 Complete summary of beaker tests in copper flash development ................. 109 Table 47 Samples from each batch of varying copper sulfate concentration copper flash
batch tests ...................................................................................................... 110 Table 48 Copper sulfate flash solutions with 1 vol% sulfuric acid inhibited with
varying concentrations of quinine, flashed for various times ....................... 111
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List of Figures
Figure 1 Proposed continuous plating line schematic ..................................................... 1 Figure 2 Concentration gradient in the solution near a surface controlled by
concentration polarization ............................................................................... 11 Figure 3 Polarization resistance data at small applied current densities in 0.52N H2SO4
at 30°C [From Graydon, J. W. "Linear polarization." Lecture.] ..................... 16 Figure 4 Polarization curve showing experimental and extracted Tafel lines for 1080
steel in deaerated 1N H2SO4. βc= -98mV, βa= 38 mV (derived from cathodic data), icorr = 1180μA/cm2 [29] ......................................................................... 18
Figure 5 Evans diagram showing the effect of anodic inhibitors on corrosion current. ............................................................................................................ 26 Figure 6 Evans diagram for the effect of cathodic inhibitors on corrosion current ...... 27 Figure 7 Evans diagram for effect of mixed inhibitors on corrosion current ................ 28 Figure 8 The quinine molecule [31]. ............................................................................. 29 Figure 9 Inhibition efficiency at different concentrations of quinine for low carbon
steel in 1 M HCl at different temperatures: (1) 20°C, (2) 30°C, (3) 40°C and (4) 50°C. ....................................................................................................... 31
Figure 10 The benzotriazole molecule [63] ................................................................... 36 Figure 11 Technic "Mini Electroless Copper Line" system ........................................... 46 Figure 12 Singleton “Mini Barrel” system ..................................................................... 46 Figure 13 Tinnerman automotive fastener used for this thesis ..................................... 47 Figure 14 Linear sweep data for uninhibited 1 vol% sulfuric acid cleaning solutions at
25°C, 50°C, and 75°C .................................................................................... 65 Figure 15 Evans diagram illustrating the effect of temperature increase ....................... 66 Figure 16 Weight losses converted to corrosion current density over time for various
uninhibited sulfuric acid cleaning solutions ................................................. 69 Figure 17 Corrosion current density (by converted weight loss) data for various
quinine concentrations in barrel-cleaned 1 vol% sulfuric acid cleaning solution .......................................................................................................... 73
Figure 18 Evans diagram illustrating the effect of quinine on iron dissolution ............. 74 Figure 19 Temperature dependence of corrosion rate in 0.03 g/L quinine inhibited
sulfuric acid solution ..................................................................................... 75 Figure 20 Linear sweep data performed in inhibited solutions at 25°C, 50°C and 75°C ........................................................................................................ 77 Figure 21 SEM images at (a) 250X and (b) 500x magnification of samples as well as
(c) surface mapping of samples plated under the following conditions: 1) uninhibited, 5.0 g/L CuSO4 for 60s, 2) inhibited, 2.5 g/L CuSO4 for 60s, 3) inhibited, 5.0 g/L CuSO4 for 60s and 4) inhibited, 5.0 g/L CuSO4 for 90s at slow RPM ...................................................................................................... 87
Figure 22 Polarization resistance for iron in 0.03 g/L quinine inhibited sulfuric acid solution at room temperature ......................................................................... 99
Figure 23 Tafel extrapolation data for a 0.03 g/L inhibited sulfuric acid cleaning solution at 25°C ........................................................................................... 100
Figure 24 Cathodic Tafel area from Figure 22 ............................................................ 100
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Figure 25 Anodic Tafel area for Figure 23 ................................................................. 101 Figure 26 Corrosion tests set up prior to addition of filter paper and chemicals ......... 104 Figure 27 Bird’s eye view of sample holder without lid for corrosion testing ............ 105
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List of Appendices Appendix 1 Corrosion Current Density Sample Calculation ....................................... 99 Appendix 2 Raw Electrochemical Data ...................................................................... 102 Appendix 3 Amercoat™ 90 Epoxy Application Procedure ....................................... 103 Appendix 4 Corrosion Test Set Up............................................................................. 104 Appendix 5 Uninhibited Hydrochloric Acid Solution Samples ................................. 106 Appendix 6 Uninhibited Sulfuric Acid Samples ........................................................ 107 Appendix 7 Inhibited Sulfuric Acid Solution Samples .............................................. 108 Appendix 8 Complete Copper Bath Beaker Test Results ........................................... 109 Appendix 9 Variation of copper sulfate in batch copper flash tests ........................... 110 Appendix 10 Summary of inhibited copper flash batch tests....................................... 111
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1. Introduction
Copper coatings are widely used as a baseline in plating technologies [1]. They
provide a conductive base for further plating, and an inexpensive corrosion resistant
coating for end-of-line products. Since the copper coating process is additive, that is
additional chemicals are added to the baths at each stage, the chemistry becomes complex
and must be discarded after use. This causes an unnecessarily large amount of waste and
is costly. By installing a continuous processing line (Figure 1) and recycling and
recovering from the flow streams, there is an opportunity to minimize the environmental
impact from the process streams. This would be highly advantageous in any industry
requiring copper coatings that are environmentally regulated.
Figure 1 Proposed continuous plating line schematic
The process must be a low cost coating process that is applicable to parts having
complex geometries and must be able to be completed in a reasonable time frame. A set
of useful criteria is outlined in Table 1.
An initial feasibility test was conducted by M. Stemp [2] to develop conditions for
a new cleaning solution, plating solution and anti-oxidizing agent. The final goal of this
project is to develop an effective copper coating system that will avoid the use of
expensive proprietary formulations and that will be suitable for components with
complex geometries. The system should provide a high gloss coating, with uniform
thickness, and controllable porosity that may be coated in a reasonable time using the
least aggressive cleaning and plating solutions possible.
Various plating techniques are possible; however initial testing determined that a
barrel coating set up should be used for the scope of this project. This will allow for
integration of the cleaning and copper flash processes developed as a preliminary step for
Cleaning
Rinse
Copper Plating
Rinse
Brightening Agent
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Stage Criteria Importance
Cleaning
Minimize effluent Decrease the total concentration of cleaning
agent in waste stream
Robust time frame Allows for a time insensitive window for
complete cleaning of multiple parts
Environmentally
conscious chemicals
Avoid use of hazardous chemicals
Plating
Minimize effluents Decrease the chemical concentration where
possible to minimize waste
Compatible chemistry
with cleaning solution
Allow for recycling of cleaning solution into
plating solution or vice-versa
Good throwing power Required for plating complex parts
Low cost reagents Improve economic feasibility
Moderate deposition rate Must be able to deposit uniform coating on all
samples within adequate time frame
Good adhesion Coating must not delaminate or flake off
Simple apparatus and
procedures
Decrease the start-up costs
Re-useable solution Improve economic and environmental
effectiveness relative to commercial systems
Brightener
Minimize Effluent Decrease waste
Environmentally
conscious chemicals
Avoid hazardous chemicals
Maintain bright copper
coating color
Ensure copper plated parts can be stored for a
reasonable time before delivery to customer
Table 1 Criteria for improving the sustainability of electroless copper plating processes
barrel plating systems. Barrel plating is the most efficient method to finish bulk parts
that do not require individual handling. Furthermore, the system allows for easy transfer
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of components in a multi-step cleaning and plating system and would allow for easy
recycling of the streams.
1.1. Objectives
The objective of this project is to develop a replacement for the
mechanical batch copper plating process on heat-treated steel fasteners currently used in
the automotive industry. For this project there were three stages; cleaning, copper
flashing and application of anti-oxidant coating. The operating window – or allowable
immersion time in solution before any adverse effects arise – should be maximized for
each step, and the ionic and organic species present and concentrations of each species
should be minimized to make each step as economically feasible as possible.
The goals for the cleaning stage were to determine the acidic cleaning medium
and inhibitor. The concentration of all acids used was to remain at a minimum to
minimize the effluents in the waste stream. If possible, the species used in the cleaning
step should contain some of the same species as those used in subsequent steps to allow
recycling of the solution. An environmentally friendly inhibitor for the cleaning solution
needed to be identified and the efficiency of the inhibitor over a range of concentrations
and temperatures had to be determined. The inhibitor was required to minimize corrosion
of the parts and provide for a time insensitive processing window.
The goal of the copper flash step was to provide a uniform visible coating while
minimizing the concentrations of all the required chemicals in a simple copper sulfate
bath. To do so, it was necessary to determine the role of each component in the solution.
Ideal plating parameters were then determined by varying plating conditions.
The final goal of this thesis was to determine an environmentally friendly anti-
oxidizing agent to be applied to the flashed samples. The chemical to be used, as well as
plating parameters were determined. A tarnishing test was evaluated and the corrosion
resistances of the coatings were determined.
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2. Background
2.1. Methods of cleaning iron
A cleaning step will be necessary to remove an oxide layer left after heat
treatment of the fastener. Below is a summary of the most commonly used methods of
cleaning iron and ferrous alloys.
2.1.1. Acid Cleaning
Various acids may be used to clean ferrous alloys. If inorganic acids are used in
low concentrations and at low temperatures, the cleaning process is referred to as acid
cleaning, while pickling typically refers to high temperature, higher acid concentration
solutions.
2.1.1.1. High-Concentration Acid Cleaning (Pickling)
Acid pickling is a fast, reliable method of cleaning oxides from an iron substrate.
It is economical and easily scaled up, making it ideal for batch processes. Unlike acid
cleaning, pickling is typically conducted at high temperatures, and while the exact
temperature will vary depending on the acid type, it usually ranges from 60-90°C [3-5].
A variety of pickling solutions are available, the most commonly used and inexpensive
solutions for ferrous alloys are hydrochloric acid and sulfuric acid solutions.
Sulfuric acid solutions are the most commonly used pickling solution. They
typically use 10-15 vol% and operate at temperatures ranging from 65-83°C.
Hydrochloric acid solutions are frequently used for continuous operations. The cleaning
solution parameters are 20-40 vol% at ambient temperature to 55°C.
Advantages of pickling include the wide range of operating parameters of the
pickling solution which may be varied to meet specific requirements (i.e. for a specific
alloy composition, a specific temperature and acid concentration may be more effective),
an inexpensive set up, easily scalable, the low cost of materials and the process may be
adapted for continuous use.
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Disadvantages to pickling include high amounts of metal loss due to the
aggressive nature of the solution. There is a potential source of hydrogen embrittlement,
especially in high-carbon steels and the process is likely to deposit smut on the substrate.
Further, excessive pitting may occur in cast irons and steels.
2.1.1.2. Low-Concentration Acid Cleaning
Low concentration acid cleaning is frequently used for final or near-final
preparation of substrates before plating [3, 4]. Typically low concentration cleaning
solutions are used on samples that are not heavily oxidized or contaminated with
organics.
It is less aggressive than acid pickling but may use the same acids, namely
sulfuric and hydrochloric acids. The cleaner composition may be limited to one or two
acids, or may contain various additives such as surfactants and inhibitors. The
application methods are typically immersion, spray or rotating-barrel methods.
Important process parameters for low acid cleaning include the temperature of the
solution, acid concentration, agitation of parts, and rinsing. Low acid cleaning, unlike
pickling, is a room temperature process. Although the efficiency of the cleaning solution
increases with increasing temperature, high temperature solutions can lead to aggressive
chemical attack on the surface of the metal. Furthermore, at high temperature many
additives such as inhibitors and surfactants risk breaking down due to poor thermal
stability. Surfaces emerging from high temperature solutions are likely to dry and streak.
Finally, the life of the cleaning solution and equipment decreases significantly in high
temperature cleaning solutions. Further discussion of temperature effects can be found in
section 2.2.2.
Acid concentration will affect the aggressiveness of the solution in terms of both
scale and oxide removal and attack on the metal substrate. The effect of acid
concentration is further discussed in section 2.2.4
Agitation is a crucial parameter in cleaning. It helps to remove surface oxides,
removes dissolved metal ions from the surface of the metal and helps prevent local pH
6
changes. Changing the degree of agitation will have a large effect on cleaning. This is
further discussed in section 2.2.5.
Rinsing plays an important role in acid cleaning by removing cleaning
byproducts. Cold water or water at room temperature is frequently used if the purpose of
cleaning is simply to remove metal oxides. If waxes or heavy greases are being removed,
then a hot water soak is used to avoid resolidification of residues.
2.1.1.3. Organic Acid Cleaning
Organic acids are used as an environmentally friendly alternative in various metal
cleaning operations. Typically acids used include citric acid, acetic acid, EDTA, formic
acid, gluconic acid and hydroxyacetic acid. These acids may be used alone or formulated
with bases and other additives such as inhibitors, surfactants and chelating agents [5].
A major advantage in organic acid cleaning is that organic acids may have
multiple functions and act as acids, buffers or chelating agents [6]. In solution, they will
release one or more hydronium ions, and typically have multiple dissociation constants
providing some buffering capacity. If there are two or more complexing sites, they may
act as chelating agents.
Organic acids remove oxides by reacting with the metal to produce salts such as
citrates and acetates, as well as dissolved metal ions and other byproducts. As these
reactions occur, hydrogen gas is produced and accumulates under the oxide layer. The
force of hydrogen gas evolution can lift oxides off the surface of the metal. The organics,
acting as chelating agents, will then complex the metal ions and remove them from the
surface into the bulk solution.
Further advantages include low corrosivity to the base metal compared to mineral
acids such as sulfuric and hydrochloric acids. Organic acid cleaning solutions typically
operate at a higher pH than mineral acids. This further reduces the corrosivity of the
cleaning solution, and makes the solutions safer to handle.
Disadvantages of organic acid cleaning include longer cleaning times due to the
decreased corrosivity of the solution, higher temperature requirements and higher costs of
the acids compared to mineral acid cleaning solutions.
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Waste disposal of organic solutions can be achieved through a variety of methods.
Biodegradation, chemical treatment and incineration are common disposal methods.
Spent solutions may also be recovered through ion exchange and reduction of metal ions
via reducing agents.
2.1.2. Electrolytic Cleaning
Electrolytic cleaning is frequently used to remove oxides from steels that are to be
electroplated in continuous processing lines [3, 7]. It is especially valuable for removing
oxides such as Cr2O3 and FeCr2O4, which are difficult to remove through acid pickling
alone. The process is typically performed at room temperature and at low sulfuric acid
concentrations (5-10 vol%). The sample is charged cathodically for 15-20 seconds at
current densities typically less than 1000A/m2 [7]. To combat smut formation if the
sample is cleaned anodically, a short cathodic charge is applied before the sample is
removed from solution.
Sulfuric acid is most commonly used for electrolytic cleaning. Hydrochloric acid
is never used as the anode reaction produces toxic chlorine gas.
2.1.3. Alkaline Cleaning
If hydrogen embrittlement is of concern, a highly caustic alkaline solution may be
used as an alternative to acidic media [7,8]. Alkaline cleaners are ideal for removal of
oils, greases, waxes, metallic oxides and dirt. The solutions contain three main
components [9]:
i. Builders: These are the alkaline salts in the solution. Cleaners may include a
blend of phosphates, silicates, carbonates and borates.
ii. Additives: These may be solvents that aid in cleaning, such as glycols and gycol
ethers, corrosion inhibitors to prevent metal oxidation and chelating agents to
soften the water and counteract any metal ion activity.
8
iii. Surfactants: These organics are the main force in removal of soils on the sample.
They act by lowering the surface tension of the cleaner at the sample’s surface,
thus allowing uniform coverage of the sample.
Alkaline cleaners are typically operated at very high alkaline salt concentrations
in the order of 120-360 g/L and at temperatures above 90°C. Barrel cleaning is quite
commonly used with an alkaline cleaner. However, it is possible to over clean a sample,
causing corrosion of the sample and localized pitting. Since the samples in this work
arrive directly from a high temperature furnace, no organic residues are expected.
2.2. Dissolution of iron in acidic media
2.2.1. Mechanism of iron dissolution
2.2.1.1. Electrode Potential In order for iron dissolution or oxidation to occur, a reduction reaction must occur
simultaneously to provide a balance of charge with respect to the exchange of electrons
[10]. In acidic media, this will typically be the hydrogen evolution reaction shown in
equation {1}. The pathway of iron dissolution is discussed in section 2.2.1.3 below. The
overall reaction may be summarized as a simple oxidation, as seen in {2}. The
summation of the half reactions {1} and {2} give the overall dissolution reaction of iron
in acidic media, {3}.
{1} 2H + + 2e− → H2
{2} Fe → Fe 2+ + 2e−
{3} 2H + + Fe → Fe2+ + H2
The Gibbs free energy associated with the overall reaction is related to the voltage
by the thermodynamic equation:
9
{4} ΔG = −nFE
ΔG ≡ Gibb’s free energy (J/mol)
n ≡ number of electrons transferred in reaction
F ≡ Faraday’s constant; 96486 C/mole
E ≡ potential (V)
Note the sign convention is such that a spontaneous reaction will have a positive
potential and a negative free energy. Half cell potentials (given for reduction reactions)
are measured against a standard hydrogen electrode, which is assigned a reference
potential of 0 volts. The potential of the overall reaction is taken as the difference
between the cathodic and anodic half-cell potentials, described in {5}.
{5} E = ec − ea
E ≡ overall cell potential (V)
ea ≡ anodic half-cell reduction potential (V)
ec ≡ cathodic half-cell reduction potential (V)
It is important to note that the half-cell potentials are all taken using the reduction
reaction of the species involved. Standard half-cell potentials are easily found in
reference tables [10] while half-cell potential in non-standard solutions may be calculated
through the use of the Nernst equation {6}.
{6} e = eo −2.3RT
nFlog
aprodi[ ]
areactj[ ]
e ≡ half-cell potential (V)
eo ≡ equilibrium half-cell potential (V)
R ≡ gas constant; 8.314 J/K•mol
T ≡ absolute temperature (K)
aprod ≡ activity of products
areact ≡activity of reactants
i ≡ stoichiometric coefficients of products
j ≡ stoichiometric coefficients of reactants
The half-cell potential is a measure of the oxidizing power of a species. The
species with the more active (negative) potential will proceed via an oxidation reaction
10
while the species with the more noble (higher) potential will proceed via a reduction
reaction.
2.2.1.2. Kinetic dissolution parameters Since electrochemical reactions produce or consume electrons, the flow of
electrons to or from a reacting surface can give a measure of the reaction rate [11]. This
proportionality is described in Faraday’s law {7};
{7} nFItam =
m ≡ mass reacted (g)
I ≡ electron flow, or current (A)
t ≡ time (seconds)
a ≡ atomic weight (g/mol)
n ≡ number of equivalents
F ≡ Faraday’s constant; 96 486 (Coulombs/mol)
The corrosion rate, or corrosion current density is determined by dividing through
by time and the surface area, giving {8};
{8} nFia
tAmr ==
r≡ corrosion rate (g/cm2s) i ≡ current density (A/cm2) A≡ area (cm2)
{9} ηc = βc logicio
{10} ηa = βa logiaio
ηc or ηa ≡cathodic or anodic overpotential (V)
ic or ia ≡ current density of given reaction (μA/cm2)
io ≡ exchange current density (μA/cm2)
βc or βa ≡ Tafel slope of cathodic or anodic reaction (V/decade)
11
At equilibrium in the absence of an external bias, the forward and reverse reaction
rates are equal, and the current is referred to as io, the exchange current density. The
overpotential, η, is the voltage relative to equilibrium required to achieve a given current
due to polarization losses in the cell and is given by η = eapplied - e. For cathodic
polarization at steady state, the overpotential can also be expressed as the well-known
Tafel equation {9} and for anodic polarization it is represented as {10}.
If the overvoltage becomes sufficiently large then diffusional rates start to limit
the reaction and a linear concentration gradient develops at the electrode surface in the
solution. Assuming 1st order diffusion according to Fick’s Law, the change in potential
becomes {11} illustrated in Figure 2, with limiting current density {12};
{11} ⎥⎦
⎤⎢⎣
⎡−=
Lconc i
inF
RT 1log3.2η
{12} iL =DznFCB
δ
iL ≡ limiting current density (μA/cm2)
Dz ≡ diffusivity of reacting species (cm2/s)
CB ≡ concentration of the uniform bulk solution (mol/cm3)
δ ≡ thickness of concentration gradient in solution (cm)
Figure 2 Concentration gradient in the solution near a surface controlled by concentration polarization
Distance
CB
Concentration Linear approximation Actual concentration gradient
δ
Co = 0
12
Finally, the corrosion current density, icorr, exists when the current density of the
anodic reaction is equal to the current density of the cathodic reaction {13}. The
corrosion potential, Ecorr is simply the potential associated with the corrosion current
density.
{13} ic = ia = ⏐icorr⏐
2.2.1.3. Iron dissolution pathway
The overall iron dissolution reaction shown in {3} does not indicate the
mechanism by which iron corrodes. Many authors agree [12, and references therein] that
the dissolution reaction shows no dependence on ferrous ion concentrations, but on a
reaction catalyzed by hydroxyl ions. This holds true even in highly acidic solutions.
Hilbert et al. [12] suggested that there is a possibility of hydroxyl generation from
water molecules by deprotonation. The cause of this reaction is claimed to be the
positive surface charge and tendency of transition metals to form complexes.
Heusler [13] and Bockris [14] proposed two separate theories for the anodic
dissolution mechanism of iron. Heusler proposed a mechanism of iron dissolution
whereby the precipitation of FeOHads would act as a catalyst for further Fe dissolution.
Bockris proposed an alternative mechanism with an intermediary species of FeOH+
produced from the FeOH taking part in the dissociation mechanism.
Lorenz [15] showed that both proposed mechanisms may occur, and that it
depends on the purity and surface morphology of the iron surface. High purity iron with
low surface activity follows the mechanism proposed by Bockris , while highly active
iron with higher surface activity follows the method proposed by Heusler.
Below are the two proposed pathways for iron dissolution. Both dissolution
pathways begin with iron reacting with water to form an adsorbed iron species a
hydroxide ion and an electron being released:
{14} Fe + H2O ↔ Fe(H2O)ads
{15} Fe(H2O)ads ↔ Fe(OH−)ads + H +
13
{16} Fe(OH−)ads ↔ (FeOH)ads + e−
For an overall first reaction step:
{17} Fe + H2O↔ (FeOH)ads + H + + e−
In high purity iron the dissolution pathway follows the mechanism proposed by
Bockris:
{18} (FeOH)ads → FeOH+ + e−
{19} FeOH+ + H+ ↔ Feaq2+ + H2O
In iron with a higher density of crystal imperfections, the dissolution pathway
follows the mechanism proposed by Heusler:
{20} Fe + (FeOH)ads → [Fe(FeOH)]
{21} [Fe(FeOH)]+ OH− → FeOH+ + (FeOH)ads + 2e−
{22} FeOH+ + H+ ↔ Feaq2+ + H2O
Both mechanisms result in the same final step, equations {19} and {22}. The
“catalyzed” mechanism, as proposed by Heusler uses the surface catalyst [Fe(FeOH)].
This catalyst likely forms at the kinks of the metal crystals or at adatoms in their vicinity
[12]. The “noncatalyzed” mechanism features two individual charge transfers, while the
“catalyzed” mechanism features one charge transfer in which two charge units are
transferred. This discrepancy in the charge transfer method is reflected in the predicted
Tafel slopes for each mechanism, as seen in Table 2. These Tafel slopes were derived
from the kinetic equations of each reaction pathway [12]. Data for steady-state could be
likened to advanced stages of cleaning, whereas nonsteady-state would be more
representative of initial reaction rates.
14
Reaction mechanism Predicted Tafel slope
Steady-state reactions Nonsteady-state reactions
Noncatalyzed 40 mV 40 mV
Catalyzed 30 mV 60 mV
Table 2 Calculated anodic Tafel slopes for the proposed reaction mechanisms in steady-state and nonsteady-state reactions. Electrochemical data derived by Tafel extrapolation for the dissolution of iron
from various sources in a variety of acidic media is provided in Table 3. Both inhibited
and uninhibited solution data is provided. The effect of inhibitors will be discussed in
section 2.4.
Source Medium Ecorr vs.
AgCl
icorr
(μA/cm2)
βa
(mV/dec)
βc
(mV/dec)
A.C Makrides
[16]
0.52N H2SO4 -0.476
–
-0.487
50 – 60 95-105
L. Cavallaro
[17]
1N HCl -0.460 67 50 130
1N H2SO4 -0.442 68 70 112
+ 10-4M thiourea -0.446 8.5 84 136
+ 10-3M thiourea -0.436 4.7 92 147
+ 0.02%
polybutanimine
-0.424 20 57 118
Table 3 Tafel data for iron in various inhibited and uninhibited acidic media
The dissolution of iron oxides in various acidic media will be discussed in section 2.2.3.
15
2.2.1.4. Corrosion rates
Corrosion testing is used for a variety of reasons [18,19], including determining
service life of equipment or a coating, screening available metals and evaluating the
corrosivity of a solution. Higher corrosion rates will indicate a highly aggressive
cleaning solution, which is to be avoided for the specified system.
For this thesis, corrosion current densities (icorr in μA/cm2) were determined
electrochemically by Tafel extrapolation in accordance with ASTM G3 - 89(2004)
Standard Practice for Conventions Applicable to Electrochemical Measurements in
Corrosion Testing [20]. The theory behind two different techniques for determining icorr,
polarization resistance and Tafel extrapolation, are discussed in sections 2.2.1.5 and
2.2.1.6 respectively. Another method of determining corrosion rate, by weight loss
analysis, is described in detail in section 2.2.1.7.
2.2.1.5. Polarization resistance
Polarization resistance provides information on the corrosion rates of a material
close to equilibrium (i.e. at small overpotentials) based on the idea that the degree of
polarization for a given system will be greater for a system with lower corrosion rates
[21-26]. Regions of linearity are observed at overpotentials within a few millivolts of
Ecorr. Polarization resistance is defined as the slope of this region:
{23} Rp =Δε
Δiapp
Rp ≡ polarization resistance (Ω/cm2)
Δε ≡ overpotential (V)
Δiapp ≡ applied current density (A/cm2)
Measurements of Rp are determined using a controlled potential. Typically the
voltage applied is ±10 mV from Ecorr, as seen in the polarization resistance curve shown
in Figure 3.
16
Rp is then used in conjunction with the Stern-Geary [22] equation to find the
corrosion current density:
{24} icorr =βaβc
2.3Rp βa + βc( )
βa ≡ anodic Tafel slope (mV/decade)
βc ≡ cathodic Tafel slope (mV/decade)
RP ≡ polarization resistance (Ω)
icorr ≡ corrosion current density (μA/cm2)
Figure 3 Polarization resistance data at small applied current densities in
0.52N H2SO4 at 30°C [From Graydon, J. W. "Linear polarization."
Lecture.]
Some automatic estimation software assigns a default value of βa = βc = 0.1.
While this will give a maximum error of a factor of two [22] a more accurate way of
determining the Tafel slope is discussed in section 2.2.1.6.
17
2.2.1.6. Tafel extrapolation
The Tafel region is defined as the linear region of a semi log polarization curve.
Conventionally, it is measured 60 mV away from Ecorr in either the cathodic or anodic
direction. For the iron system, the Tafel behavior is typically limited to one decade of
current density [22,27].
Tafel slopes indicate the rate determining step of the reaction mechanisms
occurring in a system. Changes to the cathodic and anodic Tafel slopes due to the
addition of an inhibitor may indicate the nature of an inhibitor through a change in the
mechanism of an oxidation or reduction reaction.
The corrosion potential and current density may also be determined using an
extrapolation of the Tafel slope to the Ecorr value. Typically the cathodic Tafel slope is
used for such calculations [22, 27, 28] with the Tafel line being traced back to the
determined Ecorr value. The point where Ecorr and the Tafel lines meet will give icorr, the
corrosion current density. Only the cathodic Tafel slope is used when the anodic
polarization curve does not show well-defined Tafel behavior due in part to the
dissociation of the anode (i.e. metal ions). Rapid dissociation of ions may contaminate
the solution before the anodic polarization measurements are complete or the surface may
be changed as ions are liberated.
As the potential is pushed cathodically or anodically from Ecorr, there must be a
charge conservation associated with the current. If the overpotential is negative (i.e.
moving to the cathodic reaction side), the increase in cathodic reduction rate must cause a
decrease in the anodic oxidation rate. The applied current may then be described as
follows:
{25} iapp,c = ic − ia
ic ≡ cathodic current (A)
ia ≡ anodic current (A)
iapp ≡ applied current (A)
Using formula {25}, the derived anodic Tafel slope may be calculated as the
difference between the applied cathodic current and the extrapolated Tafel slope. The
18
extrapolated Tafel slope is simply a continuation to Ecorr of the Tafel region on the
polarization diagram. An example is shown in Figure 4.
The point of intersection between the derived cathodic Tafel slope and the
experimental anodic Tafel slope will be Ecorr, with a corresponding icorr, giving the
corrosion current density.
Figure 4 Polarization curve showing experimental and extracted Tafel lines for
1080 steel in deaerated 1N H2SO4. βc= -98mV, βa= 38 mV (derived
from cathodic data), icorr = 1180μA/cm2 [29]
2.2.1.7. Comparison of weight loss versus electrochemical
methods
Weight loss data may be converted to corrosion current densities by Faraday’s
law {7}, which states proportionality between current density and mass reacted. Thus, if
19
the current density is known, the corrosion rate in terms of weight loss may be calculated
and vice-versa.
The most prominent form of expressing corrosion rates are mils per year (mpy) or
mm/yr [30]. Corrosion current density may be converted to corrosion rate by the
following:
{26} aCrnicorr
ρ=
icorr ≡ corrosion current density (μA/cm2)
a ≡ atomic weight (g/mol)
n ≡ the number of equivalents
ρ ≡ density (g/cm3)
C ≡ conversion factor equal to 0.129 for mpy and 0.00327 for mm/yr
r ≡ corrosion rate (mpy or mm/yr)
2.2.2. Effect of temperature on corrosion rate
Hudson [8] showed that for sulfuric acid and hydrochloric acid pickling solutions,
the cleaning (corrosion) rate increased with increasing temperature.
From equation {6}, it can be seen that an increase in temperature will lead to an
increase in potential. Thus, as temperature increases, Ecorr will increase. Eo is defined at
standard conditions but is also temperature dependent and will increase as temperature
increases.
Awad [31] performed Tafel extrapolation measurements on low carbon steel in
aerated hydrochloric acid solutions with and without an inhibitor. His data showed no
significant increase in potential, but a dramatic increase in corrosion current density with
increasing temperature, as seen in Table 4.
20
T (°C) Ecorr (mV
vs. SCE)
βa
(mV/dec)
βc
(mV/dec)
icorr
(mA/cm2)
20 576 125 126 3.66
30 568 125 106 6.00
40 572 136 96 16.49
50 564 143 98 19.90
Table 4 Corrosion data for low carbon steel (wt%: 0.1 C, 0.29 Mn, 0.07 Si, 0.021P) in a 1 M uninhibited hydrochloric acid solution at various temperatures
2.2.3. Effect of anion type
Various mechanisms exist for iron and iron oxide dissolution in acidic media [12-
16, 32]. For the purpose of this thesis, the two main acids of interest were hydrochloric
and sulfuric acids.
2.2.3.1. Hydrochloric Acid The dissociation of the base metal will form ferrous chloride and hydrogen gas, as
follows:
{27}
If an oxide film is present, dissociation in hydrochloric acid involves a direct
attack on the oxide film. The film will dissociate by one of the following two reactions:
{28}
{29}
21
2.2.3.2. Sulfuric Acid
The dissociation of iron oxides in sulfuric acid involves the acid penetrating the
oxide layer through cracks. The substrate is then dissolved and hydrogen gas is formed
by the following reaction:
{30}
The hydrogen gas present, along with the dissolution of FeO will aid in removal
of the remaining metal oxides, which will then dissociate in solution. The dissociation
reactions for the oxides are as follow:
{31}
{32}
2.2.4. Effect of acid concentration
For both sulfuric and hydrochloric acid cleaning solution, the corrosion rate
increases with increasing acid concentration [3-8, 33]. Reaction rate is a function of
concentration (by the rate law) regardless of the order of the reaction. Thus increasing
the concentration of the acid will increase the overall reaction rate. Since the reaction
rate is a measure of corrosivity of a solution, it follows that the corrosivity of a solution
will increase with increasing acid concentration. This may be seen in the iron dissolution
data for varying concentrations of hydrochloric acid in Table 5.
22
HCl
(mol/L)
Ecorr (mV
vs. SCE)
βa
(mV/dec)
βc
(mV/dec)
icorr
(mA/cm2)
0.1 542 79 117 1.51
0.5 569 68 119 2.51
1 553 65 122 3.80
5 525 120 130 16.60
Table 5 Electrochemical data for iron dissolution in hydrochloric acid solutions of varying concentration, performed at room temperature on Armco iron [33]
2.2.5. Effect of fluid velocity
Cathodic reduction reactions on the surface of the corroding metal will deplete the
nearby solution of H+, limiting the maximum possible current density. A schematic of
the depletion of H+ in solution is shown in Figure 2.
The maximum reaction rate that cannot be surpassed due to depletion is called the
limiting current density and is defined in {12}. By increasing solution velocity, one may
decrease δ and increase iL. If the limiting current density is increased, the maximum
possible reaction rate is also increased and the corrosion rate can increase.
For cleaning, a high flow is required both to decrease cleaning time (by increasing
corrosion rate) and to aid in removal of oxides by mechanical means [34, 35]. If the flow
is too low, there will be insufficient replenishment of hydrogen required for oxide
dissolution. However, if the flow is very high erosion may occur, which is undesirable.
2.3. Smut Formation
2.3.1. Definition of smut
A major problem with acid cleaners is the formation of smut. Smut is a term used
to denote an accumulation of reaction products on metal surfaces [36,37]. Typically
23
caused by “over cleaning” smut occurs more frequently in high-carbon and high-silicon
steels. Smut formation is a significant concern in developing the cleaning system since it
can decrease the adhesion of metal coatings by up to 50% [38].
2.3.2. Chemistry
Smut is an amorphous, highly adhesive oxide coating. Baun [36] showed that the
composition of smut on silicon containing steel was almost entirely graphite and silicon
oxide. He found that ferrous alloys and steels containing silicon are more likely to form
smut in large quantities. Further, he found that on steels and ferrous alloys contain
silicon, the smut was primarily silicon oxides and trace amounts of other metallic and
non-metallic oxides. Smuts of this nature were considerably more difficult to remove
from the substrate than simple carbon-based smut. Since the alloy used for this thesis has
a silicon content of 1.5-1.8 vol%, it is likely that it would form an adhesive silicon-based
smut. Thus smut formation should be avoided.
2.3.3. Factors affecting smut formation
Various factors affect smut formation. The most prominent is changes in
temperature. As discussed in section 2.2.2 above, reaction rate increases with
temperature. This will cause an increase in the rate of smut formation as well. Allen et al
[37] showed that for acid solutions, smut formation will increase with increasing
temperature and acid concentration. The atmosphere also has a large effect on smut
formation. Highly aerated solutions will have faster smut formation due to the
availability of oxygen. Finally, since smut is commonly dominated by silicon oxide,
higher silicon content in steels will lead to greater smutting.
24
2.3.4. Methods of controlling smut formation
Smut may be lightly bonded, in which case it may be removed by simply rinsing
or light mechanical work (for example, rubbing with a cloth). However, some smut
particularly smut with a high silicon oxide content is highly adherent, requiring
aggressive chemical treatment for removal, such as chromic acid dips [36] or highly
concentrated sulfuric or hydrochloric acid solutions [37]. Thus it is important to avoid the
formation of smut. Should it occur in the system being investigated in this work, removal
by mechanical means would be impractical due to the large number of components being
processed, and chemical removal would result in additional environmental waste
products.
Smut formation may be minimized by operating at low temperatures, with high
acid concentrations and in deaerated atmospheres. Another means of controlling smut is
by decreasing the amount of silicon in the steels used for a given application.
As smut formation increases with time [37], to avoid smut formation, a short
cleaning time frame is needed. However, the time frame should be long enough to ensure
sufficient oxide removal. As such, it is likely that an inhibitor will be needed to slow
corrosion of the metal surface and smut formation.
2.4. Inhibitors
A wide variety of inhibitors are available to reduce the corrosion rate of iron in
both hydrochloric and sulfuric acid solutions [39-56]. These inhibitors slow down the
rate of chemical attack on the surface of the metal by one of two means: either by
slowing the rate determining step of the reaction mechanism or by blocking active sites
on the surface of the metal. A minimum concentration of the inhibitor must be present,
however an excess of the inhibitor may lead to a more aggressive solution than the
uninhibited solution. Good circulation should always be present to avoid stagnant areas
where the inhibitor may be depleted.
25
Most inhibitors are considered hazardous to the environment. Only
“environmentally friendly” non-hazardous inhibitors were considered in this thesis,
limiting the type and range of inhibitors that could be successfully employed in this
study.
2.4.1. Definition and classification of inhibitor types for Fe
Inhibitors may be classified as either organic or inorganic inhibitors. Typically in
acidic solutions, sulfur-containing compounds are used for sulfuric acid solutions, while
nitrogen-containing compounds are used for hydrochloric acid solutions [39]. A list of
commonly used inhibitors is provided in Table 6. Inhibitors act by bonding to the surface
of the compound and serving as a barrier to corrosion or they may alter the anodic and/or
cathodic behavior of the surface and slow the oxidation reaction, reduction reaction or
both reactions involved in the dissolution of iron.
Acid Media Commonly used inhibitors
Sulfuric acid Phenylthiourea, di-ortho-totyl-thiourea, mercaptans, sulfides
Hydrochloric acid Pyridine, quinoline, various amines, decylamine,
phenylthiourea, dibenzylsulfoxides
Table 6 Inhibitors commonly used for industrial cleaning applications
There are three basic inhibitor types: anodic, cathodic and mixed inhibitors. A
brief description of each type is summarized below.
2.4.2. Anodic Inhibitors
Anodic inhibitors form a protective passive film at the anode, increasing
the potential of the anode and slowing corrosion. Increasing the anode potential will
cause Ecorr to increase and icorr to decrease, as seen in Figure 5. These inhibitors enhance
chemisorption of dissolved oxygen, which aids in passivating the metal. A high dose of
anodic inhibitor is typically necessary to initiate film formation in the system.
26
Since the inhibitors mask the anodic site, it is critical that they are not depleted
since below a critical limit, pitting may occur due to area effects. Area effects occur
when a large amount of anodic sites are blocked, the corrosion current stays the same, but
the unprotected areas must support a higher corrosion current density, thus increasing the
local corrosion rate.
Figure 5 Evans diagram showing the effect of anodic inhibitors on corrosion current. Anodic inhibitors have varying effectiveness in varying pH. Common anodic
inhibitors and the medium in which they are used are listed below [38]:
i. Chromate and nitrite (neutral solutions) – these inhibitors catalyze the reaction
between the metal and oxygen, creating a passive film. Chromate and nitrite are
the only anodic inhibitors that function in the absence of oxygen.
ii. Molybdate and orthorphosphate (neutral and acidic solutions) – these inhibitors
act in a similar fashion as those listed above, however they require oxygen to be
efficient.
27
2.4.3. Cathodic Inhibitors
Cathodic inhibitors are less effective than anodic inhibitors [39], however they
will not lead to excessive pitting. They form a visible film along the surface of the
cathode, which restricts the access of oxygen to the metal substrate. The film will also
Figure 6 Evans diagram for the effect of cathodic inhibitors on corrosion current block hydrogen evolution sites create a limiting current density for the reaction since it
may no longer occur uniformly over the surface of the substrate. This will decrease Ecorr
and icorr, as seen in Figure 6.
As with anodic inhibitors, cathodic inhibitors will have varying efficiency in
acidic, alkaline and neutral solutions. Common cathodic inhibitors and the media in
which they are used are listed below:
i. Zinc hydroxide and zinc phosphate (neutral and alkaline solutions)
ii. Calcium carbonate and calcium phosphate (neutral solutions)
28
2.4.4. Mixed Inhibitors
Mixed inhibitors suppress both cathodic and anodic electrochemical reactions.
They may be a combination of cathodic and anodic inhibitors or simply an inhibitor that
adsorbs to the entire surface of the substrate, causing a barrier between the substrate and
the corrosive solution. Mixed inhibitors will cause a decrease in Ecorr and icorr for the
system, as seen in Figure 7.
Figure 7 Evans diagram for effect of mixed inhibitors on corrosion current
2.4.5. Organic Inhibitors
Organic inhibitors must be adsorbed to the substrate in order to be effective [40].
Adsorption is facilitated by polar groups on the molecule, which can attach to the
substrate. The most effective polar groups include sulfur, nitrogen and hydroxyl species.
Different organics will have varied effectiveness in any given solution. Typically
mixtures of organics are used, and derived proprietary organic compounds are common.
29
2.4.6. Quinine
Quinine is an organic compound obtained from the bark of the cinchona tree
common to South America. It is used in tonic water, and as a medicinal treatment for
muscle cramps and malaria. It is an odorless white powder that has very low solubility in
water, but is soluble in acidic solutions. The structure of quinine is shown in Figure 8.
Figure 8 The quinine molecule [31].
Quinine has been proved as an effective inhibitor in the corrosion of low-alloy
steel in hydrochloric acid solutions. Awad [31] used electrochemical impedance
spectroscopy and potentiodynamic polarization to show that the inhibitive action of
quinine was due to the mechanism of physically blocking sites on the metal surface. The
adsorption mechanism of quinine is physisorption, as proven by Awad through
calculating the magnitude of various adsorption parameters, such as ∆ , ∆ , and
∆ .
Despite evidence against quinine’s effectiveness in sulfuric acid solutions [31],
early work by M. Stemp [41] proved that quinine in low concentrations is an equally
effective inhibitor in sulfuric acid solutions at low temperature. Advantages to using
quinine are its low environmental impact and its easy detection through fluorescence
[42], which should allow for a method of monitoring the quinine concentration in the
cleaning solution and also for detection of any quinine carry-over in subsequent
solutions.
30
2.4.6.1. Mechanisms of quinine inhibition
Awad [31] found that quinine generally acts as a mixed inhibitor in hydrochloric
acid solutions. He completed polarization curves at various temperatures and quinine
concentrations. An excerpt of his results is summarized in Table 7, with θ representing
T (°C) C (mM) Ecorr (mV) βa
(mV/dec)
βc
(mV/dec)
icorr
(mA/cm2)
θ
20
0 576 125 126 3.66 N/A
0.08 563 112 90 1.39 0.62
0.8 568 99 71 0.15 0.96
30
0 568 125 106 6.00 N/A
0.08 574 114 95 2.70 0.55
0.8 559 114 63 0.66 0.89
40
0 572 136 96 16.49 N/A
0.08 575 127 91 8.44 0.49
0.8 589 125 147 3.30 082
50
0 564 143 98 19.90 N/A
0.08 574 138 87 11.34 0.43
0.8 574 147 97 5.30 0.75
Table 7 Electrochemical parameters for quinine at various temperatures and
concentrations in 1 M hydrochloric acid [31].
the degree of surface coverage, which is also a measure of inhibitor efficiency. The
surface coverage was defined as the fraction of corrosion inhibition in the system:
{33}
i1 ≡ corrosion current densities in the absence of quinine (μA/cm2)
i2 ≡ corrosion current densities in the presence of quinine (μA/cm2)
31
By examining the cathodic and anodic Tafel data, it is apparent that for
any given temperature they both change as a function of quinine. This indicates that both
reactions are being inhibited. Thus Awad concluded that the inhibition method of
quinine must be by simply blocking the surface of the substrate. The corrosion potential
data is also indicative of a mixed inhibitor, as there is no significant change over the
quinine concentration range.
Awad calculated the inhibition efficiency as related to surface coverage by the
following equation [31]:
{34}
i1 ≡ corrosion current densities in the absence of quinine (μA/cm2)
i2 ≡ corrosion current densities in the presence of quinine (μA/cm2)
His findings are summarized in Figure 9.
Figure 9 Inhibition efficiency at different concentrations of quinine for low
carbon steel in 1 M HCl at different temperatures: (1) 20°C, (2) 30°C,
(3) 40°C and (4) 50°C.
Figure 9 shows that the inhibitor efficiency increases with increasing
concentration of inhibitor. The degree of surface coverage also increases with increasing
2
32
quinine concentration, thus confirming that the method of inhibition is in fact adsorption
to the surface of the substrate.
2.4.6.2. Temperature stability of quinine
As can be seen in Table 7 and Figure 9, the effectiveness of quinine decreases as
temperature increases for a given quinine concentration. This can be observed both by an
increase in corrosion current density and a decrease in surface coverage for a given
quinine concentration.
Awad studied the thermodynamic parameters for quinine adsorption and found a
free energy of adsorption equal to ~ -20 kJ/mol, a magnitude consistent with a
physisorption process. The adsorption equilibrium constants (K) of the system were
determined where:
{35} K =1
55.5exp
−ΔGadso
RT
⎛
⎝ ⎜
⎞
⎠ ⎟
K ≡ adsorption equilibrium constant (unitless)
ΔGadso ≡ free energy of adsorption (J/mol)
R ≡ gas constant ( J
K ⋅ mol)
T ≡ temperature in K
The equilibrium constant was found to decrease with increasing temperature, thus
reinforcing the theory that at high temperature the inhibitor loses efficiency by desorption
from the substrate.
33
2.4.6.3. Measurement of quinine in process control
A major concern with organic inhibitors is that their concentration in solution may
be difficult to determine analytically, especially in real time. While titrations are
appropriate for inorganic inhibitors, they are an inefficient means of determining organic
substances due to the complexity of organics.
Quinine has an advantage over other organic inhibitors as it will fluoresce in
dilute acid solutions at excitation wavelengths of 250 and 350 nm [42-44]. Through
simple fluorescence, quinine concentrations in solution may be determined. This process
is described in ASTM standard: ASTM E 579 (2004) [45]. This property could be of
great use in determining the lifetime of cleaning solutions and the quantity of inhibitor
carry-over in any subsequent plating or brightening solutions.
2.4.7. Other possible inhibitors
Inhibitor Media Reference
Jojoba oil 1M HCl [47]
Rosemary oil 2M H3PO4 [48]
Thiourea derivatives Various concentrations H2SO4 [49]
Butindiol derivatives Various concentrations H2SO4 [49]
Artemisia oil 1M HCl [50]
Natural honey 0.5M NaCl [51]
Eugenol derivatives 1M HCl [52]
Mixed halides and thiols Various concentrations H2SO4 [53]
Extract of Nypa fruticans Various concentrations HCl [54]
Extract of hella leaves Various concentrations HCl [55]
Tetramethyl-dithia-octaaza-
cyclotetradeca-hexaene
Hot, 20% H2SO4 [56]
Table 8 Environmentally friendly inhibitors researched as quinine alternates
34
Various other environmentally friendly inhibitors were examined but not tested.
These inhibitors and the media in which they were originally tested are summarized in
Table 8 .
Of the alternate inhibitors researched, the most promising ones included
rosemary, artemisia and jojoba oil. Rosemary has been found to be an effective inhibitor
in phosphoric acid [48], however the inhibitor concentrations required (up to 10 g/L)
make it a poor choice when compared to the low concentrations of quinine required.
Concentration
(g/L)
Ecorr
(mV vs. SCE)
βc
(mV/Dec)
icorr
(μA/cm2)
Picorr
(percent)
Rosemary Oil – 2M H3PO4, 298K [48]
Uninhibited -465 188 533 N/A
10 -485 162 145 73
4 -480 166 217 59
1.5 -478 187 329 38
0.6 -472 185 445 17
Artemisia Oil – 1M HCl, 298K [50]
Uninhibited -440 153 66 N/A
19 -465 140 16 76
1.9 -470 136 31 53
1.3 -480 135 40 39
0.95 -475 137 55 17
Jojoba Oil – 1M HCl, 298K [47]
Uninhibited -500 230 936 N/A
0.515 -487 270 19 98
0.386 -480 280 50 94
0.129 -475 260 95 88
0.005 -482 259 392 58
Table 9 Potentiodynamic polarization parameters for the corrosion of iron in
acidic media with varying inhibitor concentration and type.
35
Artemisia oil as an inhibitor showed inhibitive properties in hydrochloric acid
[50]. Artemisia oil acts as a cathodic inhibitor and the inhibition efficiency was shown
to increase with increasing temperature. However, like rosemary oil, high concentrations
(up to 19 g/L) of oil were required to inhibit the solution.
Jojoba oil, in small quantities, is reported to be an effective inhibitor in
hydrochloric acid solutions [47]. Less than 0.4 g/L was required to give significant
corrosion inhibition. Should an environmentally friendly alternate to quinine prove
necessary, this could be a viable alternative.
A summary of the electrochemical properties of the above mentioned inhibitors is
provided in Table 9, with Picorr being the inhibitor efficiency as calculated in section
2.4.6.1. The most promising of the alternative inhibitors appears to be jojoba oil, which
gives high inhibition efficiency (Picorr) at low concentrations. Of the examined inhibitors,
both quinine and jojoba oil have similar achievable Picorr values, but the a much lower
concentration of quinine is required to reach high Picorr values (mM) than jojoba oil (M).
2.4.8. Benzotriazole as a copper inhibitor A second inhibitor will be required in the overall plating system to act as a
protective, corrosion resistant coating to the applied copper flash. The inhibitor
investigated was benzotriazole.
Benzotriazole (BTA) is a well-established inhibitor for copper surfaces in acidic
media [58-62]. The structure of benzotriazole is shown in Figure 10. As with most
organic inhibitors, it contains a polar group with nitrogen atoms. These nitrogen atoms
act as an electron donor, enhancing chemisorption on the surface of the metal.
36
Figure 10 The benzotriazole molecule [63] The inhibition mechanism is by adsorption onto the surface of the metal. The
benzotriazole has been shown to form a polymeric surface film of variable thickness, in
the range of 40-140 Å [60-62]. For copper corrosion inhibition, there are two possible
structure of polymeric film, depending on the nature of the copper oxide present.
Cuprous oxides yield Cu(I) BTA, which consists of linear polymeric chains. Cupric
oxides yield Cu(II) BTA, which is a network structure. This polymeric film inhibits
dissolution of copper by acting as a barrier to cathodic reactions.
Concentration of BTA (M) Corrosion rate (mg/cm2h) Picorr
Blank 1.38 N/A
10-5 0.671 51.37
10-4 0.531 61.52
5 x 10-4 0.381 72.39
10-3 0.291 78.91
5 x 10-3 0.231 83.26
10-2 0.1301 90.57
5 x 10-2 0.0501 96.36
Table 10 Corrosion rate and inhibition efficiency data obtained by Kahled [58] from weight loss measurements for copper in 0.5 M HCl solutions in the absence and presence of various concentrations of BTA at 30°C
37
Kahled used weight loss measurements for copper in 0.5 M HCl solutions in the
absence and presence of various concentrations of BTA to determine the inhibitor
efficiency [58]. His results are summarized in Table 10. Inhibitor efficiency was
calculated from {34}.
Bellakhal and Dachraoui [64] found that benzotriazole is an equally effective
inhibitor in humid air plasma. They immersed copper foils in a 0.1 M benzotriazole
solution, and exposed the surface to a strongly oxidizing gliding arc plasma in humid air.
They found that the inhibitor limits the production of copper oxides and that under very
harsh oxidizing conditions the inhibitor effect remains unmodified for 40 minutes.
Beyond that time, they observed a decrease in the efficiency of the inhibitor as marked by
increased formation of copper oxides.
2.5. Copper Coating Technologies
Copper coatings are very frequently applied as an inexpensive, corrosion-resistant
coating to bulk components. It is a low-cost coating that is easily applied and acts as a
good end-of-process coating or as a conductive substrate for subsequent metallic
coatings.
A review of common copper plating processes and plating techniques are
provided in the following sections, including the thermodynamic fundamentals of the
process commonly used.
2.5.1. Fundamentals of electroless copper plating
Electroless plating is a well-established process that lends itself to the coating of
small, complex parts. The process involves a metal substrate that is brought in contact
with an aqueous alkaline solution containing copper ions. The copper in solution is
reduced onto the surface of the substrate by the following reaction (involving
formaldehyde as the reducing agent):
{36} Cu2+ + 2HCHO+ 4OH− → Cu + 2HCOO− + 2H2O+ H2
38
This is a complex, autocatalytic reaction with highly sensitive bath chemistry and
complex, multi-step mechanisms. While bath chemistries are typically proprietary and
will vary for individual use, almost all baths will include the following [65-69]
components:
i. A metal ion source
The most commonly used metal ion source for copper plating baths is copper (II)
sulfate (CuSO4) specifically, the pentahydrate copper sulfate (CuSO4⋅5H2O) salt.
ii. A reducing agent
The anodic reaction is facilitated by the selection of a reducing agent such that the
potential is more negative than the potential for copper deposition. Selection of the
reducing agent should also ensure that the copper deposits on the substrate and does not
“plate out” in solution.
iii. A complexant
As the copper plates onto the substrate, the solution becomes depleted of ions,
causing local pH variations. These local variations may lead to precipitation of the metal
in solution. Complexants prevent precipitation by depressing the free metal ion activity.
This will also allow the bath to operate at higher pH ranges. Weak or insufficient
complexant concentration will lead to precipitation and too strong or too high a
concentration of complexant will lead to insufficient metal ions for deposition.
iv. A buffer
A buffer is required to stabilize the pH of the solution, which will vary due to the
formation of H+ as the reducing agents are oxidized. Variations in pH over time can lead
to inconsistent coating properties (composition and surface roughness) and deposition
rate.
v. A stabilizer
Stabilizers adsorb to impurities in the solution and prevent unwanted copper
precipitation and decomposition of the bath. Stabilizers act by adsorbing to the active
nuclei and shielding them from the reducing agent in the plating solution.
39
vi. An exaltant (accelerator)
Complexants and stabilizers may reduce the plating rate to unprofitable levels.
Accelerators are added to the bath to increase the plating rate by driving the oxidation of
the reducing agent.
Bath parameters such as the copper concentration, iron concentration, bath
temperature and plating time will all affect the coating’s properties. Increased copper
concentration will result in higher deposition rate, but too high a concentration will cause
precipitation to occur. Higher iron concentration in the plating solution will lead to a
lower deposition rate. Both increasing the bath temperature and plating time will
increase the deposition rate.
The electroless plating process is a multi-step process, involving various cleaning
and rinse steps before plating. Table 11 shows a flow chart of a typical electroless copper
plating system [70]. Since each cleaning solution is of a different pH and composition,
there are a large number of waste streams produced, with a large variety of species in
each.
Overall, there are numerous acidic and alkaline waste streams as well as five rinse
water streams, all of which will require waste treatment. Because of the variations in
waste type (i.e. alkaline and acidic), and the variety of ions in the waste stream there is
little room for recycling of the wastes within the system. There will also be a large
variety of species in the waste that will require treatment. By reducing the amount of
species in the waste stream there is room to minimize the amount of waste treatment
required. Furthermore, by using the same species in the cleaning and flash steps, there is
room for recycling solutions within the system.
40
Solution Type Purpose Environmental Concerns
Cleaner/
conditioner
Alkaline or
Acid
Removes organics
and impurities from
substrate
May contain a variety of species
(surfactants, chelating agents,
etc.) which need to be treated in
waste stream
Rinse Water Removes residues Contaminated water, more waste
processing
Pickling Acid
Removes
impurities, prepares
surface for plating,
promotes good
adhesion
High acid concentration makes
for difficult handling and high
concentration of ions in waste
stream
Rinse Water Removes residues Contaminated water, more waste
processing
Electroless
Copper Alkaline Deposits copper
Many bath components make for
complex and costly waste
management
Rinse Water Removes residues Contaminated water, more waste
processing
10% Sulfuric
Acid Acidic
Neutralizes the
alkalinity of the
electroless copper
Another waste stream to process,
containing acids and remnants of
copper plating solution
Rinse Water Removes residues Contaminated water, more waste
processing
Anti-oxidant Neutral Prohibits oxidation Another waste stream to process
Rinse Water Removes residues Contaminated water, more waste
processing
Table 11 Process flow for electroless copper deposition with outline of environmental concerns associate with each step
41
2.5.2. Fundamentals of immersion plating Immersion, galvanic or displacement plating is one of the simplest forms of
electroless plating [57,58, 71]. It is typically an inexpensive process, and has excellent
throwing power. This process may only be used to deposit noble metals.
This process involves the deposition of one metal in solution on to a dissimilar
substrate. The more electropositive metal will be reduced, or deposited, while the more
electronegative metal will oxidize, or dissolve. In this process, the substrate acts as the
reducing agent, so unlike electroless deposition, no reducing agents are required in
solution.
For copper plating on an iron substrate, the reaction will proceed as follows:
{37} Fe + Cu2+ → Cu + Fe2+
where: EFe
o = -0.447 V vs. SHE ECu
o = +0.337 V vs. SHE Immersion plating solutions have very simple chemistries, with the most basic
consisting of a metal ion source, such a copper sulfate, and acid or base to aid in
dissociation of the substrate. Complexing agents may be used to increase the solubility
of the metal ions in solution and inhibitors may be used to slow the overall reaction.
The maximum achievable thickness is limited with immersion plating processes,
because once the substrate is covered, there is no further driving force and the deposition
reaction stops. As such, the maximum achievable thickness is approximately 1μm.
Plating rate may be increased by increasing the concentration of depositing metal ion in
solution and by stirring. The finish of the final coating may be matte or shiny.
2.5.3. Fundamentals of mechanical plating A mechanical plating system is frequently used to plate small components with
copper. The process that this thesis is attempting to replace is a mechanical plating
process described in further detail in sections 2.5.5 and 2.5.6. A basic explanation of
mechanical plating is outlined below.
42
Mechanical plating is a kinetic process where powdered metals are cold welded to
a substrate [72,73]. Typically ferrous-based parts are plated with ductile metals, such as
copper, lead, indium, zinc, cadmium and tin. This process is most suitable to small parts,
and is frequently used on screws, bolts, nuts, washers, etc. Parts are tumbled, typically in
rubber-lined barrels with water, glass bead impact media and various chemicals, which
are added in sequence.
A surface conditioner is added first. This removes oxides present on the parts.
Surface conditioners contain a mix of mineral acids, wetting agents, and inhibitors. The
conditioner also provides the proper pH for the plating reactions.
Following the conditioner is a coppering solution, which provides an adherent
copper flash. This flash acts as an adhesive base for further coatings, provides a heavier
barrier to hydrogen and gives a uniform substrate for further plating.
Promoter chemicals are then added. These will eventually clean the metal
powder, as well as control the size of metal powder agglomerates that form. It also acts
as a catalyst to the coating process.
Finally, metal powder is added. The amount of powder added controls the
thickness and weight of the final deposit. The appearance of the finished coating is
matte.
2.5.4. Fundamentals of barrel plating
Barrel plating is a method of plating components that involves a rotating vessel
that is immersed in a variety of cleaning and plating solution tanks [74, 1]. Current is
applied to interior cathodic electrical contacts in the barrel, charging the components to
be plated and allowing for current to flow for electrodeposition reactions.
Barrel plating is most commonly used for corrosion protective coatings on bulk
components and it is the most cost-effective way to plate threaded parts and fasteners.
Benefits include fast, large volume productions, easily automated processes, and
components may remain in the same vessel from one solution to the next. Further, the
rotation of the barrel agitates the solution, eliminating the need for external pumps. The
43
rotation also keeps the parts in motion, thus yielding more uniform cleaning and metal
deposits.
Various types of coatings may be accommodated in barrel plating, including zinc,
cadmium, tin, copper and precious metals.
2.5.5. Current systems used in industry
Mechanical plating is very commonly used in the automotive industry to apply a
copper flash to small components, such as fasteners and bolts. For the scope of this
thesis, a popular commercial flash system was used as a baseline for comparison of a
developed process. The flash used is the MacDermid SC-G Copper Flash.
Solution (additive) Tumble time Function
Macuguard SCF +
Foamout
12 minutes Cleans and prepares surface for subsequent
coating, Foamout reducing foaming of solution
Macuguard XL 40 8 minutes Mechanically applies copper flash
Drain
Water and polish 1-2 minutes Brightens copper flash and creates corrosion-
resistant coating
Water Rinse Dilutes polish solution for subsequent draining
Dump
Dry
Table 12 Process outline for MacDermid SC-G Copper Flash solution
This is an additive liquid process conducted in a large, non-metallic vessel [75].
The parts to be plated are added to the vessel where they are initially mixed with water,
glass impact media and a surface conditioner. The samples are tumbled at room
temperature for 12 minutes and an anti-foaming agent is added. MacDermid SC-G copper
flash solution is then added to the vessel and the samples are tumbled for an additional 5
minutes. Finally, the samples are drained, and the vessel is filled with water and a
44
polishing solution. Samples are rinsed and dried and the used solution in disposed of. A
process outline is shown in Table 12.
2.5.6. Limitations and challenges
The current system used has a few limitations. Firstly, it is a batch process, which
limits the rate of part production to the size of the vessel and the plating time. It is a
single-use, additive process meaning there is a large amount of waste produced and
copper or iron from the waste solution may not be recovered. When coating small,
intricate parts, the used mechanical media may become trapped in crevices of the parts,
causing non-uniformity of the coating and impurities on the surface of the part.
It is an electroless plating system, which has a large number of additives such as
inhibitors, accelerators, buffers and chelating agents that may be harmful to the
environment. Furthermore, since the solution is proprietary, the waste treatment is
considerably more complex as the exact chemicals and quantities of said chemicals are
unknown. As such, considerable challenges in waste management arise – such as
separation of unknown chemicals. All of these factors increase the cost of the coating
both in terms of solution cost and waste processing cost.
To reduce the cost of the plating process, a scalable, continuous, re-usable
solution should be developed. This will enable longer service life of the cleaning and
plating solutions and easier waste treatment of any effluents, as the solution’s chemistry
will be known.
2.5.7. Proposed system
The following is a brief comparison of the various plating systems described
above (from [41]).
45
Electroless Plating Immersion
Plating
Mechanical
Plating
Barrel Plating
Max. Thickness > 75μm ~ 1μm >75μm >75μm
Rate of Deposition 20-25μm films
achievable in 20 hours
or more
Minutes
(but only up to
1μm)
20-25μm films
achievable in 20-
45 minutes
20-25μm films
achievable in one
hour or less
Coating Uniformity
(thickness variation
over sample)
Excellent Excellent Good, but
dependent on
part geometry
Good, but dependent
on part geometry
Coating Adhesion Good Poor to good Good Good
Environmental health
and safety concerns
Related to cleaning
products and bath
chemistries
Limited to
cleaning
products
Limited to
cleaning
products
Related to cleaning
products and bath
chemistries
Power Supply
Required
No No No Yes
System Cost Higher, full line
required (pre-treat,
plating and rinse baths)
Higher, full
plating line
required (pre-
treat, plating and
rinse baths)
Low, due to
single barrel for
all steps
Highest, full plating
line required (pre-
treat, plating, and
rinse baths) and
power supply
Coating Cost Highest, due to use of
additional chemicals
Low Low Higher, due to use of
additional chemicals
(mitigated somewhat
by power supply)
Impact Media
Required
No No Yes No
Temperature <80°C Room to 70°C <80°C <80°C
Waste Treatment:
species to be removed
base metal, copper
source, reducing agent,
complexant, buffer,
stabilizer, accelerator
base metal ions,
other additives
TBD
proprietary
chemicals with
each batch
base metal ions,
other additives TBD
H Embrittlement Low Low No Low
Easily Scalable Yes Yes Yes Yes
Table 13 Comparison of main coating options (from [41])
For this thesis, selection of the plating technology is governed by a number of
criteria including:
46
i. Coating quality, and appearance
ii. Throwing power (coating uniformity)
iii. Deposition rate
iv. Adhesion of the deposit to the substrate
v. Cost of the plating system set up
vi. Environmental considerations
Each of the above systems meets some of the above criteria, with immersion
plating being the most feasible. Since the parts are quite small and complex, a Technic
“Mini Electroless Copper Line” system (Figure 11) with a Singleton Mini Barrel (Figure
12) was purchased as a vessel to conduct the cleaning, plating and brightener solution
development.
Figure 11 Technic "Mini Electroless Copper Line" system
Figure 12 Singleton “Mini Barrel” system
47
3. Experimental Methodology
Below is a summary of the parts used during this experiment as well as
experimental methodology used for each circuit: cleaning, plating and anti-oxidant. All
the water used for mixing solutions and rinses was ultra-pure (Type I -18M-ohm) water,
and shall henceforth be simply referred to as water.
3.1. Fasteners Used
For the span of this thesis, a spring steel alloy (AISI 9255) commonly used for
fasteners in the automotive industry was selected as the base substrate for subsequent
development of the copper flash process. Spring steels are low alloy, medium carbon
steels with high yield strength. The key component in most spring steels is silicon. The
nominal composition of the steel used was 0.52-0.6 wt% C, 1.5-1.8 wt% Si, and 0.7-1
wt% Mn with the residual being iron.
The geometry of the fastener is quite complex, as shown in Figure 13. The
surface area of the component, as provided by Tinnerman, is approximately 10.03 cm2.
That value was used as the default surface area whenever applicable.
Figure 13 Tinnerman automotive fastener used for this thesis
3.2. Cleaning Circuit
3.2.1. Beaker Tests Initial experiments were run on a beaker scale. 100mL samples of each solution
were mixed and heated on a Corning PC-101 hot plate to the desired temperature.
1.15 cm
48
Solutions were maintained within ± 5°C of the desired temperature. Prior to cleaning, as-
received samples were ultrasonically washed in water for 5 minutes. This process was
done to remove the debris on the samples accumulated from shipping and handling the
parts. Samples were then immersed in the cleaning solution, which was agitated with a
Acid concentration
(vol %)
Temperature
(°C)
Immersion Time
(s = seconds or
m= minutes)
Inhibitor
Concentration
(g/L)
Hydrochloric Acid Solution
5 25, 50, 75 10s, 30s, 1m, 2m,
5m, 15m, 30m Uninhibited
10 25, 50, 75 10s, 30s, 1m, 2m,
5m, 15m, 30m
Uninhibited and
0.03
20 25, 50, 75 10s, 30s, 1m, 2m,
5m, 15m, 30m Uninhibited
Sulfuric Acid Solution
1 25 2m, 5m, 10m, 15m Uninhibited;
0.003-0.03
5 25, 40 2m, 5m, 10m, 15m Uninhibited;
0.0003-0.3
10 25, 40 2m, 5m, 15m Uninhibited and
0.03
Table 14 Parameters for initial development of cleaning solution inhibited with quinine small Teflon stir bar at a low setting, for the specified time interval and rinsed twice by
dipping the sample in water before being air dried in an air stream. The parts were then
examined visually to determine the extent of scale removal and smut formation and
compared against samples cleaned by the MacDermid SCF cleaner and as-received
samples to determine the extent of cleaning.
49
Digital imaging was performed on the sample in accordance with the procedures
described in section 3.2.4.
A summary of the parameters used for the initial cleaning solution tests are shown in Table 14.
3.2.2. Smut rating
Degree of Smutting Rating Sample image
No smut 0
Very light smut 1
Light smut 2
Grey smut 3
Dark smut 4
Heavy dark smut 5
Very heavy dark smut 6
Very heavy white smut 7
Table 15 Evaluation of smut by appearance Smutted samples were categorized based on the color and amount of smut formed
on the substrate surface. Based on the evaluation of smut by appearance summarized by
50
Allen [37], samples were assigned a rating from 0-7 indicating the level of smut
formation. Criteria for and examples of each level are summarized in Table 15.
3.2.3. Barrel Plating Tests Once the process was ready to scale up, a small barrel set up was used. A
Technic “Mini Electroless Copper Line” system with a 6” basket made from injection-
molded integral-mesh polypropylene was used to set up a 6L tank of cleaning solution
and a 6L water rinse, as shown in Figure 11 and Figure 12. For initial copper coatings, a
6L copper plating solution and a second 6L water rinse was also set up. The barrel was
rotated at 20 rpm.
Each barrel run consisted of 50 samples (filling the barrel to approximately 50%)
with 5 samples tagged using nylon monofilament. These samples were weighed both
before and after cleaning using a Mettler AE260 scale (accuracy ± 0.1 mg) and the
weight loss measurements were used as a means to determine the extent of chemical
attack on the sample.
3.2.4. Digital Imaging Digital photographs were taken of each sample using a Nikon D40X camera with
AF Micro Nikkor 60mm lens. The samples were placed in a Medalight Digital imaging
box with all light sources (front, back, left, right and bottom of the box) at the highest
setting to ensure uniform lighting. The camera settings were as follow: F8, ISO 200,
direct sunlight light balance, high sharpening, medium high tone composition, SRGB
color mode, and normal saturation.
Once images had been captured, the exposure was adjusted as necessary to ensure
a white background to each image. Images were cropped and straightened using iPhoto
’08 version 7.1.5.
51
3.2.5. Adhesion Tests
A peel test was used to determine the adhesion of copper coatings to the surface
of the substrate. The face of the sample was covered with masking tape, and firm
pressure was applied using a palette knife. The masking tape was peeled off and the
surface of the tape and the sample was examined. If there was any transference of the
coating onto the masking tape, the sample adhesion was considered poor, while very poor
indicated full transference of the sample coating to the tape. If there was no transference,
the adhesion was considered adequate.
3.2.6. Weight Loss Tests Weight loss measurements were taken for each batch cleaned with the barrel
plating system. Ten percent of each batch (five of fifty samples) was tagged using
monofilament and each of the samples tagged was weighed after being ultrasonically
cleaned in isopropanol for 5 minutes. Samples were weighed on a Mettler AE260 scale
(accuracy ± 0.1 mg). Samples were then mixed into the batch, cleaned under the desired
parameters and separated once again. After cleaning, they were dried in an air stream
and weighed once more. The weight loss for each system was taken as the average
weight loss of the five samples measured.
3.2.7. Electrochemical Tests Polarization resistance and Tafel measurements were performed on samples in a
variety of sulfuric acid cleaning solutions. 500 mL glass electrochemical cells were used
for the tests, with a CH Instrument potentiostat, using CHI 660c analysis software. A
three-electrode set-up was used using a silver-silver chloride reference electrode in a
luggin capillary and a platinum counter electrode.
All solutions were aerated, agitated with a stir bar operating at 200 rpm on a
Variomag Electronicrührer Poly 15 stir plate, and all heated samples were heated in a
water bath. Temperature was monitored using a thermometer in the solution. The sweep
52
rate was 10mV/minute in accordance with ASTM standards G3-89(2004), G5-97(2009)
and G5 - 94(2004) [20, 26, 28].
A summary of the solutions tested is included in Table 16. A summary of how
resistance polarization and Tafel measurements were conducted may be found in
Appendix 1 and Appendix 2 , respectively.
Temperature
(°C)
Quinine Concentration
(g/L)
Acid Concentration
(vol %)
25 0, 0.3, 0.03, 0.003 1
50 0, 0.03 1
75 0, 0.03 1
Table 16 Summary of solutions subjected to electrochemical testing Anodes were prepared by soldering pieces of steel automotive fasteners to a
copper wire. Any unwanted exposed metal was masked with Amercoat™ 90HS epoxy.
Amercoat™ application methods are detailed in Appendix 3. Anode areas were
calculated through imaging, using Image J software.
3.3. Copper Plating
3.3.1. Beaker Tests Each sample was ultrasonically cleaned for 5 minutes in isopropyl alcohol and
then rinsed with water before testing. Samples were immersed in 200mL cleaning
solution (1 vol% H2SO4 + 0.003g/L quinine, bulk water) in a 250mL beaker being
agitated with a Teflon stir bar (approximately 300 rpm) for two minutes. Samples were
rinsed twice in water to remove any cleaning solution residue before being immersed in
80mL of various stirred (300 rpm, Teflon stir bar) copper plating solutions for various
time intervals. All samples were stirred on a Variomag Electronicrührer Poly 15 stir
plate. Once the sample had been immersed for the desired amount of time, they were
rinsed in water, rinsed with isopropyl alcohol to speed up the drying time and dried in an
53
air stream. Pictures of all samples were taken and catalogued. The imaging process is
described in section 3.2.4.
Inhibited copper sulfate solutions were tested using the above method with 0.003
g/L quinine.
The variables for the copper sulfate solution used are described in Table 17
below.
Sulfuric Acid
Concentration
(vol %)
Copper Sulfate
Concentration
(g/L)
Immersion time
(seconds)
0.01 5, 15, 30 30, 60, 90
0.1 5, 15, 30 30, 60, 90
1 5, 15, 30 30, 60, 90
3 5, 15, 30 30, 60, 90
0.1 + quinine 15, 30 30, 60, 90
Table 17 Copper sulfate solution parameters, uninhibited solutions and solutions inhibited with 0.003 g/L quinine
3.3.2. Barrel Plating Tests The best results from the beaker tests were then scaled up to the Technic “Mini
Electroless Copper Line” system. To conserve samples, anywhere from 10-40 samples
were used in each test. Samples were loaded into the barrel and rotated in a 1 vol %,
0.03g/L quinine solution for two minutes, rinsed in ultra pure water for one minute, and
then immersed in the copper plating solution for the specified time. All tumbling was
done at 20 rpm, save a final test conducted at 5 rpm. Once the coating process was
complete, samples were rinsed in ultra pure water for ten seconds before being air-dried.
Pictures of a selection of samples from each batch were taken and catalogued in the
manner described in section 3.2.4. A summary of the parameters tested is outlined in
Table 18.
54
Sulfuric Acid
Concentration
(vol %)
Copper Sulfate
Concentration
(g/L)
Immersion time
(minutes)
0.1
2.5 1, 3
5 1, 3
10 1,3
15 1,3
0.1 + quinine
2.5 1, 1.5, 2
5 1, 1.5, 2
15 1,2
Table 18 Summary of test parameters for barrel plating tests of uninhibited and inhibited with 0.003g/L quinine copper flash solutions
3.3.3. Adhesion Tests
Adhesion tests were performed in the same manner as described in section 3.2.5.
3.4. Anti-oxidizing Agent
The anti-oxidizing agent used for the development of the final step in the copper
flash system was benzotriazole.
3.4.1. Beaker Tests Samples were first tested in a beaker-scale plating set-up. Samples were cleaned,
rinsed, copper flashed, rinsed again and immersed in various concentrations of
benzotriazole for varying times. The process, including all variables is outlined in Table
19. All solutions were stirred at 200 rpm using a Teflon stir bar and a Variomag
Electronicrührer Poly 15 stir plate for the duration of the procedure.
55
Solution Immersion time
Purpose Composition Volume (mL)
Cleaning
1 vol% H2SO4
0.003 g/L quinine
water
200 2 minutes
Rinse Water 50 10 seconds
Copper flash
0.1 vol% H2SO4
5 g/L CuSO4
0.003 g/L quinine
water
200 1 minute
Rinse Water 50 10 seconds
Anti-oxidant
Variable:
0.1 or 0.01 M
benzotriazole in water
40
Variable:
30 seconds,
60 seconds, or
90 seconds
Rinse Water 50 10 seconds
Table 19 Process outline for beaker testing of benzotriazole anti-oxidant coating
3.4.2. Batch Tests
The process for batch testing included cleaning and copper flash solutions
identical to those outlined in Table 19. Only the 0.01 M benzotriazole solution was
scaled up to a 6L bath. The immersion times for those tests were 60 and 90 seconds. A
batch of 10 samples was used for each test run. Those samples were rotated at 5 rpm in
the barrel plating chamber for the duration of the copper flash circuit.
56
3.4.3. Copper Tarnishing Tests
Copper tarnishing tests were performed on each of the samples produced in the
beaker tests and on a sampling of the batch tests to confirm that scaling up did not affect
the anti-oxidant coating.
Samples were tested in batches of three, in accordance with ISO 4538-1978
thioacetamide corrosion test [76]. A 1L glass desiccant container was modified for the
purpose of these experiments. Images of the set up may be found in
Appendix 4. A glass dish was placed at the base of the container to flatten the bottom.
Fisher brand filter paper was cut to size so it would sit flush against the sides of the
container. A high-walled glass dish was used to hold the thioacetamide and as a support
for the sample rack, which was constructed out of Lego. Finally, a stand for the dish and
rack was also constructed out of Lego. The samples were hung above a glass container,
which had been covered with a fine layer of thioacetamide.
A solution of three parts sodium acetate to one part water (by weight) was mixed
such that it would moisten the filter paper along the walls of the vessel and have an
excess of approximately 1 cm of solution along the bottom of the vessel. For the vessel
used, 60 g of sodium acetate and 20 g of water were mixed together to yield the required
amount of salt solution.
All samples tested were photographed in the light box setup prior to corrosion
testing, as outlined in section 3.2.4.
Samples were suspended with nylon monofilament approximately 1 cm above the
thioacetamide and approximately 2 cm apart. They were sealed in a gas-tight chamber
and left for 3 hours. At the end of that time, the samples were removed and once again
photographed.
The final parameter developed for batch testing was subjected to a longer
corrosion tests – both a 3 hour and a 6 hour test was performed on the samples plated in
0.01 M benzotriazole for 60 seconds, using the test methodology described above.
57
4. Results and Discussion
If this electroless barrel plating system were to be used in industry to apply a
copper flash, thousands of parts would be plated in each load. Thus it is important that
each step have an operating window wide enough to ensure total cleaning/plating of each
part without any parts becoming over-cleaned or the integrity of the coating being
compromised. One of the major goals of each step was to widen the operating window as
much as possible while maximizing the throughput.
The first step in the process to be optimized was the cleaning step. To be
consistent with the goal of broadening the operating window, samples should all be
thoroughly cleaned in a time frame of 2-5 minutes. Solution parameters were varied until
clean samples, as defined by samples that are free of both oxides and smut, were
produced. However, smut should be minimized even at longer cleaning times to allow
for a wide margin of error. As such, longer time frames were also used in the initial
development of the cleaning solutions.
4.1. Hydrochloric Acid Cleaning Solutions
Being one of the most common, readily available and inexpensive acids used in
the industry, hydrochloric acid was chosen as the first acid to examine during the
development of the cleaning circuit. Initial tests used a broad spectrum of conditions to
determine potentially useful concentration and time values. Preliminary tests were
conducted in the beaker set up described above for 5, 15 and 30 minutes. These values
were chosen to assess the extent of scale removal and smut formation over an extended
period of time. This would give an idea of the maximum achievable levels of smut
formation and the maximum possible operating window. To assess a wide variety of
parameters - various temperatures and acid concentrations were used. A summary of the
parameters used is shown in Table 14.
58
4.1.1. Uninhibited Hydrochloric Acid Solutions
4.1.1.1. Acid Concentration Variation
The acid concentration was varied from 5-20 vol%. A summary of the cleaning
conditions of samples cleaned at room temperature is shown in Table 20 below.
Temperature HCl Acid
Concentration
(vol%)
Time
5 minutes 15 minutes 30 minutes
25°C
5
Level 1
Level 2
Level 3
10
Level 2
Level 2
Level 3
20
Level 2
Level 3
Level 4
Table 20 Smut formation in varying acid concentration in an uninhibited hydrochloric acid solution The solution is quite aggressive at room temperature, and smutting occurred at all
acid concentrations within 5 minutes. By the smut index assigned to each sample, it was
found that for a given time, smut formation increases with increasing acid concentration.
Similarly, for a given acid concentration, smut formation increases with time.
At 5 minutes cleaning time, there was very little difference in the extent of
smutting between the samples cleaned in 5 vol% hydrochloric acid, and those cleaned in
20 vol% hydrochloric acid. This information indicates that a low acid concentration may
59
be used – allowing for easier waste management. It should be noted that none of the
above solutions are acceptable cleaning solutions, as all acid concentrations and all
cleaning times presented in Table 20 produced smutted samples.
To assess the viability of a hydrochloric acid cleaning solution, it was necessary
to develop a timeframe where no smutting occurred. Keeping all solutions at room
temperature, samples were cleaned for 10 seconds, 30 seconds, 1 minute and 2 minutes in
the three acid concentrations used above. The results are shown in Table 21.
HCl Acid
Concentration
(vol%/M)
Time
10 seconds 30 seconds 1 minute 2 minutes
5
No smutting
Level 1
Level 1
Level 1
10
No smutting
Level 1
Level 1
Level 2
20
Level 1
Level 2
Level 2
Level 2
Table 21 Timeframe determinations for hydrochloric acid cleaning solution at room temperature
For all of the concentrations, a thin layer of smut is formed between 10 and 30
seconds. For a beaker set up, this is an unacceptably low working time. Thus an
inhibitor should be added to the cleaning solution to obtain a wider operating window.
60
4.1.1.2. Temperature Variation
High temperature tests were performed for all acid concentrations at temperatures
of 50°C and 75°C. The results for the 5 vol% cleaning solution are summarized in Table
22 below, the remainder of the results may be found in Appendix 5.
Temperature HCl Acid
Concentration
(vol%)
Time
5 minutes 15 minutes 30 minutes
25°C 5
Level 1
Level 2
Level 3
50°C 5
Level 3
Level 4
Level 5
75°C 5
Level 5
Level 6
Level 6
75°C 20
Level 6
Level 7
Level 7
Table 22 Temperature variations for uninhibited hydrochloric acid cleaning solutions.
Increasing the temperature of a cleaning solution leads to higher cleaning rates [3-
8], so it is expected that smutting will occur more rapidly at high temperatures. As
predicted, the smut formed for a given time increases with increasing temperature. At
61
high temperatures, a dark smut begins to form at shorter times, as seen by comparing the
level of smut formation in 5 minutes at 25°C, 50°C and 75°C.
As the temperature increases, the concentration of the acid plays a greater role in
smut formation. Unlike the samples cleaned at room temperature, there is a significant
difference between the samples cleaned for any given time in 5 vol% and 20 vol%
hydrochloric acid at 75°C. At that temperature, samples cleaned in the low acid solution
did not smut as rapidly (within half a minute), and maintained a thin, black smut layer
even at longer times. However, samples cleaned in the high acid solution smutted very
rapidly (within seconds) and developed a thicker, lighter colored smut.
The best samples produced were those cleaned at low temperature. However,
even the samples cleaned at the lowest acid concentration formed a thin layer of smut on
the surface during the operating window of 2-5 minutes. This smut layer is detrimental
as it will interfere with adhesion of the subsequent copper coating, and should be reduced
as much as possible.
4.1.2. Inhibited Hydrochloric Acid Cleaning Solution Ideally, an uninhibited solution should be developed (as it would minimize the
organic species in the waste stream). However, no suitable results were obtained with
uninhibited hydrochloric acid solutions, so 0.03 g/L quinine was added to slow down the
cleaning mechanism. This inhibitor and concentration were chosen based on Awad’s and
Stemp’s work [31 and 41].
4.1.2.1. Temperature Variation
Quinine was reported to be an effective inhibitor in small quantities and at low
temperatures in hydrochloric acid solutions [31]. To test the viability of a quinine-
inhibited solution, a 10 vol% hydrochloric acid solution was mixed with 0.03 g/L quinine
and the temperature was varied with times of 5, 15 and 30 minutes. The results are
summarized in Table 23.
62
Temperature Time
5 minutes 15 minutes 30 minutes
25°C
Level 1
Level 2
Level 3
50°C
Level 3
Level 4
Level 5
75°C
Level 5
Level 6
Level 7
Table 23 Hydrochloric acid (10 vol%) cleaning solutions inhibited with 0.03 g/L quinine At 25°C, quinine was an effective inhibitor at short times – reducing the level of
smut formation from level 2 to level 1. However, it did not eliminate smut altogether.
All other samples were smutted, especially those at high temperature. This is partially
due to the expected breakdown of quinine at high temperature as reported by Awad [31]
and a greater rate of smut formation in acid cleaning solutions at high temperature (as
noted earlier in section 4.1.1.2). While hydrochloric acid could work as a potential
cleaning solution, it is unadvisable because of the likelihood of carryover of chloride ions
to the copper flash circuit, which is based on a sulphate chemistry. An inhibited
hydrochloric acid solution would risk contaminating the copper flash solution with both
chloride ions and inhibitor molecules and of producing a mixed chloride/sulphate media
that could be more aggressive than solutions containing each ion separately. Due to the
high risk of contamination, sulfuric acid cleaning solutions were examined as an
alternative to an inhibited hydrochloric acid solution.
63
4.2. Sulfuric Acid Cleaning Solution
A summary of the acid concentrations, temperatures and inhibitors used for the
sulfuric acid cleaning trials is summarized in Table 24 below. A laboratory grade
sulfuric acid was used in preparing the solutions.
Acid concentration
(vol %)
Temperatures tested
(°C)
Quinine
concentration
5% 25, 50 0 - 0.3 g/L
10% 25, 40 0 - 0.03 g/L
Table 24 Summary of inhibited and uninhibited sulfuric acid cleaning solutions
tested
4.2.1. Uninhibited Sulfuric Acid Solution
4.2.1.1. Temperature Variations Due to the short cleaning time required for the hydrochloric acid cleaning
solution, the initial trials for the sulfuric acid cleaning solutions were modified. Samples
were immersed for the times of 10s, and 30s, and 1, 2, 5, 15 and 30 minutes. Since the
results were poor for hydrochloric acid at high temperatures, the temperatures used for
sulfuric acid were lowered. Once again, the beaker set up was used for cleaning the
samples and all samples were imaged after cleaning.
Samples cleaned in 5 vol% sulfuric acid at 50 °C were heavily smutted. The
highest temperature to be tested was thus lowered for 10 vol% sulfuric acid to 40°C.
Once again, the components cleaned in uninhibited high temperature solutions were
heavily smutted and smutting began at very short time periods (i.e. within 30 seconds).
Images of samples cleaned at 10s, 30s, 1m and 2m are shown in Table 25, showing the
extent of smutting at short times. All images, including those for samples cleaned at 5
and 15 minutes are found in Appendix 6.
64
Temp Acid
Conc.
(vol%)
Time
10 seconds 30 seconds 1 minute 2 minutes
25°C
5
No smut
No smut
No smut
Level 1
10
No smut
No smut
Level 1
Level 2
40°C 10
No smut
No smut
No smut
Level 4
50°C 5
No smut
Level 1
Level 1
Level 5
Table 25 Uninhibited sulfuric acid cleaning solutions at room temperature and elevated temperatures
Tafel measurements were taken for samples cleaned in 1 vol% sulfuric acid and
the corrosion potential, Tafel slopes and corrosion current density were found. The data
is summarized in Table 26 and Figure 14. Based on the steady-state anodic Tafel slopes
calculated, the dissolution mechanism of iron in sulfuric acid is consistent with that
proposed by Heusler’s two-electron exchange “catalyzed” pathway described in section
2.2.1.3. It should be noted that the cathodic Tafel values are anomalously high, and as
such they may not be accurately analyzed from a mechanistic perspective.
65
Temperature
(°C)
Ecorr
(mV)
βa
(mV per
decade)
βc
(mV per
decade)
icorr
(mA/cm2)
25 -450 26 ± 5 220 ± 5 2.56
50 -439 30 ± 10 200 ± 10 20.3
75 -419 55 ± 10 240 ± 10 233
Table 26 Electrochemical data for uninhibited 1vol% sulfuric acid cleaning solution
Figure 14 Linear sweep data for uninhibited 1 vol% sulfuric acid cleaning solutions at 25°C, 50°C, and 75°C By the Nernst equation {6}, the potential should increase as temperature
increases. The kinetics would also increase, so an increase in io would also be expected.
The data from Table 25 for 25°C and 75°C was plotted in an Evans diagram; the results
are shown in Figure 15.
66
Figure 15 Evans diagram illustrating the effect of temperature increase As predicted, the corrosion potential and corrosion current density increased with
increasing temperature, seen in Figure 14 and Figure 15. An interesting characteristic on
the linear sweeps is at elevated temperatures there is a secondary peak in the cathodic
region of the reaction. Cavallaro [17] noted this type of behavior for iron in sulfuric acid,
but failed to explain it.
The peak causes a change in the Tafel behavior of the sweep. At room
temperature, the cathodic reaction has a Tafel slope of approximately 220 mV/decade.
Despite this value being anomalously high, the magnitude of the change in Tafel value
was examined. The cathodic reaction at that temperature is hydrogen evolution. At high
temperature, the cathodic Tafel slope changes from a value of approximately
220mV/decade to approximately 40mV/decade at potential values of -0.53V to -0.45V
(vs. Ag/AgCl). This is indicative of either a change in the cathodic reaction or a change
in the cathodic reaction mechanism.
At high temperatures, smut formation occurs very rapidly. While setting up the
Tafel extrapolation measurements, there was a brief period where the test specimen was
immersed in solution without any current flowing. It would be vulnerable to smut
67
formation during that time. Thus at high temperatures, there will be a higher smut
(silicon oxide) concentration on the surface of the test specimen than at room
temperature, where there is no bump. This change in surface composition could account
for the peak by changing the reaction mechanism of hydrogen evolution or by changing
the cathodic reaction from hydrogen evolution to oxygen reduction.
Bockris [14], and references therein] described two possible hydrogen evolution
paths for iron in acid. The first is a coupled discharge with the following two steps:
{38} H3O+ + e− → Hads + H2O
{39} H3O+ + Hads + e− → H2(g )
The rate determining step for this reaction mechanism is determined by the
cathodic Tafel value. If the cathodic Tafel is -2RT/F, then {38} is the rate determining
step. If the cathodic Tafel is -RT/2F then {39} is the rate determining step. Thus the rate
determining step changing from reaction {38} to reaction {39} would result in a decrease
of Tafel slopes by a factor of 4.
The second hydrogen evolution mechanism consists of an atomic combination
step:
{40} 2H3O+ + 2e− → 2Hads + 2H2O
{41} 2Hads → H2(g )
Reaction {40} is just reaction {38} doubled, thus the Tafel slope for that rate
determining step will still be -2RT/F. If reaction {41} is the rate determining step, the
Tafel slope will be -2RT/3F. The rate determining step changing from reaction {40} to
reaction {41} would result in a decrease of Tafel slopes by 3.
A final possibility is that due to the increase in silicon oxide on the surface of the
test specimen, the cathodic reaction may change altogether. Since the test cell was
aerated, it’s possible that the hydrogen evolution reaction occurring at low potentials
switched to an oxygen reduction reaction as the potential increased toward Ecorr. Bockris
[14] found that for surface oxides, and adsorbed oxygen in sulfuric acid, there is a
likelihood for the oxygen reduction reaction to occur at potentials greater than 0V.
68
However, this is unlikely to be the case, as the peak occurs at potentials between -0.53V
and -0.45V.
The most likely explanation is a change in the rate determining step of the
hydrogen evolution reaction as shown in reactions {38} and {39}. The change in the
magnitude of Tafel slopes found experimentally is approximately consistent with that
seen in the aforementioned model.
Further study of this phenomenon is beyond the scope of this thesis.
For Tafel extrapolations at high temperatures, the higher Tafel value was used in
determining the corrosion current density as it had a more clearly defined Tafel region.
Since high corrosion rate relates to rapid smut formation, high temperature
uninhibited solutions are clearly not desirable since smutting will occur very rapidly.
4.2.1.2. Acid Concentration Variations Images were taken of samples cleaned at 1, 2 and 5 minutes to determine the
extent of smut formation in the desired operating window. As with samples cleaned at
high temperature for hydrochloric acid, high levels of smutting was seen at higher
temperatures. As such, only the samples produced at 25°C are shown and described in
the summary of results in Table 27.
The low acid samples showed promising results, and an even lower acid
concentration was hypothesized to be a potential candidate for the final cleaning solution.
The system was thus scaled up and a large batch of samples was cleaned using the barrel
plating apparatus in a 1 vol% sulfuric acid solution for various time intervals. Weight
loss data was taken for 10% of each batch (five samples) and compared against the
weight loss data from samples cleaned in 5 vol% sulfuric acid.
An extensive weight loss study was performed for times ranging from 10 seconds
to 30 minutes for uninhibited 5 vol% sulfuric acid cleaning solutions. Using Faraday’s
law {7}, the weight loss data was converted to a corrosion current, and subsequently to a
corrosion current density. The results, shown in Figure 16, give a model for smut
formation on the surface of the fasteners.
69
Time Acid Concentration
5 vol% 10 vol%
1 minute
No smut
Level 1 smutting
2 minutes
Level 1 smutting
Level 2 smutting
5 minutes
Level 3 smutting
Level 4 smutting
Table 27 Variation of acid concentration in uninhibited sulfuric acid cleaning
solutions at 25°C
Figure 16 Weight losses converted to corrosion current density over time for
various uninhibited sulfuric acid cleaning solutions
70
Initial measurements were conducted in triplicate, in a beaker set-up with a 5
vol% sulfuric acid solution. The agitation for these samples was not as high as samples
produced in the barrel-plating set-up for 1 vol% samples. The barrel was loaded with 50
samples, and tumbled at 20 rpm for the designated time. The variation in samples and the
weight loss profile for uninhibited acid solutions is shown in Figure 16. The data shown
for 1 vol% is an average weight loss value for the 5 samples measured, and is given as a
approximate basis for comparison.
The comparison of 5 vol% to 1 vol% can not be directly made due to the different
cleaning conditions (barrel vs. beaker set-up). To confirm that there was in fact a greater
weight loss in the higher acid solution, data for uninhibited samples produced at 5 and 10
minutes in the barrel-plating set-up were added to Figure 16. Since the cleaning would
ultimately be performed in a barrel, it was essential that any tests involving mass transfer
were performed in both beaker and barrel scale.
From Figure 16, a cleaning profile may be derived. Initially, the corrosion current
density was very high. This is representative of the initial removal of oxides from the
substrate, and the dissolution of iron into solution. As time increases, the slope begins to
decrease. This is the period during which smut begins to form. Finally, at long times
there is a plateau in the corrosion current density due to the protective barrier of smut
reducing further dissolution of iron.
It can be seen that the corrosion current densities in the barrel set up for 1 vol%
are less than those for 5 vol% acid cleaning solutions in the barrel set up. For a given
time, weight loss is greater in higher acid concentrations. It has been noted that there is a
co-relation between weight loss, cleaning rate and smut formation [38]. As weight loss
rates increase, cleaning rates will increase and the onset of smut formation will occur
more rapidly. It is thus beneficial to use a lower acid concentration in the cleaning
solution, both to minimize the effluents in the waste stream and to offset smut formation.
By comparing the beaker and barrel results for 5 vol% acid, it is apparent that
agitation plays an important role in the cleaning rate. Increasing the agitation – such as
going from beaker set up (low agitation) to barrel set up (high agitation) causes an
increase in the cleaning rate of a given solution.
71
4.2.2. Inhibited Sulfuric Acid Solution To avoid smut formation, inhibited solutions were used. The inhibitor of interest
was quinine, as earlier work [41] had shown it to be effective in sulfuric acid.
4.2.2.1. Inhibitor Concentration Variation A summary of the quinine concentrations used may be found in Table 16. The
samples were cleaned for 5 and 10 minutes, with the goal of determining a timeframe for
the onset of smutting. These times were chosen because 5 minutes was the outer limit of
the optimum timeframe and 10 minutes was well past the optimum timeframe determined
for the initial 5 vol% inhibited cleaning solution. As an additional test of the degree of
cleaning, cleaned samples were copper plated and an adhesion test was performed.
In keeping with the desire to have a low-concentration waste stream, an attempt
was made to develop a low-acid concentration cleaning cycle. Using 1 vol % sulfuric
acid and three different concentrations of inhibitor, samples were cleaned for 2, 5, 10 and
15 minutes. Samples were then flashed with a Cu coating for 1 minute after being
cleaned for the desired amount of time. The results are summarized in Table 28 below.
The adhesion of the copper flash on these samples was measured qualitatively by
performing a peel test. The peel test evaluated the cleaning of the samples, and
confirmed that highly smutted samples have poor coating adhesion.
Uninhibited low acid samples (1 vol% acid) had smutting occurring at 5 minutes.
However, adhesion of the copper flash was adequate for samples cleaned up to 5 minutes,
even in the presence of thin layers of smut. While all the inhibited samples were
completely clean for the 2 and 5 minute cleaning duration, some minor smutting occurred
at 10 minutes for both inhibitor concentrations, causing minor adherence problems and at
15 minutes all samples were smutted and failed adhesion tests.
72
Quinine
Concentration
2 minutes
cleaning
5 minutes
cleaning
10 minutes
cleaning
15 minutes
cleaning
Uninhibited
No smutting
Level 1
Level 2
Level 3
Adhesion Adequate Adequate Mediocre Very poor
0.003 g/L
No smutting
No smutting
Level 1
Level 3
Adhesion Adequate Adequate Mediocre Poor
0.03 g/L
No smutting
No smutting
Level 1
Level 3
Adhesion Adequate Adequate Adequate Very poor
Table 28 Low acid cleaning using 1 vol% sulfuric acid and various quinine
concentrations, cleaned samples (top row) and copper coated samples
(bottom row)
73
It is interesting to note that higher quinine concentrations did not have better
inhibitive properties. Rather, the higher quinine concentration appeared to attack the
surface of the sample at higher cleaning times, causing very poor adhesion. It is possible
that poor adhesion is due to the inhibition mechanism of quinine, which adsorbs to the
substrate [31]. Further evidence that high concentrations of quinine were not desirable
(in relation to the acid concentration) can be found in the converted weight loss and
electrochemical data below.
Once again, weight loss measurements were conducted in a barrel set-up. The
weight loss measurements were converted to corrosion current density to give a cleaning
profile. All concentrations of quinine added decreased the corrosion current density
when compared to uninhibited solutions. However, past 0.003 g/L quinine appears to
behave as an aggressive species.
Figure 17 Corrosion current density (by converted weight loss) data for various
quinine concentrations in barrel-cleaned 1 vol% sulfuric acid cleaning
solution
When compared to an uninhibited solution the addition of quinine in any amount
lowered icorr and increased Ecorr, as illustrated in Figure 17 and Table 29.
74
Quinine
concentration
(g/L)
Ecorr
(mV)
βa
(mV/decade)
βc
(mV/decade)
icorr
(mA/cm2)
Uninhibited -450 26.3 ± 5 223 ± 10 2.56
0.003 -447 22.3 ± 5 175 ± 10 0.84
0.03 -438 15.8 ± 5 180 ± 10 1.23
0.3 -421 22.0 ± 5 216 ± 10 1.54
Table 29 Electrochemical data for variations in quinine concentration at 25°C
Figure 18 Evans diagram illustrating the effect of quinine on iron dissolution The inhibitive action of quinine appears to be limited to concentrations no greater
than 0.003 g/L. The mechanism is by simple adsorption, which will have the effect of
blocking active sites on the anode. This will cause a decrease in the exchange current
density (io) of both the cathode and anode. As seen in Table 29, the anodic Tafel slope
does not change significantly and the cathodic Tafel slope is slightly decreased in the
presence of quinine. Once again, the Tafel values are anomalously high; however the
trend in Tafel shift will be examined. The net result will be an increase in the corrosion
75
potential (Ecorr) and a decrease in the corrosion current density (icorr), as shown
schematically in Figure 18.
At concentrations above 0.003 g/L, the efficiency of quinine as an inhibitor in
sulfuric acid solutions decreases. While all quinine concentrations tested showed
inhibition of the cleaning solution and reductions in weight loss (relative to uninhibited
solutions), lower inhibitor concentrations were found to be more effective in reducing icorr
than higher inhibitor concentrations.
4.2.2.2. Temperature Variations Preliminary tests conducted by M. Stemp showed that quinine is not an effective
inhibitor at elevated temperatures [41]. To verify this finding, Tafel measurements and
weight loss measurements were performed on 0.03 g/L quinine inhibited solutions at
room temperature, 50°C and 75°C solutions. The corrosion current density was
calculated from weight loss measurements for each temperature in both inhibited and
uninhibited solutions, with results shown in Figure 19. The corrosion data is summarized
in Table 30.
Figure 19 Temperature dependence of corrosion rate in 0.03 g/L quinine inhibited sulfuric acid solution The overall effect of temperature increase on an inhibited sulfuric acid cleaning
solution is similar to that of uninhibited cleaning solutions shown in Figure 15; both Ecorr
76
and icorr increase with increasing temperature. From Figure 19 it can be seen that quinine
does inhibit the solution at high temperatures as well as low temperatures.
Temperature
(°C) Inhibited?
Ecorr
(mV)
βa
(mV per
decade)
βc
(mV per
decade)
icorr
(mA/cm2)
25 No -450 26.3 ± 5 223 ± 10 2.56
Yes -438 15.8 ± 5 180 ± 10 1.23
50 No -439 30.2 ± 5 197 ± 10 20.3
Yes -443 15.6 ± 5 106 ± 10 9.38
75 No -419 54.5 ± 5 241 ± 10 233
Yes -421 27.3 ± 5 138 ± 10 86.1
Table 30 Electrochemical data for temperature variations in uninhibited and 0.03 g/L quinine inhibited sulfuric acid solution
Awad [31] found that in hydrochloric acid solutions at 50°C (Table 7) there was
little change in the Tafel behavior of inhibited and uninhibited solutions. He concluded
that the addition of quinine does not change the reaction mechanism of hydrogen
evolution or iron dissolution and that the inhibition mechanism was by simple blocking
of the surface by physisorption.
In sulfuric acid, the method of inhibition is also by physisorption, as supported by
the calculated activation energy value of ~ 10kJ/mol, which falls in the physisorption
range. The quoted activation energy value was calculated based on the plotted slope of
log icorr vs. 1/T. Contrary to Awad’s observations, in inhibited sulfuric acid solutions at
high temperature both the cathodic and anodic Tafel slopes change compared to
uninhibited solutions.
77
Figure 20 Linear sweep data performed in inhibited solutions at 25°C, 50°C and 75°C
As with the uninhibited solutions, a secondary peak appeared in the
cathodic reaction of samples tested at high temperature for inhibited solutions, as seen in
Figure 20.
4.2.2.3. Acid Concentration Variations
Similar to uninhibited solutions, the corrosion rate increased with increasing acid
concentration. Adequate cleaning was achieved at considerably low acid concentrations
– as low as 1 vol%. Since cleaning had already been proved possible at those
concentrations no further testing on high acid concentration was performed.
4.3. Copper Flash
4.3.1. Preliminary cleaning solution adhesion tests In order to test the feasibility of the inhibited 5 vol% sulfuric acid cleaning
solution, a copper flash was plated onto cleaned samples and the coating was compared
78
to the MacDermid process in terms of appearance and adhesion. If the copper coating had
adequate adhesion to the cleaned substrate and a uniform appearance, then the cleaning
process was deemed acceptable to be scaled up to a 6 L tank, and no further development
of the cleaning process would be necessary. Results are discussed below.
Since the MacDermid process used as a baseline for comparison is an additive
process, the copper flash step could not be isolated. It was thus necessary to develop a
preliminary copper flash solution.
The copper flash solution used was a 5 vol% sulfuric acid + 15g/L copper sulfate.
This chemistry had been proven to yield acceptable results in previous work completed
by M. Stemp [41]. Initially, the samples were cleaned for 4 minutes followed by a 5-
minute immersion in the copper flash. The flash time was chosen to match that of the
copper flash step in the MacDermid process. Adhesion to the substrate was very poor,
with heavy smutting occurring underneath the copper coating and a highly stressed
coating that flaked off during the drying process. To test whether the smutting was due
to the cleaning process or the flash, an as-received sample was immersed in the copper
flash solution for 5 minutes. Once again, heavy smutting occurred on the substrate
resulting in poor adhesion. The copper flash solution chosen was yielding highly stressed
coatings, with acid undercutting of the substrate – causing smutting to occur after the
cleaning process.
However, since the solution had been proven to yield adequate coatings, the
timeframe of the copper flash was lowered. A new sample was cleaned in the inhibited
sulfuric acid solution, and then immersed in the copper flash solution until the sample
was completely coated with copper (approximately 30 seconds). The adhesion of the
copper flash was greatly improved by short flash times. Longer flash times resulted in
acid undercutting and smutting of the substrate, as well as highly stressed coatings that
would flake off during drying, all of which contributed to poor adhesion.
All results may be seen and are discussed in Table 31, below.
The inhibited sulfuric acid cleaning solution proved suitable as a cleaning step
before copper plating. The copper flash solution used yielded highly stressed, non-
adherent samples both on cleaned and as-received samples, suggesting that the flash
solution was likely problematic, not the cleaning solution. By decreasing the flash time
79
from 5 minutes to 30 seconds, the adhesion and appearance of the coating greatly
improved.
Plating Condition and Results Resulting coating
MacDermid – no polish
The sample is bright and shiny, with a uniform Cu coating.
MacDermid – polished
The sample is not as bright as the unpolished sample. This may
be due to unclear processing instructions provided by
MacDermid. The amount of polish required was not specified.
MacDermid – polished (second trial)
A second polished sample was produced. Once again, it is not
as bright as the unpolished sample.
H2SO4 + Quinine (4min) + CuSO4 flash (5min)
The substrate was smutted by the Cu flash solution, causing a
highly stressed coating with very poor adhesion.
CuSO4 flash (5min)
Without cleaning, the Cu flash solution still caused smutting of
the underlying substrate, a highly stressed sample, and poor
adhesion.
H2SO4 + Quinine (4min) + CuSO4 flash (30 seconds)
By decreasing the time of the Cu flash, it was possible to plate a
bright, uniform coating with good adhesion.
Table 31 Initial copper flash trials used to prove suitability of inhibited sulfuric
acid cleaning solution.
80
4.3.2. Beaker Tests
For complete beaker test results, see Appendix 8. A selection of the results will
be presented in the sections below.
4.3.2.1. Acid Concentration Variation Shown in Table 32 are the acid concentration variation results for 15 g/L copper
sulfate. While a range of copper sulfate concentrations and acid concentrations were
tested the results show the overall trend of acid variation.
Table 32 Sulfuric Acid concentration variations in a beaker set-up with 15g/L copper sulfate copper flash solution All 15 g/L copper sulfate samples had a uniform copper coating at 30 seconds. At
longer immersion times, there appeared to be undercutting of the substrate, causing very
Copper Sulfate
Concentration
(g/L)
Acid
Concentration
(vol%)
Time
30 seconds 60 seconds 90 seconds
15
0.01
Adhesion Poor Poor Very poor
0.1
Adhesion Adequate Adequate Poor
1
Adhesion Adequate Poor Very poor
81
poor adhesion of the copper coating and smutting of the substrate. In some cases, the
coating flaked off during drying. The adhesion of each sample was tested using a peel
test, and the adhesion was rated as adequate (no transfer of copper coating to tape), poor
(some transfer) and very poor (nearly complete transfer of copper coating to tape).
Both low and high acid concentration had poor adhesion. The role of the acid in
this solution is to maintain the pH at a range in which both copper and iron ions will
dissolve (pH = 0 - 4). Too low a concentration will not be in the ideal pH range, while
too high an acid concentration will result in an aggressive attack on the cleaned substrate
during copper plating. The best samples were produced at 0.1 vol% sulfuric acid. This
concentration was chosen for scaling up to the barrel plating process.
4.3.2.2. Copper Sulfate Concentration Variation As seen in Table 33, an increase in copper sulfate concentration increased the
plating rate of the copper coating. By the Nernst equation {6}, it can be seen that as the
concentration of copper sulfate (reactant) increases, the potential of the reaction
increases. Higher reaction potentials have a higher driving force than lower reaction
potentials, therefore the reaction has a higher driving force at high copper sulfate
concentrations. As the surface coverage begins to increase (i.e. the copper deposits), the
potential will decrease due to a lower availability of iron from bare steel. The reaction
will begin to slow down, and continue to slow until equilibrium is reached. As such,
immersion coating will never yield a coating with complete surface coverage.
Samples produced at low copper sulfate concentrations (5 g/L) took up to 90
seconds to plate evenly while samples with higher copper sulfate concentrations (30 g/L)
took as little as 30 seconds to plate evenly. At longer plating times, the undercutting of
the substrate was still occurring, so for this system the ideal plating time would be no
longer than 30 seconds. This is an unacceptably short time frame for bulk production,
however in developing the cleaning solution it was noted that the tumbling motion of
barrel plating extended the operational window from beaker plating to barrel plating and
it is likely that operational window for copper plating time will be similarly extended. To
82
increase the plating time, the lowest copper sulfate concentration solution should be used.
As such, barrel tests began with a copper sulfate concentration no greater than 5 g/L.
Acid
Concentration
(vol%)
Copper Sulfate
Concentration
(g/L)
Time
30 seconds 60 seconds 90 seconds
0.1
5
Adhesion Adequate Adequate Poor
15
Adhesion Adequate Poor Poor
30
Adhesion Adequate Poor Poor
Table 33 Copper sulfate concentration variations in a 0.1 vol% sulfuric acid copper flash solution prior to adhesion tests with adhesion test results
4.3.3. Inhibited Copper Flash Tests The above copper sulfate solutions had a very narrow operating window. In an
effort to increase the possible plating time, an inhibitor was used. Since quinine is
already present in the cleaning solution, it was the first choice for an inhibitor in the
copper plating step. An initial concentration was chosen to match the ideal concentration
of the cleaning solution, so 0.003g/L quinine was used for preliminary testing. Results
are summarized in Table 34.
The addition of quinine to the copper flash solution helped keep the coating from
flaking off during drying and appeared to have no negative impact on the integrity of the
83
coating. The adhesion of the coating was improved at the low copper sulfate
concentration, but was worse at the high concentration. An inhibited solution should be
tested in the barrel set-up for the final plating process to evaluate the inhibitor’s ability to
increase the operating window of the copper flash step.
Table 34 Samples and adhesion results for variations in copper sulfate concentrations of copper flash solutions inhibited with quinine
4.3.4. Barrel Plating Tests
4.3.4.1. Copper sulfate concentration variations
To assess the viable operating window of the system, various copper sulfate
concentrations were tested at one and three minutes. A sample of each batch is shown in
Table 35 and a larger sample selection may be found in Appendix 9. Solutions
containing 2.5, 5, 10 and 15 g/L were tested.
A concentration of 5 g/L copper sulfate gave the most uniform coating after one
minute, however the operating window for that concentration was very narrow. At three
minutes, all of the samples showed poor adhesion with areas that had been knocked off
Quinine
Concentration
(g/L)
Copper Sulfate
Concentration
g/L
Time
30 seconds 60 seconds 90 seconds
0.003
15
Adhesion Adequate Adequate Very Poor
30
Adhesion Poor Very Poor Very Poor
84
by the parts tumbling in the barrel. Upon drying, the highest copper sulfate
concentrations used (10 and 15 g/L) showed complete delamination of coatings plated for
3 minutes and some delamination of coatings plated for 1 minute. To increase the
operating window of the copper flash, an inhibited solution was tested.
Copper Sulfate
Concentration
(g/L)
Flash time
1 minute 3 minutes
2.5
5
10
15
Table 35 Batch tests of copper sulfate concentration variations in an uninhibited copper sulfate flash solution
85
4.3.4.2. Inhibitor concentration variations Quinine was used as an inhibitor in a copper flash solution with 0.1 vol% sulfuric
acid and 15 g/L copper sulfate. Two inhibitor concentrations were tested at one and three
minutes. Results are summarized in Table 36 with expanded results in Appendix 10.
At low concentrations (0.003 g/L) quinine improved the adhesion of the coating
when compared to uninhibited, barrel plated copper coatings. The extent of coating
knocked off by the tumbling of parts against each other was decreased with the addition
of quinine in low concentrations.
As seen in the development of the cleaning solution, past a threshold value
quinine became an aggressive ion. This holds true for quinine in copper sulfate plating
solutions as well. At concentrations of 0.03 g/L, the amount of coating knocked off by
tumbled parts was comparable to uninhibited flash solutions and greater than solutions
inhibited with 0.003 g/L quinine.
Quinine
Concentration
(g/L)
Flash time
1 minute 3 minutes
0.003
0.03
Table 36 Quinine concentration variations in 5 g/L copper sulfate copper flash batch tests
86
The addition of low concentrations of quinine decreased the amount of coating
knocked off, and had a beneficial effect on the adhesion of the coating. However, the
operating window of the copper flash was not extended past 1 minute.
To obtain a wider operating window, an inhibited solution with lower copper
sulfate concentrations was used. At very low concentrations – 2.5 g/L – the operating
window ranged from 60 to 90 seconds, with the ideal time being 60 seconds. The
adhesion was adequate up to 60 seconds, and at 90 seconds there were minimal areas
where the coating had been knocked off the substrate.
Table 37 Copper sulfate concentration variations and rpm variations in inhibited copper flash solution
Copper Sulfate
Concentration
g/L
Time
30 seconds 60 seconds 90 seconds 120 seconds
2.5
NA
Adhesion Adequate Adequate Poor
5
NA
Adhesion Adequate Poor Very Poor
5 – slow rpm NA NA
Adhesion Adequate Adequate
87
1. a) b) c)
2. a) b) c)
3. a) b) c)
4. a) b) c)
Figure 21 SEM images at (a) 250X and (b) 500x magnification of samples as well as (c) surface mapping of samples plated under the following conditions: 1) uninhibited, 5.0 g/L CuSO4 for 60s, 2) inhibited, 2.5 g/L CuSO4 for 60s, 3) inhibited, 5.0 g/L CuSO4 for 60s and 4) inhibited, 5.0 g/L CuSO4 for 90s at slow RPM Further tests were conducted in 5 g/L copper sulfate solution at a lower rpm (5
rpm) for the barrel plater to see if knock-off could be further minimized. This was
Fe substrate
Fe substrate
Fe substrate
Fe substrate
88
successful in increasing the possible operating window. Samples had no knock off at 90
seconds, and very minimal knock off at two minutes. From Table 37, it can be seen that
the optimal plating parameter is at a slow rpm and 5 g/L copper sulfate. SEM images
confirmed that inhibited copper flash solutions yield coatings with smooth morphology
and good surface coverage.
The above SEM images show the surface morphology of uninhibited and
inhibited samples. The images in part c) of each sample had their contrast increased to
emphasize the steel substrate areas, which appear as darker patches on the image.
As seen in Figure 21 1c-4c the surface coverage was lowest for the sample plated
in the uninhibited solution for 60 seconds. This poor coverage is likely due to knock-off
from the tumbling in the barrel plating system. As observed earlier, when an inhibitor is
added to the plating solution, the knocking off of the copper coating is reduced. This is
confirmed by the high surface coverage observed in Figure 21 2c and 3c. The sample
coated using a slow rpm, 4a-c, had a rougher surface morphology and slightly less
surface coverage than samples 2 and 3. However, adhesion tests showed that it had
adequate adhesion, and it the operating window for that condition is higher than that of
previous conditions (i.e. samples 1-3).
Based on the adhesion and surface morphology results, the slow rpm (5rpm vs. all
others plated at 20 rpm) and inhibited solution have successfully widened the operating
window to 60-120 seconds.
4.4. Anti-oxidizing agent
4.4.1. Beaker Tests
Beaker tests were performed for initial determination of an anti-oxidant coating
that would provide resistance to general atmospheric corrosion.
The anti-oxidant of interest was benzotriazole, and solutions of two different
concentrations were mixed. Many stepwise plating procedures use a neutralizing dip
(alkaline for an acidic plating solution and acidic for an alkaline plating solution) before
the anti-oxidant step [58, 66]. Two batches of samples were produced: samples rinsed in
89
water and then dipped in anti-oxidant and samples dipped in an alkaline solution
followed by an anti-oxidant dip.
These samples were then tested in the corrosion chamber as per the test outline
described in section 3.4.3 and
Appendix 4 to determine the effectiveness of the anti-oxidant coating. Images of the
coatings before and after corrosion testing were examined for any indication of corrosion
products. The results of the coatings without an alkaline dip are shown in Table 38
below and with an alkaline dip are shown in Table 39.
Immersion time in anti-oxidant
(seconds)
Corrosion testing
Anti-oxidant solution concentration
No coating 0.01 M 0.1 M
Without alkaline dip
30
Before
After
60
Before
After
90
Before
After
Table 38 Summary of corrosion test results for 0.01M and 0.1M benzotriazole anti-oxidant solutions without preliminary alkaline dip
90
Immersion time in anti-oxidant
(seconds)
Corrosion testing
Anti-oxidant solution concentration
No coating 0.01 M 0.1 M
With alkaline dip, pH = 9
30
Before
After
60
Before
After
90
Before
After
Table 39 Summary of corrosion test results for 0.01M and 0.1M benzotriazole anti-oxidant solutions with preliminary alkaline dip As seen in Table 38 and Table 39, the samples without the alkaline dip proved to
have better corrosion resistance than those that had been dipped prior to anti-oxidant
treatment. The alkaline dip prior to the benzotriazole dip appeared to cause minor
oxidation of copper coating leading to a decrease in the luster of the coating.
Of the two different concentrations of benzotriazole used, both concentrations
were equally effective. Increased immersion time lead to better corrosion results,
however the luster of the coating was compromised, and the surface finish was not as
91
glossy when dipped in the benzotriazole rinse for 90 seconds as compared with samples
dipped for 60 seconds.
Since one of the goals of this thesis is to minimize the ionic and organic species
present, the ideal conditions are 0.01 M benzotriazole dip, no alkaline rinse and 60
second immersion time. Those parameters will be used in scaling up to a 6L bath for
final line testing.
4.4.2. Batch Tests
Using 6L basins of each solution, the cleaning, copper flash and benzotriazole dip
was applied to a batch consisting of 10 samples and rotated at 5 rpm. The process is
summarized in section 3.4.2.
Two batches of samples were produced at a dip time of 60 seconds, and one batch
was produced at a dip time of 90 seconds. The batches produced with a shorter dip time
had a more uniform finish. Accelerated corrosion tests were performed as per section
3.4.3 and
Appendix 4 on coated samples for 3 and 6 hours, with results shown in Table 40.
The samples produced at with a 60 second benzotriazole dip had very good
corrosion resistance at a 3 hour test period when compared to samples without the dip.
At a 3 hour test period, there was considerable localized corrosion, but not as uniform as
the corrosion of the untreated sample. Samples with a 90 second benzotriazole dip also
had good corrosion resistance, but the non-uniformity of the surface before the corrosion
testing made it less desirable than the 60 second dip.
After 6 hours of testing, there was considerable corrosion on both as-plated
(without dip) and anti-oxidized samples. At that testing time, samples with the
benzotriazole dip still had better corrosion resistance than the as-plated samples.
92
Benzotriazole dip time
Corrosion test
Test time Before testing After testing
60 seconds 3 hours
90 seconds 3 hours
NO DIP 3 hours
60 seconds 6 hours
NO DIP 6 hours
Table 40 Corrosion test results for benzotriazole dip batch tests.
4.5. Developed System
A summary of the parameters for the developed system compared to the
MacDermid process is outlined in Table 41.
93
Solution
Step
Developed System MacDermid Process
Composition
(bulk water)
Immersion
time
Solution
added
Tumbling
time
Cleaning 1 vol% H2SO4
0.003 g/L quinine 2-5 minutes
Macuguard
SCF +
Foamout
12 minutes
Rinse Water 10-30 seconds N/A N/A
Copper flash
0.1 vol% H2SO4
5 g/L CuSO4
0.003 g/L quinine
1-2 minutes Macuguard
XL 40 8 minutes
Rinse Water 10-30 seconds Water rinse
and drain N/A
Anti-oxidant 0.01 M benzotriazole 60 seconds Relubro165
polisher 1-2 minutes
Rinse Water 10-30 seconds Water rinse
and drain N/A
Table 41 Summary of electroless copper flash system developed for a room temperature barrel plating system All steps are conducted at room temperature, with a 50% barrel loading (barrel is
half-full) and a barrel rotation speed of 5 rpm. The total plating time of the developed
system was approximately 5-10 minutes while the MacDermid process was 21-22
minutes, not including the time taken to drain the solutions between plating and polishing
steps. Furthermore, all solutions developed are re-useable, as opposed to the single use
solutions in the MacDermid step. Thus by implementing this sustainable process, it is
possible to increase the throughput of parts by 2-4 times and the life of the solution is
extended.
94
5. Conclusions
An electroless copper coating system was developed with the intent of
minimizing the ionic and organic concentrations of each step in the process. A cleaning,
plating and anti-oxidizing process was developed using only four species, and in the
minimum possible concentrations. This process also allowed for the widest possible
operating window for each step.
Contrary to claims that quinine was an ineffective inhibitor in sulfuric acid [31],
through weight loss and electrochemical measurements it was found to be an effective
inhibitor in low concentrations. At high concentrations, it became an aggressive ion.
Quinine was also found to aid in the coating integrity and expand the operational
window when added in small concentrations to copper sulfate flash solutions.
Benzotriazole proved to be an effective inhibitor against atmospheric corrosion
for copper coatings, even in concentrations as low as 0.01M.
95
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70 J. H. Tran, Direct Copper Plating Without the Electroless Copper Solution, County
Sanitation Districts of Orange County (2003)
71 ASM Volume 5, Cleaning and Finishing of Copper and Copper Alloys, Surface
Engineering (1994) pp 611-627
72 ASM Volume 5, Mechanical Coating, Surface Engineering (1994) pp 611-627
73 G.M. Allison, Metal Finishing, Vol. 97, No. 1 (1999) pp 407-410
74 R. Singleton, Metal Finishing, Vol. 98, No. 1 (2000) pp 340-360
75 MacDermid SCF user guide. Product Code No. 17122, MacDermid Incorporated.
(2004)
76 ISO 4538-1978 Metallic Coatings – Thioacetamide corrosion test (1978)
99
Appendices
Appendix 1 Corrosion Current Density Sample Calculation
Polarization Resistance:
A range of ± 10 mV from Ecorr was used to calculate polarization resistance. The
raw data was plotted in that range and the polarization resistance was calculated by
equation {23}
Rp =Δε
Δiapp
Figure 22 Polarization resistance for iron in 0.03 g/L quinine inhibited sulfuric
acid solution at room temperature
0.43 — 0.4389 1
0.0088.9
8.99 Ω/
100
The cathodic and anodic Tafel slopes were calculated;
Figure 23 Tafel extrapolation data for a 0.03 g/L inhibited sulfuric acid cleaning solution at 25°C
Figure 24 Cathodic Tafel area from Figure 23 0.60 — 0.573 5
0.150 /
101
The anodic Tafel slope was calculated in a similar manner;
Figure 25 Anodic Tafel area for Figure 23 0.410 — 0.424
8 1
0.017 /
Then icorr was determined by applying equation {24}
2.3
0.017 0.152.3 8.9 0.017 0.15
0.00165 /cm2
1.65 /cm2
icorr calculated automatically by CHI: 1.23 mA/cm2
102
Appendix 2 Raw Electrochemical Data
Sample
RP
(Ω/cm2)
βa
(mV/dec)
βc
(mV/dec)
Ecorr
(mV)
Icorr
(mA)
Anode
area
(cm2)
icorr
(mA/cm2)
25°C
uninh 176 26.3 223 -450 0.196 0.0766 2.56
25°C
0.003 532 22.3 175 -447 0.0253 0.0302 0.84
25°C
0.03 356 15.8 180 -438 0.0804 0.0654 1.23
25°C 0.3 273 22.0 216 -421 0.126 0.0817 1.54
50°C
uninh 21.6 30.2 197 -439 1.29 0.0635 20.3
50°C inh 47.2 15.6 106 -443 0.580 0.0620 9.38
75°C
uninh 1.79 54.5 241 -419 12.9 0.0552 233
75°C inh 4.89 27.3 138 -421 5.64 0.0655 86.1
Table 42 Raw electrochemical data
103
Appendix 3 Amercoat™ 90 Epoxy Application Procedure
The following procedure was taken from the Procedure Manual found in the Surface
Engineering and Electrochemistry Lab, MB 209, University of Toronto. It was initially
prepared by Dave Anthony for S.J. Thorpe. All mixing and application of Amercoat™
must be performed in a fumehood with proper gloves and eye protection as it contains
Xylene as a solvent. Xylene vapor is both toxic and flammable.
1. Prepare a mixture of resin (white) and curing agent (translucent) in a ratio of 4:1. A
quantity of 2 mL resin and 0.5 mL curing agent is sufficient to coat 10 small anodes.
2. Open the Amercoat™ resin container and thoroughly mix the resin by hand, until no
further separation of the solvent is apparent. Once the resin is mixed, the stir bar
should be cleaned with a paper towel that is to be discarded in the fumehood.
3. Using a syringe, extract 2 mL of resin and expel into a clean weigh boat. Discard the
syringe into the waste basket in the fumehood.
4. Repeat steps 2 and 3 with the Amercoat™ curing agent, taking care to extract the
correct amount of curing agent to meet the 4:1 ratio requirements.
5. Once the curing agent and resin have been added to the weigh boat, use the end of
one of the syringes to mix the two components thoroughly.
6. Using a fine bristle brush, apply a thin coating of epoxy to the sample. Make sure to
coat all conductive surfaces that are not to be tested.
7. Using a stereomicroscope, examine the sample 1 hour after plating and re-touch if
necessary.
8. After coating the sample, it must be left in the fumehood for 24 hours to ensure the
Amercoat™ 90HS is completely cured.
9. Clean the brush thoroughly with isopropyl alcohol (IPA) after use.
10. Repeat steps 1 through 9 until 3 separate coats of Amercoat™ 90HS have been
applied to each sample. A minimum of 24 hours drying time is required between
each coating.
104
Appendix 4 Corrosion Test Set Up Corrosion tests were performed in accordance with the ISO standard ISO 4538-
1978: thioacetamide corrosion test.
This test specifies a chamber with absorbent paper along the bottom and sides.
For that purpose, Fisher brand filter paper was used to line the bottom and sides with a
3:1 (by weight) sodium acetate solution. The filter paper is not shown in the images
below, so that the framework may be seen.
Figure 26 Corrosion tests set up prior to addition of filter paper and chemicals
105
Figure 27 Bird’s eye view of sample holder without lid for corrosion testing
Samples
106
Appendix 5 Uninhibited Hydrochloric Acid Solution Samples
Temperature Acid
Concentration
(vol%)
Time
5 minutes 15 minutes 30 minutes
25°C
5
10
20
50°C
5
10
20
75°C
5
10
20
Table 43 Complete uninhibited hydrochloric acid test results
107
Appendix 6 Uninhibited Sulfuric Acid Samples Temp Acid
Conc.
(vol%)
Time
10
seconds
30
seconds
1
minute
2
minutes
5
minutes
15
minutes
25°C
5
No smut
No smut
No smut Level 1
Level 3
Level 4
10
No smut
No smut
Level 1
Level 2
Level 4
Level 5
50°C 5
No smut
Level 1
Level 1 Level 5
Level 6
Level 6
40°C 10
No smut
No smut
No smut Level 4
Smut formation very
high at 2 minutes. No
further tests
conducted
Table 44 Temperature and acid variations in uninhibited sulfuric acid cleaning solutions
108
Appendix 7 Inhibited Sulfuric Acid Solution Samples
Quinine
Concentration
Cleaning Only 1 min Cu Flash
5 min 10 min 5 min 10 min
No quinine
0.0003 g/L
0.003 g/L
0.03 g/L
0.3 g/L
Table 45 Inhibited sulfuric acid solution samples
109
Appendix 8 Complete Copper Bath Beaker Test Results
Acid
Concentration
(vol%)
Copper Sulfate
Concentration
(g/L)
Time
30 seconds 60 seconds 90 seconds
0.01
5
15
30
0.1
5
15
30
1
5
15
30
Table 46 Complete summary of beaker tests in copper flash development
110
Appendix 9 Variation of Copper Sulfate in Batch Copper Flash Tests Copper sulfate
concentration
(g/L)
Time
(minutes) Samples
2.5
1
3
5
1
3
10
1
3
15
1
3
Table 47 Samples from each batch of varying copper sulfate concentration copper flash batch tests
111
Appendix 10 Summary of Inhibited Copper Flash Batch Tests
CuSO4 Quinine Time Samples
2.5 g/L 0.003 g/L
60s
90s
120s
5 g/L 0.003 g/L
60s
90s
120s
15 g/L
0.003 g/L
60s
180s
0.03 g/L
60s
180s
Table 48 Copper sulfate flash solutions with 1 vol% sulfuric acid inhibited with varying concentrations of quinine, flashed for various times