corrosion control .pdf
TRANSCRIPT
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CORROSION CONTROL
The rate of corrosion can be controlled by eithermodifying the metal or the environment.
1. Proper Designing
A major factor in the corrosion failure of a component
is a faulty geometrical design.Some important design principles are:
1) Avoid crevices
2) Avoid residual moisture
3) Avoid galvanic corrosion
Galvanic corrosion can be prevented by the followingmethods,
a) Use an electrical insulators
b) Introduce an easily exchangeable corroding places
4) Avoid protruding parts.
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2. Cathodic Protection The principle involved in cathodic
protection is to force the metal to be
protected to behave like a cathode.
Since, there will not be any anodic area onthe metal, corrosion does not occur.
There are two types of cathodic protection.
Sacrificial anodic protection method
Impressed current cathodic protection
method.
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SACRIFICIAL ANODIC PROTECTION METHOD
In this method, the metallic structure to be
protected is made cathode by connecting it with
more active metal (anodic metal).
So that all the corrosion will concentrate onlyon the active metal.
The artificially made anode thus gradually gets
corroded protecting the original metallic
structure. Hence this process is otherwise known as
sacrificial anodic protection.
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Examples of sacrificial anode
This method is used for the protection ofships and boats.
Sheets of zinc and magnesium are hungaround the hull of the ship.
Zinc and magnesium being anodic to ironget corroded.
Since they are sacrificed in the processof saving iron (anode), they are called
sacrificial anodes.
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Protection of underground pipelines and
cables from soil corrosion.
Magnesium rods are inserted in to
domestic water boilers or tanks to preventthe formation of rusty water.
Calcium metal slag's are employed to
minimize engine corrosion.
Applications of Sacrificial Anode
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IMPRESSED CURRENT CATHODIC PROTECTION
In this method, an impressed current is
applied in the opposite direction to nullify
the corrosion current and convert the
corroding metal from anode to cathode.
This can be done by connecting negative
terminal of the battery to the metallic
structure to be protected.
Positive terminal of battery is connected to
an inert anode. inert anode used for this
purpose is graphite or platinised titanium.
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The anode is surrounded by backfill
(containing mixture of gypsum, coke,
sodium sulphate) to improve theelectrical contact between the anode and
the surrounding soil.
APPLICATION OF IMPRESSED CURRENT
PROTECTION
This type of cathodic protection is
applied to open water-box coolers, water
tanks, buried oil and water pipes,
condensers, marine piers, transmission
line towers, etc.,
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Comparison of sacrificial anode and
impressed current cathodic method
Sacrificial anodic
method
No external powersupply is necessary.
This method
requires periodical
replacement ofsacrificial anode.
Investment is low.
Impressed current
method
External powersupply must be
present.
Here anodes are
stable and do notdisintegrate.
Investment is more.
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Soil corrosioneffects are not
taken in to
account.
This is mosteconomical
method especially
when short-term
protection isrequired.
Soil corrosioneffects are taken
in to account.
This method iswell suited for
large structures
and long term
operations.
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Control of corrosion by modifying the
environment
DEAREATION
The presence of increased amount of
oxygen is harmful and increase thecorrosion rate.
Deareation involves removal of dissolved
oxygen by increase of temperature with
mechanical agitation.It also removes dissolved CO2 of water.
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In this method, moisture from the air isremoved by lowering the relative humidity ofthe surrounding air.
This is done by adding silica gel (or) alumina,which adsorbs moisture preferentially on its
surface.
DEHUMIDIFICATION
ALKALINE NEUTRALISATION
The acidic character of the corrosive
environment (due to presence of H2S, HCl, CO2,SO2, etc) can be neutralized by spraying alkalineneutralisers (like NH3, NaOH, lime etc).
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CORROSION INHIBITORS
DEFINITIONA corrosion inhibitors is a substance which when
added in small quantities to the aqueous corrosive
environment effectively decreases the rate of corrosion of the metal.
Inhibitors are classified in to three types,
ANODIC
CATHODIC
VAPOUR PHASE
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Chromates, phosphates, nitrite, nitrate,inhibit the anodic corrosion reaction byforming sparingly soluble compound with anewly produced metal ion (at the anode).
They are absorbed on the metal surfaceforming a protective film or barrier there- by
reducing corrosion rate.
This kind of corrosion rate is not fully
reliable since certain areas left uncovered bythe film can produce severe corrosion.
ANODIC INHIBITORS
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In acidic solution, the main cathodic
reaction is evolution of hydrogen.
2H+(aq) +2e- H2 (g)
In an acidic solution, the corrosion canbe controlled by slowing down the
diffusion of H+ ions through the cathode.
This can be done by adding organic
inhibitors like amines, pyridine, azoles,etc.
They absorb over the cathodic metal
surface and act as a protective layer.
CATHODIC INHIBITORS
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In a neutral solution, the cathodic reactionis,
H2O + O2 + 2e- 2OH-(aq)
The formation of OH- ions is only due to the
presence of oxygen.By eliminating the oxygen from the medium,the corrosion rate can be reduced.
O2 can be removed by adding some reducingagents like Na2SO3 or by deaeration.
Salts of Zn, Mg, Ni are employed as theyform insoluble metallic hydroxide whichforms impermeable self barriers.
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VAPOUR PHASE INHIBITORS
Vapour phase inhibitors are organicinhibitors which readily sublime and form aprotective layer on the metal surface.
Example : Dicyclohexyl ammonium nitrite,Benzotriazole.
Vapour phase inhibitors are used in the
protection of machineries, sophisticatedequipments, etc. which are sent by ships.
The condensed inhibitor can be easily wipedoff from the metal surface.
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PROTECTIVE COATING
INTRODUCTION
Protective coatings are used to protectthe metals from corrosion.
It acts as a physical barrier between thecoated metal surface and theenvironment.
They impart some special properties suchas hardness, electrical properties andthermal insulating properties to theprotected surface.
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Protectivecoatings
Inorganic coating
Metallic coating Chemical
Conversion
Organic coating
1. Paints2. Varnishes
3. Enamels
4. Ceramic
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Mechanical cleaning To remove loose scale
and rust, using hammer, wire-brushing, grinding
and polishing.
Sandblasting To clean large surface areas in
order to produce enough roughness for good
adherence of protective coating, using sand
with air stream at 25-100 atm.
Solvent Cleaning To remove oil, grease, rust
using organic solvents like alcohol, xylene,
toluene, hydrocarbons followed by cleaning hot
water or steam.
Sample Preparation
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Sample Preparation (contd..)
Alkali Cleaning To remove old paints that are
soluble in alkaline medium using chemicals l ike
NaOH, Na3PO4 etc. After cleaning, the metal is
washed with 1% chromic acid solution.
Acid pickling and etching Base metal is dipped
inside acid solution at a higher tempt for a long
duration. Acids used are HCl, H2
SO4
, H3
PO4
, HNO3
,
under dilute conditions.
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Metallic coatings
Anodic coatingGalvanization:
It is produced by anodic coating metals (Zn, Al,
Cd) on the surface of base metal (Fe) based on
the relative negative electrode potential.
Cathodic coating:
It is produced by cathodic coating metals (Sn,Cr, Ni) on Fe surface based on the relative
positive electrode potential of coat metal.
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Methods of application of
metallic coating
Hot dipping
Metal cladding
Electroplating Cu, Cr, Ni, Au, Ag
Vacuum metalizing
Metal spraying
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Hot Dipping
It is one of the common method of applying
metallic coating on the surface of base
metals.
Hot dipping is a process of coating the base
metal by immersing it in the molten coat
metal.
Examples: Galvanizing and Tinning
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Tinning: In this process tin is coated over mildsteel sheets immersed in molten tin (Sn).
The sheet is subject to acid pickling and passedthrough a bath of molten tin covered with a fluxof ZnCl2.
After coating, the sheet is passed through palmoil to protect from oxidation
Finally the sheet is passed to roller to getuniform thickness.
It is used for the coating of steel, Cu and brasssheets that store food stuffs.
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Metal Cladding
It is the process of sandwitching the basemetal between two thin layers of coating metalby hot-rolling the composite to produce a firmbonding.
The coat metals are usually metals of leastreactivity (Cu, Ni, Ag, Pt, Ti)
The cladding layer should be very thin and itsthickness is only 5% of the total composite
metal. Duraluminium sandwiched between Al sheets
and hot rolled to produce Alkad compositewhich is free from stress corrosion
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ELECTROPLATING
PRINCIPLE
Electroplating is the process in which the coatingmetal is deposited on the base metal by passing adirect current through an electrolytic solutioncontaining the soluble salt of the coating metal.
Electroplating is probably the most important andmost frequently applied industrial method of producing metallic coatings. The metal film
produced is quite uniform with little or no pinholesper unit area.
When the thickness of the deposit increases, thenumber of pinholes decreases.
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The base metal to be plated is made
cathode of an electrolyte cell, whereas theanode is either made of the coating metalitself or an inert material of good electricalconductivity.
THEORY
If the anode is made of coating metal itselfin the electrolytic cell, during electrolysis,
the concentration of electrolytic bathremains unaltered, since the metal ionsdeposited from the bath on cathode arereplenished continuously by the reaction offree anions with the anode.
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Objectives of electroplating:
(i) To increase the resistance to corrosion and
chemical attack of the plated metal.
(ii) To obtain a polished surface
(iii) To improve hardness and wear resistance
Example: Electroplating of Cu, Au, Ag, Cr, Ni, Sn etc.
Uses of electroplating:
(i) It is often used in electronic industries formaking printed circuit boards, edge connectors,
semiconductor lead-out connection
(ii) It is also used in the manufacture of jewelery,
refrigerator, electric iron etc.
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Electroplating of Cu
For electroplating of Cu on metal surface, Electrolyte: (3-5%)H2SO4 / (15-30%) CuSO4
Anode: Pure Cu metal or Graphite (inert)
Cathode: Metal to be coated
Additive: Boric acid or gelatinIonization reaction of electrolyte is observed,
CuSO4 Cu2+ + SO4
2-
On passing current, Cu2+ + 2e- Cu (at cathode)
SO42- SO4 + 2e
- (at anode)
H2SO4 2H+ + SO4
2-
Due to common ion effect, the ionization rate of Cu2+ is controlled
and the deposition process can also be controlled, with a current
density of 0.5 to 1.5 ampere/dm2.
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Factors affecting electroplating
Surface cleaning for strong adherent Concentration of electrolyte Moderate conc. Is
preferred for uniform coating
Conductivity and stability of electrolyte - Good
Thickness of the deposit for decorative purposethin coating and for corrosion protection multiple
coating.
Current density (current per unit of the base metal)
should be low for uniform controlled deposition Additives: Ensure strong adherence and mirror
smooth coating.
pH of the electrolytic bath between pH 4.0 - 5.5
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Thin-Film Coatings (m)
PVD Coating (Physical Vapor Deposition)
CVD Coating (Chemical Vapor Deposition)
Physical Vapour Deposition, PVD a group of vacuumcoating techniques that are used to deposit thin f ilm
coatings that enhance the properties and performance of
tools and machine components.
PVD coatings are used in a vast array of industries andthousands of applications as diverse as "self-cleaning"
windows, medical implants, cutting tools, decorative
fitt ings and Formula 1 racing parts.
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Thin-film property
category
Typical applications
Optical Reflective/antireflective coatings
Interference filters, Decoration (colour,
luster), Memory discs (CDs),
Waveguides
Electrical Insulation, conduction, Semiconductor
devices, Piezoelectric drivers
Magnetic Memory discs/devices
Chemical Barriers to diffusion or alloying
Protecting against corrosion or
oxidation
Gas/liquid sensors
Mechanical Tribological (wear resistant) coatings
Hardness, Adhesion, Micromechanics
Thermal Barrier layers, Heat sinks
Classifications of thin-films based on their applications
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Deposition techniques
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Deposition by Physical Vapour
Deposition (PVD)
To vacuum pump
PVD Chamber
Suction valve
9/13/2013 6:21 PM 35
Nanotechnology
N2(or)
H2
Heater
Ni Source
Ni film
H2 = 50 psi
N2 = 15 psi
Substrate
2000oC
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PVD is a process to produce a metal vapor that can be
deposited on conductive materials as a thin highly adhered
pure metal or alloy coating.The process is carried out in a vacuum chamber at high
vacuum (10-6 torr).
Single or multi -layer coatings can be applied during the same
process cycle.
Additionally the metal vapor can be reacted with various
gases to deposit Oxides, Nitrides, Carbides or Carbonitrides.
The coating method involves purely physical processessuch as high temperature vacuum evaporation or plasma
sputter bombardment rather than involving a chemical
reaction at the surface to be coated.
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Aluminium Titanium Nitride coated
Titanium Nitride coated punches
Aluminium Chromium
Titanium Nitr ide coated
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Structure of TiAlCN
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Cathodic Arc Deposition: In which an electric arc is usedto vaporize material from a cathode target. Thevaporized material condenses on a substrate, forming athin film.
Evaporative deposition: In which the material to bedeposited is heated to a high vapor pressure byelectrically resist ive heating in " low" vacuum.
Sputter deposition: In which a glow plasma discharge(usually localized around the "target" by a magnet)
bombards the material sputtering away as a vapor.
Ion plating: In which the material is heated to a highvapor pressure and a plasma is established to ionize theevaporating species. These species physically implantinto the substrate producing strong coating bond.
Different types of PVD coating
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Sputtering
Coating can be done for both conductive (dc) and non-conductive (RF sputtering) materials
DC sputtering the workpiece and substance to be
coated are connected to high voltage dc power supply.
Vacuum chamber is fil led with controlled amount ofargon gas to establish a pressure of 10-4 torr.
The supplied direct current energizes the chamber
creating a plasma between the workpiece and the
material to be coated.
The argon atoms get ionized and accelerated to bombard
on the workpiece resulting in the sputtering of atoms,
which are transported and coated on the substance
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The parts to be coated are first cleaned. The cleaningprocess varies depending on the level of quality from the
electroplater, substrate material and geometry.
The parts are loaded into the vacuum chamber on custom
fixtures designed to optimize the chamber load size andensure coating uniformity.
The vacuum chamber is evacuated to 10-6 torr (high vacuum)
to remove any contaminants in the system.
The vacuum chamber is backfi lled with an inert gas argon
and ionized, result ing in a glow discharge (plasma). This is
the gas cleaning stage and prepares the parts for the ini tial
metal deposition.
PVD Process
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A high current, low voltage arc is initiated on the target
(solid material used for deposition). The metal is
evaporated and instantaneously ionized.
These metal ions are accelerated at high energies into
the vacuum through an inert gas or reactive gas and
subsequently deposited on the part.
The basic properties of the metal being evaporated
(target) remain unchanged during the metal deposition
cycle.
Changing the volume of gas and type of gas during the
reactive deposition cycle changes the nature of the
coating.
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Zirconium nitride (ZrN) is a hard, yellow-gold colored
coating with exceptional wear and corrosionresistance, used in plumbing and door hardware
industry.
Introducing measured amounts of nitrogen into thechamber during the zirconium deposition cycle
produces zirconium nitride.
Chromium nitride is produced in much the same way.
Simply by adding an additional gas such as acetylene(C2H2), you can create chromium carbonitride. This is a
gray to black color.
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TiN 2900 HV Gold
ZrN 2800 HV Gold TiAlN 2600 HV Brown
TiCN 4000 HV Silver
CrN 2500 HV Silver
DLC 1000 to 5000 HV - Black
Coating processes are performed at 300 0C or upto 2800 0C.
The higher temperature processes usually produce optimum
coating properties but sometimes results in softening ofsubstrates especially steel.
Thickness usually in the range of 1 to 2 m.
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CVD involves the formation of a solid f ilm on a surface of a heated
substrate by means of a chemical reaction in a gas or in the vapor
phase.The complex molecule in the vapor state impinges on the hot
substrate, decomposes and forms a thin film. These reactions are
promoted by resistance, RF or infrared radiation heating.
Example: Monds Process
Ni(CO)4 Ni + 4CO150
C
CVD process
Requirements for a CVD process
1. Vacumised chamber (10-3 mbar) connected to a rotary pump
2. Complex chamber with heating facility
3. Vapour transport SS tubes ( quarter inch)
4. Substrate holder inside the chamber with heater (flat heater
900oC)
5. High Temperature valve to control the flow rate of complex vapour
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To vacuum pump
CVD Chamber
Suction valve
9/13/2013 6:21 PM 46Nanotechnology
Pre
Vap
N2(or)
H2400oC
Heater
Substrate
Ni film
H2 = 50 mm Hg
N2 = 15 mm Hg
Deposition by Chemical Vapour
Deposition (CVD)
Solidcomplex
HTV
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Step 1: The vapor source of the film-forming material
may be a solid, liquid, vapor or gas. Solid materials,having sufficient vapor pressures, need to be vaporized
to transport them at moderate temperature to the
deposition zone where the substrate is placed, and this
is normally achieved by heating.
Step 2: Another issue is the uniformity of arrival rate of
vapor sources by transport to the hot substrates. This
uniformity in transporting the vapor source varies with
the transport medium used to transport the source to thedestination (i.e. either by high vacuum or fluid (gaseous
fluid).
CVD Process steps
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Step 3: Deposition is the third step in a process of
developing thin-films, in which the actual growth of film
over the surface occurs. Deposition pattern is
determined by the source, transport of vapor and
conditions of deposition zone.
Step 4: Once the deposition is over, the next step is the
analysis of coated thin-films by various techniques.
Analysis of thin-films can be thought of as the final stageprocess of monitoring but it is important in all steps of
thin-film deposition.