201 b document
DESCRIPTION
SEGUNDA PARTE CURSO DE CORROSIÓN BÁSICA, DOD-EEUUSEGUNDA PARTE CURSO DE CORROSIÓN BÁSICA, DOD-EEUUTRANSCRIPT
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201B
Characteristics and
Impact of Materials
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IInnttrroodduuccttiioonn
Corrosion 201B discusses engineering fundamentals for controlling corrosion and
provides basic concepts and tools you will need to understand how corrosion
initiates and propagates, how it affects materials, and what you can do to
prevent or mitigate corrosion.
CCoorrrroossiioonn DDeecciissiioonn MMaakkeerrss
System managers, designers and engineers must make choices between design,
alternative materials, structural designs and manufacturing processes that have
an influence on corrosion. The environment in which the material is used will
affect how corrosion initiates and grows, and the effectiveness of corrosion
prevention and control methods.
CCoorrrroossiioonn CCoonnttrrooll FFuunnddaammeennttaallss
This section of Corrosion 201-B addresses engineering fundamentals for corrosion
control.
BBaassiicc CCoonncceeppttss ooff CCoorrrroossiioonn
Corrosion is the deterioration of materials metals, polymers, ceramics or
composites or their properties due to a reaction with the surrounding
environment.
201B Ch1 Fundamentals of Corrosion Control
Section 1: Introduction
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201B Ch1 Fundamentals of Corrosion Control
Section 1: Introduction
Material strength or durability might be lost or degraded, or the damage might be
just aesthetic. The deterioration can be due to an electrochemical reaction, a
dissolution reaction, or an interaction between the material and the environment.
The environmental effects could result from the chemical composition of an
aqueous environment such as chlorides or from factors such as moisture,
Temperature, flow rate, wet/dry cycles, stresses or abrasion. These examples
indicate that corrosion occurs many places for many reasons.
It is important to understand corrosion causes and behavior, to anticipate how
and where corrosion can occur, and to take actions to avoid corrosion induced
damage and failures. Avoiding such damage and failures depend on better corrosion
management and mitigation.
CCoosstt ooff CCoorrrroossiioonn
Why is the impact of corrosion of such great concern? The costs of corrosion are
staggering estimates generated in 2000 by the Department of Transportation
placed the cost of corrosion in the United States at more than 276 billion
dollars a year, and that figure continues to rise. By 2010 studies by the DoD,
determines Military cost of corrosion alone at over 20 billion dollars.
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Corrosion damage affects weapon systems and infrastructure performance,
availability and safety, all of which adversely impact the ability to accomplish
the military mission.
CCoorrrroossiioonn RRiisskk AAwwaarreenneessss
Better corrosion management and mitigation means there is an increased awareness
of corrosion risks. A basic technical understanding and effective implementation
program, with sound mitigation strategies and plans, can lead to sound corrosion
performance during a systems life cycle.
During a systems design phase, the objective is to identify and avoid risks, and
plan risk mitigation. During production and manufacturing, the objective is to
specify processes and practices that mitigate risk.
During the operational phase, the objective is to plan for and perform effective
corrosion inspection and maintenance to include root cause analysis, repair or
replacement.
201B Ch1 Fundamentals of Corrosion Control
Section 2: Risk-Based Corrosion Management
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RRiisskk--BBaasseedd AApppprrooaacchh MMaannaaggeemmeenntt PPrroocceessss
A risk-based approach to corrosion management incorporates an analysis of these
items:
Identify risks
Identify barriers to those risks
Determine allowable corrosion
Develop corrosion management plan for acceptable performance
Implement, validate and monitor the plan
RRiisskk--BBaasseedd AApppprrooaacchh CCoorrrroossiioonn CCoonnttrrooll
Some aspects of a risk-based approach are described here by a subject matter
expert:
To develop a risk-based approach to corrosion prevention and control,
first, you need to define what facilities and/or equipment youre trying to
protect. Secondly, once you have that defined. What are these things made of?
What materials? Is it ceramics? Is it polymers? Are there metals? What kind of
metals? There are numerous materials that can be used. Once you know the
materials that are being used and the materials you have to protect. Then, youve
got to define the environment to which they are exposed: that can be submerged;
it can be in the atmosphere. It can be severe. It can be mild. If its a
submerged situation - it could be turbulent conditions or it could be very calm
conditions. Each of those impacts the corrosion rates.
Finally, and a very important question, once you understand the operating
environments, you also need to understand the other environments impacting
corrosion. For instance, storage environments - a lot of times we have equipment
or systems and subsystems that are stored for a large portion of their life
before theyre used so when that happens, you need to understand the impact of
those storage conditions on that equipment.
201B Ch1 Fundamentals of Corrosion Control
Section 2: Risk-Based Corrosion Management
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Once you develop that first set of questions, then you need to ask a second set
of questions. First, what are the fundamental principles that we can employ to
gain corrosion protection? Once we understand the fundamentals of the science-
related or engineering -related fundamentals, we then need to look at the
concepts that we would use to protect those assets. We would also need to look at
the type of corrosion that were concerned with. For instance, if you had pitting
corrosion and if you have a situation where a system was very prone to crack
initiation - the pits would be very critical. On the other hand, if you had
uniform corrosion and it just took away the thickness of a material and we were
worried about crack growth rate, it would be uniform corrosion that we would
worried about crack growth rate, it would be uniform corrosion that we would
worry about.
So once we define the types of corrosion then we go to the corrosion
mitigation methods and practices we might use to protect those and those could be
anything from sheltering to corrosion inhibitors to coatings. Theres an array of
various types of mitigation, prevention and mitigation procedures that can be
used and they might be deployed at different points in the life-cycle of that
system. Thus, we have to decide how were gonna actually deploy or implement
those particular methods.
RRiisskk--BBaasseedd AApppprrooaacchh EEffffeeccttiivvee UUssaaggee
To effectively use the risk-based approach and answer the decisive corrosion
questions, you need to understand the corrosion behavior of metals, and the
characteristics and conditions of the corrosion environments.
201B Ch1 Fundamentals of Corrosion Control
Section 2: Risk-Based Corrosion Management
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MMeettaall EEnnvviirroonnmmeenntt BBeehhaavviioorr
A specific material in a specific environment determines corrosion behavior. For
example, steel is active in a hydrochloric acid solution and corrodes rapidly,
while it is passive in a mildly alkaline environment and the corrosion rate is
very low. Similarly, aluminum is passive in neutral environments and active in
acidic or alkaline solutions. Stainless steels are passive in many environments,
but in certain aggressive environments, passivity breaks down and localized
corrosion can cause serious damage. Likewise many nonmetallic materials will
degrade in their operating environment. An example of this would be fiberglass
composites, which deteriorate when exposed to severe UV environments, while they
remain stable if protected.
TThhrreeee BBeehhaavviioorrss ooff MMeettaallss
Environmental and Metallurgical Effects on Materials. One of three things will
happen when you put metal into an environment in this case a liquid.
If the metal is noble and is immune to the environment, there is no reaction and
no corrosion as shown on the left. In the center the metal reacts with the
environment so there is corrosion. The degradation process results in the
formation of soluble products, such as iron atoms on the metal surface going into
solution as ferrous ions. The behavior on the right is passivity, where the metal
reacts and corrosion products form on the exposed surface, but these corrosion
products provide an insoluble oxide film that can protect the metal from further
corrosion. So depending on the metal and the environment, behavior can be noble,
active, or passive.
201B Ch1 Fundamentals of Corrosion Control Section 3: Environmental and Metallurgical Effects
on Materials
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NNoobbllee aanndd PPaassssiivvee BBeehhaavviioorrss
Both noble and passive behaviors can be advantageous when developing solutions to
corrosion problems, but both have practical liabilities. Noble behavior is
functionally ideal, but noble metals are expensive and have limited strength and
ductility. Passive behavior is also advantageous as long as the metal stays
passive. If an unanticipated breakdown of the passive film occurs, pitting,
stress corrosion or crevice corrosion may result with catastrophic failures.
AAccttiivvee BBeehhaavviioorr
Active behavior results in corrosion, but a well conceived strategy provides the
foundation for effective management of active corrosion behavior. Corrosion can
be managed with improved designs and manufacturing processes, and with built-in
corrosion allowances. But corrosion management must be applied early in the
design stage. Specific corrosion management strategies include coating
susceptible materials to isolate them from the corrosive environment, application
of cathodic protection, or inducing immune or passive behavior.
IImmppoorrttaanntt CChhaarraacctteerriissttiiccss ooff MMeettaallss
Metallurgical characteristics of metals are important to describe their physical
composition and structure. These metallurgical characteristics are shown here.
201B Ch1 Fundamentals of Corrosion Control Section 3: Environmental and Metallurgical Effects
on Materials
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Inherent reactivity is a dynamic characteristic for which there is a wide range
of behavior highly noble metals such as gold, platinum, and iridium feature
very low inherent reactivity while very active metals such as magnesium feature
high reactivity with the environment.
Another important dynamic characteristic is the ability to become passive by
self-forming an insoluble protective surface layer.
MMeettaall IInnhheerreenntt RReeaaccttiivviittyy
Inherent reactivity of each metal can be ranked according to the electromotive
force series shown in the table. The metals at the top of the electromotive force
series are the least reactive while those with the lowest electromotive force are
the most reactive. Titanium and aluminum are extremely reactive and easily go
into solution. But they also can behave passively in some environments, often
protecting them from corrosion even though they are highly reactive metals.
201B Ch1 Fundamentals of Corrosion Control Section 3: Environmental and Metallurgical Effects
on Materials
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CChhaarraacctteerriissttiiccss ooff SSoolluuttiioonnss
Solutions are characterized by their pH, oxidizing potential, conductivity,
ionization potential and solubility. These properties can be measured and the
effect on metals determined quantitatively, enabling engineers to predict the
likelihood and type of corrosion for combinations of metals and solutions in
specific environments.
ppHH SSccaallee
Here we show how pH units define the acidity or alkalinity, where a pH from 1 to
7 reflects decreasing acidity, a pH of 7 indicates a neutral condition, and a pH
from 7 to 14 reflects increasing alkalinity.
OOxxiiddaattiioonn PPootteennttiiaall vvss.. ppHH
This graph combines oxidation potential with pH, where each quadrant of the graph
characterizes the particular environment in terms of oxidation or reduction
potential and relative acidity or alkalinity.
201B Ch1 Fundamentals of Corrosion Control Section 3: Environmental and Metallurgical Effects
on Materials
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OOxxiiddaattiioonn PPootteennttiiaall aanndd ppHH ooff EEnnvviirroonnmmeennttaall SSoolluuttiioonnss
The previous graph is shown here reflecting the combined oxidation potential and
pH zones of selected environmental solutions.
201B Ch1 Fundamentals of Corrosion Control Section 3: Environmental and Metallurgical Effects
on Materials
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This enables designers and engineers to relate the observed behavior of a test
solution to a common environment, and to predict its behavior in terms of that
common environment.
MMeettaall''ss CCoorrrroossiioonn TTeennddeennccyy vvss.. EEnnvviirroonnmmeennttaall OOxxiiddaattiioonn PPoowweerr
The relationship between a metals tendency to corrode and the oxidizing power of
the environment is important in designing systems to resist corrosion.
On the left the graph shows that the
tendency to corrode is highest for
the active metals at the bottom of
the scale and decreases as the metal
becomes more noble. On the right the
graph show tendency to oxidize is
highest at the top of the scale and
decreases as the oxidizing power
decreases.
For corrosion to occur, the oxidizing power of the environment has to be greater
than the metals tendency to corrode. So desecrated acid could corrode iron, and
would definitely corrode magnesium, but would not corrode copper or gold.
However, aerated acid would corrode copper, iron, and magnesium. And to corrode
gold, highly concentrated nitric acid or the equivalent would be required.
201B Ch1 Fundamentals of Corrosion Control Section 3: Environmental and Metallurgical Effects
on Materials
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MMeettaall BBeehhaavviioorrss aanndd tthhee PPoouurrbbaaiixx DDiiaaggrraamm
When the three metallic behaviors are mapped to the two important environmental
characteristics pH and oxidizing potential the result is shown here.
Iron is immune to corrosion if there
is substantial reduction potential
for any pH. And its passivity
increases with a higher oxidation
potential in alkaline and some mild
acid solutions. It is active only in
acidic solutions where the oxidation
potential varies from high to quite
low. These maps are of considerable
value to designers and engineers
attempting to reduce the impact of
corrosion on structures and
equipment.
EEnnvviirroonnmmeenntt aanndd PPoouurrbbaaiixx DDiiaaggrraamm OOvveerrllaayy
This overlay permits you to predict a metals behavior if it was introduced to
the environment in each of the boxes in the left graph. So iron would corrode in
a nitric acid solution, could be passive in a bicarbonate hydroxide environment,
and might be immune in sodium hydroxide if there was sufficient reduction
potential.
201B Ch1 Fundamentals of Corrosion Control Section 3: Environmental and Metallurgical Effects
on Materials
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PPoouurrbbaaiixx DDiiaaggrraamm:: IIrroonn vvss.. GGoolldd
This graph compares the behavior maps of iron and gold. Gold is immune over
almost the entire diagram, but if exposed to highly oxidizing environments, it
could corrode.
PPoouurrbbaaiixx DDiiaaggrraamm:: IIrroonn vvss.. AAlluummiinnuumm
This graph compares the behavior maps of iron and aluminum. This reveals that
aluminum is almost never immune, and is very active in strong acid and alkaline
environments, but can be passive in near neutral pH solutions.
201B Ch1 Fundamentals of Corrosion Control Section 3: Environmental and Metallurgical Effects
on Materials
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PPoouurrbbaaiixx DDiiaaggrraamm:: IIrroonn vvss.. TTiittaanniiuumm
This diagram compares the behavior maps of iron and titanium. Like aluminum,
titanium is almost never immune, but has a much larger passive zone, which covers
all but the high oxidation potential, high acid zone and the high reduction
potential high alkaline zone.
IIrroonn CCoorrrroossiioonn CCoonnttrrooll
This graph provides information that facilitates development of design strategies
to prevent or reduce corrosion.
If iron will be used in the
environment found at the plus
sign in the graph, the iron
would likely corrode. If the
environment could be changed to
be quite alkaline, such as at
point A in the diagram, the
iron would be in a passive zone
and could be self-protected
from corrosion. Or, if the iron
is coupled to a more active
metal like magnesium or zinc,
the reduction potential would
201B Ch1 Fundamentals of Corrosion Control Section 3: Environmental and Metallurgical Effects
on Materials
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be increased such as shown at point B and the iron would be immune a process
called cathodic protection. Alternatively, the iron could be forced into the high
oxidation potential zone where the iron displays passive behavior, such as at
point C, by coupling it to a less active metal a process called anodic
protection.
Other alternatives are to add a passivating inhibitor to the environment, which
would move the iron to a point such as D; or to substitute an alloy for the iron,
which would shift the behavior to a different zone such as at point E. These
strategies can be widely applied to other corrosion control methods and
techniques.
201B Ch1 Fundamentals of Corrosion Control Section 3: Environmental and Metallurgical Effects
on Materials
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201B Ch2 Electrochemistry of Corrosion Section 1: Electrochemical Reactions
CCoorrrroossiioonn CCeellll RReevviieeww
All corrosion results from the operation of a corrosion cell so it is essential
that you understand a corrosion cells components, behavior, and operation. This
segment will discuss and distinguish between chemical and electrochemical
reactions; identify corrosion cell components and explain the rules by which they
operate; use a water pumping system as an analogy to describe electrochemical
system types and rates of flow; and explore some useful concepts and tools to
analyze corrosion cells.
EElleeccttrroocchheemmiiccaall RReeaaccttiioonn PPrroocceessss
Electrochemical reactions are distinguished from other chemical reactions by the
generation or consumption of electrons during the reaction process. The process
of generating electrons is called oxidation, and the process of consuming
electrons is called reduction. Electrochemical cells, where oxidation and
reduction take place, are the primary mechanisms found in batteries,
electroplating, fuel cells and corrosion cells. Each of these electrochemical
cells features an anode, where an oxidation reaction occurs and a cathode, where
a reduction reaction occurs. The first three examples are productive
electrochemical cells the fourth is destructive.
CChheemmiiccaall RReeaaccttiioonn TTyyppeess
You can relate chemical reactions to events with which you might be familiar.
Dissolving sugar in your coffee is a chemical reaction. Likewise, salt added to
your soup dissolves the sodium chloride and then disassociates, which is another
chemical reaction that forms separate sodium and chloride ions. And, if you
should mix two solid chemicals, silver nitrate and sodium chloride in sequence
with water.
First, adding silver nitrate, dissolves and
disassociates into silver and nitrate ions; then
adding sodium chloride, which dissolves and
disassociates into sodium and chloride ions; the
result is four ions in solution.
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The silver and chloride ions then react and
precipitate into solid silver chloride. Likewise,
the sodium and nitrate ions react and form sodium
nitrate.
EElleeccttrroocchheemmiiccaall RReeaaccttiioonn TTyyppeess
You might also relate electrochemical reactions to familiar events. If you leave
unpainted iron items outdoors, the iron oxidizes and forms rust in this classic
corrosion reaction. If you place an iron sample in hydrochloric acid, it
dissolves into ferrous ions another oxidation reaction resulting in corrosion.
Aluminum exposed to a non-marine atmosphere reacts with the atmosphere, but forms
a protective oxide on the surface an example of passivity.
However, if you place that aluminum in a marine atmosphere, or in a strong acid
or alkaline environment, the resulting corrosion causes the aluminum to dissolve
into a non-protective film which continues to spall.
CChheemmiiccaall RReeaaccttiioonn MMeecchhaanniissmm
During reaction between chemical elements, valence refers to the number of
electrons in an atom that can be gained or lost. In the chemical reaction
example, where silver nitrate and sodium chloride react in water, silver and
sodium each have a valence of (+1) before and after the reaction, and chlorine
and nitrogen each have a valence of (-1) before and after the reaction. Here, the
chemical reaction features no oxidation or reduction, and no electron generation
or consumption occurs.
201B Ch2 Electrochemistry of Corrosion Section 1: Electrochemical Reactions
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EElleeccttrroocchheemmiiccaall RReeaaccttiioonn MMeecchhaanniissmm
On the other hand, in the electrochemical reaction example of iron and
hydrochloric acid, oxidation and reduction take place at anodes and cathodes that
form on and move about the surface of the iron.
The hydrochloric acid dissociates into hydrogen and chloride ions, and the iron
oxidizes (corrodes) at the anodes and goes into solution as a ferrous ion by
losing two ferrous electrons an electrochemical dissolution oxidation reaction.
Meanwhile, at the cathodes, hydrogen ions in solution migrate to the metal
surface and react with the free electrons to become hydrogen atoms a process
called hydrogen adsorption. Pairs of these absorbed hydrogen atoms join to form
hydrogen gas molecules a hydrogen evolution reaction.
Throughout the electrochemical reaction process, electrons flow through the metal
surface from anodes to cathodes where the electrons are consumed by the hydrogen
ions. The electrochemical cell can be described as consisting of an oxidation
half-cell and a reduction half-cell, which when combined, form the full
electrochemical corrosion cell.
CCoorrrroossiioonn HHaallff--CCeellll BBeehhaavviioorrss
A corrosion cell is composed of two half-cells, an anode and a cathode. The
behavior of each half-cell can be examined independently for effects of
temperature, pressure, chemical composition, and oxidizing potential.
201B Ch2 Electrochemistry of Corrosion Section 1: Electrochemical Reactions
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When the half-cells are coupled together to make an electrochemical cell, the
more positive half-cell will be the cathode and the more negative half-cell will
be the anode. The potential difference between the anode and the cathode drives
the amount of direct current that flows through the metal circuit from cathode to
anode, and by ionic transport through the solution back to the cathode.
The magnitude of that current is controlled by the sum of resistances throughout
the cell. The current density at the anode and cathode can differ and depends on
the size of the exposed surface area of each.
CCoorrrroossiioonn aanndd CCuurrrreenntt
Corrosion occurs at the anode, which is where the material weight loss occurs due
to the oxidation reaction generating free electrons. There is no corrosion and no
weight loss at the cathode, but the reduction reaction must consume the electrons
generated at and flowing from the anode.
The cathodic current emanating from the cathode and flowing through the solution
must equal the current flowing from the anode. Since the electrochemical reaction
rate is inversely proportional to the current magnitude, you can reduce corrosion
by increasing the circuit resistance to reduce current flow.
201B Ch2 Electrochemistry of Corrosion Section 1: Electrochemical Reactions
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EElleeccttrrooddee PPootteennttiiaall aanndd CCoorrrroossiioonn CCeellllss
The more positive corrosion half-cell forms a cathode and the more negative half-
cell forms an anode.
As mentioned above, the magnitude of the potential difference between the anode
and cathode is the driving force behind the oxidation and reduction reactions, so
the more positive the cathode in relation to the anode, the greater the driving
force.
On a scale of relative potential, shown on the left, noble metals such as gold
are the more positive and the least likely to corrode, and active metals such as
magnesium are the least positive and most likely to corrode.
The potential at the cathode determines the oxidizing power of the chemical
environment in other words how readily that environment can cause oxidation and
corrode a metal.
This scale indicates that the higher chemical concentration increases the
oxidation power, and adding air or oxygen to the chemical solution also increases
oxidation.
201B Ch2 Electrochemistry of Corrosion Section 1: Electrochemical Reactions
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EElleeccttrriicc CCuurrrreenntt,, VVoollttaaggee,, aanndd RReessiissttaannccee
An electric circuit can be compared to a water flow circuit to help understand
the relationship between electrical voltage, current and resistance.
The water pump takes in water at low pressure and ejects it at high pressure. The
battery takes in a charge at low voltage and ejects it at high voltage. Both the
water pump and the battery do work on the input to produce energy. The available
water energy per unit of volume is pressure, where energy is expressed in joules
and volume is expressed in cubic meters. The available electrical energy per unit
of charge is expressed in volts, where energy is expressed in joules and charge
is expressed in coulombs. Water flow rate is volume of water per second, and
electric flow rate, called current, is coulombs per second, called amperes. The
severe constriction in the water circuit causes a significant pressure drop. The
resistor in the electric circuit causes a significant voltage drop.
The water flow rate that results from resistance to flow is equal to the change
in pressure over the length of the resistance divided by the resistance.
The electric flow rate (current) that results from the resistor equals the drop
in voltage across the resistor divided by the resistance.
201B Ch2 Electrochemistry of Corrosion Section 1: Electrochemical Reactions
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OOhhmm''ss LLaaww
The important rule illustrated in the comparison of water and electric circuits
is Ohms law, which states that the change in energy (E) in an electrical
circuit, in volts, is equal to the current (I) in the circuit, in amperes,
multiplied by the resistance (R) of the circuit, in ohms. The formula E = I x R
can be used to determine any one variable if the other two are known.
Since current is equivalent to the corrosion rate, the corrosion reaction can be
slowed by lowering the change in E and/or increasing R.
MMaaggnniittuuddee ooff RReessiissttaannccee
Resistance must be calculated for a specific material based on its resistivity
multiplied by its length and divided by its cross-sectional area.
201B Ch2 Electrochemistry of Corrosion Section 1: Electrochemical Reactions
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Resistivity is measured in ohms centimeter. Conductivity of a material is the
reciprocal of resistivity, so a high resistivity material has low conductivity
and vice versa.
VVaalluueess ooff RReessiissttaannccee
This table compares the resistivity of metals, fluids, soils and other materials.
Some interesting comparisons show that silver has significantly less resistivity
than steel; seawater is very conductive, while distilled water has high
resistivity because of few conductive ions; and dry soil has higher resistivity
than moist soil.
201B Ch2 Electrochemistry of Corrosion Section 1: Electrochemical Reactions
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FFaarraaddaayy''ss LLaaww
Oxidation and reduction reactions in the corrosion cell result from electrolysis,
the passage of an electric current through a dissolved ionic substance causing
chemical reactions at the electrodes and separation of materials. Faraday
developed two laws concerning the material loss at the electrodes during
electrolysis.
The first law states that the mass of the material lost at the anode is directly
proportional to the electrical charge at the anode. The second law states that
the mass of the material altered at the anode is directly proportional to the
materials weight. Thus, you can calculate the quantity of electricity required
to corrode a specific amount of metal, or the amount of metal that could be
corroded using a specified current for a given time period.
CCuurrrreenntt DDeennssiittyy
Current density is the amount of current per unit area at any specific location
in a corrosion cell. Total current is the current density over a specific area
multiplied by that area.
The total anodic current in the corrosion cell must equal the total cathodic
current. That means the current density at the anode multiplied by the area of
the anode must equal the current density at the cathode multiplied by the area of
the cathode. So, if the corrosion cell contains a very large cathode and a very
201B Ch2 Electrochemistry of Corrosion Section 1: Electrochemical Reactions
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small anode, the current density on the cathode is much smaller than the current
density on the anode because the total current has to be equal. This is a key
concept in corrosion design.
CCoonnsseerrvvaattiioonn ooff CChhaarrggee
The condition where total anodic current must equal total cathodic current
illustrates the principle of conservation of charge. The total electrons
generated at the anode must equal the total electrons consumed at the cathode.
The anode and cathode potential will self-adjust until a steady-state balance is
achieved. If you need to move away from the steady state to reduce corrosion, an
external current must be applied a technique used in a number of corrosive
environments.
201B Ch2 Electrochemistry of Corrosion Section 1: Electrochemical Reactions
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CCoorrrroossiioonn CCeellll:: GGaallvvaanniicc CCoorrrroossiioonn
The galvanic corrosion cell will be used to provide a more in-depth explanation
of the corrosion cell, by examining a practical application.
In this example of a bronze fitting
connecting two buried sections of steel
pipe, the components of a galvanic
corrosion cell will be identified and the
rules of corrosion cell operation will be
applied.
Note that corrosion damage is occurring
where the current is leaving the steel
pipes surface and flowing onto the
bronze fitting.
CCoorrrroossiioonn CCeellll:: GGaallvvaanniicc CCoorrrroossiioonn CCoommppoonneennttss
When the bronze fitting and steel pipes are connected, they are in electrical
contact, and an electric circuit is completed through the ionic path in the moist
soil. So the four requirements of a corrosion cell are present one metal is the
cathode, the other is the anode, the connection between the two metals provides
the metallic current path, and the moist soil contains an electrolyte solution
that provides an ionic path.
201B Ch2 Electrochemistry of Corrosion Section 2: Galvanic Series
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Remember that the more positive half-cell will be the cathode, and that current
flows from anode into the moist soil solution and onto the cathode surface.
CCoorrrroossiioonn CCeellll:: GGaallvvaanniicc CCoorrrroossiioonn MMeecchhaanniissmm
The bronze fitting becomes the cathode because its potential is one-half volt
more positive than the steel pipe (-0.2 volts compared to -0.7 volts). The more
active steel pipe corrodes, since the oxidation reaction of iron in the steel
alloy forms soluble ferrous ions and loses two electrons. At the same time,
oxygen reduction occurs in the neutral soil, where oxygen consumes free electrons
and combines with water in the soil to form hydroxyl ions. Later, some ferrous
ions will combine with hydroxyl ions to form iron oxide (rust).
Throughout these reactions, the ferrous ions in the soil flow in an ionic current
between the steel anode and the bronze cathode positive ions from anode to
cathode, negative ions from cathode to anode. The free electrons flow through the
metal surface from the steel anode to the bronze cathode.
GGaallvvaanniicc CCoorrrroossiioonn RRaattee
The galvanic corrosion rate is controlled by potential difference and the
resistance throughout the corrosion cell electric circuit. In this case, the
driving force is one-half volt.
201B Ch2 Electrochemistry of Corrosion Section 2: Galvanic Series
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The resistance of the soil is a key factor since soil resistivity varies widely
as shown in the table. The lower the resistivity; the higher the corrosivity.
Soil with a high salt content is very corrosive as is wet sand. Dry sand is only
slightly corrosive. The outside surface area of the steel pipe anode (outside
circumference multiplied by pipe length) can be important, as can the surface
area of the bronze fitting cathode. In this case the cathode area is much smaller
than the anode, so current density at the anode will not affect corrosion.
Polarization at the steel surface, which will be covered later, is neglected
here.
RReessiissttiivviittyy ooff EEnnvviirroonnmmeenntt
The effect on the galvanic cell of soil, water or moisture in the atmosphere is
its resistance to current flow.
The so-called ohmic drop (IR drop),
which is equivalent to the potential
energy drop (E) as shown in Ohms
Law, is caused by the current flow
through the resistive soil. The
higher the soils resistivity, the
greater the ohmic drop for a given
current flow. The effect is to
reduce corrosivity in the galvanic
201B Ch2 Electrochemistry of Corrosion Section 2: Galvanic Series
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cell since corrosivity is directly proportional to the current flow rate; and
as resistance increases, current flow decreases.
On the other hand, seawater is highly
conductive, and its very low resistivity
increases the current flow rate and the
galvanic cell corrosivity. To counter
this effect, off-shore pipeline
corrosion is reduced by placing
sacrificial zinc anodes in the galvanic
circuit, where the more active zinc
corrodes rather than the less active
pipeline material. These sacrificial
anodes can be spaced more than 200
meters apart because of seawaters high
conductivity.
A thin layer of moisture (electrolyte) on a
metal surface in the atmosphere provides
high resistance, even if that electrolyte
includes chlorides. As a result, corrosion
tends to be localized where the anode meets
the cathode. For example, a stainless steel
fastener inserted in steel plate in moist
atmospheric conditions would corrode only in
the contact area because the thin layer of
electrolyte resists current flow away from
the contact point.
CCaatthhooddee AAnnooddee RReellaattiivvee RRaattiioo aanndd CCuurrrreenntt DDeennssiittyy
The relative area of a cathode compared to an anode can have a significant effect
on galvanic corrosion. Remember, total corrosion current at the anode must equal
total corrosion current at the cathode. However, the current densities in amperes
per square centimeter will vary with the difference in electrode areas. If the
cathode is large compared to the anode, the current density at the anode will be
high, and that concentrated current will significantly increase corrosion.
201B Ch2 Electrochemistry of Corrosion Section 2: Galvanic Series
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Returning to the bronze fitting connecting the buried steel pipes, the pipes form
the anode, but the surface area is much larger than the cathode so the current
density is very small.
However, if you decide to coat the steel pipe to provide added protection from
the corrosive environment, and coating defects or later damages occur, you have
created a very small exposed anode area compared to the larger bronze cathode,
and the current densities at these defects are much higher. It would be better to
coat just the bronze fitting to remove the galvanic action.
BBeenneeffiicciiaall GGaallvvaanniicc CCoorrrroossiioonn
Another alternative to protect the
steel anode is to use beneficial
galvanic action. Since zinc is even
more active than steel, you can bury
a zinc bar in the soil and connect it
with a wire cable to the steel pipe.
The 0.3-volt potential drop from the
zinc to the steel causes current to
flow from the zinc through the steel
pipe to the bronze fitting, causing
the zinc anode to corrode, which
cathodically protects the steel.
GGaallvvaanniizziinngg CCaatthhooddiicc PPrrootteeccttiioonn
Galvanizing automotive body panels provides cathodic protection of these panels
from deicing salts, marine environments and corrosive materials embedded in soil
and debris deposits. This zinc-coated metal not only corrodes slowly, but
provides galvanic protection to the underlying steel. The application of organic
and inorganic coatings over the zinc coating provides more protection.
There are also cathodic protection schemes on the market that claim anodes
installed around automobile wheel wells, and connected to the electrical system,
provide impressed current cathodic protection like that used on naval vessels.
However, this scheme lacks one important element of the electrochemical corrosion
cell the electrolyte and its ionic path in each wheel well to complete the
electric circuit. Any thin layers of moisture or other electrolyte would be
201B Ch2 Electrochemistry of Corrosion Section 2: Galvanic Series
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insufficient to carry an ionic current from the anodes to the body panel
cathodes.
SSeeccoonndd LLaaww ooff TThheerrmmooddyynnaammiiccss
Rules of how systems behave are based on the second law of thermodynamics - these
rules explain causes of system reactions, systems behavior, equilibrium and
spontaneous reaction, and the effect of this behavior on corrosion.
FFllooww ooff EEnneerrggyy
Heat always flows from a hot object to a cold object until both objects reach the
same temperature. Higher pressure outside a balloon causes the balloon to shrink
until the pressure inside the balloon equals the outside pressure. A higher
electric energy charge will flow to a lower energy point in a circuit until the
electrical energy is the same throughout the circuit.
DDrriivviinngg FFoorrccee ooff SSyysstteemmss
The driving force behind these reactions is potential difference, and that
driving force continues until there is equilibrium throughout the system. The
flow always goes in one direction from a higher energy state to the lowest energy
state until equilibrium is reached. The systems stay at equilibrium until some
external force induces a potential difference, after which they again seek
equilibrium. Thermodynamic laws that govern their behavior and determine what
constitutes the point of equilibrium; how far the system is from the equilibrium
point; and how physical phenomena such as temperature, pressure, chemical
composition, and electrochemical potential change the point of equilibrium. These
laws provide tools for determining and calculating how to adjust systems for our
benefit or how to slow or stop undesirable reactions.
SSyysstteemm RRaattee BBeehhaavviioorrss
Systems react to these laws at different rates. Linear or constant rates feature
the same amount of change every time period like depositing the same amount of
money in your savings account every month.
201B Ch3Corrosion Rates and Passivity Section 1: Understanding Corrosion Rates
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Increasing or exponential rates reflect an increasing increment in value every
time period like the sum of your savings account deposit and the interest it
accrues each month.
Decreasing or negative exponential rates show a decreasing increment in value
every time period like the remaining balance after withdrawing half of your
savings account every month.
201B Ch3Corrosion Rates and Passivity Section 1: Understanding Corrosion Rates
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Each of these behaviors can be seen in corrosion cell reactions depending on the
combination of material and environmental composition and conditions.
CCoorrrroossiioonn EExxaammppllee:: ZZiinncc iinn HHyyddrroocchhlloorriicc AAcciidd
If a zinc rod is immersed in hydrochloric acid, oxidation occurs at the anode
where two electrons are freed, zinc is consumed, and positive zinc ions are
generated. At the cathode surface, reduction occurs where free electrons are
consumed along with hydrogen ions, and hydrogen gas is generated from the
hydrogen ions and free electrons.
201B Ch3Corrosion Rates and Passivity Section 1: Understanding Corrosion Rates
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The rule is that the number of electrons consumed at the cathode must equal the
number of electrons freed at the anode.
MMiixxeedd PPootteennttiiaall TThheeoorryy
The reaction rate of zinc going to zinc ions is
a function of anode surface potential.
The diagram shows a plot of surface potential
on the Y-axis and the reaction rate (current)
on the X-axis for the anodic reaction. The
oxidation reaction of zinc producing two
positive zinc ions and two free electrons is
linear. If you know the zinc surface potential
on the Y-axis and the reaction rate (or
current) on the X-axis for the anodic reaction,
you can determine the current flow rate from
this graph.
The middle diagram shows a plot of surface
potential on the Y-axis and the reaction rate
(current) on the X axis for the cathodic
reaction. The reduction reaction of hydrogen
consuming two positive zinc ions and two free
electrons is also linear.
Since anodic current must equal cathodic
current according to the principle of
conservation of charge, superimposing the
zinc oxidation curve on the hydrogen
reduction curve shows that the anodic current
is equal to the cathodic current at the point
where two curves intersect. That point
reveals the corrosion current and the
corrosion potential values for a zinc rod in
hydrochloric acid under naturally corroding
conditions.
201B Ch3Corrosion Rates and Passivity Section 1: Understanding Corrosion Rates
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PPoollaarriizzaattiioonn
Polarization in a corrosion cell is the result of an electric current inducing a
potential difference that drives the circuit out of equilibrium. There are three
types: activation controlled polarization; concentration polarization, and ohmic
polarization.
Activation controlled polarization occurs very near an electrode surface due to
reaction resistance, which must be overcome by electrical activation energy.
Concentration polarization can occur at the anode and cathode surfaces, and
results from the exchange of reactants in the electrolyte and the electrode
material to form diffusion layers that provide high circuit resistance. Ohmic
polarization is caused by the much higher resistance in the ionic path than the
electric path in the corrosion circuit, which causes a large potential drop in
the electric path. It is important to be able to quantify the resistance caused
by polarization in order to determine the current available for the driving force
potential.
AAccttiivvaattiioonn CCoonnttrroolllleedd PPoollaarriizzaattiioonn
During the process of hydrogen adsorption, discussed in the first segment,
hydrogen ions disassociate from the electrolyte and combine with free electrons
201B Ch3Corrosion Rates and Passivity Section 1: Understanding Corrosion Rates
-
at the cathode surface to form hydrogen atoms. Pairs of hydrogen atoms then
combine to form hydrogen gas molecules. However, these reduction reactions
require activation energy, which must be sufficient to overcome the energy
barrier imposed by equilibrium conditions.
The graph depicts the barriers that must be
overcome during the reduction reactions. The
energy level of an adsorbed hydrogen atom is
X, and the energy level of a positive
hydrogen ion in solution is Y. The activation
barrier is the total energy required for a
reaction at the peak of the curve less the
energy level of the atom or ion. So, the
activation energy (Ea) required to combine a
hydrogen ion with an electron is the
difference between Ea and Y, and the
activation energy required to remove an
electron from a hydrogen atom is the
difference between Ea and X. As the curve
shows, the reduction reaction requires less
activation energy (H). For hydrogen reactions
where electrons are consumed, the more
negative the cathode potential, the faster
the reaction. The rate of change of potential
versus current is exponential as shown by the
Nernst equation.
However, when current is plotted using a logarithm scale, the relation between
current and potential difference appears linear rather than exponential. The
higher the activation barrier; the slower the reaction rate. And the height of
the activation barrier differs between metals. The curve shows that zinc has a
higher activation barrier than platinum, so platinum, though more expensive, is
used in fuel cells and other electrochemical systems where fast reactions are
essential.
CCoorrrroossiioonn RRaattee:: AAnnooddiicc aanndd CCaatthhooddiicc RRaatteess
Changing the reduction reaction affects the corrosion rate. If you shift the
reduction curve to the right, the intersection point of the oxidation and
reduction curves shifts, and the new current and electrical potential is
indicated on the horizontal and vertical axes.
201B Ch3Corrosion Rates and Passivity Section 1: Understanding Corrosion Rates
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AAccttiivvaattiioonn aanndd OOhhmmiicc PPoollaarriizzaattiioonn
Ohmic polarization and activation polarization can combine to increase the
potential drop across a galvanic corrosion cell.
The copper electrode in this diagram is the cathode because it is more positive,
and the zinc electrode is the anode. The ohmic polarization caused by the
resistances throughout the circuit and the activation polarization caused by the
reaction resistance at the electrodes cause potential drops that are additive.
201B Ch3Corrosion Rates and Passivity Section 1: Understanding Corrosion Rates
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This combined potential difference determines the current flow through the
corrosion cell circuit.
OOhhmmiicc PPoollaarriizzaattiioonn
Ohmic polarization affects the corrosion rate due to the resistance in both the
electronic and ionic paths in the electric circuit.
The graph shows current on the horizontal axis and energy levels on the vertical
axis at three levels of resistance (R3, R2 and R1) for the lower anodic curve
(starting at Ea) and the higher cathodic curve (starting at Ec). R1 represents
the resistance of seawater, R2 the resistance of tap water and R3 the resistance
of distilled water. The two curves intersect where the resistance is lowest, the
conductivity highest, and the current the greatest. At points of greater
resistance, the curves diverge due to the ohmic resistance effects, and the
current is reduced. Since current flow is directly proportional to corrosion
rate, this curve explains why corrosion occurs more readily in seawater.
AAccttiivvaattiioonn PPoollaarriizzaattiioonn
As activation polarization progresses and more hydrogen ions combine with free
electrons, the cathodic surface becomes more negative and current flows more
rapidly. Activation polarization ceases once all the hydrogen ions are able to
201B Ch3Corrosion Rates and Passivity Section 1: Understanding Corrosion Rates
-
react with free electrons, and a limiting current flow rate is reached. The
limiting current determines the diffusion rate of hydrogen ions, and as the
limiting current (iL) increases, the diffusion rate increases and the
concentration of hydrogen ions increases.
While this progression of concentration polarization is useful for certain
electrochemical processes, such as electroplating, it also promotes corrosion. As
the limiting current increases, the increasing diffusion rate also increases
corrosion.
CCoorrrroossiioonn RRaattee:: CCaatthhooddiicc RReeaaccttiioonn RRaattee
The cathodic reaction rate of a galvanic corrosion cell can influence the rate of
corrosion. For example, when a zinc bar is placed in hydrochloric acid, the
faster the positive hydrogen ions combine with free electrons to form hydrogen
atoms, the faster the corrosion rate.
201B Ch3Corrosion Rates and Passivity Section 1: Understanding Corrosion Rates
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The curves here show that as current density increases at any given surface
potential (E), expressed as the electrode potential variance from the standard
hydrogen electrode (V vs. SHE), the corrosion rate increases. Reducing the
cathodic reaction rate reduces the surface potential, the anodic reaction rate,
the current density and the corrosion rate.
201B Ch3Corrosion Rates and Passivity Section 1: Understanding Corrosion Rates
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PPaassssiivviittyy
Passivity is the phenomenon by which the surface of certain reactive metals
oxidize, and form a thin, stable, dense reaction layer that isolates the metal
substrate from the environment, and protects it from corrosion. That corrosion
resistance depends upon the durability and stability of the passive film and its
ability to reform if damaged. If the film can reform rapidly with little or no
metal loss, then corrosion resistance continues. But, if it fails to quickly
repassivate where the damage occurred, the metal substrate in those areas can
suffer from localized corrosion such as pitting, crevice corrosion, or stress
corrosion cracking. The remainder of the substrate remains protected, but the
local corrosion can propagate.
EEnnvviirroonnmmeenntt aanndd 33 MMeettaall BBeehhaavviioorrss
The surrounding environment can affect a metal in one of three ways. If the metal
is immune to the environment, it has no effect. If the metal is active, the
environment can cause corrosion. If the metal is passive, the environment can aid
in forming a protective film. Stainless steel can be used in hot water systems,
boilers and other steam generating systems by chemically treating the water to
facilitate the formation and durability of passive films. Aluminum alloys are
widely used in non-marine atmospheric environments because the aluminum develops
a passive film. Other corrosion resistant alloys (CRES) are composed of stainless
steel, nickel, chrome, iron, and other metals.
PPaassssiivvee MMeettaall CCoorrrroossiioonn RRaattee
The corrosion rates of passive metals can be extremely low, less than one micron
(a millionth of an inch) per year. It would take 16,000 years to penetrate metal
201B Ch3Corrosion Rates and Passivity Section 2: Understanding Passivity
-
the thickness of a quarter, hundreds of thousands of years to penetrate a stack
of ten quarters.
PPaassssiivvee FFiillmm DDuurraabbiilliittyy
Passive film durability is crucial. If the film breaks down and does not
repassivate after chemical, electrochemical or mechanical damage, significant
corrosion damage can result. In the laboratory experiment, a diamond-studded
scribe scrapes the passive film from a rotating electrode. The effect of the
damage is measured by the change in current flow rate. The current rate is high
during the scribing process, but almost immediately returns to a low, passive
current rate when the scribing ceases. This cycle repeats each time the metal is
scribed, showing that the metal alloy electrode remains corrosion resistant since
the passive film reforms after each instance of mechanical damage. This ability
to repassivate depends on the metal's behavior and the corrosivity of the
environment.
PPaassssiivvee FFiillmm BBrreeaakkddoowwnn SSuusscceeppttiibbiilliittyy
The susceptibility of different alloys to the breakdown of a passive film due to
crevice corrosion can be illustrated by plotting laboratory tests using the
electrochemical polarization curves. The oxidizing power (potential) of the
environment is plotted on the vertical axis, and the current density is plotted
on the horizontal scale. The upper curve depicts the behavior of a corrosion
resistant alloy composed of chromium, nickel and molybdenum.
The initial behavior is activation
polarization, and after reaching an
anodic peak, (Emax) the passive
corrosion area forms between 0 and 0.8
volt potential. Beyond that, the
behavior becomes transpassive, and even
if the potential is reversed, the
passive film readily forms. This means
that a damaged passive film will reform
in this potential range.
201B Ch3Corrosion Rates and Passivity Section 2: Understanding Passivity
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The curve for the less corrosion
resistant alloy follows the same
pattern up to the transpassive range,
but when the potential is reversed, a
large hysteresis loop forms. This
indicates that within the whole range
of passivity, there are regions where
the alloy could corrode very rapidly
if the passive film breaks down or is
damaged.
AAccttiivvee aanndd PPaassssiivvee CCoorrrroossiioonn BBeehhaavviioorrss
Active corrosion behavior can be compared to a passive corrosion behavior using
the electrochemical polarization curves featured in the previous segment.
For both active and passive metals, the variance of oxidizing power (potential)
from a reference electrode is plotted from positive to negative on the vertical
axis, and reaction rate (current) is plotted on the horizontal axis. As the
oxidizing reaction increases, the active metal reacts more quickly and corrosion
rate increases exponentially.
On the other hand, after initial active behavior, the passive metal reaches a
critical range of potential, and the corrosion rate decreases dramatically as the
metal enters the passive region. If higher oxidation conditions occur, a
transpassive range is reached where passive corrosion behavior is encountered.
The curve shows orders of magnitude lower corrosion rates in that potential
range.
201B Ch3Corrosion Rates and Passivity Section 2: Understanding Passivity
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OOxxiiddiizziinngg aanndd RReedduucciinngg RReeggiioonnss
In these diagrams, the oxidizing region is the positive region; the reducing
region is the negative region; the acidic region is pH less than 7 and the
alkaline region is pH greater than 7. The passive region for iron is the broad
hatched region in the upper right which is an oxidizing, mildly alkaline
environment. This is the environment you want to maintain if you want iron to be
passive. For aluminum to be passive, you want a neutral environment very little
to no acid or alkali. Though titanium is a very reactive material, there is a
huge passive region. But you must avoid highly acidic, highly reducing
conditions.
PPaassssiivviittyy:: WWaatteerr TTrreeaattmmeenntt
If steel is used in mildly reducing, mildly acidic water, general corrosion could
be a problem. To avoid this problem, adding phosphates or other chemicals to make
the water more alkaline changes the environment to the passive region. But if too
much alkali is added, the steel is subject to stress corrosion cracking a
condition that led to catastrophic boiler explosions in the past.
201B Ch3Corrosion Rates and Passivity Section 2: Understanding Passivity
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PPaassssiivviittyy:: LLooccaalliizzeedd CCoorrrroossiioonn
Localized corrosion poses the greatest danger to passive materials. The pictures
here show samples of pitting corrosion, crevice corrosion and stress corrosion
cracking. Each of these forms of corrosion is localized to the areas where
crevices, cracks or pits can occur. The result is a mostly pristine surface,
which is penetrated in small areas by corrosion that can eventually lead to
catastrophic failure.
PPaassssiivviittyy:: DDeessiiggnn CCaauuttiioonn
Use caution when designing a system that depends on passivity to resist
corrosion. If the passive film fails, localized corrosion can cause pitting,
stress corrosion or crevice corrosion that can result in unanticipated failures.
These can impact safety, health and the environment as well as economic factors
such as downtime and replacement costs. Passive film failure can result from
chlorides or other aggressive chemicals, electrochemical processes such as
oxidation and diffusion, or mechanical damage that causes cracks, crevices or
stagnant conditions.
PPaassssiivviittyy aanndd EEnnvviirroonnmmeennttaall CCoonnddiittiioonnss
It is important to recognize environmental conditions when using passivity for
corrosion resistance.
Acid, alkaline or oxidizing environments can all pose risks with the use of
specific metals. Strong acids or alkalis do not work with aluminum. Nitric acid
can be used with titanium, but be wary of titanium with hydrochloric acid. The
important point is to recognize that analytic tools exist, and while you may not
have access to all the analytic methods or know how to use them, you can find
competent engineers and analysts who can provide the answers needed to mitigate
these risks.
201B Ch3Corrosion Rates and Passivity Section 2: Understanding Passivity