LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
ELECTROCHEMICAL SYSTEMS,
Corrosion Science & Engineering
Supplement to CH111 course
Part II
Corrosion Science & Engineering
KG SREEJALEKSHMI
Department of Chemistry
Indian Institute of Space Science and Technology
Thiruvananthapuram
1
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Topics covered:
LECTURES 5-12
Corrosion Science; definitions - causes and consequences – significance of corrosion control classification of corrosion – theories of corrosion – chemical corrosion - fundamental components
of corrosion cell - electrochemical corrosion – galvanic cell corrosion - factors influencing
corrosion - different forms of corrosion - corrosion control.
DEFINITION
According to Ulick R. Evans, the British scientist who is considered the "Father of Corrosion
Science", "Corrosion is largely an electrochemical phenomenon, which may be defined as
destruction of a material by chemical or electrochemical agencies."
Corrosion is the degradation or deterioration of a material, usually a metal, by chemical or
electrochemical reaction with its environment. Or simply, Corrosion is extractive metallurgy in
reverse.
CAUSE OF CORROSION
Most of the metals (except noble metals Au & Pt) exist in nature in combined forms like chlorides,
silicates, oxides, carbonates etc. When these metals are separated from their ores to get them in
the pure form, they show a natural tendency to revert back to their combined forms. During this
process, they usually combine with atmospheric oxygen to form oxides or depending upon the
nature of the environment sulphides, carbonates etc may also be formed. This process of reverting
back to the combined state usually shown by a metal is termed corrosion of the metal (weeping of
metals). Such a process will result in a change in the properties of the metal which will limit its
applications.
In general terms corrosion is the chemical or electrochemical reaction between a material,
usually a metal, and its environment that produces a deterioration of the material and its
properties. The environment could be of any type such as atmosphere, water, acids, alkalies,
gases, soil etc and the interaction leading to corrosion can take place at any temperature. The term
corrosion may also be applied to weathering of timbers and concrete, leaching of glass, cracking
of plastics etc. In the modern world where miniaturization of devices and equipments are so
important, corrosion in even a very small scale may cause the damage or limit the utility of the
whole device.
Examples of corrosion
Rusting of iron (Fe3O4 formation due to reaction of iron with moisture and air)
2. Formation of basic copper carbonate [CuCO3 + Cu(OH)2] on the surface of Cu as a green
film when exposed to moist air containing CO2.
1.
2
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
In nature, all spontaneous changes occur with a release of free energy from the
system to the surroundings at constant temperature and pressure.
Corrosion is a spontaneous process where free energy is released and in the process metals return
to their stable state.
The driving force for corrosion reaction is the chemical energy - energy stored in chemical bonds
of substances – internal energy. To explain the corrosion phenomenon, we often use transition
state theory which considers that for
A+ B = (AB)* = C + D
the transition sate must be of higher free energy than the sum of the free energies of the separate
species.
For corrosion reaction, the rate constant can be shown to be related to the size of the free energy
barrier according to the equation
kcorr = A exp(G‡/RT)
where A and R are constants, and T is the absolute temperature. From the equation, as T
increases the rate constant (and hence the rate) also increases. But when the size of the barrier
(G‡) is increased, the rate constant decreases.
If we consider the reverse process, it cannot be spontaneous, because there is an increase in free
energy upon conversion of C and D into A and B. Furthermore, there is a bigger free energy
barrier for the new reactants, C and D, to cross. The Transition State Theory says that the reverse
process is possible, but occurs at a much reduced rate, represented by an equation similar to the
above equation in which the activation free energy has been increased from G‡ to ( G + G‡ ).
The reverse process is possible only on the molecular scale where the energies of an individual C
and D pair may be such as to allow the formation of the transition state. (Remember that our rule
applies to an overall free energy change for a bulk system.) The reverse process occurs at a rate
far less than the rate of the forward process, so the net reaction observed on the large scale will
always appear to be a steady conversion of A + B into C + D. For the reverse process to occur in
the bulk system, energy must be supplied to the system
(eg. as in electrolysis).
Points to remember:
Nature is always concerned with minimizing energy. Corrosion is a natural occurring process,
predicted by thermodynamics laws.
high energy states (metal) = low energy states (metals compounds)
3
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
It is this tendency of metals to recombine with elements presents in the environments that leads
to the phenomenon known as Corrosion. The low energy corrosion product is not the same as an
ore (may be similar), the energies of the ore and the corrosion product may well be comparable.
Tendency to corrode and rate of corrosion
Tendency to corrode is determined by the free energy difference G between a metal and its
corrosion product. Rate of corrosion is determined by the size of the energy barrier (the free
energy of activation)
Rate of corrosion reaction = kcorr [reactants]
kcorr =A exp (-G*/RT); A= constant; R= gas constant, T= absolute temperature
G* is the minimum amount of energy required to drive the molecules/atoms over the activation
energy barrier, so that appreciable reaction can take place.
Free energy is the single factor to determine the possibility of a corrosion reaction.
Why do gold, platinum and other precious metals not corrode ?


the energetics may not be favorable
the size of activation energy barrier may be too great (the rate is extremely slow)
Now you understand the driving force for corrosion to occur. It is also very important to have a
fairly good idea about the consequences of corrosion. The following paragraphs will provide you
with most important consequences of corrosion which will help you to understand the significance
of studying corrosion science and corrosion control.
CONSEQUENCES OF CORROSION
Consequences of corrosion can be discussed under various sub-headings among which
economic, health, safety, technological, and cultural consequences are more serious to our society.
ECONOMIC ASPECTS OF CORROSION
It is estimated that hundreds of thousands crores is lost every year due to corrosion!
Although corrosion is a slow process occurring mainly at the surfaces of metals, enormous losses
occur each year in all the countries around the world. These losses are classified into Direct and
Indirect losses.
Direct loss – include price of the corroded metal and the cost of replacement including labor.
4
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Indirect loss – loss of efficiency, shut down loses in the form of lost production, loss of product
due to leakage, wastage of metal due to overdesign and cost involved in the prevention of
corrosion.
Loses due to corrosion varies from country to country, depending upon the climatic conditions. In
India problems due to corrosion are more serious than those in cold countries due to the tropical
climate. The global cost of corrosion is estimated (as per 2022) to be US$2.5 trillion, which is ~
to 3.4% of the global GDP and about 4 per cent in India, as per the CORCON Institute of
Corrosion. According to data from the Financial Times, corrosion in its many forms is estimated
to cost the global economy $3 trillion a year due to damage to steel buildings and infrastructure.
A study from the University of Edinburgh estimated that corrosion and wear costs the UK
approximately £80 billion per annum. According to a rough annual estimate, direct loss due to
corrosion is estimated to be between Rs 250-260 crores and the money spent on its prevention is
about 45-60 crores. Thus it is very important to understand the mechanism of corrosion so that it
can be predicted and minimized whereby the money loss can be reduced.
In India, the direct loss due to corrosion amounts to Rs. 200 crores/annum and the money spent
for controlling corrosion is nearly 50 crores.
Fortunately, most useful metals react with the environment to form more or less protective films
of corrosion reaction products that prevent the metals from going into solution as ions.
HEALTH AND SAFETY ASPECTS
With the advancement of biomedical technology, there is an increase in the use of metal prosthetic
devices in the body, such as pins, rods, plates, hip joints, pacemakers, and other implants. Even
though new alloys and better techniques of implantation have been developed, corrosion continues
to create problems. Examples include failures through broken connections in pacemakers,
inflammation caused by corrosion products in the tissue around implants, and fracture of weightbearing prosthetic devices.
Another significant problem related to safety is corrosion of structures, which can result in severe
injuries or even loss of life. Safety is compromised by corrosion contributing to failures of bridges,
aircraft, automobiles, gas pipelines etc. – the whole complex of metal structures and devices that
make up the modern world.
TECHNOLOGICAL ASPECTS
The development of new technology requires new materials which are required to withstand, in
many cases simultaneously, higher temperatures, higher pressures, and more highly corrosive
environments. Corrosion problems that are less difficult to solve affect solar energy systems, which
require alloys to withstand hot circulating heat transfer fluids for long periods of time, and
geothermal systems, which require materials to withstand highly concentrated solutions of
corrosive salts at high temperatures and pressures. Another example, the drilling for oil in the sea
and on land, involves overcoming such corrosion problems as sulfide stress corrosion,
microbiological corrosion, and the vast array of difficulties involved in working in the highly
5
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
corrosive marine environment. In many of these cases, corrosion is a limiting factor preventing the
development of economically or even technologically workable systems.
CULTURAL ASPECTS
The highly polluted environments that are now prevalent in most of the countries of the world
initiate corrosive processes which will accelerate the deterioration of precious artifacts inside the
world's museums.
To explain the corrosion phenomenon observed in the nature, the different corrosion processes
are classified on the basis of the mechanisms through which they propagate. Often we explain
the real corrosion problems with the help of one or more of these mechanisms.
CLASSIFICATION OF CORROSION
CORROSION
DRY CORROSION
(CHEMICAL)
WET CORROSION
(ELECTROCHEMICAL)
CONCENTRATION CELL
CORROSION
DRY CORROSION
Occurs through direct chemical action of environment/atmospheric gases (O2, halogen, SO2, H2S, N2 or
anhydrous inorganic liquid) with metal surfaces in immediate proximity.
OXIDATION CORROSION
CORROSION BY OTHER GASES
(other than O2)
LIQUID METAL CORROSION
a. Oxidation Corrosion
Occurs due to direct action of O2 at high or low temp., usually in the absence of moisture. Highly
reactive metals (alkali & alkaline earth) are oxidized even at low temp. (Li, Na, K, Rb, Be, Ca, Sr
etc). At high temp. all metals except Ag, Au & Pt are oxidized.
6
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
Reactions
M
Mn+ + n ē
n/2 O2 + n ē
PART II
CORROSION SCIENCE
(formation of metal ion, oxidation)
n O2 -
In the first step, there is adsorption of oxygen on to the metal surface which results in the
formation of an oxide layer on the metal surface. Further diffusion (metal out or O2- in) as in the
figure below is governed mostly by the nature of the oxide layer
To explain the corrosion due to oxidation we need to understand the different types of oxides
commonly encountered in corrosion scenes.
They are
1. Stable oxide layer
2. Unstable oxide layer
3. Volatile oxide layer
4. Porous oxide layer
The stable oxide layer is characterized by fine-grained particles which tightly adhere forming
an impervious layer over the metal surface. Hence it acts as a protective layer. Al2O3 is an
example for a stable oxide layer.
The unstable oxide layer is one whose formation is reversible. For such metals, oxidation
corrosion is not possible. Inert metals like Pt are classified under this category.
The volatile oxide layer is one which is volatilized soon after the formation. This will result
in the exposure of more and more metal surface to the corroding environment. It is a continuous
process posing a very dangerous situation where the whole of the metal gets damaged. MoO3
and WO3 are examples of volatile oxides.
In the porous oxide layer category, the oxide formed will have cracks through which oxygen
can go in. After some time the whole metal may get damaged. Fe and Cr oxides are porous in
nature.
By observing the nature of the oxide formed, it is often possible to predict the corrosion
tendency of a metal. This is done by applying Pilling-Bedworth Rule.
7
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Rule to determine the protective (non-porous) or non-protective (porous) nature of the
oxide film
The PB rule states that
• If the volume of the oxide layer is greater than or at least equal to the volume of the
metal from which it is formed, the oxide layer is protective and hence non-porous
•
If the volume of the oxide layer is less than the volume of the metal from which it is
formed, the oxide layer is porous and hence non-protective
The PB ratio R is given by
R=
Volume of metal oxide formed
=
Md
Dam
Volume of metal consumed
where M and D are the mass and density of the metal oxide respectively and m and d are
the mass and density of the metal respectively. Here ‘a’ is the number of metal atoms
(stoichiometry) that go into the formation of one molecule of the oxide.
Eg; a is 1 for CuO, 2 for Cu2O, 3 for Fe3O4 etc.
Pilling-Bedworth ratio, R, for an oxide indicates its potential as a thin film material
(a) For, R<1, the film tends to be porous if it experiences volume shrinkage during film growth
(b) For 1<R<2, it has a much better coverage
(c) For R>2, large volume expansion accompanies oxide growth, and cracks can develop due
to large compressive stress
The following table gives the PB ratios for some oxides
8
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson
Learning™ is a trademark used herein under license.
(a)
Mg - porous
oxide film
(b) Al - protective,
adherent,
nonporous
oxide film
(c) Fe – layer that spills
off the
surface and
provides
poor
protection
You can find the PB ratios for given metal oxide mathematically and explain the nature
of the film formed.
Problem
The density of aluminum is 2.7 g/cm3 and that of Al2O3 is about 4 g/cm3. Molecular weight
of Al2O3 is 101.96 and that of aluminum is 26.981. Describe the characteristics of the
aluminum-oxide layer formed.
Solution
Required equation for the formation of the oxide is 2Al + 3/2O2  Al2O3
Substitute the values in the equation for R and you will find that the value is 1.28.
This oxide layer is non-porous and hence protective as observed experimentally.
In some case the thermodynamics will predict the formation of stable oxides, but the
protective nature of the oxide film can be predicted by applying PB rule only. For example
consider the case of MgO. The free energy of formation of MgO is predicting a stable oxide
layer. But if you look at the PB ratio, you will see that the MgO oxide is porous and hence
non-protective.
9
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
©2003 Brooks/Cole, a division of Thomson Learning, Inc.
Thomson Learning™ is a trademark used herein under license.
b. Corrosion by other gases (other than O2)
In the presence of dry atmosphere gases like SO2, H2S, Cl2, NOx can induce corrosion of
metallic components which are classified under dry corrosion. The corrosion products of
the reaction of these gases with metal may be protective or non-protective.
Egs: Dry Cl2 reacts with Ag and forms AgCl which is a protective layer, while with Sn
the product is SnCl4 which is volatile.
In petroleum industries at high temperatures, H2S attacks steel forming FeS scale which
is porous and interferes with normal operations.
c. Liquid Metal Corrosion
This is a major corrosion problem encountered due to flowing liquid metals and is also
called liquid metal embrittlement. In several industries, molten metal passes through
metallic pipes and causes corrosion due to dissolution or due to internal penetration. For
example, liquid metal mercury dissolves most metals by forming amalgams, thereby
corroding them. Another example is the corrosion possible in fast breeder reactors which
use liquid sodium as the coolant.
In the classic example of liquid copper metal embrittlement of steel, Cu penetrates along
the austenite grain boundaries when the carbon steel is at a temperature of 1100 °C.
(more information can be found at http://dx.doi.org/10.1590/S1516-14392004000100015)
10
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Theories of corrosion
While learning corrosion science you should keep in mind that the explanation of corrosion
phenomenon requires the combining different theories for which we will now understand
the theories of corrosion. Basically three theories are used for explaining a large proportion
of the corrosion phenomena.
– Acid theory
– Chemical corrosion (dry corrosion) theory
– Electrochemical corrosion (wet/galvanic corrosion) theory
Acid theory of corrosion considers that corrosion of a metal is due to the presence of acids
surrounding it. Eg: in the case of Fe, atmospheric CO2, oxygen and moisture contribute to
corrosion. The reactions occurring are
This theory is supported by the analysis of rust that gives the test for carbonate ion. Further, the
process of rusting is reduced by the presence of lime and caustic soda (these two can absorb CO2,
thus reducing corrosion).
Chemical corrosion (dry corrosion) theory assumes that corrosion on the surface of a metal is
due to direct reaction of atmospheric gases like oxygen, halogens, oxides of sulphur, oxides of
nitrogen, hydrogen sulphide and fumes of chemicals with metal. The extent of corrosion of a
particular metal depends on the chemical affinity of the metal towards reactive gas. Oxygen is
mainly responsible for the corrosion of most metallic substances when compared to other gases
and chemicals. We have already discussed the different categories of dry corrosion.
Electrochemical corrosion (wet/galvanic corrosion) theory considers corrosion in an aqueous
environment and in an atmospheric environment (which also involves thin aqueous layers) as an
electrochemical process because corrosion involves the transfer of electrons between a metal
11
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
surface and an aqueous electrolyte solution. It results from the overwhelming tendency of metals
to react electrochemically with oxygen, water, and other substances in the aqueous environment.
This type of corrosion occurs when the metal comes in contact with a conducting liquid or when
two dissimilar metals are immersed or dipped partly in a solution (remembers galvanic cells) and
hence called galvanic corrosion. According to this theory, there is the formation of a galvanic cell
on the surface of metals. Some parts of the metal surface act as anode and rest act as cathode. The
chemical in the environment and humidity acts as an electrolyte. Oxidation of anodic part takes
place and it results in corrosion at anode, while reduction takes place at cathode. The corrosion
product is formed on the surface of the metal between anode and cathode.
To explain the mechanism, consider corrosion of a metal in contact with aqueous solution
represented by the general half –reactions; anode reaction (oxidation) and cathode reaction
(reduction). In the case of cathode reactions, depending on the pH of the medium two reactions
are observed. In acidic medium, there is evolution of hydrogen and in basic/neutral medium there
is oxygen absorption. We can represent the reactions as
At the anode:
M
Mn+(aq) + nē
At the cathode:
 In acid environment
O2 + 4H+(aq) + 4ē
and / or
2 H+(aq) + 2ē
2 H2O
2 H2
 In alkaline/neutral environment
O2 + 2 H2O(aq) + 4ē
4 OHand / or
2H2O + 2ē
H2 + 2 OHThe metal ions can react immediately with OH- to form insoluble oxides or hydroxides that cover
the surface of the metal, or the metal ions can be released to bulk solution.
To explain the theory let’s take the most common example ie; rusting of iron
At anode: oxidation occurs
At cathode:
12
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Case I: Evolution of H2
The hydrogen ions (H+ ) are formed due to the acidic environment and the following reaction
occurs in the absence of oxygen
2H+ + 2ē →H2 ↑ (reduction)
The overall reaction is Fe + 2H+ → Fe+2 + H2
In this case, metals react in the acidic environment and are dissolved (undergo corrosion) to
release H2 gas. All metals above hydrogen in electrochemical series can undergo this type of
corrosion. In hydrogen evolution type of corrosion, anodic area is large as compared to its cathodic
area.
Case II: Absorption of O2
This type of corrosion takes place in neutral or basic medium in the presence of oxygen. The oxide
of iron covers the surface of the iron. The small scratch on the surface creates small anodic area
and rest of the surface acts as cathodic area. The following chemical reactions occur
O2 + 2 H2O(aq) + 4ē
Fe + O2 + H2O
4 OHFe 2+ + 2OH- or Fe(OH)2
Ferric hydroxide is actually hydrated ferric oxide, Fe2O3.H2O, which is yellow rust. Anhydrous
magnetite, Fe3O4 [a mixture of (FeO + Fe2O3)], is also formed, which is brown-black in colour. It
is to be noted that in electrochemical corrosion, the process of corrosion occurs at anode but the
corrosion product is formed near cathode. It is because of the rapid diffusion of Fe++ as compared
to –OH. Hence corrosion occurs at anode, but rust is deposited at or near cathode.
13
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
. Acidic environment – H2 evolution
. Basic/neutral environment – O2 absorption
From the above theory it should be apparent that there are four fundamental components in an
electrochemical corrosion cell:




An anode.
A cathode.
A conducting environment for ionic movement (electrolyte).
An electrical connection between the anode and cathode for the flow of electron current.
If any of the above components is missing or disabled, the electrochemical corrosion process will
be stopped. Clearly, these elements are thus fundamentally important for corrosion control.
14
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
It is worth to remember that there are obviously different anodic and cathodic reactions for
different alloys exposed to various environments. These half cell reactions are thought to occur (at
least initially) at microscopic anodes and cathodes covering a corroding surface. Macroscopic
anodes and cathodes can develop as corrosion damage progresses with time.
In order to predict the corrosion tendency of a metal/alloy in preference to the other we have the
galvanic series which ranks different metals/alloys on the basis of their reactivity in sea water (a
highly corrosive environment!!!)
GALVANIC SERIES
Platinum
Gold
Graphite
Titanium
Silver
316 Stainless
Steel (passive)
Nickel
(passive)
Copper
Nickel (active)
Tin
Lead
316 Stainless
Steel (active)
Iron/Steel
Aluminum
Alloys
Cadmium
Zinc
Magnesium
15
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Looking at the galvanic series you will be able to predict the corrosion tendency of metals/alloys
in combination with each other. More active members will tend to be anodic and will suffer
corrosion whereas the more cathodic component will get protected.
You will find numerous versions of the galvanic series in text books as well as in internet resources.
Depending upon the materials of your interest you can even prepare your own series (you have to
subject your samples to sea water conditions and measure the potential against a standard
electrode; say, saturated calomel electrode).
Compare the galvanic series and electrochemical series and understand the fundamental
differences/similarities between them.
EMF Series
Galvanic Series
Absolute
Relative
Quantitative
Qualitative
Pure metals only
Metals & alloys
Half-cell potential (vs SHE)
Corrosion potential (vs SCE)
Standard conditions
Any specified conditions
Based on thermodynamic analysis
Based on thermodynamic analysis
Used for theoretical calculations
Used for practical applications
Given below is another version of the galvanic series with more alloy combinations which are
usually used for structural design applications.
16
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Using the galvanic series a corrosion scientist can predict the various types of corrosions a given
sample will undergo. Remember that some metals/alloys which are found to undergo easy
oxidation as indicated by their positions in the electromotive series are sometimes found at the
cathodic end in the galvanic series. The reason for their inertness to corrosion is mostly the
formations of passive layers (mainly oxides) which will further prevent the attack of the corrosive
agents. Egs are Al, Ti etc.
17
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
One important and easily recognizable feature of galvanic corrosion is the presence of corrosion
buildup at the joint between dissimilar metals. For example, aluminum and magnesium skins that
are riveted together in an aircraft wing form a galvanic couple if moisture and contamination are
present. Which metal will act as anode and corrode??
Some more examples…
free-ed.net
Some representative examples for familiarizing yourself with the galvanic series are shown below.
You can also build different combinations and try to identify the corrosion so that gradually you
will become more and more comfortable with using the galvanic series.
Problem 1:
1. A design engineer used titanium- stainless steel (304) combination for constructing a
shaft. Is there any chance of galvanic corrosion? If yes, which metal will corrode?
18
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Solution 1:
Yes. The stainless steel will corrode, as it is below titanium in both its active and passive
states
Problem 2:
Is it advisable to have a combination of cast iron and 416 stainless steel for marine applications?
Solution 2:
For this combination there is a chance of galvanic corrosion. Here, the cast iron will corrode since
it is below stainless steel in both active and passive states. Depending on the structure made there
can be more types of corrosion.
It is to be noted that the rates of galvanic corrosion is affected by the relative areas of the cathodes
and anodes. If the cathode area is more, the specimen will suffer corrosion to a greater extent
whereas a smaller cathode will slow down the corrosion phenomenon. This can be explained on
the basis of the mechanism of galvanic corrosion. As you have seen earlier, the cathode reaction
involves the consumption of electrons. To provide more and more electrons, the anode will
undergo oxidation at a greater extent whereby the process of corrosion proceeds faster.
Problem 3:
Suppose you are given an assignment to design part of a machine. You are given the following
options
a.
Aluminium screws and stainless steel sheets
b.
Stainless steel screws and aluminium sheets
For a better design, which option will you choose and why?
Solution 3:
Since we have to combine two materials (metals/alloys) which are far apart in the galvanic
series, the combination will undergo galvanic corrosion (bimetallic or dissimilar metal
corrosion). From the galvanic series you will note that aluminium and its alloys are anodic (will
corrode) to stainless steel. Hence in the given problem aluminium screws and aluminium sheet
will corrode. Now you have to consider the areas of cathode and anode. We need a design
19
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
which will have the lowest surface area of the cathode. So we will prefer option b ie; stainless
steel screws and aluminium sheets.
Concentration cell corrosion
We can explain a large proportion of the corrosion phenomenon using the concept of concentration
cells.
Concentration cell corrosion occurs when two or more areas of a metal surface are in contact with
different concentrations of the same solution (remember the concentration cell in you
electrochemistry topics). The result is different parts of the same metal will have different electric
potential which will set up a flow of electrons and hence corrosion.
Any situation that creates a difference in the local environment between areas on a single metal
will lead to concentration cell attack. The basic mechanism is essentially the same as in galvanic
corrosion but in the case of concentration cell corrosion the driving force is the difference in
potential between a single metal exposed to different environments rather than the difference
in potential between two different metals exposed to a single environment. The rates of attack
experienced in concentration cell corrosion are affected by relative anode/cathode areas in the
same manner as in galvanic corrosion.
There are three general types of concentration cells which are very important in corrosion science.
1. Oxygen concentration cells
2. Metal ion concentration cells
3. Active-passive cells
Oxygen Concentration Cells
Dissolved oxygen is found to have a significant effect on the corrosion of many metals since
oxygen is also an active participant in the most predominant cathodic reaction in many
environments. An aqueous solution in contact with the metal surface will normally contain
dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not
20
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
allowed to diffuse uniformly into the solution, thereby creating a difference in oxygen
concentration between two points.
We can identify typical oxygen concentration cells under either metallic or nonmetallic deposits
(dirt) on the metal surface and under faying surfaces such as riveted lap joints. Crevices are
created at these regions.
Faying surface in a lap joint
Oxygen cells can also develop under wood, rubber, plastic tape, and other materials (usually used
to fasten the metal blocks) in contact with the metal surface. Corrosion will occur at the area of
low-oxygen concentration (anode). This is the usual corrosion scene you notice when you keep
metal surfaces unclean for quite a long time. The severity of oxygen concentration corrosion can
be minimized by maintaining surfaces clean, sealing the joints properly, and avoiding the use of
material that permits wicking of moisture and other corroding agents between faying surfaces.
Metal Ion Concentration Cells
In the presence of water (high humidity), a high concentration of metal ions will exist under faying
surfaces and a low concentration of metal ions will exist adjacent to the crevice created by the
faying surfaces. This will result in an electrical potential between the two points. The area of the
metal in contact with the low concentration of metal ions will act as cathode and will be protected,
and the area of metal in contact with the high metal ion concentration will act as anode and get
corroded. This condition can be eliminated by sealing the faying surfaces or by applying proper
protective coating in a manner to exclude moisture.
Active-Passive Cells
Metals that depend on a tightly adhering passive film (usually an oxide) for corrosion protection;
e.g., austenitic corrosion-resistant steel, can be corroded by active-passive cells. The corrosion
usually starts as an oxygen concentration cell; e.g., salt deposits on the metal surface in the
21
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
presence of water containing oxygen can create the oxygen cell. If the passive film is broken
beneath the salt deposit, the active metal beneath the film will be exposed to corrosive attack.
An electrical potential will develop between the large area of the cathode (passive film) and the
small area of the anode (active metal). Rapid pitting of the active metal will result. This type of
corrosion can be avoided by frequent cleaning and by application of protective coatings
Factors affecting corrosion
After learning the typical reactions involved in corrosion, we can identify primary factors which
determine the tendency of the metal to corrode and thus influence its initial rate of solution and
secondary factors which influence the rate of the subsequent reactions. Please note that secondary
factors are of no lesser importance. In fact, secondary factors influence the nature and distribution
of the final corrosion products, and hence they usually determine the ultimate rate of corrosion and
useful life of the metal in a given environment. In general, the primary factors are associated with
the nature of the metal (or alloy) itself and the secondary factors are more associated with the
specific environment. It is convenient to divide the factors affecting corrosion based on the nature
of the metal and nature of the environment although no sharp distinction can be made.
Factors Associated Mainly with the Metal
Position of metal in galvanic series: It is a measure of effective electrode potential of a metal in a
solution and hence decides the corrosion rate. A metal having higher position (anodic end) in
galvanic series undergoes corrosion when connected to another metal below it (cathodic and).
Also, more difference in the position of galvanic series will cause faster corrosion at anodic metal.
Hydrogen Overvoltage on the metal: Hydrogen overvoltage may be defined as the difference in
potential that exists between a reversible H2 electrode and an electrode in the same solution at
22
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
which H2 gas is evolved. During the passage of very small currents, the reaction 2H+ + 2ē → H2
will be attaining and maintaining equilibrium. But when stronger currents are passed across metalelectrolyte boundary, or when electrodes other than Pt coated electrodes (at which H2 is evolved)
are used, the above reaction does not take place under equilibrium conditions and an overvoltage
appears. For example, it requires an overvoltage of about 0.2V for liberating H2 gas from a Pt
electrode whereas from a Pb surface 0.7V is required.
Overvoltage () is the potential change caused by the flow of an applied current. Hydrogen
overvoltage governs the process of corrosion. This can be explained as follows. For example, when
a piece of zinc metal is placed in a normal solution of H2SO4, bubbles of hydrogen gas evolves on
zinc surface. The process is slow due to high hydrogen over voltage of zinc (0.76 V).
When we add few drops of CuSO4, reduction of hydrogen over voltage (0.34 V) happens which
accelerates dissolution of more and more Zn2+ ions, ie; corrosion of Zn happens at a faster rate.
Further, faster corrosion is observed in the presence of PtCl4 (hydrogen over voltage = 0.2 V).
The reduction in over voltage of corroding metal or alloy accelerates the rate of corrosion.
Overvoltage () = measured - corrosion
Chemical and physical homogeneity of the metal surface: Purity of metal: Pure metal resists
corrosion, while impurities in a metal form a local galvanic cell (metal as anode and impurity as
cathode) and result in the corrosion of metal. Imperfections (defects) in the lattice structure of
crystals also enhance corrosion. For alloys the system is a homogeneous solid solution, hence no
local action and no corrosion unless there is phase separation occurring during the manufacturing
processes. The defects in solids are mainly line defects, plane defects and volume defects.
Line defects
Plane defects (edge dislocation & screw dislocation)
23
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Volume defects include voids (holes) in the materials, cracks introduced during processing,
inclusion particles of foreign matter embedded in the solid. Volume defects play an important role
in corrosion mechanisms.
Physical state of the metal: Microstructure, grain boundary composition and surface condition
are important parameters determining the corrosion tendency of metals.
Cooling curve for solidification of a metal
Solidification process determines the mechanical property as well as corrosion resistance property.
Rapid cooling results in the formation of more nucleation sites and more crystals whereas slow
cooling yield few crystals.
24
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Here (a) nucleation of crystals in the melt; (b) growth of crystals into dendrites (branched crystal);
(c) complete solidification; and (d) final grain structure.
The process of solidification form dendrite is called dendritic growth. Grains refer to individual
crystals and the zones between any two grains are called grain boundaries. Small granular metal
will have more grain boundaries and will corrode faster than the larger one. We will get more
information on the topic during the seminar on intergranular corrosion.
Also the type of architecture / structure formed by a metal will have effect on the corrosion rate.
A bent metal (stress) is rapidly corroded due to stress. For a metallic sample, the sources of stress
can be intentional, residual, corrosion wedging and due to thermal cycling. More information from
stress corrosion seminar session.
Relative areas of anode and cathode: Smaller the area of anode compared to cathode will lead to
faster corrosion of anode. It is because the corrosion current at anode and at cathode will be same.
But for small anodic area the current density will be large at anode and larger cathodic area will
demand more electrons which will be provided by fast reaction at anode (oxidation), i.e. rapid
corrosion.
So when given a galvanic couple you should remember that a smaller cathode will offer more
protection to your sample.
25
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Problem:
Given two samples, steel rivet on copper bar and copper rivet on steel bar – which one will
corrode faster?
Solution: Here there is a galvanic couple and copper is cathodic to steel. Hence the steel rivets
on the copper plate will corrode very quickly: small anodic area (rivets) and large cathodic area
(copper plate). On the other hand, the copper rivets on the steel plate will corrode slowly (large
anodic area - steel plate- , small cathodic surface - copper rivet)
Nature of oxide film: We have already discussed this point in detail. If the oxide film formed is
porous and oxygen can diffuse through, more corrosion is expected. Also, remember PB ratio and
related discussions. The inherent ability of many metals to form insoluble protective film often
gives them extraordinary corrosion resistance. This phenomenon is called passivation. For
example when we dip Fe in dil. HNO3, Fe metal gets corroded. But if we dip Fe in conc. HNO3
there is formation of a passive layer and the Fe resists further corrosion.
A passive metal is one which is active in the emf series, but that which corrodes at a very low rate
(cathodic end in the galvanic series). Passivity is the property underlying the useful nature of
corrosion resistance of many structural metals including Al, Ni, and stainless steels.
Protective scales of Alumina (Al2O3), chromia (Cr2O3) and silica (SiO2) are well known passive
layers.
Problem:
High carbon steel containers are used to store/ship nitrating mixtures (con. H2SO4 + con. HNO3).
Why?
Solution:
High carbon steel contains cementite (Fe3C) which acts as cathode are readily passivated than
pure iron. This will reduce the corrosion tendency of the containers to a great extent and risk of
storage of concentrated acid mixtures is minimized.
Volatility and solubility of corrosion product: We have already discussed this point. In both the
cases, the corrosion will be faster. Examples are provided by MnO3, SnCl4 which are volatile.
Sample problem: Explain the faster rate of corrosion of Sn in chlorine atmosphere.
Solution: SnCl4 is volatile. So more and more Sn is exposed to the environment and more and
more corrosion occurs, until all the Sn is eaten away.
26
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
In case of formation of soluble corrosion product, corrosion will be enhanced by water and metal
surface will be exposed for further corrosion.
Factors Associated Mainly with the Environment
The rate at which metal samples corrode depends greatly on the environment they are exposed to
and the amount of preventive maintenance they receive. Metals that are exposed to marine
atmospheres, tropical temperatures, and industrial chemical atmospheres have the greatest rate of
corrosion. We will discuss the important environmental factors which influence corrosion process.
Temperature: With the increase of temperature, the rate of diffusion increases. Hence the rate of
corrosion also increases. At higher temperature, passive metals also become active and undergo
corrosion. But higher temperature reduces the concentration of dissolved oxygen and hence
corrosion is reduced (in case of water where oxygen is dissolved).
Humidity: Under humid conditions, gases like CO2, SO2, NOx dissolve in the moisture to form
electrolytes. Such electrolytes will cause galvanic corrosion. In case if the oxides are water soluble,
humidity washes away the corrosion products and metal surface is further corroded. Other soluble
corrosion products can also be washed away by humidity, causing further corrosion.
Oxygen concentration and oxygen concentration cell: Oxygen is one of the most important
elements responsible for corrosion and forms oxides and hydroxides (in presence of H2O) as
corrosion product on the surface of metal. Oxygen concentration cell is formed on the surface of
metal due to difference in oxygen concentration (iron rod half dipped in water corrodes due to this
effect). Dipped portion will be anode and outer portion will be cathode. The mechanism of
waterline corrosion can be explained using oxygen concentration cell.
pH value: pH value gives an idea of the acidic/basic nature of the environment. In acidic medium
(pH less than 7), corrosion is faster. In basic medium (pH > 7), some metals such as Pb, Zn, Al,
etc. form complexes and hence they corrode.
The thermodynamic information regarding corrosion behavior of any metal is stored in Pourbaix
diagram which are potential-pH diagrams. They are constructed from standard electrode potential
27
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
values and equilibrium constants and gives information regarding the most stable species at a given
potential and pH.
In the diagram there are boundaries between various zones - immunity, corrosion and passivity
zones which are influenced by ionic activity in solution. Each domain indicates a region in which
one species is the most thermodynamically stable. If the metal is the most stable species then it is
considered immune to corrosion, but if a soluble ion is most stable then the metal should corrode.
A region in which an insoluble corrosion product is the most stable species is considered passive.
Normally ionic activity values of 10-6 are employed for boundary definition; above this value
corrosion is assumed to occur.
Susceptible to
Pitting corrosion
Corrosion
Passive
E
Immune
pH
Eg:- Pourbaix diagram for iron in sea water, in terms of zones showing the type of reaction that
occurs
Pourbaix diagrams can be used to distinguish a corroding condition from a non-corroding
condition.
CORRODING CONDITION: [Mn+] > 10-6 M
IMMUNITY CONDITION: [Mn+] < 10-6 M
28
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Potential-pH diagram for Fe-H2O at 25oC; 10-6M dissolved Fe
Construction of E-pH diagram
To construct Pourbaix diagram, the following steps are required
a. Identify all possible chemical and electrochemical reactions
b. Apply Nernst equation to each possible reaction (make use of K if necessary)
c. Determine the relationship between the potential (E) and the pH of the system
d. Draw the potential as a function of pH in a chart
e. [Mn+] = 10-6 M is assumed as borderline for corrosion and non-corrosion region
f. The chart is divided into different region representing different corroding conditions
For those who are interested to know more here is a detailed description on Pourbaix diagram of
Fe.
29
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
Low E (or pE) values represent a reducing environment. High E values represent an oxidizing
environment. The pE scale is intended to represent the concentration of the standard reducing agent
(the e-) analogously to the pH scale representing the concentration of standard acid (H+). PE values
are obtained from reduction potentials by dividing Eo by 0.059.
Key to features on the diagram:
•
Solid lines separate species related by acid-base equilibria (line a): line a shows the pH
at which half of the 1 M iron is Fe3+ and half is precipitated as Fe(OH)2 ; the position of an acidbase equilibrium is dependent on the total concentration of iron ; reducing the total concentration
of Fe3+ will reduce the driving force of the precipitation ; reducing the total iron concentration
from 1 M to 10-6 M (more realistic concentrations for geochemists and corrosion engineers) shifts
the boundary from pH 1.7 to pH 4.2. In general, in more dilute solutions, the soluble species have
larger predominance areas.
30
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
•
PART II
CORROSION SCIENCE
Solid double lines separate species related by redox equilibria (lines b & c): Redox
equilibria of species not involving hydrogen or hydroxide ions appear as horizontal boundaries
(line b) ; redox species of species involving hydrogen or hydroxide appear as diagonal boundaries
because they are in part acid-base equilibria (line c) ; diagonal boundaries slope from upper left to
lower right because basic solutions tend to favor the more oxidized species ; Longer dashed lines
enclose the theoretical region of stability of the water to oxidation or reduction ((lines d & f) while
shorter dashed lines enclose the practical region of stability of the water (e & g) ; Dashed line d
represents the potential of water saturated with dissolved O2 at 1 atm (very well aerated water),
above this potential water is oxidized to oxygen:
2 H2O + 4 H+ (aq) O2 + 4 e-
Eo = +1.229 V
Theoretically water should be oxidized by any dissolved oxidizing agent Eo > 1.229 ; In practice,
about 0.5 V of additional potential is required to overcome the overvoltage of oxygen formation
(dashed line e)
Dashed line f represents the potential of water saturated with dissolved H2 at 1 atm pressure (high
level or reducing agents in solution). Below this potential water is reduced to hydrogen:
2 H+ + 2 e-
Eo = +1.229 V
In practice, an overvoltage effect prevents significant release of hydrogen until the lower dashed
line g is reached.
Application and Limitation
Applications of Pourbaix diagram include:
1. Formulation of corrosion control methods (use of cathodic protection, anodic protection,
corrosion inhibitors etc.)
2.
Identification of possible corroding states of the metal-H2O system (regions of immunity,
passivation, corrosion or cracking)
3. Prediction of most likely corrosion products (Fe2+ or Fe3+ ?) for the metal-H2O system
Practical limitations in the use of E-pH diagram are:
31
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
1. E-pH diagram is based on thermodynamic calculations at assumed standard condition
2. It is for pure metal in pure water system
3.
Lack kinetic information
4. Effect of other ionic species is ignored (eg: Cl- which are crucial for corrosion
phenomenon)
5. Effect of flow (velocity) on the stability of passive films is not taken into account
www.wou.edu/las/physci/ch412/pourbaix.htm
Factors affecting corrosion - nature of the environment continued…..
Impurity of atmosphere: Pollutants like H2S, SO2, CO2 and acid vapors cause more pollution
where they dissolve and act as electrolytes/corroding agents. In sea water (which acts as an
electrolyte) corrosion rate increases. Some suspended particles are dissolved in humidity and form
electrolyte which helps in corrosion.
Nature of ions present: Cu++ ions present in the vicinity of Fe, accelerate corrosion, while silicates
present in the vicinity resist corrosion.
Conductance effect: Due to presence of salts and water in earth, it is of con- ducting nature. More
conductance leads to more stray current and hence fast corrosion. Dry sandy soil is less conducting
and hence less corrosion, while mineralised clay soil is more conducting hence more corrosion
occurs.
Polarisation of electrodes: More the polarisation at electrodes, less current will be passed and
hence less corrosion.
You need to understand what is polarization. When a metal is not in equilibrium with a solution
of its ions, the electrode potential differs from the equilibrium potential by an amount which is
referred to as the polarisation. Overvoltage and overpotential are other terms with equivalent
meaning. The symbol commonly used for polarisation is  .
Polarisation is an extremely important corrosion parameter which can be used to make predictions
about the rates of corrosion processes.
32
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
In practical situations involving corrosion scenes, polarisation is sometimes defined as the
potential change away from some other arbitrary potential (often the mixed potential or the free
corrosion potential). Polarisation can also be expressed by E.
E = E - Ecorr
Flow of process stream: Low flow velocity will reduce corrosion of non-passive metals, while for
passive metals it may or may not be true.
General Types of Corrosion
UNIFORM CORROSION (~30% of reported failures)
Uniform corrosion occurs over the greater area of the surface of a metal at a steady and often
predictable rate. It makes the metal surface look ugly but its predictability facilitates easy control.
Uniform corrosion can be slowed or stopped if we know the five basic facts;
(1) Slow down or stop the movement of electrons
(a) Apply a coating over the surface with a non-conducting medium such as paint, lacquer
or oil
(b) Reduce the conductivity of the solution in contact with the metal. Keep the metal
surfaces dry!!! Wash away conductive pollutants regularly.
(c) Apply a current to the material (cathodic protection).
(2) Slow down or stop oxygen from reaching the surface. (coatings may help)
(3) Prevent the metal from giving up electrons.
(a)Use a more corrosion resistant metal higher in the electrochemical series.
(b)Use a sacrificial coating which gives up its electrons more easily than the metal being
protected.
(c) Apply cathodic protection.
(d) Use inhibitors.
(4) Select a metal that forms an oxide that is protective (passive) and stops the reaction.
(5) Control and consideration of environmental and temperature factors is also very important.
33
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
LOCALISED CORROSION (~70% of reported failures)
Here the corrosion process is confined over a very small region as compared to the total area of
the material/metal. The consequences of localised corrosion can be a great deal more severe than
uniform corrosion generally because the failure occurs without warning and after a surprisingly
short period of use or exposure. We need to give considerable attention to factors affecting
corrosion for this particular class of corrosion.
Listed below are some of the major classes of localized corrosions. You will listen to the seminars
by your colleagues for detailed understanding of the mechanism of each type and measures to
control them.
GALVANIC CORROSION
We have discussed this point earlier. You should also understand that for galvanic corrosion to
occur, three special features of this mechanism need to operate
(a) The metals need to be in contact electrically
(b) One metal needs to be considerably better at giving up electrons (undergo oxidation) than the
other
(c) An additional path for ion and electron movement is necessary
Prevention of galvanic corrosion is based on ensuring that one or more of the three features do not
exist. For that break the electrical contact using plastic insulators or coatings between the metals
or/and select metals close together in the galvanic series or/and prevent ion movement by coating
the junction with an impermeable material, or ensure dry environment such that liquids cannot be
trapped.
Temperature changes alter the corrosion rate of a material and a good rule of thumb is that 10oC
rise doubles the corrosion rate. If one part of component is hotter than another the difference in the
corrosion rate is accentuated by the thermal gradient and local attack occurs in a zone located
between the maximum and minimum temperatures. This is referred to as thermogalvanic
corrosion. The best method of prevention is to design out the thermal gradient or supply a coolant
to even out the difference.
34
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
PITTING CORROSION
Pitting corrosion occurs in materials that have a protective film such as a corrosion product or
when a protective coating breaks down. The exposed metal gives up electrons easily and the
reaction initiates tiny pits with localised chemistry supporting rapid attack. Control can be ensured
by:
 Selecting a corrosion resistant material (which may not be always possible)
 Ensuring a high enough flow velocity of fluids in contact with the material or frequent
washing
 Protecting/maintaining the material’s own protective film
 Use of corrosion inhibitors
 Use of a protective coating
Note: Pits can be crack initiators in stressed components or those with residual stresses resulting
from forming operations. This often leads to stress corrosion cracking.
INTERGRANULAR CORROSION
This is preferential attack of the grain boundaries of the crystals that form the metal due to the
physical and chemical differences between the interior and edges (boundaries) of the grain.
It can be avoided by:
 Selection of stabilized materials/ metals with less defects
 Control of heat treatments and processing to avoid huge temperature range at different
zones in the material
CREVICE CORROSION (CONCENTRATION CELL CORROSION)
Crevice is a narrow opening or a gap provided by a crack. If two areas of a component (especially
metallic component where bolts, screws or other joining occur) in close proximity differ in the
amount of reactive species (eg: oxygen, halides etc) available, the reaction in one of the areas is
speeded up. An example of this is crevice corrosion which occurs when oxygen cannot penetrate
a crevice and a differential aeration cell is set up. Corrosion occurs rapidly in the area with less
oxygen. The potential for crevice corrosion can be reduced by:




Selection of resistant materials
Use of sealants
Proper designs avoiding sharp corners
Use welds instead of bolts or rivets
35
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
STRESS CORROSION CRACKING
This form of corrosion is due to the combined action of a static tensile stress and corrosion which
forms cracks and eventually catastrophic failure of the component. It is noticed to be specific to a
metal material paired with a specific environment. Prevention can be achieved by:
 Design to minimise thermal and residual stresses
 Reducing the overall stress level and designing out stress concentrations
 Selection of a suitable material not susceptible to the environment
 Developing compressive stresses in the surface the material
 Use of a suitable protective coating
SELECTIVE ATTACK
This occurs in alloys such as brass when one component or phase is more susceptible to attacke
than another and corrodes preferentially leaving a porous material that crumbles. It is best avoided
by selection of a resistant material or by coating the material, reducing the aggressiveness of the
environment and by the use of cathodic protection.
STRAY CURRENT CORROSION
When a direct current flows through an unintended path the flow of electrons supports corrosion.
This can occur in soils and flowing or stationary fluids. The most effective remedies involve
controlling the current by
 Proper insulation of the structure to be protected or the removal of source of current
 Earthing sources and/or the structure to be protected.
 Applying cathodic protection or using sacrificial methods
MICROBIAL CORROSION
Here degradation of materials (metals, polymers, ceramics, concrete etc.) by bacteria, moulds and
fungi or their by-products.
It can occur by a range of actions such as:
 Attack of the metal or protective coating by acid by-products, sulphur, H2S or NH3
36
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
 Direct interaction between the microbes and metal which sustains attack.
Aerobic bacteria accelerate corrosion by oxidizing sulfur to produce sulfuric acid. The
metabolism of aerobic bacteria requires them to obtain part of their sustenance by oxidizing
inorganic compounds such as iron, sulfur, hydrogen, and carbon monoxide. The resultant
chemical reactions cause corrosion.
Prevention can be achieved by frequent cleaning of surfaces, controlling chemistry of
surrounding media and removal of nutrients, use of biocides, or by cathodic protection.
CORROSION FATIGUE
The combined action of cyclic stresses and a corrosive environment reduce the life of components
below that expected by the action of fatigue alone. This can be reduced or prevented by good
design that reduces stress concentration, removing or isolating sources of cyclic stress and
avoiding sudden changes of section.
CORROSION CAUSED BY COMBINED ACTION
This is corrosion accelerated by the action of fluid flow sometimes with the added pressure of
abrasive particles in the stream. The protective layers and corrosion products of the metal are
continually removed exposing fresh metal to corrosion. This can be prevented by reducing the
flow rate and turbulence, use of replaceable linings in susceptible areas, avoiding sudden changes
of direction and streamlining the flow or even avoiding obstructions to the flow.
Now we have discussed important points regarding corrosion, how it occurs and what can be
done for controlling corrosion. However, you will listen to class seminars for more details
regarding different forms of corrosion and corrosion control.
Given below are some case studies which may be of interesting to you. Solve the problems by
yourself.
Case studies:
1. Statue of liberty: Built in 1886, weakening of structure by 1980
Coupling of copper skin and steel supports ----predict what is happening
2. Sports car: Brass screws and steel body
37
KG SREEJALEKSHMI
IIST
LECTURE NOTES
Electrochemical systems, Corrosion science & Engineering
PART II
CORROSION SCIENCE
3. Aircraft: Graphite grease and Mg alloy (aircrafts are one of the most corrosion-prone
structures)
4. Helicopter: Mg component and steel bolts
References:
1. Revie, R. W. and Uhlig, H. H., Corrosion and Corrosion Control – An Introduction to
Corrosion Science and Engineering, 4th ed., Wiley (2008).
2. Mudali, U.K. and Raj, B. (Eds) Corrosion Science and Technology, Narosa Publishing
House (2008).
3. Jones, D.A. Principles and Prevention of Corrosion, Prentice-Hall (1996)
4. Fontana, M.G. Corrosion Engineering, 3rd ed., McGraw-Hill (1986)
5. www. corrosion.ksc.nasa.gov/
6. www. corrosion-doctors.org/
7. several academic websites…
GOOD LUCK TO ALL
38
KG SREEJALEKSHMI
IIST