ME 472
(Corrosion Engineering)
Chapter#1 ~2
Instructor: Dr. Ihsan-Ul-Haq Toor
 Office: 63-358/Phone:7493

Lecture Contents
 What is corrosion?
 Importance of the subject?
 Why Corrosion?
 Environmental factors for corrosion?
 How corrosion takes place
 Electrochemical nature of corrosion
 Forms of Corrosion
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What is Corrosion?
The destruction of a metallic material by chemical,
electrochemical, or metallurgical interaction with the
environment ”
Fig-1. Corrosion of a deserted boat (left) and a tank (right).
[1] P.R. Roberge, Corrosion Engineering Principles and Practice, first ed., McGraw-Hill, USA, 2008.
[2] Z. Ahmad, Principles of Corrosion Engineering and Corrosion Control, first ed., Elsevier, UK, 2006.
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Importance of this subject?
Almost every engineer/scientist at one time or another will be
exposed to MATERIALS /especially metals (design, synthesis,
application etc)
Examples: Petrochemical industry (oil and gas), Structures, Desalination
industry, , power industry, Nuclear industry, and so on;
Metals are the most abundant and each has different properties
(80 known elements are metals)
These different elements have been combined to develop roughly
> 40000 different alloys and still going on
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Importance of this subject?
Bettis-Besse nuclear power plant
6” of corroded arbon steel were lost
Only 1/6” stainless steel was left to
support the 2500psi inside the reactor
core
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Importance of this subject?
Human life and safety
ALOHA incident 1998/ Pilot managed to land the plane on the island of
Maui, Hawaii (one flight attendant was died)
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Importance of this subject?
Economic Impact
3~5% of GNP of Development country
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Economic Impact of Corrosion
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Economic Impact of Corrosion
 Direct loss :
Approximately 3 - 5% of GNP in developed countries. About 15 - 25% of
this expense could be avoided if currently available corrosion technology
were effectively applied.
 Cost of corrosion in U. S. about $ 276 billion in 2002 that is equivalent
to 3.1 % of GNP.
 Cost of corrosion to oil and gas producers in U. S.  $ 2 billion that are
increasing because of deeper wells and more hostile env.(500 F, H2S).
 Indirect loss
 Plant shutdown.
 Loss of product.
 Loss of efficiency
 Contamination.
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Why Corrosion?
Corrosion is natural process and
it returns the metal to its oxidized
state or combined state in the
form of chemical compound
that are similar to the mineral
from which they are extracted
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Materials & Environment
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Defining the environment is very important
Some environments are more corrosive than others,
but there can be exceptions
Moist air is more corrosive than dry air
Hot water is more corrosive than cold water
Polluted air is more corrosive than clean air
Acids are more corrosive than bases (alkalies) to steels
Salt water is more corrosive than fresh water
No corrosion will occur in a vacuum, even at very high temperatures
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How Corrosion Takes Place?
Corrosion process requires a complete
corrosion cell/circuit, which includes;
1.
2.
3.
4.
Anode
Cathode
Electrolyte
Electrical path
Galvanic Cell
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How Corrosion Takes Place?
Anodic reactions/Oxidation reaction takes place at anode
(generation of electrons)
 The electrode at which chemical oxidation occurs (or + electricity leaves the
electrode and enters the electrolyte) is called the anode
General Reaction (metal oxidation)
MMn+ + ne- (gives off electrons))
Zn  Zn2+ + 2eFe  Fe2+ + 2eAl  Al3+ + 3eFe2+  Fe3+ + e-
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(Zn corrosion)
(Fe corrosion)
(Al corrosion)
(Ferrous ion oxidation)
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How Corrosion Takes Place?
Cathodic reactions/reduction reaction (consumption of electrons)
 Electrons released at the anode travel to the cathode by a metallic
path where they react with the ions in the electrolyte and cause reduction of the
positive ions.
 The electrode at which chemical reduction occurs (or + current enters
the electrode from the electrolyte) is called the cathode .
O2 + 2H2O + 4e- → 4OH- (oxygen reduction in water/bases)
O2 + 4H+ + 4e- → 2H2O (oxygen reduction in acids)
2H2O + 2e→ H2 + 2OH- (hydrogen evolution in water/bases)
2H+ + 2e- → H2 (hydrogen evolution in acids)
Cu2+ + 2e- → Cu (metal deposition=>copper plating)
Fe3+ + e- → Fe2+ (metal/ferric ion reduction)
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How Corrosion Takes Place?
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Behavior of Water
pH = - log [H+]
Neutral= - log [1* 10-7]= 7
Acidic?
Alkaline?
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Electrochemical nature of Corrosion
Zn is placed in aerated (with O2) dilute HCl solution (acidic solution):
CR↑ or ↓?
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Corrosion of Metals
 Behavior of Active and Passive metals
Passive
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Active
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Corrosion of Metals
1: Change in Gibbs Free Energy
 The change in Gibbs free energy, ΔG , for any chemical reaction indicates the
tendency of that reaction to go.
 Reactions occur in the direction that lowers the Gibbs free energy. The more
negative the value of free energy , the greater the tendency for the reaction to
go.
2: Pilling–Bedworth Ratio
PB Ratio= Md / nmD
 M and D are the molecular weight and density, respectively, of the corrosion
product scale that forms on the metal surface during oxidation;
 m and d are the atomic weight and density, respectively, of the metal
 n is the number of metal atoms in a molecular formula of scale; for example, for
Al 2 O 3 , n = 2.
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Corrosion of Metals
2: Pilling–Bedworth Ratio
If Md / nmD < 1, film would be expected to contain cracks and pores and be
relatively nonprotective.
If Md / nmD > 1, film/scale will be in compression, protective of the underlying
metal.
If Md / nmD >> 1, the scale that forms may buckle and detach from the
surface because of the higher stresses that develop.
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Corrosion Types
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Corrosion Types
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Corrosion Types-Uniform/General
 Rates of uniform attack are reported in various units;
millimeters penetration per year (mm/y); grams per
square meter per day (gmd), inches penetration per
year (ipy), mils (1 mil = 0.001 inch) per year (mpy),
and milligrams per square decimeter per day (mdd).
 Steel, for example, corrodes at a relatively uniform rate
in seawater of about 0.13 mm/y, 2.5 gmd, 25 mdd, or
0.005 ipy. These represent time -averaged values.
 Generally, for uniform attack, the initial corrosion rate
is greater than subsequent rates .
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Corrosion Types-Uniform/General
Metals are classified into three groups according to their corrosion rates and
intended application.
A. < 0.15 mm/y ( < 0.005 ipy) — Metals in this category have good corrosion
resistance to the extent that they are suitable for critical parts, for example,
valve seats, pump shafts and impellors, springs.
B. 0.15 to 1.5 mm/y (0.005 to 0.05 ipy) — Metals in this group are satisfactory
if a higher rate of corrosion can be tolerated, for example, for tanks, piping,
valve bodies, and bolt heads.
C. > 1.5 mm/y ( > 0.05 ipy) — Usually not satisfactory.
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Corrosion Types-Pitting
 This is a localized type of attack, with the rate of corrosion
being greater at some areas than at others.
 If appreciable attack is confined to a relatively small, fixed
area of metal, acting as anode, the resultant pits are
described as deep.
 If the area of attack is relatively larger and not so deep,
the pits are called shallow.
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Corrosion Types-Pitting
 “Ratio of deepest metal penetration to average metal penetration”
as determined by the weight loss of the specimen, is called pitting
factor .
 A pitting factor of unity represents uniform attack.
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Corrosion Types- Fretting
 FC is the result of “slight relative motion (as
in vibration) of two substances in contact,
one or both being metals.
 It usually leads to a series of pits at the metal
interface.
 Metal - oxide debris usually fills the pits so
that only after the corrosion products are
removed do the pits become visible.
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Corrosion Types- Cavitation
 Cavitation – erosion is the loss of material
caused by exposure to cavitation, which is the
“formation and collapse of vapor bubbles” at a
dynamic metal – liquid interface.
 Example, in rotors of pumps or on trailing faces
of propellers.
 This type of corrosion causes a sequence of pits,
sometimes appearing as a honeycomb of small
relatively deep fissures.
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Corrosion Types- Dealloying
 Dealloying is the “selective removal of
an element from an alloy by corrosion”.
 One form of dealloying, dezincification,
is a type of attack occurring with zinc
alloys (e.g., yellow brass) in which zinc
corrodes preferentially, leaving a porous
residue of copper and corrosion
products.
 The tensile strength and ductility are
seriously reduced.
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Corrosion Types- Parting
 Parting is similar to dezincification in that one or more reactive components of the
alloy corrode preferentially, leaving a porous residue that may retain the original
shape of the alloy.
 Parting is usually restricted to such noble metal alloys as gold – copper or gold –
silver and is used in gold refining.
 For example, an alloy of Au – Ag containing more than 65% gold resists
concentrated nitric acid as well as does gold itself. However, on addition of silver to
form an alloy of approximately 25% Au – 75% Ag, reaction with concentrated HNO3
forms silver nitrate and a porous residue or powder of pure gold
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Corrosion Types- Intergranular
 This is a localized type of attack at the grain
boundaries of a metal, resulting in loss of
strength and ductility.
 Improperly heat - treated 18 - 8 stainless
steels or Duralumin - type alloys (4% Cu – Al) are
among the alloys subject to intergranular corrosion.
 At elevated temperatures, IGC can occur
because, under some conditions, phases of low
melting point form and penetrate along grain
boundaries; for example, when nickel - base
alloys are exposed to sulfur - bearing gaseous
environments, nickel sulfide can form and cause
catastrophic failures and is called sulfidation .
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Corrosion Types- Cracking
 “ A brittle failure of a metal/alloy caused by the simultaneous action of a tensile
stress and a specific corrosion environment” [Stress corrosion cracking]
 Mechanical damages of a metal caused by the presence of, or interaction with,
hydrogen are as follows : [Hydrogen induced cracking]
 When a metal is subjected to cyclic stress in a corrosive environment, the
number of cycles required to cause failure at a given stress may be reduced
well below the dotted line obtained for the same metal in air. [Corrosion
Fatigue]
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Corrosion Cells
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Faraday’s Law
 The greater the flow of electricity through the cell,
the greater the amount of METAL that corrodes.
 This relationship is showed by Faraday ’ s law:
Weight of metal reacting = kIt
where I is the current in amperes (A), t is in seconds
(s), and k is a constant called the electrochemical
equivalent g/c. [Columb=1As]
OR
m=Iat/ zF
Where;
I= Current involved during a reaction
a= Atomic weight of the material
T= time of reaction
Z= number of equivalents exchanged
F= Faraday’s constant
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Types of Cells
1: Dissimilar Electrode Cells/Galvanic cell/Voltaic cell
A potential difference exists when two dissimilar metals, electrically
connected, are immersed in a corrosive solution. A cell that
produces electricity as a result of the spontaneous cell reaction.
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Types of Cells
Cell notation
Zn(s) I ZnSO4 (aq) II CuSO4 (a) I Cu(s)
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Salt bridge: is used for:
 Electrical connection betw
een the two half cells
 Prevents mixing of the elec
trolytes
 Keep the electrical neutralit
y in each half-cell as ions flow
into and out of the salt bridge
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Types of Cells
2: Concentration Cells
-Difference in the composition/salt, content of the soil or solution, Change in oxygen
concentration
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Types of Cells
2: Concentration Cells
 Differential aeration cells can also cause pitting damage under rust (Fig. 2.5 ) and at the
water line — that is, at the water – air interface (Fig. 2.6 ). The amount of oxygen reaching
the metal that is covered by rust or other insoluble reaction products is less than the
amount that contacts other portions where the permeable coating is thinner or nonexistent.
Differential aeration cell formed
by rust on iron
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Water-line corrosion, showing
differential aeration cell
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Types of Cells
3: Electrochemical Cells
A cell in which non spontaneous reaction is
driven by an external power source.
For example: Electrolysis
M+
M
m+
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Material Selection to avoid the corrosion issue
To solve the problem, we can say that;
i) Use more corrosion resistant material
ii) Use a corrosion prevention strategy such as coating, or
cathodic protection or use inhibitors
However to decide the best suitable method, which is
effective and economical, we need to consider many things
such as
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Material Selection to avoid the corrosion issue
Estimating the corrosion
performance of a material
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Material Selection to avoid the corrosion issue
Material selection
depends on:
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Risk Management
R (risk) = P (probability) × C (consequences)
 Hence, the risk of a corrosion related failure equals the probability that such a
failure will take place multiplied by the consequence of that failure.
 Consequence is typically measured in financial terms — that is, the total cost
of a corrosion failure, including the cost of replacement, clean - up, repair,
downtime, and so on.
 Any type of failure that occurs with high consequence must be one that
seldom occurs. On the other hand, failures with low consequence may be
tolerated more frequently.
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Risk Management
R (risk) = P (probability) × C (consequences)
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