Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

Module-2
CORROSION CHEMSITRY

Introduction :
● Corrosion is defined as the destruction and consequent loss of metals through chemical or
electrochemical attack by the environment
● Corrosion, in general, is a process through which refined metals are converted into more stable
compounds such as metal oxides, metal sulfides, or metal hydroxides.
● Corrosion is usually an undesirable phenomenon since it negatively affects the desirable properties
of the metal. For example, iron is known to have good tensile strength and rigidity (especially
alloyed with a few other elements). However, when subjected to rusting, iron objects become brittle,
flaky, and structurally unsound.
● On the other hand, corrosion can be classified as an electrochemical process since it usually involves
redox reactions between the metal and certain atmospheric agents such as water, oxygen, sulphur
dioxide, etc.
Corrosion classified as
1. Dry corrosion (Chemical corrosion)
2. Wet Corrosion (Electrochemical corrosions)

1) Dry corrosion (Chemical corrosion)

Corrosion of metal occurs by direct attack of atmospheric gases such as oxygen, hydrogen sulfide, halogens
and Sulphur dioxide in the absence of moisture to form metal oxide layer.
Example : 2Ag + Cl2 AgCl2

2) Wet corrosion (Electrochemical corrosion)

Wet corrosion occurs in presence of an aqueous solution of electrolyte and atmospheric oxygen by setting up
of tiny galvanic cells on the metal surfaces.
The mechanism of the wet corrosion is explained by the Electrochemical theory of corrosion

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 Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

❖ Electrochemical theory of corrosion / WET Corrosion

Corrosion is an example of Oxidation. When a metal like Iron is exposed to atmosphere, the following
electrochemical changes occur gradually.
a) Formation of Galvanic cells - Anodic and cathodic areas are formed resulting in a large number of
minute galvanic cells.

b) Anodic Reaction: At the anodic area metal undergoes oxidation with the release of electrons

Fe → Fe2+ + 2 e-
The anodic part of the metal undergo oxidation and suffer from corrosion, cathodic part undergo
reduction and remain protected from corrosion.
c) Cathodic Reaction - The electrons flow from the anodic to cathodic area and cause reduction. There are
3 possible ways in which reduction can take place.
(i) In acidic medium and absence of oxygen: hydrogen ions are reduced to hydrogen gas.

(ii) In neutral and in the absence of oxygen: If the solution is neutral and in the absence of oxygen,
water is reduced to H2 and OH-

(iii)In neutral and aerated medium: when the solution is neutral and aerated, hydroxyl ions are formed
as follows.

(iv) Formation of corrosion product:

The hydroxyl ions migrate towards anode and react with metal ions (Mn+ ions) and forms
corrosion product. In the case of iron OH- reacts with Fe2+ ions and forms an insoluble hydrated
ferric oxide known as brown rust.

❖ Types of corrosion
1) Differential metal corrosion
2) Differential aeration corrosion
3) Stress Corrosion (not in syllabus)
4) Grain Boundary corrosion (not in syllabus)

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 Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

1) Differential metal corrosion :

(i) Differential metal corrosion arise when two dissimilar metals are in contact with each other.
(ii) The metal higher in the electrochemical series is anodic to the one below it, i.e, the metal with lower
reduction potential undergoes oxidation, whereas the metal with higher reduction potential undergoes
reduction
(iii)The rate of differential metal corrosion depends primarily on the amount of current passing from the
anode to cathode, i.e, higher the difference in potential, higher will be the rate of corrosion.
The principle of Differential metal corrosion occurring in a simple cell can be extended to a bimetallic
couple.
Let us consider a bimetallic sample of iron and copper as shown below

The standard electrode potential of Fe (– 0.44 V) is less than that of Cu (0.34 V), hence in this case iron
acts as anode and undergoes corrosion whereas copper acts as cathode and remains un-attacked.
At anode : Fe → Fe2+ + 2e-
At cathode : (either hydrogen evolution or oxygen absorption) 2 H+ + 2e- → H2↑
O2+ 2 H2O + 4 e- → 4 OH-
Products : Fe2+ + 2 OH- Fe(OH)2

Common Examples
● Steel screws in copper sheet
● Lead-tin solder around copper wire
● Buried iron pipeline connected to Zinc bar.
● Steel pipe connected to copper plumbing

2) Differential aeration corrosion :

(i) It occurs when a metal surface is exposed to differential air (or) oxygen concentration.
(ii) Part of the metal exposed to lower concentration of oxygen will have lower potential and therefore acts
as anode. This part undergoes corrosion.
(iii)The other part of the metal exposed to higher concentration of oxygen acts as cathode and remains
unaffected.

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 Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

(iv)The difference in oxygen concentration produces a potential difference and causes corrosion current to
flow from cathode to anode.
Let us consider a strip of Zn immersed partially in a solution of ZnCl2
Hence part of the metal below the water level acts as anode
undergoes corrosion and the part above the water level acts as
cathode and remain unaffected. The corrosion product is
formed between anodic and cathodic areas.
Differential aeration corrosion can be classified into two
categories :
1) Water line corrosion
2) Pitting corrosion

a) Water line corrosion

This takes place due to the formation of differential oxygen concentration cells.
The part of metal exposed to lower concentration of oxygen (dissolved oxygen) acts as anodic area.
The part of the metal above the water level is exposed to higher concentration of oxygen acts as
cathodic area and remains unaffected.
A distinct brown line is formed just below the water line due to the deposition of rust.
Water line corrosion is observed usually in steel water tanks and ships floating in sea water for a long
time

At anode : Fe → Fe2+ + 2e-

At cathode : ½ O2+ H2O + 2 e- → 2 OH-
Products : Fe2+ + 2 OH- Fe(OH)2

b) Pitting corrosion
Pitting corrosion occurs when small particles like dust, mud
etc get deposited on metals surface. The portion of metal
covered by the dust or other particles is less aerated and acts as
anode. The other portion of the metal exposed to more oxygen
of the environment act as cathodic region. Corrosion takes
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 Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

place at the portion below dust and a small pit is formed. Then the rate of corrosion increases due to small
anodic area and large cathodic area.

❖ Factors affecting corrosion

The important factors, which affect on the rate of corrosion, are

1) Nature of metal - The Metal undergo corrosion is dependent on the nature of the metal. The metals
with lower electrode potential values are more reactive than the metals with higher electrode potential
values. The more reactive metals are more susceptible for corrosion.
Example: The active metals like K, Na, Mg, Zn etc with low Electrode potential values are highly
susceptible for corrosion. The noble metals such as Silver, gold, Platinum etc., with higher electrode
potential values are less susceptible for corrosion.

2) Nature of corrosion product - If the nature of corrosion product is adherent, insoluble, stable,
non volatile and non-porous, with low ionic and electronic conductivity, then it acts as a protective
film which prevents further corrosion. The film acts as a barrier between the fresh metal surface and the
corrosive environment. On the other hand, if the nature of corrosion product is non-adherent, soluble,
unstable, conducting and porous, then the corrosion process continues even after the formation of
corrosion product.
Example: In oxidizing environment metals like Al, Cr, Ti etc. forms protective metal oxide films on their
surfaces which prevent further corrosion. Stainless steel forms a protective film of Cr2O3 on the surface.
Metals like Zn, Fe, Cd, Mg ,etc .do not form protective layer and are readily under goes corrosion .Therefore
the rate of corrosion depends on the nature of corrosion product.

3) Nature of environment (pH, temperature, conductivity)

● Temperature: Increases in temperature results in an increase in the conductance of the aqueous medium
and rate of corrosion also increases and vice versa.
● pH- Rate of corrosion increases with decrease in pH.
✔ If pH > 10, the rate of corrosion ceases due to the formation of protective coating of hydrous
oxides on the metal.
✔ If pH is between 3-10, the rate of corrosion depends upon the presence of oxygen on the
Cathodic area for reduction. Higher the concentration of O2, higher the rate of corrosion.
✔ If pH < 3 rate of corrosion is high even in the absence of air due to evolution of hydrogen at the
cathodic region.
● Conductivity - Rate of corrosion increases with increase in conductance of medium.
For example, rate of corrosion of metal will be higher in wet atmosphere than dry atmosphere due to more
conductance. Similarly metal undergo rapid corrosion in ocean water than in river water.

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 Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

CORROSION CONTROL
✔ Corrosion is a Natural process. Corrosion of metal results in huge economic loss.
✔ We cannot completely STOP the corrosion process of metals, but it is possible to minimize corrosion
effect by applying various methods.
Corrosion control methods are broadly classified into
1. Cathodic protection (Sacrificial anode method & Impressed current method)
2. Metal coating (Galvanization)

1) Cathodic protection
In cathodic protection electrons are provided from an external source so that the metal or alloy remains as
cathode.
The principle is to force a metal to be protected to behave as cathode
It is broadly divided into two:
a) Sacrificial anodic protection: SAP method
b) Impressed current cathodic protection: ICCP method

a) Sacrificial Anode Protection (SAP)

● In this method the metal to be protected from corrosion is converted into cathode by connecting into
a metal which is anodic to it.
● The base metal is usually iron, copper or brass.
● Metals like Mg, Al and Zn are more active and hence are used as anodes.
● These metals being more active acts as anode undergo corrosion and
supply electrons to the specimen.
● In this way the protected metals acts as cathode.
● Since anodic metals are scarified to protect the metal structure this
technique is called sacrificial anode method.

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 Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

Benefits
• No power source is required
• Simple to install, operate and maintain
• Additions easily installed
Example: Mg block connected to buried oil storage tanks,
Zn wire fixed to the sides of ocean-going ships are the best examples of sacrificial anode
methods.

b) Impressed Current Cathodic Protection (ICCP)

In this method, an impressed current from external source is applied in opposite direction to neutralize the
corrosion current, and convert the corroding metal from anode to cathode. Thus the anodic corroding metal
becomes cathodic and protected from corrosion. The impressed current is deriving from the external DC
source. The negative terminal of DC source is connected with metal to be protected whereas the positive
terminal of DC is connected to the anode. Here anode may be graphite, stainless steel scrap iron etc. The
impressed current protection method is used for water tanks, water & oil pipe lines, transmission line towers
etc.

https://youtu.be/QYd9ENn1nP0?si=CSwdoj6ZcBKKAuNj

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 Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

2) Metal coating
Deposition of protective metal over the surface of base metal is known as metallic coatings.

Galvanization

The process of depositing a protective layer of zinc on steel or iron is called galvanization.
It is a method used to prevent corrosion.
In this process the base metal is coated with Zinc.
Zinc coatings are generally obtained by hot dipping of the base metal in a molten zinc bath and the
process is called galvanization
Iron and steel articles are protected from corrosion usually by galvanization because of the low cost
of zinc easy application of the coating and efficient anodic protection afforded.
The primary method of galvanization is “hot dip galvanization”, which has been in use for over 150 years
Galvanization consists of hot dipping which involves the following steps.
1. The base metal surface is washed with organic solvents to remove organic impurities such as oil and
grease present on it.
2. Followed by water wash to remove excess organic solvents.
3. Then metal surface is washed with dilute sulphuric acid (pickling) to remove rust and other inorganic
deposits.
4. Followed by water wash to remove excess acids.
5. Then the base metal surface is subjected to steam zone process where we can dry the surface (Drying
process)
6. The metal is treated with mixture of aqueous solution ZnCl2 and NH4Cl which acts as flux and dried.
The flux treatment enhances the adsorption process of base metal.
7. The metal is then dipped in molten zinc maintained at 4500 C.
8. Excess zinc is released by passing the metal through rollers (or) by wiping, which makes uniform
coating of zn on base metal.
9. Finally, the base metal subjected to furnace for annealing (calcination) process at higher temperature.
This process enhances the adsorption and coating efficacy.

Advantages
Since the coating is anodic in nature the exposure of base metal will not cause corrosion.

Disadvantages
Since Zn salts are poisonous food materials cannot be stored in container.

Uses/Applications
To make galvanized articles.
Galvanized Iron objects are used as roofing sheets, buckets, wires, pipes.
To manufacture automobile parts, bolts & nuts, nails etc.

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 Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

Corrosion Penetration Rate (CPR)

The rate of material removal as a consequence of the chemical action is an important corrosion parameter,
this may be expressed as the corrosion penetration rate CPR, or the thickness loss per unit of time.
Several pieces of data must be collected to calculate the corrosion penetration rate for any given metal:
● The weight lost (the decrease in weight of the metal during the period of reference).
● The density of the metal.
● The total surface area initially present.
● The time taken for the metal to corrode.

The CPR is calculated as follows:

𝑘× 𝑊
● CPR = 𝐷 𝐴 𝑇 where k = a constant
W = total weight lost
T = time taken for the loss of metal
A = the surface area of the exposed metal
D = the metal density in g/cm³

The corrosion penetration rate is best expressed in terms of thickness or weight loss where the
surface of the metal corrodes uniformly across an area.
This method involves the exposure of a weighed piece of test metal or alloy to a specific
environment for a specific time. This is followed by a thorough cleaning to remove the corrosion
products and then determining the weight of the lost metal due to corrosion.
Where W is weight loss after exposure time T, while D and A represent the density and exposed
specimen area respectively and k is constant, its magnitude depend on the system of units used. The
CPR is conveniently expressed in terms of either mils (1/1000 inch) per year (mpy) or
millimetres per year (mm/yr).

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 Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

k=534 to give CPR in mpy (mil per year) and W, D, A and T are specified in units milligrams, gram
per cubic centimetre ,square inches and hours respectively, in second case k=87.6 for CPR in mm/yr
and unit for the other parameters are the same for first case except that A is given in square
centimetres. For most application a corrosion penetration rate less than 20 mpy (0.5 mm/yr) is
acceptable.

Numerical problem :
Q1) A piece of corroded metal alloy plate was found in a submersed ocean vessel ,it was estimated that the
original area of plate was 800 cm2 and that approximately 7.6 Kg had corroded a way during the submersion
, assuming a corrosion penetration rate of 4 mm/yr for this alloy in seawater , estimate the time of
submersion in years , the density of alloy is 4.5 g/cm3.
They have given; CPR: 4 mm/yr, K: 87.6, W: 7.6 Kg, D: 4.5 g.cm3, A: 800 cm2
We need to calculate Time ?

Q2: A steel of area 100 inch2 is exposed to air near the seashore. After 1 year it was found that the steel
sheet has lost 485 g due to corrosion. What is the value of CPR in mils/year and in mm/year ? Can such steel
sheet be applicable for the construction purpose where the steel sheet is exposed?
Given : Area (A) =100 inch , total weight lost (W) = 485g = 485 x103 mg,
T = 1 year =1 x 365 x 24 hrs, D = 7.9 g/cm3 and k=534.
CPR=(K x W)/(D x A x T)
CPR=(534 x 485 x 10^3)/(7.9 x 100 x 365 x 24)
CPR = 37.42 mils/year
To calculate the result in mmpy
Area =100 inch2 = 100 x 2.54 × 2.54 cm2 (1 inch = 2.54 cm) and k =87.6
3
87.6 𝑥 485 𝑥 10
𝐶𝑃𝑅 = 7.9 𝑥 100 𝑥 2.54 𝑥 2.54 𝑥 365 𝑥 24

CPR = 0.9516 mmpy

Since CPR is not less than 20 m/year or 0.5 mm/year, it is not applicable for the construction purpose.
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 Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

Q3: A piece of corroded steel plate was found in a submerged ocean. It was estimated that the original area
of the plate was 12 inch2 and the approximately 2.5 kg had corroded during the submersion. Assuming a
corrosion penetration rate of 100 mpy for this alloy in seawater. Estimate the time of submersion in year.
The density of steel is 7.9 g/cm3
Answer: 16.1 year
Q4: A piece of corroded steel plate was found in a submerged ocean vessel. It was estimated that the original
area of the plate was 11 inch2 and that approximately 2.3 kg had corroded away during the submersion.
Assuming a corrosion penetration rate of 200 mpy for this alloy in seawater, estimate the time of submersion
in years. The density of steel is 7.9 g/cm3
Answer: 8.03 year

ELECTROPLATING
Principle, Electroplating of Chromium- Hard and Decorative Chromium plating

Electroplating or electrodeposition is the process by which the coating metal is deposited on the base
metal by passing a direct current through electrolytic solution, containing the soluble salt of the coating
metal.

Principle of Electroplating
1. Electroplating is achieved by passing a direct current through an electrolytic solution containing metal ion
and the electrodes
2. The part to be plated is the cathode of the circuit and anode is made of metal to be plated on such part.
3. The electrodes are dipped in electrolyte containing the dissolved metals salts as well as other ions that
permit flow of electricity. The anode is connected to the positive terminal and the cathode is connected to
negative terminal of the battery. As current is passed through the circuit the metal at anode is oxidized to
forms cations.
At anode : M → Mn+ + ne- (Oxidation)
These cations (Mn+) get reduced at cathode to deposit on the other metal at cathode to form a layer.
At cathode : Mn+ + ne- → M (Reduction)

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 Engineering Chemistry – CV stream / BCHEA102 Module-2 : Corrosion Chemistry

Faradays law of electrolysis govern the amount of metal deposited.

Objectives of electroplating on metals:

• To increase corrosion resistance
• To increase resistance to chemical attack and wear resistance
• It imparts hardness
• It improves the surface properties.
• It increases the decorative and commercial values of the metal

Electroplating of chromium:
Electroplating of chromium is carried out on the pretreated Ni or Cu articles.

1) Decorative Chromium Coatings:

Decorative chromium coating is the coating of thin layer of chromium with thickness of 0.25 to 0.75µm.
thicker deposits look very dull.
2) Hard Chromium coating:
Deposition of chromium with a thickness of 0.005 to 0.75mm is referred as hard chromium coating.

Process followed for decorative and hard chromium plating :

Pre-treatment of material before electroplating: The object is degreased with organic solvent alkali
followed by acid treatment.

Post treatment: After completion of the electroplating process (around 30 minutes), the object has to be
kept in hot air oven maintained at 105- 120oC for 2 days (48 hrs). Then in the furnace at higher temperature
(600-8000 deg C) for 6 to 8hrs.

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Applications
o Electroplating of chromium is used to protect metal from corrosion
o On automobiles furnishing, house-hold fittings and surgical instrument.
o Hard chromium coatings that has high wear resistance is used in ball bearings, cutting tools, piston
rings etc.
o Black chromium coating is used on optical instruments, machine tools, electronic parts.

ELECTROLESS PLATING
It is the deposition of the metal from its salt solution on a catalytically active surface by a suitable reducing
agent without using electricity.
Example : Electroless plating with respect to copper, nickel etc.

❖ Principle of Electroless Plating

The process relies on a redox reaction. The metal ions in the plating solution are reduced to their metallic
form and deposited onto the substrate. The reaction is autocatalytic, meaning that the deposited metal itself
acts as a catalyst to sustain the deposition process.

Metal ion + Reducing agent Metal plated + oxidized products.

Note: CAS: Catalytically Active Surface

Advantages of Electroless plating process

o Electrical power and electrical contacts are eliminated.
o Semiconductors and insulators like plastics, glass, quartz, wood, and thread can be plated.
o Electroless plating baths having better throwing power.
o Irregular shapes can be uniformly coated.

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❖ Electroless plating of copper

Electroless plating of copper is a chemical process used to deposit a thin layer of copper onto a substrate
without using an external electric current. It is widely used in applications such as printed circuit board
(PCB) manufacturing and metallization of non-conductive surfaces.
Principle
The process is based on an autocatalytic redox reaction. A reducing agent in the solution reduces copper ions
(Cu2+) to metallic copper (Cu), which is deposited onto the substrate. The deposited copper layer acts as a
catalyst to continue the reaction.

Steps in Electroless Copper Plating

1. Surface Preparation
The substrate is cleaned to remove any contaminants, such as oils or oxides, that might interfere with
plating.
Non-conductive surfaces (e.g., plastics) are activated using a catalyst, typically palladium or a similar
material, to create nucleation sites for copper deposition.
2. Sensitization : The substrate is treated with a solution like stannous chloride (SnCl2​) to sensitize the
surface.
3. Activation : The sensitized surface is treated with a palladium chloride (PdCl2​) solution. The palladium
particles adhere to the surface and act as a catalyst for copper deposition.
4. Electroless Plating
The substrate is immersed in an electroless copper plating solution. The typical composition of the solution
includes:
(i) Copper source: Copper sulfate (CuSO4​)
(ii) Reducing agent: Formaldehyde (HCHO)
(iii)Complexing agent: Chelates like EDTA to stabilize the copper ions in solution
(iv)Buffer: To maintain the pH (e.g.,NaOH)
(v) Additives: To control deposition rate and enhance surface quality

Reaction Mechanism:
1. Reduction of copper ions by formaldehyde:
Cu2+ + 2 HCHO + 4 OH- Cu + H2 + 2 HCOO- + 2H2O
2. The deposited copper catalyzes further reduction of Cu2+ ions.

❖ Characteristics of Electroless Copper Plating :

● The deposited copper layer is highly uniform, even on complex surfaces.
● Good adhesion to both metallic and non-metallic substrates.
● Can deposit copper on non-conductive materials after activation.

❖ Applications :
1. Printed Circuit Boards (PCBs):
o Creating conductive pathways on non-conductive boards.
2. Electronic Devices:
o Shielding components against electromagnetic interference.

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3. Metallization of Plastics:
o Producing decorative and functional coatings.

Difference between Electroplating and Electroless Plating

Parameter Electroplating Electroless Plating

Electroplating requires an external There is no need of external power

Need of external
power source to drive the plating source in the electroless plating to
power source
reaction. drive the plating reaction.

No anode, instead the catalytic

A separate anode is used that is made
Anode surface of the substrate plays the
up of metal.
same role.

In electroplating, the substrate on the Electroless plating has no cathode,

Cathode which the coating layer is to be instead the activated surface of the
deposited forms the cathode. substrate serves the same function.

Deposition rate is primarily Deposition rate is controlled by

Deposition rate
controlled by changing electric changing the concentration of plating
control
current and plating time. solution.

Type of Electroless plating can be used for

Electroplating is limited to use for
substrate plating of both conductive and
conductive materials only.
material non-conductive materials.

Electroless plating provides

Electroplating can provide a coating
Uniformity in comparatively uniform thickness of
of uneven thickness in the case of
thickness deposition on the complex-shaped
complex geometry of the substrate.
surfaces.

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Electroplating has high operation cost Electroless plating has comparatively

Operational cost
due to electric power consumption. lower operating cost.

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