ASSIGNMENT # 1

Topic: Corrosion

Name

Muhammad Ali

REG NO

BChE-FA19-016

Subject

Engineering Safety & Maintenance

Submitted To

Dr Shahid Raza Malik

Department Of Chemical Engineering

Table of Contents
1. Definition.................................................................................1
1.1. Do All Metals Corrode?.........................................................1
1.2. Factors Affecting Corrosion..................................................2
1.3. Types of Corrosion................................................................2
1.3.1. Crevice Corrosion...............................................................2
1.3.2. Stress Corrosion Cracking..................................................2
1.3.3. Intergranular Corrosion.....................................................3
1.3.4. Galvanic Corrosion.............................................................3
1.3.5. Pitting Corrosion................................................................4
1.3.6. Uniform Corrosion.............................................................4
1.3.7. Hydrogen Grooving............................................................5
1.3.8. Metal Dusting.....................................................................5
1.3.9. Microbial Corrosion...........................................................6
1.3.10. High-temperature Corrosion...........................................6
1. Definition
It is basically defined as a natural process that causes the transformation of pure metals into
undesirable substances when they react with substances like water or air. This reaction causes
damage and disintegration of the metal starting from the portion of the metal exposed to the
environment and spreading to the entire bulk of the metal.

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 is a
diffusion-controlled process and that it mostly occurs on exposed surfaces. Therefore, in some
cases, attempts are made to reduce the activity of the exposed surface and increase a
material’s corrosion resistance. Processes such as passivation and chromate conversion are
used. However, some corrosion mechanisms are not always visible and they are even less
predictable.
Alternatively, 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.

1.1. Do All Metals Corrode?

Metals placed higher in the reactivity series such as iron, zinc, etc. get corroded very easily and
metals placed lower in the reactivity series like gold, platinum and palladium do not corrode.
The explanation lies in the fact that corrosion involves the oxidation of metals. As we go down
the reactivity series tendency to get oxidized is very low (oxidation potentials is very low).

1
1.2. Factors Affecting Corrosion
 Exposure of the metals to air containing gases like CO2, SO2, SO3 etc.
 Exposure of metals to moisture especially salt water (which increases the rate of
corrosion).
 Presence of impurities like salt (eg. NaCl).
 Temperature: An increase in temperature increases corrosion.
 Nature of the first layer of oxide formed: some oxides like Al2O3 forms an insoluble
protecting layer that can prevent further corrosion. Others like rust easily crumble and
expose the rest of the metal.
 Presence of acid in the atmosphere: acids can easily accelerate the process of corrosion.

1.3. Types of Corrosion

These are the some important types of corrosion;

1.3.1. Crevice Corrosion

Whenever there is a difference in ionic concentration between any two local areas of a metal, a
localized form of corrosion know as crevice corrosion can occur. In a simple instance, this form
of corrosion mostly occurs in confined spaces (crevices). Examples of areas where crevice
corrosion can occur are gaskets, the undersurface of washers, and bolt heads. All grades of
aluminum alloys and stainless steels also undergo crevice corrosion. This is mainly because of
the formation of a differential aeration cell that leads to the formation of corrosion inside the
crevices.

1.3.2. Stress Corrosion Cracking

Stress Corrosion Cracking can be abbreviated to ‘SCC’ and refers to the cracking of the metal as
a result of the corrosive environment and the tensile tress placed on the metal. It often occurs
at high temperatures.

2
Example: Stress corrosion cracking of austenitic stainless steel in chloride solution.

1.3.3. Intergranular Corrosion

Intergranular corrosion occurs due to the presence of impurities in the grain boundaries that
separate the grain formed during the solidification of the metal alloy. It can also occur via the
depletion or enrichment of the alloy at these grain boundaries.
Example: Aluminum-base alloys are affected by IGC.

1.3.4. Galvanic Corrosion

When there exists an electric contact between two metals that are electrochemically dissimilar
and are in an electrolytic environment, galvanic corrosion can arise. It refers to the degradation
of one of these metals at a joint or at a junction. A good example of this type of corrosion
would be the degradation that occurs when copper, in a salt-water environment, comes in
contact with steel.

3
Example: When aluminium and carbon steel are connected and immersed in seawater,
aluminium corrodes faster and steel is protected.

1.3.5. Pitting Corrosion

Pitting Corrosion is very unpredictable and therefore is difficult to detect. It is considered one of
the most dangerous types of corrosion. It occurs at a local point and proceeds with the
formation of a corrosion cell surrounded by the normal metallic surface. Once this ‘Pit’ is
formed, it continues to grow and can take various shapes. The pit slowly penetrates metal from
the surface in a vertical direction, eventually leading to structural failure it left unchecked.
Example: Consider a droplet of water on a steel surface, pitting will initiate at the center of the
water droplet (anodic site).

1.3.6. Uniform Corrosion

This is considered the most common form of corrosion wherein an attack on the surface of the
metal is executed by the atmosphere. The extent of the corrosion is easily discernible. This type
of corrosion has a relatively low impact on the performance of the material.

4
Example: A piece of zinc and steel immersed in diluted sulphuric acid would usually dissolve
over its entire surface at a constant rate.

1.3.7. Hydrogen Grooving

This is a corrosion of the piping by grooves that are formed due to the interaction of a corrosive
agent, corroded pipe constituents, and hydrogen gas bubbles. The bubbles usually remove the
protective coating once it comes in contact with the material.

1.3.8. Metal Dusting

Metal dusting is a damaging form of corrosion that occurs when vulnerable materials are
exposed to certain environments with high carbon activities including synthesis gas. The
corrosion results in the break-up of bulk metal to metal powder. Corrosion occurs as a graphite
layer is deposited on the surface of the metals from carbon monoxide (CO) in the vapor phase.
This graphite layer then goes on to form meta-stable M3C species (where M is the metal) that

5
usually moves away from the metal surface. In some cases, no M3C species may be observed.
This means that the metal atoms have been directly transferred into the graphite layer.

1.3.9. Microbial Corrosion

Microbial corrosion which is also known as microbiologically influenced corrosion (MIC), is a
type of corrosion that is caused by microorganisms. The most common one is
chemoautotrophs. Both metallic and non-metallic materials either in the presence or absence
of oxygen can be affected by this corrosion.

1.3.10. High-temperature Corrosion

High-temperature corrosion as the name suggests is a type of corrosion of materials (mostly
metals) due to heating. Chemical deterioration of metal can occur due to a hot atmosphere that
contains gases such as oxygen, sulfur, or other compounds. These compounds are capable of
oxidizing the materials (metals in this case) easily. For example, materials used in car engines
have to resist sustained periods at high temperatures during which they can be affected by an
atmosphere containing corrosive products of combustion.
Corrosion Examples, Reactions and Effects

6
7