Corrosion: Definition and significance of corrosion, mechanism of chemical (dry) and electrochemical (wet) corrosion, galvanic corrosion, concentration corrosion and pitting corrosion. Protection from corrosion; protective coatings-galvanization and tinning, cathodic protection, sacrificial anode modifications in design.

1. Corrosion: Definition and significance of
corrosion, mechanism of chemical (dry) and
electrochemical (wet) corrosion, galvanic
corrosion, concentration corrosion and pitting
corrosion. Protection from corrosion;
protective coatings-galvanization and tinning,
cathodic protection, sacrificial anode
modifications in design
Unit: 3

9. Antiques and monument corrosion

10. (a) The Statue of Liberty is covered with a copper skin, and was originally
brown, as shown in this painting. (b) Exposure to the elements has resulted in
the formation of the blue-green patina seen today.
Patina: is a green or brown film on the surface of bronze or similar metals, produced by
oxidation over a long period.

17. Cathodic Protection
(CP)

18. Also called Sacrificial Anode.

19. Ship Corrosion: Sacrificial Anode

24. Anodic protection is referred as Anodic Control is a
technique to control the corrosion of a metal surface by
making it the cathode of an electrochemical cell and to
connect it with more active metal which is anode.
Application of Anodic Protection
Anodic protection is used for ships and carbon steel storage
tanks containing extreme pH environments including
concentrated sulfuric acid and 50 percent caustic soda where
cathodic protection is not suitable due to very high current
requirements.

25. Carbon steel storage
tanks containing
extreme pH environments

28. Difference Between Cathode and Anode

30. Difference Between Electrochemical Cell and Electrolytic Cell

31. Electrochemical Series
The arrangement of elements in order of increasing electrode
potential values is called “Electrochemical Series”.
The Electrochemical Series is also called “Activity Series”.
The series has been established by measuring the potential of
various electrodes versus standard hydrogen electrode (SHE).

43. Mechanism of Dry Corossion
-

44. Galvanic corrosion, also known as
bimetallic corrosion, is an electrochemical
process whereby one metal corrodes in
preference to another metal that it is in contact
with through an electrolyte. Galvanic
corrosion occurs when two dissimilar metals
are immersed in a conductive solution and are
electrically connected.
Galvanic Corrosion

49. Concentration Corrosion
It is due to the electrochemical attack on metal surface, exposed
to an electrolyte of varying concentrations or of varying aeration.
It occurs when one part of metal is exposed to a different air
concentration from the other part.
This causes a difference in potential difference between
differently aerated areas. It has been found experimentally that
poor oxygenated parts are anodic.

50. Concentration Corrosion

51. Pitting Corrosion
Pitting corrosion is very localized form of corrosion, which causes
small holes in the metal affected. Pitting corrosion can be started
by a small scratch or damage to a protective coating.
Pitting is more common in alloys that are protected against
corrosion by using a passivation layer. These include stainless
steel, Nickel alloys and Aluminium alloys etc.
Pitting corrosion is a localized form of corrosion by which
cavities or holes are produced in the material. Pitting is
considered to be more dangerous than uniform corrosion damage
because it is more difficult to detect, predict and design.

52. 1 2
3 4

53. Different Types of Pitting Corrosion

54. Protection from Corrosion

55. Protection from corrosion by protective coatings

57. A protective coating is a layer of material applied to the surface of
another material with the intent of inhibiting or preventing
corrosion. A protective coating may be metallic or non-
metallic. Commonly used materials in non-metallic protective
coatings include polymers, epoxies and polyurethanes. Materials
used for metallic protective coatings include zinc, aluminum and
chromium.
A zinc coating over steel is an example of a reactive protective
coating. The zinc reacts with the atmosphere more readily than
the steel, thus preventing the oxidation of the steel. Zinc
protective coatings are typically applied by the electroplating or
hot dipping method. A protective coating can provide water
resistance or electrical properties that the material did not have
prior to the protective coating being applied.

60. Galvanization or galvanizing is the process of applying a protective zinc coating
to steel or iron, to prevent rusting.
Galvanizing, or galvanization, is a manufacturing process where a coating of zinc
is applied to steel or iron to offer protection and prevent rusting.
There are several galvanizing processes available, but the most commonly offered
and used method is called hot-dip galvanizing in which the parts are submerged
in a bath of molten hot zinc.
The zinc coating of hot-dipped galvanized steel will last in the harshest soil is 35
to 50 years and in less corrosive soil 75 years or more.
Although humidity affects corrosion, temperature itself has less of an impact.
Galvanized zinc coatings respond well in extreme cold and hot temperatures.
Galvanization

62. Tinning is the process of thinly coating sheets of wrought iron or steel with tin,
and the resulting product is known as tinplate.
Tinning (Sn Plating)
Tinplate is a thin steel sheet coated by tin. It has an extremely beautiful metallic
luster as well as excellent properties in corrosion resistance.

65. 1 Francs = 82 Indian Rupees

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