2. 1. Materials are Precious resources.
2. Engineering design is incomplete without
knowledge of corrosion.
3. Applying knowledge of corrosion protection
can minimize disasters.
4. Corrosion- may contaminate stored food,
dairy products, etc.
5. Corrosion products cause pollution.
6. Artificial implants for the human body.
Why study Corrosion???
3. • Corrosion is a process of formation of the compound
of pure metal by the chemical reaction between metallic
surface and its environment.
• It is an oxidation process. It causes loss of metal.
• Hence, disintegration of a metal by its surrounding
chemicals through a chemical reaction on the surface
of the metal is called corrosion.
INTRODUCTION
4. •Corrosion is the slow process of decay of the metal,
due to the attack of the atmosphere gases on the
surface of the metal, resulting in the formation of
metallic compounds such oxides, hydroxides,
carbonates, sulphides etc.
6. Dry or Chemical Corrosion
It is due to the direct chemical action of
environmental atmosphere gases such as 0₂, H₂S,
SO₂, N₂, halogens or anhydrous inorganic liquids with
metal surfaces.
Wet or Electrochemical Corrosion
It is due to the existence of separate "anodic" and
"cathodic" areas in the system between which current
flows through the conducting liquid and the anode
gets oxidized and wasted.
7. WET / AQUEOUS CORROSION
Based on the appearance of the corroded metal, wet
corrosion may be classified as:-
(i) Uniform or General
Corrosion over the entire exposed surface at a uniform rate.
(ii) Galvanic or Two-metal
When two dissimilar metals are joined together and
exposed,the more active of the two metals corrode
faster and the nobler metal is protected.
8. (iii) Crevice Corrosion
Intensive localized corrosion within crevices &
shielded areas on metal surfaces.
(iv) Pitting
A form of extremely localized attack causing
holes in the metal.
Most destructive form.
9. Factors Affecting Corrosion
👉 Purity of metal.
👉 Physical State of Metal.
👉 Solubility and volatility of corrosion product.
👉 The position of metals in the Electrochemical Series.
👉 Nature of the corroding environment.
(i) Temperature
(ii) Humidity of the air
(iii) Impurities
(iv) Influence of pH
10. Prevention of Corrosion
👉 Maintaining the purity of metal.
👉 Alloying with other elements.
👉 Modification of environment.
👉 Protective Coating.
1. Metallic Coating
2. Non-metallic Coating
3. Organic Coating
11. 1. Metallic Coating
(i) Using less active metal.
(ii)Using a more active metal.
2. Non-Metallic Coating
(i) Phosphate coating by Alkaline solution of phosphate.
(ii) Chromate coating using chromate solutions.
3. Organic Coating
(i) Plastics, polythene, rubber etc, are used for coating to
prevent corrosion.
(ii) Mainly on articles like, ship, submarines. Etc.
