The document discusses different types of corrosion including mechanical erosion, chemical corrosion, and electrochemical corrosion. It provides examples of how different metals corrode via chemical reactions with oxygen, sulfur dioxide, carbon dioxide, chlorine, fluorine, and hydrogen sulfide. The corrosion of iron into rust is given as a specific example of electrochemical corrosion where iron acts as the anode. Methods to prevent corrosion are also outlined, including metal purification, alloying, proper design, use of non-metallic materials, metallic coatings, inhibitors, cathodic protection, and protective coatings like paint, varnish, oils, and grease.

1. Corrosion
Mechanical Erosion
Chemical M - ne Mn+
(i) By O2 : a) More stable oxide, more corrosion Al2O3 , SnO2
b) Volatility of oxide MoO3
c) Porous oxide Na, K, Li / O
(ii) By SO2 ,CO2 , Cl2, F2, H2S :
a) Unpermeable Shield (AgCl), corrosion stops
b) M + H2S  MS + H2 Enters interstices, increase pressure
makes M brittle

2. Electrochemical
Rusting
ANODE :Fe – 2e Fe 2+
Air H2O H+ OH-
Fe (OH)2
Air
O2
Fe (OH)3
Cathode : H + + e  H
Preventive Measures
1. Metal Purification
2. Alloying
3. Proper Designing
4. Non-Metallic Materials
5. Metallic Coating Galvanizing, Tinning, Sherardizing

3. Preventive Measures
6. Inhibitors
Anodic Oxidising Agents, Na2CrO4
Cathodic Ca(HCO3)2, Na3PO4,
7. Cathodic Protection: Anode is made cathode
e.g. To protect steel, Zn and Mg are used
912 BC8. Protective coating
PAINT
VARNISH
OILS
GREASE