Question

(a) What do you understand by corrosion?
(b) Write electro-chemical theory of corrosion (Rust).
(c) Write prevention (two) of corrosion.

Answer 1

(a) Corrosion: Process by which the layers of undesirable compounds are formed on the surface of a metal on its exposure to atmospheric conditions are called corrosion.
(b) Electrochemical theory of corrosion: It is widely accepted electrochemical reaction. A number of galvanic cells are formed in impure iron. In rusting of iron pure iron acts as a anode while impure part of iron acts as cathode. A thin layer formed by dissolved oxygen and carbon dioxide in moisture acts as solution of electrolyte. Oxidation and reduction occur at anode and cathode, respectively.
$Fe\to {Fe}^{2+}+2e^{-}$ (Anode)
$2H+2e^{-}\to H_2$ (Cathode)
Thus all metals above hydrogen in electrochemical series after corrosion evolve hydrogen.
(c) Prevention of corrosion:
(i) By coating metal surface with oil or grease: A thin layer of oil or grease is coated on the surface of the metal.
(ii) By painting the metal surface: A layer of point is coated on the surface of the metal.