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Question
Define corrosion of metals. Explain the electrochemical theory of wet corrosion, giving its mechanism.
Solution
1. Any process of destruction and deterioration of any material is known as corrosion.
2. When metal is in immediate contact of aqueous /acidic/alkaline/natural/electrolytic solutions, the short circuited galvanic cells get set all along the surface of metal. This gives rise to corrosion which proceeds by electrochemical principles.
3. Wet corrosion is more common than dry corrosion.
4. The co-ordinating metals behave like galvanic cells thereby the parts of metal acting as an anode is consumed while the other part which is acting as cathode remains unchanged.
Mechanism:
Corrosion, by this mechanism occurs usually if environment to the metal is acidic.
Example:
Pipe lines made of iron metal get corroded if industrial waste material, or solutions of non- oxidising acids is transported through them.
The following reaction occurs:
Anode `(e-)/`> Cathode
`fe → fe ^+2+2e^- → 2H^+ +2e^- → H_2 `
From above reactions, we can say that the flow of electrons take place from anode to cathode. These electrons are gained by cathodic reaction, and at cathodic H+ ions are eliminated as H2 gas. The overall mechanism can be represented as
`fe+2H^+→ Fe^+2+H^2`
Thus displacement of H2 ions from acidic solutions by metal ions takes place. Thus all metals have tendency to get dissolved in acidic solution with simultaneous equations of the H2 gas.
