Destruction or deterioration or disintegration of a metal by chemical or electrochemical reaction with its environment is called corrosion.
oxygen absorption mechanism ::
This type of corrosion will take place if the medium contains oxygen.
• E.g. rusting of iron.
• The surface of iron is generally coated with a thin film of iron oxide. However if this iron oxide film develops some cracks, anodic areas are created on the surface of the exposed iron metal while the rest metal surface forms the cathode & neutral aqueous solution like NaCl acts as an electrolyte.
• At anode iron undergoes oxidation as follows,
Anodic reaction: Fe → Fe+2 + 2e- ( oxidation)
• Fe2+ is released in aqueous medium and electrons travel from anode to cathode. Dissolved oxygen from electrolyte undergoes reduction as follows.
Cathodic reaction: H2O + 1/2O2 + 2e- → 2(OH) - (Reduction)
The Fe+2 ions at anode & OH- ions at cathode combine to form compound Fe++ + 2 OH- → Fe(OH)2 (brown rust)
If enough oxygen is present, ferrous hydroxide is easily oxidized to ferric hydroxide
2 Fe(OH)2 +1/2 O2 + H2O → 2Fe(OH)3↓ (Yellow Rust )
• If supply oxygen is limited the corrosion product is black anhydrous magnetite (Fe3O4)
• Here the anodic area is smaller than the cathodic area thus the rate of corrosion is faster as large cathodic areas demand more electrons and small anodic areas undergo corrosion to greater extent.
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