Oxidation corrosion is brought about by the direct attack of oxygen at low or high temperature on metal surfaces in the absence of moisture. Alkali metals (Li, Na, K etc.,) and alkaline earth metals (Mg, Ca, Sn, etc.,) are rapidly oxidized at low temperature. At high temperature, almost all metals (except Ag, Au and Pt) are oxidized. The reactions of oxidation corrosion are as follows.
Oxidation takes place at the surface of the metal forming metal ions.
M → M2+ + 2e-
2) Oxygen is converted to oxide ion (O2-) due to the transfer of electrons from metal. n/2 O2 + 2ne-→ n O2-
3) The oxide ion reacts with the metal ions to form metal oxide film.
2 M + n/2 O2 → 2 Mn+ + nO2-
The nature of the oxide formed plays an important part in oxidation corrosion process.…show more content… Corrosion
(i) Stable layer:
A stable layer is fine grained in structure and can get adhered tightly to the parent metal surface. Hence, such layer can be of impervious nature (i.e., which cuts-off penetration of attaching oxygen to the underlying metal). Such a film behaves as protective coating in nature, thereby shielding the metal surface. The oxide films on Al, Sn, Pb, Cu, Pt, etc., are stable, tightly adhering and impervious in nature.
(ii) Unstable oxide layer:
This is formed on the surface of noble metals such as Ag, Au, Pt. As the metallic state is more stable than oxide it decomposes back into the metal and oxygen. Hence oxidation corrosion is not possible with noble metals.
(iii) Volatile oxide layer:
The oxide layer film volatilizes as soon as it is formed. Hence always a fresh metal surface is available for further attack. This causes continuous corrosion. MoO3 is volatile in