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Describe the types of oxide film formed in oxidation corrosion

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The extent of corrosion depends upon the type of oxide film formed by metals. Oxide films formed are classified into three categories,

 

Stable Oxide film: A stable oxide film consists of fine grains in its structure & it can get adhered tightly to the metal surface 

 

Unstable Oxide film: - Where the oxide film formed is unstable & it decomposes back into metal & oxygen.  e.g. Oxides of metals like Gold, Silver, and Platinum etc. Hence metal forming unstable oxide film does not get covered.

Volatile oxide film:- When the oxide film formed is volatile it vaporizes as soon as it is formed, the underlying metal surface is exposed for further attack of oxygen & thus causes continue & excessive corrosion. e.g. Molybdenum metal form its oxide MoO3, which is volatile

 

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