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
