Top layer of any metal is always getting exposed to atmosphere and thus it will be the first part of any metal where corrosion starts. In some cases if such corrosion product is insoluble then this corrosion layer act as barrier for further corrosion to occur but if this corrosion product is unstable in that case corrosion will remain continue until entire metal get corroded. The mechanism of corrosion reaction is best explained by Zaki Ahmed as follows;