The breakdown of the air-formed oxide film on iron upon immersion in solutions of pH 6–13

The breakdownof the air-formed oxide film on iron is greater upon immersion in corrosive than inhibitive solutions; it is retarded by dissolved oxygen and in some solutions marginally affected by the age, or thickness,of the film. The breakdown is not due to direct solution, as with aluminium, but to reductive dissolution, a two-stage process, in which ferric ions are first reduced to ferrous ions, which then pass into solution as hydrated cations. It follows that inhibition will occur if either process is impeded.