It is well known that when a metal is in contact with an aqueous solution, its surface state varies strongly according to the circumstances existing. This surface, which sometimes remains actually metallic, often becomes covered with a deposit or film of oxide or salt, the formation of which can protect the metal against corrosion.
With regard to iron, for example, it is established that protection manifested in certain cases results from the presence of a film containing ferric oxide (Fe2O3); this film can consist of pure Fe2O3 or a mixture of F2O3 and something else. For example, in the course of its anodic passivation in dilute sulfuric acid, iron becomes covered, as Evans1 has shown, with Fe2O3 containing an excess of oxygen. In many hard and aerated waters, like those of the majority of city water...
