CORROSION is defined as the oxidation of a substance; it may be produced by chemical or electrochemical means. The following facts are difficult to explain on a purely electro-chemical theory of corrosion: (a) Certain depolarisers do not increase corrosion, but actually inhibit it; (b) the conductivity of electrolytes is not directly connected with the amount of corrosion; (c) Lambert's pure iron is readily attacked by sodium chloride solution and dilute acids; and (d) the presence of ions of the corroding metal sometimes increases corrosion. The order of corrodibility of metals in distilled water, certain salt solutions, and non-electrolytes is different from their order in the electro-chemical list; this suggests that there are factors interfering with the electrochemical action. Such factors are scale formation, and the nature and distribution of the products of corrosion. The effects of strain and impurity in the metal are considered on the electro-chemical view to be of fundamental importance. Experiments on Lambert's pure iron and lead showed that the effect of strain is a minor and ephemeral factor in corrosion in neutral solutions; a trace of impurity appears to assist local corrosion, but the amount of corrosion is not proportional to the amount of impurity. The effect of a trace of impurity is probably a trigger action. The main function of oxygen in corrosion is not that of a depolariser, but rather to oxidise the metal directly, and also in some cases the products of corrosion.