The velocity of corrosion from the electrochemical standpoint

It is now agreed that the wet corrosion of metals is usually an electrochemical process, accompanied by the flow of electric currents over measurable distances. Direct combination, without appreciable flow of current, is indeed met with in some cases, for instance, in corrosion by a solution of a halogen. But direct attack by dissolved oxygen would lead to the production of a film of oxide, which, being sparingly soluble, would restrain further action. Indirect electrochemical action by oxygen, in the presence of a salt, will, however, often yield soluble bodies as the primary products; thus iron in sodium chloride solution containing oxygen, fields ferrous chloride at the anodic areas and sodium hydroxide at the cathodic areas. Where these primary products meet, in presence of oxygen, they yields a mixture of hydrated iron oxides known as "rust"; but such secondary products—although almost insoluble— are usually precipitated at a sensible distance from the metal, and do not protect it from further attack. In cases where electrochemical action would lead to a sparingly soluble salt as the direct product, the attack does not develop; thus lead, although rapidly attacked by a nitrate solution in presence of oxygen, is hardly affected by a sulphate solution. The electric currents involved in the corrosion-process may be set up:— ( a ) By differences in the metallic object ( e. g ., on a "bimetallic" specimen, where one metal serves as cathode and the other as anode; or on iron partly covered with mill-scale, which serves as cathode, the exposed metal acting as anode).