Introduction.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, yields ferrous chloride at the anodic areas and sodium hydroxide at the cathodic areas. Where these primary products meet, in presence of oxygen, they yield a mixture of hydrated iron oxides known as " rust " ; but such secondary products-although almost insolubleare 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 u p :-(a) By differences in the metallic object (e.g., on a " bimetallic 55 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). (b) By differences in the liquid, notably differences in oxygen-concentration, which cause the " aerated " part of the metal to be cathodic and the " unaerated " to be anodic.These " differential aeration currents "f can themselves, however, be referred to differences in the state of repair of an " invisible " oxide-film present on * J. A. N. Friend and J. S. Tidmus, ' J. Inst. Met.,' vol. 31, p. 182 (1924). t J. Aston, ' Trans. Amer. Electrochem. Soc.,' vol. 29, p. 449 (1916) the metallic surface.* On some materials (aluminium or stainless steel) this film confers definite protection from many liquids. Even ordinary iron or steel exhibits passivity to certain reagents after oxidising treatment, but the film produced by exposure to air gives no permanent protection. For such films tend to break down locally, and only at places where oxygen (or an oxidising agent) is present in excess do they remain " in repair. "f Thus the film on ordinary iron, although not " protective 55 in any real sense, does serve to divert corrosion to the parts least accessible to oxygen. The potential is slightly higher at the " aerated " places, where the film is kept in good repair ; J these parts are therefore cathodic towards the " unaerated places," and electric currents flow between the aerated and unaerated areas ; corrosion only con tinues so long as oxygen is supplied to the cathodic region, but the attack actually occurs at the anodic parts which are ...
