The colours due to thin films on metals

Abstract

In recent papers it has been shown that the corrosion of metallic objects partly covered with liquid is usually connected with electric currents set up through differences in the oxygen concentration at different parts of the metallic surface. When, for instance, a drop of sodium chloride solution is placed on a metallic surface, a current passes between the surface of the metal at the periphery of the drop and the central portion. The metal in the interior, where the concentration of oxygen is least, constitutes the anode and suffers corrosion. The peripheral zone, to which oxygen has direct access, forms the cathode, and suffers little or no corrosion in the ordinary sense of the word; the immunity, or “passivity,” of this outer zone, and the local alteration of the potential, which causes the current to flow between the outer and inner portions, is best ascribed to the formation of an oxide film or oxygen film over the “aerated” part of the surface. On iron this film is invisible, presumably because it is too thin to affect the optical properties of the surface; on copper, on the other hand, there may be seen upon the portions of the surface nearest to the edge of the drop, a series of bright rings roughly concentric with one another, green, rose and yellow in colour. It seems likely that these colours are actually due to a thin film of cuprous oxide which protects the underlying metal from further attack, and which also, by altering the potential, stimulates corrosion at the central parts to which oxygen does not diffuse readily. Since the anodic product of copper in a chloride solution is cuprous chloride, an efficient absorbent of oxygen, it follows that when once this salt has begun to accumulate at the central portion of the drop, the corrosion due to “differential aeration currents” will be stimulated.