Effects of Anions on the Dissolution Kinetics of Metals

In acidic solutions , , and ions increase the true anodic dissolution rate of cadmium and of indium amalgam considerably. This is accounted for by the direct participation of these ions in elementary processes of ionizing metal atoms. The chemoadsorptive interaction of halide ions with surface atoms of a metal takes place at potentials much more negative than the dissolution potential of the metal. The extent of "filling" the surface with adsorbed anions increases considerably with shift of potential to more positive values. Some concepts are developed about the mechanism of dissolution of metal with direct participation of the solution components. These concepts explain both the dissolution rate acceleration by ions that specifically adsorb and the passivation of a metal surface by adsorbed oxygen from water. The mechanism of formation and development of pits on the surface of a corroded metal has been considered, zirconium dissolution in chloride, bromide, and iodide solutions being an example. It is postulated that a necessary condition for formation of pits is a greater affinity of the corroded metal for oxygen than for the attacking anion and that formation of pits is a result of the substituted adsorption of halide ions for passivating oxygen at certain sites on the metal surface.