Influence of an Applied Field on the Tarnishing of Silver and Copper in Iodine Vapour

Abstract

The tarnishing of silver and copper in iodine vapour at 30°c and at pI₂ = 62·5 Nm⁻² has been studied under an applied electric field and under short-circuited conditions. The electric field was applied across the growing iodide layer by pressing a platinum mesh over the outer surface, to act as one electrode, the metal specimen being the other electrode. Under the influence of an electrical field of either polarity, and also under short-circuited conditions, the growth for both systems, Ag–I₂ and Cu–I², conformed to a parabolic law as occurs under normal iodination.

In the Ag–I² system, increased rate of tarnishing was observed both under cathodic (specimen negative) and short-circuited conditions, compared to that under normal iodination; under anodic (specimen positive) conditions, the rate is decreased. This behaviour under the action of a field has been explained on the basis of positive hole migration. The increased rate under short-circuited conditions can be explained on the basis of cation migration through the film. An exploratory run with a Ag–4·1% Cd alloy at 100°c showed a decrease in rate under cathodic conditions. This suggests that the rate determining step changes from hole migration to ion migration, with a large increase in inherent defect concentration, as a result of the dissolution of divalent Cd in the growing iodide layer.

In the Cu–I² system, an increased rate of growth was found under cathodic, anodic and short-circuited conditions as compared with that under normal iodination. This has been explained on the basis of cation migration as in normal iodination; the parabolic growth has been assumed to be a resultant of both the mechanisms of Mott–Cabrera and of Wagner.