Photocurrents on passive and active-passive metals

Abstract

Metals generally corrode faster in the light than they do in the dark because of the interaction of photons with the passive film. In order to explain this photo-effect, the passive film is modelled as a semiconductor. This study shows the relationship between the corrosion currents and the photocurrents (light-induced corrosion) of passive and active-passive metals. (An “active” metal generally corrodes freely, while a “passive” metal corrodes very slowly due to protection by a passive film. An “active-passive” metal has a transition where it changes from “active” to “passive” behavior.) The question is what impact do the semiconductor properties of the passive film have on the corrosion rates of the base metal? Photocurrents and corrosion currents were measured on Zr, Ti, Zn, Sn, Fe and Cu immersed in 1N sulfuric acid and/or 1N KOH. The results show that the passive region corresponds to depleted n-type semiconductor. The onset of the photocurrent (flat-band potential) always occurs after the onset of passivity. Non-reversible photocurrent voltammograms often signify the formation of a new oxide film. It is postulated that the active dissolution region corresponds to the degenerate passive film.