Growth of Thick Anodic Oxide Films on Nickel in Borate Buffer Solution

Abstract

Anodic galvanostatic charging of nickel in pH 7.65 borate buffer solution produces a finely porous oxide film which is distinctly different from the passive film of . The porous film which is probably can grow to thicknesses in excess of 1000Å. It is reduced to a lower oxidation state during galvanostatic reduction, giving a distinct cathodic potential arrest. The oxide, however, remains on the surface during the cathodic treatment and reapplication of the anodic charging current results in an anodic arrest that is almost the mirror image of the cathodic arrest. The reversible conversion charge increases with increasing time of polarization in the evolution region, in conjunction with an increase in porous film thickness as determined by x‐ray emission and Auger electron spectroscopies. In fact, the conversion charge is a close monitor of porous film thickness over a wide range of thicknesses and has been used to study film growth kinetics as a function of temperature, charging rate, and prior electrode treatment. The kinetics of porous oxide growth are interpreted in terms of an outer porous film growing on top of an inner compact film, the latter probably being the passive film. It is suggested that breakdown and repair events in the compact inner film generate the Ni2+which is incorporated into the porous film, and that the stability of the passive film thus determines the growth rate of the porous film. This model explains the influences of temperature and electrode surface condition on oxide growth rate as well as the existence of breakaway‐type kinetics and its dependence on charging rate.