The mechanism of formation of pre-passive films on nickel: a chronoellipsometric study

Abstract

The authors have previously shown that the anodic passivation of nickel in acid solutions is a two-stage process; the first stage of which involves the formation of a pre-passive film. In this study, the mechanism of formation of the precursor film on nickel in a 0.5 M K₂SO₄ (pH = 3.1) solution has been investigated by means of the technique of chronoellipsometry which consists of the ellipsometric in situ monitoring of the anode surface during a galvanostatic experiment. The main finding is that, before the pre-passive film forms, there must elapse an induction time which is affected by the current density and mass-transport factors. The nature of the dependence of the induction time upon current density indicates that the pre-passive film is formed by a dissolution-precipitation mechanism. It is proposed that the pre-passive film consists of precipitated Ni(OH)₂.