The mechanism of passivation of nickel as determined by an automatic recording ellipsometer

Abstract

The process of anodic passivation of nickel in acid solutions has been investigated by means of an automatic recording ellipsometer in which the light emerging from a rotating analyzer has been electronically processed to yield a continuous read-out of the changes of polarization state of light reflected from a nickel anode during a galvanostatic transient. Digital computer analysis of the ellipsometric data has revealed: (a) the existence of an induction time for film formation; (b) a linear growth law for the film during a time interval commencing with the induction time and ending before the passivation time; (c) the occurrence of passivation when the absorption coefficient of the film jumps from a near-zero value to a fractional value; and (d) continuous changes of the refractive index of the film during the period of film growth. These phenomena are consistent with a model in which (a) a pre-passive film forms and thickens by a dissolution–precipitation mechanism, (b) the growth of the film is accompanied by its electrochemical oxidation, (c) the film growth–oxidation process continues until the pre-passive film is transformed into an electronically-conducting material, and (d) the conversion of the pre-passive film into an electronic conductor constitutes the essence of the so-called active–passive transition.