The Effect of F− on the Anodic Passivation of Nickel

The anodic behavior of nickel is studied in solution of p H 3.0 and 5.0. The presence of F − in solution interferes with the potentiostatic establishment of passivity, with currents only falling to values in the mA cm −2 range. In the absence of any prior cathodic reduction, potentiostatic immersion of electropolished nickel into F − solution results in an initial decay of the current to a fairly low value followed by an increase to higher steady‐state values. The delay in establishing steady‐state conditions is dependent on the solution p H, with the delay time increasing with increasing p H. Similar delays are observed even on initially oxide‐free surfaces if the potential of anodization is sufficiently high. The delays in establishing steady‐state conditions are especially noticeable in potential switching experiments, and a "stable" and "unstable" state of the surface can be defined for each potential. The results are explained in terms of a surface oxide film that contains an amount of incorporated F − that is determined by the potential and [F − ] in solution. The delays are believed to be associated with the slowness of the rate of F − incorporation and exclusion from the oxide film lattice relative to changes in the oxygen component of the film. Auger electron spectroscopy provided a semiquantitative indication of the amount of F − incorporated into the film, and the results suggest that the amount increases with the potential of anodization.