Pitting of nickel during anodic galvanostatic charging in Na2SO4 solutions

The anodic galvanostatic charging technique has been used to study the pitting of nickel in acid and neutral Na₂SO₄ solutions containing various concentrations of Cl^?. Pit initiation is indicated by a change in potential in the cathodic direction, this potential being substantially higher than that required to sustain pitting. The steady-state pitting potentials are in the passive, rather than the active, potential region, a result which can be explained by the highly localized nature of the pitting process. Increasing the Cl^?] lowers the potential for pit initiation as well as the steady-state pitting potentials. The influence of anodic charging rate (i^g_a) on pit initiation depends on the Cl^?] in solution; at high Cl^?] the charging curves are similar except for the potential shift with i^g_a, while at low Cl^?] pit initiation is more difficult with higher values of i^g_a. The minimum Cl^?] required for pit initiation increases with decreasing solution temperature, ie with decreasing solution aggressiveness. The potential for pit initiation tends to be higher at lower solution temperatures and/or at higher pH's Cl^?], anodic charging rate, solution temperature and pH influence pit initiation by affecting the efficiency of oxide repair at local breakdown sites in the passive oxide film.