A.c. corrosion of nickel in sulphate solutions

Abstract

The corrosion of nickel under the influence of alternating current (a.c.) in pH 1–7 Na₂SO₄ solutions has been investigated with an alternating voltage (a.v.) modulation technique. The anodic polarization curves and pitting behaviour of nickel were examined with the superimposition of sinusoidal, square, and triangular a.v. over a range of a.v. magnitudes of 0–5000m V r.m.s., and a.v. frequencies of 20–5000 Hz. It was found that a.v. reduced the passive d.c. potential regime by increasing the critical current density prior to passivation and by shifting the active-passive transitional potential toward the positive direction. In acidic sulphate solutions, a.v. increased the current density in the passive regime in a way similar to the addition of chloride ions. A.v. destroyed the passivity and enhanced the pitting corrosion of nickel at both the passive and transpassive d.c. potentials. The passivity of nickel was destroyed regardless of the a.v. waveforms. Triangular a.v. caused the severest destruction of passivity; this was followed by sinusoidal and square wave a.v.