Influence of chloride ions on the open circuit potentials of chromium in deaerated sulfuric acid solutions

Abstract

Open circuit potential measurements and cyclic voltammetry of chromium in deaerated aqueous H2SO4 solution of pH 1 without and containing NaCl in the concentration range 0.5 - 4 M revealed that chromium exhibits two stable open circuit potentials having the character of a Wagner-Traud corrosion potential. One, E corr.1, was established on the passive surface formed by exposing Cr previously to air or passivated potentiostacially in a controlled manner, and the second one, E corr,2, at the bare Cr surface obtained by prolonged cathodic activation. There is a small difference in the E corr,1 values as a function of the passive layer properties. Addition of NaCl accelerates the hydrogen evolution reaction on the passive surface to some extent, while the same reaction on the bare surface was not affected by NaCl. On the other hand, presence of NaCl accelerates the anodic reaction on the bare surface, and it activates the dissolution of the passive layer so that the passive currents increase with addition of NaCl. This effect is so large that at concentration of NaCl larger than 3 M, the destruction of the passive layer was so fast that in a matter of seconds the Cr was activated, and the only one stable corrosion potential observed was E corr.2. No pitting of Cr in the presence of NaCl was observed up to the transpassive potentials.