Pitting on the crevice wall prior to crevice corrosion: Iron in sulfate/chromate solution

Experimental results are presented for the induction period that precedes the onset of crevice corrosion. In situ visual examination and ex situ microscopy revealed that the first corrosive attack on the (passive) crevice wall occurs as a pit near the bottom of the crevice. Additional pits and corrosion product form higher and higher on the crevice wall during the induction period. The measured current is initially low in the passive range (10 μA), increasing gradually, but the measured current does not record the large increases in anodic current that produce the pits. The latter result and the observation that gas bubbles form within the pits indicate that the pitting initially is a local cell process of metal dissolution and cathodic reactions on the sample, including H2 formation which occurs inside the pits in this iron/sulfate-chromate solution (pH 8.8, room temperature). Then, a transition from merging pits to corrosive attack across the width of the crevice wall occurs accompanied by a much steeper increasing current. This latter morphology (horizontal boundary that moves up the wall towards the crevice opening, heaviest attack just below this boundary and passive crevice wall above this boundary) is representative of stable crevice corrosion of the IR type reported in the literature.