Crevice corrosion of 444 stainless steel in a mildly acidic chloride solution - Detection of an anodic peak prior to onset of corrosion

The crevice corrosion of stainless steel in chloride solutions has been studied extensively and multiple explanations can be found in the literature to describe the actual mechanism. The current study attempts to provide insight on the topic by focusing on the role of electrode potential. The crevice corrosion of a passive system was investigated: Type 444 stainless steel (UNS S44400). Tests were conducted in a variety of Cl^- solutions including pH 2, 0.6M NaCl solution in which the alloy demonstrated broad passivity and a high pitting potential.

The occurrence of crevice corrosion without pitting in this system was demonstrated with conclusive evidence including a high measured current and both in-situ and post testing photographic evidence showing severe material loss by crevice corrosion.

Through the use of a novel applied potential shift method, an anodic peak was detected before the initiation of crevice corrosion. Thus an anodic peak has been detected in a passive system prior to the onset of corrosion. This observation suggests that while a change in the composition of the solution is required to develop the anodic peak, an additional step is necessary to initiate crevice corrosion. The IR mechanism defines this step as the initiation event, IR>Δφ*, when the crevice wall in contact with the acidified crevice solution is activated and corrosion begins.