Crevice Corrosion of Type 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 (UNS S44400) stainless steel. Tests were conducted in a variety of Cl- solutions including pH 2, 0.6 M sodium chloride (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 clearly with conclusive evidence, including a high measured current and both in situ and ex situ 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. Therefore, an anodic peak has been detected in a passive system prior to the onset of corrosion. This observation suggests that while solution compositional change is required to develop the anodic peak, another step is necessary to initiate crevice corrosion. The IR mechanism defines this step as the initiation event, IR > Delta Phi*, when the crevice wall in contact with the acidified crevice solution is activated and corrosion begins.