Revisiting the crevice corrosion of stainless steel and aluminum in chloride solutions---The role of electrode potential

Although the crevice corrosion of stainless steel and aluminum in chloride solutions has been studied extensively, there is little agreement on the mechanism of crevice corrosion for these materials. The present study attempted to resolve the ambiguity of the crevice corrosion mechanism by first reviewing previous relevant studies and then, by conducting an in-depth study of crevice corrosion with a focus on electrode potential and the IR mechanism. The crevice corrosion of aluminum (99.999 wt. %, AA2024) and stainless steel (304 and 444) in neutral and mildly acidic chloride solutions was investigated. For each alloy system, polarization curves were generated in simulated crevice solutions and potentiostatic crevice corrosion experiments with varying crevice gap opening dimensions were performed. Current and electrode potential along the crevice wall were measured. In-situ photographs of the developing crevice corrosion were obtained and in-situ and ex-situ pH measurements were performed. For the stainless steels, potentiodynamic and potentiostatic methods, including a novel applied potential shift method, were used in an attempt of detect the development of an anodic peak in a passive system, prior to the onset of crevice corrosion.;Crevice corrosion of aluminum in a pH 6, 0.6M NaCl solution was not found to occur under open circuit or polarized conditions. Polarization curves in simulated crevice solutions did not show an anodic peak. Based on the polarization behavior, the IR mechanism correctly predicts the absence of crevice corrosion for this system.;Crevice corrosion was observed for aluminum in a mildly acidic (pH 3), chloride free solution and found to be consistent with the IR mechanism. In this solution, crevice corrosion resulting in severe material loss was observed and a high current (10 mA) was measured. This is the first time crevice corrosion of aluminum, as indicated by significant material loss and high measured current, has been reported. Also of importance, is the fact that the corrosion occurred in the absence of pitting corrosion. The cause of corrosion in this case can be explained by alkalization of the crevice due to a high rate of hydrogen evolution reaction, driven by a potential drop within the crevice.;For stainless steels, crevice corrosion was clearly demonstrated in a neutral 0.6M NaCl solution and in two mildly acidic simulated crevice solutions: pH 2, 0.6M NaCl and pH 1, 3M NaCl. The conclusive evidence of the occurrence of crevice corrosion was a high measured current and photographic evidence of showing severe material loss within the crevice and no attack on the uncreviced surfaces. All observations of the propagation of crevice corrosion for 304 and 444 were consistent with the IR mechanism.;Additional efforts to understand the mechanism of stainless steel crevice corrosion focused on studying the initiation. No evidence was found to suggest initiation occurs due to a reduction of the pitting potential from chloride accumulation. Initiation of crevice corrosion was found to occur after the development of an anodic peak. This is the first time an anodic peak has been detected prior to the onset of crevice corrosion. This novel observation provides proof that an anodic peak can develop in a passive system. Since the IR mechanism requires an anodic peak, this observation provides convincing evidence for the applicability of IR mechanism in a passive system. Equally important, the observation shows that while compositional change is required to develop the anodic peak, another step is necessary to initiate crevice corrosion. Crevice corrosion mechanisms that rely solely on compositional change are inadequate in that they do not provide an explanation for the additional step. However, the IR mechanism clearly defines this additional step as the initiation event, IR>Deltaphi*, at which point the crevice wall in contact with the acidified crevice solution is activated and crevice corrosion begins.;In summary, crevice corrosion of aluminum and stainless steels was clearly demonstrated and found to be consistent with the IR mechanism.