Experimental and theoretical study of inclusions during initiation of crevice corrosion

he initiation sites of crevice corrosion on stainless steel are generally recognized to be sulfide inclusions (MnS), although the mechanism by which the initiation occurs remains unknown. Controlled solution experiments showed that the products of the MnS electrochemical oxidation reaction were elemental sulfur and thiosulfate. Spectroelectrochemical cell experiments on MnS quantified the electrochemical production of thiosulfate. This finding linked the metallurgical and solution phases and suggested a depassivation mechanism based on a critical crevice solution composition. The range of thiosulfate and chloride ion concentrations in which the stainless steel oxide film remained passive was determined. All other species present in the crevice were shown not to influence depassivation. From the proposed mechanism a mathematical model was developed to determine the position and time of breakdown in the crevice based on transport of species by migration and diffusion, electrochemical kinetics, and potential field theory. Instrumented artificial crevice cell experiments were conducted to test the validity of the model and the proposed mechanism. The crevice cell geometry had a variable crevice height of 2.7 to 9.2...