Mass Transport and Electrochemical Phenomena Influencing the Pitting and Repassivation of Stainless Steels in Neutral Chloride Media

摘要

Mass transport and electrochemical parameters affecting pitting stability and repassivation in neutral chloride media were measured using the artificial pit electrode technique and the effect of different variables was studied. Sensitivity analysis showed that accurate estimation of the critical pit stability product, (i~*x)_(crit) is necessary for correctly estimating the maximum pit size that can be attained on a stainless steel surface under a thin electrolyte film. The value of (i~*x)_(crit) is directly related to the surface concentration and is some fraction of the value when a salt film is present (i~*x)_(sf). A graphical method to determine the surface concentration at the edge of pit stability was developed in terms of diffusive transport by relating repassivation potential (E_(rp)) to pit stability. It was seen that the value of 60-80% commonly accepted in the literature as the minimum degree of saturation for pitting stability may in fact overestimate the actual value. Further, it was noted that redox reactions involving dissolved minor alloying elements may significantly affect the measurement of E_(rp) in some systems.