Quantitative analysis of repassivation kinetics of ferritic stainless steels based on the high field ion conduction model

Abstract

The effects of the alloying elements (Cr, Mo, W, and Ni) on the repassivation kinetics of ferritic stainless steels in deaerated MgCl2 solution at 50 degrees C were examined using a rapid scratching electrode technique. The repassivation kinetics of the alloys was analyzed in terms of the current density flowing from the scratch, i(t), as a function of the charge density that has flowed from the scratch, q(t). Repassivation on the scratched surface of the alloys occurred in two kinetically different processes; passive film initially nucleated and grew according to the place exchange model in which log i(t) is linearly proportional to q(t), and then grew according to the high-field ion conduction model in which log i(t) is linearly proportional to 1/q(t). The slope determined from the log i(t) versus 1/q(t) plot for stainless steels in chloride solution was found to be a parameter representing the repassivation rate, stress corrosion susceptibility and protectiveness of the passive film. The influences of alloying elements (Cr, Mo, W and Ni) on the repassivation kinetics of Fe-Cr stainless steels were quantitatively examined by the effects of these elements on the slope, and both the protectiveness of the passive film and the stress corrosion susceptibility of the alloys were discussed in terms of the value of the slope. (C) 2000 Elsevier Science Ltd. All rights reserved.