Effects of tungsten and nickel on repassivation rate of stainless steels in chloride solution by electrochemical method

Abstract

The recent development of the method for analyzing repassivation rate by a rapid scratch test and high field conduction model makes it possible to predict SCC susceptibility more quantitatively but it is impossible to distinguish the repassivation rates with a little difference due to the inaccurate measurement of the scratched surface area. In addition, the electrochemical method previously reported as a method to remove passive film also makes it difficult to analyze repassivation rate by the high conduction model due to hydrogen evolution at the removing process. This study is to propose the new test conditions based on an electrochemical method and to get a better understanding of alloying elements on the repassivation rate of stainless steels. From the scratch test and the proposed electrochemical (potential step chronoamperometry: PSC) test, the function of nickel and tungsten affecting the repassivation rate is due to blocking the passivation by metal not participating in the formation of passive film and stabilizing the film by molybdenate (MoO42-) with accelerating the dissolution rate of iron by tungsten, respectively. In addition, the PSC test can be approved a technique for simulating the repassivation rate by the comparison of the results from two tests. Finally, the relationship between protectiveness of passive film and repassivation rate was mentioned.