Effect of NH4+ on the pitting corrosion behavior of 316 stainless steel in the chloride environment

Abstract

• The effect of NH 4 + on pitting growth of 316 stainless steel are clarified. • Pitting parameters are statistically analyzed in NH 4 + -containing environment. • The high applied potential can promote the rupture of the residual passive film. • NH 4 + can promote the metastable pitting proliferation and transition process. • The pitting growth is controlled by diffusion. Statistics analysis method is used to study the corrosion process of 316 stainless steel (316 SS) in NH 4 + -containing environments. The pitting corrosion develops through three stages: nucleation, metastable pitting propagation and stable pitting propagation. The research results show that NH 4 + does not significantly promote the nucleation process and the number of nucleation sites. However, the hydrolysis of NH 4 + makes the solution weakly acidic and further slightly increases the degree of acidification of the solution, NH 4 + can significantly promote the growth process of metastable pitting corrosion that has been formed. Positive shift potential can stimulate more active points on the surface of 316 SS, promote pitting initiation, and increase the nucleation number of metastable pitting. In the growth and propagation process of metastable pitting, the high applied potential can promote the rupture of the residual passive film above pits. In the process of stable pitting growth and propagation, the pit growth is controlled by diffusion and has nothing to do with applied potential and solution ions, and the pit growth is mainly along the depth direction.