Counterintuitive passivation role of manganese in stainless steel under high potential in neutral NaCl solution

Abstract

The passivation behavior of a cast Fe-20.7Cr-17.7Mn-20.2Co-1.7Si alloy in a 3.5 wt% NaCl solution was investigated with an inclined focus on the passivation role of Mn. A two-stage passivation process was observed: initial Cr-based passivation, followed by Mn-based secondary passivation around 710 mV/SCE. As the potential increases, oxides in the passive film gradually transform into hydroxides, and the film itself becomes more defective. MnO2 formation drives the secondary passivation, enhancing corrosion resistance by reducing defects and increasing the thickness of the passive film. This study provides insights into the unexpected passivation behavior of Mn at high potentials.