A bipolar model of the passivity of stainless steels—II. The influence of aqueous molybdate

Abstract The passive films formed on Fe-19Cr-9Ni in solutions of 0.3 M NaCl (pH 5.4) and 0.3 M NaCl + 0.16 M Na 2 MoO 4 (pH 9.0) have been characterized by variable angle XPS. Additional studies were conducted to determine the effect of the pH change on the passive film and which agent was responsible for the deprotonation found in the film produced in the molybdate solution. It is proposed that the MoO 4 2 anions reinforce the action of CrO 4 2− in enhancing the bipolarity of the film and thereby significantly increasing the deprotonation of the inner Cr(OH) 3 region, forming a more protective Cr 2 O 3 barrier layer.