An investigation on the effect of molybdenum alloy element and molybdate inhibitor on the stable and metastable pits and its correlation with the pit morphology

Abstract

BackgroundMolybdenum (Mo) has been introduced as a useful alloying element by increasing the resistance to pitting corrosion of steels. To prove this opinion, the effect of this element on pitting corrosion parameters in corrosive environments can be investigated. In addition, adding a molybdate (MoO42−) inhibitor to the media can answer the question of whether there is a synergistic effect between molybdenum in the alloy and molybdate in the environment to increase corrosion resistance or not. The study holds relevance by suitable select materials based on the presence of corrosion-resistant alloying elements and inhibitors based on the reduction of environmental acidity or the occurrence of polymerization reactions for industries frequently affected by pitting corrosion. MethodsIn this paper, the influence of the molybdenum alloying element and molybdate inhibitor on the metastable and stable pits of 316 SS and 304 SS in chloride and molybdate-bearing media is studied. The evaluation is conducted using potentiodynamic and potentiostatic techniques on a flat sample. The morphology of the pits is observed through scanning electron microscopy in solutions of NaCl and Na2MoO4. FindingsThe results suggest that the presence of molybdate ions in chloride solution leads to an increase in the pitting potential and repassivation potential of both steels. Furthermore, molybdate is suggested to decrease the peak current, radius, and stability product of metastable pits. A comparative analysis between the corrosion behavior of 316 SS and 304 SS suggests that the addition of molybdenum enhances the resistance to pitting corrosion and facilitates the repassivation process. Additionally, adding molybdate in chloride solutions results in smaller stable cavity sizes without altering their appearance.