Nitrate on localized corrosion of carbon steel and stainless steel in aqueous solutions

Abstract

Nitrate is significant for corrosion process, which has been extensively studied for decades, while some issues remain unclear. Adsorption and reduction mechanisms are widely proposed but separately in literature, and results are inconsistent regarding different corrosion modes and conditions. Adsorption mechanism of nitrate, mostly as competitive adsorption against chloride, is discussed regarding electrode potential, concentration, temperature and alloy composition. However, it has to be based on a premise that nitrate cannot stimulate corrosion, which cannot be fully interpreted by adsorption mechanism. This inadequacy is made up by nitrate reduction mechanism, consuming protons and producing water to lower aggressiveness. Based on these mechanisms, nitrate effect on localized corrosion of carbon steel and stainless steel in aqueous solutions are reviewed in details, including pitting, crevice corrosion and cracking. Nitrate tends to passivate salt-covered pit, but not salt-free pit since proton migration across salt layer is unavailable. However, we suggest that sufficient nitrate may passivate salt-free pit through reduction alone, while further work is required and suggested. Effect of nitrate on crevice corrosion is relatively consistent, which tends to inhibit dissolution against chloride. Nitrate reduction and adsorption are also important for cracking, mostly initiated from intergranular corrosion for carbon steels due to imperfect passivation, and from localized corrosion for stainless steels. Competition between localized corrosion and cracking can interpret inhibitive/non-inhibitive/promoting effect of nitrate. Corrosion products within a local site dissolving in nitrate are also discussed, which inhibit mass transport but enhance stability of dissolution. Nitrate is generally classified as inhibitor but described as dangerous inhibitor.