Impressed current cathodic protection of stainless steel in molten NaOH at 600 °C

This study explores the feasibility of impressed current cathodic protection to mitigate corrosion of AISI 316L stainless steel in molten NaOH at 600 °C. Dynamic polarization and chronoamperometry with and without impressed current were conducted, followed by gravimetric analysis and characterization of corrosion products. Results indicate that impressed current cathodic protection reduces the corrosion rate of AISI 316L at potentials near the equilibrium potential (−0.1 V vs. OCP and −0.2 V vs. OCP), however a significant increase in corrosion rate was observed at −0.4 V vs. OCP. It is reasoned that the increased cathodic dissolution at −0.4 V vs. OCP is attributed to the formation of sodium oxide (Na₂O), which subsequently reacts with iron to form sodium ferrite (Na₄FeO₃) being soluble in the salt. After chronoamperometry for 48 h stainless steel formed a two-phase corrosion product layer, composed of an oxide phase consisting of chromium, iron, nickel, sodium, manganese, and molybdenum. Metallic bands comprised of iron were observed within the corrosion product layer without cathodic protection and at −0.1 V vs. OCP, whereas both nickel and iron metallic bands and nodules were observed after −0.2 V vs. OCP and −0.4 V vs. OCP.