Comparative study of the pitting corrosion resistance, passivation behavior and metastable pitting activity of NO7718, NO7208 and 439L super alloys in chloride/sulphate media

Abstract

Comparative study of the corrosion resistance and passivation characteristics of NO7718, NO7208 and SS439L super alloys in 4 M H2SO4 media at 0–6% NaCl concentration was done by potentiodynamic polarization, open circuit potential measurement and optical microscopic analysis. The metal alloys showed relatively high resistance to corrosion as variation in Cl− ion concentration had no significant influence on their corrosion rate results. SS439L showed the weakest resistance to pitting despite its relatively low corrosion rate. Passivation behavior disappeared after 2% NaCl concentration. NO7208 displayed exceptional pitting resistance characteristics, however, its metastable pitting and repassivation domain responds to changes in Cl− ion concentration. The passivation domain of NO7718 significantly decreased with increase in Cl− ion concentration due to delayed repassivation of the protective film after metastable pitting. NaCl concentration caused significant negative shift in open circuit potential (OCP) for NO7718 and SS439L alloys, compared to NO7208. The OCP of NO7208 at comparatively positive values contrasts those of NO7718 and SS439L alloys. Micro analytical studies shows a highly corrosion resistant morphology for NO7208 with numerous macro-pits present but their effect on the overall corrosion resistance of the nickel alloy is negligible. The corrosion pits on SS439L coupled with the appearance of grain boundaries are deleterious to the overall viability of the alloy. Enlarged corrosion pits are present on NO7718 due to the action of Cl− ions. Keywords: Corrosion, Pitting, Passivation, Alloy