Corrosion polarization and passivation behavior of selected stainless steel alloys and Ti6Al4V titanium in elevated temperature acid-chloride electrolytes

Abstract

Abstract The corrosion polarization behavior of 439ll ferritic (439ll), 316L austenitic (316L), and NO7208 nickel-chromium-aluminum-iron (NO7208) stainless steels, and Ti6Al4V titanium (Ti6Al4V) alloys was studied in 4 M H2SO4 + 5% NaCl solution at 35 and 70°C. Corrosion rate (C R) of the alloys were generally higher at 70°C. NO7208 and 439ll alloy exhibited higher resistance to general corrosion at 35°C (0.067 and 0.050 mm/year) while Ti6Al4V was the most reactive (0.506 mm/year). Passivation behavior was evident on the plots of NO7208 and Ti6Al4V alloys. NO7208 pitted at 1.04 V with passivation range of 0.17 V. Metastable pitting occurred at 0.02 V and ceased at 0.19 V. Pitting was absent from the polarization plot of Ti6Al4V though it exhibited metastable pitting at −0.39 V and passivated at −0.21 V. At 70°C, NO7208 alloy exhibited the lowest C R (0.392 mm/year), while Ti6Al4V was the most reactive at 21.868 mm/year. C R of the alloys increased by 97.63%, 91.18%, 82.83%, and 97.69% at 70°C. Corrosion potential of the alloys shifted cathodically at 35 and 70°C signifying dominant cathodic processes. Ti6Al4V exhibited passivation behavior at 70°C with no pitting evidence. Open circuit potential measurement showed that Ti6Al4V was the most electronegative and NO7208 alloy was the most electropositive due to the significant growth of its protective oxide. Grain boundary corrosion was visible on 439ll and 316L at 35°C and total surface deterioration at 70°C. Pseudo corrosion pits were visible on NO7208 and Ti6Al4V alloy at 35°C. At 70°C, total surface degradation was visible on Ti6Al4V and grain boundary corrosion on NO7208.