Understanding localized corrosion of Ni- and Fe-based alloys in 280 °C mildly acidic sulfate environments

The influence of sulfur on the localized corrosion susceptibility of Alloys 600 (Ni-16Cr-9Fe), 690 (Ni-30Cr-10Fe), and 800 (Fe-32Ni-21Cr) was investigated and compared in 280 °C mildly acidic sulfate solutions. Variations in chromium content impacted the development of passivity and localized corrosion resistance of the alloys. Alloy 690, with the highest chromium content, demonstrated superior passivity and pitting resistance, with sulfur incorporated in the surface oxide layer. In contrast, Alloys 600 and 800 showed increased susceptibility to intergranular corrosion and pitting, respectively, with sulfur present at the oxide-metal interface (penetrated past the surface oxide). The density of pits or intergranularly attacked regions per unit area increased with a decrease in chromium content. Microscopic analysis of the passive films on each alloy revealed nanoscale differences in adhesion, oxide chemistry, and pitting behavior/susceptibility. These findings demonstrate the role of alloying elements and the effect of sulfur on a passive film.