Interstitial nitrogen enhances corrosion resistance of an equiatomic cocrni medium-entropy alloy in sulfuric acid solution

Abstract

The corrosion resistance of the equiatomic CoCrNi medium-entropy alloy (MEA) and its 0.5 atomic % nitrogen alloyed variant in 0.1 M H2SO4 solution was investigated and compared with that of the 316 L stainless steel as a reference material. All of the investigated materials showed single-phase face centered cubic (FCC) microstructures, and nitrogen was fully dissolved in the solid solution structure of the CoCrNi MEA. Both the nitrogen-free and nitrogen-doped MEAs showed significantly higher corrosion resistance (lower corrosion currents and rates) than the 316 L steel. Compared to the nitrogen-free CoCrNi, the interstitial nitrogen dissolved in solid solution causes a significant improvement in the pitting corrosion resistance of the CoCrNiN. Under the same testing condition, pitting corrosion was not observed in the CoCrNiN alloy, while the CoCrNi MEA and 316 L steel showed distinct pitting cavities. The better anti-corrosion performance of the CoCrNiN compared to that of the CoCrNi is correlated with a higher fraction of chromium oxide in the passive films.