Investigating the Long-Term Synergistic Benefits of Al on Cr(Iii) in the Passive Films of Feconi-Cr-Al Ccas in Sulfuric Acid

Abstract

This work explores the synergistic effects of Al on Cr(III) passivation and re-passivation behaviors in solid solution (FeCoNi)100-x-yCrxAly (at.%) complex concentrated alloys (CCAs). Homogenized alloys containing 10 at.% Cr fixed below the stainless steel threshold of 12 at.% achieved excellent passivation by forming a Cr(III) rich film when alloyed with small amounts of Al. For example, 10 at.% Cr containing CCAs with 6 at.\% Al can attain comparable re-passivation properties as seen in 304L stainless steel in sulfuric acid. Potentiostatic re-passivation over 10 ks exposure revealed the benefit of Al addition leading to higher Cr(III) cation fractions in the film, better corrosion protection, and faster film formation tendencies. Two underlying mechanisms are proposed to explain the effect. The observed Al-Cr synergy can be understood as a type of third element effect behavior of Al with enrichment of Cr(III) in the passive films because of a small but beneficial presence of Al (III). Further, we examine the potential effects of various alloying elements in CCAs by investigating binary and ternary alloys, focusing on their long-term passivation behavior and their influence on the chemical short-range ordering of Cr-Cr pairs by Al addition. These could potentially contribute to the design of corrosion-resistant CCAs in the future.

Keywords: Compositionally Complex AlloysAqueous PassivationThird element effectcorrosion resistanceXPSFirst principles