Investigating the synergistic benefits of Al on Cr(III) in the passive films of FeCoNi-Cr-Al compositionally concentrated alloys in sulfuric acid

RIP2025-00064: The synergistic effects of Al on Cr(III) passivation in a family of (FeCoNi)100-x-yCrxAly (at.%) complex concentrated alloys (CCAs), where x = (0, 4, 10, 13, 16) and y = (0, 3, 6, 9, 13) were investigated. Homogenized solid solution alloys containing 10 at.% Cr intentionally selected below the stainless steel threshold of 12 at.% achieved excellent passivation by forming a Cr(III) rich film containing Al(III) when alloyed with small amounts of Al. For example, CCAs containing 10 at.% Cr and 6 at.% Al can attain comparable re-passivation properties as seen in 304 L stainless steel in sulfuric acid. Potentiostatic re-passivation over 10 ks exposure revealed the benefit of Al addition leading to greater Cr(III) cation fractions in the film, better corrosion protection, and faster film formation. Two underlying mechanisms are proposed to explain the effect. The observed Al-Cr synergy can be understood as a type of third-element effect due to Al where enrichment of Cr(III) in the passive films occurs due to the small but beneficial presence of Al (III). Al is also a secondary passivator detected in the passive film. Moreover, the potential effects of other alloying elements in CCAs were examined by investigating binary and ternary alloys containing Cr and Al. These studies also focused on the influence of Cr-Cr pairs by Al additions on Cr-Cr clustering in solid solution during their steady-state passivation, facilitated by chemical short-range ordering.