Passivity and pitting of single-phase FeCoNi-Cr-Al compositionally complex alloys: An Al-Cr Third Element Effect in sulfuric acid containing chloride

The synergistic effects of Al on Cr(III) passivation in a family of single phase FCC (FeCoNi)100-x-yCrxAly (at%) compositionally complex alloys (CCAs), where x = (10, 13, 16) and y = (0, 3, 6, 9) were investigated for both passivation and resistance to chloride induced local breakdown of the passive film. Homogenized solid solution alloys containing Cr and small amounts of Al achieved excellent passivation compared to Cr containing CCAs alone by enriching both Cr and some Al on the surface in chloride containing sulfuric acid solutions. For example, an alloy containing 10 at% Cr and 9 at% Al demonstrated better repassivation as well as resistance to chloride induced pitting corrosion than a CCA containing just 10 at% Cr. Further, compared to 304L stainless steel containing 20 at% Cr, a CCA containing 16 at% Cr and 3 at% Al attained better passivation and improved pitting resistance in 1 M NaCl + 0.1 M H2SO4(aq.) solution. Two underlying mechanisms are proposed to explain the beneficial effects on passivity. The observed Al-Cr synergy can be understood as a type of third element effect. Enhanced passivation is enabled by surface enrichment of Cr promoted by Al enrichment, resulting in mixed solid-solution Al(III)-Cr(III) oxides. Further, the study also considers the influence of Cr–Cr pairs brought about be Al addition, facilitated by chemical short-range ordering.