Factors governing passivation behavior of Fe-Cr-Al-Ti alloys in sulfate containing acidified solutions: Uncovering the many roles of Ti

Passivation induced corrosion resistance of non-equiatomic BCC Fe-Cr-Al-Ti alloys, including critical passivator concentration, the roles of each alloying element, and possible synergies between the passivating components, i.e., Ti, Cr, and Al are presented. Two solid-solution BCC alloy series: Fe-8Cr-8Al-xTi and Fe-xCr-{16-x}Al-8Ti, where x was varied from 0 to 16 at.%, were investigated in acidified 0.1 M Na₂SO_4(aq) solutions. Short- and long-term passivation behaviors and oxide passive film chemistry were characterized. Alloys with at least 4 at.% Ti with 8 at.% both Cr and Al; or 8 at.% Ti with a Cr/Al ratio higher than 0.5 exhibited excellent corrosion resistance in pH 1 solution having a lower currrent density than 304L stainless steel. Ti additions to Fe-Cr-Al alloys were predicted to not only function as a passivating species but also to act as a species that alters the chemical short-range order by clustering of Cr-Cr pairs in their 1^st nearest neighbors’ arrangement. This could be forecast using a “Coherent Ordering Descriptor” based on Monte Carlo simulations supported by first-principles Density Functional Theory calculations. The possibility of a new Ti containing highly corrosion-resistant low-cost stainless steel with only 4 at.% Ti and 8 at.% Cr while using 88 at.% inexpensive Fe and Al is suggested. Two underlying mechanisms describing the enhanced passivation behaviors are discussed based on the chemical short-range ordering of Cr atoms, and passive film stability provided by mixed oxide species, suggesting concepts useful for designing new lightweight corrosion-resistant alloys.