Design of corrosion-resistant alloys for preventing oxidation-induced nanoscale Cr-depletion by inclusion engineering

Abstract

Due to the preferential oxidation of Cr in various corrosion-resistant alloys, Cr-depletion extensively forms around the oxides and impairs passivation ability. This harmful effect has been widely recognized, but available solutions are limited. Here, to prevent such oxidation-induced Cr-depletion in corrosion-resistant alloys, we proposed a general strategy based on inclusion engineering using oxygen-preferred elements in alloy design. The inclusion-Cr depletion-passivation relationship was systematically studied in 3.5 wt% NaCl using commercially cast original, Si-modified, Al-modified, and Ti-modified CoCrFeMnNi alloys, in which the prominent inclusions were identified as MnCr2O4, MnO-SiO2, AlN, and TiN, respectively. Unlike the original alloy, all modified alloys prevented Cr-depletion with improved corrosion resistance; however, the Si-modified alloy exhibited the most remarkable improvement due to few nitride by-products. Thermodynamic analysis on the free energy changes of potential reactions revealed that all the additives were preferential to be oxidized over Cr, whereas Al and Ti nitridation reactions have the most negative free energies compared to all oxidation reactions, which validated experimental results. From experiments and thermodynamics, an effective method was proposed, and its application was discussed with three newly-designed alloys prepared using arc melting. We believe the proposed strategy can be widely used in designing alloys to prevent oxidation-induced Cr-depletion.