Corrosion behaviour of HVOF-sprayed TiC + high-entropy alloy (HEA) hardmetal coatings

Hardmetal coatings with a High-Entropy Alloy (HEA) matrix have been shown to provide intermediate wear performance between conventional WC-CoCr and Cr3C2-NiCr compositions. Thus, they may overcome the traditional dichotomy between a more performing material that is however based on strategic (W, Co) and carcinogenic (Co) elements, and one based on less critical materials but with poorer performance. However, little is known about their corrosion response in aqueous environments. Therefore, in this work, we selected four High-Velocity Oxygen Fuel (HVOF)-sprayed HEA + 60 vol% TiC coatings, namely Cr20Mn25Fe40Ni15 + 60TiC, Al14(Cr20Mn25Fe40Ni15) + 60TiC, Al0.5CuCrFeNi2 + 60TiC and CrMnFeCoNi+60TiC (matrix compositions in atomic ratios), and we investigated their corrosion behaviour in a common model environment, a 3.5% NaCl (wt./vol.) aqueous solution, through electrochemical polarization and chronoamperometry tests. We found that the electrochemical polarization responses of all HEA-based hardmetals were much more similar to each other than their widely different matrix compositions would have suggested. Coupling with the TiC hard phase, indeed, had a levelling effect, promoting pseudo-passivation of compositions where the matrix alone would hardly exhibit any passivity, such as Al14(Cr20Mn25Fe40Ni15), while worsening the response of better-passivating matrices. There were, however, differences in the long-term stability of the pseudo-passive state, studied by potentiostatic chronoamperometry. Al0.5CuCrFeNi2 + 60TiC and CrMnFeCoNi+60TiC experienced progressive destabilization during 3 h of testing at +0.15 V vs. OCP, while Cr20Mn25Fe40Ni15 + 60TiC and Al14(Cr20Mn25Fe40Ni15) + 60TiC developed a more stable pseudo-passive film based on Fe, Cr, and Ti oxides (from XPS analyses), which was destabilized only at higher anodic overpotentials of +0.20 and + 0.25 V vs. OCP.