Microstructures and Corrosion Behaviors of Non-Equiatomic Al 0.32 CrFeTi 0.73 (Ni 1.50−x Mo x)(x = 0, 0.23) High-Entropy Alloy Coatings Prepared by the High-Velocity Oxygen Fuel Method.

The non-equiatomic Al_0.32CrFeTi_0.73(Ni_1.50−xMo_x) (x = 0, 0.23) high-entropy alloy (HEA) coatings were prepared by the high-velocity oxygen fuel (HVOF) method. The microstructures and corrosion behaviors of the HVOF-prepared coatings were investigated. The corrosion behaviors were characterized by polarization, EIS and Mott-Schottky tests under a 3.5 wt.% sodium chloride aqueous solution open to air at room temperature. The Al_0.32CrFeTi_0.73Ni_1.50 coating is a simple BCC single-phase solid solution structure compared with the corresponding poly-phase composite bulk. The structure of the Al_0.32CrFeTi_0.73Ni_1.27Mo_0.23 coating, combined with the introduction of the Mo element, means that the (Cr,Mo)-rich sigma phase precipitates out of the BCC solid solution matrix phase, thus forming Cr-depleted regions around the sigma phases. The solid solution of large atomic-size Mo element causes the lattice expansion of the BCC solid solution matrix phase. Micro-hole and micro-crack defects are formed on the surface of both coatings. The growth of both coatings' passivation films is spontaneous. Both passivation films are stable and Cr₂O₃-rich, P-type, single-layer structures. The Al_0.32CrFeTi_0.73Ni_1.50 coating has better corrosion resistance and much less pitting susceptibility than the corresponding bulk. The corrosion type of the Mo-free coating is mainly pitting, occurring in the coating's surface defects. The Al_0.32CrFeTi_0.73Ni_1.27Mo_0.23 coating with the introduction of Mo element increases pitting susceptibility and deteriorates corrosion resistance compared with the Mo-free Al_0.32CrFeTi_0.73Ni_1.50 coating. The corrosion type of the Mo-bearing coating is mainly pitting, occurring in the coating's surface defects and Cr-depleted regions. [ABSTRACT FROM AUTHOR]