Tailoring microstructure and corrosion behavior of CoNiCrFeMoBSi high-entropy alloy coatings via Mo addition

In this paper, Fe was strategically replaced by Mo in the laser cladded Co20Ni20Cr16Fe19-xMoxB15Si10 (x = 0, 4, 8, 12, 16 at.%) high-entropy alloy (HEA) coatings via tuning Mo content and their corrosion behaviors were explored in 1 M H2SO4 solution. As Mo content increases, the microstructures of the coatings evolve from BCC, FCC and (Co, Cr, Ni, Fe)2B phases to BCC and FCC phases, followed by BCC, FCC and a phases, and finally transform to BCC, FCC, a and (Co, Cr, Ni, Fe, Mo) Si2 phases. The addition of Mo triggers the following evolutions in the order of Cr-deplete regions, uniform distribution of constituents and (Mo, Si)-rich regions. The corrosion resistance of the HEA coatings firstly enhances and then slightly decreases as a function of Mo content. The HEA coating with 8 at.% Mo demonstrates the utmost passive region of 1.117 V, the minimal Icorr of 2.755 mu A cm 2, the minimum Ip of 5.164 mu A cm 2, and the maximum passive film thickness of 13.4 nm. The exceptionally high corrosion resistance of the coating is due to the formation of less defective and denser passive films enriched with Cr2O3 and Mo6+ oxides. Significant evolutions in microstructure, corrosion behavior, and the corresponding corrosion mechanisms of the HEA coatings as a function of the Mo content were discussed.