Spray parameters optimization, microstructure and corrosion behavior of high-velocity oxygen-fuel sprayed non-equiatomic CuAlNiTiSi medium-entropy alloy coatings

In this study, a novel Cu55Al20Ni12Ti8Si5 (at.%) medium-entropy alloy (MEA) was designed, and the Cu-based MEA coating was deposited on the AISI 1020 steel substrate via high-velocity oxygen-fuel (HVOF) spraying process. With the porosity of the coatings as objective, Taguchi method was employed to optimize the three influential HVOF spray parameters. Phase composition, microstructure, microhardness of the optimized coating was characterized by means of XRD, SEM/EDS and STEM techniques, Vickers microhardness tester, respectively. The corrosion behavior of the coatings, as-cast Ni-Al bronze and AISI 1020 steel was investigated in 3.5 wt% NaCl solution by using electrochemical test methods. The XPS was adopted to characterize the composition of corrosion products, and the corrosion mechanism was further discussed. The results illustrated that oxygen flow had a dramatic impact on the porosity of the coatings, and the optimized coating can be prepared under oxygen flow of 802.3 L/min, kerosene flow of 0.38 L/min and spray distance of 320 mm. Detailed characterization indicated that the optimized coating was comprised of mainly Cu-rich BCC phase and a few granular NiTi-rich B2-ordered phase with a porosity of 0.42% ± 0.06% and a microhardness of 470 ± 31 HV0.2. The electrochemical test results revealed that the optimized coating exhibited superior corrosion resistance relative to that of all orthogonal coatings, as-cast Ni-Al bronze and AISI 1020 steel on account of its low porosity and primarily single-phase microstructure. Additionally, the long-term corrosion behavior of the optimized coating was controlled by electrochemical reaction that promoting the formation of protective film on the coating surface, which can efficiently improve the corrosion resistance of the coating.