Cavitation erosion mechanisms of HVOF-sprayed Ni-based cermet coatings in 3.5% NaCl environment

Monel K-500 is a nickel-based alloy that is a widely used material in industries like hydraulic, chemical and marine. In its marine application, such as in seawater, Monel alloy suffers from high cavitation and corrosion. In one of the approaches to control these degradations, thermal spray coatings are recommended to enhance the resistance against these degradations. The current study assesses the synergic effect of cavitation erosion (CE) and corrosion on two HVOF-sprayed nickel-based cermet coatings, WC-10Ni5Cr (WC-NiCr) and WC-18Hastelloy C (WC-H), on Monel K-500 substrate. CE tests were conducted for 15 h in a corrosive (3.5% NaCl) environment. It was observed that WC-NiCr coating reduced the CE losses in Monel alloy by 45%. In pure cavitation (DI water) conditions, the better CE of a coating originates from its better combination of microhardness and indentation toughness. Whereas, in a corrosive medium, the CE behavior of a coating depends upon its electrochemical properties too. Electrochemical corrosion test on both the coatings revealed that WC-NiCr coating had better corrosion results than that of WC-H coating due to its higher corrosion potential (Ecorr) value and slightly lower corrosion current density which may influence the CE performance of the investigated coatings. In-depth study of the erosion mechanism for each of the coatings was conducted using Scanning Electron Microscopy (SEM). It was observed that due to the effect of the corrosive medium, several pits and pores were generated over the coating surface. These defects provided additional nucleation sites for CE in the coatings resulting in a high level of erosion losses in the coatings. Considering the effects of cavitation and corrosion simultaneously, HVOF-sprayed WC-NiCr coating is recommended to enhance the cavitation-corrosion resistance in Monel K-500 alloy.