The effect of post-treatment of a high-velocity oxy-fuel Ni-Cr-Mo-Si-B coating - Part 1: Microstructure/corrosion behavior relationships

The microstructure and aqueous corrosion characteristics of a Ni-Cr-Mo-Si-B high-velocity oxy-fuel (HVOF) coating have been assessed. It has been show n that postprocessing by vacuum fusion has a significant effect on the coat ing microstructure by increasing the type and concentration of hard phase p articles. The principal hard phases in the as-sprayed condition and vacuum- sealed condition are chromium carbides, whereas molybdenum-containing borid e phases are also present after vacuum fusion. Vacuum-fusion post-treatment eliminates splat boundaries, which can act as sites, where preferential co rrosion can occur and, hence, the dominant corrosion mechanisms change. In as-sprayed and vacuum-sealed coatings, localized attack at splat particle b oundaries and crevice corrosion dominate, whereas in vacuum-fused coating, the principal mechanism of corrosion is "micropitting" as a result of the h ard phase loss.