The corrosion behaviour of high velocity oxy-fuel (HVOF) sprayed Ni-Cr-Si-B coatings

摘要

1. Very similar corrosion behaviour was observed on as sprayed Ni-Cr-Si-B coatings supplied by two commercial spraying contractors. The coatings exhibited passive characteristics during electrochemical testing upon initial exposure to saline solution but suffer corrosion attack upon extended exposure. 2. Corrosion of the Ni-Cr-Si-B coating without vacuum-fusion post-treatment occurs by a series of micropit-ting and macropitting mechanisms associated with the loss of hardphase particles and individual splats. 3. Post-treatment of HVOF sprayed Ni-Cr-Si-B coating by polymer impregnation has been shown to have no effect on the corrosion resistance. 4. Vacuum fusion treatment of the as-sprayed coating is beneficial in many aspects such as lowering both the coating and interfacial porosities, increasing the coating density and hardness and reducing the roughness of the as-sprayed surface. 5. Vacuum fusion also alters the microstructure and microchemistry of the coating. 6. Vacuum-fused coatings corrode by different mechanisms such as matrix corrosion and micropitting than do the as-sprayed or vacuum-sealed coatings but these mechanistic differences do not result in any significant difference in corrosion resistance.