HVOF sprayed Ni–Mo coatings improved by annealing treatment: microstructure characterization, corrosion resistance to HCl and corrosion mechanisms

Abstract

In this study, HVOF spraying technology was employed to deposit the Ni–Mo alloy coating on nodular cast iron substrate. Post–spray annealing treatment was used to modify the microstructure and corrosion property of Ni–Mo coating. The microstructural evolution and corrosion resistance to HCl of both the as-sprayed and annealed Ni–Mo coatings were investigated. Results show that an unoxidized Ni–Mo coating with the porosity of 0.62% and bonding strength of 58.8 MPa can be achieved by HVOF. Electrochemical tests indicate that the influence of temperature on corrosion behavior of the as-sprayed Ni–Mo coating in 15wt.% HCl solution follows the order: 40 °C > 60 °C > 20 °C. With increasing the concentration of HCl solution, corrosion resistance of the as-sprayed Ni–Mo coating decreases. Post–spray annealing treatment has a significant effect on the microstructural evolution of Ni–Mo coating, including interfacial bonding quality, coalescence of micro-voids, phase transformation within coating and element diffusion reactions from substrate into coating. Annealing treatment accelerates the diffusion of Fe and C from nodular cast iron substrate into Ni–Mo coating with forming Mo2C and NiFe phases, which results in the enrichment of Mo. The 850 °C annealed Ni–Mo coating possesses the best corrosion resistance in 15wt.% HCl solution, which is due to the improved interfacial bonding and the low extent of Fe/C diffusion from substrate into coating, whereas the dramatical reduction of corrosion resistance of the 950 °C annealed case can be mainly attributed to the large-scale formation of NiFe and Mo2C phases within coating.