The Effects of Heat Treatment on the Corrosion Behavior of HVOF-sprayed WC-17 wt % Co Coatings

Abstract

A WC-17 wt % Co coating was deposited onto a ST37 mild steel substrate by HVOF-spray technique and then vacuum heat treated at 1100°C. The cross sectional microstructure of the produced coating was characterized before and after heat treatment by optical microscopy. X-ray diffraction (XRD) was also utilized to evaluate the effects of heat treatment on the phase composition of the WC-17Co coating. To study the electrochemical corrosion behavior of both as-produced and heat treated samples, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods were performed in 3.5 wt % NaCl solution. Our results reveal that the WC-17Co coating had a dense structure with an average thickness of about ~500 μm. However, heat treating the coating resulted in an adhesion failure and therefore partial separation of the coating from the substrate. The as-produced coating was also composed of both crystalline WC and amorphous phases whilst the heat treated layer was fully crystalline. According to the corrosion tests, the WC-17Co coating improved the corrosion resistance of the substrate. However, heat treating the coating at 1100°C decreased anticorrosion performance which was due to the precipitation of η-phases with different electrochemical potential than the WC phase together with the formation of microgalvanic cells between the crystalline phases with different compositions.