Corrosion behavior of thermal sprayed WC cermet coatings having various metallic binders in strong acidic environment

Abstract

A series of the electrochemical and long-term immersion tests was carried out in a strong sulfuric acid (5 wt.% H2SO4) solution on thermal sprayed WC cermet coatings having various kinds of metallic binder in order to examine the effect of composition of binder materials on the corrosion behavior. In the present study, the coatings were processed via a high velocity oxygen fuel (HVOF) spraying technique with WC–Co, WC–Co–Cr, WC–CrC–Ni and WC–Ni composite powders. The experimental results revealed that a considerable galvanic corrosion occurred between WC particles and metallic binders in the aerated 5 wt.% H2SO4 solution. The corrosion resistance of the coatings containing Cr was better than that of the coatings without Cr. For the coatings without Cr, general corrosion occurred in binder materials in addition to galvanic corrosion between WC particles and metallic binders. By contrast, the formation of passive film in the form of surface oxide in the coatings containing Cr suppressed the binder material dissolution into the solution. However, the overall corrosion resistance of the WC–CrC–Ni coating was inferior to that of the WC–Co–Cr coating due to the presence of microcracks which act as the infiltration paths of the solution. Conclusively, it is found that chemical composition of metallic binder materials and control of microcracks are the most important factors influencing the corrosion resistance of the HVOF sprayed WC cermet coatings in the strong acidic environment. Also, the wear–corrosion results such as surface morphologies and friction coefficients have been presented.