Corrosion mechanism of HVOF thermal sprayed WC-CoCr coatings in acidic chloride media

Abstract

HVOF thermal sprayed WC cermet coatings exhibit excellent abrasive and erosive wear resistance due to the presence of high volume fraction of WC particles bounded by a tough cobalt or cobalt-chromium alloy binder. However, less information is at present available on the corrosion response of these coatings in strong acidic environment. In this study, the corrosion behaviour of the HVOF WC-CoCr coatings was investigated by electrochemical polarization technique in 0.1¿N hydrochloric (HCl) acid solution at 25¿°C. The coating morphology was studied by scanning electron microscopy (SEM) and the relationships between the microstructure and corrosion mechanism were investigated using small-angle X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy measurements. The analysis of the corroded coating surface showed that during anodic polarization, the corrosion attack of the WC-CoCr coating began with active oxidation of the binder phase followed by the formation of a pseudo-passive layer composed by anhydrous Cr-oxides (CrO), Co-oxides (CoO/Co3O4) and W-oxides (WO3). At higher potentials the corrosion was governed by the hydration of tungsten oxide (WO3·xH2O) and the extension of the oxidation to the WC particles.