WC10Co4Cr coatings deposited by HVOF on martensitic stainless steel for use in hydraulic turbines: Resistance to corrosion and slurry erosion

Abstract

The corrosion resistance and slurry erosion wear of WC-10Co-4Cr coatings sprayed by the High Velocity Oxy-Fuel Flame (HVOF) method on an AISI 410 stainless steel substrate, usually employed in hydraulic turbines, were studied. Studies on the materials tested in this work in relation to dry wear and cavitation have been found. However, they do not represent the conditions found in hydraulic turbines. In this work, the wear behavior of the coatings was assessed by erosion tests in aqueous medium containing erodent particles simulating environmental and operating conditions of hydraulic turbines. By using different thermal spray parameters, different levels of porosity were generated in the coated layer. Particle velocity and temperature were monitored during deposition. From the characterization by X-ray diffraction and EDS the phases WC, W2C and the CoCr metal matrix were detected in the layer. The mechanism of material removal by wear was evaluated by mass loss as a function of time and coating properties. Different wear regimes were observed as the thickness of the layer was progressively reduced. The erosion depletion in aqueous medium occurs preferentially in the metal matrix of the coating. Corrosion behavior in aqueous medium containing 3.5 wt% NaCl was also investigated by potentiodynamic polarization tests. Results show that in the coating, corrosion occurred preferentially on the metal matrix - essentially cobalt - the most active element of the coating, while the nonmetallic phases (WC, W2C) are site of the cathodic reaction. The stainless steel substrate is cathodically protected by the metal matrix of the coating.