Investigations of microstructure and corrosion resistance of WC-Co and WC-Cr3C2-Ni coatings deposited by HVOF on magnesium alloy substrates

Abstract

Magnesium alloys have found widespread application in various industries, mainly due to the combination of low density, the best structural material known today, and high strength to weight ratio. However, the use of magnesium alloys is limited due to their poor wear and corrosion behavior. In this study, two commercial powders, agglomerated and sintered WC-Co and WC-Cr3C2-Ni were thermally sprayed to deposit coatings onto two magnesium alloy substrates (AZ31 and AZ91) with a combination of two different spraying distances (320 and 400 mm). The main aim of this work was to investigate the microstructure and corrosion resistance of the coatings deposited by High Velocity Oxy Fuel (HVOF) spraying. The morphology and microstructure of the sprayed coatings were analyzed by Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD). Additionally, porosity, microhardness and surface roughness were examined. Furthermore, potentiodynamic polarization was used to characterize the corrosion behavior of the deposited coatings. The experimental results showed that manufactured coatings are characterized by a dense microstructure with porosity typical for coatings sprayed with the HVOF method. Moreover small cracks and unmelted carbide particles embedded in the metal matrix were found in the as-sprayed coatings. Based on the results of the potentiodynamic studies it was found that the corrosion resistance of AZ31 and AZ91 magnesium alloys is not very high in a sodium chloride solution. The polarization resistance of AZ31 is very low (241.3 Ω∙cm2) and the corrosion current density is high (57.4 μA∙cm−2), whereas for AZ91 these values are 17.8 Ω∙cm2 and 1612.1 μA∙cm−2, respectively. The corrosion potential of the AZ31 and AZ91 alloy equal −1.56 V which is characteristic for magnesium alloys in a NaCl solution. For both type of substrates, better corrosion resistance has been obtained for the WC-Co coating. The Rp value was nearly 150 times higher than the one for uncoated AZ91 alloy, whereas for AZ31 the polarization resistance was about 17 times higher (Rp = 4.2 kΩ∙cm2) in comparison to non-covered alloy. In summary, all deposited coatings improved the corrosion resistance of the magnesium alloy. However, the WC-Co one protects the magnesium substrate against corrosion more effectively than the WC-Cr3C2-Ni.