Corrosion behaviour of WC-VC-Co hardmetal in various acidic and chloride containing media

• Main Author: Konadu, David Sasu
• Format: Others
• Language: en
• Published: 2010
• Online Access: http://hdl.handle.net/10539/8631

Cemented carbides have been used for milling and cutting, as a result of their high hardness and toughness, but their poor resistance to chemically aggressive environments calls for some modifications to improve their corrosion resistance. VC with a grain size of less than 2 μm was added, which improved the hardness and toughness compared to the straight grade. In this work, the corrosion behaviour and surface films of WC-VC-Co hardmetals were investigated in acids and neutral chloride salts as VC content was increased, while keeping the binder content constant. Characterisation of the microstructure and composition of the alloys, before and after corrosion, were done with scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. The electrochemical techniques used for the investigation were: open circuit potential (OCP), potentiodynamic anodic polarisation and chronoamperometry. Increasing VC content improved corrosion resistance in HCl solution, but little change was experienced in H2SO4 according to corrosion rate values. The surface film formed on all the test specimens in HCl was WO3.H2O. In H2SO4, all the specimens formed WO3.xH2O and at high VC content, VOSO4.H2O. Increasing VC content makes the OCP in H2SO4 more negative than in the base alloy. The OCP in HCl only becomes more negative at very high VC additions. An increase in VC content did not affect the variation of current during chronoamperometric tests in the acidic media. Different tungsten oxides formed as passive films in NaCl. Corrosion resistance decreased with increasing VC content for the WCVC- Co alloys in NaCl. In synthetic mine water (SMW), all specimens showed continuous anodic dissolution as there was minimal influence of increasing VC additions. There was no trend in the variation of the OCP values as the VC content varied and increasing VC content resulted in an increase in the value of current density during chronoamperometry in the neutral salts. The lack of effect on the corrosion resistance is attributed to the fact that VC does iv not possess passivation characteristics. In the neutral salts, there was formation of green V2+ solid compound, accompanied by oxygen reduction.