| Summary: | 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.
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