Resumo:
Cemented carbides are formed by the association of very fine powdered particles of
hard carbides with tough metals. They are generally composed by tungsten carbide, which
provides high hardness and wear resistance, and cobalt, which increases the material's
tenacity. Due to this good combination of properties, they can be applied in several
engineering areas, such as machining, mining and construction industry, reducing costs and
increasing the useful life of tools, when compared to conventional steel. Cobalt is the most
used binder in cemented carbides due to its superiority in relation to other binders in several
factors, such as: good wettability in WC, high solubility of WC in cobalt at the sintering
temperature and, above all, the width of the carbon window. However, several researches
have been carried out searching for alternative binders to cobalt, as the corrosion resistance of
conventional tungsten carbide/cobalt cemented carbides is not satisfactory in certain
applications, such as the chemical and food industries. Thus, this study compared the
corrosion behavior of WC-NiSi, WC-NiAl cemented carbides with that of conventional
cemented carbide WC-Co. Samples were characterized by optical microscopy, scanning
electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and Vickers
microhardness tests. Microscopy of all the samples showed a typical carbide microstructure,
with an appropriate distribution of the binder by the faceted WC grains and without the
presence of the undesirable graphite and η phases. The addition of aluminum and silicon to
the nickel binder considerably increased the cemented carbide hardness, with the silicon
sample providing hardness values similar to cemented carbide WC-Co. All samples showed
continuous decreases in weight loss rates during the immersion corrosion test period, with the
WC-NiAl sample showing the best corrosion resistance in this test. In the electrochemical
tests of open circuit potential, potentiodynamic polarization and electrochemical impedance
spectroscopy, the samples with nickel binder showed nobler potentials, decrease in current
density values and a total impedance higher than the sample with cobalt. The WC-NiAl
sample showed the best corrosion resistance, with the best response in both the immersion test
and in the electrochemical tests.
