Avaliação da corrosão eletroquímica dos metais duros WC-Ni-Al, WC-Ni-Si e WC-Co produzidos por metalurgia do pó convencional

Abstract

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.