Estudo comparativo da resistência à corrosão de metais duros WC-NiMo, WC-NiCr e WC-Co

Abstract

Cemented carbide is a composite material consisting of hard ceramic particles embedded in a ductile metal matrix, called the binder phase. These materials exhibit a unique combination of mechanical properties, having several engineering applications. Currently, cobalt is the binder that dominates the cemented carbide market due to better mechanical properties. However, due to the high cost, scarcity and the search for cemented carbides with greater corrosion resistance, an alternative binder phase has been researched. Nickel is an exciting and promising candidate but, the mechanical properties of WC-Ni cemented carbides are relatively inferior to those of WC-Co cemented carbides. Thus the addition of alloying elements, such as molybdenum and chromium, has been studied in order to promote the strengthening of the WC-Ni alloys by using solid solution techniques. The aim of this work is to evaluate the corrosion resistance of WC-NiMo, WC-NiCr and WC-Co cemented carbides, obtained by conventional powder metallurgy. The sintered sample was characterized, , before and after corrosive tests, by the analysis in optical microscopy (OM), scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD. Microhardness was measured by the Vickers microhardness method. The corrosion resistance of the cemented carbides was investigated by open circuit potential, potentiodynamic polarization, electrochemical impedance spectroscopy and static immersing corrosion test, in a 3.5 wt.% NaCl solution. The WC-Ni cemented carbides showed microstructures similar to those of conventional cemented carbides, WC-Co. However, pores and binder islands were observed in the microstructures, which leads to inferior mechanical properties, such as Vickers microhardness. The corrosion tests had promising results, the WC-NiMo e WC-NiCr cemented carbide showed superior corrosion resistance and higher passivation capacity than the WC-Co cemented carbide. The WC-NiMo cemented carbide was the material with the best performance, as it maintained good mechanical properties and presented the noblest corrosive behavior among the studied materials.