Corrosion Behavior of VN20 Alloy Doped with Graphite or Chromium and Vanadium Carbides
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CORROSION BEHAVIOR OF VN20 ALLOY DOPED WITH GRAPHITE OR CHROMIUM AND VANADIUM CARBIDES V. A. Vynar
UDC 539.621
We study the corrosion properties of a composite material based on tungsten carbide on a nickel binder and doped with graphite or chromium and vanadium carbides. It is shown that the introduction of graphite in the VN20 composite decreases its corrosion resistance in a 3-% NaCl solution. The addition of 2 and 4% graphite makes the corrosion current densities 2 and 2.5 times higher, respectively. Doping of the composite with vanadium carbide in amounts of 0.15 and 1% increases the corrosion currents, whereas the introduction of 1% of chromium carbide weakens these currents by about an order of magnitude. Keywords: nickel, tungsten carbide, graphite, vanadium carbide, chromium carbide, electrode potential, corrosion current, dissolution.
The powder composites based on tungsten carbide are characterized by high characteristics of hardness and wear resistance. For this reason, they are used in numerous branches of industry for the production of various elements of the tools and structures, namely, nozzles, sealing rings, sliding bearings, and other parts of the friction couples [1–3]. It is assumed that the corrosion resistance of hard alloys based on tungsten carbide in acid and neutral electrolytes depends on the corrosion resistance of the binding metal because the oxidizing potential of tungsten carbide is more positive than the binding potential [4, 5]. As a result, a galvanic couple in which the anodic reaction runs on the surface of the binder is formed in the electrolyte. The dissolution of the binder is intensified due to the unfavorable ratio of the area of surface anodic sections to the area of cathodic sections. Tungsten carbide is practically insoluble in these media because the dissolution of the binding component of the composite material guarantees its cathodic protection. Therefore, the major part of studies is aimed at the improvement of the corrosion resistance of the binder [6, 7]. The addition of graphite to VN20 alloys significantly decreases the friction coefficient and guarantees the long-term service life of antifriction couples [8]. However, its influence on the corrosion resistance of VN20 alloy has not been studied yet. In what follows, we study the influence of alloying with graphite and/or vanadium and chromium carbides on the corrosion resistance of a powder composite based on tungsten carbide. Materials and Experimental Procedure We studied specimens of VN20 alloy and the composites obtained by adding 2 and 4% of graphite (VN20Gr2 and VN20Gr4) and 0.15 or 1% of chromium and vanadium carbides to the base VN20 alloy. The chemical compositions and properties of the indicated materials are presented in Table 1. Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine; e-mail: [email protected]. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 50, No. 5, pp. 87–90, September–October, 2014. Original article submitted August 4, 2014. 1068-
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