Effect of Alloying Components on the Tribocorrosion Properties of Tungsten-Carbide Cermets
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EFFECT OF ALLOYING COMPONENTS ON THE TRIBOCORROSION PROPERTIES OF TUNGSTEN-CARBIDE CERMETS V. I. Pokhmurs’kyi,1 Kh. B. Vasyliv,1, 2 V. A. Vynar,1 V. M. Dovhunyk,1 I. V. Koval’chuk,1 and O. P. Khlopyk1
UDC 539.926:627.527
We study the corrosion and tribocorrosion properties of tungsten-carbide metal composites alloyed with graphite and chromium and vanadium carbides in a 3% NaCl solution. It is shown that the addition of 2–4% of graphite to the VN20 composite intensifies local corrosion processes because graphite inclusions serve as additional cathodic sections. This deteriorates the tribocorrosion behavior of the alloys, including the alloys alloyed with chromium and vanadium carbides because the corrosion processes in the contact zone lead to the crumbling of carbide inclusions playing the role of abrasive. We revealed the high wear resistance of the (VN20 + 1% VC)–(VN20 + 1% Cr3C 2 ) friction couple in a 3% NaCl solution. After friction, the corrosion current density decreases on both contacting surfaces by almost an order of magnitude, which corresponds to the formation of secondary structures guaranteeing corrosion protection. Keywords: cermet composites, chromium carbides, vanadium carbides, corrosion, tribocorrosion.
Cermet composites of the WC–Ni system have high operating characteristics, in particular, high hardness, and high wear and thermal resistances. They are used in the production of the components of heavily loaded friction units. Under the working conditions, these friction couples, as a rule, operate in contact with conducting media, such as tap water, seawater, oil products, household or industrial wastes, etc. Therefore, it is necessary to guarantee their corrosion and tribocorrosion resistance [1–3] determined by the corrosion resistance of binding metal because the oxidation potential of WC is more positive than the oxidation potential of nickel [4, 5]. As a result, a galvanic couple is formed in the electrolyte and the anodic reaction runs on the surface of the nickel component. The dissolution of nickel in the contact zone is nonuniform due to different ratios of the anodic and cathodic sections. Tungsten carbide is practically insoluble in electrolytes because the dissolution of the nickel component guarantees its cathodic protection [5, 6]. In order to increase the corrosion resistance of cermet composites, nickel is replaced with cobalt or its mixtures with chromium and molybdenum. Moreover, small amounts of the carbides of transition metals (TiC, Cr3C 2 , NbC, TaC, Mo 2C , VC) are also added to the compositions [7–9]. In the case of addition of the carbide phases, the atoms of metals may dissolve in the binder separately or in combination with tungsten atoms, which affects its electrochemical characteristics [5]. It is known that the addition of about 0.5 wt.% of Cr3C 2 chromium carbide noticeably improves the corrosion resistance of alloys of the WC–Co system [5]. The effect of the alloying with low concentrations of VC vanadium carbide is neutral [5] but the addition of ∼ 10 wt.% of v
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