Abstract
In this paper, the electrochemical corrosion resistance of near-nano and nanostructured WC-Co cemented carbides was investigated. WC powders with an average grain size dBET in the range from 95 nm to 150 nm and with an addition of vanadium carbide (VC) and chromium carbide Cr3C2 as grain growth inhibitors were used as starting powders. The mixtures with 6 wt. % and 9 wt. % Co were consolidated by two different processes; sintering in hydrogen atmosphere and the sinter-HIP process. WC-Co samples were researched by direct current and alternating current techniques in the solution of 3.5% NaCl at room temperature. Corrosion parameters such as corrosion potential (Ecorr), corrosion current density (jcorr) and polarization resistance (Rp) were determined by electrochemical techniques. From the conducted research, it was found that the consolidation processes and microstructural characteristics—grain growth inhibitors, grain size of the starting WC powders and η-phase—influenced the electrochemical corrosion resistance. η-phase enhanced the formation of a passive layer on the samples’ surfaces, thereby reducing the tendency of the sample dissolution and increasing the stability of oxides forming therewith a passive layer on the sample surface.
Highlights
Corrosion resistance and electrochemical properties are not a prime requirement of cemented carbides, these properties are very important in industrial use since the application area of cemented carbides is rapidly growing due to their superior mechanical properties, high hardness and high toughness, and good wear resistance [1,2,3,4]
The corrosion mechanism occurring in cemented carbides depends on many influential factors which can be classified into two basic groups; one referring to the environment and another one to microstructural characteristics
Grain growth inhibitors in the starting powders influenced the electrochemical corrosion resistance; samples with only 0.27% vanadium carbide (VC) showed better corrosion resistance compared to samples resistance; samples with only 0.27% VC showed better corrosion resistance compared to samples with an addition of 0.26% VC and 0.45% Cr3C2
Summary
Corrosion resistance and electrochemical properties are not a prime requirement of cemented carbides, these properties are very important in industrial use since the application area of cemented carbides is rapidly growing due to their superior mechanical properties, high hardness and high toughness, and good wear resistance [1,2,3,4]. Cemented carbides are used in the chemical and petroleum industry for applications such as flow control components, oil and gas inserts such as drill bits, drilling rigs, brag bit profiles, chisel buttons and metal forming tools where they are exposed to an aggressive environment which requires erosion and corrosion resistance [3,5]. Cemented carbides are susceptible to oxidation behavior, electrochemical corrosion and wet corrosion which can cause wear problems [3,4]. The corrosion mechanism occurring in cemented carbides depends on many influential factors which can be classified into two basic groups; one referring to the environment and another one to microstructural characteristics. Conventional WC-Co cemented carbides are characterized by poor corrosion resistance in acid or neutral aqueous solutions for pH ≤7 due to the susceptibility of Co to wet corrosion [3]. In neutral and acidic medium, the selective dissolution of the Co matrix occurs while WC particles are not affected by the corrosion attack; while at alkaline pH values, the passivation of the Co matrix occurs and WC is dissolved [3,4,5,6]
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