Influence of Cr_3C_2, TiC and TaC on Corrosion Behaviour and of Cemented Carbides at Different pH-Values

摘要

Corrosion resistance of cemented carbides has been investigated by potentiodynamic polarization measurements with a standard three electrode cell. Mechanically polished hard metals with different refractory metal carbide additions namely Cr_3C_2, TiC and TaC have been investigated. Experiments were done in following media at room temperature: Sulphuric acid, neutral sodium sulphate and sodium hydroxide. Influence of pH on corrosion behaviour was studied with different hard metals. Free corrosion potential was shifted to more noble values with a decrease in pH due to higher current densities of the cathodic reaction. In sodium hydroxide passivity has been obtained for all tested samples between potentials of -700 and -200 mV_(SCE). At about -800 mV_(SCE) a small active peak has been obtained in pure WC-Co hard metal due to cobalt oxidation. Additions of TiC and TaC had almost no influence on electrochemical behaviour in the active range. Cr_3C_2 addition suppresses the active peak which is due to passivation properties of dissolved chromium in the binder phase. In neutral sodium sulphate and sulfuric acid a pseudo-passive area has been found as already described by other authors. TiC and TaC additions decrease current densities in both media by about one order of magnitude. Dissolved chromium in the binder has a similar effect in sulfuric acid and decreases corrosion current densities by three decades in sodium sulfate solution. Behaviour and function of refractory metal carbide additions are explained by their precipitation and dissolution behaviour in hard metals.