Effect of pH and Calcium (II) Ion on the Corrosion of Tungsten Carbide Bearing Hard Alloy in Water at Elevated Temperatures

Abstract

Tungsten carbide bearing hard alloys have been used as rolls in the hot working process of steel. Wear corrosion of the roll surface by cooling water is a serious problem in this process. The effect of pH and Ca (II) on the corrosion of hard alloy was investigated in water at elevated temperatures. Immersion test was carried out at 100 and 150°C in the aqueous solution of pH at 3.5 to 10 and Ca (II) concentrations of 0 to 0.003 moldm^-3. Analysis of the dissolved metal ions revealed that the corrosion of hard alloy was characterized by the preferential dissolution of binder alloy in acidic media and of tungsten carbide in alkaline media. The dissolution rate of tungsten carbide was markedly decreased in the solutions containing Ca (II): the corrosion inhibition has resulted from the formation of crystalline CaWO₄ on the specimen. Effect of temperature, pH and Ca (II) ion on the solubility of CaWO₄ was investigated. It was considered that the corrosion of hard alloy in alkaline solution was controlled by the dissolution/precipitaiton equilibrium of CaWO₄.