CORROSION, EROSION-CORROSION, AND THE FLEXURAL STRENGTH OF WC-CO HARDMETALS

The corrosion of commercial hardmetals containing 6, 8 and 10% Co with small additions of Cr3C2 and VC, in solutions of pH 1, 3, 5 and 10, and in a cutting fluid of pH 8.6, has been investigated using weight loss, atomic absorption spectroscopy, and metallographic techniques. For a surface area 420 mm2, at 20°C, the amount of cobalt dissolved in pH 5 and 10 solutions was less than 0.05 mg per day and in the cutting fluid was less than 0.05 mg in 7 days. Salt in the water increased the corrosion rate in the pH 10 solution by over five times. Corrosion was highest in the hardmetal containing VC. Corrosion in a pH 1 solution at 20 °C for 24 h (weight loss about 1 mg) did not affect the mean strength of the hardmetals. Erosion by alumina balls during corrosion increased the corrosion rate in the pH 1 solution by 16 to 29 times, formed a thick layer of WC denuded of cobalt, and reduced the strength of the eroded-corroded hardmetals by 0.54 to 0.76 times.