Corrosion of Cemented Carbides

Abstract

Corrosion tests on cemented carbides in acids at room temperature and at 95°C were carried out by the authors and it was shown that cemented carbides were attacked predominantly due to the dissolution of Co as the binder metal. In the present work, relation between corrosion rates of WC-Co alloy and potential differences of hot-pressed WC-electrode vs Co-electrode in NaCl aq. (0~25 wt%) were examined at 19°C. The corrosion rate of WC-Co alloy in NaCl aq. decreased with decreasing of the potential difference between WC- and Co-electrodes and the concentration (2 wt%) of NaCl aq. showing the maximum corrosion rate likely corresponded with that showing the maximum potential difference (410 mV). Therefore, it could be qualitatively noted that the corrosion of WC-Co alloy in NaCl aq. was caused by local galvanic action between WC grains and Co matrix on the surface of the alloy. In this case, because soluble oxygen played as a depolarizer, increased content of soluble oxygen in NaCl aq. increased the corrosion rate : Co: Co → Co++ + 2e- WC: 2Na+ + 2e- → 2Na(on WC), 2Na + 2H2O → 2Na+ + 2OH- + 2H(on WC) Depolarization reaction : 2H(on WC) + 1/2 O2(in NaCl aq.) → H2O. © 1972, Japan Society of Powder and Powder Metallurgy. All rights reserved.