THERMODYNAMIC AND EXPERIMENTAL-STUDY OF CARBON FORMATION ON CARBIDES UNDER HYDROTHERMAL CONDITIONS

Thermodynamic analyses for the corrosion of SiC, TIG, TaC, NbC, WC and B4C in the temperature range 300-1000 degrees C in high-pressure steam have been conducted using a Gibbs energy minimization computer code, The calculated results are compared to experimental results for corrosion of these carbides in a hydrothermal fluid above the critical point. The formation of carbon has been predicted and observed for all these carbides, except B4C. Carbon is formed at low H2O:carbide molar ratios. Lower pressure also promotes carbon formation. For a high H2O:carbide molar ratio, the carbon will be oxidized to CO and CO2. The hydrothermal method can be used for producing carbon coatings of nanometre to micrometre thickness on the surface of carbides.