CORROSION OF CHEMICALLY VAPOUR DEPOSITED TITANIUM CARBIDE ON AN INERT SUBSTRATE

Abstract
The corrosion properties of chemically vapour deposited titanium carbide (TiC) have been studied. Any contribution to the corrosion current from the substrate was avoided by using an inert substrate. The TiC was deposited onto substrates of Al₂O₃ from a gas mixture containing TiCl₄, CH₄ and H₂ at temperatures between 1020°C and 1080°C. Differences in film thickness and morphology were measured for samples deposited at different temperatures and gas flows. Film thicknesses varied between 0.5 and 1.5 µm and grain sizes between 0.1 and 0.8 µm. Anodic polarization curves were registered in 1 M hydrochloric acid (HCl). HCl of a lower concentration did not activate the samples. The corrosion potentials of the samples were used as starting points for the polarization curves. Films deposited on polished Al₂O₃ at 1020°C reached stable potentials of 250 mV (vs. SCE) after one hour. Films deposited onto unpolished substrates had lower values of corrosion potentials (- 100 to 0 mV vs. SCE) which reached stability after about three hours. It is suggested that the passivating film on TiC, deposited on polished substrates, achieved a higher protective quality in the solution, due to self-healing. The passivating film on TiC, deposited on unpolished substrates, on the other hand, did not succeed to reach this high quality. Anodic polarization curves for the different films showed only slight differences. The curves showed two passivation peaks at 800 and 1900 mV (vs. SCE), respectively. An extremely high oxygen overpotential (> 5V) and a poor tendency to transpassive corrosion (< 6V vs, SCE) were indicated by the curves.