Influence of passivity on the tribocorrosion of carbon steel in aqueous solutions

The degradation of DIN 34CrNiMo6 steel sliding against alumina was investigated in NaOH and in borate buffer solutions of pH 8.4 under electrochemically applied passive potentials. Small amounts of chromate or chloride were added to the borate solutions to vary the nature of the passive film. Film thickness and composition were characterised using surface analysis (Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS)). For comparison, tribocorrosion experiments were carried out at appropriate applied cathodic potentials, where no iron oxidation is thermodynamically possible, and therefore no passive film is formed. Results show the crucial role played by surface oxidation not only with respect to wear accelerated corrosion processes but also on surface mechanical phenomena. Depending on the nature of the solution, passive carbon steel exhibits either subsurface cracking or plastic flow accompanied by wear. In absence of passive film, wear is negligible and large plastic flow of the metal occurs, independent of the nature of the solution.