Mechanism of corrosion resistance degradation of Cr-plated parts due to heat treatment and its prevention

Abstract

The mechanism of decrease in the corrosion resistance of Cr-plated parts when they are subjected to a heating process and measures for preventing the degradation are studied. When postfinishing is performed after the Cr-plating process, cracks in the Cr layer are closed as a result of plastic flow on the topmost surface layer. When the residual stress in the Cr layer becomes compressive due to the postfinishing, the cracks are completely closed, resulting in a high corrosion resistance. The residual stress in the Cr layer changes from compressive to tensile following heat treatment. When the residual stress becomes tensile, the cracks in the Cr layer which were closed during postfinishing open once again, resulting in a decrease in the corrosion resistance. We speculated that the change of residual stress from compressive to tensile is caused by the shrinkage of the Cr layer due to heat treatment and the difference in the coefficients of thermal expansion between the substrate (steel) and Cr layer, as well as the release of residual stress generated during processing. In order to prevent the decrease of corrosion resistance under heat treatment, it is effective to apply compressive stress which exceeds the level of change in residual stress due to heat treatment to the Cr layer during postfinishing, after the Cr plating process.