Effects of alumina addition and heat treatment on the behavior of Cr coatings electroplated from a trivalent chromium bath

To study the effect of Al2O3 particles on corrosion behavior of Cr-C coating, low carbon steel substrates are electroplated in Cr(III) baths without and with Al2O3. The results demonstrate that the introduction of Al2O3 particles eliminates the cracks within coatings as a result of the reduction of internal stress. Therefore, the corrosion resistance of Cr-C-Al2O3 composite coating is better than that of Cr-C coating. Heat treatment is also utilized to improve hardness of the Cr-C-Al2O3 composite coating. The hardness increases with heat treatment temperature. The highest hardness (HV 1520) is found for Cr-C-Al2O3 composite coating annealed at 600 °C due to the precipitation of Cr23C6 and Cr2O3 during the treatment. However, the corrosion resistance decreases after annealing at 600 °C. The Cr-C-Al2O3 composite coating with improved hardness can be obtained by annealing the coating at 400 °C. Cr-C-Al2O3 coatings annealed at such temperature are free of through-deposit cracks. It leads to the corrosion resistance of Cr-C-Al2O3 annealed at 400 °C is comparable to that of as-deposited one.