Duplex coating for improvement of corrosion resistance in chromium deposit

Amorphous bright chromium deposit was electroplated in a modified Sargent bath containing formamide. It was evaluated that this film had no cracks and carbon incorporated was distributed uniformly from the examination of cross-section of deposit with scanning electron microscope and Auger electron spectroscopy. Hardness increased from Hv 1300 to 1700 after annealing at 600 °C owing to crystallization. This Cr–C amorphous film was applied to chromium deposit as an inner layer in order to enhance corrosion resistance. Besides, plasma nitriding on chromium deposit was carried out in mixed gas atmosphere of NH3 95% and CH4 5% as a post-treatment. Surface nitride layer, CrxN, and carbide layer at the interface between chromium deposit and substrate were formed without any cracks after nitriding. Duplex coating of Cr–C/Cr and Cr/CrxN was achieved and salt spray test was performed for elucidation of corrosion resistance. Corrosion resistance of double layers was improved remarkably by an inner layer of Cr–C and by an outer layer of CrxN. These crack-free layers were considered to play a key role to enhance corrosion resistance of chromium electrodeposit.