An investigation of trivalent chromium electrodeposition on the surface of high-carbon tool steel

• Main Authors: Chun-Ching Hsu, 許俊清
• Other Authors: Ching-An Huang
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/29624709732369468607

博士 === 長庚大學 === 機械工程研究所 === 94 === Presently, chromium deposits could be obtained from trivalent chromium baths using formate and acetate, or glycine complex at 30℃. However, bath stability, relatively low plating current efficiency and deposition rate of trivalent chromium electroplating are still not well solved. In this study, the chromium deposit obtained from trivalent chromium plating bath with different complex formers was conducted. Meanwhile, the property of deposit was evaluated by means of hardness measurement, morphology examination, microstructure charization and electrochemical test. Based on the experimental results, a suitable and practicable trivalent plating bath was proposed. With the bath, high current efficiency of electroplating was recognized. The highest current efficiency of trivalent chromium electroplating can reach as high as 85% at 50 - 70 A/dm2 and 30℃. This improvement is attributed to suitable complex formers and conducting salts added in the plating bath. The annealing behavior of hard chromium deposit was studied with x-ray diffractrometer, hardness and corrosion resistance tests. The highest hardness of chromium deposit could be raised to ca. 1400 Hv after the annealing at 600C for 1 hour. According to the results of X-ray diffractrometer, hardness increase is resulted from precipitation of Cr7C3 in the deposit during annealing. The corrosion resistance of trivalent chromium deposit could be increased after annealing at 700C for 1 hour in the air furnace. Formation of within deposit cracks in the deposit is the main ground for improving its corrosion resistance. Without Cr2O3 within cracks, steel substrate was preferred etched through cracks developed in the deposit. The behavior of both chromium-nickel deposit and chromium rich –nickel rich multilayer obtained from the proposed plating bath were reported at last section. The chemical composition of chromium-nickel deposit depends strong on plating current density. At plating current density of 10 A/dm2, the nickel-rich deposit was obtained, while at 40 A/dm2, the chromium-rich deposit was formed. Hence, pulsed-current electroplating was used to develop chromium- and nickel-rich multilayer with a thickness modulation of tens of nanometers. Microstructures of chromium-nickel multilayers were investigated by transmission electron microscopy and auger electron spectrometer.