The Research Of New Trivalent Chromium Plating For Decoration

In this paper, three types of bath for trivalent chromium plating are gained: chloride-glycine bath, chloride-U bath and sulfate-oxalic bath.The main salt in chloride-glycine bath and the chloride-U bath is chromic chloride. And the formic, acetate, glycine or U is used as the complexing agents, the bromide is the stabilizer. The study shows that the two chloride baths have low potential of the Hull cell, wide range of allowed current density, high stability, long life, great throwing power, good covering power and stable deposit rate. Their deposits have excellent brightness, fine and uniform crystal, great adhesion with bright nickel deposit, high impact resistance and equal corrosion resistance.The main salt in sulfate-oxalic bath is chromic sulfate. And the formic, oxalic and U is used as the complexing agents, the A is the stabilizer. It won't generate the chlorine while in the deposition, so it is good for the environment. The study shows that the bath wide range of allowed current density, high stability, long life, great covering power and stable deposit rate. Their deposits have excellent brightness, fine and uniform crystal, great adhesion with bright nickel deposit, high impact resistance. While its corrosion resistance is some worse than the deposit from hexavalent chromium bath.The study of cathodic polarization shows that the acetate and other additives can decrease the overpotential of deposition, so it would promote the deposition of chromium.The topography of the deposit shows the deposit from the chloride-U bath is microcrack. The longer the deposit time is, the more distinct the microcrack of the deposit is and the bigger the size of the crystal of the deposit is. While the deposit time is 20min, the corrosion resistance is the best. It may be caused by that while the deposit time is 20min, the microcrask is most in the unit area.The U could restrain the generation of the chlorine, and it is semi-quantitative analyzed by the starch iodide paper for the first time. The study of anodic polarization shows that the generation of the chlorine is the main reaction. The over potential of chlorine generation in graphite anode is higher than the one in the coating anode. So, coating anode is easier for the generation of chlorine. While the anode is graphite, the U as the additive will enhance the brightness of the deposit, and increase the over potential of chlorine generation. Hence it can restrain the generation of chlorine.