STRUCTURE OF Zn-Cr ALLOY ELECTRODEPOSITED ON STEEL AT HIGH CURRENT DENSITIES

摘要

Zn-Cr alloy coating with around 15 wt percent Cr has been found to have much higher chemical corrosion-resistance to steel than conventional zinc plating, while its structure has not been clarified. The Zn-Cr alloy plating is performed at high current densities around 1 A cm~(-2) and for 10 to 20 sec using acidic aqueous sulfate solutions with some additives in the iron-steel industry. Due to the high current densities, it seems reasonable to consider that the Zn-Cr alloy plating proceeds under an unsteady-state condition and that the resulting coatings have a non-equilibrium phase and have a structure change in the direction of thickness. In this research, the crystal structure of the Zn-Cr alloy coating electrodeposited at high current densities and change of the structure in the direction of thickness were examined by X-ray diffraction. The coatings containing under 20 and 35-40 wt percent Cr had hcp and bcc structure, respectively, while the coatings containing 20-35 wt percent Cr comprised two phases with the hcp and the bcc structure. The structure of the coating varied in the direction of thickness; both the hcp and bcc phases coexisted in the vicinity of the substrate, while the hcp phase became predominant with increasing distance from the substrate.