Effect of Thiocyanate Ions on the Electrodeposition Behavior of Zn-iron-group Metal Alloys in Sulfate Baths

The electrodepositon of Zn-iron-group metal binary alloys, which has been applied in the production of highly corrosion-resistant alloy-plated steel sheet, was conducted in sulfate baths. The current density-dependence of the alloy composition demonstrated a typical feature of an anomalous codeposition in sulfate baths. As the present authors reported previously, the alloy deposition behavior was divided with respect to the current density into four regions. The presence of thiocyanate (SCN-) ions in the baths changed the alloy deposition behavior both in lower current density region I where normal codeposition occurred with extremely poor current efficiency and in the region II where anomalous codeposition occurred with high current efficiency. The partial current efficiency of iron-group metal was found to increase in regions I and II. The boundary current density between these two regions, which was also known as the transition current density, was also found to increase. It was concluded that the catalytic action of SCN- ions served to explain well the changes in the electrodeposition behavior.