Abstract
Zinc-iron-group metal alloys were electrodeposited from sulfate solutions. In a wide range of plating conditions, the deposition behavior of each metal showed the main features of anomalous codeposition. Evaluation of the pH in the vicinity of cathode and of the surface film resistance revealed the following mechanism of anomalous codeposition in which electrochemically less-noble zinc is deposited preferentially. During anomalous codeposition of zinc with iron-group metal, the electrodeposition of zinc proceeds with preceding zinc hydroxide formation on the cathode, which results from a rise in pH in the vicinity of cathode. On the other hand, since the critical pH for iron-group metal hydroxide precipitation is not attained in the cathode layer, iron-group metal deposition occurs by the direct discharge of iron-group metal ions through the zinc hydroxide film, which makes the iron-group metal deposition difficult. Consequently, the superiority of iron-group metal over zinc in nobility is canceled out by the film resistance of zinc hydroxide, resulting in the preferential deposition of zinc in anomalous codeposition.
