In an alkaline zincate bath, the electrode reaction of Zn deposition proceeds in accordance with the following equations; Zn(OH)
42-→Zn(OH)
2+2OH
- and Zn(OH)
2+2e→Zn+2OH
-. It is controlled by the charge-transfer reaction and the diffusion of Zn(OH)
42- ion towards the interface from the bulk, while Co deposition is controlled by the diffusion of Co (tetren)
2+ ion.
The influence of plating variables on the composition of deposited alloys was examined. As a result, Zn-Co alloy deposition from this zincate bath was found to be an anomalous codeposition, in which Zn, the electrochemically less noble metal, deposits preferentially. The mechanism of anomalous codeposition, that is, the preferential deposition of Zn was explained on the basis of Zn (OH)
2 film formation. In the absence of additives, columnar crystals perpendicular to the substrate grew remarkably, while the addition of brighteners notably improved the surface state of alloy deposits, and their morphology changed to a smooth and fine structure. Moreover, the additives caused the preferred crystal orientation of alloy deposits. The corrosion resistance of Zn-Co alloy deposits was highly superior to that of Zn deposits, and the performance was much improved when chromated.
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