Zn–Co electrodeposits: Heterogeneous structure and anomalous deposition

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Abstract
Structural characterization of electrodeposited Zn-Co alloys with Co contents in the range 1·5–6·6 wt% has been carried out by transmission electron microscopy and scanning electron microscopy. Particular attention has been paid to the fine structure of these deposits which appears as a nanolaminated composite. The deposits contain small Zn crystallites encapsulated within an oxide layer. The Co has been shown to exist at the oxide-metal interface in the Zn-6·6 wt% Co coating. With the Zn–1·5 wt% Co coating the Co has also been found to be associated with the oxide. The presence of Co at the Zn–ZnO interface in the Zn-6·6 wt% Co coating appears to disturb the epitaxial linkage between the two phases. Lattice mismatch in the Zn–6·6 wt% Co also produces microstresses and the observed cracks. A hydroxide oscillation model is proposed to explain the laminated structure in Zn–Co deposits, as well as the associated anomalous deposition behaviour. Thus the deposition occurs through an intermediate hydroxide layer and this encourages Zn deposition rather than Co which is the more noble metal. The hydroxide layer undergoes a cyclic decay and replenishment during deposition and it is this pattern of behaviour which encourages the formation of each Zn and ZnO lamination. Zn crystallites are formed by the reductions of the hydroxide, and ZnO arises through its dehydration. Preferential deposition of Co occurs in the short period before the hydroxide is replenished and further Zn2+ reduction can take place. At the very early stage of deposition, Co enrichment within fine Zn-ZnO crystallites occurs. This could result from the need to set up appropriate conditions for anomalous deposition.

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