Abstract
The effect of boric acid as additive in electrodeposition of Zn–Co alloy coatings from an acidic sulphate electrolyte containing NH4Cl, Na2SO4 and citric acid Na3C6H5O7, at ambient temperature, without agitation has been investigated. The deposition process was investigated by cyclic voltammetry. It was found that the current density first decreased and then increased, due to adsorption of a boric acid complex and/or changes in the morphology, but the initial deposition potential was not affected. The addition of boric acid to the bath led to the formation of improved Zn–Co deposits, composed of coalesced globular fine grains smaller than ∼2 μm in diameter. Also, all of the Zn–Co deposits studied consisted of η phases. The Co content in the Zn–Co deposits produced in the presence of boric acid increased from 2·5 to 4·3 wt-%. It is suggested that Zn–Co deposits produced in the bath containing 60 g L−1 boric acid probably offer sacrificial protection to the steel substrate.
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