Effect of additives on electrodeposition of zn∑zr oxide composite from dispersed particles-free solution

Electrodeposition of ZnZr oxide composite was performed from an unagitated sulfate solution containing Zn^2＋, Zr ions and additives such as NO₃ ions and polyethylene glycol（PEG）at pH 2 and 313 K under galvanostatic conditions, and the effect of additives on the codeposition of Zr oxide and polarization property and micro structure of deposits was investigated. Zr content in deposits obtained at all the current densities increased significantly with addition of 2.0 g・dm³ of NaNO₃. ZnZr oxide deposited from the solution containing NaNO₃ showed massive structure composed of fine crystals without Zn platelets crystals, and a lot of large cracks occurred between the massive crystals. EDX analysis revealed that Zr codeposited on the massive crystals as fine concavoconvex oxide. The corrosion current density of ZnZr oxide deposited from the solution containing NaNO₃ was almost same as that of pure Zn deposits, showing no improvement of corrosion resistance with codeposition of Zr oxide. On the other hand, Zr content in deposits obtained from the solution containing PEG increased significantly with increasing current density above 1000 A・m². With addition of 1000 mg・dm³ of PEG, the Zn platelets crystals disappeared, and the deposits were composed of fine meshlike crystals with preferred orientation {1010}Zn plane, resulting in smooth surface. The cathodic current density for the reduction of dissolved oxygen on the ZnZr oxide deposited from the solution containing PEG was smaller than that of pure Zn deposits, as a result, the corrosion current density of ZnZr oxide deposits was smaller than that of pure Zn deposits. The increase in Zr content in deposits with NO₃ ions and PEG is attributed to acceleration of hydrolysis reaction of Zr ions.