Effect of organic additives on electrodeposition behavior of Zn from zincate solution containing potassium hydroxide and its micro structure

Zn was electrodeposited on an Fe electrode at a current density of 50-5000 A.m-2, charge of 4 × 104 C.m-2, and temperature of 313K in an unagitated zincate solution containing 0.62 mol.dm-3 of ZnO, 4.0 mol.dm-3 of KOH or NaOH, and organic additives. The effects of KOH and NaOH on the deposition behavior of Zn in the solution containing the organic additives and on the microstructure of the deposits were investigated. In a solution containing a straight-chain polymer composed of a quaternary ammonium cation (PQ) and a quaternary ammonium salt with a benzene ring (QA), the current efficiency for Zn deposition in a high-current-density region (1000-5000 A.m-2) to produce glossy films was higher with KOH than that with NaOH. At high current densities above 1000 A.m-2, the Zn deposition approached the diffusion limitation of ZnO22- ions. With the addition of PQ and QA, the diffusion of ZnO22- ions was significantly suppressed, and the degree of suppression was smaller with KOH than that with NaOH. The polarization resistance at 200A.m-2, which was investigated through alternating current impedance, revealed that the adsorption ability of PQ and QA onto the cathode was smaller with KOH than that with NaOH. Since the suppression effect of the additives on the Zn deposition was smaller with KOH than that with NaOH, the current efficiency for Zn deposition in the high-current-density region was larger with KOH. The upper limit of the current density needed to produce glossy films was smaller with KOH than that with NaOH, and spongy thin films were partially observed on platelet crystals obtained at high current densities in the KOH solution. The C content resulting from the additives in the deposited Zn was smaller with KOH because the adsorption ability of PQ and QA onto the cathode was smaller with KOH than that with NaOH.