Effect of Organic Additives on Electrodeposition Behavior of Zn from Zincate Solution Containing Potassium Hydroxide and Its Micro Structure

Abstract

Electrodeposition of Zn was performed on an Fe electrode at a current density of 50-5000 A·m−2 and a charge of 4 × 104 C·m−2 in an unagitated zincate solution at 313 K 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 solution containing organic additives and the microstructure of the deposits were investigated. In solution containing a quaternary ammonium cation (PQ) and a quaternary ammonium salt with a benzene ring (QA), the current efficiency for Zn deposition at high current density region of 1000 to 5000 A·m−2 to produce the glossy films was higher with KOH than that with NaOH. At high current densities above 1000 A·m−2, Zn deposition approaches the diffusion limitation of ZnO22− ions. With additions 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 200 A·m−2 investigated by AC 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 additives on the Zn deposition is smaller with KOH than that with NaOH, the current efficiency for Zn deposition at high current density region is larger with KOH. The upper limit of current density to produce the glossy films was smaller with KOH than that with NaOH, and the spongy thin films were partially observed on the platelets crystals obtained at high current density in KOH solution. The content of C resulting from the additives in deposited Zn was smaller with KOH. These phenomena are attributed to the adsorption ability of PQ and QA onto the cathode being smaller with KOH.