Effect of organic additives on the electrodeposition behavior of Zn from an alkaline zincate solution and its microstructure

Abstract

Electrodeposition of Zn was performed on an Fe electrode at a current density of 205000 A·m-2 and a charge of 4 × 104 C·m-2 in an unagitated zincate solution at 313K containing 0.62 mol·dm-3 of ZnO, 4.0 mol·dm-3 of NaOH, and organic additives. The effects of organic additives on the deposition behavior of Zn and the microstructure of the deposits were investigated. Glossy films were obtained by depositing at current densities higher than 1000 A·m-2 from the solution containing additives of a straight-chain polymer composed of a quaternary ammonium cation (PQ) and a quaternary ammonium salt with a benzene ring (QA). The polarization curve was separated into partial polarization curves of Zn deposition and hydrogen evolution by using the galvanostatic data of Zn deposition. The overpotentials of the charge transfer of Zn deposition and that of ZnO22- ion diffusion increased with the addition of PQ and QA. The increase in overpotential was considerable at potentials less noble than -1.5V. Zn deposition reached the diffusion limit of ZnO22- ions at potentials less noble than -1.5V, indicating that the diffusion of ZnO22- ions was suppressed considerably by PQ and QA. With the addition of PQ and QA, C, N, and H were codeposited with Zn, which demonstrated that the additives of PQ and QA were incorporated into the deposited films. Zn crystallite size decreased with increasing current density. At a high current density of 5000 A·m-2, the crystallite size decreased with the addition of PQ and QA, and the surface of the film was smooth. The orientation index of the {1010} plane of Zn deposited from the solution containing PQ and QA increased with increasing current density. The changes in the crystallite size and crystal orientation of deposited Zn were explained by the deposition overpotential.