Effect of Organic Additives on Electrodeposition Behavior of Zn from Alkaline Zincate Solution and Its Micro Structure

Electrodeposition of Zn was performed on a Fe electrode at a current density of 20-5000 A·m−2 and a charge of 4 × 104 C·m−2 in an unagitated zincate solution containing 0.62 mol·dm−3 of ZnO, 4.0 mol·dm−3 of NaOH and organic additives at 313 K. The effects of organic additives on the deposition behavior of Zn and the microstructure of the deposits were investigated. The films deposited at current densities higher than 1000 A・m−2 from the solution containing additives of a straight-chain polymer composed of quaternary ammonium cation (PQ) and a quaternary ammonium salt with benzene ring (QA) exhibited the gloss. The polarization curve was separated into a partial polarization curve of Zn deposition and that of hydrogen evolution by using the galvanostatic data of Zn deposition. The overpotentials of charge transfer of Zn deposition and that of ZnO22− ions diffusion increased with additions of PQ and QA. The increase in overpotential was remarkable at potential less noble than −1.5 V. Zn deposition reached the diffusion limit of ZnO22− ions at potential less noble than −1.5 V, indicating that the diffusion of ZnO22− ions was greatly suppressed by PQ and QA. With additions of PQ and QA, the C, N and H were co-deposited with Zn, showing that the additives of PQ and QA were incorporated in deposited films. The size of the Zn crystallite decreased with increasing current density. At a high current density of 5000 A・m−2, the crystallite size decreased furthermore with additions of PQ and QA and the film showed the smooth surface. The orientation index of the {1010} plane of Zn deposited from the solution containing PQ and QA increased with current density. The change of the crystallite size and crystal orientation of deposited Zn were explained related to the deposition overpotential.