Insight into the role and its mechanism of polyacrylamide as an additive in sulfate electrolytes for nanocrystalline zinc electrodeposition

Electrodeposition of nanocrystalline zinc coating is greatly significant in improving the mechanism, wear and corrosion properties of conventional coarse-grained zinc coating and its formation is usually by the synergetic effect of two or more additives on electrodeposition reaction. However, the multicomponent additives not only make the electrolytes complicate and unstable, but also difficult to analyze the effect mechanism of each additive on electrodeposition of nanocrystalline zinc coating. In this study, nanocrystalline zinc coating is electrodeposited from a simple sulfate bath with polyacrylamide as the only additive. The influence and effect mechanism of polyacrylamide on electrodeposition of nanocrystalline zinc coating are investigated. It is found that the addition of polyacrylamide does not change the instantaneous nucleation mechanism with three-dimensional diffusion-controlled growth nucleation of zinc, however an increase in the zinc deposition potential is observed in the presence of polyacrylamide compared with electrodeposition of zinc coating from the additive-free bath. The zinc deposition overpotential increase is due to the absorption of polyacrylamide on the surface of the electrode, specifically the synergistic adsorption of its polymer chain and amine group, which is beneficial for increasing the free energy to form new nuclei, thereby results in a higher nucleation rate and smaller grain size.