The influence of Al substrate intermetallic precipitates on zinc electrodeposition

Abstract

The cathodic reaction during zinc electrodeposition is usually characterized by cyclic voltammetry on the assumption that homogeneous nucleation is taking place on the cathodic substrates. In this present study, this premise was examined for pure Al and its alloys, including AlFe, AlSi, AlFeSiMg and the commercial alloy AlFeSi, by systematic observations of the substrates in the scanning electron microscope, using various modes of detection such as secondary electrons, back scattered electrons, for electron channelling contrast, and X-ray energy dispersive spectrometry. The results clearly show that zinc nuclei form heterogeneously on precipitates rather than on the alumina-coated matrix. Under constant potential, alloys with precipitates had deposition rates a few orders of magnitude larger than in the case of high purity Al. Normalization of the electrochemical data with the volume fraction of precipitates indicates that the deposition rates and morphology of the deposits are affected by the chemistry of the precipitates. The effectiveness of the alumina barrier was demonstrated by the observation that the specific features of the microstructure, such as grain boundaries and plastically deformed substrate, as observed by electron channelling contrast, did not act as locations for zinc nucleation. The high rate of electrodeposition on Al alloys was determined to be due to the presence of intermetallic particles, which significantly improve the electronic conductivity through the alumina film. For the AlFeSiMg alloy, two types of intermetallic particles, one rich in Fe and the other in Si, were identified. The voltammetry study indicates that Fe-rich particles provide more favorable sites for zinc electrodeposition. In order to examine the influence of Fe and Si particles, binary alloys of AlFe and AlSi were tested and their deposition characteristics, together with the integrated charge accumulation, were compared. These studies clearly revealed that a one-to-one correlation exists between the intermetallic particles and Zn nucleation locations.