THE INFLUENCE OF AL SUBSTRATE INTERMETALLIC PRECIPITATES ON ZINC ELECTRODEPOSITION

The cathodic reaction during zinc electrodeposition is usually charact erized by cyclic voltammetry on the assumption that homogeneous nuclea tion is taking place on the cathodic substrates. In this present study , this premise was examined for pure Al and its alloys, including Al-F e, Al-Si, Al-Fe-Si-Mg and the commercial alloy Al-Fe-Si, by systematic observations of the substrates in the scanning electron microscope, u sing various modes of detection such as secondary electrons, back scat tered electrons, for electron channelling contrast, and X-ray energy d ispersive spectrometry. The results clearly show that zinc nuclei form heterogeneously on precipitates rather than on the alumina-coated mat rix. tinder constant potential, alloys with precipitates had depositio n rates a few orders of magnitude larger than in the case of high puri ty Al. Normalization of the electrochemical data with the volume fract ion of precipitates indicates that the deposition rates and morphology of the deposits are affected by the chemistry of the precipitates. Th e effectiveness of the alumina barrier was demonstrated by the observa tion that the specific features of the microstructure, such as grain b oundaries and plastically deformed substrate, as observed by electron channelling contrast, did not act as locations for zinc nucleation. Th e high rate of electrodeposition on Al alloys was determined to be due to the presence of intermetallic particles, which significantly impro ve 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 tha t Fe-rich particles provide more favorable sites for zinc electrodepos ition. In order to examine the influence of Fe and Si particles, binar y alloys of Al-Fe and Al-Si were tested and their deposition character istics, together with the integrated charge accumulation, were compare d. These studies clearly revealed that a one-to-one correlation exists between the intermetallic particles and Zn nucleation locations.