Electrodeposition of Al-Mg alloys from chloride-based molten salts

Aluminum-magnesium (Al-Mg) alloys were potentiostatically electrodeposited on copper using Al anode at 180 °C from 62AlCl3+17NaCl+15KCl+6MgCl2 electrolyte. The depositions are categorized into two groups based on current density-time curves and deposit compositions. Group 1 corresponds to overpotentials from −1.05 to −1.10 V, and Group 2 from −1.15 to −1.30 V. Morphologies within Group 1 possess almost pure Al and coarsen with overpotential; while those in Group 2 possess Al and Mg, and refine with overpotential. Based on electrochemical, compositional, and spectral analyses, a generic deposition scheme is proposed. Mathematical variables in this scheme are adjusted rendering it suitable for both groups. Sources of Al for deposition are Al2Cl7-, and AlCl4-; that of Mg is MgCl2 (possibly forming complexes in electrolyte). Al anodic dissolution occurs significantly in Group 1. Na(AlCl4) formation and Cl2 evolution occur at anode in Group 2. The proposed scheme is used to devise the following corollaries: (i) Al deposits preferentially in both groups; (ii) Mg deposition is prerequisite for Na(AlCl4) formation; (iii) Deposited Mg is curtailed to maximum of ~4.02 at.% by Na(AlCl4); and (iv) A minimum threshold exists for Al/Mg deposit composition ratios. The scheme and corollaries answer questions of practical and fundamental importance.