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

Abstract

Abstract Aluminum-magnesium (Al–Mg) alloys were potentiostatically electrodeposited on copper using Al anode at 180 °C from 62AlCl 3  + 17NaCl + 15KCl + 6MgCl 2 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 $${{\text{Al}}_{2}\text{Cl}}_{7}^{-}$$ , and $${\text{AlCl}}_{4}^{-}$$ ; that of Mg is MgCl 2 (possibly forming complexes in electrolyte). Al anodic dissolution occurs significantly in Group 1. Na(AlCl 4 ) formation and Cl 2 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(AlCl 4 ) formation; (iii) Deposited Mg is curtailed to maximum of ~ 4.02 at.% by Na(AlCl 4 ); and (iv) A minimum threshold exists for Al/Mg deposit composition ratios. The scheme and corollaries answer questions of practical and fundamental importance.