The electrodeposition of aluminium from molten aluminium chloride/sodium chloride

The electrodeposition of aluminium from an AlCl₃/NaCl melt (80 wt % purified A1C1₃, 20 wt % NaCl at 175°C) has been studied and conditions established for the production of good quality electroplate on steel. To obtain a coherent, dendrite-free plate at current densities up to 16 mA/cm², the presence of not less than about 0.07 wt % hydrogen chloride was found to be essential. Under these conditions the current efficiency was 85%. Continued plating resulted in a progressive increase in current efficiency up to practically 100% and a progressive deterioration in plate quality. Addition of more HCl at this stage restored the original conditions. These observations explain inconsistencies in previously reported values for current efficiency and maximum current density. The mechanism of the action of HCl was not established but it is suggested that it may have an action akin to chemical polishing by dissolving off the high spots on the already deposited aluminium. Alternatively, the electrolytic reduction of hydrogen preferentially on active sites may block these off and so prevent the build-up of aluminium dendrites.