Investigation of Subvalent Ion Effects During Aluminum Anodization in Molten NaCl-AlCl[sub 3] Solvents

Abstract

Anode current yield variations with current density have been measured in the nominal I: 1 and I: 2 molar NaCI-A/CI3 molten electrolytes by an anodic stripping procedure. In the range of apparent current densities 0.03-6.30 mA cm-2, the brutto electrode reaction was found to be a faradaic three-electron oxidation. Subvalent ion effects could not be positively established. Experimental difficulties in removing final traces of impurities limit the precision of coulometric determinations in these systems; however, the anodic stripping procedure does provide a sensitive means of detecting corrosion losses of aluminum. It has recently been reported (1, 2) that the anode current yields for aluminum dissolution in molten NaC1-A1Cls solvents are appreciably higher at low current densities than required for the three-electron oxidation reaction AI(0)-3e-~ At(liD [1] As early as 1857 (3), high current yields for the anodic dissolution of aluminum were recorded, and they have since been observed for a variety of electrolytes; aque-ous (4, 5), organic (6-8), and nonaqueous (1, 2, 9). In general, the alternative explanations to account for these deviations from a faradaic three-electron oxidation are (i) the formation of subvalent aluminum ions, (ii) spallation or electrophoretic removal of colloidal aluminum particles, and (iii) chemical corrosion, i.e., direct electrolyte or impurity attack on the anode surface. Delimarskii et al. (1) found that the anode current yields in the NaC1-A1CI3 (l:2M) electrolyte at 160~ were dependent on the anodic current densities for the range 10-150 mA cm-2 and, for all experiments, the anode current efficiencies were usually >100% and exceeded the cathode current efficiencies (weight losses corresponding to faradaic three-electron oxidation of aluminum are 100%). From studies in the equimolar NaC1-A1Cls melt at 300~ Storozhenko (2) determined current yields >100% for the anodic dissolution of annealed aluminum anodes and, similarly, the current yields increased with decreasing anodic current densities, to reach a value 230% at the minimum investigated current density of 0.11 mA cm-2. Delimar-skii and co-workers considered that chemical corrosion or losses from spallation were unable to explain the magnitude of their anode current yields, whereas the results of Storozhenko are stated to have been corrected for losses due to chemical corrosion. Accordingly , both sets of data were interpreted by the assumption that considerable quant…