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 reactionAs 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; aqueous (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. Delimarskii 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 quantities of univalent aluminum ion are formed under the conditions of anodization, especially at current densities of 10 mA cm -2 and lower.It was of interest to examine this phenomenon in more detail as the presence of free or complex univalent aluminum ion in larger than trace quantities might have significant consequences in many of the chemical or electrochemical studies, in particular 9 Electrochemical Society Active Member. Key words: fused salts, NaC1-A1CI~ melt, aluminum electrode, subvalent aluminum, Al(I) ion. chloroaluminate solvents, e.g., materials corrosion in aluminum deposition ...

Anodic Dissolution of Aluminum in the Aluminum Chloride-1-Ethyl-3-methylimidazolium Chloride Ionic Liquid

“…During electrolysis, the anode rod sometimes developed a water-insoluble black coating. The formation of this black surface film on Al anodes in chloroaluminate molten salts and ionic liquids has been reported by many workers 8,11,12,15,16 and has been attributed to finely divided Al resulting from the disproportionation of subvalent Al species produced during an initial one-electron oxidation reaction. 15,16 Gale and Osteryoung 11 investigated this phenomenon in some detail in acidic AlCl 3 -NaCl, but were unable to confirm the participation of subvalent Al ions.…”

“…Arrhenius plots of log j 0 versus 1/T that were constructed from this data were linear, and G a,0 # was estimated from the slopes of these plots with the expression 22 G a,0 # = −2.303R ∂ log j 0 /∂ (1/T ) [11] Given the independence of log j 0 on the ionic liquid composition, it seemed valid to average the values of G a,0 # obtained at different melt compositions at each electrode and given temperature. The results were 18.0 ± 0.9, 17 ± 5, and 16.…”

“…2,[8][9][10][11][12] But only few such investigations have been undertaken in the related room-temperature ionic liquids, e.g., AlCl 3 -EtMeImCl 13 and AlCl 3 -BuMeImCl, 7 with the former study conducted in the basic (Cl − -rich) ionic liquid. However, the kinetics and mechanistic aspects of the anodization reaction remain unclear.…”

Electrochemistry—I

“…Treatment of molten chlorides, 134 chloroaluminates. 135 Purified hydrogen chloride may be obtained by scrubbing the commercial gas with sulfuric acid which absorbs sulfur dioxide and water vapor. Final traces of moisture are removed by passing the gas through a mass of granular aluminum chloride.…”

Electroless Growth of Aluminum Dendrites in NaCl ‐ AlCl3 Melts

“…As many authors observed, an aluminum anode was found to dissolve at a higher current efficiency than expected for a three-electron reaction. This was initially attributed to the formation of subvalent aluminum ions (7), but subsequently to the corrosion of aluminum after electrolysis caused by impurities in both NaC1-A1C13 (8) and BuPyC1-A1C13 (9-11) melts. However, there appears to be another type of problem in connection with the stability of aluminum after electrolysis, i.e., the spontaneous growth of aluminum dendrites on aluminum deposits after electrolysis.…”