The Valency of Aluminum Ions and the Anodic Disintegration of the Metal

Abstract

It appears from coulometric measurements that Al in Al‐amalgam may go into solution in basic electrolytes exclusively as a trivalent ion. Experiments made in aqueous solutions of normalNaOH , HNO3 , KNO3 , and K2Cr2O7 showed that at anodic current densities between 20 and 300 ma/cm2 dark, nearly black films, which constantly broke down, are formed on the Al electrode. Microscopic examinations revealed the presence of a multitude of metallic particles entangled in the hydrated Al‐oxide flakes, causing the dark color of the latter. Depending on the conditions, the diameter of the metallic particles and leaflets varied from almost 10−5 to 10−2 mm. Such a disintegration of Al was also observed when the partially amalgamated metal merely came in contact with distilled H2O : dark flakes of hydroxide, containing a multitude of smallest Al particles, were formed. There are indications that the disintegration of Al is caused by the forcible breakdown (e.g., by the current or Hg) of the oxide layers firmly sticking to the surface of the metal. On breaking down of the film the smallest Al particles which remain on the film are torn from the meal. The observed deviation from Faraday's law and the negative difference effect, may be explained in terms of this behavior, without recourse to the hypothesis of lower valency Al ions.

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

“…Hydrofluoric acid could not be considered suitable due to the great chemical activity of the system. Though other acid electrolytes have been examined, phosphoric acid is the electrolyte most commonly used in hydrocarbon air fuel cells (4,5).…”

“…19.63. 5 Downloaded on 2015-07-01 to IP Gravimetric coulometry.--The current efficiency for Al deposition was determined with a Sargent coulometric current source. Aluminum foil samples (10 rail, Alfa Inorganics, m5N purity) were weighed and used as cathodes at a current density of 1.4 mAcm -$ (geometrical area) for 20,000 sec (i = 48.25 mA).…”

“…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; aqueous (4,5), organic (6)(7)(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.…”