The weight gains of Al specimens immersed for short times in water at 50°–70°C were measured. An analysis of these data and published results at other temperatures indicated that the kinetics of pseudoboehmite film growth in the temperature range 50°–100°C is determined initially by the nucleation and growth of hydrolysis sites on the amorphous oxide surface, and subsequently by solid‐state diffusion through the pseudoboehmite layer. At 40° C, pseudoboehmite and bayerite were observed to grow simultaneously at first, with bayerite crystallization eventually becoming the dominant process. The two oxide phases occupied completely separate layers.

Electrochemical etching of Al with alternating current produces a high density of cubic etch pits covered with a film of oxide or hydroxide. Micrographs of the metal and film structures are presented and a model is proposed to explain the observations. Calculations demonstrate the feasibility of film deposition from acid solution when a high cathodic current density is applied. A critical feature is that the high aluminum salt concentration near the metal surface hinders proton mobility.

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The effect of a hydrated oxide on the kinetics of anodic oxidation of aluminum has been studied and the properties of the resulting composite oxides compared with those of purely anodic oxides. The presence of a hydrate layer reduced the amount of anodic oxide needed to support a given voltage and increased the current efficiency to as much as 100%. Water was lost from the oxide during anodization, probably as the result of transformation of boehmite to ~-A1203, which was incorporated into the barrier layer. The dielectric strength was greater for a composite oxide than for an oxide produced in the absence of a hydrate, though the dielectric constants were probably not very different. The impedance characteristics of a composite oxide indicated fewer microfissures than in a purely anodic oxide.The formation of an oxide film on aluminum by the sequential processes of reaction in hot water followed by anodic oxidation produces a composite oxide with some unique properties. This process has been of technological importance for some time in the manufacture of electrolytic capacitors (1, 2), but has received scant attention in the scientific literature. Altenpohl (3) briefly mentioned that the anodic oxide grows underneath the hydrate and that the latter is partially consumed by the growing film. He suggested that water is eliminated from the consumed portion of the hydrate whenever this portion crystallizes into ~/-A1203 under the influence of a high electric field. Unfortunately, no experimental evidence accompanied this discussion. Burger and Cheseldine (4, 5) also advanced the idea that the barrier portion of the composite oxide consists of both anodic oxide and converted hydrate, but their data were insufficient to confirm this.The lack of information on the composite oxide prompted us to perform the investigation reported here of the effect of a hydrated oxide on the kinetics of anodic oxidation, and the properties of the resultant composite oxide. Our data support the hypothesis that some hydrate is dehydrated during anodization and becomes converted to barrier oxide. Moreover, details of this process are presented that have not been available previously. Experimental ProceduresCommercially etched 99.97% pure aluminum foil was used throughout. The increased surface area improved the sensitivity of weight measurements and the "corrugations" increased the rigidity and strength of the isolated oxide, greatly simplifying handling procedures. Any change in surface area over the range of oxide thickness studied was sufficiently small to have no effect on the interpretation of results. Sample size was 6.4 x 7.6 cm for weight and charge measurements and smaller samples, 2.6 x 3.6 cm, were used for impedance measurements. The foil was not treated in any way prior to hydration in boiling water at atmospheric pressure2 Anodic oxidation was at 2 ma/cm 2 in 100 g/1 boric acid at 90~The boric acid was electronic grade, and distilled deionized water with a resistivity ca. 504),000 ohm-cm was used throughout. Foils were rinsed...

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