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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Properties of pseudoboehmite films, produced by the reaction of aluminum foil with boiling water, were studied by a variety of techniques. For reaction times of 1-30 rain the average film porosity decreased linearly with increasing film weight. The surface area of freshly prepared films was similar to that of alumina gel, but decreased on aging in air. Aging in 1N NaNO~ inhibited the area decrease. The series capacitance decreased on aging in air or nitrate electrolyte. It was determined that with the sample immersed in electrolyte the measured dielectric properties were those of an inner region impermeable to electrolyte. This layer was not the same as a typical barrier oxide. The films exhibited a low frequency dielectric dispersion. From the temperature dependence of the loss maxima the activation energy and relaxation time for the process were found to be 13.4 kcal/mole and 6.2 x 10 -15 sec. These are in the range found for other systems containing hydroxyl chains or bound water.Films produced by the reaction of aluminum or aluminum oxide with water are of importance in corrosion (1), the sealing of porous anodic oxides (2), and the manufacture of electrolytic capacitors (3). A recent paper by Vedder and Vermilyea (4) has greatly extended our knowledge of the formation and physical properties of these porous hydrous films. They identified the film produced by reaction with boiling water as pseudoboehmite, a poorly crystallized oxide with a structure close to that of boehmite but containing more hydroxyl than corresponds to the formula A1OOH. The composition of the film will vary, but reaction at 100~ has always been found to result in an average water content of 27% (3, 5, 6).Some new data on film thickness and surface area are presented here, but the primary purpose of this paper is to report on the dielectric properties of films produced by the reaction of aluminum with boiling water. This was of interest to us because these films are often incorporated into the dielectric layer of electrolytic capacitors (3). There may be more general interest in the comparison of the dielectric properties of this film with those of other materials such as boehmite, ice, and silica gel. ProcedureThe hydrous films were grown on 99.99% A1 foil specimens that had been cleaned by chemical polishing in a mixture of 15 parts 70% HNO3 + 85 parts 85% H~PO4 for 2 min at 85~ followed by a 10 min etch in 1N NaOH at room temperature. The reaction with boiling, distilled deionized water was in Pyrex beakers for times up to 30 min. For reaction times of 3 min or less, two samples were prepared consecutively and then the water discarded. For longer reaction times fresh water was used with each sample.Series capacitance and RC were measured with a resistance ratio arm bridge. The maximum amplitude of the a-c signal was 0.2V rms. A large capacitance (1300 ~f) was in series with the signal generator to block the passage of d-c current through the measuring cell. Two measuring cell designs were used. For measurements at a frequency of 120 Hz...

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