Some investigators of anodic oxide coatings on aluminum state that they are amorphous and others state that they are crystalline in structure. The present paper describes a variety of experiments which indicate that an x‐ray diffraction pattern corresponding to that of gamma alumina is obtained when the formation potential is above about 100 volts. The formation of a crystalline coating does not seem to be a characteristic of the electrolyte, for crystalline coatings were obtained with a variety of electrolytes. In some way, high electric stress seems to favor the formation of an ordered oxide lattice.
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Taylor et al. (1945) studied this question.