The morphology of anodic aluminum oxide films formed in neutral aqueous 3% ammonium tartrate was investigated using transmission and replica electron microscopy and transmission stereoscopy. After an initiation period of approximately 90 min, corresponding to the appearance of a minimum in the potentiostatic anodic current, development of a porous structure was observed. The film structure was comparable to that formed in conventional pore‐forming electrolytes at shorter times, with approximately polygonal oxide cells, each containing a central pore. Pore depth and diameter both increased in a systematic manner with anodizing time; a hypothetical model relying on field‐assisted dissolution of oxide, with local field enhancement due to the pore geometry, is used to explain this dependence. At longer times, the pore depth increases linearly with anodizing time, also consistent with this model and with the observed establishment of a stable pore structure. The stable pore size and oxide cell size are proportional to formation voltage in accord with the geometric argument forwarded to explain pore coalescence and the formation of stable terminal pores.
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Choo et al. (1975) studied this question.