Abstract
When pure aluminum is passivated at constant applied voltage in solutions ranging from pH 0–7, the current density representing film growth falls during the first few seconds in a manner that is qualitatively and nearly quantitatively the same for all solutions. The fall of current is succeeded by a rise to a constant value; the rise occurs the earlier, and is the greater, the lower the pH. The fall is interpreted as caused by barrier‐layer formation, and the rise as caused by pore initiation when the field across the barrier layer has fallen low enough to allow the entry of protons into the film. Electron micrographs of the film surface show that very few pores are present on specimens that have not been anodized long enough for the initial current fall to halt, and that the pore system is fully established as soon as the succeeding rise is complete. A mechanism for pore formation is proposed.