A new image of porous aluminium oxide

New electron microscopy data for porous aluminium oxide show that simultaneously with the appearance of a gel-like nascent oxide a gas evolution takes place. In a stationary state, the nearly straight-line succession of gas bubbles leads to the formation of ‘cylindrical’, pores in the anodized aluminium film. The new data, with earlier evidence, sustain the discontinuousmodel of aluminium porous oxide growth, in which the electrical breakdown of the barrier layer is the rate-determining step. The gas can be one of the electron-avalanching breakdown consequences. This seems to be the easiest explanation for the development of porous aluminium oxide, even if it contradicts the usual image of the continuous displacement of the ‘solid state’ barrier layer in front of the porous oxide.