Abstract
The pitting behaviour of aluminium electrodes in hydrochloric acid under an applied alternating voltage has been examined by scanning and transmission electron microscopy and electrical methods. Pit formation is though to occur by local attack of metal at pre-existing flaws in the air-formed film on the aluminium substrate. Merging of larger pits and undermining of the surface film produces an overall pitted and convoluted surface topography with an overlying covering of relatively thin, probably discontinuous, hydrated alumina. The hydrated alumina is apparently produced during the negative half cycle of the alternating voltage waveform by a dissolution/precipitation mechanism caused by the production of a high local pH due to hydrogen evolution, or alternatively by protonation and hydration of the initial air-formed film.
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