During a.c. electrograining of aluminium in HCl electrolyte, a non-uniform etch film is developed over the aluminium surface. The film is deposited as colloidal material, containing fine, amorphous Al(OH)3 particles, together with appreciable quantities of water (about 90%); additionally, metallic aluminium particles, resulting from undermining of the substrate during pit propagation, are resident within the film. By examination of etch film development as a function of a.c. waveform frequency, it is evident that the film develops during negative half cycles when sufficient boundary layer alkalinity occurs. Thus, for a.c. waveform frequencies below about 250 Hz, hydrogen evolution causes a significant local increase of pH allowing the formation of aluminate ions. The aluminate ions are soluble only in the high pH interfacial region and precipitate as Al(OH)3 upon contact with the bulk acid electrolyte.
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