AC electrograining of aluminium

SummaryThe AC electrograining of aluminium has been investigated by electrochemical and surface analytical techniques. It is evident that the relatively uniform pitted and convoluted surface topography develops through anodic dissolution in the positive half-cycle and an etch film deposits during the negative half-cycle of the sinusoidal current waveform. The final topography arises from the merging of approximately hemispherical pits, which develop through the initiation of fine cube-shaped pits during each positive cycle. The effective size of the cube-shaped pits is governed by the charge passed during the positive half-cycle, which can be altered by variation of the frequency of the sinusoidal waveform; such modification of the cubic pit size influences the final appearance of the electrograined surface.The etch film, covering the macroscopic aluminium surface, arises from the local pH increase during the negative half-cycle, where anodic dissolution of aluminium as aluminate ions is feasible; aluminium hydroxide is precipitated where the high pH solution meets the lower pH bulk solution.