The mechanism of electrograining aluminium sheet in nitric/boric acid electrolyte

Aluminium sheet is roughened commercially in various electrolytes to form a pitted substrate suitable for lithographic printing plates. Although studies on hydrochloric acid systems have been published, little has been reported on the equally important nitric acid based electrolytes.

Observations were made using high resolution scanning electron microscopy on a commercial lithographic sheet alloy (AA1050A) that had been a.c. electrograined for very short times in a nitric/boric acid electrolyte. These were combined with electrochemical information to elicit the mechanism by which a.c. electrograining proceeds.

It was determined that graining is governed by anodizing processes. Pitting proceeds at flaws in the oxide film. At the start of the anodic cycle the exposed metal, which may be in existing pits or new sites, is anodized forming an oxide layer that replicates the shape of the pit, thus forming a cup shaped structure. As the temperature rises the rate of dissolution overtakes the barrier film growth rate, which causes detachment of the anodic oxide cup and effectively direct metal dissolution occurs. When the potential falls, repassivation of the pit occurs.

Repetition of this process causes a successive increase in pit diameter as each oxide cup is formed and the cups are forced out of the pit in a stacked formation. As the pit grows the oxide film on the surrounding metal surface peels back around the periphery of the pit.

The cathodic half cycle influences pit initiation.