Anodic oxidation process for aluminium and aluminium alloys in chromic acid electrolytes (30-50g/l CrO3,40°C). The voltage is gradually increased over a period of 10 minutes to 40 V, kept constant at 40 V for the next 20 minutes and increased to 50 V during the next 5 minutes, leaving 5 minutes.
The process is used, for example, in aviation for corrosion protection of Al components, but also as a pretreatment for bonding Al alloys. One of the advantages of the chromic acid electrolyte is that no corrosive electrolyte is left behind in the gaps of riveted joints, rearranged edges, etc., because the chromic acid residues diluted with the rinsing water do not corrode the aluminium; hexavalent chromium rather acts as a corrosion inhibitor for aluminium. The brown colour of the electrolyte also reveals cracks and porous areas on workpieces, which is an advantage for quality assurance.