STA: Sulpho-tartaric anodic oxidation
Sulpho-tartaric anodic oxidation is the emanation of an Alenia patent improved by Airbus. The aim is to replace chromic anodic oxidation, (CAO).
Description of the process
It is generally performed in a bath containing about 40 g/l of sulphuric acid and 80g/l of tartaric acid, at a temperature of 35 to 40°C. The constant voltage is 14/15V. The cycles of approximately 25 minutes include a voltage rise of 5 minutes and then a steady state of 20 to 25 minutes. The sulpho-tartaric anodizing forms very thin layers, 2 to 7 µm (thicknesses similar to those formed during a CAO). It creates good corrosion resistance thanks to the adhesion of the paint films, painting is performed on an unsealed layer. Its influence on the fatigue characteristics is moderate (reduction in the order of 20 to 30% compared to the bare substrate, similar to that induced by a CAO).
All grades of aluminium alloys and for all modes of processing. However, the alloy greatly influences the level of performance obtained and the appearance.
In the case of unpainted parts requiring high corrosion resistance the anodizing cycle is extended to approximately 15 minutes (long cycle STA) to obtain a thicknesses of 5 to 7 µm and then sealing is done in 2 steps, the first consisting of impregnation with a trivalent chromium based solution followed by hydrothermal sealing in water at 98°C
Sectors concerned: aeronautical / aerospace (recommended by Airbus)
Choice criteria - Limitations
Via the deletion of the chromium hexavalent, which is toxic and carcinogenic, this process complies with the REACH Regulation.