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).

  • Appearance: colourless to slightly iridescent
  • Thickness: 2 to 7 µm
  • Surface condition: slight degradation of the roughness
  • Reduction in fatigue: comparable to that of the CAO, depending on the alloy, may be compensated for by prior shot peening

Substrates

All grades of aluminium alloys and for all modes of processing. However, the alloy greatly influences the level of performance obtained and the appearance.

Variants

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

Applications

  • Corrosion resistance.
  • Paint adhesion base.
  • Unlike CAO it cannot be applied as a preparation before bonding. In this case we use the Phosphoric anodic oxidation (PAO /AAP) recommended by Boeing or the Phospho-Sulphuric anodic oxidation (PSA) recommended by Airbus.

Sectors concerned: aeronautical / aerospace (recommended by Airbus)

Choice criteria - Limitations

  • The parts of an assembly must be treated separately. Riveted, screwed or articulated assemblies cannot be treated by this process because the possible acid residues are corrosive. For the same reason, the process is little or not at all used on cast alloys.
  • Treatment leads to a significant reduction in the fatigue limit of the components. For most aeronautical applications, chromic anodizing is preferred.

Environmental impact

Via the deletion of the chromium hexavalent, which is toxic and carcinogenic, this process complies with the REACH Regulation.

Reference systems

  • NF EN 4704: Aerospace series
  • ISO 9227: Corrosion test in artificial atmospheres - Salt spray tests
  • EN ISO 2409: Paint and varnish - Test grid