SAO: Sulphuric anodic oxidation

This is generally implemented in a bath containing 150 to 250 g/l of sulphuric acid, at a temperature between 16 and 24°C, at a current density of 1.2 to 1.8 A/dm2.


Description of the process

he duration of treatment is 20 to 60 min, the thickness of the oxide film obtained depends on the chemical composition of the alloy and varies according to the application sought and the industrial sector concerned. It is usually 5 to 25 µm. The treatment leads to a swelling equivalent to 33% of the thickness of the film and a slight increase in the surface roughness. The oxide layer is mainly composed of a porous structure. Sealing treatment by hydration of the porous layer is necessary to achieve the maximum corrosion resistance, but paint adhesion decreases significantly. It is possible to colour the coating, a bright metallic colour is obtained by chemical colouration: black, red, blue or green.

Electrolytic colouring, by the deposit of a mixture of oxides and metals, produces shades of gold, bronze and black that are very solid under light. The colouring operation is located before the sealing.

  • Appearance: metallic grey
  • Thickness: 5 to 25 µm
  • Microhardness: 200 / 250 Hv 0.050
  • Surface condition: slight degradation of the roughness
  • Electrical insulation: 500 to 1500 volts
  • Coefficient of friction: good under medium load
  • Reduction in fatigue: 5 to 40 % in alternating traction, depending on the alloy, may be offset by prior shot peening
  • Corrosion resistance:
    - without sealing: > 100 h (BS ISO 9227 NSS)
    - sealing with nickel salt: > 300 h
    - sealing with chromium VI salt > 500 h

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

  • Use of baths composed of mixtures of sulphuric and organic acid.
  • ontinuous processes (sheet metal coils) and pad application (new parts or repairs)

Applications

Corrosion resistance, erosion or abrasive wear and good performance on scratching

Applications : automotive, building, navigation, household appliances, aerospace, cosmetics, food applications, etc.

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

The process used has no critical influence on the environment, it contains no heavy metals or chlorinated solvents. Consequently, so far there is no threat to the continuation of this process.

Reference systems

  • EN 2284: aeronautical sector
  • EN 12 373-1: building sector
  • EN 12 373-2 / 17: Methods of measurement of the anodic coating
  • ISO 9227-NSS: neutral salt spray test
  • EN 12 373-18: pitting corrosion rating system