SAO(ti): Sulphuric anodic oxidation (titanium)

Compared with chromic, sulphuric and hard anodizing carried out on aluminium alloy, the structure of the film obtained on titanium by sulphuric anodizing is different. In this latter case, it is a barrier type film, i.e. thin, compact, and non-porous.


Description of the process

In most cases, this treatment is carried out on pure titanium, T40 or T60, or on titanium alloy, TA6V, TA5Zr, TU2, etc.

The treatment parameters can be adjusted as follows:

  • H2SO4 concentration: 150 to 300gr/l
  • Temperature: 15 to 30°C
  • Current density: 0.2 to 2.0 A/dm2
  • Time: 5 to 30 min

However, we should explain that these operating conditions are only an indication and that different acids can be used.

Unlike in the case of lightweight alloys, to the extent that there is no "porous part” over the "barrier part” neither the sealing nor is the colouring operations are performed.

Hence, the treatment is generally confined to an alkaline degreasing operation (degreasing in chlorinated solvent is generally avoided in order not to risk weakening the titanium by inclusion of hydrogen), a stripping operation in a fluo-nitric bath and the anodizing itself.

After the anodizing process a further treatment will often be applied. It could be paint or a dry lubricant such as MoS2 or PTFE based varnish.  It should be noted that titanium surfaces treated by anodizing are very sensitive to handling and stain easily.

Anodizing allows us to obtain oxide films of controlled thickness, and the colour is the result of the optical interference of white light.

The optical interference effect comes from the fact that the incident light is partly reflected, and partly transmitted and refracted in the oxide film. The light reaching the metal / oxide interface is again partly absorbed, but mostly reflected in the oxide film. Several reflections may take place, during which it dephasing occurs.  The beam that finally emerges provides optical interference resulting in a light of reduced wavelength; in other words, coloured light reaching the eye.

The thickness of the anodic film depends on the voltage applied and, as the colour depends on the thickness of the film, this colour can be controlled by the anodizing voltage. The following table shows the relation between the formation voltage and the colour of the anodic films obtained in a sulphuric medium.

Voltage Thickness Colour of the oxide film
2 volts 25 Å silver
10 volts 180 Å pale gold
14 volts 242 Å dark gold
18 volts 260 Å dark gold / purple
22 volts 349 Å blue / purple
30 volts 610 Å light blue
  • Appearance: usually metallic blue
  • Thickness: in the order of a tenth of a micron.
  • Surface condition: no degradation
  • Friction coefficient: behaviour unchanged compared with the base metal
  • Reduction in fatigue: little data available
  • Corrosion resistance: does not alter the metal’s very good intrinsic resistance. Care should be taken over the risk of galvanic coupling with another metal. Titanium is more noble than most metals.

In most cases, anodizing is conducted in such a way as to obtain a blue tinted film. However, there are exceptions for decorative films (jewellery, eyewear, etc.) or technical films which are thick and of different colours.

Substrates

All grades of alloys and for all modes of processing (rolled, forged, cast, extruded or machined)

Variants

Use of chromic acid or phosphoric acid baths. Variants of voltage cycles.

Applications

Bonding base for organic coatings (paint, glue, etc.). Tracking in relation to other metals (stainless steel, etc.) by the colour obtained. Colouring for aesthetic reasons for jewellery, eyewear, sports items, etc.

Sectors concerned: Aeronautical, mountings, etc.

Environmental impact

This process is not affected by ELV, RoHS or REACH regulations.

Reference systems

  • NF EN 2808: Anodizing of titanium and titanium alloys; aerospace sector