Titanium anodizing
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle:
Compared to chromic, sulfuric and hard anodizing on aluminum alloys, the structure of the film obtained on titanium by sulfuric anodizing is different. In this last case, it is a barrier type film, i.e. thin, compact and non porous.
In most cases, this treatment will be performed on pure titanium, T40 or T60, or on alloyed titanium, TA6V, TA5Zr, TU2.....
As an example, the processing parameters can be set as follows:
150 to 300gr/l
H2SO4 concentration
15 to 30°C
Temperature
0.2 to 2.0 A/dm2
Current density
5 to 30 minutes
Time
It should be noted, however, that these operating conditions are only indicative and that, in particular, different acids may be used.
Since there is no "porous part" over the "barrier part", the sealing operation is not practiced, contrary to the case of light alloys, nor is the coloring operation.
The treatment will therefore generally be limited to an alkaline degreasing operation (degreasing in a chlorinated solvent medium is usually avoided to avoid the risk of embrittling the titanium by inclusion of hydrogen), a pickling operation in a fluoro-nitric bath and then the anodizing itself.
After the anodizing treatment, a complementary treatment is often applied. This can be a paint or a dry lubricant such as MoS2 or P.T.F.E. varnish.
It should be noted that anodized titanium surfaces are very sensitive to handling and stain easily.
The anodizing process produces oxide films of controlled thickness and the color results from the optical interference of white light.
The optical interference effect results from the fact that the incident light is partly reflected, 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 can take place, during which a phase shift occurs. The beam that finally emerges, provides the optical interference resulting in light of reduced wavelength, ie, colored light reaching the eye.
The thickness of the anodic film depends on the applied voltage and, as the color is a function of the film thickness, this color can be controlled by the anodizing voltage. The following table shows the relationship between the formation voltage and the color of anodic films obtained in sulfuric medium.
In most cases, the anodization will be carried out in order to obtain a blue colored film. Nevertheless, exceptions exist for decorative films (jewelry, eyewear...) or technical films which are of different thickness and color.
SUBSTRATE
All grades of alloys and for all types of transformation (rolled, forged, cast, extruded, machined)
VARIANTS
Use of baths made of chromic acid or phosphoric acid.
Variations of tension cycles.
APPLICATIONS
Bonding base for organic coatings (paints, glues, etc.)
Identification in relation to other metals (stainless steel, etc.) due to the color obtained
Coloring for aesthetic purposes in jewelry, eyewear, sports equipment....
Sectors concerned: aeronautics, fasteners.....
ENVIRONMENTAL IMPACT
This process is not impacted by ELV, RoHS or REACH regulations.
REFERENCE
NF EN 2808 anodizing of titanium and titanium alloys, aerospace sector
Implementation
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