Anodizing commercially pure titanium surfaces

Anodizing is a special kind of surface treatment for titanium which strengthens and densifies the existing thin passive layers, resulting in a spectrum of surface colors ranging from silver and gold through blue and purple to pale blue.

Anodizing is performed in a variety of electrolytes. Dilute sulfuric and phosphoric acids are frequently used. As the thickness of the anodized coating depends on the voltage used it can be precisely and reproducibly controlled via the voltage.

This process is comparable with aluminum anodizing.

Particularly adherent coatings are achieved by submerging the workpiece in a solution of 80% phosphoric acid, 10% sulfuric acid and 10% water and applying a direct current. The workpiece should be connected as the anode (+ pole). A sheet of stainless steel or same-grade titanium can be used as the cathode (- pole).

Using a controllable dc source it is possible to create red-gold (15 V), yellow-gold (65 V) and violet (85 V) surfaces. Surface oxidation can also be carried out in a 20% phosphoric acid solution, in which blue (10 V), yellow (20 V) and violet (30 V) surfaces can be created.

The color coatings adhere firmly to the substrate and are resistant to abrasion. The colors themselves remain constant if the surface is given a transparent protective coating.

Oxide coatings can be deposited on the surface of virtually any titanium material, although parts subjected to anodic oxidation require a certain pretreatment to produce firmly adhering, bright and uniformly colored coatings.

For example, as the surfaces to be treated must be free of impurities, the following steps are recommended prior to anodizing:

1. Degrease using suitable degreasing agents
   (this step may be omitted if impurities are only slight),
2. Rinse in water,
3. Treat in an aqueous solution of 20 vol.-% HNO₃
   (65 % nitric acid) and 2 vol.-% HF (40 % hydrofluoric acid),
   immersion period 0.5 to 2 minutes depending
   on required surface structure,
4. Rinse in water,
5. Anodize.

The relevant precautions must always be taken when working with caustic fluids.

Hydrofluoric acid is highly toxic, extremely caustic, water miscible, hygroscopic and highly reactive. Its fumes are heavier than air and have a pungent odor. They react with certain metals, giving off hydrogen. The acid causes slow-healing wounds on skin, eyes and mucous membranes and inflammations of the nailbed (no warning effect!) which can be extremely painful.

The burns often remain invisible on the skin for several days!

Further information on the anodizing of titanium can be found in the attached information material. For more detailed information we recommend the publications listed below:

1. A. Aladjem "Anodic oxidation of titanium and its alloys" Journal of Materials Science 8 (1973), p. 688/704
2. J.-L. Delplancke, M. Degrez, A. Fontana, R. Winand "Self-colour anodizing of titanium" Surface Technology 16 (1982), p. 153/62
3. F. Climent Montoliu, R. Capellades Font "Die anodische Oxidation von Titan" Galvanotechnik 75 (1984), H. 6, p. 686/91
4. H. Yamaguchi, K. Miki, H. Satoh, K. Shimogori "Characteristics of anodized titanium and its application" Sociètè Francaise de Métallurgie, Vol. 4, p. 1817/22 Proceedings of the Sixth World Conference on Titanium Cannes, June 6-9, 1988 Edited by P. Lacombe, R. Tricot, G. Bèranger Published by les èditions de physique
5. A.K. Sharma, H. Bhojaraj "Anodizing Titanium for Space Applications" Titanium 1990 - Products and Applications, Vol. 1, p. 282/89 Proceedings of the Technical Program from the 1990 International Conference Published by Titanium Development Association

Deutsche Titan, Nov. 2000

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