Method of Activating Titanium Surfaces

The present invention relates to a method of activating titanium surfaces for the subsequent cladding with metallic coatings in the course of which the surface is first of all wet-blasted with finely granular Al.sub.2 O.sub.3 and then treated with a fluoride-containing solution for a period of a few minutes at room temperature.

One such method is known from British Pat. No. 13 07 649. In this prior-art method, activation is effected in an acetic-acid/hydrofluoric-acid solution after the wet-blasting and etching. However sufficient bond strengths upon the subsequent cladding cannot be obtained with such an activation bath since a heat treatment must be carried out subsequently.

German Patent Application P No. 30 08 314.8-45 which is the priority application of (U.S. patent application Ser. No. 238,627) has proposed a method of this type in which the wet blasting and etching are followed by an activating of the surface with a solution of chromic acid, hydrofluoric acid and arsenic or antimony compounds at temperatures of 35.degree. to 100.degree. C. for a period of 15 to 50 minutes. This proposed method gives excellent bonding properties upon the subsequent cladding with metallic coatings. However, when arsenic and antimony solutions are used, the danger of side reactions is not entirely excluded, which then change the bath so that the desired activating function is not definitely assured. The monitoring and control of the bath is thereby made difficult.

An object of the present invention is so to provide an improved method of this type in which the activating bath can be controlled precisely and can be supplemented without difficulty, particularly after standing for a long time.

In accordance with the invention, this object is achieved in that the wet-blasting and etching are followed by activating the surface with a solution of chromic acid, fluoric acid, and hexafluosilicic acid at temperatures of 35.degree. to 100.degree. C. for a period of 15 to 50 minutes.

It has been found that the reproducibility of the activation can be improved by the method of the invention. There are improvements also upon use in series manufacture. The activating of the titanium surface is effected in the manner that the surface is increased in its roughness. As another advantage of the method of the invention, the possibility exists, after activation has been effected (roughening of the surface), to store the titanium part untreated even for a long period of time since the newly-forming oxide skin is not detrimental for further treatment due to the bonding process by the enlarged surface.

A particularly long-lasting activation can be obtained if the hexafluosilicic acid, hydrofluoric acid and chromic acid are present in the solution for the activation of the surface in the following molar ratios (referred to the constituent elements):

Within these limits an excellent activation was obtained in an illustrative embodiment with the following molar concentrations:

H.sub.2 SiF.sub.6 : 0.6 mol/L

HF: 0.3 mol/L

CrO.sub.3 : 1.8 mol/L

The hexafluosilicic acid concentration should advantageously be selected between 0.1 and 2.0 mol/L.

In the method of the invention, silicon is present as an oxide, acid, fluoride compound, or fluorine-complex compound for the hexafluosilic acid of the activating solution.

It has been found that when this method is used, metal layers can be deposited with particularly high bond strength on titanium surfaces, obtaining bond strengths of about 70 N/mm.sup.2. As in the method of German application P No. 30 08 314.8-45 which is the priority application of (U.S. application Ser. No. 238,627), all method steps can be carried out at temperatures below 60.degree. C. so that the covering with wax necessary for selective cladding is possible. In this way selective cladding can be very considerably simplified. Finally another advantage of the method of the invention is that absorption of hydrogen by the base material is excluded.

In the method of the invention, the fluoride-containing solution which serves as an etching agent comprises a solution of nitric acid and hydrofluoric acid present in respective concentrations of 400 g/l and 5 g/l.