GB2150603A

GB2150603A - Method for the surface treatment of articles of titanium and a bath therefor

Info

Publication number: GB2150603A
Application number: GB08430209A
Authority: GB
Inventors: Dr Christopher Matz
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1983-12-01
Filing date: 1984-11-29
Publication date: 1985-07-03
Also published as: FR2568270A2; GB8430209D0; FR2568270B2; GB2150603B; DE3427543C2; DE3427543A1

Abstract

Workpieces of titanium or titanium alloys are surface treated in an alkaline bath comprising an alkali hydroxide (preferably 7.5m NaOH) and a titanium complexing agent (preferably 0.33m hydroxy carboxylic acid). An impurity ion complexing agent, preferably a complex ion such as ethylenediamine tetraacetic acid, and/or a builder, such as sodium silicate, may also be provided. The treatment may be anodisation.

Description

SPECIFICATION Method for the surface treatment of articles of titanium and a bath therefor The invention relates to a method for the surface treatment of articles, such as workpieces, of titanium or titanium alloys by dipping the workpieces in an alkaline bath.

Titanium materials have acquired great importance in many fields of engineering, such as in aircraft construction, because these materials are superior to other materials because of their low specific gravity and their high strength even at high temperatures. Despite these advantages, the use of titanium materials is not without problems, however, because opposed to the advantages outline above there are also disadvantages such as low resistance to wear and a strong tendency towards stress corrosion. It is therefore necessary to subject the titanium materials to a pretreatment in order that these materials may be able to be machined and used in the required manner without disadvantageous effects.

It is known to use acid baths or alkaline baths for the pretreatment of titanium materials. Thus materials can be removed from structural members for example (chemically milled) or titanium components can be pretreated for following working or processing such as the application of coatings, diffusion welding or adhesive connections. In the case of acid baths, hydrofluoric acid baths are employed but their use is decreasing because of the danger of this working medium. In addition, strict regulations have to be observed in disposing of spent hydrofluoric baths and these have an adverse effect on the economy of the pretreatment.

In the case of alkaline methods, two baths have hitherto been known, namely the bath known under the firm name of "Turco 5578" and a bath consisting of a mixture of sodium hydroxide and hydrogen peroxide. The first bath is only suitable for short treatment times, however. for example for the pretreatment before adhesive measures, because with a longer treatment period, an unwanted incrustation forms on the titanium material. It is true that the second alkaline bath permits longer treatment times but this method requires heavy expenditure on checking and attention because of the unstable bath components.

The present invention seeks to provide a method for the pretreatment of articles and workpieces of titanium or titanium alloys which permits a simple operation, free of problems, without great expenditure on control and supervision.

According to a first aspect of the present invention there is provided a method for the surface treatment of workpieces of titanium or titanium alloys by dipping the workpieces in an alkaline bath, wherein the alkaline bath comprises an alkaline hydroxide, and a titanium complexing agent.

In preferred methods the alkaline bath also comprises an impurity ion complexing agent and/or a builder.

According to a second aspect of the present invention there is provided an alkaline bath for surface treating titanium-containing articles, comprising an alkali hydroxide and a titanium complexing agent.

The above method ensures, through the bath components, a high removal of material without the formation of incrustations and in addition the surface of the treated components is very well suited for the adhesion of such components, for the application of coatings such as surface protection and for diffusion welding.

Caustic soda solution (NaOH) can be used for the alkali hydroxide while hydroxy carboxylic acid or its salts, for example sodium tartrate, can be used for the titanium complexing agents.

Complexons, preferably ethyledediamine tetraacetic acid (EDTA), can be used for the impurity ion complexing agents. A complexon is an especial class of complexing agent. The indication of amount m means mols per litre of the particular bath. It is advisable to heat the alkaline bath, namely between 30"C and 1 10'C. Particularly favourable operating conditions exist at 70"C to 80"C, and especially at substantially 75"C. As an immersion time, a period of between 5 and 90 minutes, preferably 30 minutes has proved advani,nc:15.

As mentioned above, the method ac3oidirry to the invention is an alkaline method, that is to say it is free of hydrofluoric acid. In addition, what is important is that no incrustation forms on the treated titanium componet,ts and that the surfaces produced are very well suited for adhesion, coating or for diffusion welding.

A builder such as a phosphate, silicate c: borate, preferably sodium silicate with a concentration of 0.02 m, may be admixed with the bath components. This bath component prevents major variations in the caustic removal of material under difficult mordant conditions, for example in the case of extremely thin or small workpieces.

In addition, the bath can also be used for the anodising of titanium materials, as a result of which a further improvement in the surface treatment of titanium components can be achieved. In the case of such a mode of operation only the alkali hydroxide and the titanium complexing agent must be present in the bath while the impurity ion complPxlc- agent can be added optionally. The builder can likewise be added optionally.

In the anodising mode of operation the alkaline bath is at a voltage of 3V to 50V preferably 1 or. The bath is preferably simul taneously heated to between room temperature and 60"C, preferably 30"C. The preferred treatment time is 1 5 minutes, but the treatment times within the range 2 to 90 minutes can be employed advantageously. The voltage regulation of the anodising proces may be replaced by a process controlling the amount of current, preferably at 1A/dm2.

Claims

1. A method for the surface treatment of workpieces of titanium or titanium alloys by dipping the workpieces in an alkaline bath, wherein the alkaline bath comprises an alkali hydroxide and a titanium complexing agent.

2. A method as claimed in claim 1, wherein the concentration of alkali hydroxide is in the range 0.5 to 10m.

3. A method as claimed in claim 2, wherein the concentration of alkali is substantially 7.5m.

4. A method as claimed in any preceding claim wherein the concentration of the titanium complexing agent is in the range 0.1 to 1m.

5. A method as claimed in claim 4, wherein the concentration of the titanium complexing agent is substantially 0.33.

6. A method according to any preceding claim, wherein the alkaline both also comprises an impurity ion complexing agent.

7. A method according to claim 6, wherein the concentration of the impurity ion complexing agent is in the range 0.01m to 1 m.

8. A method according to claim 7, wherein the concentration of the impurity ion complexing agent is substantially 0.067m.

9. A method according to claim 7, wherein the concentration of the impurity ion complexing agent is substantially 0. ism.

10. A method as claimed in any preceding claim, wherein the caustic soda solution (NaOH) is used as the alkali hydroxide.

11. A method as claimed in any preceding claim, wherein hydroxy carboxylic acid or a salt thereof is used as the titanium complexing agent.

1 2. A method as claimed in claim 11, wherein sodium tartrate is used as the titanium complexing agent.

1 3. A method as claimed in any preceding claim, wherein a complexon is used as the impurity ion complexing agent.

14. A metnod according to claim 13, wherein the complexon used is ethylenediamine tetraacetic acid (EDTA).

1 5. A method as claimed in any preceding claim, wherein a builder is admixed with the bath components.

1 6. A method as claimed in claim 15, wherein 'as a builder there is used a phosphate, silicate or borate.

17. A method as claimed in claim 16, wherein as a builder there is used sodium silicate (Na2SiO3).

1 8. A method as claimed in claim 17, wherein the concentration of the sodium silicate is substantially 0.02m.

1 9. A method as claimed in any preceding claim, wherein the alkaline bath is heated.

20. A method as claimed in claim 19, wherein the alkaline bath is heated to a temperature between 30"C and 1 10"C.

21. A method as claimed in claim 20, wherein the temperature is within the range 70 to 80"C.

22. A method as claimed in claim 21.

wherein the temperature is substantially 75'C.

23. A method as claimed in any preceding claim, wherein the treatment period amounts to between 5 and 90 minutes.

24. A method as claimed in claim 23, wherein the period is substantially 30 minutes.

25. A method as claimed in any of claims 1 to 18, wherein the alkaline bath is at a voltage of 3V to 50V in order to anodise the workpiece.

26. A method as claimed in claim 25, wherein the voltage is substantially 10V.

27. A method as claimed in claim 25 or 26, wherein the bath is simultaneously heated to between room temperature and 60"C.

28. A method as claimed in claim 27, wherein the temperature is substantially 30"C.

29. A method as claimed in any of claims 25 to 28, wherein the treatment time amounts to between 2 and 90 minutes.

30. A method as claimed in claim 29, wherein the treatment time amounts to substantially 1 5 minutes.

31. A method as claimed in any of claims 25 to 30, wherein the voltage regulation of the anodising process is replaced by a process controlling the amount of current.

32. A method as claimed in claim 31, wherein the amount of current is controlled substantially at 1 A/dm2.

33. A method of surface treatment substantially as herein described.

24. As an independent invention the additional feature of any of claims 2 to 32.

35. An alkaline bath for surface treating titanium containing articles comprising an alkali hydroxide and a titanium complexing agent.

36. An alkaline bath for surface treatment substantially as herein described.