Electropolishing process for titanium

Abstract

The present invention relates to a method of electrochemical polishing of surfaces of titanium or titanium-containing alloys, such as Nitinol. An electrolyte is used that comprises methanesulfonic acid and one or more alkanediphosphonic acids. These alkanediphosphonic acids can optionally be substituted with hydroxy and/or amino groups. A further aspect of the present invention relates to the use of said electrolyte for the electropolishing of titanium or titanium-containing alloys.

Description

The present invention relates to a method of electrochemical polishing of surfaces of titanium or titanium-containing alloys. This method is especially suitable for alloys that have a titanium content of at least about 50 mol. %, for example the nickel-titanium alloy Nitinol. For this, an electrolyte is used that comprises methanesulfonic acid and one or more, optionally substituted alkanediphosphonic acids. The present invention also relates to the use of said electrolyte for the electropolishing of surfaces of titanium and/or titanium-containing alloys, such as Nitinol.

BACKGROUND OF THE INVENTION

Electrochemical polishing or electropolishing, as well as the brightening and deburring of metal surfaces, is a process that is frequently employed in industry for treatment of the surfaces of metal workpieces and objects. This treatment is used for improving surface quality, for example by removing burrs from edges and surfaces, by smoothing, cleaning and brightening, both for decorative and for technical purposes. Furthermore, electropolishing can relieve stresses in the outer layers of the material.

The workpieces that are to be processed are arranged on suitable, electrically conducting holding devices, or alternatively in baskets or drums made of electrically conducting material. These devices, together with the workpieces, are immersed in a polishing bath that contains an electrolyte, and connected as the anode, and a direct current is applied. Under the action of the current and the electrolyte, metal is removed from the surface of the material, thus smoothing and deburring the surface. Then the workpiece is taken out of the polishing bath and rinsed.

The following mixtures are used in the state of the art for the treatment of titanium and titanium alloys:

1. Perchloric acid and acetic anhydride;
2. Hydrofluoric acid, sulfuric acid and acetic acid;
3. Hydrofluoric acid, sulfuric acid and acetic anhydride;
4. Sulfuric acid, hydrofluoric acid, phosphoric acid and ethylene glycol;
5. Sulfuric acid, ammonium difluoride and hydroxycarboxylic acids.

Examples are described for example in WO 01/00906 A1 and DE 103 20 909 A1.

All of these electrolytes are indeed capable of achieving satisfactory electropolishing results on surfaces of pure titanium and a selection of titanium alloys, but they are only partially able to produce a satisfactory quality of electropolishing of Nitinol, a nickel-titanium alloy, which consists of about 50 mol. % Ni and about 50 mol. % Ti and is often also called “shape memory alloy”.

Furthermore, all of these electrolytes have the disadvantage that their use involves substantial technical and health risks. Thus, for the electrolyte mixture according to No. 1 there is a risk of explosion if it is handled incorrectly, whereas the other electrolytes pose a considerable health risk, in particular because of their content of fluorides.

The use of these electrolytes, and processes in which these electrolytes are used, in an industrial context requires expensive equipment and precautions for maintaining the process parameters, and for protection of labor and of the environment. Generally the execution of these electropolishing processes additionally requires an expensive cooling system, so that the generally very low working temperatures can be maintained and, when using fluoride-containing electrolytes, enclosure of the electropolishing plant, and exhaust air purification.

EP 1 354 986 A2 describes a device and a method for the electropolishing of titanium and titanium alloys (titanium degree 1 to 10), which uses an electrolyte of sulfuric acid and alcohols. Apart from the health risk through the apparently preferred use of methanol and the associated formation of highly toxic, carcinogenic dimethylsulfate during the process, this method has the further disadvantage that because this electrolyte is highly flammable, expensive cooling of the electrolyte to temperatures below 15° C. is necessary, together with automatic extinguishers for fire protection.

Patent application DE 100 37 337 A1 describes a method and a holder for the electropolishing of objects made of titanium alloys or nickel-titanium alloys such as Nitinol, in which the electrolyte comprises formamide and sulfamic acid.

Therefore there is a considerable need for a method of electropolishing, with which titanium and titanium-containing alloys such as Nitinol can be smoothed and deburred efficiently and at high quality, without causing any appreciable pollution or dangers for humans and the environment, and making expensive cooling equipment and safety precautions unnecessary.

DESCRIPTION OF THE INVENTION

The present invention relates to a method of electropolishing for the electrochemical smoothing and/or deburring of surfaces of titanium or titanium-containing alloys, that does not have the aforementioned disadvantages. These methods are based on the use of an electrolyte that comprises methanesulfonic acid and one or more alkanediphosphonic acids, wherein the one or more alkanediphosphonic acids can optionally be substituted with one or more hydroxy and/or amino groups.

In a preferred embodiment of the method of this invention, the concentration of methanesulfonic acid in the electrolyte is at least 95 wt. %, based on the total weight of the electrolyte. As a rule, therefore, concentrated methanesulfonic acid with a content of at least 98 wt. %, for example methanesulfonic acid with a content of more than 99 wt. %, is used, to which the one or more alkanediphosphonic acids are added as pure substance(s).

An especially suitable alkanediphosphonic acid for use in the method described here is 1-hydroxyethane-1,1-diphosphonic acid (HEDP, also called etidronic acid). Thus, HEDP alone or in combination with other alkanediphosphonic acids in concentrated methanesulfonic acid, as defined previously, can be used as the electrolyte for use in the method described here.

The concentration of the one or more alkanediphosphonic acids in the electrolyte is preferably between 1 and 50 g/kg electrolyte, for example between 3 and 25 g/kg electrolyte. It is especially preferable for the one or more alkanediphosphonic acids to be used between 5 and 20 g/kg electrolyte.

In one embodiment of the present invention, the electrolyte consists substantially of methanesulfonic acid and one or more alkanediphosphonic acids.

This means that no other substances are added and other such substances are only present in small amounts, for example of less than 3 wt. %, e.g. through contamination of the methanesulfonic acid or of the alkanediphosphonic acids, or on account of operation in the course of electropolishing (for example stripped ions). It is thus also preferable that the electrolyte that is used according to the present method should contain little if any water. The water content of the electrolyte is therefore preferably at most 2 wt. %, and better still less than 1 wt. %, based on the total weight of the electrolyte.

When carrying out a method according to the present invention, the electropolishing preferably takes place at a temperature in the range from 20° C. to 70° C., for example between room temperature and 60° C., and in particular at a temperature between 25° C. and 50° C. The anodic current density at which the workpieces are electropolished can be selected from a wide range. This is preferably in the range from 2 to 50 A/dm², in particular between 5 and 30 A/dm². The applied voltage can often be in the range from 10 to 35 V.

The duration of the electropolishing process naturally depends on the particular roughness of the workpiece being processed and the desired smoothing or deburring. The optimal time of action can be determined at no great expense by a person skilled in the art by means of routine experiments as a function of the current density used, the temperature, the electrolyte and the equipment used. As a rule, treatment of the workpiece for a few minutes is sufficient.

Following the electropolishing process, the treated workpiece is taken out of the electropolishing bath and rinsed with water, preferably deionized water. It is not decisive that the electrolyte be removed from the workpiece immediately. The electrolyte described here does not attack the treated surface, which facilitates the processing and does not impose any additional requirements on the equipment that is used.

The electrolyte described here can be used both for pure titanium and for titanium-containing alloys. These titanium-containing alloys can in particular include titanium at a proportion of at least 50 mol. %. An important alloy of this kind, which includes titanium at a proportion of about 50 mol. %, is the nickel-titanium alloy Nitinol, which is also called “shape memory alloy”. In tests with the electrolyte described here it was found that, in particular, workpieces made of Nitinol can also be electropolished with a method according to the present invention efficiently and with a good result. The use of an electrolyte that comprises methanesulfonic acid and one or more alkanediphosphonic acids, for the electropolishing of surfaces of titanium, titanium-containing alloys and nickel-titanium alloys, such as Nitinol, is a further aspect of the invention described here.

In contrast to the electrolytes that were used previously in the state of the art, the use of these electrolytes does not require any technically demanding equipment, instead they can be used in usual industrial electropolishing plant, as used for example for the treatment of special steel. In particular an electrolyte, as used according to the present invention, is not combustible, is not especially corrosive and is easy to handle. With normal handling, there is no increased risk for the people operating the electropolishing plant or working in the vicinity of this plant, or for the environment. In particular, the electrolyte described here does not release any harmful gases or vapors.

The method and electrolyte described here are not only easier to use in the treatment of titanium-containing surfaces, they also make it possible to achieve smoothing or deburring of the surfaces that is at least equal, if not even superior in many cases, to the methods that are described in the state of the art.

The invention will be explained in more detail in the following examples. However, these examples only represent possible embodiments of the electropolishing method described here and should in no way imply any restriction to the conditions used here.

EXAMPLES

Example 1

Titanium alloy TiAlV4

A plate with the dimensions 50×50×1.0 mm with a ground surface and roughness of Ra=0.8 μm was electropolished in an electrolyte consisting of 990 g of 100% methanesulfonic acid and 10 g of 1-hydroxyethyl-1,1-diphosphonic acid.

The operating variables were as follows:

Temperature: 30° C.

Current density: 20 A/dm²
Polishing time: 7 min
Prior to the treatment, the plate was degreased, rinsed with water and dried. After electropolishing, the plate was taken out of the electrolyte and, after a waiting time of 5 min, rinsed in deionized water and dried. The surfaces had a bright finish, without etch attack, and had roughness of Ra=0.3 μm. The edges were smooth and burr-free.

Example 2

Nitinol

Nitinol wire with a diameter of 0.8 mm was electropolished in an electrolyte according to Example 1.

The operating variables were as follows:

Temperature: 25° C.

Current density: 10 A/dm²
Polishing time 4 min

The wire was pretreated and posttreated according to Example 1.

The result was a bright, smooth surface without etch attack on the structure.

Claims

1. A method of electropolishing and/or electrochemical deburring of surfaces of titanium or titanium-containing alloys, wherein the electrolyte used comprises methanesulfonic acid and one or more alkanediphosphonic acids, and the one or more alkanediphosphonic acids can optionally be substituted with hydroxy and/or amino groups.

2. The method as claimed in claim 1, wherein the concentration of methanesulfonic acid in the electrolyte is at least 95 wt. %, based on the total weight of the electrolyte.

3. The method as claimed in claim 1, wherein the one or more alkanediphosphonic acids comprise 1-hydroxyethane-1,1-diphosphonic acid.

4. The method as claimed in claim 1, wherein the concentration of the one or more alkanediphosphonic acids is between 1 and 50 g/kg electrolyte.

5. The method as claimed in claim 1, wherein the concentration of the one or more alkanediphosphonic acids is between 5 and 20 g/kg electrolyte.

6. The method as claimed in claim 1, wherein the electrolyte consists substantially of methanesulfonic acid and of one or more alkanediphosphonic acids.

7. The method as claimed in claim 1, wherein it is carried out at a temperature between 20° C. and 70° C.

8. The method as claimed in claim 1, wherein the method is carried out at an anodic current density of 2-50 A/dm2.

9. The method as claimed in claim 1, wherein the method is carried out at an anodic current intensity of 5-30 A/dm2.

10. The method as claimed in claim 1, wherein the titanium-containing alloys include titanium at a proportion of at least about 50 mol. %.

11. The method as claimed in claim 10, wherein the titanium-containing alloy is Nitinol.

12. Use of an electrolyte as claimed in claim 1 for the electropolishing of surfaces of titanium and/or titanium-containing alloys.

13. The use as claimed in claim 12, wherein the titanium-containing alloy is Nitinol.