ELECTROPOLISHING PROCESS

Abstract

The present invention relates to a method for the electropolishing of surfaces of metals and metal alloys. Said method is characterized in particular in that it can be applied to a wide range of metals. Thus, it is suitable for the electropolishing of metal surfaces comprising iron, tungsten, magnesium, aluminum or an alloy of these metals. The electrolyte used in the method comprises methanesulfonic acid and at least one alcoholic compound selected from aliphatic diols of general formula CnH2n(OH)2 with n=2-6 and alicyclic alcohols of general formula CmH2m-1OH with m=5-8.

Description

FIELD OF THE INVENTION

The present invention relates to a method for the electrochemical polishing of surfaces of metals and metal alloys.

BACKGROUND OF THE INVENTION

The purpose of the process of electrochemical polishing or electropolishing is to produce metal surfaces of high purity, and to smooth and deburr the metal surfaces. Smoothing in the micro-range can also produce gloss of the surfaces so treated. Furthermore, electropolishing can also remove any stresses from the outer layers of the material.

There are a great many different electropolishing processes that can be used for the processing of various metals or metal alloys. As a rule these processes are based on the use of electrolytes that comprise a concentrated inorganic acid such as phosphoric acid or sulfuric acid or a mixture of concentrated inorganic acids, often with additives for further enhancing the action of the electrolytes so as to obtain smoother and shinier metal surfaces. Some examples of such additives are chromic acid, hydrofluoric acid, amine fluorides or organic additives, such as alcohols, amines, glycerol, etc.

However, a feature that is common to all of these existing electrolytes, which are widely used industrially, is that they can only be used successfully for certain metals and/or alloys and thus have a very restricted application profile. For the processing of various metals or alloys it is therefore often necessary to maintain a corresponding number of different electrolytes. Often the individual electrolytes must be kept strictly separate and in particular must not be mixed, as any mixing may damage them and make them unusable. Sometimes this can even result in certain constituents of the electrolytes reacting together and, for example, releasing substances that are hazardous or injurious to health. Furthermore, the requirements on execution of the process and equipment of the electropolishing installations are also often very varied on account of the different electrolytes, so that several installations have to be maintained for different materials.

The electrolytes usually employed are often hazardous materials, which depending on their particular toxicity, flammability and/or danger class are subject to special requirements and regulations with respect to their storage and use and require appropriate precautions in connection with environmental protection and labor safety. This in turn causes considerable expenditure and the associated costs.

The ideal solution for these problems would be an electropolishing process that is equally suitable for the processing of all metals and metal alloys and is largely harmless with regard to the associated environmental impact and labor safety.

An electrolyte that largely meets the requirements for universal application has long been known from the state of the art. This is a mixture of perchloric acid and acetic anhydride. Often, however, this mixture cannot be used industrially owing to the associated explosion risk, or can only be used with considerable expenditure on safety measures.

Patent application WO 01/71068 A1 discloses electrolytic polishing processes that can apparently be used for a wide range of metals or metal alloys. These electropolishing processes use, among other things, an electrolyte of methanesulfonic acid and methanol. This electrolyte has, however, the serious drawback that because of its high proportion of more than 80% of highly volatile methanol it is a health hazard, and presents a risk of fire and explosion. Therefore such a process can as a rule only be used at very low temperatures, for example max. 10° C., or with an expensive system for trapping the resultant vapors and carrying them away. Furthermore, suitability of this process for carbon steels, magnesium, magnesium alloys or aluminum-silicon alloys has not been disclosed.

The method disclosed in patent application U.S. 2005/0045491 A1 also appears to have a relatively wide range of applications, but once again, suitability for magnesium-containing metal surfaces or those comprising aluminum-silicon alloys is not disclosed. The electrolyte used contains at least 75 wt. % of an alkylene glycol, the remainder comprising a chloride salt of alkali metals and/or alkaline-earth metals.

SUMMARY OF THE INVENTION

The present invention relates to an electropolishing process that can be used successfully for a wide range of metals and metal alloys and is substantially harmless with respect to labor safety and environmental protection. The electrolyte used for this comprises methanesulfonic acid and at least one alcoholic compound, which is selected from aliphatic diols and alicyclic alcohols.

The method is suitable for the electropolishing of surfaces of such diverse metals as iron, tungsten and light metals, and of surfaces of alloys of these metals. In particular it is suitable for surfaces of iron or an iron alloy, such as ferronickel, special steel (stainless steels) or carbon steel, which can be electropolished according to the present process both in the hardened and in the unhardened form; of tungsten or a tungsten alloy, of magnesium, a magnesium alloy, aluminum or an aluminum alloy, as well as an aluminum-silicon alloy. An alloy of a particular metal means alloys in which this metal is the main constituent of the alloy, based on the weight of the constituents of the alloy. Often said metal (or metals) comprises more than 50 wt. % of the alloy.

The electrolyte used in the electropolishing process according to the present invention is a solution comprising methanesulfonic acid and at least one alcoholic compound, the at least one alcoholic compound being selected from the group comprising aliphatic diols of general formula C_nH_2n(OH)₂ with n=2-6 and alicyclic alcohols of general formula C_mH_2m-1OH with m=5-8. In particular, the alcoholic compound can comprise at least one aliphatic diol of general formula C_nH_2n(OH)₂, with n=3, 4, 5 or 6. Moreover, all isomers of these aliphatic diols can be used, provided the two hydroxyl groups are bound to different carbon atoms. Examples that may be mentioned are the compounds 1,2-propanediol, 1,2-butanediol or 1,4-butanediol.

In a special embodiment, the electrolyte contains, as alcoholic compounds, both at least one aliphatic diol of general formula C_nH_2n(OH)₂ and at least one alicyclic alcohol of general formula C_mH_2m-1OH, where n=2-6 and m=5-8.

The alicyclic alcohols of the present invention also comprise all isomers satisfying the general formula C_mH_2m-1OH with m=5-8. All carbon atoms can form the ring structure, such as in cyclopentanol, cyclohexanol, cycloheptanol and cyclooctanol; it is also possible, however, for one or more carbon atoms to form a hydroxyalkyl and/or one or more alkyl side chain(s). Electrolyte solutions comprising cyclohexanol are especially preferred.

In a preferred embodiment, the electrolyte according to the method for electropolishing of the present invention comprises a mixture comprising 5-93% methanesulfonic acid and 95-7% of the at least one alcoholic compound. These percentages and all others in the present application relate, unless stated otherwise, to the weight of the respective substances and solutions. It is especially preferred for the electrolyte to comprise 10-80% methanesulfonic acid and 90-20% of the at least one alcoholic compound. Thus, the electrolyte can comprise for example 20-50% methanesulfonic acid and 50-80% of the at least one alcoholic compound.

In particular, the method for electropolishing according to the present invention is characterized in that apart from methanesulfonic acid and alcoholic compounds, no other additives are required for the electrolyte. It should be mentioned in particular that the electrolyte used in this method contains neither chromic acid or chromates, nor perchloric acid or its salts. Moreover, the method does not use any highly volatile additives such as methanol, ethanol or esters, the high vapor pressure of which presents a particular challenge for labor safety both with respect to their flammability and their toxicity. In addition, the electrolyte does not contain any hydrofluoric acid and also on this basis is largely problem-free in operation.

Preferably, the electrolyte used in the method according to the present invention contains no water or only small amounts of water. Thus, the water content of the electrolyte should not exceed a proportion of 10% water. Furthermore, the electrolyte does not require any addition of salts to increase its conductivity.

In a special embodiment of the present invention the method is carried out at a temperature between 40° C. and 100° C. It is especially preferred for the method to be carried out at a temperature between 60° C. and 100° C. Since the electrolyte of the present method does not contain any highly volatile constituents, higher temperatures can be used, for example temperatures up to 80° C., up to 90° C., up to 100° C. or even higher, without the need for special precautions, for example for reliable capture and removal of vapors that form. The possibility that the method can also be carried out at higher temperatures makes it possible, on the one hand, for the electropolishing process to be carried out if necessary in a relatively short time, and on the other hand it means that expensive removal of the heat released in the electropolishing process becomes unnecessary. Accordingly, expensive cooling becomes largely or completely unnecessary. If cooling is used, it therefore does not have to satisfy high performance requirements.

There is also considerable freedom in the choice of anodic current density in the method presented here. Depending on the particular metal, values between 3 and 40 A/dm² of the surface to be polished are preferred, and values in the range 5-30 A/dm² are especially preferred. Tungsten or tungsten alloys in particular permit the use of higher anodic current densities of for example about 30-40 A/dm². However, the other materials described here can also be electropolished successfully at higher anodic current densities. For surfaces containing iron, aluminum and magnesium, however, anodic current densities of about 5-20 A/dm² are generally entirely sufficient.

The duration of the electropolishing operation depends of course on the metal being processed, the roughness of the workpiece to be polished, the desired amount of metal removal and the desired smoothing of the workpiece surfaces, and the temperature and the current density.

In addition to its wide applications, the method according to the invention possesses other important advantages over the existing electropolishing processes. Thus, the electrolyte used is not chemically aggressive and therefore after switching off the electropolishing current, as well as during the subsequent rinsing operations, it remains substantially inert with respect to the surfaces being electropolished. The surfaces are not chemically attacked and etched, so that the quality of the electropolished surfaces is maintained and no special measures are required for removing the electrolyte as quickly as possible from the treated workpiece. This is particularly important in the processing of workpieces with low corrosion resistance, for instance ordinary steel, magnesium, aluminum and their alloys.

In addition to the method itself in all its aspects presented here, a further aspect of the present invention relates to the electrolytes described above that are used in this method.

The invention is explained in more detail in the following examples. These examples only represent possible embodiments of the electropolishing process described here and of the electrolytes used therein, and do not in any way imply a restriction to the conditions used here.

EXAMPLES

1

Treated surface: Special steel, Material No. 1.4301

Electrolyte: 37% methanesulfonic acid+63% 1,2-propanediol

Temperature: 80° C.

Anodic current density: 10 A/dm²

Duration: 15 min

Result: Mirror finish

2

Treated surface: tool steel (carbon steel)

Electrolyte: 37% methanesulfonic acid+63% 1,2-pentanediol

Temperature: 80° C.

Anodic current density; 20 A/dm²

Duration: 10 min

Result: highly polished

3

Treated surface: tungsten

Electrolyte: 50% methanesulfonic acid+50% 1,2-propanediol

Temperature: 80° C.

Anodic current density: 40 A/dm²

Result: highly polished

4

Treated surface: magnesium

Electrolyte: 20% methanesulfonic acid+40% 1,2-propanediol+40% cyclohexanol

Temperature: 60° C.

Anodic current density: 10 A/dm²

Duration: 8 minutes

Result: highly polished

5

Treated surface: aluminum-silicon alloy AlSi₂O

Electrolyte: 20% methanesulfonic acid+80% 1,2-butanediol

Temperature: 80° C.

Anodic current density; 10 A/dm²

Duration: 12 min

Result: highly polished

6

Treated surface: aluminum-magnesium alloy AlMg₁

Electrolyte: 50% methanesulfonic acid+50% 1,2-propanediol

Temperature: 80° C.

Anodic current density: 10 A/dm²

Duration: 10 min

Result: highly polished

Claims

1. A method for the electropolishing of surfaces of metals, comprising exposing a metal surface to an electrolyte in the presence of an electric current,

wherein the metal surface is selected from iron, tungsten, a light metal, and an alloy thereof, and

wherein the electrolyte comprises: methanesulfonic acid, and at least one alcoholic compound selected from the group comprising aliphatic diols of general formula CnH2n(OH)2 with n=2-6 and alicyclic alcohols of general formula CmH2m-1OH with m=5-8.

2. The method as claimed in claim 1, wherein the surface comprises iron or an iron alloy, hardened or unhardened.

3. The method according to claim 2, wherein the iron alloy is ferro-nickel, special steel, or carbon steel.

4. The method as claimed in claim 1, wherein the surface comprises tungsten or a tungsten alloy.

5. The method as claimed in claim 1, wherein the surface comprises magnesium, a magnesium alloy, aluminium, or an aluminum alloy.

6. The method as claimed in claim 1, wherein the surface comprises an aluminum-silicon alloy.

7. The method as claimed in claim 1, wherein the alcoholic compound comprises at least one aliphatic diol of general formula CnH2n(OH)2 with n=3-6.

8. The method as claimed in claim 7, wherein the aliphatic diol comprises 1,2-propanediol and/or 1,2-butanediol.

9. The method as claimed in claim 1, wherein the electrolyte comprises 5 to 93 wt. % methanesulfonic acid and 95 to 7 wt. % of the alcoholic compound or compounds.

10. The method as claimed in claim 1, wherein the electrolyte comprises 10 to 80 wt. % methanesulfonic acid and 90 to 20 wt. % of the alcoholic compound or compounds.

11. The method as claimed in claim 1, wherein the alcoholic compound comprises at least one aliphatic diol and at least one alicyclic alcohol.

12. The method as claimed in claim 1, wherein the alicyclic alcohol comprises cyclohexanol.

13. The method as claimed in claim 1, wherein the electrolyte does not contain any chromic acid or chromates.

14. The method as claimed in claim 1, wherein the method is carried out at a temperature between 40 and 100° C.

15. The method as claimed in claim 1, wherein the method is carried out at a temperature between 60 and 100° C.

16. The method as claimed in claim 1, wherein the method is carried out at an anodic current density of 3 to 40 A/d m2.

17. An electrolyte for the electropolishing of surfaces of metals, which are selected from iron, tungsten and light metals, and from alloys of these metals, wherein the electrolyte comprises

methanesulfonic acid and

at least one alcoholic compound consisting of an aliphatic diol of general formula CnH2n(OH)2 with n=2-6 or an alicyclic alcohol of general formula CmH2m-1OH with m=5-8.

18. The electrolyte as claimed in claim 17, wherein the alcoholic compound comprises at least one aliphatic diol of general formula CnH2n(OH)2 with n=3-6.

19. The electrolyte as claimed in claim 17, wherein the aliphatic diol comprises 1,2-propanediol and/or 1,2-butanediol.

20. The electrolyte as claimed in claim 17, wherein the electrolyte comprises 5 to 93 wt. % methanesulfonic acid and 95 to 7 wt. % of the alcoholic compound or compounds.

21. The electrolyte as claimed in claim 17, wherein the electrolyte comprises 10 to 80 wt. % methanesulfonic acid and 90 to 20 wt. % of the alcoholic compound or compounds.

22. The electrolyte as claimed in claim 17, wherein the alcoholic compound comprises at least one aliphatic diol and at least one alicyclic alcohol.

23. The electrolyte as claimed in claim 17, wherein the alicyclic alcohol comprises cyclohexanol.