EP1923490B1 - Electropolishing method - Google Patents

Description

The present invention relates to a method for electrochemically polishing surfaces of metals and metal alloys. The electrolyte used includes methanesulfonic acid and at least one alcoholic compound selected from aliphatic diols and alicyclic alcohols. This method is suitable for metal surfaces consisting of iron, tungsten, magnesium, aluminum or an alloy of these metals.

Background of the invention

The process of electrochemical polishing or electropolishing serves to produce metal surfaces of high purity, to smooth and deburr metal surfaces. Smoothing in the micro range can also bring about a gleaming of the surfaces treated in this way. In addition, by electropolishing any stresses in the outer material layers can be removed.

There are a variety of different electropolishing processes that can be used to process various metals or metal alloys. In general, these methods are based on the use of electrolytes comprising a concentrated mineral acid such as phosphoric acid or sulfuric acid or a mixture of concentrated mineral acids, to which additives are often added to further enhance the effect of the electrolytes and thus smoother and more lustrous metal surfaces receive. Examples of such additives are, for example, chromic acid, hydrofluoric acid, amine fluorides or organic additives, such as, for example, alcohols, amines, glycerol, etc.

However, all these conventional, often industrially used electrolytes have in common that they can be used successfully only for certain metals and / or alloys and thus have a very limited use profile. For processing different metals or alloys, it is therefore often necessary to have available a corresponding number of different electrolytes. In this case, the individual electrolytes often have to be kept strictly separate from one another and in particular must not be mixed, since they could be damaged by possible mixing and thus become unusable. Occasionally, this may even cause certain components of the electrolytes to react with each other and, for example, release harmful or dangerous substances. In addition, the requirements for the execution of the process and the equipment of the electropolishing often also very different due to the different electrolytes, so that several facilities must be kept for different materials.

The electrolytes commonly used herein are often hazardous materials that are subject to special requirements and regulations in terms of their respective toxicity, flammability and / or hazard class with regard to their storage and use, and that require appropriate environmental and occupational safety precautions. This in turn causes a considerable effort and associated costs.

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

An electrolyte has long been known from the prior art, which largely meets the requirements for universal usability. It is a mixture of perchloric acid and acetic anhydride. However, due to the risk of explosion associated with its use, this mixture is frequently not industrially applicable, or can be used only with considerable safety expense.

The patent application WO 01/71068 A1 discloses electrolytic polishing processes which apparently can be used for a wide range of metals or metal alloys. These electropolishing processes use, inter alia, an electrolyte of methanesulfonic acid and methanol. However, this electrolyte has the serious disadvantage that it is hazardous to health, fire and explosive due to its high content of more than 80% of volatile methanol. Therefore, such a method can be operated usually only at very low temperatures, for example of at most 10 ° C, or with a complex system for safe capture and discharge of the resulting vapors. In addition, a suitability of this method for carbon steels, magnesium, magnesium alloys or aluminum-silicon alloys is not disclosed.

A relatively wide range of applications also appears in the patent application US 2005/0045491 A1 disclosed methods, but also does not disclose suitability for magnesium-containing metal surfaces or those of aluminum-silicon alloys. The electrolyte used contains at least 75 wt .-% of an alkylene glycol, the remainder of a chloride salt of the alkali and / or alkaline earth metals.

Subject of the invention

The subject of the present invention is an electropolishing process which can be applied to a wide range of metals and metal alloys with good success and is largely harmless in terms of occupational safety and environmental protection. The method is suitable for electropolishing surfaces of metals as different as iron, tungsten, magnesium and aluminum, as well as surfaces of alloys of these metals. In particular, it is suitable for surfaces of iron or an iron alloy, such as nickel iron, stainless steel (stainless steels) or carbon steel, which can be electropolished in both hardened and uncured form according to the present process; tungsten or a tungsten alloy, magnesium, a magnesium alloy, aluminum or an aluminum alloy, such as an aluminum-silicon alloy. An alloy of a certain metal is understood to mean alloys in which this metal is the main constituent of the alloy, based on the weight of the constituents of the alloy. Frequently, this metal (or these metals) comprises more than 50% by weight of the alloy.

The electropolishing method according to the present invention uses as electrolytes a solution comprising methanesulfonic acid and at least one alcoholic compound, wherein the at least one alcoholic compound is selected from the group consisting of aliphatic diols of the general formula C _n H _2n (OH) ₂ with n = 3-6 and alicyclic alcohols of the general formula C _m H _2m-1 OH with m = 5-8. In particular, the alcoholic compound may comprise at least one aliphatic diol of the general formula C _n H _2n (OH) ₂ , where n = 3, 4, 5 or 6. All isomers of these aliphatic diols can be used as long as the two hydroxyl groups are bonded to different carbon atoms. Examples of these are, for example, the compounds 1,2-propanediol, 1,2-butanediol or 1,4-butanediol.

In a specific embodiment, the electrolyte contains as alcoholic compounds both at least one aliphatic diol of the general formula C _n H _2n (OH) ₂ and at least one alicyclic alcohol of the general formula C _m H _2m-1 OH, where n = 3-6 and m = 5-8.

The alicyclic alcohols of the present invention also include all isomers satisfying the general formula C _m H _2m-1 OH with m = 5-8. In this case, all carbon atoms can form the ring structure, as in cyclopentanol, cyclohexanol, cycloheptanol and cyclooctanol; but it is also possible that one or more carbon atoms form a hydroxyalkyl and / or one or more alkyl side chain (s). Particularly preferred are electrolyte solutions comprising cyclohexanol.

In a preferred embodiment, the electrolyte according to the electropolishing method of the present invention comprises a mixture consisting of 5-93% methanesulfonic acid and 95-7% of the at least one alcoholic compound. These percentages, as well as all others in the present application, unless otherwise stated, relate to the weight of the respective substances and solutions. It is particularly preferred that the electrolyte consists of 10-80% methanesulfonic acid and 90-20% of the at least one alcoholic compound. For example, the electrolyte may comprise 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 in addition to methanesulfonic acid and alcoholic compounds, no further additives for the electrolyte are needed. It should be mentioned in particular that the electrolyte used in this process contains neither chromic acid or chromates, nor perchloric acid or salts thereof. Also, the process does not use any volatile additives such as methanol, ethanol or esters, whose high vapor pressure presents a particular challenge to occupational safety in terms of both flammability and toxicity. In addition, the electrolyte contains no hydrofluoric acid and is also for this reason considerably less problematic in operation.

Preferably, the electrolyte used in the processes according to the present invention contains no or only small amounts of water. Thus, the water content of the electrolyte should not exceed a proportion of 10% water. In addition, the electrolyte does not require the addition of salts for Leitwerterhöhung.

According to the present invention, the process is carried out at a temperature between 60 ° C and 100 ° C. Since the electrolyte of the present process contains no volatile constituents, such higher temperatures, for example temperatures up to 80 ° C, up to 90 ° C, up to 100 ° C or even higher, can be used without special precautions, for example for safe collection and discharge incurred vapors are necessary. The possibility that the method can also be carried out at relatively high temperatures, on the one hand allows the electropolishing process to be carried out in a relatively short time if required, and on the other hand makes it unnecessary to dissipate the heat released in the process of electropolishing. Thus, a complex cooling largely or even completely be dispensed with. If a cooling system is used, it does not have to meet any higher performance requirements.

The anodic current density of the method presented here is, depending on the respective metals, at values between 3 and 40 A / dm ^{2 of} the surface to be polished, preferably at 5-30 A / dm ² . In particular, tungsten or tungsten alloys allow the use of higher anodic current densities of, for example, about 30-40 A / dm ² . But the other materials described here can be successfully electropolished at higher anodic current densities. For iron, aluminum and magnesium-containing surfaces, however, anodic current densities of about 5-20 A / dm ² are usually sufficient.

Of course, the duration of the electropolishing process depends on the metal being worked, the roughness of the workpiece to be polished, the desired removal rate and the desired smoothing of the surfaces of the workpiece, as well as the temperature and the current density.

In addition to its broad applicability, the process according to the invention has further significant advantages over conventional electropolishing processes. Thus, the electrolyte used is chemically non-aggressive and therefore behaves after switching off the electropolishing, as well as during the subsequent rinsing processes the electro-polished surfaces largely inert. The surfaces are not chemically attacked and etched, so the quality of the Electropolished surfaces are maintained and no special precautions are required to remove the electrolyte as quickly as possible from the machined workpiece. This is especially important in the machining of workpieces with low corrosion resistance, as they have about normal steel, magnesium, aluminum and their alloys.

In addition to the process itself in all its aspects set forth, another aspect of the present invention is directed to electrolytes according to claim 5.

The invention is explained in more detail in the following examples. These examples are merely possible embodiments of the electropolishing process described herein and the electrolytes used herein, and are not intended to imply any limitation to the conditions used herein.

Examples

• 1.
  Treated surface: stainless steel Wst. Nr. 1.4301
  Electrolyte: 37% methanesulfonic acid + 63% 1,2-propanediol
  Temperature: 80 ° C
  Anodic current density: 10 A / dm ²
  Duration: 15 min
  Result: mirror gloss
• Second
  Treated surface: tool steel (carbon steel)
  Electrolyte: 37% methanesulfonic acid + 63% 1,2-pentanediol
  Temperature: 80 ° C
  Anodic current density: 20 A / dm ²
  Duration: 10 min
  Result: high gloss
• Third
  Treated surface: tungsten
  Electrolyte: 50% methanesulfonic acid + 50% 1,2-propanediol
  Temperature: 80 ° C
  Anodic current density: 40 A / dm ²
  Result: high gloss
• 4th
  Treated surface: magnesium
  Electrolyte: 20% methanesulfonic acid + 40% 1,2-propanediol + 40% cyclohexanol Temperature: 60 ° C
  Anodic current density: 10 A / dm ²
  Duration: 8 minutes
  Result: high gloss
• 5th
  Treated surface: aluminum-silicon AlSi ₂ O alloy
  Electrolyte: 20% methanesulfonic acid + 80% 1,2-butanediol
  Temperature: 80 ° C
  Anodic current density: 10 A / dm ²
  Duration: 12 min
  Result: high gloss
• 6th
  Treated surface: aluminum-magnesium alloy AlMg ₁
  Electrolyte: 50% methanesulfonic acid + 50% 1,2-propanediol
  Temperature: 80 ° C
  Anodic current density: 10 A / dm ²
  Duration: 10 min
  Result: high gloss

Claims (5)

1. A method for electro polishing of surfaces of metals, which are selected from iron or an iron alloy,
   tungsten or an tungsten alloy,
   magnesium or a magnesium alloy, and
   aluminum or an aluminum alloy;
   with an electrolyte, comprising

   - 10-80 percent by weight methane sulfonic acid and

   - 90-20 percent by weight of at least one alcoholic compound selected from the group consisting of aliphatic diols of general formula C_nH_2n(OH)₂ with n = 3-6 and alicyclic alcohols of general formula C_mH_2m-1OH with m = 5-8,

   wherein the method is carried out at a temperature between 60 and 100 °C and an anodic current density of 3-40 A/dm².
2. The method of claim 1, characterized in that the aliphatic diol comprises 1,2-propanediol and/or 1,2-butanediol.
3. The method of claim 1 or 2, characterized in that the alicyclic alcohol comprises cyclohexanol.
4. The method of anyone of the preceding claims, characterized in that the electrolyte does not contain any chromic acid or chromate.
5. An electrolyte for carrying out the method according to anyone of the preceding claims, characterized in that the electrolyte consists of 10 to 80 percent by weight of methane sulfonic acid and 90 to 20 percent by weight of at least one alcoholic compound, selected from the group, consisting of aliphatic diols of the general formula C_nH_2n(OH)₂ with n=3-6 and alicyclic alcohols of the general formula C_mH_2m-1OH with m=5-8.