ELECTROPOLISHING PROCESS

PROCEDE DE POLISSAGE ELECTROLYTIQUE

English 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 C n H2n(OH)2 with n = 2-6 and alicyclic alcohols of general formula C m H2m-1OH with m = 5-8.

French Abstract

La présente invention a trait à un procédé de polissage électrolytique de surfaces de métaux et dalliages métalliques. Ledit procédé est caractérisé notamment en ce quil est possible de lappliquer sur une vaste gamme de métaux. Ainsi, il convient au polissage électrolytique de surfaces métalliques contenant du fer, du tungstène, du magnésium, de laluminium ou un alliage de ces métaux. Lélectrolyte utilisé dans le procédé comprend de lacide méthanesulfonique et au moins un composant alcoolique choisi parmi les diols aliphatiques de la formule générale C n H2n(OH)2 avec n = 2-6 et des alcools alicycliques de la formule générale C m H2m-1OH avec m = 5-8.

Claims

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 iron, tungsten, magnesium, aluminium or alloys
thereof,
wherein the electrolyte comprises
- 10-80 wt.% methanesulfonic acid, and
- 90-20 wt.% of at least one alcoholic compound consisting of an
aliphatic diol of
formula C n H2n (OH)2 with n = 3-6, an alicyclic alcohol of formula C m H2m-
1OH with m = 5-8, or a
combination thereof, and
wherein the method is carried out at a temperature between 60 and 100°C
and at an
anodic current density of 3-40 A/dm2.
2. The method as claimed in claim 1, wherein the metal surface comprises
iron or an iron
alloy, hardened or unhardened.
3. The method as claimed in claim 2, wherein the iron alloy is ferro-
nickel, special steel, or
carbon steel.
4. The method as claimed in claim 1, wherein the metal surface comprises
tungsten or a
tungsten alloy.
5. The method as claimed in claim 1, wherein the metal surface comprises
magnesium, a
magnesium alloy, aluminium, or an aluminum alloy.
6. The method as claimed in claim 1, wherein the metal surface comprises an
aluminum-
silicon alloy.
7. The method as claimed in any one of claims 1 to 6, wherein the aliphatic
diol is 1,2-
propanediol, 1,2-butanediol, or a combination thereof.
8. The method as claimed in any one of claims 1 to 7, wherein the alicyclic
alcohol is
cyclohexanol.
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9. The method as claimed in any one of claims 1 to 7, wherein the alcoholic
compound
consists of at least one aliphatic diol of formula C n H2n (OH)2 with n = 3-6.
10. The method as claimed in any one of claims 1 to 9, wherein the
electrolyte does not
contain any chromic acid or chromates.
11. An electrolyte for the electropolishing of a surface of a metal,
said metal consisting of iron, tungsten, magnesium, aluminium, or an alloy
thereof, and
wherein the electrolyte comprises 10-80 wt.% methanesulfonic acid, and 90-20
wt.% of at
least one alcoholic compound consisting of an aliphatic diol of general
formula C n H2n (OH)2 with
n = 3-6, an alicyclic alcohol of general formula C m H2m-1 OH with m = 5-8, or
a combination
thereof
12. The electrolyte as claimed in claim 11, wherein the aliphatic diol is
1,2-propanediol, 1,2-
butanediol, or a combination thereof.
13. The electrolyte as claimed in claim 11 or 12, wherein the alicyclic
alcohol is
cyclohexanol.
14. The electrolyte as claimed in claim 11 or 12, wherein the alcoholic
compound consists of
at least one aliphatic diol of general formula C n H2n(OH)2 with n = 3-6.
15. The electrolyte as claimed in any one of claims 11 to 13, wherein the
alcoholic compound
comprises at least one aliphatic diol and at least one alicyclic alcohol.
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Description

CA 02614783 2007-11-13
ELECTROPOLISHING PROCESS
The present invention relates to a method for the electrochemical polishing of
surfaces of metals and metal alloys. The electrolyte used for this comprises
methanesulfonic acid and at least one alcoholic compound, which is selected
from
aliphatic diols and alicyclic alcohols. Said method is suitable for metal
surfaces
comprising iron, tungsten, magnesium, aluminum or an alloy of these metals.
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.
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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 US 2005/0045491 Al 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 method is suitable
for the
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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 CnH2i(OH)2 with n= 2-6 and alicyclic
alcohols of
general formula C,,,HZm_IOH with m = 5-8. In particular, the alcoholic
compound can
comprise at least one aliphatic diol of general formula CõH2õ(OH)Z, 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 CnH2õ(OH)2 and at least one
alicyclic alcohol of
general formula CmH2m_IOH, where n = 2-6 and m = 5-8.
The alicyclic alcohols of the present invention also comprise all isomers
satisfying
the general formula CmH2,,,_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
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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 performa.nce 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/dmZ are
especially preferred. Tungsten or tungsten alloys in particular permit the use
of higher
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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
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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/dm 2
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
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6.
Treated surface: aluminum-magnesium alloy AlMg1
Electrolyte: 50% methanesulfonic acid + 50% 1,2-propanediol
Temperature: 80 C
Anodic current density: 10 A/dmZ
Duration: 10 min
Result: highly polished
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