U.S. patent application number 12/073793 was filed with the patent office on 2008-09-11 for electropolishing process for titanium.
This patent application is currently assigned to POLIGRAT GmbH. Invention is credited to Olaf Bohme, Siegfried Piesslinger-Schweiger.
Application Number | 20080217186 12/073793 |
Document ID | / |
Family ID | 39365667 |
Filed Date | 2008-09-11 |
United States Patent
Application |
20080217186 |
Kind Code |
A1 |
Bohme; Olaf ; et
al. |
September 11, 2008 |
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.
Inventors: |
Bohme; Olaf; (Erding,
DE) ; Piesslinger-Schweiger; Siegfried;
(Vaterstetten, DE) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW, SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
POLIGRAT GmbH
Munchen
DE
|
Family ID: |
39365667 |
Appl. No.: |
12/073793 |
Filed: |
March 10, 2008 |
Current U.S.
Class: |
205/674 |
Current CPC
Class: |
C25F 3/26 20130101 |
Class at
Publication: |
205/674 |
International
Class: |
C25F 3/18 20060101
C25F003/18 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 9, 2007 |
DE |
10 2007 011 632.4 |
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.degree. C. and 70.degree. C.
8. The method as claimed in claim 1, wherein the method is carried
out at an anodic current density of 2-50 A/dm.sup.2.
9. The method as claimed in claim 1, wherein the method is carried
out at an anodic current intensity of 5-30 A/dm.sup.2.
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.
Description
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] Examples are described for example in WO 01/00906 A1 and DE
103 20 909 A1.
[0006] 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".
[0007] 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.
[0008] 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.
[0009] 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.degree. C. is necessary, together with automatic extinguishers
for fire protection.
[0010] 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.
[0011] 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
[0012] 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.
[0013] 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).
[0014] 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.
[0015] 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.
[0016] In one embodiment of the present invention, the electrolyte
consists substantially of methanesulfonic acid and one or more
alkanediphosphonic acids.
[0017] 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.
[0018] When carrying out a method according to the present
invention, the electropolishing preferably takes place at a
temperature in the range from 20.degree. C. to 70.degree. C., for
example between room temperature and 60.degree. C., and in
particular at a temperature between 25.degree. C. and 50.degree. 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/dm.sup.2, in particular
between 5 and 30 A/dm.sup.2. The applied voltage can often be in
the range from 10 to 35 V.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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
[0025] A plate with the dimensions 50.times.50.times.1.0 mm with a
ground surface and roughness of Ra=0.8 .mu.m was electropolished in
an electrolyte consisting of 990 g of 100% methanesulfonic acid and
10 g of 1-hydroxyethyl-1,1-diphosphonic acid.
[0026] The operating variables were as follows:
Temperature: 30.degree. C.
[0027] Current density: 20 A/dm.sup.2 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 .mu.m. The edges were smooth
and burr-free.
Example 2
Nitinol
[0028] 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.degree. C.
[0029] Current density: 10 A/dm.sup.2 Polishing time 4 min
[0030] The wire was pretreated and posttreated according to Example
1.
[0031] The result was a bright, smooth surface without etch attack
on the structure.
* * * * *