U.S. patent application number 11/630137 was filed with the patent office on 2008-11-13 for method for removing a coating from a component.
This patent application is currently assigned to SIEMENS AKTIENGESELLSCHAFT. Invention is credited to Ursus Kruger, Ralph Reiche, Raymond Ullrich, Gabriele Winkler.
Application Number | 20080277288 11/630137 |
Document ID | / |
Family ID | 34972912 |
Filed Date | 2008-11-13 |
United States Patent
Application |
20080277288 |
Kind Code |
A1 |
Kruger; Ursus ; et
al. |
November 13, 2008 |
Method For Removing A Coating From A Component
Abstract
Method for removing coatings which have been applied to
components such as turbine blades. After the end of the product
life cycle of the turbine blades has been reached, these coatings
can be removed to reuse the turbine blades after recoating. The
coating, containing in particular chromium oxide compounds is
removed via a stripping bath by adding alkanolamine compounds or
salts containing such compounds as the inhibitor. These compounds
advantageously prevent new chromium oxide compounds from being
produced during the stripping of the turbine blades or the chromium
oxide compounds present in the coating are removed effectively, so
that the chromium oxide compounds cannot have an adverse influence
on the removal rate of the stripping process. Advantageously
reduced treatment times can thereby be achieved for the stripping
process. By adding 2% triethanolamine, the treatment time in
hydrochloric acid for example can be reduced to below one hour.
Inventors: |
Kruger; Ursus; (Berlin,
DE) ; Reiche; Ralph; (Berlin, DE) ; Ullrich;
Raymond; (Schonwalde, DE) ; Winkler; Gabriele;
(Berlin, DE) |
Correspondence
Address: |
YOUNG & THOMPSON
209 Madison Street, Suite 500
ALEXANDRIA
VA
22314
US
|
Assignee: |
SIEMENS AKTIENGESELLSCHAFT
Munich
DE
|
Family ID: |
34972912 |
Appl. No.: |
11/630137 |
Filed: |
June 13, 2005 |
PCT Filed: |
June 13, 2005 |
PCT NO: |
PCT/DE2005/001090 |
371 Date: |
January 7, 2008 |
Current U.S.
Class: |
205/717 |
Current CPC
Class: |
B08B 3/12 20130101; B08B
3/08 20130101; C23F 1/26 20130101; C23F 1/44 20130101; C23G 1/106
20130101; C25F 5/00 20130101 |
Class at
Publication: |
205/717 |
International
Class: |
C25F 1/00 20060101
C25F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2004 |
EP |
04015424.7 |
Oct 29, 2004 |
DE |
10 2004 053 135.8 |
Claims
1-5. (canceled)
6. A method for electrochemically removing a coating containing
chromium and/or chromium oxide compounds from a component in which
the coated component is introduced into a stripping bath and is
removed again after the treatment time has elapsed, an alkanolamine
compound or a salt containing such a compound being added to the
stripping bath as an inhibitor, characterized in that
triethanolamine or one of its salts is added as the inhibitor, the
coating being removed from a turbine blade.
7. The method as claimed in claim 6, characterized in that carbonic
acids and/or aldehyde compounds and/or unsaturated alcohols are
added as further inhibitors.
8. The method as claimed in claim 6, characterized in that the
coating is an MCrAlY coating and/or a thermal barrier coating.
9. The method as claimed in claim 7, characterized in that the
coating is an MCrAlY coating and/or a thermal barrier coating.
Description
[0001] The invention relates to a method for removing a coating
containing chromium and/or chromium oxide compounds from a
component with the aid of a stripping bath.
[0002] According to EP 1 094 134 A1, such a stripping operation can
be carried out for example by means of an electrochemical process.
This involves immersing the component from which the coating is to
be stripped in an electrolyte and thereby forming the working
electrode. By applying a voltage to the corresponding counter
electrodes, the coating on the component that is to be stripped is
dissolved.
[0003] Another method for stripping coatings from components is
described in EP 1 314 797 A2. This is a chemical stripping method.
The component from which the coating is to be stripped is immersed
in a solution comprising hydrochloric acid and an inhibitor
reducing the extent to which the base material of the component is
attacked. The stripping bath formed by the hydrochloric acid makes
the coating dissolve.
[0004] With the methods mentioned it is possible for example to
strip coatings from turbine blades when they reach the end of their
intended service life. This is because subsequent recoating allows
the turbine blades to have a further life cycle, for which reason
there is great interest in the effective stripping of coatings from
turbine blades. When stripping coatings from turbine blades, it is
necessary to remove not only the coatings that have been used, such
as MCrAlY coatings or thermal protective coatings (known as thermal
barrier coatings, hereafter TBCs), but also the contaminants that
have occurred on the coating over the time during which the turbine
blade has been in operation, preferentially comprising complex
crystalline compounds. Compounds of the Ca--Mg--Al--Si--O system
(hereafter referred to as CMASs) may be mentioned in this
connection. Other compounds that are difficult to remove comprise
what are known as thermally grown oxides (hereafter TGOs) on the
base material of the coated component, such as for example
Cr.sub.2O.sub.3 or Cr.sub.xCo.sub.yO compounds. These compounds
have a high chemical stability, for which reason they can only be
removed by the stripping bath if they are left for a considerable
time. Furthermore, such compounds may also be formed during the
stripping process, causing the stripping procedure to take even
longer. The compounds increasingly cover the surface of the
component to be stripped, for which reason the removal rate during
stripping drops as the treatment time increases. According to EP 1
314 797 A2, it is therefore proposed to carry out the stripping
process in a number of steps, with the surfaces being mechanically
worked between each of the treatment steps, so that the
contaminants that occur or have not yet been removed can be
removed.
[0005] The object of the invention is to provide a method for
removing coatings containing chromium and/or chromium oxide
compounds which allows short treatment times for the stripping
process.
[0006] This object is achieved according to the invention by a
method for removing a coating containing chromium and/or chromium
oxide compounds from a component in which the coated component is
introduced into a stripping bath and is removed again after the
treatment time has elapsed, an alkanolamine compound or a salt
containing such a compound being added to the stripping bath as an
inhibitor. This is because it has surprisingly been found for
alkanolamine compounds and salts containing them as inhibitors that
they not only reduce an attack on the base material of the coated
component by the stripping bath but also suppress the formation of
TGOs or the dissolving of TGOs and CMASs. This advantageously
allows a stripping process with comparatively high removal rates to
be maintained over a longer treatment time than is possible with
the inhibitors that are used according to the prior art. This
allows the method according to the invention to be used more
cost-effectively, in particular there is no need for the stripping
procedure to be interrupted for an intermediate mechanical working
step, or at least allows the treatment times between these
mechanical working steps to be prolonged. This advantageously
reduces the overall time that is necessary for performing the
stripping process, for which reason the method can be used more
cost-effectively. Furthermore, instances of the base material of
the coated component being damaged as a result of mechanical
working steps can be avoided, whereby the number of reject
components during stripping is reduced, which additionally
contributes to improved cost-effectiveness of the method. It has
been found to be advantageously particularly effective to use
triethanolamine or one of its salts, such as triethanolamine
borate, as the inhibitor.
[0007] It is advantageous to add carbonic acids and/or aldehyde
compounds and/or unsaturated alcohols as further inhibitors, the
effect of which as an inhibitor during stripping is already known.
The mixtures of inhibitors advantageously improve the overall
effect of the stripping bath as a whole.
[0008] The method according to the invention can be used
particularly advantageously for the stripping of coatings from
turbine blades, it being possible in particular for MCrAlY coatings
and/or thermal barrier coatings to be removed from the turbine
blades. The MCrAlY coating often serves here as an intermediate
layer between the turbine blade and the thermal barrier coating
(TBC). It goes without saying that said coating systems can also be
removed from any other components if necessary (for example from
feed pipes of gas turbines).
[0009] Further details of the invention are described below with
reference to the drawing. The single FIGURE shows a diagram which
represents the course of various exemplary embodiments of the
method according to the invention. The exemplary embodiment of the
method was that of removing an MCrAlY coating from turbine blades
that have been in operation, a five percent hydrochloric acid being
used as the stripping bath. Furthermore, in a method conducted in
parallel, triethanolamine of a one percent or two percent
concentration was added. During the treatment time, the stripping
bath was exposed to ultrasound at a frequency of 36 kHz.
[0010] The exemplary embodiments of the method according to the
invention involved electrochemical stripping, the potential at the
turbine blade being kept at 0.100 V SCE and 0.340 V SHE. The
diagram shows the current I in amperes A measured at the turbine
blade over the treatment time t in hours h.
[0011] This current can be used as a direct measure of the removal
rate at the turbine blade.
[0012] It is found that, with the addition of one percent or two
percent triethanolamine (TEA), the current density for dissolving
can be increased by a multiple in comparison with the use of pure
hydrochloric acid. This allows removal of the coating from the
turbine blade to be achieved in a correspondingly shorter time.
This is evident in the drawing from the drop in current. The
necessary treatment times can be reduced as a result to below one
hour.
* * * * *