U.S. patent application number 09/738449 was filed with the patent office on 2002-06-20 for feedback controlled airfoil stripping system with integrated water management and acid recycling system.
Invention is credited to Janowsky, Glenn T., Jaworowski, Mark R., Kryzman, Michael A., Riewe, Curtis H., Shovlin, Christopher C..
Application Number | 20020074240 09/738449 |
Document ID | / |
Family ID | 24968070 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020074240 |
Kind Code |
A1 |
Jaworowski, Mark R. ; et
al. |
June 20, 2002 |
FEEDBACK CONTROLLED AIRFOIL STRIPPING SYSTEM WITH INTEGRATED WATER
MANAGEMENT AND ACID RECYCLING SYSTEM
Abstract
The present invention relates to a feedback controlled stripping
system with integrated water management and acid recycling system.
The system comprises a stripping tank containing an electrolyte
bath stripping solution for removing a coating from at least one
workpiece immersed in the stripping solution while a controlled
absolute electrical potential is maintained on the at least one
workpiece with respect to a reference electrode also immersed in
the stripping solution, a rinse tank for rinsing the workpiece(s)
after removal of the workpiece(s) from the stripping tank, and a
distillation unit for receiving electrolyte containing dissolved
metals from the stripping tank, for purifying the electrolyte
received from the stripping tank, and for returning the purified
electrolyte to the stripping tank. In a preferred embodiment, the
stripping tank, the rinse tank, and the distillation unit are
mounted to a skid. The system further includes a control module. A
process for using the system to remove a coating from a workpiece
is also described.
Inventors: |
Jaworowski, Mark R.;
(Glastonbury, CT) ; Shovlin, Christopher C.;
(Wethersfield, CT) ; Janowsky, Glenn T.;
(Coventry, CT) ; Riewe, Curtis H.; (Manchester,
CT) ; Kryzman, Michael A.; (W.Hartford, CT) |
Correspondence
Address: |
Bachman & LaPointe, P.C.
Suite 1201
900 Chapel Street
New Haven
CT
06510-2802
US
|
Family ID: |
24968070 |
Appl. No.: |
09/738449 |
Filed: |
December 15, 2000 |
Current U.S.
Class: |
205/673 ;
204/237; 205/717 |
Current CPC
Class: |
C25F 5/00 20130101; C25F
1/00 20130101 |
Class at
Publication: |
205/673 ;
205/717; 204/237 |
International
Class: |
C25F 005/00 |
Claims
What is claimed is:
1. A coating removal system comprising: a stripping tank containing
an electrolyte bath stripping solution for removing a coating from
at least one workpiece immersed in said electrolyte bath while a
controlled absolute electrical potential is maintained on said at
least one workpiece with respect to a reference electrode immersed
in said electrolyte bath; a rinse tank containing a rinse solution
for rinsing said at least one workpiece after completion of removal
of said coating from said at least one workpiece; and a
distillation unit for receiving electrolyte from said stripping
tank containing dissolved metals, for purifying said electrolyte
received from said stripping tank, and for returning said
electrolyte in a purified form to said stripping tank.
2. A coating removal system according to claim 1, wherein said
stripping tank has at least one counter electrode for applying a
current to each workpiece immersed in said stripping tank.
3. A coating removal system according to claim 2, wherein said at
least one counter electrode comprises a counter electrode array
designed to provide a symmetrical potential distribution to each
said workpiece.
4. A coating removal system according to claim 3, wherein said
counter electrode array is formed from an electrically conductive
material and said reference electrode is a hydrogen reference
electrode array.
5. A coating removal system according to claim 3, wherein said
counter electrode array has a front wall, a rear wall, two side
walls connecting the front and rear walls, and at least one insert
extending between said front and rear walls.
6. A coating removal system according to claim 5, further
comprising a fixture for holding said at least one workpiece and
immersing said at least one workpiece in said electrolyte bath
stripping solution and at least one buss strip affixed to one of
said walls of said counter electrode array for contact by said
fixture, whereby current is delivered to each said workpiece.
7. A coating removal system according to claim 1, wherein said
stripping tank, said rinse tank, and said distillation unit are
mounted on a skid.
8. A coating removal system according to claim 1, wherein said
distillation unit has: a boiler for receiving used electrolyte from
said stripping tank and for evaporating said used electrolyte
leaving dissolved metals in the boiler; and a condenser for
receiving said evaporated electrolyte and condensing said
electrolyte into a purified liquid form.
9. A coating removal system according to claim 8, wherein an acid
return line connects said condenser to said stripping tank so that
said purified electrolyte may be returned to said stripping tank by
gravity.
10. A coating removal system according to claim 9, wherein said
distillation unit further has: a solenoid actuated valve for
purging collected metals from said boiler; and an electrodeless
conductivity probe and a conductivity meter for controlling said
valve.
11. A coating removal system according to claim 1, wherein said
rinse tank has: a conductivity probe for monitoring the quality of
the rinse solution in said rinse tank; a circulating pump within
the rinse tank; and a filter for removing dissolved metals from
said rinse solution.
12. A coating removal system according to claim 1, further
comprising a power supply and a first digital multimeter for
measuring the potential between said reference electrode and said
at least one workpiece and for supplying a signal representative of
said potential to a computer.
13. A coating removal system according to claim 12, further
comprising said power supply having an adjustable current output
for maintaining a predetermined target voltage between said
reference electrode and said at least one workpiece and said
computer being used to modify a power supply current set point as a
function of a change in potential between the reference electrode
and the at least one workpiece.
14. A coating removal system according to claim 12, further
comprising a shunt resistor electrically connected to a counter
electrode or the at least one workpiece in said stripping tank and
to the power supply and a second digital multimeter for monitoring
actual current in said stripping tank and supplying a signal
representative of said monitored current to said computer.
15. A coating removal system according to claim 14, further
comprising a power supply shorting resistor for allowing fine
adjustments to said cell current as cell resistance increases.
16. A coating removal system according to claim 13, further
comprising a first conductivity probe in said stripping tank for
monitoring electrical conductivity and temperature of the
electrolyte bath, a data acquisition system for receiving data from
said first conductivity probe, and said computer being connected to
said data acquisition system and being programmed to determine acid
concentration in the strip solution.
17. A coating removal system according to claim 16, further
comprising: a second conductivity probe in said rinse tank for
monitoring the quality of the rinse solution in the rinse tank and
for producing a second signal representative of said rinse solution
quality; and said data acquisition system receiving said second
signal from said second conductivity probe and notifying an
operator of said rinse solution quality.
18. A coating removal system according to claim 1, wherein said
electrolyte bath in said stripping tank contains from about 3% to
about 15% volume of an acid selected from the group consisting of
nitric acid and hydrochloric acid.
19. A process for removing a coating from a workpiece using an
electrochemical bath and for regenerating and recycling said
electrochemical bath comprising the steps of: stripping the coating
from the workpiece by immersing the workpiece in said
electrochemical bath for a sufficient period of time to remove the
coating from the workpiece while the workpiece in the
electrochemical bath is maintained with a controlled absolute
electrical potential with respect to a reference electrode; and
regenerating said electrochemical bath by atmospheric
distillation.
20. The process according to claim 19, wherein said regenerating
step comprises: introducing used electrolyte from said bath
containing dissolved metals into a boiler; evaporating said used
electrolyte while leaving the dissolved metals in said boiler;
condensing said evaporated electrolyte to return said electrolyte
to a liquid phase; and reintroducing said electrolyte into said
electrochemical bath.
21. The process according to claim 20, wherein said reintroducing
step comprises reintroducing said electrolyte using a gravity
feed.
22. The process according to claim 20, further comprising purging
said dissolved metals in a concentrated form from said boiler.
23. The process according to claim 19, further comprising removing
said workpiece from said electrochemical bath after completion of
the stripping of the coating and immersing said workpiece into a
rinse solution in a rinse tank.
24. The process according to claim 19, further comprising
monitoring rinse solution quality and notifying an operator when
said quality is unacceptable.
25. The process according to claim 19, further comprising
maintaining a predetermined target voltage during the stripping
step by adjusting a current output of a power supply supplying a
current to the workpiece and to an electrode array in a tank
containing said workpiece and said electrochemical bath.
26. The process according to claim 25, wherein said predetermined
target voltage maintaining step comprises adjusting a power supply
set point as a function of a change in said potential.
27. The process according to claim 25, further comprising
monitoring cell current within said tank containing said workpiece
and said electrochemical bath.
28. The process according to claim 27, wherein said cell current
monitoring step comprises providing a shunt resistor and measuring
the voltage across said shunt resistor using a digital
multimeter.
29. The process according to claim 28, further comprising providing
a shorting resistor and using said shorting resistor to make fine
adjustments to said cell current as cell resistance increases 30.
The process according to claim 25, further comprising monitoring
conductivity and temperature of said electrochemical bath and
determining metal loading of said electrochemical bath solution
using said monitored conductivity and temperature.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a system for stripping
coatings from workpieces which has an integrated water management
and acid recycling system and a process for using same.
[0002] Gas turbine engines in aircraft are taken out of service at
periodic intervals and regular maintenance service is performed on
them. Part of the regular repair sequence for the blades and vanes
(individual or together referred to hereafter as "airfoils") of
these engines includes the removal and then replacement of the worn
coatings from their surfaces. These coatings are usually either an
aluminide coating or an MCrAlY coating. The underlying base metal
of airfoils are generally made of either a nickel base alloy or a
cobalt base alloy. These coatings provide airfoils with a barrier
to the hot corrosive environment in which airfoils operate.
[0003] In the past, these aluminide and MCrAlY coatings were
removed from airfoils by soaking the parts either in nitric acid
solutions (to remove aluminide-type coatings) or in hydrochloric
acid solutions (to remove MCrAlY-type coatings) in high acid
concentrations for up to six hours at elevated temperatures. This
soaking process has several disadvantages associated with it.
[0004] This soaking process is extremely labor intensive and can
produce non-uniform and unpredictable results. It can also damage
or destroy airfoils if improperly carried out. Furthermore, each
airfoil part requires extensive masking to protect areas sensitive
to the acid soaking solution. Such areas include internal surfaces
and the root section of the airfoil. These masking operations are
costly, add significant time to the repair process and, if not
properly carried out, can lead to damaged or destroyed parts. Still
further, these soaking processes may result in extensive amounts of
acidic waste solution that must be properly disposed of as well as
have a long cycle time and require relative large amounts of energy
to heat the acidic solutions.
[0005] A better airfoil stripping process is needed by the engine
maintenance and repair industry. This better airfoil stripping
process should be one that has a reduced cycle time; requires
reduced amount of labor; requires less masking and lower operating
temperatures; produces less hazardous waste effluent; requires less
heating energy; produces uniform and predictable stripping results
so that fewer parts are damaged, destroyed or require recycling.
Such a stripping process has been presented in allowed co-pending
U.S. patent application Ser. No. 09/216,469, filed Dec. 18, 1998,
entitled FEEDBACK CONTROLLED STRIPPING OF AIRFOILS. In this
process, a coating is electrochemically stripped from an airfoil by
immersing the airfoil in an electrochemical acid bath for a
sufficient period of time to remove the coating from the airfoil
while maintaining a controlled absolute electrical potential with
respect to a reference electrode on the airfoil surface.
[0006] In order to make the stripping process commercial, the
expended stripping solution and the waste water that is created
from rinsing the stripping solution must be managed. In the past
this has required the use of large industrial waste water treatment
plants.
SUMMARY OF THE INVENTION
[0007] Accordingly, it is an object of the present invention to
provide a feedback controlled stripping system with an integrated
water management and electrolyte recycling system which can be used
to strip or remove coatings from a wide variety of workpieces.
[0008] It is a further object of the present invention to provide a
process for utilizing the system to strip coatings from a wide
variety of workpieces.
[0009] The foregoing objects are attained by the system and the
process of the present invention.
[0010] In accordance with the present invention, a feedback
controlled stripping system is provided with integrated electrolyte
recycling. This allows protective coatings to be removed from
turbine blades, vanes, and other workpieces, as well as permitting
brazing and solder compounds to be removed from metals, in cold,
dilute acid without masking by the use of controlled potential
stripping. The integration of a recycling system based on acid
distillation stabilizes the chemistry of the stripping solution
while minimizing the volume of chemical waste created by the
process. The integration of zero waste water discharge equipment
allows the system to be located in facilities lacking central waste
water treatment plants.
[0011] The coating removal system of the present invention broadly
comprises a stripping tank containing an electrolyte bath stripping
solution for removing a coating from at least one workpiece
immersed in the electrolyte bath while a controlled absolute
electrical potential is maintained on the at least one workpiece
with respect to a reference electrode immersed in the electrolyte
bath; a rinse tank containing a rinse solution for rinsing the at
least one workpiece after completion of removal of the coating from
the at least one workpiece; and a distillation unit for receiving
used electrolyte from the stripping tank containing dissolved
metals, for purifying the electrolyte received from the stripping
tank and for returning the electrolyte in a purified form to the
stripping tank. In a commercial embodiment, the stripping tank,
rinse tank, and distillation unit are mounted on a skid. The
coating removal system further comprises a control module for
operating the system.
[0012] A process for removing a coating from a workpiece using an
acid bath stripping solution and for regenerating the stripping
solution broadly comprises the steps of stripping a coating from a
workpiece by immersing the workpiece in an electrochemical acid
bath for a period of time sufficient to remove the coating from the
workpiece while the workpiece in the electrochemical bath is
maintained with a controlled absolute electrical potential with
respect to a reference electrode and regenerating the
electrochemical acid bath by atmospheric distillation of the
electrochemical acid bath.
[0013] Other details of the system and the process of the present
invention, as well as other objects and advantages attendant
thereto, are set forth in the following detailed description and
the accompanying drawings in which like reference numerals depict
like elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematic representation of a feedback
controlled stripping system with integrated water management and
electrolyte recycling system in accordance with the present
invention;
[0015] FIG. 2 illustrates a module for allowing an operator to
control the system;
[0016] FIG. 3 illustrates a counter electrode array used in the
stripping tank in the system of FIG. 1;
[0017] FIG. 4 illustrates a fixture for holding the workpieces to
be treated;
[0018] FIG. 5 illustrates a data acquisition and control system
used in the system of FIG. 1; and
[0019] FIG. 6 is a schematic representation of a distillation unit
used in the system of FIG. 1 to recycle the acid stripping
solution.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] The phrase "controlled absolute electrical potential with
respect to a reference electrode" as used herein means the
electrical potential as measured between the airfoil (as a working
electrode) and a non-polarized reference electrode in a three-wire
electrode setup in the electrochemical acid bath is controlled to
affect a suitable rate of stripping of the coating from airfoil
base metal.
[0021] A phrase "controlled electrical current density on the
airfoil surface" as used herein means the electrical current is
measured as the current flow between the airfoil and the counter
electrode in the electrochemical acid bath while the absolute
potential of the airfoil is monitored with respect to a
non-polarized reference electrode also present in the
electrochemical acid bath.
[0022] The phrase "three wire electrode setup" as used herein
refers to the use of an airfoil as the working electrode while also
have at least one counter electrode and non-polarized reference
electrode in the electrochemical acid bath.
[0023] The word "coating" as used herein refers to a coating
applied to an airfoil such as a barrier coating, a solder or braze
joining compound applied to a metal part, an electroplated coating
applied to a steel component, and the like.
[0024] The technique used in the present invention to strip
coatings from workpieces such as turbine blades and vanes and other
metal objects and/or to remove braze or solder compounds from
metallic workpieces is based on the application of an external
anodic current to the workpieces, which results in an increase in
the potential of the workpieces. Thus, the rate of the acidic
stripping process is increased significantly while being able to
operate at either lower acid concentrations, at lower operating
temperatures and/or at shorter periods of time than conventional
soaking processes. This use of less aggressive solutions or lower
temperatures or shorter reaction times or combinations thereof
allows for use of less costly and less complex masking materials.
Furthermore, upon removal of the coating material, the
electrochemical current may be automatically stopped or reversed to
obtain the desired stripping effect without going too far and thus
destroying or damaging the workpiece.
[0025] The present invention can be carried out using controlled
absolute potential stripping. The coatings that may be removed by
this process include one or more aluminide-type coatings or one or
more MCrAlY-type coatings or mixtures thereof. Examples of
MCrAlY-type coatings include NiCoCrAlY, NiCrAlY and CoCrAlY. The
technique of the present invention may also be used to remove braze
and/or solder joining compounds from metallic components.
[0026] The controlled potential stripping preferably uses a
constant absolute electrical potential on the workpiece in the acid
bath. The constant potential provides activation energy for
dissolution of the coating/brazing/solder material, and also causes
a difference in the intrinsic corrosion current density between the
workpiece base material and the coating/brazing/soldering material.
Alternatively, it may be desired in some situations to employ a
variable absolute potential with respect to a reference electrode.
By controlling the absolute potential of the workpiece, the coating
removal rate will vary over time (i.e. will be smaller as more is
removed). This embodiment provides good selectivity for
coating/braze/solder removal, but requires a complex potentiostatic
power supply. Accordingly, controlled absolute potential stripping
is preferred where selectivity is the primary concern.
[0027] It is desirable to select the optimum electrical potential
for conducting this electrochemical reaction. This optimum level
may be found by measuring the current density of coated and
stripped workpieces to find the optimum point where the selectivity
of stripping the coated/brazed/solder material from the workpiece
metal is greatest.
[0028] Preferably, the electrochemical tank may be of any standard
acid resistant material. An external anionic current may be applied
to the workpieces which may be fully or partly immersed in the
acidic electrolyte bath in the tank. The working electrodes for the
baths will be the workpieces themselves. One or more counter
electrodes (preferably, standard graphite electrodes) will be
placed in the bath. A reference electrode (an Ag/AgCl or a hydrogen
reference electrode) is also placed in the bath. Specially, the
workpiece may be first suitably masked (which may be less than the
masking required for the conventional soaking process) to cover any
acid sensitive surfaces. The workpieces are preferably affixed to
an insulating fixture at the root section or base portion of the
workpiece. The root or base sections may not be immersed in the
bath and unlike the conventional soaking stripping process, thus do
not require masking. The insulating fixture holding one or more of
the workpieces is preferably made of titanium or any other suitable
noble metal. Alternatively, the workpiece may be completely
immersed after masking the root or base section and other acid
sensitive surfaces.
[0029] In operation one or more of the root or base sections of the
coated/brazed soldered workpieces are preferably clamped into the
titanium fixture or other type of insulating fixture. The
workpieces are then partly or fully immersed in the acidic
solution. The electrical current is applied with the absolute
electrical potential of the workpiece being controlled. The
reference electrode is used to measure or monitor the electrical
potential of the workpiece in the bath. In the case of controlled
electrical potential stripping, the reference electrode is
connected to potentiostat/galvanostat whereby the degree of
stripping may be monitored.
[0030] The electrochemical stripping bath may contain any suitable
acidic solution. Preferably, the acid is either a nitric acid or
hydrochloric acid. Any suitable acid concentration up to
concentrated solutions may be used. Aqueous acid concentrations
containing about 3% to about 15% by volume technical grade acid in
water (most preferably nitric or HCl) are preferred because of the
greater selectivity achieved with them over more concentrated acid
solutions.
[0031] The electrochemical operations used to carry out the present
process may be carried out for any suitable amount of time and at
any temperature to remove the coating/solder/braze from the
workpiece without harming the underlying base metal of the
workpiece. Preferably, these stripping operations may be carried
out at room temperature and for about 15 to about 300 minutes.
These conditions are lower and shorter than the conventional
soaking processes.
[0032] The end point of the stripping process may be predetermined
by any standard end-point technique. These include a linear
extrapolation of the current/time curve to the time corresponding
at zero current; a predetermined ratio of the initial current to
the measured current; by predetermined alternating current (AC) or
voltage measurements; or by a predetermined absolute quantitative
end-point value of current where the process will stop or be
reversed.
[0033] The present invention is further described in detail by
means of the following Examples.
EXAMPLES 1-4
Example 1
Controlled Potential Stripping of Aluminide Coating
[0034] Six airfoils (PW4000 2.sup.nd stage blades fabricated with a
single crystal nickel-based superalloy base metal) bearing an
aluminide coating, (approximately 0.001" thick) were clamped by
their root section into a titanium fixture. These coated airfoils
were engine-run for 5,000-11,000 hours. These six airfoils were
immersed in the tip-down orientation in a tank containing a
solution of 5% by volume concentration hydrochloric acid in water
at room temperature. The blades were submerged to their platform
level so that the acid solution contacted the areas requiring
coating removal but not the root section.
[0035] The acid tank also contained an insert comprised of three
graphite plates that functioned as counter electrodes. The tank
also contains a silver/silver chloride reference electrode, (e.g.
Model A6-4-PT available from GMC Corrosion of Ontario, Calif.).
[0036] The blades under open circuit conditions were initially at a
potential of -350 mV vs. Ag/AgCl. The potential of the blades with
respect to the Ag/AgCl reference electrode was adjusted using an
external power supply to a controlled value of +200 mV (that has
been determined experimentally to provide the greatest selectivity
between -350 mV and +500 mV for coating removal). The current flow
between the blades and the counter electrode assembly was monitored
(by the extrapolated zero-point algorithm based on numeric
differentiation of the current/time waveform) to determine the
point in time when the aluminide coating would be completely
removed. The coating was completely stripped after 45 minutes, and
the current flow was discontinued, and the airfoils were removed
from the stripping bath.
[0037] The completeness of the coating removal was verified
non-destructively through heat-tinting one of the six airfoils at
1050.degree. F. in air to produce a characteristic blue color.
Additionally, another airfoil was sectioned and examined
metallographically to verify the completeness of coating removal
and the absence of base metal attack.
Example 2
Controlled Potential Stripping of MCrAlY Coating
[0038] Six airfoils (PW4000 1.sup.st stage blades fabricated with a
single crystal nickel-based superalloy base metal) bearing a
NiCoCrAlY coating,(approximately 0.004" thick) were clamped by
their root section into a titanium fixture. These coated airfoils
were engine-run for about 5,000 to 11,000 hours. The six airfoils
were immersed in the tip-down orientation in a tank containing a
solution of 5% by volume concentration hydrochloric acid in water
at room temperature. The blades were submerged to their platform
level so that the acid solution contacted the areas requiring
coating removal, but not the root section.
[0039] The tank of solution contained an insert comprised of three
graphite plates that functioned as counter electrodes. The tank
also contained a silver/silver chloride reference electrode used in
Example 1.
[0040] The blades under open circuit conditions were initially at a
potential of -350 mV vs. Ag/AgCl). The potential of the blades with
respect to the Ag/AgCl referenced electrode was adjusted using an
external power supply to a controlled value +105 mV (that has been
determined experimentally to provide the greatest selectivity
between -350 mV and +500 mV for coating removal). The current flow
between the blades and the counter electrode assembly was monitored
(by the extrapolated zero-point algorithm based on numeric
differentiation of the current/time waveform) to determine the
point in time when the aluminide coating would be completely
removed. When the coating was completely stripped, the current flow
was discontinued, and the airfoils were removed from the stripping
bath.
[0041] The completeness of the coating removal was verified
non-destructively through heat-tinting one of the airfoil parts at
1050.degree. F. in air to produce a characteristic blue color.
Additionally, another airfoil was sectioned and examined
metallographically to verify the completeness of coating removal
and the absence of base metal attack.
[0042] Having described the stripping process, attention is now
turned to implementing use of the process in a commercial
environment. Referring now to FIG. 1, a commercial system 10 in
accordance with the present invention is shown. The commercial
system comprises a stripping tank 12 containing an acid electrolyte
bath stripping solution, a zero discharge rinse tank 14 containing
a rinse solution such as water, and a distillation unit 16 for
recycling and regenerating the stripping solution integrated on a
containment skid 18.
[0043] The stripping tank 12 contains the acid bath stripping
solution (not shown), a reference electrode 20, and an
electrodeless conductivity probe 24, such as a conductivity meter,
for monitoring the quality of the stripping solution. In a
preferred embodiment, the reference electrode 20 is actually a
hydrogen reference electrode array. The stripping tank contains a
counter electrode array 32, such as that shown in FIG. 3, for
providing a symmetrical solution potential distribution to each
workpiece 33 immersed wholly or partly in the stripping solution.
The counter electrode array 32 has four walls 60, 62, 64, and 66
formed from graphite or any other suitable electrically conductive
material and a pair of inserts 70 and 72 also formed from graphite
or any other suitable electrically conductive material secured to
the walls 60 and 62 by corner pieces 74 or any other suitable means
known in the art. The counter electrode array 32 is designed to
symmetrically enclose the workpiece(s) 33 from which the coating is
being stripped. While the counter electrode array 32 has been shown
as having a pair of inserts, the array could have just one insert
or it could have more than two inserts.
[0044] The rear wall 62 of the array 32 has buss strips 36 running
along its top. The buss strips 36 are preferably formed from grade
2 titanium plate or some other suitable electrically conductive
material.
[0045] Referring now to FIG. 4, the workpiece(s) 33 to be
introduced into the stripping solution are clamped into the fixture
34 using workpiece holders 35. The workpiece holders 35 may
comprise any suitable means known in the art. The fixture 34
delivers current from the buss strips 36 to each workpiece 33. The
fixture 34 may be moved towards and away from the stripping tank 12
using any suitable means known in the art such as a crane or a
hoist movable along a track (not shown). The fixture 34 may also be
used to transport the workpiece(s) 33, after the stripping
operation has been completed, to the rinse tank 14 where they are
rinsed to remove any residual stripping solution or metals.
[0046] The rinse tank 14 contains a conductivity probe 26 for
monitoring the quality of the rinse water in the tank. The rinse
tank 14 also contains a filter 28, such as a mixed-resin ion
exchange filter, and a circulating pump 30 to purify the rinse
water of acid and dissolved metal. The filter 28 preferably
operates at all times. If the conductivity of the rinse water in
the rinse tank exceeds a predetermined value as measured by the
probe 26, the operator is notified that corrective action is
required, i.e. replacement of the filter 28. Optionally, the system
may be interlocked until the filter 28 is changed.
[0047] Referring now to FIG. 6, acid recovery and acid regeneration
is accomplished by atmospheric distillation of the used stripping
solution in the distillation unit 16 using a low cost acid
distillation system sized for the stripping application. In this
distillation system, the used acid solution is gravity fed from the
strip tank 12 via line 91 to a boiler 90 in the distillation unit
16 where the acid is vaporized, leaving the dissolved metals in the
used acid solution in the boiler 90. The thus generated acid vapor
travels up into a condenser 92 where it is condensed back into the
liquid phase. From here, the purified acid returns to the strip
tank 12 by gravity via return line 93. The dissolved metals
accumulate in the boiler 90 effectively concentrated to
approximately 100 grams per liter total metal, resulting in a
10.times.reduction in waste over the typical stripping operation.
The concentrated dissolved metals are periodically purged from the
boiler 90. The purging is accomplished by a solenoid actuated valve
94 controlled by an electrodeless conductivity probe 96 and a
conductivity meter 98.
[0048] As shown in FIG. 2, the system 10 is controlled by a module
40 containing a computer 42, a data acquisition unit 44, and a
programmable power supply 46. The computer 42 may comprise any
suitable computer known in the art which has been programmed in any
language to carry out the functions hereinafter discussed. An
operator interface 47 including a keyboard 48, a mouse (not shown),
a CRT 52, push button controls (not shown), and a signal light tree
56 is built into the module 40. The module 40 may be mounted on the
skid 18 or may be a stand alone module separate from the skid
18.
[0049] Referring now to FIG. 5, the data acquisition and control
system employed in the system 10 of the present invention is
illustrated. A digital multimeter 80 is provided to measure the
voltage potential between the reference electrode 20 and the
workpiece(s) 33 from which the coating is to be/being stripped. A
predetermined target voltage is maintained during the stripping
process by adjusting the current output of the DC power supply 46,
which is preferably operated in a constant current mode.
[0050] The value of the adjustment required to maintain the target
voltage is determined by the computer 42 preferably using an
algorithm which tracks the change in the cell current versus the
change in potential between the reference electrode 20 and the
workpiece(s) 33. The operating mode of the power supply 46 is thus
monitored to prevent invalid adjustments. When the power supply 46
approaches its voltage output limit, it automatically switches into
a constant voltage mode. No power supply adjustments are made under
these conditions until the voltage output decreases and the power
supply 46 switches back into a constant current mode. The stripping
cycle end point is determined by the computer 42 preferably using a
multiple regression analysis of elapsed time and cell amperage.
[0051] Actual cell current in the stripping tank 12 is monitored by
measuring the voltage across a shunt resistor 84 via a second
digital multimeter 86. The shunt resistor 84 is electrically
connected to a counter electrode or a workpiece in the stripping
tank and to the power supply 46. A power supply shorting resistor
88 is provided in the control system to allow for finer adjustments
to the cell current as the cell resistance increases due to the
presence of an increased presence of dissolved metals in the
stripping solution. A conductivity probe 24 in the stripping tank
12 is used to monitor the conductivity and temperature of the
stripping solution and to transmit a first signal representative of
these properties to the data acquisition system 44. The metal
loading of the stripping solution may be determined in any manner
known in the art by the computer 42 such as through the use of a
linear regression analysis of the solution temperature and
conductivity. The acid concentration of the stripping solution may
be determined by the computer 42 using an algorithm based on
solution conductivity.
[0052] As previously discussed, the conductivity probe 26 in the
rinse tank 14 notifies the operator whether the rinse solution is
within acceptable limits or not. As shown in FIG. 5, the probe 26
transmits a second signal representative of the state of the rinse
solution acceptability to the data acquisition system 44. If the
rinse solution is not acceptable, the filter 28 is changed and
operated until the rinse solution returns to an acceptable
state.
[0053] The operator interface 47 includes a set of interactive
screens for selecting the parameters for the strip cycle, and
digital inputs from front panel pushbuttons/selector switches to
provide various other control features. For example, the operator
interface 47 may include a "Cycle Start" pushbutton to begin a
strip cycle; a key operated "Run/Stop" selector switch to provide a
level of security against unauthorized use; and a "Controls On"
pushbutton used to energize all subsystems. Two "Emergency Stop"
latching mushroom buttons, one on the operator console and another
on the strip tank 12, may be provided to de-energize the power
supply 46. The Christmas tree 52 provides a visual indication of
system status. A green light may indicate that a cell setup is in
progress (part loading, etc.). A yellow light may indicate that a
strip cycle is in progress. A red light may indicate that the
system is idle.
[0054] The above describes a system which may use a type of
reference electrode arrangement (i.e. a Ag/AgCl reference electrode
placed in close proximity to the workpiece to avoid the need for
solution IR corrections). The reference electrode may also be
placed further away from the workpiece for ease of use with the
resulting potential being corrected for this IR effect. An array of
platinum or hydrogen reference electrodes can also be used, each in
close proximity to a given workpiece, to more precisely monitor and
control large numbers of parts being strapped at one time. These
latter two arrangements can also be employed together to reduce the
time required to saturate the solution with hydrogen. The remote
Ag/AgCl electrode and IR correction to monitor and control the
potential until such time as sufficient hydrogen has been generated
to switch over to the Pt array electrode control methodology may be
used in the system of the present invention.
[0055] The system of the present invention may be used in a wide
variety of environments to remove a wide variety of coatings. For
example, the system 10 could be used to remove thermal barrier
coatings, aluminide coatings, and MCrAlY coatings from turbine
blades and vanes and other airfoils. The system could also be used
to remove solder or braze joining compounds from metal workpieces.
Still further, the system of the present invention could be used to
remove electroplated coatings from steel members.
[0056] While various mathematical techniques have been discussed
herein to carry out certain functions and analyses, it should be
apparent to those skilled in the art that other mathematical
techniques may be used to carry out the analyses and functions set
forth hereinbefore.
[0057] While the invention has been described above with reference
to specific embodiments thereof, it is apparent that many changes,
modifications,-and variations can be made without departing from
the inventive concept disclosed herein. Accordingly, it is intended
to embrace all such changes, modifications and variations which
fall within the broad scope of the attached claims.
* * * * *