U.S. patent application number 12/325808 was filed with the patent office on 2010-05-27 for method for electrolytic stripping of spray metal coated substrate.
Invention is credited to James Coulas, Jovica Marjanovic, Robert A.R. Mills.
Application Number | 20100126878 12/325808 |
Document ID | / |
Family ID | 42195232 |
Filed Date | 2010-05-27 |
United States Patent
Application |
20100126878 |
Kind Code |
A1 |
Marjanovic; Jovica ; et
al. |
May 27, 2010 |
Method for Electrolytic Stripping of Spray Metal Coated
Substrate
Abstract
A method for stripping a fused thermal spray metal coating from
the surface of a soft metallic substrate. The steps for the method
include: immersing the coated metallic substrate in an aqueous
solution of chromic acid, peroxide, and a second acid; immersing a
metal cathode in the aqueous solution; applying a positive
potential to the fused spray metal coated substrate and a negative
potential to the metal cathode to generate a direct current between
the substrate and the cathode; where the current is applied for a
sufficient time to remove the coating. This method permits the
electrochemical removal of fused and impermeable thermal spray
metal coatings.
Inventors: |
Marjanovic; Jovica;
(Calgary, CA) ; Coulas; James; (Calgary, CA)
; Mills; Robert A.R.; (Calgary, CA) |
Correspondence
Address: |
QUARLES & BRADY LLP
411 E. WISCONSIN AVENUE, SUITE 2040
MILWAUKEE
WI
53202-4497
US
|
Family ID: |
42195232 |
Appl. No.: |
12/325808 |
Filed: |
December 1, 2008 |
Current U.S.
Class: |
205/720 ;
205/717 |
Current CPC
Class: |
C25F 5/00 20130101 |
Class at
Publication: |
205/720 ;
205/717 |
International
Class: |
C25F 3/06 20060101
C25F003/06 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 27, 2008 |
CA |
2,645,387 |
Claims
1. A method for stripping a fused spray metal coating from the
surface of an iron based substrate, comprising the steps of:
immersing the fused spray metal coated iron based substrate in an
aqueous solution comprising chromic acid, peroxide, and a second
acid, the second acid being sulfuric acid, phosphoric acid or a
mixture thereof; immersing a metal cathode in the aqueous solution;
and applying a positive electrical potential to the substrate and a
negative electrical potential to the cathode for creating a direct
electrical current between the cathode and the anode through the
fused spray metal coating for removal of the fused spray metal
coating from the steel substrate.
2. The method according to claim 1, wherein the positive and
negative electrical potentials are maintained until complete
removal of the fused spray metal coating.
3. The method according to claim 1 or 2, wherein the spray metal
coating is a NiCr, a WC, or a MoS.sub.2 alloy.
4. The method according to claim 1, 2 or 3, wherein the substrate
is steel, mild steel, common steel, or iron.
5. The method according to any one of claims 1 to 4, wherein the
spray metal coating is an impermeable coating.
6. The method according to any one of claims 1 to 5, the aqueous
solution comprising: 6 to 38 parts by volume of a 20 to 25 wt %
aqueous chromic acid solution; 2 to 8 parts by volume of about a 35
wt % aqueous peroxide solution; and 100 parts by volume is
55.degree.-35.degree. Baume sulfuric acid.
7. The method according to any one of claims 1 to 6, wherein the
peroxide is hydrogen peroxide or barium peroxide.
8. The method according to any one of claims 1 to 7, wherein the
chromic acid is H.sub.2CrO.sub.4.
9. The method according to any one of claims 1 to 8, wherein the
current is 20 to 80 mA/cm.sup.2 (20 to 80 A/ft.sup.2).
10. The method according to any one of claims I to 9, the aqueous
solution further comprising a catalyst.
11. The method of claim 10, wherein the catalyst is hydrogen
peroxide.
12. The method according to any one of claims 1 to 11, wherein the
aqueous solution is maintained at a temperature of 30 to 80.degree.
C. (90.degree. F. to 180.degree. F.).
13. The method of claim 12, wherein the solution is maintained at
50 to 60.degree. C. (120.degree. F. to 140.degree. F.).
14. The method according to any one of claims 1 to 12, the cathode
comprising an anodically deposited coating of lead, carbon or
titanium or oxides thereof, covering a conductive material having a
backing of non-conductive material.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to a method for
electrochemically stripping a spray metal coating from an
electrically conductive material.
BACKGROUND OF THE INVENTION
[0002] Surface treatment of a substrate is known to control
friction and wear, improve corrosion resistance, appearance, and/or
alter the dimensions of the substrate. There are a variety of
surface treatments available, including painting, laminating,
physical vapor deposition and chemical vapor deposition. In
addition, two common surface treatments used on metal substrates
are electroplating and thermal spray coating (also known as
hardfacing).
[0003] Electroplating or plating is an electrochemical process in
which a metal coating is deposited on a substrate by passing a
current through the substrate placed in an electrolytic bath
containing metal ions. Electroplated coatings are widely used for
depositing a corrosion- or wear-resistant metal on the
substrate.
[0004] Thermal spraying is a coating process that consists of a
heat source and a coating material feedstock. The coating material
can often be in a powder- or wire-form, and is sprayed onto a
substrate in a molten state. The thermal spraying process welds the
coating material to the substrate. Thermal spraying methods using
high velocity processes such as flame spray, plasma, high velocity
oxygen fuel (HVOF), electric-arc, and detonation gun (D-gun), are
widely used.
[0005] The plasma spray, HVOF and detonation gun (D-gun) processes,
use different approaches for melting a metal powder, and propelling
the resulting metal droplets onto the surface to be coated. They
produce a coating layer made of consecutive layers of solidified
metal droplets. The result is a rather porous coating with
different degrees of coating oxides included in the coating.
[0006] Flame spray, also known as oxy-acetylene combustion spray,
uses the basic principles of a welding torch to propel molten
particles onto the substrate. The coating material can be either a
wire- or powder-form. Usually nitrogen is used as a carrier to
conduct metal powder into the centre of the combustion zone in the
torch.
[0007] Thermal spray metal coatings are naturally somewhat porous
or permeable, since the molten metal droplets upon impact with the
substrate don't necessarily merge into a continuous, impermeable
coating. To achieve an impermeable coating, thermal spray metal
coatings are often fused after being applied to enhance bond
strengths and coating density. The fusing process at least
partially melts and bonds coating particles to each other,
resulting in a non-porous coating virtually free from coating
oxides found in non-fused spray coatings. This optional step
changes the process from a "cold spray" method to a
"spray-and-fuse" method. Cold spray coatings exhibit lower bond
strengths than most other thermal spray processes. Spray-and-fuse
coatings are used in applications where excessive wear and/or
corrosion combined with high stresses on the coating/substrate are
a problem.
[0008] Methods for the removal of electroplated coatings
essentially involve a process known as reverse electroplating, the
electrical reversal of the plating process. Reverse electroplating
involves applying an electric current through an electrolytic bath,
wherein the positive electrical potential is applied to a coated
metallic substrate and a negative electrical potential is applied
to a cathode. A direct current is generated between the substrate
and the cathode, removing the coating. Electroplated coatings can
usually be completely removed without damage to the substrate,
given sufficient care and attention.
[0009] The prior art describes electrochemical methods for removing
electroplated coatings. U.S. Pat. No. 4,356,069 (Cunningham)
describes a method wherein electroplated chrome and nickel coatings
can be stripped using an electrochemical process. The patent
discloses a stripping composition which aids in the removal of
chromium and nickel ions from the surface of a base metal, such as
zinc, steel, aluminum, brass or copper which had been previously
plated with chrome and/or nickel".
[0010] Electrochemical processes have generally been accepted in
the art as unsuited for the removal of fused thermal spray coatings
since the coatings were considered insufficiently porous to be
susceptible to the known stripping compositions. In fact,
Cunningham explicitly states that the process is for use on metals
previously plated (column 2, line 35). The term "plated" is known
in the art to refer to the deposition of a coating of metal using
electrolysis. Cunningham does not mention thermal spray coatings in
general, or flame spray-and-fuse coatings in particular.
Furthermore, the harsh stripping compositions generally used were
expected to result in the corrosion of the metal substrate,
especially iron substrates.
[0011] Up to this point, there has been no reason to expect that
the fused thermal spray coatings would be susceptible to stripping
composition attack due to their lack of porosity and that the
extremely harsh chemical stripping composition necessary for
electrochemical removal of fused coatings would not damage the
underlying substrate if used for removing fused thermal spray metal
coatings from soft metal substrates. The prior art does describe
electrochemical methods for removing thermal spray metal coatings,
but not fused coatings, and the disclosed methods are limited to
situations where a person of skill in the art would expect that the
metal substrate would not be corroded by an acidic electrolytic
bath.
[0012] Thermal spray or electroplated coatings generally become
worn during use, and the substrates are often of sufficient value
that it is desirable to reuse or recycle them. For reuse, it is
necessary to strip the coating, re-coat the substrate, and place it
back in service. This requires a means of economically and quickly
stripping the coating without damaging the base metal substrate of
the object it is also sometimes desirable to strip the coating and
re-coat the substrate if, in the process of coating a part, the
part no longer meets specifications. This, also, must be
accomplished without damaging the substrate.
[0013] Traditionally, removal of thermal spray coatings from a
substrate has been accomplished using grinding or other mechanical
means, such as sand, shot, or grit blasting. However, parts with
complex shape cannot be ground within the desired tolerances, and
the other mechanical methods are also not sufficiently precise.
Moreover, the removal of thermal spray coatings using mechanical
means can result in irreparable damage to the substrate.
[0014] It is frequently necessary or desirable to roughen the
surface of a substrate rather deeply before thermal spray metal
coating in order to achieve the necessary bonding strength of the
coating to the substrate. Therefore, coatings must be completely
removed before the substrate is re-coated with thermal spray
coatings. Since the thermal spray coating cannot be completely
removed by mechanical means without also removing a considerable
amount of the substrate material, serious and often unacceptable
changes to the dimensions of the substrate can occur.
[0015] U.S. Pat. No. 4,128,463 (Formanik) discloses a process
wherein titanium or titanium alloy substrates coated with
flame-spray metal WC coatings are stripped using an electrolytic
cell. Titanium and titanium alloys are known in the art to be
highly resistant to corrosion and would not be expected to be
damaged during the reverse electroplating process. It is,
therefore, not unexpected that reverse electroplating could be used
to remove a spray metal coating without damaging the titanium
substrate. However, Formanik does not discuss or even suggest that
the process could be used for other substrates. The process
disclosed by Formanik is only described for use in removing WC
coatings from a titanium or a titanium alloy, substrate, and no
soft metal substrates such as iron or steel. More importantly,
Formanik never discusses fused spray metal coatings and whether the
disclosed process could be used for the removal of fused thermal
spray metal coatings.
[0016] U.S. Pat. No. 4,886,588 (Curfman) discloses a process
wherein spray metal coatings can be electrochemically removed from
a soft metal substrate, in this case aluminum. However, aluminum is
not as corrosion resistant as titanium. Thus, it is not surprising
that the focus of the process described by Curfman is to prevent
corrosion of the substrate by using aluminum corrosion inhibitors
in the reverse electroplating process. It is taught by Curfman that
preventing the corrosion of the metal substrate by the highly
corrosive electrolytic bath used for reverse electroplating is of
great importance (see column 2, lines 7-11).
[0017] Neither Formanik nor Curfman disclose a process for the
removal of fused thermal spray coatings or other coatings of low
permeability (e.g. flame spray-and-fuse coatings, plasma spray
coatings, or HVOF coatings). In fact, Formanik describes the
process as mechanically loosening the coatings by electrolytically
generating hydrogen gas on the surface of the substrate (see column
1, lines 61-64). For this mechanical loosening to occur, the
coating must be permeable. Formanik and Curfman neither disclose
nor suggest the electrochemical removal of low permeability, fused
coatings from steel, mild steel, common steel or iron.
SUMMARY OF THE INVENTION
[0018] It is an object of the present invention to obviate or
mitigate at least one disadvantage of the references discussed
above.
[0019] It has been surprisingly discovered that electrochemical
removal of fused thermal spray metal coatings is possible.
[0020] It is surprising that electrochemical conditions were
discovered that completely remove flame spray-and-fused coatings
economically from a soft metal substrate, without altering the
substrate.
[0021] In a first aspect, the present invention provides a method
for stripping a fused spray metal coating from the surface of a
soft metallic substrate. The steps for the process comprise:
immersing the coated metallic substrate as an anode in an aqueous
solution of chromic acid, peroxide, and sulphuric acid or
phosphoric acid; immersing a metal cathode in the aqueous solution;
applying a positive potential to the fused spray metal coated
substrate and a negative potential to the metal cathode to generate
a direct current between the substrate and the cathode; whereby the
current is preferably applied for a sufficient time to completely
remove the coating.
[0022] In various embodiments, the spray metal coating is a NiCr,
WC or MoS.sub.2 alloy. In various embodiments, the metallic
substrate is steel, common steel or iron. In various embodiments,
the aqueous solution is made up of: 6 to 38 parts by volume of a 20
to 25 wt % aqueous chromic acid solution; 2 to 8 parts by volume of
about a 35 wt % aqueous peroxide solution; and 100 parts by volume
of 55.degree.-35.degree. Baume sulphuric acid. While any peroxide
is acceptable, the aqueous solution preferably comprises hydrogen
peroxide or barium peroxide. In further embodiments, the chromic
acid is H.sub.2CrO.sub.4, and the positive potential applied
generates a current of 20 to 80 mA/cm.sup.2 (milli Amperes per
square centimeter) (20 to 80 A/ft.sup.2).
[0023] In another aspect, the aqueous solution further includes a
catalyst, which, in the embodiment above, is peroxide. In a further
aspect, the temperature of the aqueous solution is maintained at a
temperature between 30.degree. C. and 80.degree. C., (90.degree. F.
and 180.degree. F.) and preferably between 50.degree. and
60.degree. C. (120.degree. F. and 140.degree. F.).
[0024] In a further embodiment, the cathode is solid lead, carbon,
titanium or any other conductor inert to the electrolyte solution.
The cathode preferably has a conductive material, the conductive
material having a non-conductive backing and a coating of lead,
carbon, titanium or other conductor inert to the electrolyte
solution which has been electrochemically deposited on the
conductive material. Cathodes of this embodiment are lighter and
more easily maneuvered than solid metal cathodes.
[0025] Other aspects and features of the present invention will
become apparent to those ordinarily skilled in the art upon review
of the following description of specific embodiments of the
invention.
DETAILED DESCRIPTION
[0026] Generally, the present invention provides a method and
system for electrochemically removing fused spray metal coatings
from metallic substrates. For the purpose of illustrating the
preferred embodiment, there will be discussed a method for the use
of a stripping composition, which is adapted for the
electrochemical removal of chrome and/or nickel spray metal
coatings. However, other alloys such as WC or MoS.sub.2 can also be
removed by this process.
[0027] The electrochemical stripping of a coating depends upon the
object being placed in a bath containing chemicals such that one of
the coating components is either soluble or will react with a
chemical in the bath to form a soluble compound. Preferably, the
reaction between the chemicals in the bath and the coating should
be slow, otherwise the process would be difficult to control and
there would be greater risk of damaging the substrate.
[0028] In some cases, the reaction will not proceed because
compounds are formed that coat the surface and stop the reaction.
In others, the process would be uneven because the reaction would
be catalyzed by impurities in places. The application of an
electrical potential overcomes these problems with the correct
placement of the electrodes.
[0029] The application of an electrical potential enables a weak
reaction to be accelerated and controlled to prevent damage to the
substrate. In some cases, corrosion inhibitors are helpful to
protect the substrate.
[0030] A final advantage of electrochemical stripping is that the
concentration of the chemicals in the bath are maintained because
the material being removed is redeposited at the other electrode
and the chemical stripper is thereby rejuvenated. This also helps
in the control of the process because the rate of stripping is more
or less constant.
[0031] The stripping composition of the present invention is an
aqueous, electrolyte solution having the following ingredients:
chromic acid (CrO.sub.3), a peroxide, preferably hydrogen peroxide
(H.sub.2O.sub.2), a second acid, preferably sulfuric acid
(H.sub.2SO.sub.4) or phosphoric acid (H.sub.3PO.sub.4), and water
(H.sub.2O). The method of preparing the stripping composition is
also important since exothermic reactions are involved and the
order of addition of the ingredients is significant.
[0032] Thus, it is preferred that the chromic acid solution be
initially formed having a 20-25% CrO.sub.3 concentration by weight
in water. In the presence of water, CrO.sub.3 forms
H.sub.2CrO.sub.4. Preferably 225-1500 ml (6-40 parts by volume) of
this chromic acid solution is added to each gallon (100 parts by
volume) of sulfuric acid. In terms of dry weight of CrO.sub.3 added
to each gallon of H.sub.2SO.sub.4, this ranges from 2-10 ounces,
and is preferably 24 ounces. The sulfuric acid itself is an aqueous
solution of H.sub.2SO.sub.4 and water. Preferred is a 50.degree.
Baume solution, i.e. one having a 62.2% H.sub.2SO.sub.4
concentration by weight and a specific gravity of 1.53. Other
sulfuric acids in the 55.degree.-35.degree. Baume range may be used
as well. Phosphoric acid at this concentration may be used as well.
The chromic acid solution is added to the sulfuric acid preferably
at between room temperature and 50.degree. C.
[0033] Because an elevated temperature may be used and since the
reaction between CrO.sub.3 and H.sub.2SO.sub.4 is exothermic, the
solution should thereafter be cooled. Once cooled, the peroxide is
added. Preferred is a hydrogen peroxide which has a 35%
H.sub.2O.sub.2 concentration by weight in water. Technical grades
of hydrogen peroxide at this concentration are available. Other
concentrations may also be used, as may other peroxides, such as
barium peroxide. The preferred amount is 100-300 ml (2.5-8 parts by
volume) of this peroxide solution. Although more may be added
without deleterious effect, it is not necessary to achieve the
result desired.
[0034] The stripping solution is now complete in terms of active
ingredients. Clearly, it may contain amounts of any number of
inactive ingredients as well. The exact chemistry of the products
within the stripping solution after mixing is not known. In the
preferred embodiment, however, it is believed that a trivalent form
of chromium such as chromium sulfate or chromium dichromate is
formed and that the trivalent chromium is oxidized at the anode to
form a hexavalent chromium. In the process, the chromium metal
plated on the base metal substrate of the anode will be stripped
therefrom. The peroxide is believed in the process to form an
intermediate oxide layer on the surface of the base metal substrate
which will protect the substrate metal from attack by the sulfuric
acid.
[0035] A chromic acid solution is prepared having a CrO.sub.3
concentration of 25%. 20 parts per volume of this solution are
gradually added to 50.degree. Baume sulfuric acid at room
temperature. The rate of addition is controlled to prevent
overheating of the resulting composition. Alternatively, the
composition is cooled during addition of the chromic acid solution.
4 parts per volume of a 35% aqueous hydrogen peroxide solution are
added once the composition is at room temperature to finish the
stripping composition. A lead cathode is inserted into the
stripping composition and a steel PCP pump rotor with a fused
thermal spray coating of NiCr is suspended in the stripping
composition. A positive electrical potential is applied to the
coated rotor and a negative electrical potential is applied to the
lead cathode. A current is applied for 30 minutes for complete
removal of the fused NiCr coating.
[0036] In the preceding description, for purposes of explanation,
numerous details are set forth in order to provide a thorough
understanding of the embodiments of the invention. However, it will
be apparent to one skilled in the art that these specific details
are not required in order to practice the invention.
[0037] The above-described embodiments of the invention are
intended to be examples only. Alterations, modifications and
variations can be effected to the particular embodiments by those
of skill in the art without departing from the scope of the
invention, which is defined solely by the claims appended
hereto.
* * * * *