Note: Descriptions are shown in the official language in which they were submitted.
CA 02645387 2008-11-27
METHOD FOR ELECTROLYTIC STRIPPING OF SPRAY METAL COATED
SUBSTRATE
FIELD OF THE INVENTION
The present invention relates generally to a method for electrochemically
stripping a
spray metal coating from an electrically conductive material.
BACKGROUND OF THE INVENTION
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).
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.
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.
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.
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
-1-
CA 02645387 2008-11-27
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.
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.
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.
The prior art describes electrochemical methods for removing electroplated
coatings.
U.S. Patent 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".
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.
-2-
CA 02645387 2008-11-27
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.
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.
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.
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.
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.
-3-
CA 02645387 2008-11-27
U.S. 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.
U.S. 4,886,588 (Curfinan) 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
Curf nan 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).
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
It is an object of the present invention to obviate or mitigate at least one
disadvantage
of the references discussed above.
-4-
CA 02645387 2008-11-27
It has been surprisingly discovered that electrochemical removal of fused
thermal
spray metal coatings is possible.
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.
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.
In various embodiments, the spray metal coating is a NiCr, WC or MoS2 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 - 35 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 H2CrO4, and the positive potential
applied
generates a current of 20 to 80 mA/cm2 (milli Amperes per square centimeter)
(20 to 80
A/ft).
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 C and 80 C, (90 F and 180 F) and
preferably
between 50 and 60 C (120 F and 140 F).
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
-5-
CA 02645387 2008-11-27
deposited on the conductive material. Cathodes of this embodiment are lighter
and more
easily maneuvered than solid metal cathodes.
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
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 MoS2 can also be removed
by this
process.
The electro-chemical 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.
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.
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.
A final advantage of electro-chemical 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.
-6-
CA 02645387 2008-11-27
The stripping composition of the present invention is an aqueous, electrolyte
solution
having the following ingredients: chromic acid (CrO3), a peroxide, preferably
hydrogen
peroxide (H202), a second acid, preferably sulfuric acid (H2SO4) or phosphoric
acid (H3PO4),
and water (H2O). 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.
Thus, it is preferred that the chromic acid solution be initially formed
having a 20-25%
Cr03 concentration by weight in water. In the presence of water, Cr03 forms
H2CrO4.
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
Cr03 added to
each gallon of H2SO4, this ranges from 2-10 ounces, and is preferably 2-4
ounces. The
sulfuric acid itself is an aqueous solution of H2SO4 and water. Preferred is a
50 Baume
solution, i.e. one having a 62.2% H2SO4 concentration by weight and a specific
gravity of
1.53. Other sulfuric acids in the 55 - 35 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 C.
Because an elevated temperature may be used and since the reaction between
Cr03
and H2SO4 is exothermic, the solution should thereafter be cooled. Once
cooled, the peroxide
is added. Preferred is a hydrogen peroxide which has a 35% H202 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.
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
-7-
CA 02645387 2008-11-27
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.
A chromic acid solution is prepared having a Cr03 concentration of 25%. 20
parts per
volume of this solution are gradually added to 50 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.
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.
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.
-8-
