Note: Descriptions are shown in the official language in which they were submitted.
2056~3
SUBSTRAT~ OF IMPROV~D PLASMA SPRAY~D SURFACE MORPHOLOGY
Back~round of the Invention
The adhesion of coatings applied directly to the
surface of a substrate metal is of special concern when
the coated metal will be utilized in a rigorous
industrial environment. _Car~ful attention is usually
paid to surface treatment and pre-treatment operation
prior to coating. Achievement particularly of a clean
surface is a priority sought in such treatment or
pre-treatment operation.
Representative of a coating applied directly to a
base metal is an electrocatalytic coating, often
containing a precious metal from the platinum metal
group, and applied directly onto a metal suCh as a valve
metal. Within this technical area of electrocatalytic
coatings applled to a base metal, the metal may be
simply cleaned to give a very smooth surface. U.S.
Patent No, 4, 797,182. Treatment with ~luorine compounds
may produce a smooth surfac~ . U. S . Patent No .
3,864,163. Cleaning might include chemical degreasing,
A
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electrolytic degreasing or treatment with an oxidi2ing
acid. U.S. Patent 3,864,163.
Cleaning can be followed by mechanical roughening
to prepare a surface for coating. U.S. Patent NO,
3,778,307. If the mechanical treatment is sandblasting,
such may be followed by etching. U.S. Patent No .
3,878,08~. Or such may be followed by flame spray
application of a fine-particled mixture of metal
powders. U.S. Patent No. 4,849,085.
Another procedure for anchoring the fresh coating
to the substrate, that has found utility in the
application of an electrocatalytic coating to a valve
metal, is to provide a porous oxide layer which can be
formed on the base metal. For example, titanium oxide
can be flame or plasma sprayed onto substrate metal
before application of electrochemically active
substance, as disclosed in U.S. Patent No. 4,140,81~.
Or the thermally sprayed material may consist of a metal
oxide or nitride or So forth, to whiCh
electrocatalytically active particles have been pre-
applied, as taught in U.S. Patent No. 4,392,927.
It has however been found difficult to provide
long-lived coated metal articles for serving in the most
rugged commercial environments, e.g., oxygen evolving
anodes for use in the present-day commercial
applications utilized in electrogalvanizing,
electrotinning, electroforming or electrowinning. Such
may be continuous operation. They can involve severe
conditions including potential surface damage It would
30 be most desirable to provide coated metal substrates to
serve as electrodes in such operations, exhi~iting
extended stable operation while preserving excellent
coatlng adheslon~ It would also be hlghly deslrable to
provide such an electrode not only from fresh metal but
35 als~ fr~m rec~ated metal.
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SummarY of the Invention
There has now been found a metal surface which
provides a locked on coating of excellent coating
adhesion. The coated metal substrate can have highly
desirable extended lifetime even in most rigorous
industrial environments. For the electrocatalytic
coatings, the invention can provide for well anchored
coatings of uniform planarity, even when utilizing
gouged and slmilarly disflgured substrate metal.
In one aspect, the invention is directed to an
electrode of a valve metal substrate having a valve metal
containing surface adapted for enhanced coating adhesion,
said surface comprising a plasma spray applied valve
metal surface, which plasma spray applied surface
provides a profilometer-measured average ~urface
roughness of at least about 250 microinches and an
average surface peaks per inch of at least about 40,
based upon a profilometer upper threshold limit of 400
microinches and a profilometer lower threshold limit of
300 microinches.
In another aspect, the invention is directed to the
method of preparing a planar metal surface for enhanced
coating adhesion, which surface has been gouged and
thereby exhibits loss of planarity, which method
comprises:
Plasma spraying the gouges of said sur~ace with a
valve metal to establish metal surface planarity, and
then, plasma spraying the surface to be coated, in~luding
the plasma sprayed gouges to provide a profilometer-
30 measured average ~urfa~e roughness of at least about Z50
microinches and an average surface peaks per inch of atleast about ~o, ba~ed upon a profilometer uppeL thre~hold
limit ~ 400 microinches and a p~o~ilometer lower
threshold llmit of 300 mlcrolnches.
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In a still further aspect, the invention is directed
to a cell for electrolysis having at least one electrode
of a metal article as further defined herein. When the
metal articles are electrocatalytically coated and used
as oxygen evolving electrodes, even under the rigorous
commercial operations including continuous
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electrogalvanizing, electrotlnning, copper foil plating,
electroforming or electrowinning, and including sodium
sulfate electrolysis such electrodes can have highly
desirable servlce llfe. Thus the invention is also
directed to such metal articles as are utilized as
electrodes.
Description of the Preferred Embodiments
The metals of the substrate are broadly
contemplated to be any coatable metal. For the
particular application of an electrocatalytic coating,
the substrate metals might be such as nickel or
mângane~e, but will most always be valve metals,
including titanium, tantalum, aluminum, zirconium and
niobium. Of particular interest for its ruggedness,
corrosion resistance and availability is titanium. As
well as the normally available elemental metals
themselves, the suitable metals of the substrate can
include metal alloys and intermetallic mixtures, as well
as ceramics and cermets such as contain one or more
valve metals.. For example, titanium may be alloyed
with nickel, cobalt, iron, manganese or copper. More
specifically, Grade 5 titanium may include up to 6.75
weight% aluminum and 4.5 weight% vanadium, grade 6 up to
6~ aluminum and 3% tin, grade 7 up to 0.25 weight~
palladium, grade 10, from 1~ to 13 weight~ molybdenum
plus 4.5 to 7.5 weight% zirconium and so on.
By use of elemental metals, alloys and
intermetallic mixtures, it is most particularly meant
the metals in their normally available condition, i.e.,
having minor amounts of impurities. Thus for the metal
of particular interest, i.e., titanium, various grades
of the metal are available including those in which
other constltuents may be alloys or alloys plus
impurities. &rades of titanium have been more
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specifically set forth in the standard specifications
for titanium detailed in ASTM B 265-79.
Regardless of the metal selected and how the metal
surface is su~sequently processed, the substrate metal
advantageously is a cleaned ~urfa~e. Thi~ may be
obtained by any of the treatments used to achieve a
clean metal surface, but with the provision that unless
called for to remove an old coating, and if etching
might be employed, as more specifically detailed
hereinbelow, mechanical cleaning is typically minimized.
ThuS the usual cleaning procedures of degreasing, either
chemical or electrolytic, or other chemical cleaning
operation may be used to advantage.
Where an old coating is present on the metal
surface, such needs to be addressed before recoating.
It iS preferred for best extended performance when the
finished article will be used with an electrocatalytic
coating, such as use as an oxygen evolving electrode, to
remove the old coating. In the technical area of the
invention which pertains to electrochemically active
coatings on a valve metal, chemical means for coating
removal are well known. Thus a melt of essentially
basic material, followed by an initial pickling will
suitably reconstitute the metal surface, as taught in
U.S. Patent No. 3,573,100. Or a melt of alkali metal
hydroxide containing alkali metal hydride, which may be
followed by a mineral acid treatment, is useful, as
described in U.S. Patent No. 3,706,600. Usual rinsing
and drying steps can also form a portion of these
operations.
When a cleaned surface, or prepared and cleaned
surfa~e ha~ been obtained, and particularly for later
applying an electrocatalytic coating to a valve metal in
the practice of the present invention, surface roughness
is then obtained. ThiS will ~,,e achieved by means which
include plasma spray appli~;~cion, usually of particulate
valve metal, most especially tltanium powder. However~
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as described herelnbelow, although the metal will be
applied in particulate form, the feed metal, i.e., the
metal to be applied, may be in different form such as
wire form. This should be understood even though for
convenience, application will typically be discussed as
metal applied in particulate form. In this plasma
spraying, the metal is melted and sprayed in a plasma
stream generated by heating with an electric arc to high
temperatures an inert gas, such as argon or nitrogen,
optionally containing a minor amount of hydrogen. It is
to be understood by the use herein of the term "plasma
spraying" that although plasma spraying i5 preferred the
term i~ meant to include generally thermal spraying such
as magnetohydrodynamic spraying, flame spraying and arc
spraying.
The spraying parameters, such as the volume and
temperature of the flame or plasma spraying stream, the
spraying distance, the feed rate of particulate metal
constituents and the like, are chosen so that the
particulate metal components are melted by and in the
spray stream and deposited on the metal substrate while
still substantially in melted form so as to provide an
essentially continuous coating (i.e. one in which the
sprayed particles are not discernible) having a
foraminous structure. Typically, spray parameters like
those used in the examples give satisfactory coatings.
Usually, the metal substrate during melt spraying is
maintalned near ambient temperature. This may be
achieved by means such as streams of air impinging on
the substrate during spraying or allowing the substrate
to air cool between spray passes.
The partlculate metal employed~ e.g.l titanium
powder, has a typical particle size range of 20 - 100
microns, and preferably has all particles within the
~5 range of 40 - 80 micron~ for efficient preparation of
surface roughness. Particulate metals having ~if~eren~
particle sizes should ~e equally suita~le so long as
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they are reddily plasma spray applled. The metallic
constituency of the particulates may be as above-
described for the metals of the substrate, e.g., the
titanium might be one of several grades most usually
grade 1 titanium. It is also contemplated that
mixtures may be applied, e.g., mixtures of metals or of
metals with other subsituents, which can include metal
oxides, for example a predominant amount of metal with a
minor amount of other substituents.
It is also contemplated that such plasma spray
applications may be used in combination with etching of
the substrate metal surface, with each treatment most
always being applied to different portions of a surface.
If etching is used, it is important to aggressively etch
the metal surface to provide deep grain boundaries and
well exposed, three-dimensional grains. It iS preferred
that such operation will etch impurities located at such
grain boundaries.
Particularly where an old coating has been present
and the coated substrate has been in use, e . g., as an
anode in electrogalvanizing, the metal article can be
disfigured and can have lost surface planarity.
Typically, such disfiguring will be in nicks and gouges
of the surface. For convenience, all such surface
disfigurement, including nicks, scrapes, and gouges, and
burns where metal may actually be melted and resolidify,
will generally be referred to hereln simply as "gouges."
These may or may not be filled with a metal filling. If
the overall surface were to be subsequently etched
before recoating, the filled zones can be expected to
yield poor etch results. Also, gouging of the substrate
may be extensive, ~r the substrate from its heat history
and/or chemlstry may not achieve desirable results in
etching. It may, therefore, be especially desirable to
simply plasma spray the entire surface which can
overcome these substrate deficiencie5. It is also
contemplated that it may be useful to combine plasma
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spray appli~ation with etching in some situations.
Thus, gouges and the like may be filled by plasma spray
technique. U~ually, the areas of the surface which are
not disfigured will flrst be etched, then the planar,
etched areas can be masked, and the gouges remaining
will be filled and/or surface treated by plasma spray
application. That is, plasma spray can be used to fill
and reactivate a gouge, or it simply can be used ~o just
reactivate gouges without necessarily restoring surface
planarity. By reactivation is meant the plasma spray
application to prepare the gouge for subsequent
treatment. Hence, the entire surface will have the
needed roughness for coating, and if desired it may in
the same processing be refur~ished to desirable
planarity.
When etching is utilized the heat treatment history
of the metal can be important. For example, to prepare
a metal such as titanium for etching, it can be most
useful to condition the metal, as by annealing, to
diffuse impurities to the grain boundaries. ThUS, by
way of example, proper annealing of grade 1 titanium
will enhance the concentration of the iron impurity at
graln boundaries. Where the suitable preparatlon
includes annealing, and the metal is grade 1 titanium,
the titanium can be annealed at a temperature of at
least about 500~C. for a time of at least a~out 15
minutes. For efficiency of operation, a more elevated
annealing temperature, e.g., 600~-800~C. is advantageous.
Where etching is employed, it will be with a
sufficiently active etch solution to develop aggressive
grain boundary attack. Typical etch solutions are
acidic solutions. These can be provided by
hydrochloric, sulfuric, perchloric~ nitric, oxalic,
tartarlc, and phosphorlc aclds as well as mlxtures
~5 thereof, e.g., aqua regia. Other etchants that may be
utilized include caustic etchants such as a solution of
potassium hydroxide/hydrogen peroxide in combination, or
20569~3
a melt of potassium hydroxide with potassium nitrate.
For efficiency of operation, the etch solution is
advantageously a strong, or concentrated solution, such
as an 18-22 weight% solution of hydrochloric acid.
Moreover, the solution is advantageously maintained
during etching at elevated temperature such as at 80~C.
or more for aqueous solut1ons, and often at or near
boiling condition or greater, e.g., under refluxing
condition. Following etching, the etched metal surface
can then be subjected to rinsing and drying steps to
prepare the surface for coating. A more detailed
discussion of the etching and annealing can be found in
Canadian Patent Application Serial No. 2,018,670.
For the plasma spray applied surface roughness, it
is necessary that the metal surface have an average
roughness (Ra) of at least about 250 microinches and an
average number of surface peaks per inch (Nr) of at
least about 40. The surface peaks per inch can be
typically measured at a lower threshold limit of 300
microinches and an upper threshold limit of 400
microinches. A surface having an average roughness of
below about 250 microinches will be undesirably smooth,
as will a surface having an average number of surface
peaks per inch of below about 40, for providing the
needed, substantially enhanced, coating adhesion.
Advantageously, the surface will have an average
roughness of on the order of about 400 microinches or
more, e.g., ranging up to about 750-1500 microinches,
with no low spots of less than about 200 microinches.
Ad~antageously, for best avoidance ~f surface
smoothness, the surface will be free from low spots that
are less than about 210 to 220 mlcrolnches. It ls
preferable that the surface have an average roughnes~ of
from about 300 to about 500 microinches.
Advantageously, the surface has an average number of
peaks per inch of at least about 60~ but which might be
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on the order of as great as about 130 or more, with an
average from about 80 to about 120 being preferred. It
is further advantageous for the surface to have an
average distance between the maximum peak and the
maximum valley (Rm) of at least about 1,000 microinches
and to have an average peak height (Rz) of at least
about 1,000 microinches. All of such foregoing surface
characteristics are as measured by a profilometer. More
desirably, the surface for coating will have an Rm value
of at least about 1,500 microinches to about 3500
microinches and have a maximum valley characteristic of
at least about 1,500 microinches up to about 3500
microinches.
After the substrate has attained the necessary
surface roughness, it will be understood that the
surface may then proceed through various operations,
including pretreatment before coating. For example, the
surface may be subjected to a cleaning operation, e.g.,
a solve~t wash. Or it may be subjected to a subsequent
etching or hydriding or nitriding treatment. Prior to
coating with an electrochemically active material, it
has been proposed to provide an oxide layer by heating
the substrate in air or ~y anodic oxidation of the
substrate as described in U.S. Patent No. 3,234,110.
European patent application No. 0,090,42s published October
5, 1983 proposes to platinum electroplate the substrate to
which then an oxide of ruthenium, palladium or iridium is
chemideposited. Various proposals have also been made
in which an outer layer of electrochemically active
material is deposited o~ a sub-layer which primarily
serves as a protective and conductive intermediate.
U.K. Patent No. 1,~44,540 dlscloses utlllzing and
electrodepo~ited layer of ~obalt or lead oxide under a
ruthenium-titanium oxide or similar active outer layer.
Various tin oxide based underlayers are disclosed in
U.S. Patents No~ 4,272,354, 3,882,002 and 3,950,240.
After providing the necessary surface roughness followed
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by any pretreatment operation, the coating most
contemplated in the present invention is the application
of electrochemically active coating.
As representative of the electrochemically active
5 coatings that may then be applied to the etched surface
of the metal, are those provided from platinum or other
platinum group metals or they can be represented by
active oxide coatings such as platinum group metal
oxide~, magnetite, ferrite, cobalt spinel or mixed
10 metal oxide coatings. Such coatings have typically been
developed for use as anode coatings in the industrial
electrochemical industry. They may be water based or
solvent based, e.g., using alcohol solvent. Suitable
coatings of this type have been generally described in
one or more of the U.S. Patent Nos. 3,265,526,
3,63Z,498, 3,711,385 and 4,528,084. The mixed metal
oxide coatings can often include at least one oxide of a
valve metal with an oxide of a platinum group metal
including platinum, palladium, rhodium, iridium and
20 ruthenium or mixtures of themselves and with other
metals. Further coatings in addition to those
enumerated above include manganese dioxide, lead
dioxide, palatinate coatings such as MXPt3O4 where M is
an alkali metal and X is typically targeted at
25 approximately 0.5, nickel-nickel oxide and nickel plus
lanthanide oxides.
It is contemplated that coatings will be applied to
the metal by any of those means which are useful for
applying a liquid coating composition to a metal
30 substrate. Such methods include dip spin and dip drain
techniques, brush application, roller coating and spray
application such as electrostatic spray. Moreover spray
application and combination techniques, e.g., dip drain
with spray application can be utilized. with the
~5 above-mentloned coatlng composltlons for provldlng an
electrochemically active coating, a modified dip drain
operation can be mo5t 5erviceable. Following any of the
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foregoing coating procedures, upon removal from the
liquid coating compo~ition, the coated metal surface may
simply dip drain or be subjected to other post coating
technique such as forced air drying.
Typical curing conditions for electrocatalytic
coatings can include cure temperatures of from about
300~C. up to about 600~C. Curing tlmes may vary from
only a few minutes for each coating layer up to an hour
or more, e.g., a longer cure time after several coating
layers have been applied. However, cure procedures
duplicating annealing conditions of elevated temperature
plus prolonged exposure to such elevated temperature,
are generally avoided for economy of operation. In
general, the curing technique employed can be any of
those that may ~e used for curing a coating on a metal
substrate. Thus~ oven curing, including conveyor ovens
may be utilized. Moreover, infrared cure techniques can
be u~eful. Preferably for most economical curing, oven
curing is used and the cure temperature used for
electrocatalytic coatings will be within the range of
from about 450~C. to about 550~C. At such temperatures,
cur1ng times of only a few minutes, e.g., from about 3
to 10 minutes, will most alw ys be used for each applied
coating layer.
The following examples show ways in which the
invention has been practiced, as well as showing
comparative examples. However, the examples showing
ways in which the invention has been practiced should
not be construed as limiting the invention.
~XAMPL~ 1
A tltanlum nut ls welded to the back of each sample
plate having an approximate 7.5 cm2 ~ample face and each
being unalloyed ~rade 1 titanium. The sample plates
were then mounted to a large back plate to provide a
mosaic of sample plates. This mounting scheme served to
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provide a large array of sample plates which could be
handled as a unit in ensuing operations. The sample
plates were grit blasted with aluminum oxide, then
rinsed in acetone and dried.
A coating on the sample plates of titanium powder
was produced using a powder having average particle size
of 50 - 60 microns. The sample plates were coated with
this powder using a Metco plasma 6pray gun equipped with
a GH spray nozzle. The spraying condltions were: a
current of 500 amps: a voltage of 45 - 50 volts; a
pla~ma gas consisting of argon and helium; a titanium
feed rate of 3 pounds per hour; a spray bandwidth of 6.7
millimeters (mm)i and a spraying distance o~ 64 mm, with
the resultlng titanium layer on the titanium sample
plates having a thickness of about 150 microns.
The coated surface of the ~ample plate~ were then
subjected to surface profilometer measurement using a
Hommel model TlOOO C instrument manufactured by
Hommelwerk GmbH. The plate surface profilometer
measurements were determined as average values computed
from three separate measurements conducted by running
the instrument-in random orientation across the coated
flat face of the plate. This gave average values as
measured on three sample plates for surface roughness
(Ra) of 448, 490 and 548 microinches, respectively for
the three plates, and peaks per inch (Nr) of 76, 6~ and
76, respectively for the three plates. The peaks per
inch were measured within the threshold limits of 300
microinches (lower) and 400 microinches ~upper).
EXAMPLE 2
A sample of titanium whiCh had been previously
coated with an electrochemically actiYe coating~ was
blasted wit~ alumi~a powder to rem~ve t~e pre~ious
coating. By thls abrasive method, it was determined by
~5 X-ray fluorescence that the prevlous coatlng had been
Trademark
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removed. ALter removal of any residue of the abrasive
treatment, the resulting sample plate was etched. It
was etched for approximately 1 hours by immersion in 20
weight percent hydrochloric acid aqueous solution heated
to 95~C. After removal from the hot hydrochloric acid,
the plate was again rinsed with deionized water and air
dried. Under profilometer measurement conducted in the
manner of Example 1, the resulting average values for a
flat face surface of the sample were found to be 180
(Ra) and 31 (Nr).
The sample then received a coating of plasma spray
applied titanium using the titanium powder and the
application procedure as described in Example 1. Under
profilometer measurement conducted in the manner of
Example 1, the resulting average values for a flat
surface of the sample were found to be 650 (Ra) and 69
(Nr).
