Note: Descriptions are shown in the official language in which they were submitted.
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METAL SUBSTRATE OF IMPROVED SURFACE MORPHOLOGY
Background 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. Careful attention is usually paid to surface
S 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
applied to a base metal, the metal may be simply cleaned
to give a very smooth surface. U.S. Patent No,
15 4,797,182. Treatment with fluorine compounds may produce
a smooth surface. U.S. Patent No. 3,864,163. Cleaning
might include chemical degreasing, electrolytic degreasing
or treatment with an oxidizing 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,
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such may be followed by etching. U.S. Patent No.
3,878,083. Or pickling with an non-oxidizing acid can
produce a rough surface for coating. U.S. Patent No.
3,864.163. Such picklinq can follow degreasing. U.S.
patent No. Re. 28,820. The pickling may readily etch
titanium to a surface roughness within the range of
150-200 or more microinches. "Titanium as a Substrate for
Electrodes", Hayfield, P.C.S., IMI Research and
Development Report.
If there is a pre-e~isting coating present on the
substrate metal, the metal can be treated for coating
removal. For an electrocatalytic coating, such treatment
may be with a melt containing a basic material used in the
presence of an oxidant or oxygen. Such can be followed by
pickling to reconstitute the original surface for
coating. U.S. Patent No, 3,573,100: Or if a molten
alkali metal hydroxide bath is used containing an alkali
metal hydride, this is preferably followed by a hot
mineral acid treatment. U.S. Patent No. 3,706,600. It
has also been proposed to prepare the surface without
stripping the old coating. U.S. Patent No. 3,684,543.
More recently, this procedure has been improved by
activation of the old coating, prior to application of the
new. U.S. Patent No. 4,446,245.
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.
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,
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electroforming or electrowinning. Such may be continuous
operation. They can involve severe conditions including
potential surface damage. It would be most desirable to
provide coated metal substrates to serve as electrodes in
such operations, exhibiting extended stable operation
while preserving excellent coating adhesion. It would
also be highly desirable to provide such an electrode not
only from fresh metal but also from recoated metal.
SUMMARY OF THE INVENTION
There has now been found a metal surface which
provides an excellent, locked on coating of outstanding
coating adhesion. The coated metal substrate can have
highly desirable extended lifetime even in most rigorous
industrial environments. For the electrocatalytic
coatings, the invention may provide for lower effective
current densities and also achieve substrate metal grains
desirably stabilized against passivation.
In one aspect, the invention is directed to a metal
article having a surface adapted for enhanced coating
adhesion, such surface being free from deleterious affects
of abrasive treatment while having desirable surface grain
size, which surface has three-dimensional grains with deep
grain boundaries, such surface having been etched
including the etching of impurities located in the grain
boundaries at the surface of the metal, which
intergranular etching provides a profilometer-measured
average surface roughness of at least about 250
microinches and an average surface peaks per inch of at
least about 40, basis a profilometer upper threshold limit
of 400 microinches and a profilometer lower threshold
limit of 300 microinches.
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In another aspect, the invention is directed to the
method of preparing a surface of an impure valve metal for
enhanced coating adhesion on such surface, which method
comprises subjecting the surface to elevated temperature
annealing for a time sufficient to provide an at least
substantially continuous intergranular network of
impurities, including impurities at the surface of such
metal; cooling the resulting annealed surface; and etching
intergranularly the surface at an elevated temperature and
with a strong acid or strong caustic etchant; while
maintaining the surface at least substantially free from
the deleterious effects of abrasive surface treatment.
In a still further aspect, the invention is directed
to a metal article having a surface adapted for enhanced
coating adhesion, said surface having, as measured by
profilometer, an average roughness of at least about 250
microinches and an average surface peaks per inch of at
least about 40, basis the lower and upper threshold limits
mentioned hereinbefore. Such surface most desirably also
has an average distance between the maximum peak and the
maximum valley of at least about 1,000 microinches and an
average peak height of at least about 1,000 microinches.
When the fully prepared metals are
electrocatalytically coated and used as oxygen evolving
electrodes, even under the rigorous commercial operations
including continuous electrogalvanizing, electrotinning,
electroforming or electrowinning, such electrodes can have
highly desirable service life. Also, such metals as
electrodes may provide an effectively lower current
density, which will aid in prolonging the life of the
electrode, when used as above discussed or, for example,
in water or brine electrolysis.
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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 manganese, 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. For
example, titanium may be alloyed with nickel, cobalt,
iron, manganese or copper. More specifically, Grade 5
15 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 10 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 constituents may
be alloys or alloys plus impurities. In titanium, iron
may be a usual impurity. Its maximum concentration can be
expected to vary from 0.2 weight percent for grades 1 and
11 up to 0.5% for grades 4 and 6. Additional impurities
that may be found throughout the grades of titanium
include nitrogen, carbon, hydrogen and oxygen. Since
beta-titanium located at the titanium grain boundaries can
be susceptible to etching, such beta-titanium is
considered herein for purposes of this discussion as an
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impurity. Thus etching of an impurity as discussed herein
may include etching of a phase of the metal itself. In
addition to the beta-titanium, the titanium metal of
particular interest may have beta-phase stabilizers, some
of which may be present in extremely minor amounts in the
manner of an impurity and include vanadium, niobium,
tantalum, molybdenum, ruthenium, zirconium, tin, hafnium
and mixtures thereof. Grades of titanium have been more
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 subsequently processed, the substrate metal
advantageously is a cleaned surface. This 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, mechanical cleaning is typically
minimized and preferably avoided. 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.
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3,706,600. Usual rinsing and drying steps can also form a
portion of these operations.
When a cleaned surface, or prepared and cleaned
surface has been obtained, and particularly where applying
an electrocatalytic coating to a valve metal, it is most
always contemplated in the practice of the present
invention that surface roughness will be achieved by means
of etching. In the invention context of etching, it is
important to aggressively etch the metal surface to
provide deep grain boundaries providing well exposed,
three-dimensional grains. It is preferred that such
operation will etch impurities located at such grain
boundaries. For convenience, a metal having etchable
grain boundary impurities may be referred to herein as a
metal having a correct "metallurgy". It is however
contemplated that other roughening technique, which can be
used in addition to or along with the roughness achieved
by etching, such as plasma spraying of one or more of a
valve metal or valve metal oxide, including valve metal
suboxides, onto the metal surface can provide the surface
roughness characteristics. These characteristics, as
measured by profilometer, are more particularly described
hereinbelow.
Where etching has been selected to achieve surface
roughness, an important aspect of the invention involves
the enhancement of impurities of the metal at the grain
boundaries. This is advantageously done at an early stage
of the overall process of metal preparation. One manner
of this enhancement that is contemplated is the inducement
at, or introduction to, the grain-boundaries of one or
more impurities for the metal. For example, with the
particularly representative metal titanium, the impurities
of the metal might include iron, nitrogen, carbon,
hydrogen, oxygen, and beta-titanium. Although impurities
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introduction procedures that might be used can include
surface deposition, e.g., vapor deposition, which might be
followed by a heat treatment for surface impurity
diffusion, one particular manner contemplated for impurity
enhancement is to subject the titanium metal to a
hydrogen-containing treatment. This can be accomplished
by exposing the metal to a hydrogen atmosphere at elevated
temperature. Or the metal might be subjected to an
electrochemical .hydrogen treatment, with the metal as a
cathode in a suitable electrolyte evolving hydrogen at the
cathode.
Another consideration for the aspect of the invention
involving etching, which aspect can lead to impurity
enhancement at the grain boundaries, involves the heat
treatment history of the metal. For example, to prepare a
metal such as titanium for etching, it can be ~ost useful
to condition the metal, as by annealing, to diffuse
impurities to the grain boundaries. Thus, by way of
example, proper annealing of grade l titanium will enhance
the concentration of the iron impurity at grain
boundaries. Where the suitable preparation 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 about 15 minutes. For
efficiency of cperation, a more elevated annealing
temperature, e.g., 600-800C. is advantageous.
Annealing times at such more elevated temperatures will
typically be on the order of 15 minutes to 4 hours.
Alternatively, a short, high temperature anneal, e.g., on
30 the order of 800C. for a few minutes such as 5-10
minutes, may be continued, after rapid or slow cooling, at
a quite low temperature, with Z00-400C. being
representative, for several hours, with 10-20 hours being
typical. Suitable conditions can include annealing in
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air, or under vacuum, or with an inert gas such as argon.
Subsequent cooling of the annealed metal can appropriately
stabilize the grain boundaries for etching. Stabilization
may be achieved by controlled or rapid cooling of the
metal or by other usual metal cooling technique including
quenching. For convenience, a metal having such
stabilization may be referred to herein as a metal having
a desirable "heat history".
For enhancing coating adhesion for the invention
aspect of etching, it can be desirable to combine a metal
surface having a correct grain boundary metallurgy as
above-discussed, with an advantageous grain size. Again,
referring to titanium as exemplary, at least a substantial
amount of the grains having grain size within the range of
from about 3 to about 7 is advantageous. Grain size as
referred to herein is in accordance with the designation
provided in ASTM E 112-84. Size for titanium grains helow
about 3 produce a high percentage of broad grains which
detract from advantageous coating adhesion. G-rain sizes
above about 7 are not desired for best three-dimensional
grain structure development. Preferably for titanium,
the grains will have size within the range from about 4
to about 6.
After the foregoing operations, e~g., cleaning, or
coating removal and cleaning, and including any desired
rinsing and drying steps, followed by any impurity
enhancement for grain boundary etching, the metal surface
is then ready for continuing operation. Where such is
etching, it will be with a sufficiently active etch
solution to develop aggressive grain boundary attack.
Typical etch solutions are acid solutions. These can be
provided by hydrochloric, sulfuric, perchloric, nitric
oxalic, tartaric, and phosphoric acids as well as mixtures
thereof, e~g., aqua regia. Other etchants that may be
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utilized include caustic etchants such as a solution of .
potassium hydroxide/hydrogen peroxide in combination, or 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 1~-22 weight% solution of hydrochloric acid.
Moreover, the solution is advantageously maintained during
etching at elevated temperature such as at 80C. or more
for aqueous solutions, and often at or near boiling
condition or greater, e.g., under refluxing condition.
Following etshing, the etched metal surface can then be
subjected to rinsing and drying steps to prepare the
surface for coating.
Regardless of the technique employed to reach the
desired rouyhness, e.g., plasma spray or intergranular
etch, 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 250 microinches or more, e.g., ranging up to
about 750-1500 microinches, with no low spots of less than
about 200 microinches. Advantageously, for best
avoidance of surface smoothness, the surface will be free
from low spots that are less than about 210 to 220
microinches. It is preferable that the surface have an
average roughness of from about 300 to about 500
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microinches. Advantageously, the surface has an average
number of peaks per inch of at least about 60, but which
might be 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 maximu~ 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.
As representative of the electrochemically active
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 oxides,
magnetite , ferrite, cobalt spinel or mixed 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, 3,632,498, 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 ruthenium or mixtures of themselves
and with other metals. Further coatings in addition to
those enumerated above include manganese dioxide, lead
dioxide, platinate coatings such as MXPt3O4 where M
is an alkali metal and X is typically targeted at
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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
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
above-mentioned coating compositions for providing an
electrochemically active coating, a modified dip drain
operation can be most serviceable. Following any of the
foregoing coating procedures, upon removal from the liquid
coating composition, 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
20 300C. up to about 600C. Curing times 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
be used for curing a coating on a metal substrate. Thus,
oven curing, including conveyor ovens may be utilized.
Moreover, infrared cure techniques can be useful.
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 450C.
to about 550C. At such temperatures, curing times of
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only a few minutes, e.g., from about 3 to 10 minutes, will
most always be used for each applied coating layer.
The following examples show ways in which the
invention has been practised, 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.
EXAMPLE 1
There is used a titanium plate measuring 2 inches by
6 inches by 3/8 inch and being an unalloyed grade 1
titanium, as determined in accordance with the
specifications of ASTM 8 265-79. This titanium sheet thus
contained 0.20 percent, maximum, iron impurity.
This plate, which was a fresh grade 1 titanium plate,
was degreased in perchloroethylene vapors, rinsed with
deionized water and air dried. It was then etched for
approximately 1 hour by immersion in 20 weight percent
hydrochloric acid aqueous solution heated to 95C.
After removal from the hot hydrochloric acid, the plate
was again rinsed with deionized water and air dried. By
this etching, the plate achieves a weight loss of 500-600
grams per square meter of plate surface area. This weight
loss is determined by pre and post etching weighing of the
plate sample and then calculating the loss per square
meter by straight forward calculation on the basis of the
surface area of both large flat faces of the plate.
The surface structure of the sample plate, on both
broad surfaces, is then examined under a stereo microscope
under magnification varying during the study from 40X to
60X. Such plate surface can be seen to have a well
defined, three dimensional, grain boundary etch.
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The etched surface was then subjected to surface
profilometer measurement using a Hommel model T1000 C
instrument manufactured by Hommelwerk GmbH. The plate
surface profilometer measurements as average values
computed from eight separate measurements conducted by
running the instrument in random orientation across on
large flat face of the plate. This gave average values
for surface roughness (Ra) of 393 microinches, peaks per
inch (Nr) of 86 and an average distance between the
maximum peak and the maximum valley (Rz) of 2104. The
peaks per inch were measured within the threshold limits
of 300 microinches (lower) and 400 microinches (upper).
C~MPARATIVE EXAMPLE 2
A titanium plate sample of unalloyed grade 1
titanium, but from a different batch than the plate sample
of Example 1, was etched under the identical canditions of
Example 1. Visually, the resulting etched surfaces of the
titanium plate sample, as viewed in the manner of Example
1, were found not to have a well defined grain boundary
etch. Subsequent profilometer measurements, conducted in
the manner of Example 1, provided average values of 157
(Ra), 31 (Nr) and 931 (Rz). Because of the lack of well
defined grains as determined visually, plus the lack of a
well defined, three dimensional grain boundary etch as
determined by profilometer measurement, this plate sample
was a comparative sample.
EXAMPLE 2
A second sample plate from the same batch of
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unalloyed titanium as was used for the plate sample of
Comparative Example 2, was subjected to annealing
operation. In this operation, the sample was placed in an
oven and the oven was heated until the air temperature
reached 700C. This air temperature was then held for
15 minutes, cooled to 450C., and held for 30 minutes.
Thereafter, while the sample was maintained in the oven,
the oven air temperature was permitted to cool to about
200C. in a period of 1.5 hours. The sample was then
removed for cooling to room temperature.
The resulting test sample was then etched in boiling
18 weight percent HCl for one hour, then rinsed and dried
as described in Example 1. Subsequently, under visual
examination in the manner of Example 1, the etched sample
plate was seen to have a highly desirable, three
dimensional grain boundary etch. This was confirmed by
profilometer measurements which provided average values of
398 (Ra), 76 (Nr) and 2040 ~Rz).
EXA~PLE 3
A grade 1 titanium plate sample prepared in the
manner of Example 1, and having highly desirable three
dimensional and well defined grain boundary etching as
described in Example 1, was provided with an
electrochemically active coating of tantalum oxide and
iridium oxide and using an aqueous, acidic solution of
chloride salts, the coating being applied and baked in the
30 manner as described in Example 1 of U.S. Patent 4,797,182.
The resulting sample was tested as an anode in an
electrolyte that was a mixture of 285 grams per liter
(g/l) of sodium sulfate and 60 g/l of magnesium sulfate.
The test cell was maintained at 65C. and operated at a
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current density of 15 kiloamps per square meter
(kA/m ). About once per week the electrolysis was
briefly interrupted. The coated titanium plate anode was
removed from the electrolyte, rinsed in deionized water,
air dried and then cooled to ambient temperature. There
was then applied to the coated plate surface, by firmly
manually pressing onto the coating, a strip of
self-adhesive, pressure sensitive tape. This tape was
then removed from the surface by quickly pulling the tape
away from the plate. After 3000 hours of operation,
including approximately 18 tape tests, the coated anode
continued to exhibit excellent coating adhesion to the
underlying titanium substrate.
COMPARATIVE EXAMPLE 3
A sample of titanium which had been previously coated
with an electrochemically active coating, was blasted with
alumina powder to remove the previous coating. By this
abrasive method, it was determined by X~ray fluoroescence
that the previous coating had been removed. After removal
of any residue of the abrasive treatment, the resulting
sample plate was etched in the composition of Example 1 in
the manner of Example 1. Under visual inspection as
described in Example 1, it was seen that there was no
evidence of desirable grain boundary etching.
Furthermore, under profilometer measurement, the resulting
average values were found to be 137 tRa), 12 (Nr) and 841
(Rz).
The sample was nevertheless coated with the
electrocatalytic coating of Example 3 in the manner as
described in Example 3 and utilized as an anode also in
the manner as described in Example 3. After 91 hours of
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operation, the sample was removed and the coating adhesion
tested utilizing the tape test of Example 3. In this
test, and after only the 91 hours of testing, the tape
removed the majority of the coating exposing the
underlying substrate and thus terminating further testing.
