Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF ELECTROPLATING A POROUS BODY
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Porous bodies are known to be difficult to
electroplate interiorly. The problem is intensified
with an increasing ~ t of the voids interiorly
~5 of a porous body where the plating deposit is desired
`~ to be made on the enclosing wall surfaces thereof.
This is particularIy 50 in cases where the body to be
interiorly plated is a porous electrode intended~ for
electrochemlcaI usage, and which contains an abun- .
dance of exceedingly ~ine, internal pores many of
which are of miniscule size of less than 10 microns
to as sinall as 0.1 micron.
: :
Frequently, particular:Ly with electrodes, -
a heavy plating deposit is~neither needed nor wanted
~-~ 15 (and may even be deleterious) on the exposed exterior
faces of the porous body to be plated and therefore
represents a waste of the expensive plating material.
Also, standard electroplatiny techniques tend to
cause a build-up of plating deposit on the exterior
surfaces of the~porous bodies, especially around and
about the egress sites o the pores. This is often
substantial enough to cause blocking of the por~s
on the exterior surface resulting in a serious dis-
advantageous if not inoperative condition, particularly
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where electrodes are involved. Furthermore, the
usual means employed to plate porous bodies inte-
riorly thereof are not always effective in leaving
a conservatively thin, but adequate deposition on
the interior surfaces of the pores to be plated.
This is economically undesirable when plating is
performed with expensive coating materials such
as silver. Silver or other relatively expenslve
noble or non-noble metals are often used for their
enhanced catalytic effect on the less costly base
I metal bodies of such porous electrodes.
Attempts to overcome these difficulties
have not met with success and have of-ten required
resorting to complicated and expensive procedures
to avoid or minimize the indicated problems. For
example, it has been proposed to pump a plating bath
through a porous body to improve the application of
internal coatings to the porous body. Such procedure
is difficult, however, and not entirely reliable for
realizing the desired results. Illustrative of such
previous efforts are U.S. Patent Nos. 3,359,469 and
3,787,244 and Canadian Patent ~o. 921,111.
Accordingly, the pr0sent invention essentially
resides in a process for electroplating porous bodies.
More particularly, the present invention resides in
a process for electroplating porous electrodes for
use in electro-chemical processes, particularly for
the use in chlor-alkali cells. Electroplating is
performed in such a way as to preclude or minimize
the deposition of a coating on the exterior surfaces
of the porous body so as to cause a substantial and
excessive blockage of the pores on the outer surface
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of the porous body. The process of the present inven-
tion provides for improved economy and deposition of
an adequate and effective quantity of a uniformly thin
plating layer on the interior surfaces of the porous body.
The present invention particularly resides in a
process for applying a coating on the interior wall sur-
faces of at least a portion of a porous electroconductive
body having a multiplicity of void spaces, comprising:
the steps of at least substantially filling
the void spaces in the porous body with a plating medium
containing an electrodepositable substance which forms
said coating when the porous body is subjected to passage
therethrough of an electrical current;
immersing the porous body in an electro-
conductive non-plating liquid medium, and
applying~a direct current electrical potential
to said porous body to cause current to flow through said
electroconductive liquid medium and porous body to apply
said coating on the internal wall surfaces of said body.
FIGURE 1 is a flow-diagram and schematically
illustrates one procedure for implementing the invention;
and
FIGURES 2 and 3 are graphical presentations
showing plots of experimental results obtained to demon-
state the invention.
According to the present invention porous
bodies are interiorly electroplated by providing
substantially the entire volume of a plating bath
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~rom which the coating is to be deposited within all or
at least a substantial portion of the porous body during
the time that an electroplating current is applied to
thereby effect the desired plate deposition on the wall
~I surfaces of the internal voids in the porous body.
According to the invention, an efficient, effective and
sparing application of the plating is made on only the
desired waLl surfaces within the porous body such that
the plating exhibits good uniformity and quality. ~t
`~ the same time, exterior surface plating is minimized to
substantially reduce pore blockage which is normally
associated with prior electroplating procedures. The
latter problem can be a serious detriment to porous
bodies, particularly where such porous bodies are used
as electrodes in electrochemical applications.
Thus, in conventional electroplating the article
to be plated is placed in a solution which contains the
ion of the metal to be plated. Often, the anode is made
of the same metal as the metal to be deposited as a
coating on the cathode during electroplating, e.g., a
silver anode in a silver plating solution. This materially
helps to keep a constant concentration of metal ions in
solution through anode dissolution as metal ions plate
ou~ on the cathode. Since metal lon migration into the
interstices of a porous body is relatively slow and is
retarded by pores o~ a decreasingly smaller size, the
concentration of metal ions within the body voids decreases
with time during the plating as compared to the concentra-
tion of metal ions of the plating bath. Unavoidablyl the
plating rate is much faster on the exterior body sur
faces where metal ions in the bath, per se, are in
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proportionally greater abundance to cause a much
heavier plating on the exterior surface compared
to the internal voids.
With particular reference to FIGURE 1 of the
Drawing, there is shown one way of carrying out the
procedure o the present invention. An electroplatable
metallic porous body (5) (such as an electrode) is
suitably pretreated, if necessary, to ready it for the
plating operation. This may include chemical treatments,
for example, degreasing, washing and cleaning or drying.
Body (5) contains a plurality of internal voids (6)
to give it a somewhat sponge-like structure. According
to the present invention, it is intended to plate
the wall surfaces of the voids within the porous
- 15 body without sealing of the open pores on the ex-
terior surface of the porous body. To this end,
the electrode is immersed at a filling station
(4) in a plating bath solution (7~ within con-
tainer (8).
The body (5) is preferably kept in the
bath until the void spaces in the porous body are
saturated with the plating solution. Accordingly,
enough time should be allowed ~or immer~ion of the
porous body in the solution to permit ade~uate pene-
tration of and filling of the pores by the solution.
Although the body is shown in a vertical posi~ion,
penetration of the solution into the pores is also
facilitated by having the body tilted in any position
other than the vertical to minimize or avoid air
entrapment in the porous body. The physical posi-
tioning of the body and/or vigorous circulation of
the plating solution may also help to achieve more
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effective and quicker penetration of the solution
into the porous body. Incomplete penetration and
saturation of the solution into the pores of the
body would result in less than total plating of
the pore surfaces within the body.
Other methods of obtaining satisfactory
saturation of the plating solution into the pores
may also be resorted to, such as by spraying or
forced filling manipulations.
Though~not shown in the drawings, it should
be understood that one may also use the procedure of
the present invention to plate only a portion of the
internal pores of a porous body. For example, an
outer portion of the porous body may be saturated
with the plating solution by immersion of the body
into the solution for a predetermined limited time
period without obtaining complete saturation of the
plating solution into the pores of the body. Alter-
natively, only a portion of the porous body may
be immersed into the plating solution, such as one
side or the lower portion of the body -to saturate ~
~- only that portion of the porous body before proceeding
;~ 2s with the electrolytic plating procedure in accordance
with the process of the present invention.
There are several techniques which may be
used to fill the internaI pores in only a portlon of
the porous body. For example, one may apply a gas or a
non-plating solution to one side of the porous body
- while applying the plating solution to the other side
;~ of the porous body. By varying the pressure of each,
it is possible to selectively control the proportion
of voids which are saturated with the plating solution.
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The porous body can be formed of any
desired electroplatable material depending sub-
stantially on the particular use to which the body
is subjected. Porous bodies for use as electrodes
are frequently fabricated from metals such as iron,
steel alloys (particularly the corrosion~resisting
or so-called "stainless steel" types) copper, titanium
or alloys of these metals, although there obviously is
no limitation on the metals used for electrodes or any
other porous bodies to be plated. Likewise, depending
on the substrate of the porous body to be plated, any
suitable and compatible plating solution may be employed.
After the porous body has been immersed
~bo the plating solution for a predetermined period
of time, it is transferred from the filling station
(4) to a plating station (9). Care should be taken
in this transfer to prevent or minimize the loss of
plating solution due to leakage or spillage of the
solution from the pores of the body (6). This can
be accomplished by holding the body in a yosition
;~ to minimize such loss when taking the body out of
; the plating solution. Alternatively, a covering
member may be held closely against the pore openings
~ on the exposed surfaces of the body to prevent or
`~ 25 minimize the leakage of the plating solution from
the pores of the body. Bodies, such as electrodes,
having extremely small pores are not too troublesome
and can be manipulated without leakage or spilling
of the plating solution when they are outside of
the plating bath.
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At plating station (9), a non-plating,
electroconductive liqui.d (10) is provided in a
vessel or container (11). It is generally satis-
factory for the current-carrying liquid (10) to
be an appropriately formulated aqueous saline sol-
ution, i.e., one containing a sufficient amount
of a suitable and compatible ionizabla salt that
does not react with the plating solution and is
adapted to adequately transport and conduct the
electrical current necessary for plating. In other
words, the saline solution (10) is intended to more
or less function as a fluid electrical brush for
the porous body to be plated and it should be sub-
stantially if not entirely free rom reducible ions
that would tend to interfere with the desired internal
;~ plating procedure.
An anode (12) is positioned inside of
vessel ~11) together with means (not shown) to
receive and mount the porous body saturated with
the plating solution. An electrical circuit is
established between the anode and the porous body,
which functions as the cathode through electric
line (14) connected to a suitable direct curren-t
power source (13) which, in turn, is connected by
electric line (15) to the anode (12). It is pre-
~ ferred to completely submerge the saturated body
-; in the saline solution (10) for the plating pro-
cedure, although there are circumstances when only
partial immersion will suffice such as while the
porous body is saturated with the plating solution
over a portion of its surface.
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It is necessary that the electrode counter
to the porous body being plated (~hich, as is primarily
described, is anodic although that is not necessarily
the case) be inertO In other words, the counter-
electrode should not dissolve in the saline solution
so as to yield platable ions in the solution. Instead,
the counterelectrode material should be selected so
as to be capable of allowing for gas evolution or some
other non-interference-provoking reaction in the elec-
trolysis process.
,
plating current is applied to the porous
body to cause a deposition of the metal ions from
the plating solution within the void spaces or pores
(6) of the saturated porous body onto the interior
walls of the pores. The current is applied through
the saline solution (10) for a period of time suffic-
ient to accomplish plating and at a relatively lower
current level as compared to the current level
used in conventional electroplat:ing~ the reduction
generally being such that the current rate in the
practice of the present invention is from 5 to 40
percent, pre~erably less than 10 percent, of
normally utilized electroplating currents in
standard plati~g procedures util:izing the same
metal substrate and platiny materials. However,
the current density should be sufficient to per-
form the plating quickly enough to minimize or
avoid diffusion of the plating solution (7) into
the saline solution (10) and vice versa, during
the plating procedureO
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While the optimum current density employed
will vary from system-to-system and also depend
upon the size and material, for example of such
porous bodies to be plated and particularly upon the
level of plate deposit desired, it is usually
desirable to apply a current density of at least
about 0.05 amp/in2 (0.008 amp/cm2). A current
density at a level much lower than 0.05 amp/in2
may require too much time to allow for deleterious
plating solution/saline solution diffusion and
mixing. Upper acceptable current density levels
are reached ~ excessive formation and evolution
of hydrogen occurs. For most purposes, a current
density of about 0.1 amp/in2 (0.015 amp/cm2) is
found satisfactory. It will be u~derstood, however,
that the precise current level to be employed for
~ any given situation is readily determinable by
-~ persons skilled in the art.
-~ 20 After the plating procedure is completed,
~ the plated porous body is removed from the saline
`~ solution (10) and given a washing, drying or other
post-plating treatment such as may be needed to
finish the porous body for final intended use.
2s
In some instances, the quantity of the
coating applied to the porous body"~a single~pass
plating procedure may not be adequate. In such
a situation, the desired thic~ness of the plating
deposit can easily be achieved by a repetition of
the plating procedure for as many times as necessary
to achieve the desired results.
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It is obvious that a plurality of porous
bodies can be plated simultaneously in following
the procedure of the present invention.
In an example of the present invention, a
flat, disc-like body of porous nickel having a 2 1/2
inch (6.35 cm) diameter was washed thoroughly with
acetone and air dried at about 110C. The porous
body ~made of a commercial, pressed and sintered
powdered nickel electrode stock~ had a thickness
of 70 mils ~0.178 cm) and an average diameter pore
size of 10 microns. It had a porosity of 80 percent
' by volume.
'/ The porous body was saturated in an aqueous
plating solution containing 50 g/l AgCN (silver cyanide~
and 100 g/l KCN (Potassium cyanide). The porous body
saturated with the solution was electrically con-
nected as the cathode in an electrolytic cell con-
;~ taining as the current carrying medium a l/lOth
molar (0.1 M) aqueous solution of sodium perchlorate
(NaC104~. A platinum (Pt) electrode was inserted
into the cell to serve as the anode. An electric
current of 0.1 amp/in2 was pa~sed ~or 30 minutes
through the cell to deposit a silver coating on
the inner walls of the pores in the nickel body.
;..
To demonstrate the efficacy of the obtained
plating, the plated and thus catalyzed electrode was
tested in an experimental cell along with an unplated
electrode made of the same porous nickel stock.
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Each of the electrodes was mounted for
evaluation as a depolarized cathode in a standard
electrolytic test cell having an expanded titanium
mesh anode provided with a coating of titanium oxide
and ruthenium oxide. Anode-to-cathode spacing was
9/32 inch (0.714 cm) with an intermediate Nafion~
ion exchange membrane separator in the cell. The
anolyte was 300 g/l NaCl and the catholyte 100 g/l
NaOH; with the cell operated at a temperature of
about 60C and at a gas pressure on the back side
of the cathod maintained at between 2 and 2-1/2 psig.
.,. ~ ~
The applied current density was 0.5 amp/in .
The test was carried out to determine the
performance of the electrode over increasing time
periods and the voltage savings realized in comparing
cell operation with both nitrogen and oxygen gases
applied to the electrode. The differences (arrived
at by subtraction of the voltage values obtained
from nitrogen (i.e., inert gas) operation at any
, ~ .
-~ given point of measure and those from oxygen (i.e.,
active gas) operation at the same point of measure)
provided a reliable indication oE voltage savings
obtained as well as a corresponding depolariæation
effect upon use of the electrode as a cathode.
.
The results obtainea are graphically
depicted in Figures 2 and 3. Figure 2 particularly
shows the performance of uncatalyzed (uncoated)
porous nickel as a depolarized cathode while Figure
~ 3 illustrates the performance of a porous nickel body
- coated in accordance with the practice of the present
invention.
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Good results were obtained with other
porous bodies plated in accordance with the pro-
cedure of the present invention when employed
for other electrochemical and diverse purposes.
It will be understood that the plating
procedure of the present invention is applicable to
materials other than the metals specifically iden-
tified herein. Moreover, coating solutions other
than the solutions specifically identified herein
and which may even be organic in nature but which
are electro-depositable from such appropriate
solutions and suspensions are useable in the plating
procedure, particularly in electroplating procedures,
of the present invention.
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