Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
1. Field of the Invention: - This invention relates to
the production of caustic and chlorine in a chlor-alkali
cell, especially a cell of the diaphragm or membrane type,
and more particularly, the invention relates to the
cathode members used in such cells. The invention provides
a method for coating a steel cathode with a dense, non-po-
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rous electroless coating of nickel having a thickness of1 to 15 microns.
2. Description of the Prior Art: - The usual practice is
to use for the cathode in a chlor-alkali cell a piece of
uncoated steel wire mesh or expanded metal. It has been
known in fuel-cell technology that the performance of steel
cathodes can be improved by providing them with a coating
of nickel; a nickel-coated steel cathode exhibits a lower
hydrogen overvoltage and makes it possible to operate a
cell with less electrical power than would otherwise be
required, but the effect has been known to be short-lived.
Now that the chlor-alkali industry has largely adopted
the use of dimensionally stable anodes, it is possible to
operate a chlor-alkali cell continuously for many months;
with the nickel coatings of the prior art, it would not be
possible to maintain the energy savings for very long,
particularly if the power to the cell is ever interrupted.
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Prior to our invention, it has been proposed that
cathodes of a chlor-alkali cell be improved by being pro-
vided with a plasma-sprayed coating of nickel, despite the
considerable expense of using plasma-spraying equipment and
the disappointing results obtained with nickel-coated steel
cathodes in the fuel-cell art. It has not been obvious
that satisfactory results could be obtained by the use of
the method now proposed, which is technologically far less
demanding to practice.
k lo In the art of electroless plating, it has long
been known that nickel can be plated onto steel by using
a solution of a nickel salt such as nickel chloride or
nickel sulfate, and a chemical reducing agent such as
sodium hypophosphite or sodium borohydride. The coating
proceeds relatively rapidly, but the resulting coating
tends to be spongy or porous. Before a cathode for a
chlor-alkali cell could be used, after having been provided
with such a coating, it would be necessary to "anneal" the
cathode, i.e., heat the cathode in order to cause the
coating to become consolidated and non-spongy. The anneal-
ing step adds considerably to the cost of preparing`the
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In the electroplating of nickel, it has long been
i known that it is desirable or necessary to supply ammonia
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along with the nickel salt, or to start with a double salt
such as nickel ammonium sulfate. Such electroplated coatings
can be produced far more rapidly than the electroless
coatings made according to our invention, but we consider
that such coatings are not satisfactory for our purposes
because such coatings corrode too rapidly.
Summary of the Invention
A steel cathode for a chlor-alkali cell is made
by providing it with a dense, non-porous electroless
coating of nickel having a thickness of 1 to 15 microns.
Such a coating is produced by immersing the cathode for
about five hours in a bath maintained at a temperature in
the range of 20 to 70 Centigrade, the bath containing a
nickel salt such as nickel (II) chloride or nickel (II)
sulfate, water, a complexing agent such as ammonia,ethylene-
diamine, citric acid, or glycolic acid, and a reducing agent
such as sodium borohydride or hydrazine hydrate. Moreover,
by controlling the temperature of the electroless nickel
coating bath so that the rate of coating is quite slow but
the nickel coating produced is hard and non-porous, there
is obtained a steel cathode which exhibits a lower hydrogen
overvoltage and maintains such lower hydrogen overvoltage
for a long period, such as for many months. This makes it
possible to achieve considerable savings of power in the
production of chlorine and caustic in a chlor-alkali cell
provided with such cathodes.
107~9SZ
escription of the Preferred Embodiments
Before applying the electroless coating of nickel
to a steel cathode, it is important that the steel cathode
be carefully cleaned. Otherwise, the coating produced is
likely to be non-adherent and otherwise unsatisfactory. The
cleaning may be done in various ways well known to those
skilled in the art. Satisfactory results may be obtained
by immersing the cathode for several minutes in a solution
of hydrochloric acid.
An electroless nickel coating bath is prepared.
g One necessary ingredient of such a bath is a
; suitable nickel salt. Satisfactory results may be obtainedwith nickel (Il) chloride or nickel (II) sulfate or nickel
~` ammonium sulfate. Such a salt is present in the bath to
an extent such as 10 to 40 grams per liter.
A second ingredient of such a bath is water.
A third ingredient of such a bath is a suitable
! complexing agent, such as ammonia, ethylenediamine, glycolic
acid, citric acid, and the like. Such a complexing agent
~ 20 is generally present to an extent such as 2 to 20 percent
,,~ of the amount of water present. Satisfactory results have
, - been obtained with the use of ~70 to 570 milliliters of
concentrated ammonia per liter of water. Satisfactory
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results have also been obtained with the use of 50 grams of
ethylenediamine per liter of water.
Still another ingredient oE the electroless
coating bath is a reducing agent, such as sodium boro-
hydride, hydrazine hydrate, sodium hypophosphite, sodium
dithionite, or the like. Such a reducing agent is ordi-
narily present in a relatively small amount,such as two to
fifty grams per liter of water. The performance of the
coated steel electrode, in respect to retaining its lower
hydrogen overvoltage, appears to be dependent in part upon
the reducing agent selected. Particularly good results
, have been obtained with the use of hydrazine hydrate, which
is therefore preferred. In one test conducted with the use
of sodium dithionite as a reducing agent, there was ob-
tained a coated cathode that gave a declining voltage
1~ saving.
$ After the electroless plating bath has been com-
posed, it is brought to and maintained at a proper tempera-
ture. The temperature to be selected de-pends to some exten~
upon the particular choice of ingredients used in the bath.
Temperatures as low as 20 degrees Centigrade and as high a6
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70 degrees Centigrade are sa~isEac~ory in some circum-
stances; dependin~ upon th~ id~ntlty, acti.vity, and pro-
por~ion l~rcsen~ of redu~ln~ a~ent ancl comp]exing agen~, ~
temperature is to be selecte'd which is not so low that it
becomes necessary to prolong the plating operation unduly,
nor so high as to give a rapidly developing, spongy, non-
adherent deposit. Those skilled in the art will be able,
with a minimum of simple experimentation, to select a suit-
able temperature. For baths using ammonia as the complexing
agent in combination with sodium borohydride or hydrazine
hydrate as the reducing agent, a temperature such as 30-40
' Centigrade is usually preferred, although in one instance
the use of 40 Centigrade gave a spongy, non-adherent
coating.
The time of the immersion of the steel cathode in
the electroless plating bath may be varied over wide limits,
such as 2 to 24 hours. In general, satisfactory results
have been obtained with immersions of 5 hours. There is
'~, produced a hard nickel coating having a thickness of about
~' 20 1 to 15 microns, typically about 12 microns.
~' Desirably, but not necessarily, provision is'made
i for the agitation of the steel cathode while it is immersed
in the platlng bath.
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After having been coated with nickel in an elec-
troless plating bath as specified above~ the steel cathode
is permitted to dry, and it is then installed in a chlor-
alkali cell and used in the same manner as steel cathodes
are customarily used in such cells, in accordance with the
prior art. It is found that when such a nickel-coated
steel cathode is used in place of an uncoated one, the
voltage required to operate the cell under a given set of
conditions is reduced, for example, by about 0.15 or 0.16
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10 volt. This is to be compared with a usual voltageJ using
an uncoated steel cathode, of about ~.2 volts; thus, it
. amounts to a saving of power in the operation of the cell
of about 4.7 percent. Because of the tremendous quantities
of electrical power used in the electrolysis of brine, the
saving of even one percent of such amount is usually econom-
P- ically significant.
` The invention described above is further illus-
trated by the following specific examples and comparison
tests.
EXAMPLE 1
A piece of steel mesh measuring 76.2 millimeters
by 152.4 millimeters was cleaned in hydrochloric acid. A
~- bath was prepared which contained 18 grams of NiCl2, 800
milliliters of water, and 450 milliliters of concentrated
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ammonia. Three grams of sodium borohydride were added, just
before immersing the steel mesh in the solution. The tem-
perature of the solution was maintained at 28 Centigrade.
The steel mesh ~as vibrated vertically with respect to its
plane at a rate o 5 to 10 cycles per second, and it was
left immersed in the bath for a period of 5 hours. This
produced a uniform, hard coating, 12 microns thick, covering
the piece. When subsequently installed in a chlor-alkali
cell operating at a temperature of 80 Centigrade and pro-
ducing a weak cell liquor containing 18 percent by weight
of sodium hydroxide, the cell operated with a voltage 0.1
volts lower than that required for similar operation with
an uncoated steel-mesh cathode.
Comparison Test A
v Example 1 was repeated, except that the tempera-
ture of the bath was maintained at 18 Centigrade. No
appreciable coating developed on the piece of steel mesh.
Comparison Test B
Example 1 was repeated, except that the bath was
, 20 maintained at 40 Centigrade. After only 2 hours of immer-
sion, the test piece had developed a spongy black coating,
which could be wiped off easily.
EXAMPLE 2
A piece of steel mesh was cleaned; as in Example
1. A bath was prepAred which contained 18 grams of NiC12,
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107~95Z
~00 milliliters of waterJ 300 milliliters of ammonia, and
30 grams of hydrazine hydrate. The bath was maintained at
30 Centigrade. The steel piece was immersed in the bath
for a period of 5 hours, and then removed and tested in a
chlor-alkali cell. The voltage was 0.16 volt lower than
with an uncoated steel cathode, and the lower voltage
remained steady for many months.
Comparison Test C
Example 2 was repeated, except that 3 grams of
sodium dithionite were used instead of hydrazine hydrate.
A good coating was obtained, but the voltage saving declined
steadily when the cathode was used, becoming insignificant
after a period of 2 months.
EXAMPT~ 3
Example 2 was repeated, except that 50 grams of
ethylene diamine was used in place of the ammonia, and 75
grams of hydrazine hydrate instead of 30 grams, and the
temperature of the bath was raised to 40 Centigrade. A
~ cathode giving excellent performance was produced.
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While there have been shown and described herein
certain embodiments of the invention, it is intended that
there be covered as well any change or modification therein
which may be made without departing from the spirit and
0l scope of the invFntion.
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