U.S. patent application number 14/396305 was filed with the patent office on 2015-04-02 for surface modified stainless steel cathode for electrolyser.
This patent application is currently assigned to Chemetics Inc.. The applicant listed for this patent is Chemetics Inc.. Invention is credited to Paul Kozak, Bin Lan, David Summers.
Application Number | 20150090586 14/396305 |
Document ID | / |
Family ID | 49482079 |
Filed Date | 2015-04-02 |
United States Patent
Application |
20150090586 |
Kind Code |
A1 |
Kozak; Paul ; et
al. |
April 2, 2015 |
SURFACE MODIFIED STAINLESS STEEL CATHODE FOR ELECTROLYSER
Abstract
Sodium chlorate is produced industrially via electrolysis of
brine and is thus an energy intensive process. An improved cathode
for this and other industrial processes is a low nickel content
stainless steel whose surface has been suitably modified. With an
appropriate amount of surface roughening, the cathode provides for
improved overvoltages during electrolysis while still maintaining
corrosion resistance.
Inventors: |
Kozak; Paul; (Surrey,
CA) ; Summers; David; (Vancouver, CA) ; Lan;
Bin; (Burnaby, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Chemetics Inc. |
Vancouver |
|
CA |
|
|
Assignee: |
Chemetics Inc.
Vancouver
CA
|
Family ID: |
49482079 |
Appl. No.: |
14/396305 |
Filed: |
April 15, 2013 |
PCT Filed: |
April 15, 2013 |
PCT NO: |
PCT/CA2013/050289 |
371 Date: |
October 22, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61637244 |
Apr 23, 2012 |
|
|
|
Current U.S.
Class: |
204/286.1 ;
204/293; 451/39 |
Current CPC
Class: |
B24C 11/00 20130101;
C25B 11/04 20130101; B24C 1/06 20130101; C25B 11/02 20130101; C25B
11/0415 20130101; C25B 1/265 20130101 |
Class at
Publication: |
204/286.1 ;
204/293; 451/39 |
International
Class: |
C25B 11/04 20060101
C25B011/04; B24C 1/06 20060101 B24C001/06; B24C 11/00 20060101
B24C011/00; C25B 1/26 20060101 C25B001/26 |
Claims
1. A cathode for an industrial electrolyser comprising a stainless
steel electrode comprising less than about 6% by weight nickel and
characterized by a surface roughness R.sub.q between about 1.0 and
5.0 micrometers.
2. The cathode of claim 1 wherein the stainless steel electrode is
characterized by a surface roughness R.sub.q less than about 2.5
micrometers.
3. The cathode of claim 1 wherein the stainless steel is a ferritic
stainless steel.
4. The cathode of claim 3 wherein the stainless steel is selected
from the group consisting of 430, 430D, 432, and 436S grades of
ferritic stainless steel.
5. The cathode of claim 1 wherein the stainless steel comprises a
stabilizing dopant selected from the group consisting of Cu, Mo, N,
Nb, Sn, Ti, V, and W.
6. The cathode of claim 3 wherein the ferritic stainless steel
comprises Mo dopant.
7. The cathode of claim 3 wherein the ferritic stainless steel
comprises Sn dopant.
8. The cathode of claim 3 wherein the ferritic stainless steel
comprises Ti dopant.
9. The cathode of claim 3 wherein the ferritic stainless steel
comprises V dopant.
10. The cathode of claim 1 wherein the stainless steel is a duplex
stainless steel.
11. The cathode of claim 10 wherein the stainless steel is selected
from the group consisting of S31803, S32101, S32205, S32304,
S82441, S82011, and S82122 grades of duplex stainless steel.
12. The cathode of claim 1 wherein the stainless steel comprises
less than about 0.03% by weight carbon.
13. The cathode of claim 12 wherein the stainless steel comprises
less than about 0.005% by weight carbon.
14. The cathode of claim 1 wherein the stainless steel comprises
less than about 0.03% by weight phosphorus and less than about
0.003% by weight sulfur.
15. The cathode of claim 1 comprising an electrolysis enhancing
coating applied to the low nickel content stainless steel
electrode.
16. An industrial electrolyser comprising the cathode of claim
1.
17. The industrial electrolyser of claim 16 wherein the industrial
electrolyser is a sodium chlorate electrolyser.
18. The industrial electrolyser of claim 16 wherein the cathode is
welded to a carrier plate made of carbon steel or stainless
steel.
19. The industrial electrolyser of claim 18 wherein the cathode is
welded to a carrier plate made of stainless steel and the
electrolyser is absent a cathodic protection unit.
20. A method of reducing the overvoltage of an industrial
electrolyser cathode during electrolysis of brine while maintaining
resistance of the cathode to corrosion, the method comprising
roughening the surface of a stainless steel cathode comprising less
than about 6% by weight nickel to a surface roughness R.sub.q
between about 1.0 and 5.0 micrometers.
21. The method of claim 20 wherein the roughening comprises
sandblasting the cathode surface with aluminum oxide powder.
Description
TECHNICAL FIELD
[0001] The present invention pertains to cathode electrodes for use
in industrial electrolysis, such as electrolysis of brine to
produce chlorate product. In particular, it pertains to surface
modified, low nickel content, stainless steel cathodes for such
use.
BACKGROUND
[0002] Sodium chlorate is produced industrially mainly by the
electrolysis of sodium chloride brine to produce chlorine, sodium
hydroxide and hydrogen. The chlorine and sodium hydroxide are
immediately reacted to form sodium hypochlorite, which is then
converted to chlorate. In the overall electrolysis process, complex
electrochemical and chemical reactions are involved that are
dependent upon such parameters as temperature, pH, composition and
concentration of electrolyte, anode and cathode potentials and
over-voltages, and the design of the equipment and electrolytic
system. The choices of cell parameters such as electrode sizes,
thickness, materials, anode coating options and off-gas are
important to obtain optimal results.
[0003] The choice of material and configuration for the cathode
electrode in the chlorate electrolyser is particularly important
with regards to the efficiency of the electrolysis and to the
durability of the cathode in the harsh conditions in the
electrolyser. Material and design combinations are selected so as
to obtain the best combination possible of overvoltage
characteristics during operation, along with corrosion and blister
resistance, cost, manufacturability, and durability
characteristics. If cathodes comprising coated substrates are
employed, substrate compatibility with the coatings must be taken
into account. Preferably any improved cathode electrode is able to
replace those in current electrolyser designs, without requiring
other major design and material changes to other components like
the carrier plates to which they are attached by welding.
[0004] Conventional chlorate electrolyser efficiency might be
improved via improvements to the overpotential found at the cathode
during the electrolysis process. According to a typical breakdown
of losses incurred within the electrolyser, the cathode
overpotential accounts for approximately 38% (430 mV) of the total
loss, with the other key losses relating to electrolyte resistance,
anode overpotential, metallic resistance and "dichromate effect"
(which results from film formation on the cathode when sodium
dichromate is employed as a buffer and for suppressing the
reduction of hypochlorite and chlorate ions at the cathode.)
[0005] In commercial mono-polar and hybrid chlorate electrolyser
designs, the cathodes are typically uncoated carbon steel types
like Domex grade steel, C1008 and Stahrmet.RTM.. The latter
Stahrmet.RTM. cathodes use a select steel with specific elemental
composition in order to prevent and/or reduce hydrogen blistering
and embrittlement when in service. Such cathodes perform reasonably
well in combination with conventional DSA.RTM. (Dimensionally
Stable Anode) anodes in terms of cell voltage and overpotential
over the normal range of operating conditions (e.g. current
densities from 2.5 to 4.0 kA/m.sup.2 and temperatures from 60 to
90.degree. C. They are also a relatively low cost component of the
electrolyser.
[0006] Uncoated carbon steel electrodes however are susceptible to
corrosion (rusting) which results in cathode thinning, undesirable
metal ions entering the electrolyte, and decreased cathode life,
even under normal operating conditions with cathodic protection.
Over the expected life of the electrolysers, there are typically
shutdowns and power interruptions which accelerate the corrosion of
the cathodes. Metal ions in the electrolyte deposit on the
electrodes and can affect both the anode and cathode performance
simultaneously via this type of fouling, yielding symptoms of both
elevated cell potential and oxygen production, and resulting in
higher operating cost. The cathodes will show predominantly
pitting-type surface erosion distributed more or less uniformly
throughout the working area. This type of corrosion is typical for
carbon steel cathodes exposed to hypochlorite. Since the scale has
to be removed during servicing and before re-use, the cathodes need
to be mechanically cleaned (e.g. by sand-blasting) and acid-washed.
A significant amount of material (mostly iron) is typically removed
in this treatment such that carbon steel cathodes require a
substantial corrosion allowance to compensate for the loss of
material, thereby resulting in a requirement for thicker cathodes
and thus reduced active electrode area per unit volume. Further,
when cathodes are refurbished and put back into service, the gaps
between cathodes and anodes in the electrolyser will increase
causing an increase in voltage.
[0007] As an alternative, other materials may be considered for use
as chlorate electrolyser cathodes. However, unlike in the related
industrial chlor-alkali electrolysis process (in which sodium
chloride brine undergoes electrolysis to form sodium hydroxide,
hydrogen and chlorine products), cathodes based on nickel or which
comprise a significant amount of nickel cannot be employed. The
presence of nickel results in an increase in the rate of
hypochlorite decomposition and thereby reduces product yield and
produces higher levels of oxygen than normal. This presents a
safety concern since the oxygen can potentially combine with the
hydrogen that is present to achieve unsafe, explosive mixtures.
Thus, cathodes which are nickel free or at least have low nickel
content (e.g. less than about 6% by weight) are used for chlorate
electrolysis.
[0008] Certain grades of stainless steel (e.g. ferritic, martensic,
duplex, and precipitation-hardened) are low nickel content grades
of stainless steel and can offer advantages over carbon steel with
regards to their corrosion resistance characteristics. However,
these types of stainless steels, and in fact stainless steels in
general, at least as they are typically prepared for commercial use
exhibit substantially higher overvoltages than carbon steel when
used as a cathode in chlorate electrolysis.
[0009] As a further alternative, various coatings have been
suggested in the art for purposes of preparing coated substrates
for use as electrodes in brine electrolysers. For instance,
Canadian patent application CA2588906 discloses nanocrystalline
alloys for use as coatings for chlorate electrolysis. RuO.sub.2
type coatings have also been suggested as electrode coatings in
brine electrolysis. However, cathodes using carbon steel substrates
cannot be easily coated with typical precious metal and mixed oxide
coatings containing Ru, Ir, Ti, or the like. When applied using
conventional methods, there are adhesion and degradation issues. In
turn, this results in durability issues for the coatings as they
simply exfoliate or "flake off" when the underlying carbon steel
substrate is corroded. (In U.S. Pat. No. 7,122,219, attempts were
made to address this problem for electrodes intended for
chlor-alkali electrolysis.) It has been very difficult to obtain a
carbon steel coated cathode that matches the expected 5 to 8 year
lifetime of standard commercial anodes in chlorate
electrolysers.
[0010] It is generally known that appropriate surface treatments of
metal substrates (e.g. sandblasting) can result in improved
adhesion of applied coatings. And it is known (e.g. as disclosed in
U.S. Pat. No. 6,017,430) that grit-blasting the cathodes used in
the electrolysis of aqueous alkali metal chloride solutions can
reduce the hydrogen overvoltage at the cathode by increasing its
surface area. However, it is also well known that surface
smoothness is important for better corrosion resistance of
stainless steels. Since stainless steels resist corrosion best when
they are clean and smooth, low surface roughness has been
particularly sought for use in extremely corrosive environments,
e.g. the environment within a brine electrolyser.
[0011] Roughness is characterized in various ways in the industry.
Roughness parameters such as arithmetic mean of roughness, denoted
as R.sub.a, and mean square of roughness, denoted as R.sub.q, are
commonly used to quantify surface roughness and are determined by
standardized methods. In addition, surfaces may also be
characterized by more qualitative terminology, such as "finish". A
No. 4 finish stainless steel is a general purpose polished finish,
is duller than the other common finishes, and is commonly used for
work surfaces or the like where appearance and cleanliness is
important (e.g. for equipment used in the food, dairy, beverage,
and pharmaceutical industries). As per ASTM A480, the R.sub.a of a
No. 4 finish may generally be up to 0.64 micrometers. R.sub.a may
be approximately 80% of R.sub.q and so the R.sub.q of a No. 4
finish would be somewhat less than 1 micrometer.
[0012] However, while there is a correlation between these various
roughness characteristics and other characteristics such as
appearance and corrosion resistance, two surfaces can have the same
R.sub.a (and/or the same R.sub.q) and yet have a different
appearance or resistance to corrosion depending on how the surface
condition was obtained. For instance, such characteristics can vary
depending on whether the finish is directional or random (e.g. was
obtained by belt abrasion or by sandblasting respectively) and on
other factors such as orientation.
[0013] While the industrial chlorate electrolysis process is quite
advanced, there still remains a desire for ever greater efficiency,
electrolyser lifetime, and reduction in cost.
SUMMARY
[0014] The present invention addresses these needs by providing
improved electrolysis cathodes which exhibit both desirable
overvoltage and corrosion resistance characteristics. For instance,
overvoltages similar or better to those seen with carbon steel
cathodes can be obtained along with corrosion resistance similar to
that expected from cathodes made with conventional stainless
steels. Such cathodes are useful for chlorate electrolysis and may
be for other industrial electrolysis processes.
[0015] Surprisingly, cathodes made with certain nickel free or low
nickel content (e.g. less than about 6% by weight) stainless steels
can achieve both these characteristics if the surface has been
modified or treated so as to obtain a certain surface roughness.
Low nickel content stainless steels potentially suitable for this
purpose include certain ferritic, martensitic, duplex, and
precipitation-hardened stainless steels. It can be advantageous to
employ a stainless steel comprising one or more stabilizing
dopants. Suitable dopants include Cu, Mo, N, Nb, Sn, Ti, V, and W.
It can also be advantageous to employ a stainless steel with low
carbon content, e.g. less than about 0.03% by weight and preferably
less than about 0.005% by weight in certain embodiments.
[0016] In particular, the low nickel content stainless steel can be
a ferritic stainless steel such as a 430, 430D, 432, or 436S grade
of stainless steel or a ferritic stainless steel comprising a Mo,
Sn, Ti, and/or V dopant. Ferritic grades of stainless steel
typically contain impurities of phosphorus and sulfur. It can be
preferable for the stainless steel to comprise less than about
0.03% by weight phosphorus and less than about 0.003% by weight
sulfur. Further, the low nickel content stainless steel can be a
duplex stainless steel such as a S31803, S32101, S32205, S32304,
S32404, S82011, or S82122 lean/low alloy grade of duplex stainless
steel.
[0017] A surface roughness R.sub.q in the range from between about
1.0 and 5.0 micrometers has been found to be suitable with regards
to overvoltage and may also provide improved corrosion resistance.
In particular, a ferritic stainless steel with a R.sub.q less than
about 2.5 micrometers appears suitable.
[0018] The surface modified stainless steel can be used directly
(uncoated) as a cathode in an industrial electrolyser, such as a
sodium chlorate, potassium chlorate or sodium perchlorate
electrolyser. For use in such an embodiment, the cathode can be
welded to a carrier plate made of carbon steel or stainless steel.
Advantageously, if the cathode is welded to a carrier plate made of
an appropriate stainless steel and the remainder of the
electrolyser is also made of an appropriate stainless, the
electrolyser does not need to employ a cathodic protection
unit.
[0019] Alternatively the surface modified stainless steel can be
used as a substrate in a cathode which comprises an electrolysis
enhancing coating applied to it. The surface modification can
improve the adhesion of a suitable electrolysis enhancing coating.
And further, although the overvoltage advantage of the surface
modified substrate may not be immediately necessary or observed in
a new coated cathode, when the coating eventually wears away, the
underlying surface modified stainless steel substrate is exposed.
At this time, the exposed substrate now exhibits the combined
overvoltage and corrosion resistance advantages of the invention
and thereby extends the useful life of the cathode over that of the
current industry standard Stahrmet.RTM..
[0020] Thus, the overvoltage of a chlorate electrolyser cathode can
be reduced during electrolysis of brine, while maintaining
resistance of the cathode to corrosion, by roughening the surface
of a low nickel content stainless steel cathode to a surface
roughness R.sub.q between about 1.0 and 5.0 micrometers. A variety
of roughening methods may be employed, for instance sandblasting
the cathode surface with aluminum oxide powder.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 compares the mini-cell voltage versus current density
plots for several representative surface modified SS430 cathode
samples, a comparative SS430 sample and a conventional Mild steel
sample.
[0022] FIG. 2 plots the mini-cell voltages observed at several
representative current densities as a function of the surface
roughness for the SS430 cathode samples in the Examples.
[0023] FIG. 3 compares the mini-cell voltage versus current density
plots for various RuO.sub.2 coated, surface modified SS430 cathode
samples to a conventional Mild steel sample.
[0024] FIG. 4 compares the mini-cell voltage versus current density
plots for several representative surface modified ferritic cathode
samples to a conventional Mild steel sample.
[0025] FIG. 5 compares the plot of electrolysis pilot cell voltage
versus days of operation at normal conditions for a cell comprising
a conventional carbon steel cathode to those of cells comprising a
SS430 cathode and a doped ferritic cathode which have been surface
treated in accordance with the invention.
DETAILED DESCRIPTION
[0026] Unless the context requires otherwise, throughout this
specification and claims, the words "comprise", "comprising" and
the like are to be construed in an open, inclusive sense. The words
"a", "an", and the like are to be considered as meaning at least
one and not limited to just one.
[0027] In addition, the following definitions are intended. In a
numerical context, the word "about" is to be construed as meaning
plus or minus 10%.
[0028] Stainless steel refers to a steel alloy with a minimum of
10.5% chromium content by mass.
[0029] Surface roughness R.sub.q refers to the mean square of
roughness as determined according to standards JIS2001 or ISO1997
and are what were used in the Examples below.
[0030] And herein, an electrolysis enhancing coating refers to a
coating on an electrode in a chlorate electrolyser which results in
a reduction in overvoltage during normal operation. Various such
coating compositions are known in the art and typically comprise
noble metal compositions such as RuO.sub.2.
[0031] In otherwise conventional electrolysers for the industrial
production of chlorate, certain low nickel content stainless steels
have unexpectedly been found to be improved materials for use as
cathode electrodes if their surfaces have been appropriately
modified. Such cathodes show desirable overvoltage characteristics
that are similar to or better than those obtained with carbon
steel, while maintaining the desirable corrosion resistance
expected from conventional stainless steel.
[0032] Suitable stainless steels are nickel free or have nickel
content less than about 6% by weight. Several classes of stainless
steels meet this requirement including ferritic, martensitic,
duplex, and precipitation-hardened stainless steels. In addition,
it can be of advantage to employ one or more stabilizing dopants in
the stainless steel. Suitable such dopants include Cu, Mo, N, Nb,
Sn, Ti, V, and W. It may also be of advantage to employ a stainless
steel with low carbon content or very low carbon content, namely
less than about 0.03 or less than about 0.005% by weight carbon
content. (Carbon is known to promote hydrogen embrittlement by
reaction with hydrogen to form methane Thus, the more carbon
present in a hydrogen evolution cathode, the more likely it may be
for methane to form in the cathode substrate. Accumulation of
methane in grain boundaries or defects (such as inclusions of the
sulfide or oxide type) in the substrate can cause blistering and
embrittlement of the substrate.)
[0033] In particular, ferritic stainless steels can be suitable and
are distinguished by the primary alloying element being chromium
(ranging from about 10.5 to 27 wt %), which provides a stable
ferritic structure at all temperatures. Due to their low carbon
content, ferritic stainless steels have limited strength but can
have good ductility and they work harden very little. The toughness
of these alloys is quite low, but this is not an essential
requirement for use as a cathode in an electrolyser. Unprotected, a
Cr-rich ferritic stainless steel eventually corrodes in hot
chlorinated liquor but not as quickly as carbon steel does. The
Cr-rich stainless steel hydrogen release over-potential is higher
than that for carbon steel. The Cr-rich stainless steel in contact
with carbon steel does not appear to corrode quicker since the
former does not act as a sacrificial anode for the latter. This is
important for implementation as a replacement or upgrade for a
carbon steel cathode in commercial electrolysers since the cathode
side of the carrier plate in the electrolyser may still be carbon
steel and thus a ferritic stainless steel will be compatible
therewith. Cromgard.RTM. is an example of a potentially suitable
ferritic stainless steel having about 12% Cr content and exhibiting
good weldability. Alternatively of course, carrier plates may be
employed that are also made of a suitable grade of stainless steel,
thereby eliminating all carbon steel present and thus any issue
with use of dissimilar metals.
[0034] Testing has shown that ferritic grades including 430, 430D,
432, and 436S can be suitable. And in particular, certain extra low
interstitial ferritic type stainless steels comprising dopants have
shown marked improvement in electrolyser overvoltage. It is also
expected that other ferritic grades would be suitable, including
444 grade which comprises Mo, Nb, and V dopants (in exemplary
amounts of about 1.8, 1.6, and 0.06% by weight respectively) and
434, 439, 441, 442 and 446 grades of stainless steel.
[0035] Other low nickel content ferritic or martensitic stainless
steel alloys may contain molybdenum, providing them with corrosion
resistance far superior to conventional carbon steel in most
chemical environments. There are many types of these alloys which
contain other elements like Mn, Si, Al, Se, Cb, Cu, Ta, N, and W
which may offer additional benefits with regards electrical
conductivity, surface activity, manufacturability and/or durability
for such applications. For instance, duplex stainless steel, also
known as ferritic-austenitic stainless steel, in which the Cr range
is from about 4-18 wt % has better welding characteristics than
ferritic stainless steel. Certain duplex stainless steel alloys,
such as UNS numbers S32101, S32304, and S82441 grades (e.g.
commercial LDX 2101.TM., LDX 2304.TM. or LDX 2404.TM. respectively)
along with S31803, S32205, and S82122, can be expected to offer
advantages including superior corrosion resistance,
manufacturability (also having better welding characteristics than
ferritic stainless steel), and commercial availability in addition
to performance advantages.
[0036] In order to obtain overvoltages similar to or better than
that obtained with carbon steel, the surface of a conventional low
nickel content stainless steel has to be roughened, typically such
that its surface roughness R.sub.q is greater than about 1.0
micrometers. For instance, the surface roughness R.sub.q of a
conventional 430 grade of ferritic stainless steel intended for use
in the Examples below was less than 0.1 micrometers as-obtained.
Its surface was suitably roughened using a sandblasting method and
aluminum oxide powder.
[0037] Any of various methods known in the art may be contemplated
for roughening the stainless steel surface. For instance, along
with sandblasting, alternative abrasion techniques (e.g. table
blasting, belt blasting, cylinder blasting) and methods including
chemical etching, micro-machining, and micro-milling can also be
used to suitably increase surface roughness. However, as is also
known in the art, the surface characteristics may vary according to
the detailed method used. For instance, the surface characteristics
obtained via sandblasting can vary according to the type of powder
used (e.g. aluminum oxide, sodium bicarbonate, silicon carbide,
glass bead, crushed glass), powder particle size, nozzle size,
pressure, distance, angle, and so on. And processes like
photochemical machining allow for the milling and grinding of the
surface to more precise depths and to larger R.sub.q values.
[0038] While increased surface roughness of the low nickel content
stainless steel is required in order to obtain a desirable
overvoltage, excessive roughness may result in unacceptable
corrosion characteristics. Based on the Examples below, surface
roughness R.sub.q values up to 5.0 micrometers may still be
acceptable. In certain cases, values up to about 2.5 micrometers
may be preferred. It may however be necessary to maintain the
cathodic protection provided to the cathode as a result of normal
operation of an electrolyser or provide alternative means of
protection during instances of power outages or shutdown.
[0039] Surface modified low nickel content stainless steel cathodes
can replace present conventional carbon steel cathodes while
advantageously providing better durability, cost and performance.
Such cathodes can be welded successfully to standard carbon steel
carrier plates for use in industrial electrolysers as a substitute
for conventional carbon steel cathodes. Welding can be accomplished
via different combinations of filler wire (e.g. welding rod),
shielding gases, backup purge, and welding parameters (including
current, voltage, and rate). Thus, major electrolyser design
changes need not be implemented for either refurbished electrolyser
cells and for new electrolyser systems. Further, it may be possible
to incorporate cathodes of the invention in future designs (e.g. of
the bipolar type).
[0040] Alternatively, if the industrial electrolyser is made
entirely of an appropriate stainless steel and thus for instance
the cathodes are welded to carrier plates made of stainless steel,
the electrolyser may do without cathodic protection and thus may
not need to employ a cathodic protection unit.
[0041] Other advantages of the invention include the energy savings
obtained from the lower cathodic overpotential. And with better
corrosion resistance of some grades, thinner cathode embodiments
may be considered yielding more product per unit volume of
electrolyser and/or allowing for reduced size and cost for the same
level of output. It is also likely that such surface modified
cathodes will be more compatible with electrolysis enhancing
coatings in terms of adhesion and durability due to the "anchoring
effect" created by the rougher finish and avoidance of the failure
mechanism associated with carbon steel corrosion. And even if no
significant advantage was obtained, once a coating has worn away or
otherwise failed, the underlying surface modified stainless steel
substrate would be expected to continue providing for normal
operation and survive substantially longer than a conventional
carbon steel substrate, thereby extending the useful life of such
coated cathodes.
[0042] The following Examples have been included to illustrate
certain aspects of the invention but should not be construed as
limiting in any way.
Examples
Mini-Cell Testing
[0043] A series of cathode material samples was tested in a
laboratory mini-cell under static conditions but otherwise similar
to those experienced in a commercial chlorate electrolyser. The
mini-cell construction used a cathode material sample as the cell
cathode and used a conditioned DSA.RTM. as the cell anode. Both of
the electrodes were flat sheets. The active test surface area was
about 2 cm.sup.2 and the gap between them was 5.8 mm. The
electrolyte was an aqueous solution of
NaClO.sub.3/NaCl/Na.sub.2Cr.sub.2O.sub.7 in concentrations of
450/115/5 gpl. The electrodes were immersed in the electrolyte at a
test temperature of 80.degree. C. Unlike commercial electrolysers,
the electrolyte was not circulating during testing and no
continuing brine feed was supplied.
[0044] Where indicated, the various cathode material samples were
surface modified and their roughness measured prior to assembling
into the mini-cell. Fresh electrolyte was then added, heated to the
test temperature, and polarization testing was performed which
involved ramping the current density applied from 0.5 up to 6
kA/m.sup.2 while recording the cell voltage. The test was then
stopped and the sample electrode inspected for evidence of
corrosion.
[0045] Surface roughness, R.sub.q, was determined using a Mitutoyo
Surftest SJ210. Six surface roughness samplings were performed at
random locations on each cathode material sample over a sampling
length of 2.5 to 6 inches and the maximum deviations from the mean
line determined for each sampling. The R.sub.q reported was the
square root of the arithmetic mean of the squares of these six
deviations.
[0046] The unmodified cathode material samples tested included:
[0047] Stahrmet.RTM. mild steel with a measured R.sub.q of 2.16
.mu.m (denoted "Mild steel" in the Figures and Tables) [0048] 420A
grade stainless steel (SS420A) with a supplier's 2D mill finish and
having a measured R.sub.q of 0.26 .mu.m (denoted "SS420A-0.26
.mu.m" in the Figures and Tables) [0049] 430 grade stainless steel
(SS430) with a supplier's bright mill finish and having a measured
R.sub.q of 0.06 .mu.m (denoted "SS430-0.06 .mu.m" in the Figures
and Tables) (Note: both stainless steel samples had similar low
nickel content, i.e. <0.25 wt %, and both comprised amounts of
Mn, S, P, Si, Cu and Mo. The SS420A grade had C and Cr contents of
0.25% and 12.83% by weight and also had a trace amount of Al. The
SS430 grade had C, Cr, and N contents of 0.04%, 16.64%, and 0.03%
by weight.)
[0050] Surface modified cathode material samples were prepared by
taking similar SS420A and SS430 samples as above and subjecting
them to a manual sandblasting process using a 120 grit aluminum
oxide powder. The surface modified samples tested included: [0051]
SS420A sandblasted to a measured surface roughness R.sub.q of 1.73
.mu.m (denoted "SS420-1.73 .mu.m" in the Figures and Tables) [0052]
a series of SS430 samples sandblasted to various surface
roughnesses R.sub.q ranging from 0.86 to 4.62 .mu.m (denoted
"SS430-0.86 .mu.m" to "SS430-4.62 .mu.m" in accordance with their
surface roughness in the Figures and Tables)
[0053] Further, RuO.sub.2 coated, surface modified SS430 cathode
material samples were prepared with a range of RuO.sub.2 loadings.
Cathode material samples were made by initially sandblasting 430
stainless steel samples as above to and then coating in-house using
RuCl.sub.3 solution followed by a heat treatment procedure.
Specifically, samples were degreased, rinsed, and then etched with
a 10% HCl solution for 5 minutes at room temperature. After rinsing
again and drying, a solution of RuCl.sub.3 in an organic solvent
was applied. The coated samples were dried and then heat treated at
about 420.degree. C. for 20 minutes. More than one application of
coating and heat treatment was used to obtain the greater loading
amounts.
[0054] The RuO.sub.2 coated, surface modified samples prepared and
tested are summarized in Table 1 below:
TABLE-US-00001 TABLE 1 RuO.sub.2 coated, surface modified samples
Sample name R.sub.q (.mu.m) RuO.sub.2 loading (g/m.sup.2) RuO.sub.2
#1 1.6 2.77 RuO.sub.2 #2 1.55 4.33 RuO.sub.2 #3 1.45 5.54 RuO.sub.2
#4 1.45 6.1
[0055] Mini-cells comprising each of these cathode material samples
were then assembled and subjected to polarization testing over a
range of current densities from 0.5 to 6 kA/m.sup.2 at 80.degree.
C.
[0056] Table 2 summarizes the data obtained for the conventional
Mild steel sample, the SS420A-0.26 sample, and the surface modified
cathode sample SS420-1.73 .mu.m. Table 2 shows the laboratory
mini-cell voltage for each cathode sample at the various current
densities tested. As is evident from the data, the cell with the
unmodified SS420A-0.26 .mu.m cathode operated at a substantially
greater cell voltage or overvoltage than the cell with the
conventional Mild steel cathode. However, the cell with the surface
modified SS420-1.73 .mu.m cathode operated at even somewhat lower
cell voltages than the cell with the conventional Mild steel
cathode. Specifically at 4 kA/m.sup.2, the unmodified SS420A-0.26
.mu.m cathode cell voltage was 150 mV higher than the Mild steel
cell voltage, while the surface modified SS420-1.73 .mu.m cathode
cell voltage was 25 mV less than the Mild steel cell voltage.
TABLE-US-00002 TABLE 2 Cell voltage versus current density for
SS420 samples tested Current density Cell voltage (volts)
(kA/m.sup.2) Mild steel SS420-0.26 .mu.m SS420-1.73 .mu.m 0.5 2.53
2.60 2.46 1.0 2.67 2.76 2.62 1.5 2.78 2.88 2.74 2.0 2.87 3.00 2.84
2.5 2.97 3.10 2.93 3.0 3.06 3.20 3.03 3.5 3.16 3.31 3.12 4.0 3.25
3.40 3.22 4.5 3.34 3.49 3.31 5.0 3.41 3.57 3.41 5.5 3.50 3.67 3.50
6.0 3.60 3.76 3.59
[0057] Following testing, the cathode samples were inspected. Both
SS420 samples were found to have corroded substantially
however.
[0058] Table 3 summarizes the data obtained with the series of
SS430 samples sandblasted to various surface roughnesses and
compares them to the comparative unmodified SS430 and mild steel
cathode samples. The laboratory mini-cell voltage for each cathode
sample at the various current densities tested are shown.
TABLE-US-00003 TABLE 3 Cell voltage versus current density for
SS430 samples tested Cell voltage (volts) Current SS430 SS430 SS430
SS430 SS430 SS430 SS430 SS430 SS430 SS430 SS430 SS430 SS430 density
Mild 0.06 0.86 1.05 1.15 1.70 1.76 1.81 2.14 2.25 2.49 2.82 3.48
4.62 (kA/m.sup.2) steel .mu.m .mu.m .mu.m .mu.m .mu.m .mu.m .mu.m
.mu.m .mu.m .mu.m .mu.m .mu.m .mu.m 0.5 2.53 2.73 2.61 2.57 2.53
2.45 2.47 2.45 2.46 2.51 2.49 2.52 2.56 2.50 1.0 2.67 2.91 2.78
2.72 2.68 2.60 2.60 2.60 2.62 2.65 2.63 2.66 2.71 2.65 1.5 2.78
3.02 2.92 2.85 2.80 2.71 2.71 2.71 2.73 2.77 2.74 2.78 2.83 2.76
2.0 2.87 3.13 3.02 2.96 2.90 2.82 2.82 2.81 2.84 2.86 2.84 2.88
2.94 2.87 2.5 2.97 3.22 3.13 3.06 3.01 2.91 2.91 2.91 2.94 2.96
2.94 2.98 3.04 2.96 3.0 3.06 3.31 3.24 3.17 3.09 3.00 3.01 3.00
3.03 3.06 3.05 3.08 3.13 3.05 3.5 3.16 3.39 3.33 3.26 3.19 3.08
3.10 3.09 3.13 3.15 3.13 3.17 3.23 3.15 4.0 3.25 3.48 3.42 3.35
3.28 3.19 3.18 3.18 3.22 3.24 3.23 3.27 3.32 3.24 4.5 3.34 3.55
3.51 3.44 3.37 3.27 3.28 3.27 3.32 3.33 3.33 3.35 3.41 3.34 5.0
3.41 3.66 3.59 3.53 3.46 3.36 3.36 3.36 3.39 3.42 3.41 3.45 3.51
3.43 5.5 3.50 3.71 3.70 3.62 3.54 3.45 3.46 3.46 3.47 3.50 3.49
3.55 3.60 3.53 6.0 3.60 3.80 3.77 3.70 3.62 3.54 3.55 3.55 3.56
3.59 3.58 3.63 3.68 3.61
[0059] FIG. 1 compares the mini-cell voltage versus current density
plots for several representative surface modified SS430 cathode
samples, the comparative unmodified SS430-0.06 .mu.m sample and the
conventional Mild steel sample. (A line through the data for the
Mild steel sample is provided as a guide to the eye.) As can be
seen in FIG. 1, the cell with the unmodified SS430-0.06 .mu.m
cathode also operated at a substantially greater overvoltage than
the cell with the conventional Mild steel cathode. As for the
surface modified SS430 samples, the overvoltage generally improved
with increasing surface roughness up to a R.sub.q of about 1.70
Mini-cells with SS430 cathodes having surface roughnesses less than
or about 1.15 .mu.m had lower operating voltages than the cell with
the unmodified SS430-0.06 .mu.m cathode but were not as low as the
cell with the conventional Mild steel cathode. However, mini-cells
with SS430 cathodes having surface roughnesses of about 1.70 .mu.m
or greater had similar or lower operating voltages than the cell
with the conventional Mild steel cathode. The increase in surface
roughness to 1.81 .mu.m (not shown in FIG. 1 but see Table 3)
however did not seem to significantly reduce the operating cell
voltage further. Specifically at 4 kA/m.sup.2, the unmodified
SS430-0.06 .mu.m cathode cell voltage was about 230 mV higher than
the Mild steel cell voltage, while the SS430-1.81 .mu.m cathode
cell voltage was about 70 mV less than the Mild steel cell
voltage.
[0060] FIG. 2 plots the mini-cell voltages observed at several
representative current densities as a function of surface roughness
of the SS430 cathode samples. Specifically, the mini-cell voltages
at 2, 3 and 4 kA/m.sup.2 are plotted. As would be expected, the
mini-cell voltage increases with current density used. And
initially, the mini-cell voltage decreases with surface roughness.
However, unexpectedly the mini-cell voltages at each current
density seem to be at their lowest at surface roughnesses of about
1.8 .mu.m.
[0061] FIG. 3 compares the mini-cell voltage versus current density
plots for the various RuO.sub.2 coated, surface modified SS430
cathode samples to the conventional Mild steel sample. As is seen
in FIG. 3, every cell with a RuO.sub.2 coated, surface modified
SS430 cathode operated at a substantially lower cell voltage than
the cell with the conventional Mild steel cathode. However, based
on the testing performed, the amount of RuO.sub.2 loading did not
affect the cell voltage significantly. At 4 kA/m.sup.2, the
RuO.sub.2 coated, surface modified SS430 cathode cell voltages were
substantially lower than the Mild steel cathode cell voltage, i.e.
about 240-280 mV lower.
[0062] Following all the above SS430 and RuO.sub.2 coated sample
testing, there was no visible corrosion observed on any of the
samples.
[0063] Another series of ferritic cathode material samples was
obtained, surface modified, and tested in a laboratory mini-cell as
described above and/or were corrosion tested as described later
below. The samples here included the following: [0064] 430 grade
stainless steel with a composition of 0.042% C, 0.36% Si, 0.48% Mn,
0.031% P, 0.0015% S, 16.13% Cr, 0.15% Ni, 0.041% N by weight, the
remainder being Fe and having a measured R.sub.q of 2.13 .mu.m
after sand blasting (denoted "SS430" in FIG. 4) [0065] 430D grade
stainless steel with a composition of 0.005% C, 0.1% Si, 0.11% Mn,
0.025% P, 0.002% S, 16.39% Cr, 0.29% Ti by weight, the remainder
being Fe and having a measured R.sub.q of 2.1 .mu.m after sand
blasting (denoted "SS430D" in FIG. 4) [0066] 432 grade stainless
steel with a composition of 0.004% C, 0.1% Si, 0.08% Mn, 0.022% P,
0.001% S, 17.20% Cr, 0% Ni, 0.18% Ti, 0.01% N, 0.48% Mo, 0.02% Cu,
by weight, the remainder being Fe and having a measured R.sub.q of
1.89 .mu.m after sand blasting (denoted "432" but not shown in FIG.
4) [0067] 436S grade stainless steel with a composition of 0.005%
C, 0.1% Si, 0.09% Mn, 0.022% P, 0.002% S, 17.2% Cr, 0.23% Ti,
0.011% N by weight, the remainder being Fe and having a measured
R.sub.q of 2.09 .mu.m after sand blasting (denoted "SS436S" in FIG.
4) [0068] LDX2205 grade stainless steel with a composition of
0.018% C, 0.38% Si, 1.54% Mn, 0.023% P, 0.001% S, 22.50% Cr, 5.70%
Ni, 0.017% N, 3.10% Mo by weight, the remainder being Fe and having
a measured R.sub.q of 1.73 .mu.m after sand blasting (denoted
"LDX2205" but not shown in FIG. 4) [0069] a first doped grade of
stainless steel with a composition of 0.004% C, 0.12% Si, 0.10% Mn,
0.024% P, 0.001% S, 14.4% Cr, 0.11% Sn, 0.20% Nb+Ti combined,
0.010% N by weight, the remainder being Fe and having a measured
R.sub.q of 2.35 .mu.m after sand blasting (denoted "Doped-1" in
FIG. 4) [0070] a second doped grade of stainless steel with a
composition of 0.005% C, 0.07% Si, 0.06% Mn, 0.020% P, 0.001% S,
16.4% Cr, 0.31% Sn, 0.22% Nb+Ti combined, 0.010% N by weight, the
remainder being Fe and having a measured R.sub.q of 2.14 .mu.m
after sand blasting (denoted "Doped-2" in FIG. 4)
[0071] FIG. 4 compares the mini-cell voltage versus current density
plots obtained for these surface modified ferritic and surface
modified doped ferritic cathode samples to that of the conventional
Mild steel sample of FIG. 1. (No test was performed on the 432
sample and thus it does not appear in FIG. 4. And only the voltage
at 4 kA/m.sup.2 was obtained on the LDX2205 sample and thus it too
does not appear in FIG. 4. This voltage for the LDX2205 sample was
3.18 volts.) In all measured cases, the results for the surface
modified samples were comparable to or better than the conventional
Mild steel sample.
[0072] To obtain additional information relating to corrosion, the
aforementioned samples including the conventional Mild steel sample
were also subjected to a corrosion test in which individual samples
were exposed to corrosive, circulating "hypo" electrolyte from a
pilot scale chlorate reactor. (The "hypo" comprised an approximate
4 g/L solution of HClO and NaClO, which circulated at a flow rate
of 60 L/h, at about 70.degree. C., and was obtained from the
reactor operating at a current density of 4 kA/m.sup.2.) The
samples were approximately 80 mm.times.35 mm in area and about 3 mm
thick and they were exposed to the electrolyte for a period of up
to 5 hours. Corrosion rates were then determined based on the loss
of weight from the samples resulting from this exposure (recorded
as weight loss per unit area and time). Table 4 summarizes some of
the corrosion rates observed.
TABLE-US-00004 TABLE 4 Corrosion rates observed Sample Mild steel
430 430D 432 436S LDX2205 Doped-1 Doped-2 Corrosion rate
(g/m.sup.2-h) 31.2 30.6 37.2 32.9 27.7 0.01 39.7 24.4
[0073] The corrosion rates for all the samples tested was
considered acceptable. (Note that the corrosion rate measured for
the LDX2205 sample was very low. While correct, other testing
suggested that attention should be paid to crevice corrosion as it
may be much more significant.)
[0074] These examples suggest that SS430, SS430D, SS436, and doped
ferritic stainless steel based cathodes might be appropriately
surface modified so as to provide similar or better overvoltage
performance to that of a conventional mild steel cathode in a
chlorate electrolyser, while still maintaining an acceptable
resistance to corrosion.
Pilot Cell Testing:
[0075] Comparison testing was performed in larger pilot scale
electrochemical cells on a surface modified SS430 cathode (having a
composition similar to that of the SS430 sample of FIG. 4), a
surface modified Doped-2 type cathode (having a composition similar
to that of the Doped-2 sample of FIG. 4) and on a conventional
Stahrmet.RTM. mild steel cathode under the same conditions to those
experienced in a commercial chlorate electrolyser. The pilot cells
employed flat sheet cathodes that were 19 square inches in active
area, the same commercially available anodes (DSA with a RuO.sub.2
coating), and an electrolyte comprising an aqueous solution of
sodium chlorate, sodium chloride, and sodium dichromate and having
NaClO.sub.3/NaCl/Na.sub.2Cr.sub.2O.sub.7 concentrations of
450/110/5 gpl. Electrolyte flowed through the cell at a rate of 0.8
litre/amp-hour and was controlled to a pH of 6.0. During the
testing the temperature ranged from 80.degree. C. to 90.degree. C.
and the current density from 2 kA/m.sup.2 to 4 kA/m.sup.2. The
pilot cell voltage was recorded during testing and also the oxygen
concentration in the off-gases generated by the cell was monitored.
Oxygen is an undesirable by-product in this type of electrolysis. A
higher oxygen concentration in the off-gases is indicative of lower
current efficiency (i.e. more energy being consumed to produce the
same amount of sodium chlorate). Further, higher oxygen
concentrations pose a safety concern when mixed with hydrogen gas
also being produced. (Many factors can affect oxygen concentration
including both electrode materials. While this is not a direct
indicator of electrode corrosion it is a very important criterion
to consider with regards to electrode selection.)
[0076] The cathodes tested again included a conventional
comparative Stahrmet.RTM. mild steel cathode with a measured
R.sub.q of 2.16 .mu.m, a SS430 cathode which had been sandblasted
to a measured R.sub.q of 1.54 .mu.m, and a Doped-2 type stainless
steel cathode which had been sandblasted to a measured R.sub.q of
1.91 .mu.m.
[0077] Initially, all cells were conditioned by operating at a
reduced temperature and current density (80.degree. C. and 2
kA/m.sup.2) from those used during normal production electrolysis
(90.degree. C. and 4 kA/m.sup.2). Over the course of 1-6 days, the
temperature and current density were increased to the 90.degree. C.
and 4 kA/m.sup.2 values normally used for production electrolysis.
Operation continued at these settings while the cell voltages
stabilized. During conditioning, the cell voltages drift up over
the first two to three weeks or so of operation. This is a normal
effect and is due to conditioning of the DSA.RTM. anode and cathode
polarization. FIG. 5 compares the pilot cell operating voltages
versus days of operation at normal conditions after the cell
voltages had stabilized. (In FIG. 5, the voltages from day 12 and
onwards are shown. Note: the comparative mild steel cathode had
been preconditioned for up to an additional 12 days.). As is
evident in FIG. 5, the cell with the surface modified SS430 cathode
has a markedly lower cell voltage than the comparative cell. After
12 days of conditioning, the surface modified SS430 cathode based
cell was operating at 3.18 Volts and the oxygen concentration in
the off-gases was a low 1.7%. Remarkably, the cell with the surface
modified Doped-2 series cathode had an even lower cell voltage than
that of the surface modified SS430 cathode and its superior
performance was maintained for more than 85 days of operation.
[0078] To obtain an indication of the corrosion resistance of the
surface modified SS430 cathode under these normal pilot cell
operating conditions, approximately 1200 ml of the electrolyte from
the cell was filtered through 934-AH Glass Microfiber filter paper.
No discoloration was seen in the filter paper suggesting no
evidence of corrosion in the electrolyte after 20 days of pilot
cell operation under normal operating conditions.
[0079] Again with regards to the surface modified SS430 cathode
evaluation, testing continued at normal production electrolysis
conditions for a total of 46 days during which time there was
cathodic protection. Thereafter, the pilot cell was subjected to a
power interruption test. This test evaluates corrosion resistance
in the event of a shutdown of the electrolyser during which time
there is no cathodic protection. The test comprised shutting off
power three times for five minute periods with five minute periods
of normal operation in between. Again, an electrolyte sample was
taken and filtered through filter paper. This time, evidence of
cathode corrosion was observed. However, unlike that observed on
mild steel cathodes, the corrosion pattern on the SS430 cathode was
localized (e.g. pitting) and not over the entire surface. Thus an
improvement over mild steel is indicated and it would be expected
that coatings over the majority of the SS430 surface would be
unaffected.
[0080] An indication of the corrosion resistance of the surface
modified Doped-2 cathode was obtained in a like manner by filtering
electrolyte from its pilot cell and checking for residue and
discoloration. Again, electrolyte samples were taken after normal
pilot cell operating conditions and also after a power interruption
test. In this case, the pilot cell was operated normally for 137
days while still maintaining a markedly low cell voltage under 3.21
volts. An electrolyte sample was then taken, and the cell was
subjected to the power interruption test, after which another
electrolyte sample was taken. Again, no discoloration was seen in
the filter paper suggesting no evidence of corrosion in the
electrolyte after 137 days of normal pilot cell operation. And
again, evidence of corrosion was seen after the power interruption,
but again the corrosion pattern on the Doped-2 cathode was
localized, the discoloration of the filter paper was modest, and an
improvement over mild steel is indicated.
[0081] This example demonstrates a significantly improved
overvoltage for the cells comprising the surface modified SS430 and
Doped-2 series cathodes as well as improved corrosion
resistance.
[0082] All of the above U.S. patents, U.S. patent applications,
foreign patents, foreign patent applications and non-patent
publications referred to in this specification, are incorporated
herein by reference in their entirety.
[0083] While particular elements, embodiments and applications of
the present invention have been shown and described, it will be
understood, of course, that the invention is not limited thereto
since modifications may be made by those skilled in the art without
departing from the spirit and scope of the present disclosure,
particularly in light of the foregoing teachings. For instance,
while the preceding description and examples were directed at
chlorate electrolysers, the invention might instead be useable for
chlor-alkali production, hydrogen electrolysis, desalination of
seawater or other industrial electrochemical applications used for
chemical production requiring an active, low cost, chemically
resistant cathode electrode material (e.g. conversion of carbon
dioxide to liquid fuels and industrial chemicals). Such
modifications are to be considered within the purview and scope of
the claims appended hereto.
* * * * *