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.

Claims

1-19. (canceled)

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 employing a stainless steel cathode comprising less than about 6% by weight nickel wherein the surface of the stainless steel cathode has been roughened to a surface roughness Rq 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.

22. The method of claim 20 wherein the stainless steel electrode is roughened to a surface roughness Rq between about 1.0 and less than about 2.5 micrometers.

23. The method of claim 20 wherein the stainless steel is a ferritic stainless steel.

24. The method of claim 23 wherein the ferritic stainless steel is selected from the group consisting of 430, 430D, 432, and 436S grades of ferritic stainless steel.

25. The method of claim 20 wherein the stainless steel comprises a stabilizing dopant selected from the group consisting of Cu, Mo, N, Nb, Sn, Ti, V, and W.

26. The method of claim 23 wherein the ferritic stainless steel comprises Mo dopant.

27. The method of claim 23 wherein the ferritic stainless steel comprises Sn dopant.

28. The method of claim 23 wherein the ferritic stainless steel comprises Ti dopant.

29. The method of claim 23 wherein the ferritic stainless steel comprises V dopant.

30. The method of claim 20 wherein the stainless steel is a duplex stainless steel.

31. The method of claim 30 wherein the duplex stainless steel is selected from the group consisting of S31803, S32101, S32205, S32304, S82441, S82011, and S82122 grades of duplex stainless steel.

32. The method of claim 20 wherein the stainless steel comprises less than about 0.03% by weight carbon.

33. The method of claim 32 wherein the stainless steel comprises less than about 0.005% by weight carbon.

34. The method of claim 20 wherein the stainless steel comprises less than about 0.03% by weight phosphorus and less than about 0.003% by weight sulfur.

35. The method of claim 20 wherein the stainless steel cathode comprises an electrolysis enhancing coating.

36. The method of claim 20 wherein the industrial electrolyser is a sodium chlorate electrolyser.

37. The method of claim 20 wherein the stainless steel cathode is welded to a carrier plate made of carbon steel or stainless steel.

38. The method of claim 37 wherein the cathode is welded to a carrier plate made of stainless steel and the industrial electrolyser does not comprises a cathodic protection unit.

39. A method for electrolyzing brine with an industrial electrolyser, the method comprising the step of employing a stainless steel cathode comprising less than about 6% by weight nickel, wherein the surface of the stainless steel cathode has been roughened to a surface roughness Rq between about 1.0 and 5.0 micrometers

40. The method of claim 39 wherein the brine comprises sodium chloride and the method produces sodium chlorate.

Description

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 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, LDX 2304 or LDX 2404 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 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 NaCl0.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 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 mild steel with a measured R.sub.q of 2.16 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 m (denoted SS420A-0.26 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 m (denoted SS430-0.06 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 S420A 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 m (denoted SS420-1.73 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 m (denoted SS430-0.86 m to SS430-4.62 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 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 (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 C.

[0056] Table 2 summarizes the data obtained for the conventional Mild steel sample, the SS420A-0.26 m sample, and the surface modified cathode sample SS420-1.73 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 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 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 m cathode cell voltage was 150 mV higher than the Mild steel cell voltage, while the surface modified SS420-1.73 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 m SS420-1.73 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 Current Cell voltage (volts) density SS430 SS430 SS430 SS430 SS430 SS430 SS430 SS430 SS430 SS430 SS430 SS430 SS430 (kA/m.sup.2) Mild steel 0.06 m 0.86 m 1.05 m 1.15 m 1.70 m 1.76 m 1.81 m 2.14 m 2.25 m 2.49 m 2.82 m 3.48 m 4.62 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 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 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 m. Mini-cells with SS430 cathodes having surface roughnesses less than or about 1.15 m had lower operating voltages than the cell with the unmodified SS430-0.06 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 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 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 m cathode cell voltage was about 230 mV higher than the Mild steel cell voltage, while the SS430-1.81 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 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 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 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 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 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 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 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 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 C., and was obtained from the reactor operating at a current density of 4 kA/m.sup.2.) The samples were approximately 80 mm35 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 31.2 30.6 37.2 32.9 27.7 0.01 39.7 24.4 rate (g/m.sup.2-h)

[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 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 C. to 90 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 mild steel cathode with a measured R.sub.q of 2.16 m, a SS430 cathode which had been sandblasted to a measured R.sub.q of 1.54 m, and a Doped-2 type stainless steel cathode which had been sandblasted to a measured R.sub.q of 1.91 m.

[0077] Initially, all cells were conditioned by operating at a reduced temperature and current density (80 C. and 2 kA/m.sup.2) from those used during normal production electrolysis (90 C. and 4 kA/m.sup.2). Over the course of 1-6 days, the temperature and current density were increased to the 90 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 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.