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• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/042—Electrodes formed of a single material
  • C25B11/046—Alloys
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
  • B24C1/00—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods
  • B24C1/06—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods for producing matt surfaces, e.g. on plastic materials, on glass
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
  • B24C11/00—Selection of abrasive materials or additives for abrasive blasts
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/24—Halogens or compounds thereof
  • C25B1/26—Chlorine; Compounds thereof
  • C25B1/265—Chlorates
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/02—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/055—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material
  • C25B11/057—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material consisting of a single element or compound

Definitions

• the present invention pertains to cathode electrodes for use in industrial electrolysis, such as electrolysis of brine to produce chlorate product.
• it pertains to surface modified, low nickel content, stainless steel cathodes for such use.
• 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.
• 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.
• 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.
• 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.)
• the cathodes are typically uncoated carbon steel types like Domex grade steel, CI 008 and Stahrmet®.
• Stahrmet® 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® (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 2 and temperatures from 60 to 90 °C. They are also a relatively low cost component of the electrolyser.
• 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.
• a significant amount of material 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.
• chlorate electrolyser cathodes 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.
• nickel content e.g. less than about 6% by weight
• Certain grades of stainless steel are low nickel content grades of stainless steel and can offer advantages over carbon steel with regards to their corrosion resistance characteristics.
• 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.
• Roughness is characterized in various ways in the industry. Roughness parameters such as arithmetic mean of roughness, denoted as R a , and mean square of roughness, denoted as R 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 a of a No. 4 finish may generally be up to 0.64 micrometers.
• R a may be approximately 80% of R q and so the R q of a No. 4 finish would be somewhat less than 1 micrometer.
• two surfaces can have the same R a (and/or the same R q ) and yet have a different appearance or resistance to corrosion depending on how the surface condition was obtained.
• 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.
• 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.
• 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.
• 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.
• 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.
• a surface roughness R 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.
• a ferritic stainless steel with a R q less than about 2.5 micrometers appears suitable.
• 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.
• the cathode can be welded to a carrier plate made of carbon steel or stainless steel.
• the electrolyser does not need to employ a cathodic protection unit.
• 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.
• 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®.
• 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 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.
• Figure 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.
• Figure 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.
• Figure 3 compares the mini-cell voltage versus current density plots for various Ru0 2 coated, surface modified SS430 cathode samples to a conventional Mild steel sample.
• Figure 4 compares the mini-cell voltage versus current density plots for several representative surface modified ferritic cathode samples to a conventional Mild steel sample.
• Figure 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.
• Stainless steel refers to a steel alloy with a minimum of 10.5% chromium content by mass.
• Surface roughness R q refers to the mean square of roughness as determined according to standards JIS2001 or IS01997 and are what were used in the Examples below.
• 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 Ru0 2 .
• 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.
• 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.
• 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.
• 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.
• 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. 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.
• 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.
• LDX 2101TM, LDX 2304TM or LDX 2404TM 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.
• the surface of a conventional low nickel content stainless steel has to be roughened, typically such that its surface roughness R q is greater than about 1.0 micrometers.
• the surface roughness R 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.
• any of various methods known in the art may be contemplated for roughening the stainless steel surface.
• alternative abrasion techniques e.g. table blasting, belt blasting, cylinder blasting
• methods including chemical etching, micro-machining, and micro- milling can also be used to suitably increase surface roughness.
• the surface characteristics may vary according to the detailed method used.
• 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.
• processes like photochemical machining allow for the milling and grinding of the surface to more precise depths and to larger R q values.
• 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).
• filler wire e.g. welding rod
• shielding gases e.g. shielding gases
• backup purge e.g.
• welding parameters including current, voltage, and rate
• the electrolyser may do without cathodic protection and thus may not need to employ a cathodic protection unit.
• 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.
• 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 2 and the gap between them was 5.8 mm.
• the electrolyte was an aqueous solution of NaC10 3 /NaCl/Na 2 Cr 2 0 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.
• 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 2 while recording the cell voltage. The test was then stopped and the sample electrode inspected for evidence of corrosion.
• Surface roughness, R 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 q reported was the square root of the arithmetic mean of the squares of these six deviations.
• 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.
• Ru0 2 coated, surface modified SS430 cathode material samples were prepared with a range of Ru0 2 loadings.
• Cathode material samples were made by initially sandblasting 430 stainless steel samples as above to and then coating in-house using RuCl 3 solution followed by a heat treatment procedure. Specifically, samples were degreased, rinsed, and then etched with a 10% HC1 solution for 5 minutes at room temperature. After rinsing again and drying, a solution of RuCl 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.
• the Ru0 2 coated, surface modified samples prepared and tested are summarized in Table 1 below:
• 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 2 at 80°C.
• 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 ⁇ . 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 ⁇ 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 ⁇ cathode operated at even somewhat lower cell voltages than the cell with the conventional Mild steel cathode.
• the unmodified SS420A-0.26 ⁇ cathode cell voltage was 150 mV higher than the Mild steel cell voltage, while the surface modified SS420-1.73 ⁇ cathode cell voltage was 25 mV less than the Mild steel cell voltage.
• 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.
• Figure 1 compares the mini-cell voltage versus current density plots for several representative surface modified SS430 cathode samples, the comparative unmodified SS430-0.06 ⁇ 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 Figure 1, the cell with the unmodified SS430-0.06 ⁇ 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 q of about 1.70 ⁇ .
• Mini-cells with SS430 cathodes having surface roughnesses less than or about 1.15 ⁇ had lower operating voltages than the cell with the unmodified SS430-0.06 ⁇ cathode but were not as low as the cell with the conventional Mild steel cathode.
• mini-cells with SS430 cathodes having surface roughnesses of about 1.70 ⁇ 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 ⁇ (not shown in Figure 1 but see Table 3) however did not seem to significantly reduce the operating cell voltage further.
• the unmodified SS430-0.06 ⁇ cathode cell voltage was about 230 mV higher than the Mild steel cell voltage, while the SS430-1.81 ⁇ cathode cell voltage was about 70 mV less than the Mild steel cell voltage.
• Figure 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 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 ⁇ .
• Figure 3 compares the mini-cell voltage versus current density plots for the various Ru0 2 coated, surface modified SS430 cathode samples to the conventional Mild steel sample.
• Figure 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 Figure 1. (No test was performed on the 432 sample and thus it does not appear in Figure 4.
• 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 HCIO and NaCIO, 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 2 .
• the samples were approximately 80 mm x 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.
• Pilot cell testing 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 Figure 4), a surface modified Doped-2 type cathode (having a composition similar to that of the Doped-2 sample of Figure 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 Ru0 2 coating), and an electrolyte comprising an aqueous solution of sodium chlorate, sodium chloride, and sodium dichromate and having NaC10 3 /NaCl/Na 2 Cr 2 0 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 2 to 4 kA/m 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).
• 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.)
• 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.
• 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.

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