NZ205086A

NZ205086A - Manufacture of low-hydrogen-overvoltage cathodes for chloralkali cells

Info

Publication number: NZ205086A
Application number: NZ205086A
Authority: NZ
Inventors: H C Bhedwar
Original assignee: Du Pont
Priority date: 1982-07-30
Filing date: 1983-07-29
Publication date: 1986-05-09
Also published as: IN157836B; EP0100659A1; CA1235386A; ZA835530B; AU559813B2; JPS5941486A; KR840005497A; AU1738483A; JPS6116424B2

Description

<div class="application article clearfix" id="description"> New Zealand Paient Spedficaiion for Paient Number £05086 205086 R ' % : 19 jUU^Vl -*L ji Priority D3te(s): -'P.'&.'S.P Complete Spacification Filed: Class: C9&&JXJQ& /.. .C.Q.^T/ *?**/• • "VkFM'fstiii -ElMM'O" Plication Date: F.G- Journal, No: Patents Form No. 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "PROCESS FOR MAKING RAWEY NICKEL COATED CATHODE, AND PRODUCT THEREOF" -i-,WE E.I. Du Pont De Nemours and Company, a corporation organised and existing under the laws of the State of Delaware, U.S.A. located at 10th & Market Streets, Wilmington, Delaware, U.S.A. hereby declare the invention, for which-I-/we pray that a patent may be granted to-me-/us, and the method by which it is to be performed, to be particularly described in and by the following statement (followed by page J A.} -1- i R TITLE 205086 PROCESS FOR MAKING RANEY NICKEL COATED CATHODE, AND PRODUCT THEREOF BACKGROUND OF THE INVENTION 5 This invention relates to a process for making an improved low hydrogen overvoltage cathode of the Raney nickel type for use in chloralkali electrolysis, and to the cathode so made. Cathodes of the Raney nickel type are known 10 in the art, e.g., U.S. 4,116,804, U.S. 4,169,025 and U.S. 4,251,344. However, Raney nickel coated v ■ cathodes of the prior art, while initially exhibiting good performance characteristics, suffer certain defects. Such Raney nickel coating on the cathode 15 substrate has been of low durability, i.e., easily mechanically damaged and lost even by, e.g., water jets. For example, when such a cathode is used in a diaphragm-type chloralkali cell wherein an asbestos diaphragm is deposited directly onto the cathode 20 surface, such diaphragm gradually loses its permeability due to clogging of the pores, usually after about 9 to 15 months, and must be removed and replaced; removal is ordinarily accomplished by spraying with jets of water, and during such 25 spraying, the jets of water have caused loss of some of the Raney nickel by flaking or spalling. When the replacement diaphragm is deposited and baked, ordinarily at 355°C in air for 4 hours, some deactivation of the Raney nickel coating by oxidation 30 occurs. This progressive spalling and deactivation J of Raney nickel after multiple washes and bakes is responsible for a progressive loss of the voltage benefit provided by the Raney nickel, i.e., the operating voltage of the cathode progressively 35 increases and approaches the operating voltage of the AD-5271 cathode substrate. 1 2Q&P 8 Accordingly, it is a principalj ' this invention to provide an improved Raney nickel coated cathode having a more durable Raney nickel surface than those previously known, and a process 5 for making same. Other objects will appear hereinafter. SUMMARY OF THE INVENTION Briefly, according to the present invention, there is provided a process for making a durable, 10 low-hydrogen-overvoltage cathode for chloralkali use which comprises applying a nickel/aluminum alloy composition containing 56 to 59 % by weight of nickel onto a suitable cathode substrate by plasma spraying, followed by leaching with caustic to remove aluminum. 15 More specifically, there is provided in accordance with the invention a process for making a low-hydrogen-overvoltage cathode for a chloralkali cell which comprises (a) plasma spraying a coating of a 20 nickel/aluminum alloy composition containing 56 to 59% by wt. nickel onto a foraminous substrate of iron, ferrous alloy, nickel or nickel alloy, and (b) leaching aluminum from said coating by 25 contacting the article prepared in step (a) with a caustic solution. There are also provided in accordance with the invention a Raney nickel coated cathode made by said process, a chloralkali cell containing said 30 cathode, and a process for electrolysis of an alkali metal chloride in said cell. DETAILED DESCRIPTION OF THE INVENTION In the present invention a selected nickel/aluminum alloy composition is applied to an 35 electrically conductive cathode substrate by plasma spraying. 2 3 2050 8 6 It is a requirement of the present invention tha.t the nickel/aluminum alloy composition employed for deposition on the cathode substrate contain about 56 to 59% by wt. nickel. Nickel/aluminum alloys in 5 this composition range contain the intermetallic compound t^Al^ as the predominant phase, but also contain minor amounts of NiAl and NiAl^, the exact amounts of these varying with the specific overall composition. Preferably, the nickel/aluminum 10 alloy composition contains 56.5 to 57.5% by wt. nickel, most preferably substantially-57% by wt. nickel. The nickel/aluminum alloy is suitably employed in the form of particles or granules, 15 preferably 36 to 90 microns (165 to 400 mesh) in size, more preferably 36 to 53 microns (270 to 400 mesh) in size. The electrically conductive cathode substrate can have various configurations, e.g. , 20 flat, tubular, fingers, etc., but in any case should be foraminous, e.g., woven wire screen, expanded sheet metal or punched sheet metal. Iron, ferrous alloys such as mild steel and stainless steel, nickel and nickel alloys are particularly suitable materials 25 for this substrate. It is another requirement of the present invention that the nickel/aluminum alloy composition be deposited on the cathode substrate with the use of plasma spraying, also termed plasma arc spraying. 30 Plasma spraying is a known technique for depositing metal particles onto a substrate. Briefly, streams of a gas and the metal particles are separately fed to a spray nozzle, (torch), and there is an electrical power input to the gas stream which ionizes the gas; 35 recombination of the positive ions and electrons in 3 205086 the gas stream after mixing with the metal^ particles , ' releases energy which heats and partially melts the metal particles, and the partially melted metal particles adhere to the substrate on contact with 5 it. Various gases, including nitrogen and argon, are suitable gases for this purpose. Argon is a preferred gas. The power input need only be sufficient to partially melt the metal particles, e.g. 20 to 30 kilowatts in apparatus of ordinary size. 10 In most cases it is preferred to coat the fabricated cathode component rather than to coat the foraminous sheet material used to fabricate the cathode, because welding and other assembly techniques are more easily accomplished with the 15 substrate in the uncoated condition. Also, it is best to clean the surface of the substrate before coating, as by sandblasting, or blasting with other grit such as alumina. The cathodes of this invention can be either 20 one-side or two-side coated. However if only one side is coated it is preferred to coat the side which will face the membrane or diaphragm in order to obtain the maximum reduction in cell voltage. Thickness of the nickel/aluminum alloy 25 coating applied to the cathode substrate of about 13 to 508 microns (0.5 to 20 mils) is suitable, and is preferably about 127 to 254 microns (5 to 10 mils). Following deposition of the nickel/aluminum alloy composition on the substrate, the Raney nickel 30 •surface is developed by contacting the coated article with any strong base such as a caustic solution so as to leach aluminum from the coating. The concentration of the caustic is not critical; aqueous caustic solution containing 5 to 15% by wt. NaOH is 35 suitable for this purpose, and 5 hours to 1 day is 4 205086 adequate time for the leaching. A 10% by wt. aqueous caustic solution for 16 hours is typical. Similarly, the temperature for leaching is not critical; temperatures from room temperature up to the boiling 5 point of the caustic solution are suitable. For coatings of indicated thickness, about 70 to 90% of the aluminum will ordinarily be removed during such leaching; some aluminum appears to remain even after.. much longer leaching time, but is believed not to be 10 deleterious to performance of the resulting cathode. This leaching step is also referred to as activation, and can be carried out prior to or after assembly of the cell. The Raney nickel cathode so made, if permitted to dry, heats up upon contact with air, due 15 to the pyrophoric character of Raney nickel. The Raney nickel coated cathode so made not only exhibits low hydrogen overvoltage, but the coating has improved durability against flaking and spalling, as is shown hereinbelow in Example 9 and 20 Comparative Examples B, C and D. As shown therein, the nickel/aluminum alloy composition applied as the coating in the process of the present invention provides an activated Raney nickel cathode surface which unexpectedly is much more resistant against 25 spalling and flaking than those derived from alloys which contain 55% by wt. nickel or less, which spall at a commercially unacceptable rate. Also as shown therein, although all the compositions tested initially showed good reductions in hydrogen 30 overvoltage, the improvement in hydrogen overvoltage unexpectedly was maintained at a higher level following wash/bake cycles in the case of the nickel/aluminum alloy composition applied as the coating in the process of the present invention than 35 did those containing lesser (55% by wt. nickel and 5 205086 lower) and greater (62% by wt. nickel) amounts of • nickel. The cathode made by the process herein can be used as the cathode in known types of 5 electrochemical cells which comprise a cathode compartment, a cathode disposed within said cathode compartment, an anode compartment, an anode disposed within said anode compartment, and a separator disposed between the anode and cathode compartments. 10 It is especially useful in chloralkali cells of such description. The separator can be, e.g., a liquid permeable diaphragm; such diaphragm can be fabricated of, e.g., asbestos fibers or perfluorinated polymer fibers. The separator can alternatively be a 15 membrane, e.g., of highly fluorinated or perfluorinated ion-exchange polymer; membrane of such polymer containing sulfonate and/or carboxylate groups is now well known in the chloralkali art, e.g., U.S. 4,192,725, U.S. 4,065,366 and 20 U.S. 4,178,218. As is now known in this art, such membrane cells can be operated in both narrow gap (1-3 mm spacing from anode to cathode) and zero gap (both electrodes in contact with the membrane) configurations. 25 In a chloralkali process for electrolysis of an alkali metal chloride such as sodium chloride in such a cell, an aqueous alkali metal chloride solution is introduced into the anode compartment, an electric current is passed through the cell, an 30 aqueous alkali metal hydroxide solution is removed from the cathode compartment and chlorine is removed from the anode compartment. In a membrane cell, water (or dilute alkali metal hydroxide at startup) is generally introduced into the cathode compartment, 35 and spent alkali metal chloride solution is removed 6 205086 from the anode compartment. In a diaphragm cell, all ' of the unreacted alkali metal chloride solution percolates through the diaphragm into the cathode compartment, and is removed along with the product 5 caustic. The present invention has numerous advantages when compared to the prior art. These include (1) high resistance to flaking, spalling and mechanical damage of the activated coating; (2) 10 resistance to oxidizing atmospheres, such as heating in air, e.g., during baking of a diaphragm thereon; (3) low hydrogen overvoltage compared to mild steel cathodes and Ni cathodes; (4) good tolerance to moderate amounts of contamination by iron 15 overplating; (5) a variety of substrates including mild steel, stainless steel and nickel can be used; (6) low cost and convenient application to preconstructed cathode components; and (7) the substrate is rarely heated above 200°C, so that 20 warping is minimal. To further illustrate the innovative aspects of the present invention, the following examples are provided. In the examples, abbreviations are used as 25 follows: PTFE refers to polytetrafluoroethylene; TFE/EVE refers to a copolymer of tetrafluoro-ethylene and methyl perfluoro(4,7-dioxa-5-methyl-8-nonenoate); 30 TFE/PSEPVE refers to a copolymer of tetra- fluoroethylene and perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride); EW refers to equivalent weight. 35 7 8 EXAMPLES Example 1 2050 8 6 A commercial cathode screen made of 0.23 cm (0.092 inch) diameter steel wire spaced on 0.42 cm 5 centers (6 mesh) was sandblasted, and then plasma arc coated on one side with a 178-203 micron (7-8 mil) th ick- coating of a nickel aluminide alloy whose composition was nominally 57% Ni, 43% Al; particle size range was 100% smaller than 90 microns (i.e., 10 through 165 mesh) to 30% smaller than 42 microns (i.e., through 325 mesh). Spraying conditions were: Metco Type 7 MC (TMB) Torch-7 07 Nozzle Metco 3MP Dual Power Feeders 3 3 Torch Gas - 2.1 m /hr (75 ft /hr) nitrogen 15 Volts - 60 Amperes - 425 Kilowatts - 27.3 No. of Passes - 5. The screen surface was cooled with 20 compressed air jets. / A sample portion of the screen was cut out, and a 7.62 cm (3 inch) diameter circular section cut from the sample. The circle was welded into a stainless steel cathode holder such that the coated 25 side would face toward the anode in the subsequently assembled cell, leached in 10% caustic, and a PTFE fiber-asbestos diaphragm (TAB Diamond Shamrock composition) was deposited on the Raney nickel coated surface of the screen and baked. Performance of this 30 cathode was as follows, with typical steel cathode performance given for comparison. Current Density, amp/dm2 Actual Current, amps 35 Cell Voltage, volts Advantage vs. Steel, mV Ex. 1 Steel 18.3 18.3 8.3 8.3 2.85 3.10 250 8 t- ■ i Example 2 2 0 50 B 6 A cathode finger from a Diamond Shamrock MDC-55 cathode assembly was sprayed on one side with nickel aluminide of the same composition as in 5 Example 1. Before the finger was sprayed, it was degreased and sandblasted. Spraying conditions were: Metco Type 7 MC (TMB) Torch-707 Nozzle Metco 3MP Dual Power Feeders 3 3 Torch Gas - 2.1 m /hr (75 ft /hr) nitrogen 10 Volts - 62 Amperes - 450 Kilowatts - 27.9 No. of Passes - 4. The screen surface was cooled with an air 15 blast. A sample was cut from this screen, leached in 10% caustic, coated with a TAB diaphragm directly on the Raney nickel coated surface, and tested in a 7.62 (3-inch) diameter diaphragm cell with the Raney 20 nickel coated surface facing the anode. Performance of the coated sample at two different current densities was: Ex. 2 2 Current Density, amp/dm 18.3 11 25 Actual Current, amps 8.3 5.0 Cell Voltage, volts 2.83 2.54 The advantage for the cathode of Example 2 vs. the steel cathode shown for comparison in Example 1 was 270 raV. 30 Example 3 The fingers for three Diamond Shamrock MDC-55 cathode assemblies (cathode cans) were plasma sprayed with nickel aluminide of the same composition as in Example 1 under the conditions outlined under 35 Example 2, only on the sides of the fingers which 9 10 2050S6 were subsequently coated with the diaphragm coating and"which would face toward the anode. Each can was then leached for about 16 hours in cold 10% caustic solution. The cans were then washed, and a Diamond 5 Shamrock TAB diaphragm deposited and baked on each. Each can was assembled into a chloralkali cell and operated in a cell line. All the cans performed similarly. Performance compared to steel was: Ex. 3 Steel 2 10 Current Density, amp/dm 11.3 11.3 Actual Current, kiloamps 62 "62 Cell Voltage, volts 2.70 2.88 Advantage vs steel, mV 180 Example 4 15 Samples of Type 304 stainless steel screen of 0.18 cm (0.072 inch) diameter wire spaced on 0.42 cm centers (6 mesh) were welded into stainless steel cathode rings and sprayed with a 0.015 cm (6-mil) thick coating of nickel aluminide of the same 20 composition as in Example 1, only on that side to which the diaphragm coating was subsequently applied and which would face toward the anode in the subsequently assembled cell. The coating was leached with 10% caustic. A TAB diaphragm was deposited and 25 baked onto the screens and the cathode assembly operated in a 7.62 cm (three-inch) cell. After initial operation, the diaphragms were removed, new diaphragms installed, and a second run made. Data are: 30 v Ex. 3 Ex. 4 Steel 1st Cycle 2nd Cycle Average 2 Current Density, amp/dm 18.3 18.3 18.3 Actual Current, kiloamps 8.3 8.3 8.3 Cell Voltage, volts 2.99 2.89 3.11 35 Advantage vs steel, mV 120 220 10 11 20 50 8 6 Example 5 - Preparation of a cathode for evaluation in a narrow gap membrane cell. A cathode for a laboratory membrane cell was prepared by cutting out a 7.62 cm (3-inch) diameter disk of a flattened expanded metal having dimensions of 2.54 cm LWD (long way diamond) x 0.635 cm SWD (short way diamond) x 0.16 cm strand width x 0.12 cm thick of a type 200 nickel. A length of 0.635 cm nickel tubing was welded to the center of the disk to serve as a current conductor. A coating of an alloy of 57% + 0.5% by wt. Ni and 43% + 0.5% by wt. Al was applied to both sides of this cathode by plasma spraying. The alloy was in the form of a fine powder having a particle size of 42 to 110 microns (i.e., between 140 mesh and 325 mesh). Conditions of application were as follows. Power - 40 V and 580 A (23.2 kw) Arc Gas - 2.63 m3/hr (93 CFH) argon at 3.448 x 10^ Pa (50 psi) 10.16 cm (4 inch) TTWD (torch to work piece distance) The coating thickness was 203 microns (8 mils) on both sides of the expanded metal. The coating was activated by immersing the cathode in a solution of 10% NaOH at room temperature for 16 hours. The cathode was then water rinsed and kept wet until installed in a cell. Example 6 - Performance of a two-side coated cathode for evaluation in a finite gap membrane cell. The cathode of Example 5 was mounted in a 2 small cell having an active area of 45 cm , together with a reinforced fluorinated ion exchange membrane and a dimensionally stable anode. The membrane comprised a 38 micron (1.5 mil) layer of 11 12 TFE/EVE having an equivalent weight of 1080, a 100 micron (4 mil) layer of TFE/PSEPVE having an equivalent weight of 1100, a layer of fabric having both PTFE filaments and polyethylene terephthalate 5 filaments, and a 25 micron (1 mil) layer of TFE/PSEPVE having an equivalent weight of 1100, and was treated with an aqueous bath containing 11% KOH and 30% dimethylsulfoxide to hydrolyze the functional groups to carboxylate and sulfonate groups. The 10 membrane was mounted with the carboxylate side toward the cathode. The electrodes were positioned so that there was a 3 mm gap between them. The cell was constructed so that a hydraulic head corresponding to approximately 25.4 cm (10 inches) of water on the 15 cathode side pressed the membrane against the anode. The cell was operated at 90°C with a current density 2 of 31 A/dm . A saturated solution of purified sodium chloride was fed to the anolyte chamber at such a rate as to maintain a concentration of 200 g/1 20 NaCl. Water was added to the catholyte chamber to maintain the concentration of caustic produced at 32 + 1%. After 7 days, the cell was performing at 96% current efficiency and 3.17 volts. The performance 25 remained unchanged after 110 days of operation. Comparative Example A Example 6 was repeated except that the cathode was made from uncoated mild steel expanded mesh. After 7 days, the cell was operating at 96.4% 30 current efficiency and 3.51 volts. This is 340 mV higher than the activated cathode of Example 6. Example 7 - Performance of a one-side coated cathode in a finite gap membrane cell. Example 5 was repeated except that the 35 coating was applied only to the one face of the 12 13 cathode which faced the membrane in the cat-hode was then installed and operated in a test cell under the same conditions as Example 6. After 7 days, the cell was operating at 97.6% current efficiency and 3.20 volts. After 30 days, the cell voltage was 3.18 volts and remained unchanged after 110 days of operation. This is 330 mV lower than the mild steel cathode of Comparative Example A. Example 8 - Performance of a two-side coated cathode in a zero gap membrane cell. A cathode prepared as per Example 5 was mounted in a small cell having an active area of 2 45 cm . The membrane comprised a 38 micron (1.5 mil) layer of TFE/EVE having an equivalent weight of 1080 and a 100 micron (4 mil) layer of TFE/PSEPVE having an equivalent weight of 1100, was coated on both sides with an inorganic nonconductive layer comprised of ZrC^ particles bonded with a solution in ethanol of a 950 equivalent weight copolymer of TFE/PSEPVE and dried, and was treated with KOH as in Ex. 6 to hydrolyze the functional groups to carboxylate and sulfonate groups. A dimensionally stable anode was used and the electrodes were positioned close together with the membrane between them such that there was no gap between the membrane and either electrode. The membrane was mounted with the carboxylate side toward the cathode. The cell was operated at 90°C with a 2 current density of 31 A/dm . A saturated solution of purified sodium chloride was fed to the anolyte chamber at such a rate as to maintain the concentration at 200 g/1 NaCl. Water was added to the catholyte chamber to maintain the concentration of caustic produced at 32 + 1%. .17 hrJ 13 2050 86 14 After 7 days, the cell was performing at 97.8% current efficiency and 3.09 volts. Example 9 and Comparative Examples B, C and D Samples of mild steel cathode screen made of 5 .0.23 cm (0.092 inch) diameter wire spaced on 0.42 cm centers (6 mesh), cleaned by blasting with 700 micron (24 mesh) alumina grit at 5.5 x 10^ Pa (80 psi), were plasma sprayed at the following conditions: Volts - 40 10 Amperes - 620 Power - 24.8 kw TTWD (torch to workpiece distance) -6.3-7.6 cm (2.5 - 3 inches) 3 Arc gas - 2.63 m /hr (93 CFH) argon at 15 3. 45 x 105 Pa (50 psi) Four different nickel/aluminum alloy compositions were used, having nickel/aluminum weight ratios of 52/48 (Ex. B) , 55/45 (Ex. C) , 57/43 (Ex. 9) and 62/38 (Ex. D). All had particle sizes of 42-110 20 microns (140 to 325 mesh). The coating thickness applied was 152-178 microns (6-7 mils). The plasma spray coated cathodes were activated by leaching successively in 2% aqueous caustic at 25°C, 5% aqueous caustic at 25°C, 10% 25 aqueous caustic at 25°C, and 10% aqueous caustic at 80°C, each stage being carried out until hydrogen evolution ceased before proceeding to the next stage. Each cathode (circular disk, 8.25 cm or 3.25 inches in diameter) was tested for performance in a 30 small chloralkali cell as follows. The cell had anode and cathode compartments separated by a membrane of a perfluorinated ion exchange polymer having sulfonate groups; the area of membrane in use was circular, 3.5 cm in diameter. The anode was a 35 platinum screen. A reference standard calomel 14 205086 15 electrode and the cathode to be tested were placed in the' cathode compartment, with the calomel electrode as close as possible to, or in contact with, the cathode under test. The anode compartment was filled 5 with a sodium chloride solution (250g NaCl/liter of solution); the cathode compartment was filled with a sodium chloride/sodium hydroxide solution containing about 13% sodium hydroxide (3 liters of the above NaCl solution, 1.5 liters water and 585g NaOH). 10 Electrolysis was carried out at a current density of 2 15.5 A/dm at ca. 90°C, until stable electrode potentials were attained. The difference in potential between the test cathode and reference calomel electrode was measured with a high impedance 15 digital voltmeter at intervals throughout the electrolysis period. The performance was compared against an uncoated mild steel cathode, and the results given below are reported as the millivolts advantage for the coated cathode vs. mild steel. 20 Each test cathode was then subjected to a sequence of water spray at 1.03 x 10^ Pa (1500 psi) for 3 minutes on each side of the cathode to simulate the conditions of removal of a spent asbestos diaphragm and baking at 355°C for 4 hours to simulate 25 the baking a newly applied asbestos diaphragm, even though no asbestos was actually removed or deposited. The amount of Raney nickel coating which spalled off during the water spraying was collected by filtration and weighed. The cell performance of 30 the washed and baked cathode was again evaluated. This sequence was carried out ten times for each test cathode. The results are summarized in Tables I and II. 35 15 bjL. 20508 , REDUCTION IN HYDROGEN ' " ' ' OVERVOLTAGE VS. MILD STEEL CATHODE at 15.5A/dm2(l asi) (in millivolts) Nickel aluminide coating composition (% by wt.) Number of Ex. B Ex. C Ex. 9 Ex. D Wash/Bake Cycles 52/48 55/45 57/43 62/38 1 163 173 160 150 2 153 162 15 3 146 3 153 150 146 141 4 143 146 14 6 136 5 137 143 143 136 6 133 137 13 8 132 7 133 118 132 128 8 117 120 12 6 103 9 108 105 123 98 10 99 98 116 80 20 25 30 35 16 </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 17 Table II 20 SO 8 6 CUMULATIVE COATING SPALL DURING 1.03 X 107 Pa (1500 psi) WATER JET SPRAY (in milligrams) Nickel aluminide coating composition (% by wt.) Number of Ex. B Ex. C Ex. 9 Ex. D Wash/Bake Cycles 52/48 55/45 57/43 62/38 1. 6.7 6.1 4.4 0.7 2 12.9 13.1 11.9 8.3 3 13.6 14.7 16.8 21.4 4 45 22.2 25.5 25.8 5 45. 8 27.3 25.5 26.5 6 45.8 63.7 32.6 28.2 7 48.2 63.7 3 2.6 28.2 8 54. 2 69.2 32.7 29.2 9 77.0 91.2 36.3 29.9 10 84.0 91.6 37.3 34.7 INDUSTRIAL APPLICABILITY The present invention is useful whenever Raney nickel coated electrodes are desired. It finds use especially in the chloralkali field where low hydrogen overvoltage cathodes are needed. The cathodes are more durable than those of the prior art, and have other advantages set forth above. 17 WHAT^WE CLAIM IS:- 18 2QS0SS 10 15 20 25 30 vCftlrl^

1. Process for making a low-hydrogen-overvoltage cathode for a chloralkali cell which comprises (a) plasma spraying a coating of a nickel/aluminum alloy composition containing 56 to 59% by wt. nickel onto a foraminous substrate of iron, ferrous alloyr nickel or nickel alloy, and (b) leaching aluminum from said coating by contacting the article prepared in step (a) with a caustic solution.

2. The process of Claim 1 wherein said nickel/aluminum alloy composition used in step (a) is in the form of 36 to 90 micron particles.

3. The process of Claim 2 wherein the coating made in step (a) is 13 to 508 microns thick.

4. The process of Claim 3 wherein said nickel/aluminum alloy composition used in step (a) is in the form of 36 to 53 micron particles.

5. The process of Claim 4 wherein the coatingvnade in step (a) is 127 to 254 microns thick.

6. The process of Claim 5 wherein said plasma spraying is carried out with an argon or nitrogen gas plasma generated with 20 to 30 kilowatts of power.

7. The process of Claim 5 wherein said step (b) is carried out for a time sufficient to remove 70-901 by weight of the aluminum from said coating.

8. The process of Claim 5 wherein said caustic solution in step (b) is 5 to 15% by wt. aqueous NaOH, and.said contacting is for a time of 5 hours to 1 day. said nickel/aluminum alloy composition contains 56.5 to 57.5% by wt. nickel.

9. The process of Claim 1, 3 or 5 wherein 18 4 FEB 1986 • 2.0 50 8 6 19

10. The process of Claim 9 wherein said nickel/aluminum alloy composition is substantially 57% by wt. nickel.

11. The cathode made by the process of 5 Claim 1, 3 or 5.

12. The cathode made by the process of Claim- 2, 4, 6, 7 or 8.

13. The cathode made by the process of Claim 9. 10

14. The cathode made by the process of Claim 10.

15. In a chloralkali cell comprising a cathode compartment, a cathode disposed within said cathode compartment, an anode compartment, an anode 15 disposed within said anode compartment, and a separator disposed between said anode and cathode compartments, the improvement which comprises said cathode being a cathode of Claim 11.

16. The chloralkali cell of Claim 15 wherein 20 said separator is a liquid permeable diaphragm.

17. The chloralkali cell of Claim 16 wherein said diaphragm comprises asbestos fibers.

18. The chloralkali cell of Claim 15 wherein said separator is a membrane of highly fluorinated 25 ion-exchange polymer.

19. In a process for electrolysis of an alkali metal chloride in an electrochemical cell divided into anode and cathode compartments by a separator, wherein an aqueous alkali metal chloride 30 solution is introduced into the anode compartment of said cell, an electric current is passed through said cell, and aqueous alkali metal hydroxide solution is removed from said.cathode compartment and chlorine is removed from said anode compartment, the improvement 35 which comprises employing as said electrochemical cell the chloralkali cell of Claim 15. 19 v * 20. 205686

20. A process as claimed in Claim 1 for making a low hydrogen-overvoltage cathode for a chloralkali cell substantially as herein described in any one of Examples 1 to 9. BALDW ATTORNEYS FOR THE APPLICANTS i--t r 24 FEB 1986" </div>