US3491014A

US3491014A - Composite anodes

Info

Publication number: US3491014A
Application number: US798574*A
Authority: US
Inventors: Giuseppe Bianchi; Patrizio Gallone; Antonio E Nidola
Original assignee: Oronzio de Nora Impianti Elettrochimici SpA
Current assignee: De Nora SpA
Priority date: 1969-01-16
Filing date: 1969-01-16
Publication date: 1970-01-20
Anticipated expiration: 1987-01-20

Description

Jan. 20, 1970 a. BlANcHi ETAL COMPOSITE ANODES 3 Sheets-Sheet 1 Filed Jan. 16, 1969 Jan. 20, 1970 G. BIANCHI ETAL COMPOSITE ANODES 3 Sheets-Sheet 5 Filed Jan. 16, 1969 12 3 35. uzo .8 555: .firue E 315m :0

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VENTORS BIAN cm GALVLONE NEY J (SHN) 110A United States Patent Int. Cl. B01k 3/06 US. Cl. 204242 Claims ABSTRACT OF THE DISCLOSURE Composite anodes having a metal base selected from the group consisting of magnetite, lead dioxide, manganese dioxide and high silicon-iron alloy, preferably containing molybdenum or chromium and having an outer layer of a platinum group metal or a ceramic, semi-conductor coating, which anodes are useful in electrolytic cells, especially for the production of chlorine.

PRIOR APPLICATION The present application is a continuation-in-part of our copending, commonly assigned US. patent application Ser. No. 555,026, filed June 3, 1966, now abandoned.

STATE OF THE ART Chlorine is produced in electrolytic cells by the electrolysis of an aqueous solution of sodium chloride (brine). Chlorine is released at the anode and sodium is released at the cathode and converted into sodium hydroxide.

Anodes are usually of graphite since it is relatively inexpensive and easy to shape. However, graphite is rather rapidly consumed during the electrolysis process (7.5 pounds for each ton of chlorine), requiring frequent replacement of anodes in order to maintain the desired electrode gap with time consuming dismantling and reassemblying of the cells being necessary.

Anodes of platinum group metals, such as platinum, iridium, rhodium, ruthenium, palladium or alloys of these metals, either as such or on a support have also been used. One particular type of support is titanium or tantalum. These anodes are more advantageous than graphite, in that they are not consumed as rapidly as graphite and the desired electrode gap is maintained over longer periods. They are thus dimensionally more stable and do not require as frequent replacement.

The platinum type anodes, however, have the disadvantage that after a short period of use, the anode potential increases which causes an increased energy consumption in the electrolytic process. In addition, although paltinum anodes are not consumed as rapidly as graphite, due to the high cost of platinum type metals, the depletion at the anode is still sufficient to add a substantial expense to the operation of the cell.

Various other types of anodes have been proposed such as anodes of an iron or steel base coated with magnetite to prevent erosion. However, due to the difference in the coefiicient of expansion of the base and the coating, the coating flakes off exposing the iron base which is then rapidly depleted.

OBJECTS OF THE INVENTION It is an object of this invention to provide an anode which is relatively inexpensive and is not materially consumed in the electrolytic process and is, therefore, dimensionally stable.

Another object is to provide an anode having good 3,491,014 Patented Jan. 20, 1970 electrical properties which does not have a large potential increase with time and use.

It is a further object of this invention to provide an anode having the desirable stability and other characteristics of an iron base and a platinum surface without the disadvantages of prior anodes.

These and other objects and advantages of the inven tion will become obvious from the following detailed description.

THE INVENTION The electrodes of the invention which achieve the above objects and overcome the disadvantages of the prior art are comprised of a metal base selected from the group consisting of mangetite, manganeses dioxide, lead dioxide and high silicon-iron having a silicon content of about 14 to 16% which may have alloyed with it about 3 to 4% of either molybdenum or chromium or other metals, preferably from the sixth group of the periodic table, the remainder being iron, which base is coated with a thin layer of platinum or a platinum group metal or alloys of the platinum group metals or a ceramic, semi-conductor coating.

The high intrinsic corrosion resistance of high Si-Fe alloys (14.5 Si) has long been known. The most significant discovery relating to the addition of other elements was the profound effect of 3% molybdenum on the resistance of these alloys to hot hydrochloric acid and various chlorides. This alloy is described in US. Patent No. 1,972,103. It is further known that extra resistance to hot hydrochloric acid is attained by adding 3 to 4% M0 instead of raising the silicon content beyond about 14 to 16%. It is thus possible to avoid the extreme fragility characteristic of the 17% Si-Fe alloy, as would otherwise be required for hot HCl. A relatively recent discovery is that chromium can be used as a constituent of high siliconiron, in the place of molybdenum. It is also known that a 14.5% Si, 3.5% Mo-Fe alloy has good behavior in contact with free, wet chlorine.

The improved behavior of the high silicon-iron alloy including molybdenum has been recognized also in its use as anode material for cathodic protection in sea water. However, we have now also found that under such conditions a slight amount of noble metal of the platinum family is suflicient to reduce the anodic corrosion rate to negligible amounts. Indeed, just as in the case of titanium, even a partial coverage with a noble metal allows the silicon-iron substratum to acquire and keep passivation also under chlorine discharge, provided that the current density is not so high as to raise the anodic potential beyond a trans-passivity limit. It has now been found that in concentrated sodium chloride solution, the trans-passivity limit for the 15% Si-3% Cr-Fe alloy is about 1.8 v.

The high silicon-iron alloys can be used for electrolytic cell anodes in the chlor-alkali industry as a passive support for a thin film of noble metal of the platinum group, such as platinum, iridium, ruthenium, rhodium and palladium or alloys thereof.

The anodes are useful in various types of electrolytic cells. However, the molybedenum and chromium content alloys are more suitable for use in diaphragm cells because of the possibility of catalytic evolution of hydrogen with these metals in mercury cells. The anodes are also useful for cathodic protection of underwater metal objects such as ships, piers, etc.

The metal base anodes of the invention provided with a ceramic semi-conductor coating, have a semi-conductive mixed metal oxide coating over part or all the metal base sufficient to conduct an electrolysis current from the base to an electrolyte over long periods of time without passivation or increase in overvoltage. The mixed metal oxide is comprised of titanium oxide or doped titanium oxide or tantalum oxide or doped tantalum oxide or mixed metal oxides from adjacent groups in the periodic table.

It has long been known that rutile or titanium dioxide and tantalum oxide have semi-conducting properties, either when doped with traces of other elements or compounds which disturb the lattice structure and change the conductivity of the titanium dioxide or tantalum oxide, or-

when the lattice is disturbed by the removal of oxygen from the titanium dioxide or tantalum oxide crystal. Titanium dioxide has been doped with tantalum, niobium, chromium, vanadium, tin, nickel and iron oxides and other materials to change the electrical conducting or the semi-conducting properties of the titanium dioxide, and by changing the stoichiometric balance by removing oxygen from the crystal lattice. Likewise, Ta O films have had their conductivity altered by ultraviolet radiation and by other methods, but no one has suggested the use of doped titanium dioxide or tantalum oxide to provide a conductive or semi-conductive face on the base metal electrode for use in electrochemical reactions. Other metal oxides when intimately mixed and heated together have the property of forming semi-conductors, particularly mixed oxides of metals belonging to adjacent groups in the Periodic Table.

Various theories have been advanced to explain the conductive or semi-conductive properties of doped or undoped titanium dioxide, also for Ta O See, for example, Grant, Review of Modern Physics, vol. 1, page 646 (1959); Frederikse, Journal of Applied Physics, Supplement to vol. 32, No. 10, page 221 (1961) and Vermilyea, Journal of the Electrochemical Society, vol. 104, page 212 (1957), but there appears to be no general agreement as to what gives doped titanium dioxide and tantalum oxide their properties of semi-conduction. When other mixed metal oxides are used to produce semi-conductors, it is possible that oxides of one metal belonging to an adjacent group in the Periodic Table penetrates into the crystal lattice of the other metal oxide by solid solution to act as a doping oxide which disturbs the stoichiometric structure of the crystals of one of the metal oxides to give the mixed oxide coating its semi-conducting properties.

In general, it is preferred to make a solution of the semi-conductor metal and the doping composition in such form that when applied and baked on the cleaned base metal electrode the solution will form TiO plus doping oxide or Ta O plus doping oxide or other metal oxide plus doping oxide and to bake this composition on the base metal electrode in multiple layers so as to form a solid solution of the TiO T a O or other metal oxide and the doping oxide on the face of the electrode which will have the desired semi-conducting properties and will continue chlorine discharge without increase in overvoltage over long periods of time. Any solutions or compounds which on baking will form TiO plus doping oxide, Ta O plus doping oxide or otherv metal oxide plus doping oxide may be used, such as, chlorides, nitrates, sulfides, etc., and the solutions given below are only by way of example.

Overvoltage as used above may be defined as the voltage in excess of the reversible or equilibrium which must be applied to cause the electrode reaction to take place at the desired rate. Chlorine overvoltage varies with the anode material and its physical condition. It increases with anode current density but decreases with increase in temperature.

Titanium dioxide, tantalum oxide and other metal oxide semi-conductor faces may be produced by doping titanium dioxide, tantalum oxide or other metal oxide crystals with various doping compositions or by disturbing the stoichiometric lattice by removing oxygen therefrom to cause the TiO Ta O or other metal oxides to become semiconductive. Because of the tendency of the TiO Ta O or other metal oxide crystals to become reoxidized, it is preferred to form the semi-conductive faces on our elec- 4 trodes by the use of doping compositions which in baking form solid solutions with the TiO Ta O or other metal oxide crystals which are more resistant to change during electrolysis processes. However, semi-conducting coatings produced by withdrawing oxygen from the T iO Ta O or other oxide lattices to cause lattice defects or deficiencies may be used on the electrodes of the invention.

Various doping materials which introduce impurities into the TiO and Ta O crystals to make them semiconductive, may be used to increase the conductivity and electrocatalytic properties of the TiO and Ta O layer 011 the electrode, W02, P205, Sb O5, V205, T3205, Nb205, B203, Cr2O B60, N320, CaO, SrO, R1102, 1102, PbOg, OsO PtO AuO AgO SnO A1 0 and mixtures thereof. The best results have been secured with doping compositions for TiO which have the tetragonal rutiletype structure with similar unit cell dimensions and approximately the same cationic radii (0.68 A.). Thus, RuO (0.65 A.) and IrO (0.66 A.) are especially suitable doping compositions as well as other oxides of metals of the platinum group (i.e., platinum, palladium, osmium and rhodium). IrO forms solid solutions in TiO up to about 5 mole percent IrO when heated together at 1040 C. At lower temperatures, the amount of H0 which will form solid solutions in TiO is lower but the amount of platinum metal oxide group which is not in solid solution continues to act as a catalyst for chlorine discharge.

Oxides of metals from Group VIII of the Periodic Table of elements as well as oxides of metals of Group VB, Group VI-B, oxides of metals from Group I-B and oxides of elements from Group V-A, as well as mixtures of these oxides capable on baking of forming solid solution crystals with TiO and Ta O and of interrupting the crystal lattice of Ti0 and Ta O may be used to form semi-conductor and electrocatalytic coatings on the base metal electrodes.

In forming semi-conductor coatings for base metal electrodes from other metal oxides, it is preferable to use mixed oxides of metals, or materials which form mixed oxides of metals, from adjacent groups of the Periodic Table, such as, for example, iron and rhenium; titanium, tantalum and vanadium; titanium and lanthanum. Other oxides which may be used are manganese and tin; molybdenum and iron; cobalt and antimony; rhenium and manganese and other metal oxide compositions.

The percentage of the doping compositions may vary from 0.10 to 50% of the TiO Ta O or other metal oxide and surprising increases in conductivity of the TiO Ta O or other metal oxide facing can be gotten with as little as 0.25 to 1% of the doping composition to the TiO Ta O or other metal oxide in the conductor face on the electrode. It is preferred, however, to use sufiicient excess of the doping metal oxide to provide a coating on the anodes which will catalyze chlorine discharge without material overvoltage.

Examples of suitable ceramic semi-conductive mixed metal oxides are 54.8% TiO 22.6% IrO and 22.6% RuO; 10% RuO, 10% IrO and Ta O 15% RuO, 5% AuO and 80% TiO 35% RuO, 55% TiO and 10% SnO; 45% RuO, 54% TiO and 1% A1 0 45% RuO, 50% TiO and 5% Ta O etc.

The conductive coating of the invention may be applied in various ways, and to various forms of base anodes, such as solid rolled massive plates, perforated plates, slitted, reticulated plates, mesh and rolled mesh, woven wire or screen, rods and bars or similar metal plates and shapes. The preferred method of application is by chemideposition in the form of solutions painted, dipped or sprayed on or applied as curtain or electrostatic spray coatings, baked on the anode base, but other methods of application, including electrophoretic deposition or electro-deposition, may be used. Care must be taken that no air bubbles are entrapped in the coating and that the heating temperature is below that which causes warping of the base material.

The spectrum of doped Ti samples shows that the foreign ion replaces the Ti ion on a regular lattice site and causes a hyperfine splitting in accordance with the nuclear spin of the substituting element.

In all applications, the metal base is preferably cleaned and free of oxide or other scale. This cleaning can be done in any way, by mechanical or chemical cleaning, such as, by sand blasting, etching, pickling or the like.

This invention may be further understood from the following detailed description of preferred embodiments and by reference to the drawings, in which:

FIG. 1 is a perspective of a diaphragm cell showing location of anodes;

FIGS. 2 and 3 are graphs of anode potential against current;

FIG. 4 is a graph of anode potential against time; and

FIG. 5 is a graph of anode weight loss against time.

The embodiment of a diaphragm cell, illustrated in FIG. 1, has vertical anodes imbedded in a lead base and cathodes separated from the anodes by diaphragms.

The anodes of this invention may be used in various electrolysis processes and the cell shown in FIG. 1 is only for the purpose of illustrating one application of anodes and process.

The base of the cell of FIG. 1 consists of a shallow cast iron pan 1, housing flat copper grids, not shown. In contact with these grids are lateral rows of anodes 2, which extend vertically from the base 1 and are secured in the base and electrically connected to the copper grids by having molten lead poured around the base of the anodes and the copper grids.

The cathode assembly which rests on the base is a rectangular steel shell 3, having an inner section consisting of lateral rows of double metal screens 4 upon which a diaphragm of asbestos fiber is deposited. When the cathode assembly 3 is positioned on the base 1, the rows of screens 4 fit alternately between the rows of anodes 2 to form the anolyte and catholyte sections of the cell. Readily accessible electrical connections for anodes 2 are made by lugs 5 which extend from the copper grids in base 1 to a copper grid bar 6 around the outside of the cathode assembly 3. The cell assembly is completed by a concrete head 7 on top of the cathode assembly 3. Cells of this type are known commercially as D3 cells.

Brine is introduced to the cell by brine feed assembly 8. Cell liquor is taken out through conduit 9, having a standpipe to maintain brine level in the cell. Chlorine is withdrawn through outlet 10 and hydrogen through outlet 11. The anodes 2, instead of the usual graphite, are made of a platinum coated high silicon-iron composition, preferably about Si, about 3% Mo, about 0.6 to 0.7% carbon and the remainder iron. Other alloy ingredients, such as chromium, may be used in place of molybdenum. This composition anode may be used in other form of diaphragm cells than that illustrated in FIG. 1, and with proper design may be used in mercury cells and for other electrolysis purposes and for corrosion control.

The platinum coating is of the order of about 1 micron in thickness and may be deposited on the high siliconiron anodes by electroplating, chemical deposition, from a platinum containing solution, spraying or any other method which will produce a substantially uniform and substantially non-porous coating. In place of platinum, other platinum group metals and alloys thereof, with platinum and with each other, may be used.

The following specific examples of preparation of the improved anodes and tests thereof are given to enable persons skilled in the art to better understand the invention and are not intended to be limitative.

EXAMPLE I Composite anodes were prepared as follows. The two alloys used for comparison testing had the following composition:

6 Alloy A: Iron Percent Silicon 15 Alloy B: Iron Silicon 15 Chromium 3 The samples were shaped as discs 3 cm. in diameter and 0.5 cm. thick, with a central hole 0.8 cm. diameter.

Surface preparation Two alternative methods of surface preparation, one chemical and the other mechanical, were used as follows:

(a) Chemical preparati0n.Acid pickling at 40 C. for 5 minutes in the following solution contained in a polyethylene vessel:

HNO (RP) 86% 10% ml. HF (RP) 10% 10 ml. H O (distilled) 30 ml.

(b) Mechanical surfacing by the grinding wheel, followed by sand blasting-The platinum deposits were made by electroplating and had more uniform appearance when the substratum was prepared according to alternative (b). Accordingly, all samples subjected to the subsequent tests were prepared by mechanical surfacing only.

Platinum plating.-Bath composition:

The electroplating time was established after determining the current efiiciency under the above noted conditions, so as to obtain a platinum deposit of 28.8 g./m. or about 1.25 microns Pt. The current efficiency was 23 to 25% with reference to the reaction: I

Pt(NH ++[-2e'- Pt+2NH The anodes prepared for testing were as follows:

(A) Fe-Si (B) Fe-Si-Pt c Fe-Si-Cr (D) Fe-Si-Cr-Pt EXAMPLE II Anodic polarization curves The anodic curves on the samples were determined at an anodic current density of 0.75 a./cm. (4.83 per sq. in.).

The solution was kept at 300 g. NaCl per liter and at 70 C., pH 2 to 3.

The results obtained on the two samples B and D, with and without chromium, are illustrated in FIGS. 2 and 3, respectively. Whereas in the presence of 3% Cr (Sample D, FIG. 3), the overtension has increased by less than 0.1 v. after 816 hours at said current density, the overtension on the sample without chromium (Sample B, FIG. 2) was almost 0.3 v. higher than the initial value (1.4 v.) after 744 hours. The potential rise in time for both samples is represented in FIG. 4. No corrosion phenomena were visible on the chromium alloyed Sample D, but the other Sample B produced a yellow coloring on the solution, due to dissolved iron ions. The corrosion phenomenon which was also checked by analytical determination, began after 192 hours of operation and subsided after 336 hours. Thereafter, it became observable again after 744 hours of operation.

7 EXAMPLE 111 Chemical corrosion tests The behavior of several samples in chlorinated brine at 70 C., is shown in FIG. 5, where weight losses are plotted versus time. The unplatinized Sample E without Cr had by far the highest corrosion rate (about 4 mg./hr. cm. however on Sample F made of the same alloy, and platinum coated on one face only, the initial corrosion rate was less than 0.1 mg./ hr. cm. and tended to become negligible after 720 hours.

In alloys G and H, containing chromium, the corrosion rate became negligible in about 120 hours, while its initial value (Sample G) was practically reduced to nil by platinizing one face of the sample.

EXAMPLE IV I This example shows the comparison of the terminal effect exerted by electrode resistance on current density in (1) a graphite anode, (2) a platinum plated titanium anode and (3) a platinum plated silicon-iron anode (Anode B in Example I).

The anodes (1) graphite, (2) platinum plated titanium and (3) platinum plated silicon-iron were evaluated in a Type D-2A Cell, basically similar to the cell illustrated in FIG. 1.

Each anode had a working face 69 cm. in length and 15 cm. wide, with the anode-to-cathode distance being about 1 cm. The results are tabulated in Table I. Calculations were made by the method of C. W. Tobias, Journal Electrochemical Society, 100, 459 (1953).

TABLE I.TERMINAL EFFECT FOR DIFFERENT ANODE STRUCTURES IN D-2A CELL High silicon- Iitanium iron Graphite (It coated) (Pt coated) 1 (cm) 69 G9 69 (1. (cm.) 1 1 1 ll 0. 0145 0. 0145 0. 0145 g (ohm. cm 800Xl- 60 10- 90x10" t (thick. cm 3. 18/0. 1 0. 2 1. 2 1 F0 [1' (av; 1. 094/1. 3 1. 14 1. 0409 iu-l /1' rm) 0. 95/0. 86 0. 93 0. 98 i (1-o)/i(x-I) 1. 15/1. 5 1. 22 1. 06

The values shown in Table I illustrate the ratios of current density i at the bottom edge of the anode to its value i at the upper edge.

These ratios indicate that there is a considerable unevenness in current distribution for all the structures considered, with the exception of the platinum coated high silicon-iron anode, for which the current density was only 6% higher at the bottom than at the top.

In particular, it can be noted that the unbalance in the case of the graphite anodes is by itself sufiicient to explain the higher consumption rate in the lower portion of graphite anodes, aside from the further disturbance caused by the blanketing effect of the rising gas bubbles, whereby the current distribution is made to depart even farther from uniformity.

As regards the platinum coated titanium anodes, even when the titanium structure is 2 mm. thickness the difference between lower and upper current density values is still as great'as 22%. Such difference is too high, especially if the additional effect of the gas blanketing bubble action is considered; indeed, this would bring about different rates of platinum consumption and passivation at different places along the anodes, with a consequent decrease in the platinum coating life.

In the platinum coated high silicon-iron anodes, the terminal effect calculated for a 12 mm. thickness, which is recommended for mechanical safety, shows that such thickness should not be materially diminished if the unevenness in current distribution is to be kept within narrow limits. A

An economic comparison shows that the initial cost of A magnetite anode plate,"'with a surface of 50 cm. projected area, was cleaned by boiling at reflux temperature of 110 C. in a 20% solution of hydrochloric acid for 40 minutes. It was then given a liquid-coating containing the following materials: l

Ruthenium as RuCl -H O mg. (metal)-.. I 10 Iridium as (NH lrCl mg. (metal) 10 Titanium as TiCl ccmg. (metal) 56 Formamide (HCONH c drops "-10-12 Hydrogen peroxide (H 0 30%) do 3-4 The coating was prepared by first blending or mixing the ruthenium and iridium salts containing the required amount of Ru and Ir in a 2 molar solution of hydrochloric acid (5 ml. are suflicient for the above amounts) and allowing the mixture to dry at a temperature not higher than 50 C. until a dry precipitate is formed. Formamide is then added to the dry salt mixture at about 40 C. to dissolve the mixture. The titanium chloride, TiCl dissolved in hydrochloric acid (15% strength commercial solution), is added to the dissolved Ru-Ir salt mixture and a few drops of hydrogen peroxide (30% H 0 are added, sufiicient to make the solution turn from the blue color of the commercial solution of TiCl to an orange color.

This coating mixture was applied to both sides of the cleaned titanium anode base, by brush, in eight subsequent layers. After applying each layer, the anode was heated in an oven under forced air circulation at a temperature between 300 and 350 C. for l0.to 15 minutes, followed by fast natural cooling in air between each of the first seven layers, and after the eighth layer wasapplied the anode was heated at 450 C. for one hour under forced air circulation and then cooled.

The amounts of the three metals in the. coating correspond to the weight ratios of 13.15% Ir, 13.15% Ru and 73.7% Ti and the amount of noble metalv in the coating corresponds to 0.2 mg. Ir and 0.2 mg. Ru per square centimeter of projected electrode area. It is believed that the improved qualities of this anode are due to the fact that although the three metals in the coating mixture are originally present as chlorides, they are co-deposited on the base in oxide form. Other solutions which will deposit the metals in oxide form may, of course, be used.

EXAMPLE VI A magnetite anode plate of the same size as in Example V was submitted to the cleaning and etching procedure as described above and then given a liquid coating containing the following materials:

Ruthenium as RuCl '-'H O mg. (metal) 10 Iridium as IrCl mg. (metal) c 10v Tantalum as TaCl -4 mg. (metal) Isopropyl alcohol drops 5 Hydrochloric acid (20%) ml 5 The coating Was prepared by first blending or mixing the ruthenium and iridium salts in 5 ml. of 20% HCl. The volume of this solution was then reduced .to about one-fifth by heating at a temperature of C. The required amount of TaCl was dissolved in boiling 20% HCl so as to form a solution containing about 8% TaCl by weight. The two's'olutions were mixed together and the overall volume reduced to about one-half by heating at 60C. The specified quantity of isopropyl alcohol was then added.

The coating mixture was applied to both sides of the cleaned anode base in eight subsequentlayers and following the same heating and cooling procedure between each coat and after the final coat as described in Example V. v

The amounts of the three metals in the coating correspond to the weight ratios of Ru, 10% Ir and 80% Ta and the amount of noble metal in the coating corresponds to 0.2 mg. Ir and 0.2 mg. Ru per square centimeter of projected electrode area.

EXAMPLE VII A manganese dioxide anode plate was submitted to a cleaning and etching procedure and then given a liquid coating containing the following materials:

Mg./cm. (metal) Ruthenium as RuCl -3H O 0.8 Titanium as TiCl 0.89 Tantalum as TaCl 0.089

The coating mixture was prepared by first blending the dry ruthenium salt in the commercial hydrochloric acid solution containing TiCl Tantalum was then added in the above proportion and in the form of a solution of 50 g./l. TaCl in HCl. The blue color of the solution was made to turn from blue to orange by introducing the necessary amount of hydrogen peroxide, which was followed by an addition of isopropyl alcohol as a thickening agent. The coating mixture was applied to both sides of the anode base by electrostatic spray coating in four subsequent layers. The number of layers can be varied and it is sometimes preferable to apply several coats on the area facing the cathode and only one coat, preferably, the first coat, on the area away from the cathode. After applying each layer, the anode was heated in an oven under forced air circulation at a temperature between 300 and 350 C. for 10 to '15 minutes, followed by fast natural cooling in air between each of the first three layers and after the fourth layer was applied the anode was heated at 450 C. for one hour under forced air circulation and then cooled.

The amounts of the three metals in the coating correspond to the weight ratios of 45% Ru, 50% Ti, 5% Ta.

EXAMPLE VIII The coating mixture consisted of an HCl solution containing the following salts:

Mg./cm. (metal) Manganese as Mn(NO 0.5

Tin as SnCl -5H O 0.5

The solution was prepared by first blending the two salts in 0.5 ml. of 20% HCl for each mg. of overall salt v amount, and then adding 0.5 ml. of formamide. The solution was heated at 40-45 C. until reaching complete dissolution, and then applied in six subsequent coatings on the pre-etched base with a thermal treatment after each layer as formerly described.

EXAMPLE 1X EXAMPLE X Using the same procedure as in Example VIH, the following binary mixture was applied to a base electrode:

Mg./cm. (metal) Cobalt as C001 0.5 Antimony as SbCl (C0OI-I) (CHOH) 0.

10 EXAMPLE x1 Four coating types were tested, each of which con- (commercial) 0.7 Lanthanum as La(NO -8H O 0.088 Tin as SnCl -5H O 0.15 platinum as PtCl -nH O (commercial) 0.85 Sample No. 2:

Titanium as TiCl in HCl solution (commercial) 0.7 Lanthanum as La(NO -8H O 0.088 Tin as SnCl -5H O 0.15 Rhodium as (NH RhCl 0.85 Sample No. 3:

Titanium as TiCl in HCl solution (commercial) 0.7 Aluminum as AlCl -6H O 0.088 Tin as SnCl -5H O 0.15 Iridium as IrCl 0.85 Sample No. 4:

Titanium as TiCl in HCl solution (commercial) 0.7 Aluminum as AlCl -6H O 0.088 Tin as SnCl -5H O 0.15 Palladium as PdCl 0.85

While certain specific embodiments and preferred modes of practice have been set forth above, it will be recognized that this is mainly for the purpose of illustrating the invention to persons skilled in the art, and that various changes and modifications may be made without departing from the spirit of the disclosure or the scope of the appended claims.

We claim:

1. An electrode comprising a metal base selected from the group consisting of magnetite, manganese dioxide, lead dioxide and high silicon-iron having a silicon content of 14 to 16% and optionally containing a metal from the sixth group of the Periodic Table, said base being coated with a thin layer selected from the group consisting of at least one platinum group metal and a ceramic semi-conductor coating.

2. The electrode of claim 1 wherein the base is high silicon-iron containing from about 3 to 4% of a metal selected from the group consisting of chromium and molybdenum.

3. The electrode of claim 1 wherein the layer consists of metals and alloys of metals from the platinum group on said base.

4. The electrode of claim 1 wherein the base is high silicon-iron containing from 3 to 4% of a metal from the sixth group of the Periodic Table.

5. The electrode of claim 1 wherein the base is iron containing about 14 to 16% of silicon and the layer is a platinum group metal.

6. The electrode of claim 1 wherein the base is iron containing about 14 to 16% of silicon and the layer is a ceramic, semi-conductive layer of mixed metal oxides.

7. The electrode of claim 6 wherein the mixed metal oxides is a doping metal oxide and an oxide selected from the group consisting of tantalum oxide and titanium dioxide.

8. The electrode of claim 6 wherein the ceramic, semiconductive layer contains titanium dioxide.

9. The electrode of claim 8 wherein the other metal oxide is a platinum metal oxide.

10. In an electrolytic cell, an anode comprising an iron base containing about 14 to 16% of silicon and about 3 to 4% of a metal selected from the group consisting of chromium and molybdenum and a coating selected 9/1934 Parsons 1483 XR 3/1938 Ihrig 29-196 XR 3,376,209 4/1968 Sabins 204290 XR JOHN H. MACK, Primary Eraniirle'r W D' R- JORDAN, Assistant Examiner" ""iisfbl. 511R: