DE1964294A1 - Electrolytic anode - Google Patents

Description

Patent attorneys
Dr. Ing.Walter Abitz Dr. Dieter F. Morf Dr. Hans-A. Browns

8Munchen86.PtenzeiauMtr.2S

December 22, 1969 B-1000 / B-IOOO-a

ENUEIHAR]) MINERALS & CHEMICALS CORPORATION Newark, New Jersey V.St.A.

Electrolytic anode

The erfindungegemäße anode for the electrolysis of brines composed of a substrate made of corrosion resistant valve metal, a thin porous adhesive outer coating of a refractory oxide, which is inert to the electrolysis cell environment and, arranged between the substrate and the outer coating thin layer of ruthenium oxide ·

The invention relates to new anodes for cells used for electrolysis used by saline solutions and particularly relates to anodes coated with a platinum group metal have electrolytic valve metals, and a method for making such anodes.

The anodes according to the invention are particularly suitable for cells which are used for the production of chlorine and sodium hydroxide by the electrolysis of an aqueous sodium chloride solution. Graphite anodes are commonly used in the art in these cells. Although the graphite anodes are not entirely satisfactory because their wear rates are high and contaminants such as CO _{2 are introduced} into the product, no satisfactory substitute has been found.

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Platinum group metal coated catalytic valve or valve. Tubular metals have been suggested as substitutes for graphite anodes. These metal anodes provide several possible Advantages compared to the usual graphite anodes, e.g. low overvoltage, low erosion speeds and higher purity of the products. However, the economic benefits gained by such anodes must be sufficiently high in order to to compensate for the high cost of these metal anodes. Previously proposed anodes could not meet this requirement Therefore, the economic exploitation of platinum group anodes has heretofore been limited.

One problem is the life of the metal anodes. One factor that contributes to shortening the anode life is the so-called "undercut" effect. For economic For reasons, the noble metal coatings consist of very thin films, so that there is a risk of exposure of the substrate. This is special essential when using coatings with low overvoltage, which are inherently porous. Although corrosion-resistant, the valve metals are attacked through the pores of these coatings. thereby shortening the service life of the anodes.

Another problem is the loss of precious metal during operation of the eIIe. Although the loss occurs gradually, because the precious metals are very expensive and because the erosion of the thin coating shortens the anode life, it is costly. The loss of precious metal can result from mechanical wear. At the desired in technical installations high current densities, the increased flow velocity and the excessive gassing for mechanical exhaust wear 'use at. In the case of mercury cells, a contributing factor is the amalgamation of the precious metals

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Another problem with mercury cells is the "short circuit" ("shorting") the cell when the 3 precious metal comes into contact with the mercury, which leads to amalgamation, Change in the surface of the anodes, with calibration from it resulting adverse change in the electrolytic properties and cell stoppage.

Another consideration that is very important In the case of the highly competitive manufacturing processes, such as the electrolysis of salt solutions, the one with the Power consumption associated with anodes. Electricity costs represent a significant percentage of the total production costs, and even a small reduction in power consumption produces a substantial economic benefit.

One object of the invention was to provide metal anodes with improved physical and electrical properties to manufacture. Another object of the invention was a method for the electrolysis of salt solutions, which is essential lower production costs is feasible.

According to the invention, the electrolysis of salt solutions can be carried out with a significantly lower power consumption. This is made possible by the use of an improved anode. The anode not only reduces the power consumption in the cell, but it also has a long service life and low metal losses due to mechanical wear and amalgamation. The resistance to amalgamation makes the anode particularly suitable in mercury cells.

The anode of the E _r invention comprises a corrosion-resistant metal substrate *, a Rutheniumoxydüberzug and a thin porous adherent coating of a refractory oxide on the opposite

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the electrolytic cell environment is inert. Examples of suitable ones Refractory oxides are oxides of Si, Ti, Ta, Nb, Hf, Zr, W, Al, and combinations thereof. Oxides of Si, Ti and Ta proved proved to be particularly suitable »The refractory oxide coating is porous. It is especially important in terms of the exterior. Silica coating that it has a large surface.

The corrosion resistant ones used for electrolytic anodes Metal substrates called valve metals are known in the art. They are cheaper than the metals Platinum group and have properties that make them corrosion-resistant to the anodic environments in the electrolytic cells do. Examples of suitable corrosion-resistant valve metals are Ti, Ta, Nb, Hf, Zr, W, Al and their alloys « It is also known that the valve metal can be used as a layer on a base metal such as copper, which is a good electrical conductor, but is corrosive to the environment, is arranged, and such modifications are in the the invention.

The coating of refractory oxide is not only the contact of the noble metal layer with the electrolyte down, but it also reduces penetration of JSlektrolytenventilmetal3s and limits therefore the extent of "Unters chneidungs ¹¹ effects (" undercutting "effect). Another advantage is" in That "shorting" and the difficulties associated therewith are reduced to a minimum, surprisingly, however, despite the dielectric properties of the outer coating, these advantages are obtained without sacrificing the desired electrical properties of the noble metal anodes. Indeed, the outer, porous coating of silicon oxide with a large surface area improves the electrolytic properties of the thin, non-precious metal coatings.

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Another advantage of the anode according to the invention is that the porous outer coating with a large surface area is conductive to the development of gas. The fire-resistant oxide coating is also suitable for improving the mechanical interrelationship between the ruthenium oxide deposits applied in a relatively large amount,

The anodes of the invention are made by first a ruthenium oxide layer on the base metal substrate is formed and then a refractory oxide coating is deposited on the ruthenium oxide.

Many methods are known for forming continuous films of ruthenium oxide on a metal substrate. For example, after the etching and cleaning, the surface of the base metal becomes ruthenium metal or a rutheniura salt. deposited on the substrate, and then the coated substrate is subjected to elevated temperatures in an oxidizing atmosphere. The ruthenium metal or salt is deposited in a variety of known ways, e.g., the ruthenium metal can be deposited in a finely divided dispersion in an organic carrier or by placement, atomization, or vacuum deposition, and the ruthenium salt can be dispersed or dissolved in an organic or by application of this salt aqueous medium are deposited. The conversion to the oxide is accomplished then by baking the coating in an oxygen containing atmosphere, eg air, at a temperature above about 400 ⁰ C. The burning time depends on the temperature, the oxidizing atmosphere used and the thickness of the applied Rutheaiummetallüberzuga. Typically, a suitable ruthenium oxide is formed by burning the metal film in air at 500 ⁰ C for about 5 minutes, can also jedooh temperatures in the range of

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400 ° to 1000 ⁰ C and even higher can be used.

The outer, porous, refractory oxide coating with a large surface area is made from a dispersion or solution _; which contains a precursor compound of the refractory oxide or the refractory oxide in a very fine particle size. It has been found to be particularly advantageous to deposit the refractory oxide coatings from batches which contain a compound of the refractory metal in an organic carrier, the batches developing into refractory oxides on firing. It has also proven to be particularly beneficial to deposit the porous outer coatings from a dispersion or solution which contains a hydrophilic refractory oxide in a very fine particle size, for example with a surface area of at least 30 m / g. The coatings are baked at temperatures of over approximately 400 ^° C. to bring about the bond. If they are fired at temperatures below about 400 ^° C., the coatings are not sufficiently adhesive. According to preferred processes, the refractory oxide is deposited from an aqueous colloidal solution or from an organic resin preparation. Preferred temperatures for forming an adhesive coating are formed 600 to 1000 ⁰ C and higher, for example, suitable coatings were at 120O ⁰ C. With regard to the silica coating, it is particularly important that it have a large surface area. It was found that coatings formed from colloidal hydrophilic silica were adherent, porous and had a large surface area.

More than one coating of the refractory oxide can be applied. In general, the refractory oxide coatings are effective at a thickness of up to about 508 χ 10 ™ (200 microinhes). Diekere coatings are often not

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enough. On the other hand, several thin coatings can also be produced by depositing alternating layers of ruthenium oxide and refractory oxide, whereby a hard, permanent multi-layer coating is formed on the substrate. The temperature range for firing the successive alternating layers is in the range of about 400 ° to 1000 ⁰ C, and there may be even higher temperatures are used. It is not necessary that all layers be fired at the same temperature. In general, however, the final layer is preferably fired at a itmperatur above 800 ⁰ C. Coatings with improved adhesiveness and uniformity can be produced, for example, at temperatures of 1200 C ₁ without unduly sacrificing the electrical properties of the anode. The electrical properties vary with the crystalline structure of the Ti substrate, e.g. Ti grades with greater ultimate strength are more resistant to oxidation during firing in air at high temperatures. Properties also vary with the amount of cold rolling the Ti was subjected to prior to plating and with the thickness of the RUO2 coatings. Although the multilayer coatings are effective at a thickness in excess of 508 χ 10cn (200 microinches), there is no benefit in forming thicker coatings because they are durable even in extremely thin layers.

The following examples serve to explain the invention, without limiting them. It is to be noted that modifications within the scope of the invention will become apparent to those skilled in the art self-surrender »

Examples 1, 2 and 3 show comparative tests in diaphragm and mercury electrolysis cells using different ones Anodes, for each anode a plate made of commercially available

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Lichem pure (Ditan of 12 _S 7 χ 76 χ 1.6 mm (1/2 χ 3 x 0.063 in) for coating by etching in concentrated hydrochloric acid for a period of 18 hours at room temperature and cleaning in fluoroboric acid In di.e-Ben three examples, the coatings are produced as follows · ^!

Production of the RuO ₂ coatings

An aqueous solution of RuCl ,, containing 10.35 wt .- # Ru is applied to one side of a titanium plate using a brush. Subsequent coatings are applied, each being ^{baked at 500 °} G in air for 5 minutes until a coating of the desired thickness is obtained. A ruthenium resinate solution containing 4 G-ew, - # Ru can be applied. According to another different method, an alcohol-based analytical measure is used. This line of paint consists of 1 g of RuCl, 1 ml of linalool and 30 ml of 2-propanol. X-ray analysis of the samples prepared in a similar manner by baking the deposited coating in air under the specified conditions indicated that a major portion of the ruthenium had been converted to ruthenium oxide.

Manufacture of porous silicon dioxide coatings

After the RuOg layer has been formed, it is _{coated with SiO 2} using a composition containing hydrophilic colloidal silicon dioxide. Li;.! Ox HS, an aqueous colloidal silica solution, is used in the approaches. The anaesthetics contain about $ 10 colloidal silica and $ 90 water. Film-forming additives, eg sodium titanium «t _? Silicate or borate can be incorporated into the colloidal Biliciumdios ^ d solution in small amounts. Z »B. Suitable coatings are made up of a $ 10 composition

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daler silica, 0.5 $ sodium titanate and 8.5 ₀ 5 ^ water. Successive coatings of silicon dioxide are applied and baked at 500 ° for 5 minutes until a coating of the desired thickness is obtained.

The thickness of the coating is determined gravimetrically.

example 1

It. two samples are produced which have a RuOg coating corresponding to A3 x 10 ~ co (17 mioroinohes) Hu metal on a titanium substrate «

Sample B is used as it is made

Sample A is coated with 290x10 cm (100 microinches) of SiO ₂ using the method described above. The silica has a surface area of about 70 m / g.

Samples A and B were used as anodes in a laboratory diaphragm cell for the electrolysis of 25 # NaCl solution. The experiments were conducted at a temperature of 35 ⁰ C and a current density of 107 A / dm (1000 amperes per square foot, ASP) is performed. The chlorine overvoltage is determined with a standard Luggin capillary probe and the results are shown in Table 1.

Table 1

Sample _ A

Oversug. RuO

Thickness of the Ru in the coating ₉ 10 °° cn 43 ι (microinches)

Initial overvoltage at

107 A / dm ² (1000 ASP) (millivolts) 70 59

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Table 1 (continued) sample . AWAY

Initial cell potential

at 107 A / dm ² (1000 ASP)

(V) * 3.50 3.85

Overvoltage after 210 hours

at 107 A / dm ² (1000 ASP)

(mV) 15 *

Cell potential after 210 hours

at 107 A / dm ² (1000 ASP)

(V) 3.80 *

* After 210 hours, sample B does not give the specified current density at their initial cell potential. After increasing the cell potential, rapid disintegration took place, both des Coating as well as the substrate.

This example illustrates the superior electrical and wear _{properties of the anode according to the invention with an outer SiO 2} coating, compared to an anode with an EuOp layer but no silicon dioxide coating.

- Example 2

Samples _f similar to those described in Example 1 were prepared. Sample G consists of a titanium substrate with a RuOp coating of a thickness corresponding to 43 x 10 cm (17 microinches) Ru, sample D consists of a titanium substrate with a RuGg layer corresponding to 43 x 10 cm (17 microinches) Ru and one On top of this, a silicon dioxide overspill of 254 x 10si (100 million inches). Each sample is protected with a backbone, so that a transfer pad of Q ₉ 31 β es (0 ₅ 049 in) remains etched out. The pro "bon G iiiia 3 then wait as. anodes in a small 3 @ llsa imj ^^ s- AnssMimg of a Mercury taiciia as Kais & c-d® .mia cäias? 23 ^ ± S ⁱ sa Ia0I ^ 3.GSiaig as Mskb varwendcits Di® Ιπθοθε. cardan in fslg ^ nd ©? f / si'id ο1 «ιε-ί £ ί Pw eilberku ^ a a chluB wab arworf @ n3

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The exposed area of the Uberzucoprobe is allowed to develop chlorine at .107 A / dra ² (1000 ASF) in the brine, and then it is immersed in the mercury pond and the change in current density is measured RuOg-Schicht and the outer SiOg coating layer is much less affected by short-circuit than the same coating without the outer SiOp protective coating.

It is noted that, since the resistance to short circuit is higher, the anodes according to the invention in closer spatial proximity to a mercury cathode _? can be arranged without the risk of a short circuit and with lower power requirements at the same time.

example 3

Samples similar to those in Example 1 b @@ chrieto @ n ₉ , with the exception that the Ru (^ - layer is thinner _p are made. Two samples are made _g j Ij 3 ©? J © ils © lasa according to FIG x 10 oi (2 mieruisokeB ] g " Bm contained on a titanium subbrewate.

Sample E is used as it is

Sample F is coated with 432 χ 10 Om (170 micro inches) SiO ₂ using the method described above,

You samples E and F are used as anodes in eia ©? Laboratory ³ diaphragm cell was used and tested for the C-Silor overvoltage using the method described in Example 1 above. You cell rant it? Use of the sample F, ά.'η * the e ^ fiMuagsgeiaaSe Aasd® ₀ has an initially "ChI- overpamyaig wmi 22Q ml unä ®in gellpotentia

BATH ORIGINAL

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of 4.30 V. The cell using sample E as an anode shows uneven behavior. The coating of sample E has poor adhesion and it is believed that the inconvenient results are due to this simple adhesion of the coating and also to the inadequate protection _{brought about by the RuO 2} coating of this thickness.

This experiment shows the improved physical and electrical properties of the anodes according to the invention. These improvements not only enable a cell to operate with lower power requirements, but also illustrate also the increased service life of the anodes, since these have thinner coatings of noble metal than anodes without such covers can be operated

Example 4

Two plates of commercially available pure titanium measuring 12.7 x 76 χ 1.6 mm (1/2 χ 3 x 0.063 in) are produced for coating by sandblasting the surfaces with aluminum oxide grit and then cleaning them with an emery cleaner. Both plates are then coated on both sides with a mixture of 11.5 ruthenium chloride, 42.3 # 2 propanol and 46.2 6 linalool, based on weight. The coated substrates are heated to 300 ° to 400 ⁰ C for 1 to 2 minutes and then for 5 minutes at 50O ⁰ C in an oven with free access of air to form a coating RuOg-fired.

Sample G is made by applying the ruthenium approach and the heat treatment is repeated twice so that a total of 3 coatings of ruthenium oxide are applied.

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Sample H is prepared by covering the first butylic oxide layer with a porous silicon dioxide coating. The porous Slliciumdioxydübersug is thereby manufactured ₉ that an aqueous colloidal SilieiUMdiosqralösung from ^ belögen on da3 weight 31.656 Ludox HS, (the $ 30 contains SiOg), 0.5 $ latrium "titanate and 67.9 $ water applied WIMO Baa with silica coated substrate is heated währeai 5 mewed to 500 ⁰ C. Then the doorway d @ s liafbrlng © ns and burning alternating coating © of buthenium oxide and silicon dioxide is repeated twice «

The samples have the following composition: 1

coating coarse G sample H

RuO ₉ 100 ^ $ 43.3

SiOg © 56 _s l $

The gravimetric ^ »^ ewiohts ^ uaaliffi © eatspric-M ia" b@ld.e21 Mllea 15.3 x 10 cm (6.1 microineiiee) Eii = Metallo Bas GronioSit an SiOg in the sample H corresponds to 147 x 10 "° @ m (58 Mioroinohes) SiO ₂ .

Samples G and H are used as anodes in a laboratory diaphragm cell and tested for Ohlor overvoltage using the method described in Example 1. Sample G with 3 coats of Rutheniumosiyd has an initial Chlorüb @ £> spazmung "won 155 mV and a Zelipotential of 4.20 Y ₈ Brob® _E:!, Sin © Mefers Me at" EuO ₂ = Siög "coating which according to the Irfiadung established τΜτ & Φ ₉ had an initial Cliis-iHiö @ rspam «ag won 10 ml iisä © ia ^ © llpotential. Of 4.30 V. Pernsr Is-fe ä © v M@hr®eJiioJa.t« => lmO «- Sio «= - coating which is applied in weohselndsn Sohiehtsn" urde _v more adhesive ala the RuOg-öbergixg of? Sample 0 »

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fr

Example 5

Dia samples I and J are made as follows! Two plates of commercially pure titanium, from. 25 χ 7 1.02 mm (1 χ 3 χ Q ₉ CHO in) are coated aiam in a manner similar to that produced in Example 4 b © 3 cshrisbenen, about 12 10 "om (5 microis-mc ^) euthenium oxide Warasn was done on both sides of this document by five assignments of its approach ₉ which is composed of 6 $ rutile sniurachloridf 44 $ 2-propanol and 50 $ linalool _£ , with burning after every contribution to the formation of the lithium oxide coating a formed in the firing conditions i'roben be both ⁰ 500 G for 10 minutes in. air, however, the sample J is anschließersd at 1200 ⁰ O for 30 seconds in air fired examination represents color aeigt® that beld © Proban bestandan mainly of Hutheniumoxyd "

B-side samples are then approx. 250 se 10 "-cm (100 mioroin-

g überEOgeiij, woaei j © but the sample J Wirde then exposed ainsz ⁵ 12OQ temperature of ⁰ G in air for 30 seconds, which is deposited as in Example 4

In order to determine the adhesion of the coatings, the following simple sand test was carried out - each sample? / Was weighed to an accuracy of about 0.1 mg. Sin white rubberized electrical. BaM was pressed firmly on both strings of the samples " vmü then removed <* This sr work ogaag mrd further awsisaal

g jswsils i

Ids Prolj-JH. ^ srisji Qzmi In Aoat-sn gespal-fej ϊ3γ? ε8ο1ι®ε.,) eat. lisis ~

llt miss. 3iaen Strr® το: · hsi ^ Qi? Air dried? Ä @ 2i ciis B ^ Lb-iii aiuruo ^ fs ^ ogsa ϊΐΚ-Q die Senge iss iagüsj äiia? Ah, Bi: ifo ?? ::. S -i'i ~ ii- "y> alt ₉ the

-: U £ f © lgsa: i

"14 -

@ £ »98317U98

BATH ORIGINAL

0.0074 0.0078 0.0250 0.0257 0.0324 0.0335 0.0045 0.0013 89.2 # 96.7 #

B-1000 / B-10000-a

rehearse

• Weight from RuOp on a surface
from 25 x 76 fflm (1 χ 3 in)

Weight of SiO ₂

Weight of the compound
Coating

Weight of the tape test
removed coating

Weight # of after the tape test
remaining coating

The results of this experiment show that the coatings formed by firing at higher temperatures, for example, 120O ⁰ C have improved adhesion.

The characteristic overvoltages dieaer coated anodes depend partially on the strength of the intermediate Rutheniumoxydschicht from · anodes with a Ruthcniumoxydiiberzug of, for example 102 χ 10 "cm (40 microinches) and a porous outer Siliciumdioxjdüberzug that at high temperatures of, for example, 120O ⁰ G in the above Weiae baked do not only have excellent adhesion, but also excellent electrical properties.

This example not only illustrates a method for producing the RuOp and SiOp coatings by deposition of alternating layers of RuOg and SiO _2, but also shows the improved physical and electrical properties of anodes according to the invention.

Example 6

Sample K was prepared in the following W _e ise!

A sheet of commercially available fine titanium measuring 25 x 76 χ 1 »02 mm (1 χ 3 χ 0.040 in) is pre-

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ο parried. The titanium sheet is then in air at 500 C during Fired for 10 minutes to produce a conductive titania surface coating. The purpose of training this Coating consists in improving the spreading of the aqueous ruthenium salt solution.

An aqueous solution containing 25 wt% ruthenium chloride (40 ru) in water is used to apply about 10.1 χ 10 cm (4 microinches) of ruthenium in one application. The coated substrate is fired in air at 700 ⁰ C for 5 minutes and results in the ruthenium oxide-Uberzug.

A commercial Tantalresinatansatz, resinate No. 7522 of Engelhard Minerals 5 Chemicals Corp. is then applied over the Rutheniumoxydschicht and fired in air at 65O ⁰ C for 5 minutes.,. The tantalum resinate paint fired in this way deposits a layer of tantalum oxide. Two applications of the tantalum resinate are applied and each fired separately so that a tantalum oxide coating about 25 x 10 cm (10 microinches) thick is deposited.

The application of a coating of ruthenium oxide under subsequent deposition of two coatings of the tantalum oxide is repeated three times so that an overall deposition is achieved of ruthenium oxide in all layers results in approximately 40.6 χ 10 "cm (16 microinheses) of ruthenium.

Sample K is compared to a low overvoltage anode having a 102 10 Gm (40 microinches) coating of Pt-Ir on a titanium base in a laboratory diaphragm cell as described in Example 1. The sample, be K has a cell potential of 1.350 V and an anode potential

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of 5.50 V. The comparison anode of low overvoltage has a cell potential of 1.240 V and an anode potential from 5.50 V.

The sample K is cut into sections of 2.5 x 2.5 µm (1 χ 1 in) and one section is used as an anode in a laboratory chlorine cell of the diaphragm type at 107 A / dm (1000 ASP) for 110 hours. The change in thickness after use of the cell is determined by the difference in the thickness of the coating used as anode and an unexamined section of 2.5 x 2.5 µm, measured by X-ray fluorescence. After 110 hours of use, the cut showed a decrease in thickness of 0.5 10 egg (Q ₉ 2 microinches). This rate of wear corresponds to 0.041 g of ruthenium loss per ton of chlorine formed.

The outer coating of santal oxide improves the ruthenium oxide separation insofar as it "improves the adhesion of the ruthenium oxide" by inhibiting the electrolysis reaction. It also makes di @ AnoS @ - less prone to mercury algamation »

Example 7

The sample L is produced in the following way A sheet of commercially available pure Ϊitan is used as in Bel ·? game 6 with a thin surface coating of titanium oxide, which is developed on the base by firing in air, prepared. The ruthenium oxide layer is as in Example 6 developed, but 3 coatings d @ s Rutheniumohloridansatses applied and fired so that a ruthenium oxide coating corresponding to about 30.5 x 10 * om (12 microinches) of ruthenium is obtained will. The surface connection of this coating is not good.

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. Two coats of a titanium approach with a content of 7,29b titanium, titanium resinate No. 9428 of Engelhard Minerals & Chemicals Corp., are applied and fired in air at 65O ⁰ C for 5 minutes each, so that a protected gravimetric thickness of about 15 - 25 χ 10 "cm (6-10 microinches) titanium oxide is obtained. The cohesion of the sample L is excellent.

In the examination of the sample L and a comparative anode of low overvoltage mit102 χ 10 "cm (40 microinches) Pt-Ir in a chlorine cell, besehrieben as above _s denotes the sample L with that of the comparison anode of low overvoltage comparable electrical Eigenschaftenι *

Strength of

Ruthenium oxide over anode cell train al3 Ru ₁ 10 ~ ° cm potential potential sample (microinches) (volt) (volt)

L 30.5 (12) I ₉ 260 5.80

Vargl. 102 (40) 1.210 5.75

The outer titanium oxide coating not only improves the adhesion and cohesion of the ruthenium oxide, but also, the electrical properties of the anode coated with titanium oxide are comparable to a conventional one Low overvoltage anode. The coating is less prone to mercury amalgamation than the usual costly. Metal coatings and has a longer lifespan.

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Claims (1)

B-1000 / B-IOOO-a i9 December 22, I969 Claims 1. Anode for the electrolysis of salt solutions consisting of a corrosion-resistant valve metal pad, a thin adhesive porous outer coating layer made of a refractory oxide, the refractory oxide being inert to the Electrolytic environment, and a thin layer of ruthenium oxide between the base and the outer coating. 2. Anode according to claim 1, characterized in that the outer refractory oxide coating is less than 500 χ 10 cm (200 microinchea) thick. 3. Anode according to claim 1 or 2, characterized in that the refractory oxide coating layer consists of an oxide of at least one of the elements Si, Ti, Ta ₁ Nb, Hf, Zr, W or Al. 4 * anode according to claim 3, characterized in that the refractory oxide from silica ". stands" 5. Anode according to claim 3 »dadurah-characterized that the Refractory oxide consists of titanium oxide. 6. Anode according to claim 3 & characterized in that dac Refractory oxide consists of tantel oxide. 7. Electrolytic cell consisting of a mercury cathode and an electrolytic anode, characterized in that the anode made of a corrosion-resistant valve metal base, a thin adherent porous outer cover from a fire r solid oxide, which is inert towards the cell environment, and a thin layer of ruthenium oxide between the base and the outer coating consists of " - 19 - 009831 / U98 ORIGINAL BATHROOM B-1000 / B-10000-a % Q 8. Process for the electrolysis of an aqueous alkali metal chloride solution in a cell with a metal anode of the platinum group, wherein the solution comes into contact with the anode and the Alkali chloride solution is electrolyzed by conducting a current through the solution to a cathode, characterized in that that an anode made of a corrosion-resistant valve metal base, a thinly adhering porous outer surface Coating made of a refractory oxide, which is inert towards the cell environment, and a layer of ruthenium oxide between the backing and the outer cover, is used 9 · Process for the production of an electrolytic anode, which consists of a corrosion-resistant metal base, a thin adhesive porous outer coating of a refractory oxide and a thin layer of ruthenium oxide between the base and the outer coating, characterized in that a) a thin film of ruthenium oxide is formed on the substrate and b) on the ruthenium oxide coating a thin, porous outer coating of high surface area is deposited on the refractory oxide will. 10. The method according to claim 9 »characterized in that the Rutheniumoxydsohicht by depositing a coating from the base, which contains a ruthenium metal or a ruthenium salt, and burning the coated base to a temperature of at least about 400 ⁰ C, is formed. 11. The method according to claim 10, characterized in that the refractory oxide layer is deposited on the thin layer of ruthenium oxide and the anode at a temperature - 20 - 009831 / U98 B-1000 / fc-10000-a "U temperature of at least about 40O ⁰ O is burned. ι 12 * Method according to claim 11, characterized in that \ the refractory Oxydsohicht is deposited on the layer of ruthenium oxide and the anode at a temperature of higher than 600 ⁰ C is burned. 13. The method according to claim 11, characterized in that the refractory oxide coating by applying a coating of a hydrophilic colloidal solution of the refractory
Oxide is deposited on the ruthenium oxide layer, and the applied coating is burned with the deposition of a thin adherent porous refractory oxide coating. 14 * Method according to claim 13t characterized in that a solution containing hydrophilic colloidal silica is used. 15. The method according to claim 11, characterized in that the refractory oxide coating is deposited by applying an approach which contains a compound of the refractory metal in an organic carrier, the
Approach when firing a coating from the appropriate
refractory oxide is deposited, and the applied resin coating is burned with the deposition of the refractory oxide coating 16. The method according to claim 15t, characterized in that the batch contains a tantalum resinate 17 · Method according to claim 15, daduroh marked that 4-part approach contains a pure titanium * - 2Y - BATH ORIGINAL B-1000 / B-1000-a 18. "Process for the production of anodes according to claim 9, wherein the anode has a multilayer coating of ruthenium oxide and the inert refractory oxide, characterized in that, that a second layer of ruthenium oxide is deposited on the refractory oxide coating and subsequently a second refractory layer is deposited on the second ruthenium oxide layer. 19. The method according to claim 9 »characterized in that the base metal is first fired in air to develop a thin oxide coating on the base metal and then the thin layer of ruthenium oxide is formed on the substrate. 20 · The method according to claim 19 »characterized in that the base is made of titanium and titanium oxide is formed on the surface. 21 · Anode, produced according to the method of claims 9 to 20 » \ - 22 - 0091-31 / Uti