NO138664B - ELECTRODE FOR USE IN ELECTROCHEMICAL PROCESSES, ESPECIALLY AS ANODE IN CHLORALKALYEL ELECTROLYSIS - Google Patents

Abstract

Electrode for use in electrochemical processes, especially as an anode in chlor-alkali electrolysis.

Description

The present invention relates to valve metal electrodes

with a semi-conductive surface of titanium dioxide or tantalum oxide or other metal oxides, which is sufficiently conductive to

avoid the passivation that takes place in valve metal electrodes or in valve metal electrodes with a coating of a platinum group metal, when these are used in electrolysis processes, such as e.g. production of chlorine, and where the semiconducting titanium dioxide or tantalum oxide or other metal oxide coating is sufficiently conductive that the electrolysis current can pass from the electrode to an electrolyte with high amperage and low overvoltage for chlorine discharge for longer periods of time.

Electrodes according to the present invention can be used

for the electrolysis of lithium, sodium and potassium chlorides, bromides and iodides, and more generally for the electrolysis of halides, for the electrolysis of other salts subjected to decomposition under electrolytic conditions, for the electrolysis of HCl solutions,

for electrolysis of water etc. They can also be used for other purposes, e.g. in processes where an electrolytic current is passed through an electrolyte for the purpose of decomposing the electrolyte, to effect organic oxidations and reductions, for cathodic protection and in other electrolytic processes. They can be used in mercury or diaphragm cells, and they can have other forms than those specifically described here. Electrodes according to the present invention are particularly useful as anodes for the electrolysis of sodium chloride solutions in mercury cells and diaphragm cells, as they have the ability to release chlorine at low anode voltages as long as one has a semiconducting surface of titanium dioxide, tantalum oxide or other metal oxides, and the electrodes

has low wear (loss of conductive metal per tonne of chlorine produced).

Valve metals such as titanium, tantalum, zirconium, molybdenum, niobium and tungsten have the ability to conduct the current in the anodic direction and to resist a passage of current in the cathodic direction, and they are sufficiently resistant to the electrolytes and the conditions that occur in a electrolytic cell, e.g., when this is used for the electrolytic production of chlorine and caustic soda. In the anodic direction, however, their resistance rises very quickly due to the formation of an oxide layer on the surface, so that it is no longer possible to conduct the current to the -electrolyte in significant quantities without increasing the voltage, which very soon does uneconomical to use uncoated valve metal anodes during electrolysis.

When valve metal anodes are coated with platinum or a platinum group metal, these are also passivated, which in turn causes a rapid rise in voltage after being used for a short period of time at a sufficiently high current density during chlorine discharge. The reduced catalytic activity of the electrode means that the anodic oxidation of the dissolved chloride ion to molecular chlorine gas will only occur at increasingly higher overvoltages.

Attempt to avoid this passivation (after relatively

short service life) by providing titanium or tantalum anodes with a coating of a platinum group metal deposited by an electrolysis or by thermal processes, and where the metal essentially covered the entire surface of the titanium anode, has not been very successful. The coating did not always stick properly, and the consumption of the platinum group metal was very high, which made the results unsatisfactory.

It has long been known that rutile or titanium dioxide

and tantalum oxide have semiconducting properties, either when they were doped with traces of other elements or compounds that disrupt the crystal structure and change the conductivity of the oxide, or when the lattice is disrupted by removing oxygen from titanium dioxide or the tantalum oxide crystals. Titanium dioxide has been doped with tantalum, niobium, chromium, vanadium, tin, nickel and iron oxides and other compounds, thereby changing the conducting or semi-conducting properties. The stoichiometric balance in the titanium dioxide has also been changed by removing oxygen from the crystal lattice. Similarly, the wiring has been changed

the ability in Ta20^ films by ultraviolet irradiation or by other methods, but it has not been proposed so far to use doped titanium dioxide or tantalum oxide to thereby provide a conductive or semi-conductive surface on a valve metal electrode, which is to be used in electrochemical reactions.

Various theories have been proposed to explain the conducting or semi-conducting properties in doped or undoped titanium dioxide, and the same applies to Ta 2 O 2 . In this regard, see e.g. in 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 seems to be no general agreement on

what makes doped titanium dioxide and tantalum oxide acquire semiconducting properties. When other mixed metal oxides are used to produce semiconductors, it is possible that oxides of a metal belonging to a closely related group in the periodic table penetrate into the crystal lattice of another metal oxide by a solution in solids, and there act as a doping oxide which disturbs the stoichiometric structure in the crystals from one of the metal oxides, whereby a mixed oxide coating with semi-conducting properties is obtained.

It is previously known anode coating containing

primarily a platinum group metal oxide and titanium dioxide. IN

these known coatings are the platinum group metal oxides the main conductive materials and the titanium dioxide (or other film-forming metal oxides) dilutes and protects the platinum group metal oxides from cell contact and improves the attachment of the coating to the anode base material, but is a poor conductor. With the present invention, the titanium dioxide is made into a semi-conductor and improves the conductivity of both the titanium dioxide and the platinum group metal oxides in the coating. Using the present invention, increased conductivity is achieved by the titanium dioxide or tantalum pentoxide content in the coatings and the amount of the more expensive platinum group metal oxide in the coatings can be reduced, and further the chlorine overvoltage in the coatings is reduced and the coatings' service life is increased. When using aluminum oxide, the amount of ruthenium oxide can be reduced from 16 to 8 g/m 2 (a reduction of 50%) and furthermore tin oxide can be combined

with e.g. oxides of aluminum or cobalt, give a similar reduction in the amount of ruthenium oxide, Oxides of tantalum, iron, cobalt, vanadium and chromium, also allow a reduction in the amount of ruthenium oxide and give the coatings a longer service life. It is also possible to "tailor" the coatings for particular applications to achieve either a low chlorine overvoltage with a higher oxygen overvoltage, or the opposite (low oxygen overvoltage and high chlorine overvoltage). Higher C^ overvoltage gives less contamination by'Cl2 and higher Cl,,- efficiency. By using different doping agents in the compositions, the resulting coating can, in other words, be given improved special characteristics. Tin oxide can e.g. replace ruthenium oxide up to at least 50% and improve the hardness and adhesion of the coatings to the titanium base material compared to compositions not containing tin oxide.

On the basis of what is thus recognized with regard to the state of the art and the mentioned advantages with other metal oxides, the purpose of the present invention has been to provide an electrode with a valve metal core where this metal core is partially or completely covered with a coating of a semi-conducting mixed metal oxide in sufficient quantity to conduct an electrolysis current from said core to an electrolyte for long periods of time without passivation. The purpose has also been to achieve a better attachment of the metal oxide to the metal electrode than has been the case with the known platinum group coatings.

According to the present invention, an electrode is thus provided for use in electrochemical processes, especially as anode-chloralkali electrolysis, with a core of titanium or tantalum and an oxide coating resistant to electrolyte and electrolysis products on the surface of the core, and this electrode is characterized by the oxide coating consists of an oxide of a platinum group metal and a mixture of metal oxides which form a semi-conductive coating on the core, said mixture of metal oxides forming the semi-conductive coating comprising a material from the group consisting of titanium dioxide and tantalum pentoxide and at least one doping oxide from the group consisting of an oxide of silver, tin, chromium, lanthanum, aluminium, cobalt, antimony, molybdenum, nickel, iron, tungsten, vanadium, phosphorus, boron, beryllium, sodium, calcium, strontium, lead, copper and bismuth, and mixtures thereof, wherein the percentage of doping oxide is between 0.10 and 50% of titanium dioxide or tantalum pentoxide and wherein the ratio between platinum group metal and non-precious metals in the oxide coating is between 20:10 and 85:100, where said percentages are based on the metal in the oxides.

It is usually preferred to prepare a solution of the semiconducting metal and the doping composition in such a form that when it is applied and burned into the cleaned valve metal electrode, the solution will form TiO^ as well as doping oxide, or Ta20,- plus doping oxide,

and then burn this composition in

in the valve metal electrode in several layers so that a solid solution is formed between TiO2 or TiO^

and the doping oxide on the surface of the electrode, whereby a semi-conductive coating will be obtained which will be able to produce a chlorine discharge without an increase in the overvoltage for long periods of time. You can use all solutions or compounds which will form Ti02 and doping oxide or Ta20^ and doping oxide during combustion,

e.g. chlorides, nitrates, sulphides, etc., and the solutions given below are only intended as examples.

By the term "overvoltage" here is meant an excess of voltage in relation to the reversible electromotive force or the electromotive equilibrium force, which must be applied in order to cause the electrode reaction to occur at the desired rate. The chlorine overvoltage varies with the anode material and its physical condition. It increases with the anode current density, but decreases with an increase in temperature.

Semiconducting surfaces of titanium dioxide or tantalum oxide can be produced by doping titanium dioxide or tantalum oxide with various doping compositions, or by disturbing the stoichiometric crystal lattice by removing oxygen, whereby TiO? or Ta20tj can become semiconducting. As Ti02 and Ta^

tends to be reoxidized, then it is preferred to prepare the semi-conducting surface on the electrodes by using doping compositions which form solid solutions with Ti02 or T<a>205 during a burn-in, as such

solutions are more resistant to changes during electrolysis processes. However, in the present electrodes, one can also use semiconducting coatings produced by extracting oxygen from TiO 2 or Ta^O^, in order to thereby produce lattice deficits or lattice defects.

In order to make Ti02~ and Ta205~ crystals semiconducting, one can use various doping compounds, and as such "'can be mentioned W02, P205, Sb^', V 2O 5, Ta205, Nb205, .iB^Og, Cr203, BeO, Na20, CaO, SrO, RuC>2, Ir02, Pb02, 0s02, PtO^,,.Au6?2, Ag20, .Sn02, Al20.j/ and mixtures of these. For Ti02 with a tetragonal rutile structure, the best results have been obtained with doping compositions with equal unit cell dimensions and approximately the same cation radius (0.68 Å).. Ru02 (0.65 Å) and Ir02 (0.66 Å) are consequently very well suited as doping compositions as well as other metal oxides from the platinum group (i.e. . platinum, palladium, osmium and rhodium), when the titanium in the coating solution is converted to the pertitanate, as described in Examples I-IV and VI-X. C. At lower temperatures, smaller amounts of Ir02 will form solid solutions with Ti02, but the amount of oxide that is not included in this solution will still work e as a catalyst for the chlorine discharge.

To produce electrocatalytic and semi-conductive coatings on valve metal electrodes of e.g. titanium and tantalum, you can e.g. use oxides of metals from group VIII in the periodic table, as well as oxides from metals in group VB, group VIB, group IB and group VA, as well as mixtures of these oxides, when these are capable of forming solid solution crystals upon combustion with Ti02 and Ta20^ or disturb the crystal lattice in Ti02 and Ta20,-.

The amount of the doping composition used varies from 0.10 to 50% by weight of the TiO 2 or ^ 2G 5 used in

and even with amounts as small as from 0.25 to 1% by weight, a surprising increase in conductivity can be achieved in TiO 2 and Ta 2 C>5,. Mon

however, prefer to use a sufficient excess of the doping metal oxide to thereby obtain a coating that will catalyze the chlorine discharge without any particular overvoltage.

The conductive coating can be applied in different ways,

and on various forms of titanium and tantalum anodes, e.g. in the form of solid plates, perforated plates, divided plates, different types of cloths, nets, rods or elements of varying shape. The preferred form of application is for the solutions to be painted, dipped or sprayed onto the electrodes and then burned into them, but other methods can also be used, including electrophoretic deposition or electrodeposition. Certain care must be taken so that no air bubbles are embedded in the coating, and that the heating temperature is below the temperature at which the base material shifts or twists.

The spectrum of doped TiC^ samples shows that the foreign ions replace the Ti ion in regular crystal positions and cause a hyperfine splitting in accordance with the nuclear spin of the substituting element.

For all applications, it is preferred that the metal base surface is cleaned and free of oxide or other contaminants. This cleaning can be carried out in any way, by mechanical or chemical cleaning, e.g. by sandblasting, etching, grinding or the like.

Example I

Titanium trichloride in HCl was mixed with methanol, after which the TiCl-j was converted to pertitanate by an addition of ^C^. This conversion is indicated by a color change from TiCl^

(purple) to I^O,. (orange) , An excess of ^ 2°2 ^or ^ was used to ensure a complete conversion to pertitanate. Sufficient RuCl^^^O was dissolved in methanol to give the final and desired ratio of TiO 2 to RuO 2 - The solutions of pertitanic acid and ruthenium trichloride were mixed, and the resulting solution was applied to both sides of a cleaned titanium anode surface by brushing. The coating was applied in a series of layers with an intermediate burn-in at approx. 350°C for 5 min.

After you had obtained a coating with the desired thickness or weight per area unit, the coating was given a final heat treatment at approx. 450°C for 15 minutes to 1 hour. The molar ratios between TiO 2 and RuO 2 can vary from 1:1 to 10:1. These numbers correspond to a ratio of 22.3:47 to 51:10.8 wt% Ti:Ru.

Anodes prepared in the above manner will resist immersion in amalgam and have high electrochemical activity in chlorine cells even after very long periods of operation.

The thickness of the coating can be varied according to the electrochemical needs. A typical coating yielding 46 mg of ruthenium metal and 80 mg of titanium metal in the coating of 38.7 cm 2 of anode surface can be prepared by using 117.9 mg of RuCl^^t^O

(39% Ru metal) and 80 mg titanium metal as TiCl3 (80 mg Ti dissolved in dilute HCl in sufficient excess to maintain acidic conditions). Methanol is added to the titanium trichloride solution, and this is then oxidized with H^C^ to produce the pertitanate. The resulting solution can be painted on a titanium anode surface in several layers with drying or baking at 350°C for 5 minutes between each application. 13 applications were required before the entire solution was applied. Finally, a heat treatment was carried out at 450°C for 1 hour. The molar ratio between Ti and Ru, or TiO 2 to RuO 2 , was 3.65:1. Example II

2

A titanium anode plate with a surface area of 50 cm was cleaned by boiling it at 110°C for 40 minutes in a 20% hydrochloric acid solution. It was then coated with a coating containing the following compounds:

The coating was prepared by first mixing xuthenium-

and the iridium salts containing the required amount of Ru and Ir in a 2-molar solution of hydrochloric acid (5 ml is sufficient), after which the mixture was allowed to dry at a temperature not exceeding 50°C until a dry precipitate formed. The dry salt mixture was at approx. 40°C then added formamide. The titanium chloride dissolved in hydrochloric acid (15% strength) was added to the dissolved Ru-Ir salts, after which a few drops of hydrogen peroxide (30% H202) were added, whereby the solution changed color from blue to orange.

This mixture was then coated on both sides of the cleaned titanium anode in 8 successive layers. After each application, the anode was heated in an oven under air circulation at a temperature of between 300 and 350°C for 10-15 minutes. After each heating, anodes were cooled in air. After application of the eighth layer, the anode was heated to 450°C for 1 hour under vigorous air circulation and then cooled.

The amounts of metal in the coating correspond to a weight ratio of 13.15% Ir, 13.15% Ru and 73.7% Ti, and the content of precious metals in the coating corresponds to 0.2 mg Ir and 0.2 mg Ru per cm 2 of the electrode area. The improved qualities of this anode are believed to be due to the fact that although the three metals in the coating were originally present as chlorides, they are deposited on the titanium surface in oxide form. In this case, you can of course also use other solutions that will deposit the metals in oxide form. During an accelerated test, the above anode showed a weight loss of 0 after three current reversals, a weight loss of 0.152 mg/cm 2 after three amalgam immersions compared to a weight loss of 0.93 mg/cm<2 > for a similar titanium anode coated with ruthenium oxide . After 2000 hours of operation, the above anode showed a weight increase of

0.7 mg/cm 2 , while a similar anode coated with a layer of platinum or ruthenium oxide showed a significant weight loss. The weight gain was apparently stabilised.

Example III

A mixture was applied to a cleaned titanium anode of the same dimensions as in ex. II and by the same procedure. The applied mixture had the following composition:

The method of application was the same as described in ex. II.

The amount of metal in this mixture corresponds to a weight ratio of 22.6% Ir, 22.6% Ru and 54.8% Ti, and the content of noble metal

2 oxides in the coating correspond to 0.4 mg Ir, and 0.4 mg Ru per cm of the active electrode area. After 2300 hours of operation, this anode showed a weight gain of 0.9 mg/cm 2 , and this weight gain had apparently stabilized.

Example IV

Before it was coated, a titanium anode was after a pre-etch as described in ex. II, immersed in a mixture of 1-molar solution of. HgOg and a 1-molar solution of NaOH at 20 to 30°C for two days. The titanium surface was thereby transformed into a thin layer of black titanium oxide.

The same application mixture was used as in ex. II, except that isopropyl alcohol was used as solvent instead of formamide. The use of isopropyl alcohol resulted in a more even distribution of the coating films on the black titanium oxide surface than when formamide was used.

Example V

A titanium anode plate of the same size as in the previous examples was subjected to cleaning and etching as described above, and given a liquid coating containing the following compounds:

The mixture was prepared by first mixing the ruthenium and iridium salts in 5 ml of 20% HCl. The volume of this solution was then reduced to a fifth by heating to a temperature of approx. 85°C. The required amount of TaCl^ was dissolved in boiling 20% HC1, so that a solution containing approx. 8% by weight TaCl^. The two solutions were mixed, and the total volume was reduced to approx. half when heated to 60°C. The specified amount of isopropyl alcohol was then added.

The mixture was applied to both sides of the cleaned titanium anode in 8 successive layers, and the same heating and cooling as in ex. II.

The amounts of metal in the coating correspond to a weight ratio of 10% Ru, 10% Ir and 80% Ta, and the amount of precious metals in the coating corresponds to 0.2 mg Ir and 0.2 mg Ru per cm 2 of the electrode area. During accelerated testing, this anode showed a weight loss of

0.0207 mg/cm 2 after three current reversals and a loss of 0.0138 mg/cm 2 after two immersions in amalgam. After 514 hours of operation, the anode showed a weight loss of 0.097 mg/cm 2 .

Example VI

A titanium anode plate of the same size as in the previous examples was, after cleaning and etching, applied with a coating containing the following compounds:

The mixture was prepared by first mixing ruthenium

and the guli salts in the indicated amount in a 2-molar hydrochloric acid solution (5 ml) and then leave the mixture to dry at a temperature of 50°C. The TiCl^ solution was then added to the Ru-Au salt mixture, and a few drops of hydrogen peroxide were then added,

which caused the solution to change color from blue to orange. Isopropyl alcohol was then added in the indicated amount. The mixture was applied to both sides of the cleaned titanium anode in 8 successive layers, using the same heating and cooling method as described in ex. II.

The amounts of metal in the coating correspond to a weight ratio of 15% Ru, 5% Au, 80% Ti, and the amount of precious metals in the coating

2

corresponds to 0.225 mg Ru and 0.075 mg Au per cm of the electrode area. During accelerated testing, this anode showed a weight loss of 0.030 mg/cm 2 after three current reversals and a loss of 0.04 3 mg/cm 2 after two amalgam immersions. After 514 hours of operation, the anode showed a weight change of +0.2 mg/cm 2 .

Example VII

A titanium anode plate was subjected to cleaning and etching

and then applied a coating composed of the following compounds:

The mixture was prepared by first mixing the dry ruthenium salt in commercial hydrochloric acid solution containing 15% TiCl 2 . The tantalum was then added in the above amount,

and in the form of a solution of 50 g/l TaCl5 in 20% HC1. The titanium chloride was oxidized by adding hydrogen peroxide, after which isopropyl alcohol was added as a thickener. The mixture was applied to both sides of the titanium anode by electrostatic spraying in four successive layers. The number of layers can be varied, and it is sometimes preferred to impose more

coating on the side facing the cathode, and only one coating, preferably the first coating, on the side facing away from the cathode. After each application, the anode was heated in an oven under strong air circulation at a temperature between 300 and 350°C for 10 to 15 minutes. After each heating, it was cooled in air. After the fourth layer was imposed,

the anode was heated to 450°C for 1 hour under strong air circulation and then cooled..

The amounts of metal in the coating correspond to a weight ratio of 45% Ru, 50% Ti and 5% Ta.

During accelerated testing, this anode showed no weight loss after two current reversals and after two immersions in amalgam. Each current reversal consisted of a sequence of 5

anodic polarizations at 1 A/cm 2 , each lasting two minutes and being followed by a cathodic polarization at the same current density and for the same duration. After more than 1500 hours of operation at 3 A/cm 2 in a saturated sodium chloride solution, the anode potential was 1.41 V.

Example VIII

A titanium anode plate was subjected to cleaning and etching

and then applied a coating containing the following compounds:

The mixture was prepared by first mixing the dry ruthenium salt in commercial hydrochloric acid containing 15% TiClg. The tin tetrachloride was then added, after which sufficient hydrogen peroxide was added whereby the solution changed color from blue to orange. Isopropyl alcohol was added as a thickener. The mixture was applied to both sides of a pre-cleaned and pre-etched titanium anode in four successive layers, and each layer was subjected to a conventional heat treatment as described in Ex. VII. The amounts of metal in the coating correspond to a weight ratio of 35% Ru, 55% Ti, and 10% Sn. During accelerated testing, this anode showed a weight loss of 0.09 mg/cm 2 after two current reversals as described in ex. VII, as well as a weight loss of 0.01 mg/cm 2 after immersion in amalgam. After more than 1500 hours of operation in a concentrated NaCl solution at 2 A/cm^ and a temperature of 60°C, the anode potential was 1.42 V,

Example IX

A pre-cleaned titanium anode plate was applied to a mixture consisting of a hydrochloric acid solution containing the following salts:

The mixture was prepared by first mixing the ruthenium and the titanium salts in the commercial hydrochloric acid solution of TiCl^, as described in the previous examples. Aluminum chloride was added in the above quantity, after which the solution was treated with hydrogen peroxide as in ex. VII, whereupon isopropyl alcohol was added as a thickener. The mixture was applied to a pre-cleaned and pre-etched titanium anode in four successive layers, applying the coating on both sides of the anode and on

the exposed area between the top and bottom surfaces of the anode. The heat treatment after each layer was as described in ex. VII.

The amount of metal in the coating corresponds to a weight ratio of 45% Ru, 54% Ti and 1% Al. After a current reversal and two immersions in amalgam, the total weight loss was 0.1 mg/cm 2. After more than 1500 hours of operation in a concentrated sodium chloride solution at a temperature of 60°C and at an anodic current density of 3 A /cm 2 , the anode potential was 1.42 V.

Example X

A titanium plate was subjected to cleaning and etching and then applied a coating containing the following compounds:

The mixture was prepared by first mixing the dry ruthenium salt in a commercial hydrochloric acid solution containing 15% TiCl3. The tantalum was then added in the form of a solution containing 50 g/l TaCl5 in 20% HCl. The blue color of the solution was changed to orange by adding the required amount of hydrogen peroxide, after which isopropyl alcohol was added as a thickener. The mixture was applied to the tantalum anode using a brush in four successive layers. After each application, the anode was heated in an oven under strong air circulation at a temperature between 300 and 350°C for from 10 to

15 minutes. Before the next application, the anode was then cooled. After the fourth layer was applied, the anode was heated to 450°C

for 1 hour under vigorous air circulation and then cooled.

The amounts of metal in the coating correspond to a weight ratio of 45% Ru, 50% Ti and 5% Ta.

An X-ray diffraction analysis indicates that the coatings

on the above-mentioned anodes is in the form of semi-conducting rutile, where the doped oxides have diffused into the rutile crystals in solid solution, whereby a veritil metal anode with a semi-conducting surface of rutile has been obtained, which has the ability to oxidize dissolved chloride ions to molecular chlorine. The coatings can be applied and attached to tantalum electrodes in a similar manner.

While semi-conducting surfaces can be applied to titanium or tantalum surfaces with other doping compositions, tests carried out so far have shown that by using the above-mentioned compositions and coating methods with titanium or tantalum oxide and only iridium, i.e. without ruthenium oxide, a coating with low activity and higher chlorine discharge potential.

Example XI

Electrodes were produced with five different coatings, each consisting of a four-component salt mixture containing a ruthenium salt.

Each sample was prepared by first mixing the ruthenium salt in a commercial hydrochloric acid solution of TiCl^ and then adding hydrogen peroxide in the required amount to obtain

a color change from blue to red. This mixture was then added to the other salts in the indicated amounts together with 0.56 ml of isopropanol for each mg of total amount of metal. The five mixtures were applied to five separate titanium plates in five successive coatings. A heat treatment was carried out at 350°C for 10 minutes between each application. After the last coating, the anode was heated at 450°C for 1 hour.

Anodic tests were carried out in a saturated NaCl solution at 60°C and a current density of 1 A/cm. The electrode potentials obtained were the following:

Example XII

Four coating types were tested, and each consisted of a four-component salt mixture including a noble metal salt. Sample No. 1

The four mixtures were applied to four separate titanium and four separate tantalum plates in five consecutive swipes. Intermediate and final heat treatments are those specified in ex. XI. The anode potentials measured under the same conditions as stated in the previous example were as follows:

Anodes prepared according to Examples 1-10 showed the following advantages when compared with titanium anodes coated with platinum group metals by electrolytic deposition or chemical deposition:

The weight losses for samples according to the present invention were determined under simulated operating conditions and compared with weight losses determined under the same conditions on titanium anodes coated with a Pt-Ir alloy. The tests were carried out in a saturated NaCl solution at 65°C and an anodic current density of 1 A/cm^. The anode potentials were measured using a Luggin tip against a saturated calomel electrode and converted to the normal hydrogen electrode scale. The results obtained are indicated in Table II. The weight change was positive for most samples according to the present invention, and this is an indication that the coating, instead of gradually being worn down and thereby losing its content of metal oxides, tends to have a protective semi-conductive surface applied, and this increase in weight becomes stable very quickly, which is shown by sample C.

If one compares the results in table I with the results in table II, it is clear that even the best precious metal alloys have a higher wear rate, and this is not only due to metal spalling, but also to a lowering of the content of precious metals in the coating. The amount of precious metals in such coatings which is necessary to obtain satisfactory anode activity and a sufficiently long operating time is 5-10 times greater than in the semiconducting rutile or tantalum oxide coatings used in the present invention.

The average thickness of the final coating is 1.45 um, and the ratio of platinum group metals to doping metals in oxide coatings on catalytically active semiconductor coatings according to the invention 1 to 10 varies from 20 to 100 and from 85 to 100.

Although certain theories have been stated in the above to describe the invention, these are only intended as explanations, and one is not bound by these theories if it should turn out that the invention works according to a different principle.

In the present invention, the term "oxide" is understood to mean oxides of titanium and tantalum, either in the form of TiC^ and or other oxides of these metals, as well as oxides of other metals which are able to form semiconducting coatings with oxides from adjacent groups in the periodic table, and the term "precious metals" refers to the platinum group metals, gold and silver. The titanium dioxide can either be in the form of rutile or anatase.

The electrode itself can be made of a valve metal or any metal capable of withstanding the corrosive conditions that occur in an electrolytic chlorine cell, e.g. high silicon iron ("Duriron"), cast or pressed magnetite, etc.

However, it is preferred to use titanium or tantalum. Electrodes according to the present invention can be used in any gaseous or liquid electrolyte, and then especially in aqueous salt solutions or in molten salts. They are dimensionally stable and are not consumed during the electrolytic process when they e.g. used in alkali halide electrolytes, e.g. sodium chloride solutions of the type used for the production of chlorine and sodium hydroxide, and where the present electrodes may form the anodes, while the cathodes may be of mercury, steel or other suitable conductive materials. In mercury cells of the type that e.g. is indicated in the U.S. Patent No. 3,042,602 or No. 2,958,635, or in diaphragm cells of the type disclosed in U.S. Pat. patent no. 2,987,463, the present electrodes can be used as anodes, and they can be used instead of graphite anodes of the type shown in these patents, and which have hitherto been used in cells of this type.

The semi-conductive coatings conduct the electrolysis current from the anode to the electrolyte through which the current goes to the cathode.

Claims (4)

1. Electrode for use in electrochemical processes, especially as an anode in chloralkali electrolysis, with a core of titanium or tantalum and an oxide coating resistant to electrolyte and electrolysis products on the surface of the core, characterized in that the oxide coating consists of an oxide of a platinum group metal and a mixture of metal oxides forming a semi-conductive coating on the core, said mixture of metal oxides forming the semi-conductive coating comprising a material from the group consisting of titanium dioxide and tantalum pentoxide and at least one doping oxide from the group consisting of an oxide of silver , tin, chromium, lanthanum, aluminium, cobalt, antimony, molybdenum, nickel, iron, tungsten, vanadium, phosphorus, boron, beryllium, sodium, calcium,, strontium, lead, copper and bismuth, and mixtures thereof, the percentage of doping oxide is between 0.10 and .50% of titanium dioxide or tantalum pentoxide and the ratio between platinum group metal and non-precious metals in the oxide coating is between 20:10 and 85:100, where said percentages are based on the metal in the oxides. 2. Electrode according to claim 1, characterized in that the dopant oxides consist of tin together with one or more oxides from the group consisting of oxides of tantalum, lanthanum, chromium, aluminium, iron, cobalt and nickel. 3. Electrode according to claim 1, characterized in that the dopant oxide contains tantalum in an amount of 1-15%. 4. Electrode according to claim 1, characterized in that the dopant oxide contains tin in an amount of 5-30%.