Method Of Making An Electrode

Abstract

An electrode and method of manufacture thereof in which a film-forming base is provided with an electrically conductive coating which comprises a mixture containing at least one chemical compound of the film-forming metal and at least one chemical compound of at least one other metal.

Description

BACKGROUND OF THE INVENTION

This invention relates to electrodes for use in electrolytic processes, for example the chlor-alkali electrolysis of brine. Other uses include chlorate, chlorite and hypochlorite production and cathodic protection. These specified uses are given as examples only.

SUMMARY OF THE INVENTION

In accordance with the invention, an electrode for use in electrolytic processes comprises an electrically conductive base of which at least the surface is formed of a "film-forming" metal or a "film-forming" alloy, and an electrically conductive coating on at least part of the surface of the base, the coating comprising a mixture containing at least one chemical compound of the film-forming metal or at least one chemical compound of at least one metallic constituent of the alloy and at least one chemical compound of the or each of at least one other metal.

The term "film-forming" as used in this specification refers to the type of metal or alloy which will form an oxide film when immersed in the electrolyte to which it is to be subjected, the oxide film preventing further corrosive attack upon the metal or alloy. Examples of "film-forming" metals are titanium, tantalum, niobium and zirconium.

The term "mixture" as used in this specification includes within its ambit compounds and solid solutions of the constituents concerned.

In accordance with the invention also, a method of manufacturing an electrode for use in electrolytic processes comprises taking an electrically conductive base of which at least the surface is formed of a film-forming metal or a film-forming alloy, and applying to at least part of the surface of the base an electrically conductive coating comprising a mixture containing at least one chemical compound of the film-forming metal or at least one chemical compound of at least one metallic constituent of the alloy and at least one chemical compound of the or each of at least one other metal.

Preferably the mixture comprises at least 50 percent of at least one chemical compound of the film-forming metal or of at least one metallic constituent of the alloy, with not more than 50 percent of at least one chemical compound of the or each of at least one other metal.

Preferably, the film-forming metal or alloy is titanium or a titanium-base alloy whereby said at least one chemical compound of the film-forming metal or of at least one metallic constituent of the alloy is at least one chemical compound of titanium. Alternatively, the film-forming metal or alloy can be tantalum or niobium or film-forming alloys including those elements. Zirconium can also be used provided that, in service, it will not contact halides. A suitable film-forming alloy is titanium-0.15 wt. percent palladium.

Preferably also the whole of the base is formed of the film-forming metal or alloy, but if required the base may comprise an electrically conductive core which is protected from corrosion by the electrolyte by an impervious layer of the film-forming metal or alloy which thereby provides the surface of the base. The core can be provided to enhance the electrical conductivity of the base, or to reduce its cost. A suitable core material is copper.

Preferably also said at least one chemical compound of the or each of at least one other metal is at least one chemical compound of at least one of the Group VIII metals. The Group VIII metal can be a metal of the platinum group, by which is meant ruthenium, rhodium, palladium, osmium, iridium and platinum, or it can be iron, cobalt or nickel. If more than one metal of Group VIII are used, examples are platinum with iridium or ruthenium, platinum with iron, and iron with cobalt and nickel. Other metals then those of Group VIII can be used, for example manganese.

The mixture may also comprise the metals concerned as well as chemical compounds of each of them. Thus, as an example, if the metals are the film-forming metal titanium and ruthenium, the mixture will comprise at least one chemical compound of titanium with at least one chemical compound of ruthenium, and can include some titanium metal and some ruthenium metal.

The chemical compounds are preferably all oxides, although one or more of them may be borides, carbides, nitrides, fluorides, sulphides, aluminides, or silicides.

Preferably further, the coating also comprises an underlayer beneath said mixture, at least 95 percent of said underlayer consisting of at least one chemical compound of the film-forming metal or of at least one metallic constituent of the alloy. Preferably, said at least one chemical compound of the underlayer is the same chemical compound or compounds as the chemical compound or compounds of that metal or those metals in the mixture.

Each chemical compound of at least one other metal may also be a chemical compound of the said film-forming metal or of at least one metallic constituent of the film-forming alloy.

There are various methods of producing the electrodes of this invention, each method depending upon the following sequence of stages:

1. The material and condition of the electrically conductive base.

2. An initial treatment of the base.

3. The coating applied to the base.

4. Any treatment to the coating or to a number of coatings.

5. Any subsequent treatment.

Thus, regarding Stage 1, in all of the experiments which are referred to in the following examples, the base of the electrode was chosen to be wholly a film-forming metal. Examples are commercially pure titanium and commercially pure tantalum. The chosen metal was fabricated into the form of the required specimen electrodes.

For Stage 2, various alternatives can be used, and it must be borne in mind that titanium normally has a surface film of titanium dioxide having a rutile structure. A satisfactory method of removing substantially all of this rutile film is an etch in a 10 percent solution of oxalic acid for 16 hours at 80.degree. C. Thus, the term "etching" as used in this specification refers to this treatment with oxalic acid. For tantalum the oxide film can be prepared for coating by vapor blasting. Various possible methods of carrying out Stage 2 for titanium and tantalum are as follows:

a. A heat-treatment of titanium under vacuum at 700.degree. C. for about 30 minutes. On subsequent exposure to air the surface is probably covered with an oxide film to a thickness of about 20 A.

b. Titanium as etched.

c. An anodic treatment of a titanium-base at 20 volts for a matter of seconds in an electrolyte which is typically 5 percent sulphuric acid. The electrolyte composition is not critical and others which may be used include phosphoric acid, phosphoric/sulphuric/water mixtures and ammonium sulphate.

d. The same anodic treatment for titanium as (c), but at 100 volts. Again the treatment is for only a matter of seconds as the voltage is increased to 100 volts and then decreased to zero. Treatments (c) and (d) produce a titanium dioxide film of up to 2,000 A. in thickness, the anatase modification being usually formed.

e. A heat-treatment of titanium in air at about 450.degree. C. for about 30 minutes.

f. A heat-treatment of titanium in air at about 600.degree. C. for about 30 minutes. The air heat-treatment increases the thickness of the naturally occuring rutile-type coating, but it probably does not exceed 2,000 A.

g. For titanium, no treatment except a degreasing operation.

h. For tantalum, a vapor-blasting treatment.

After this initial treatment of the titanium or tantalum surface, Stage 3 can be carried out with deposition onto different specimen electrode bases, of any one of aluminum, chromium, cobalt, germanium, iridium, iron, lead, manganese, nickel, palladium, platinum, ruthenium, selenium, tin and tungsten metals. This metal deposition can be carried out by vaporizing the coating metal in vacuum alongside the titanium or tantalum specimen. The thicknesses achieved can be varied, but preferably each treatment is carried out with the intention of producing a thickness of about 100 A. For the platinum metal coatings on titanium, measurements were taken on specimen electrodes which showed thicknesses of 25, 100 and 300 A., and for nickel a thickness of 400 A. In addition, nickel, cobalt and iron can be deposited on a single titanium specimen as successive layers in that order, each of about 100 A. in thickness.

Coatings of the platinum group metals can also be applied by the use of suitable organic metal paints. For ruthenium, an alcohol solution of ruthenium chloride with a suitable reducing agent can be used. This is referred to as "RuCl.sub.3 paint." Alternatively, these paints can be used as a mixture with organic titanium paint for titanium specimens.

For tantalum specimens coatings can be applied as mixed resinate paints of tantalum and ruthenium with tantalum metal to ruthenium metal ratios of 1:1, 2:1 and 3:1.

For stage No. 4, for the metals applied by evaporation, this treatment can be one of the following:

a. A heat-treatment in vacuum at temperatures from 450.degree.-800.degree. C. for about 30 minutes. This is preferably used for titanium samples which have already been subjected to an oxidizing treatment of the titanium surface.

b. A heat-treatment in air at temperatures in the range 200.degree.-800.degree. C. for about 30 minutes.

c. An anodic treatment in sulphuric acid at 20, 40, 60 or 100 volts. For platinum metal coatings, these can be oxidized by connecting the specimen as an electrode and immersing it in a 6 percent brine solution, and then subjecting it to an alternating current of 5 volts at 50 cycles per second for about 30 minutes.

For the painted surfaces, each paint layer can be subjected to a heat-treatment in air for 10 minutes at 250.degree. C., and then 20 minutes at 450.degree. C. Two coats of paint are preferably applied in each case with this heat-treatment applied after each coat. Alternatively, alternate paint layers of titanium paint and RuCl.sub.3 paint can be applied to titanium bases, the same heat-treatments being used. Four layers are preferably applied altogether.

For stage No. 5, electrodes can be given a 20 minute treatment in an equal parts ammonia-butane mixture at 450.degree. C. Another last stage treatment which can be given is immersion in an oxidizing bath of molten commercial grade sodium nitrate at from 450.degree. C. up to about 600.degree. C. Typically immersion is extended for about 30 minutes, although times of up to about 60 hours can be used.

In a further method of producing the required electrode of the invention, an electrically conductive base of which at least the surface is of a film-forming metal or alloy is first subjected to a preparation process and then has precipitated thereon the required mixture of chemical compounds. This may be carried into effect by treating the film-forming metal or alloy with an acid corrosive thereto for sufficient time to dissolve some of the film-forming metal or alloy, adding to the acid a source of ions of the required other metal or metals, and causing precipitation of a mixed oxide of the film-forming metal or one constituent of the film-forming alloy and of the other metal or metals on to the base.

As an example, a titanium-base can be treated with boiling sulphuric acid for at least 1 hour, and ferric chloride is then added to the solution followed immediately by an oxidizing agent such as potassium chlorate. The sulphuric acid prepares the titanium surface for coating, and dissolves some titanium as Ti.sup.-.sup.-.sup.- ions. Oxidation converts the Ti.sup.-.sup.-.sup.- and the Fe.sup.-.sup.-.sup.- ions to Ti.sup.-.sup.-.sup.-.sup.- and Fe.sup.-.sup.-.sup.-.sup.- ions which are unstable and will coprecipitate as a mixed titanium and iron oxide.

Alternative methods of oxidizing the solution are the uses of palladium, the application of an external current with the titanium base positive with respect to a cathode, and bubbling air through the solution.

DESCRIPTION OF PREFERRED EXAMPLES OF THE INVENTION

Example 1

A commercially pure titanium base was fabricated and then subjected to a vacuum treatment at about 700.degree. C. for about 30 minutes. After exposure of the base to air, a metallic coating of manganese was applied by vaporizing a manganese sample in vacuum alongside the base.

The coated base was treated in air at about 450.degree. C. for about 30 minutes to produce a specimen electrode provided with a coating containing a mixture of titanium and manganese oxides. The coating contained more titanium oxide than manganese oxide and may contain some manganese metal.

The electrode thus produced was given a conductance test by being connected as an anode in a 22 percent by weight solution of brine at room temperature. A titanium cathode was located 5 cm. from the anode, and 5 volts of direct current were applied.

The specimen electrode was electrically conductive, initially passing a current of 2.5 kiloamperes/m..sup.2. The average current density between 5 and 60 minutes operation was 0.6 ka/m..sup.2.

Example 2

The materials and processes of Example 1 were followed with the use of a nickel layer about 400 A. in thickness instead of manganese. The resulting coating contained a mixture of titanium and nickel oxides.

Using the same conductance test, the electrode initially passed a current of 2.5 ka/m..sup.2. The average current density between 5 and 60 minutes operation was 2.1 ka/m..sup.2, and between 1 and 10 hours 0.9 ka/m..sup.2.

Example 3

A titanium base was anodized at 20 volts in a 5 percent sulphuric acid electrolyte for a few seconds to produce an oxide film of about 2,000 A. in thickness. The base was then provided with a cobalt coating and heat-treated as described for manganese in Example 1 to produce mixed coating containing titanium and cobalt oxides.

The described conductance tests were used, the current densities being 1.8, 1.4 and 1.2 ka/m..sup.2 initially, from 5-60 minutes and from 1-10 hours respectively.

The initial value of the potential between the brine solution and the specimen electrode (initial overpotential) was also measured and was found to be 2,150 millivolts.

Example 4

A titanium base was oxidized in air at about 450.degree. C. for about 30 minutes. This produces a thickened oxide film up to about 2,000 A. thick. The base was then provided with an iron coating about 100 A. in thickness by vacuum deposition, as described in Example 1.

The coated base was subjected to vacuum at about 450.degree. C. for about 30 minutes to diffuse some of the oxygen content of the titanium oxide film into the iron coating. In this way there was produced a specimen electrode having an underlayer of which at least 95 percent was a titanium oxide, and a coating on the underlayer comprising a mixture of oxides of iron and titanium.

Using the conductance test described, current densities of 3.1, 2.0 and 0.06 ka/m..sup.2 were measured initially, from 5-60 minutes and from 1-10 hours respectively.

Example 5

A titanium base was etched in oxalic acid using the etching procedure described above, and was then provided with subsequent layers of nickel, cobalt and iron, each layer being about 100 A. in thickness. The layers were each deposited in turn by vacuum deposition as described in Example 1. The heat-treatment at 450.degree. C. of Example 1 was then applied to produce a coating on the titanium base comprising a mixture of oxides of titanium, nickel, cobalt and iron.

The described conductance tests were used, the current densities being 3.1 and 1.9 ka/m..sup.2 initially and from 5-60 minutes respectively. The initial overpotential was found to be 350 millivolts.

Example 6

The procedures of Example 5 were followed with the substitution of platinum for the vacuum deposition.

After air oxidation, the coating comprises a mixture of oxides of titanium and platinum, and some platinum metal.

Using the conductance test, current densities of 3.4, 2.9 and 0.3 ka/m..sup.2 were measured initially, from 10-100 hours and from 200-300 hours respectively.

Example 7

A titanium base was air oxidized as described in Example 4 and was then coated with platinum and air oxidized as described in Example 6.

Conductance test measurements taken at the same times as those of Example 6 showed 2.5, 2.1 and 1.6 ka/m..sup.2 respectively.

Example 8

A titanium base was anodized as described in Example 3, and then provided with a platinum coating and air oxidized as described in Example 6.

Conductance test measurements taken initially and from 10-100 hours showed current densities of 2.5 and 2.1 ka/m..sup.2 respectively.

Example 9

A titanium base was anodized as described in Example 3 with the use of a potential of 100 instead of 20 volts. The base was then provided with a platinum coating as described in Example 6.

The coated titanium base was subjected to the same vacuum treatment as described in Example 4.

Conductance test measurements taken initially and from 10-100 hours showed current densities of 2.5 and 1.8 ka/m..sup.2 respectively. The initial overpotential was measured and it was found to be 580 millvolts.

Example 10

A titanium base was etched as described in Example 5 and was provided with a palladium coating by the evaporation of a palladium sample alongside the base in vacuum.

The coating was oxidized in air at about 450.degree. C. as described in Example 1. This produced a mixed oxide coating on the surface of the base of titanium and palladium, the coating containing some palladium metal.

Conductance test measurements taken initially, from 10-100 hours and from 200-300 hours showed current densities of 3.7, 1.5 and 1.2 ka/m..sup.2 respectively.

Example 11

A titanium base was provided with a coating as described in Example 10, except that air treatment was carried out at 350.degree. C. The initial overpotential was found to be 340 millivolts.

Example 12

A titanium base was etched as described in Example 5, and was then provided with two coats of an organic palladium paint. For each coat of paint the base was subjected to a heat-treatment in air for 10 minutes at 250.degree. C. and then 20 minutes at 450.degree. C. This produced a coating on the titanium base comprising a mixture of titanium and palladium oxides.

Conductance test measurements were taken initially, from 10-100 hours and from 200-300 hours and showed current densities of 4.6, 3.4, and 1.2 ka/m..sup.2 respectively. The initial overpotential was measured and found to be 151 millivolts.

Example 13

A titanium base was etched as described in Example 5 and was then provided with two coats of an organic palladium paint mixed with an organic titanium paint. For each coat of paint, the base was subjected to a heat-treatment in air for 10 minutes at 250.degree. C. and then 20 minutes at 450.degree. C. This produced a coating on the titanium base comprising a mixture of titanium and palladium oxides.

Conductance test measurements were taken initially, from 10-100 hours and showed current densities of 2.8 and 1.5 ka/m..sup.2 respectively. The initial overpotential was measured and found to be 400 millivolts.

Example 14

In this example the same processes as those described in Example 12 were followed, except that each paint layer was provided with a single heat-treatment in air at 650.degree. C. for about 20 minutes.

Conductance test measurements taken initially, and from 10-100 hours showed current densities of 3.7 and 2.1 ka/m..sup.2 respectively.

Example 15

In this example the same processes as those described in Example 12 were followed, except that a ruthenium organic paint was used instead of a palladium organic paint.

Conductance test measurements taken initially, from 10-100 hours and from 200-300 hours showed current densities of 4.6, 3.7 and 2.5 ka/m..sup.2 respectively. The initial overpotential was found to be 3 millivolts.

Example 16

In this example the same processes as those described in Example 13 were followed, except that ruthenium paint was used instead of palladium paint.

Conductance test measurements over the same periods showed current densities of 3.1 and 2.1 ka/m..sup.2 and the initial overpotential was 190 millivolts.

With mixtures of organic paints it is believed that mixed oxides of the constituent metals are formed in the coatings on the electrode bases.

Example 17

A titanium base was oxidized as described in Example 4, and was provided with a painted coating as described in Example 15 of ruthenium organic paint.

A single heat-treatment in air at 450.degree. C. for about 20 minutes was used for each coat.

The specimen electrode was then subjected to a 20 minutes treatment in an equal parts ammonia-butane mixture at 450.degree. C.

The initial overpotential was found to be 28 millivolts.

Example 18

In this example the processes of Example 17 were followed except that prior to the ammonia-butane treatment, the electrode was immersed in an oxidizing bath of molten commercial grade sodium nitrate at about 450.degree. C. for about 30 minutes.

The initial overpotential was found to be 190 millivolts.

Example 19

In this example the processes of Example 17 were followed, except that, as an organic paint, there was used an alcohol solution of ruthenium chloride with a reducing agent. The electrode was not subjected to the ammonia-butane treatment.

The initial overpotential was found to be 115 millivolts.

Example 20

In this example, the processes of Example 19 were followed, there being an additional oxidizing stage in the immersion of the electrode in the sodium nitrate bath described in Example 18.

The initial overpotential was found to be 27 millivolts.

Example 21

A titanium base, after being degreased, was provided with two coats of a mixed paint of organic titanium and an alcohol solution of ruthenium chloride with a reducing agent. Each coat was treated in air at 450.degree. C. for about 20 minutes.

The resulting electrode had a coating comprising a mixture of oxides of ruthenium and titanium.

The initial overpotential was found to be 54 millivolts.

Example 22

In this example the processes of Example 21 were followed with the addition of a sodium nitrate bath treatment at 450.degree. C. for 30 minutes.

The initial overpotential was 10 millivolts.

Example 23

A titanium base was degreased and was provided with alternate coats of ruthenium chloride paint described above and an organic titanium paint. Each coat was subjected to an air treatment at 450.degree. C. for about 20 minutes. The first coat was of titanium paint, and four coats were applied altogether.

The initial overpotential was found to be 20 millivolts.

Example 24

In this example, the processes of Example 23 were followed with the addition of a final treatment in a molten sodium nitrate bath at 450.degree. C. for about 30 minutes.

The initial overpotential was found to be 17 millivolts.

Example 25

A tantalum base was degreased and vapor blasted, and was then provided with four coats of a mixture of equal parts of ruthenium and tantalum organic paints. The metal to metal ratio of ruthenium to tantalum was approximately 1:1.

Each coat was subjected to an air treatment at about 250.degree. C. for about 10 minutes and further treatment at about 450.degree. C. for about 20 minutes.

A conductance test was carried out between the specimen electrode as an anode and a titanium cathode, and with an applied potential of 6 volts the initial current passed was 1.5 amps and measurements taken after 10 hours and 100 hours showed currents of 1.43 and 1.12 amps.

Example 26

In this example the processes of Example 25 were followed except that the paint was a 2:1 ratio of tantalum to ruthenium paints. Thus the metal to metal ratio was about 2:1.

For the conductance test again at 6 volts, the initial current passed was 1.3 amps.

Examples 25 and 26 were compared with a control of a platinum electroplated titanium base of the same dimensions as the electrodes of Examples 25 and 26. In the same conductance test the control initially passed a current of 1.3 amps, and the same current was still flowing after 10 hours.

The initial overpotentials of the electrodes of Examples 25 and 26 compared favorably with the initial overpotential of the control at the usual current densities of about 6 ka/m..sup.2.

Example 27

A titanium base was degreased and treated in boiling 7 percent sulphuric acid for about 1 hour. This dissolved some titanium as Ti.sup.-.sup.-.sup.- ions, and prepared the titanium surface for coating.

The sulphuric acid solution was then provided with 100 ml. of 0.5 molar solution in water of ferric chloride to provide a source of Fe.sup.-.sup.-.sup.- ions in the solution, and this was immediately followed by the addition of 75 ml. of a 0.1 molar solution in water of potassium chlorate to the solution. The oxidizing effect of potassium chlorate is believed to convert the Ti.sup.-.sup.-.sup.- and the Fe.sup.-.sup.-.sup.- ions to Ti.sup.-.sup.-.sup.-.sup.- and Fe.sup.-.sup.-.sup.-.sup.- which, because of their instability react with water from the solution to coprecipitate as the relatively insoluble mixed titanium and iron oxide on the titanium base.

The electrode so formed was tested in a saturated sodium chloride solution at room temperature, current passing with an applied voltage of 8 volts being 1.5 amps. A platinum electrode plated titanium electrode of the same dimensions used as a control passed a current of 1.2 amps.

The current passed by the electrode of this example was still at the same level after more than 70 hours, and there was no loss in weight which indicated that no dissolution of the electrode was taking place.

In every case, the coating produced upon the electrode comprises a mixture of oxides of titanium or tantalum and oxides of the metal concerned. The portion of oxide of the nontitanium metal varied between 5 and 50 percent of the overall oxide composition of the coating. In some cases the coating also comprises the metal concerned as a metal and not an oxide.

In the cases where a rutile coating was permitted to remain, or was produced on a titanium surface, the mixture of oxides was provided with an underlayer consisting almost entirely of rutile titanium. Any other substances in this underlayer were present by way of contamination, for example because of original impurity, or by diffusion from the mixture, or were titanium metal.

Claims

We claim:

1. A method of manufacturing an electrode for use in electrolytic processes comprising providing an electrically conductive base of which the surface is formed of a film-forming metal or alloy selected from the group consisting of titanium, tantalum, niobium and zirconium and alloys based upon at least one of these metals, treating the base with an acid which is corrosive to the film-forming metal or alloy to dissolve some of the film-forming metal or alloy from the base to provide ions of the film-forming metal, adding to the acid a source of ions of at least one other metal, providing an oxidizing means for producing oxides of said film-forming metal and of said other metals and precipitating upon the base a mixture of oxides of said film-forming metal and of said other metal.

2. A method according to claim 1 wherein the base is of titanium, the acid is boiling sulphuric acid, ferric chloride is added to the acid to provide a source of iron ions, and precipitation of a mixed oxide of titanium and iron is effected upon the base.

3. A method according to claim 2 wherein the oxidizing means is the use of potassium chlorate.

4. A method as in claim 2 wherein the oxidizing means is air bubbled through the acid.

5. A method as in claim 2 wherein the oxidizing means is an external electrical current with the titanium positive with respect to a cathode.