NO137324B - PROCEDURES FOR PRODUCING ELECTRODES SUITABLE FOR USE IN ELECTROLYTICAL PROCESSES. - Google Patents

Description

The present invention relates to a method for producing electrodes suitable for use in electrolytic processes. Examples of such electrolytic processes are chlorine-alkali electrolysis, per-site electrolysis, electro-plating and cathodic protection.

By the term "film-forming metal" is understood herein a metal or an alloy which has anodic polarization properties similar to those exhibited by titanium and titanium alloys. The film-forming metals and alloys referred to in the present specification are titanium and titanium alloys, tantalum and tantalum alloys, niobium and niobium alloys, and zirconium and zirconium alloys.

It has previously been proposed, i.a. in British patent

no. 925 080, a method for producing an electrode comprising a core of titanium and a porous coating of a metal from the platinum group, where the titanium core is provided with a barrier layer by anodization or by oxidation before the coating is applied to the core.

In the said patent it is stated that the advantages of such a method are that it avoids the necessity, before coating with a metal from the platinum group, to remove the oxide film which usually

as occurs on titanium, further the certainty that the titanium will be protected against corrosion by the barrier layer, which would be of importance if the platinum metal coating was damaged, and further that one avoids any need to remove the barrier layer when a fresh coating of a metal from the platinum group is to be applied, and finally the ease with which an adherent coating of the platinum group metal can be provided.

The present invention involves significant improvements when it comes to the application of the barrier layer to the film-forming metal in the manufacture of electrodes of the aforementioned kind, and significant advantages are achieved. The invention thus relates to a method for the production of electrodes suitable for use in electrolytic processes, where on an electrode substrate in which at least the surface consists of a film-forming metal of titanium, tantalum, niobium, zirconium and/or alloys based on at least one of these metals, an electrically conductive, electrolyte-resistant layer containing a metal from the platinum group or an oxide of a metal from the platinum group is applied, and the method is characterized in that a further layer consisting of an oxide of a film-forming metal, using a solution containing metal ions of the film-forming metal, from which the layer of an oxide of the film-forming metal is separated. The layer of an oxide of film-forming metal is preferably of the same metal as that which forms the surface of the electrode substrate.

Preferred embodiments of the method according to the invention are indicated in the subclaims. According to such an embodiment, one or more layers each consisting of an oxide of the film-forming metal are separated from the solution before applying the layer containing a metal from the platinum group or an oxide of a metal from the platinum group. Preferably, the layer or at least one of the layers of an oxide of the film-forming metal is precipitated from a solution by chemical cleavage of a dissolved compound of the film-forming metal. Alternatively, the layer or at least one of the layers is precipitated by an oxide of a film-forming metal by electrolysis of the solution.

As an example of the chemical cleavage method in connection with titanium should be mentioned: an aqueous solution is prepared containing trivalent titanium ions. These are oxidized, e.g. by bubbling air through the liquid, whereby the titanium is obtained in the tetravalent state. When the electrode substrate has been immersed in the solution, the solution is heated to near the boiling point, which has the effect of hydrolysing the titanium so that titanium dioxide is precipitated on the titanium surface. It is assumed that the reactions that take place here can be symbolized by the following, simplified reaction equations:

Using a 10% by weight sulfuric acid containing 1000 ppm titanium in the tetravalent state, the rate of weight increase of the titanium dioxide film is uniform and is approximately 5 g/m 2 /day. This rate may be varied by boiling the solution, or by using the solution at a temperature just below its boiling point, but the titanium dioxide must not be applied at too great a rate, as a non-adherent deposit may then form. The maximum weight of the titanium dioxide film that can be satisfactorily obtained is in the range of 20 to 30 g/m 2 ; above this amount a white non-adherent film forms and peeling may occur.

When the desired weight of titanium dioxide has been precipitated, preferably in the range of 2.5 to 20 g/m 2 , a coating of oxidized metal from the platinum group is applied. In the case of ruthenium oxide, a coating material is used which is heated or baked in at 500°C for 20 minutes in air. An optimal coating material is obtained by dissolving 60 g/l of RuCl^•3H20 in a suitable alcohol saturated with ammonium chloride. The material can, for example, applied with a broom so that it dries within a few minutes of application.

As an example of an electrolysis method in connection with titanium, an aqueous solution containing trivalent titanium ions is also prepared here. An electrode is immersed in the solution and connected as anode to a cathode of a suitable metal, which is also placed in the resolution. With the solution close to the boiling point, a voltage of typically 12 volts provides a well-adherent coating on the electrode substrate. A coating speed of approx. 2 g/m 2/hour.

Preferably, the layer containing a metal of the platinum group or an oxide of a metal of the platinum group is applied by thermal cleavage of an iron-on coating. The iron-on coating is split, e.g. by heating it at 450 to 700°C in an oxygen-containing atmosphere for a time from 5 minutes to 1 hour. The coating can be subjected to a subsequent heat treatment in an ok-pathogenic atmosphere for up to 20 hours. If the coating contains ruthenium, the brazing temperature can go up to and include 700°C.

Typical examples of the invention will now be described

in the following:

EXAMPLE 1

An electrode substrate consisting of commercial grade titanium with a low content of impurities in the annealed and chemically cleaned state is coated, without chemical etching in this example, with a titanium oxide layer of 10 g/m 2. On top of this layer

2

15 g/m of oxidized ruthenium is precipitated.

The chemical precipitation of titanium dioxide is achieved in the following way: Titanium of commercial purity is dissolved in heated 10% by weight sulfuric acid to a purple/blue solution

3+

which is typical for a Ti solution. This solution is diluted to a concentration of 1000 ppm titanium per litres, and the titanium ions are oxidized to the tetravalent state by bubbling air through the solution until the purple color disappears and the solution becomes colourless. When this solution is close to the boiling point, or boils weakly, the titanium electrode substrate is suspended vertically in the solution for a time during which chemical precipitation of titanium dioxide takes place. An immersion period of 24 to .48 hours results in a deposit of 5-10 g/m 2'. After the coating period, the electrode is washed in water, brushed carefully with a nylon brush to remove any loose coating and dried.

The oxidized ruthenium is obtained by applying a solution containing 60 g/l of Rud^^^O dissolved in an alcohol and saturated with ammonium chloride. The material is applied to the titanium with the help of a nylon brush with several coats. The heating or burn-in is carried out after every other coat <p>g consists in exposing the coating to circulating air at 500°C for 20 min.

The electrode of Example 1 was subjected to an electrolytic test in which a coated blade edge or strip, 30 mm x 1 mm, was placed 2 mm above a mercury surface. Salt solution with a concentration of 22 g/l at 70°C was passed between the electrodes, and the test surface was made anodic to the mercury at a real current density of 4 amps/cm 2. The electrolysis time at a low cell voltage exceeded that obtained by depositing the same amount of oxidized ruthenium on an etched titanium surface without applying the chemically deposited titanium dioxide layer. This is a demanding test with a short anode-to-mercury distance and a high current density.

EXAMPLE 2

An electrode consisting of commercial grade titanium

with a low content of contaminants are used in air tu.t annealed and: chemically cleaned condition. The surface of the electrode substrate was degreased with. trichlorethylene vapor, but not etched, and two layers of oxidized-ruthenium were applied by brushing and heating-or burning in with. it. method described in example 1. 10 g/m of titanium dioxide is deposited on this surface using the technique described in example 1.

To complete the coating, a further 4 g/m 2 of oxidized ruthenium is applied by the method used to deposit the first two layers. A layer of such a sandwich construction gave good performance in electrolytic testing at the current working method. described above in example 1.

EXAMPLE 3

I., this example became a way of working very similar to it. as

is described - in example 2 used, except that - the first precious metal coating on the titanium was carried out with: 70/30 platinum/iridium. One layer of "Hanovia IRI" material was applied which was dried for 15 minutes at 250°C followed by heating for 20 minutes at 4:50°C, both heatings with air. On this layer, 10 g/m 2 of titanium dioxide is precipitated using the sulfuric acid method; and. finally 4 g/m 2 of ruthenium metal as oxidized ruthenium. This coating also gave good electrolytic properties.

Electrodes - prepared using examples 1, 2 and. 3 turned out to exhibit a. chlorine overpotential with a current density of around 1 ampere/cm 2 , which can at least be compared with good results. the. which is achieved with a titanium electrode equipped with: several layers of pure ruthenium oxide.

In modifications of examples 1, 2 and 3, film-forming metals other than titanium can be used for the electrode substrate, e.g. niobium, tantalum and zirconium when the chlorine environment is avoided. In addition, applied layers of titanium dioxide can be replaced with layers of oxides of other film-forming metals. It is not necessary that the same film-forming metal is used in the applied oxide layer as in the electrode substrate. Thus, a layer of titanium dioxide can be applied to an electrode substrate made of tantalum.

Furthermore, the film-forming metal oxide can: be applied from;

a spread-on composition by heating in air a spread-on--

material containing a suitable organic compound. For titanium, an application material containing isopropyl titanate can be used.

Furthermore, the electrode substrate can either consist exclusively of the film-forming metal, or it can be equipped with a core of another metal, e.g. copper, to improve electrical conductivity.

EXAMPLE 4

An electrode substrate made of titanium, of commercial purity, is etched in 10% by weight oxalic acid for 8 to 16 hours. A positive potential of 12 volts is then applied to the substrate in front of a lead cathode, and the substrate and the cathode are immersed in a 7% sulfuric acid solution containing 5. g/l of titanium as Ti 3+ ions... The solution is heated to and maintained at 90°C. A coating of a titanium dioxide layer is deposited on the electrode substrate at a rate of approx. 2 g/m 2 /hour. A coating of 15 g/m² was formed.

After the coating, the electrode substrate is washed in water

and dried. The titanium dioxide coating proved to adhere very well to the titanium substrate.

The electrode substrate is then given an electrically conductive coating by dissolving iridium chloride and ruthenium chloride in n-butyl alcohol to form an application composition, and this is applied in several layers to the electrode substrate over the titanium dioxide, each layer is dried and every second layer is heated in air at 500°C for 20 minutes. The electrically conductive coating showed good adhesion to the titanium dioxide and was a mixture of iridium metal or

-oxide and ruthenium oxide, which were present in an amount of approx.

15 g/m . The ratio between iridium and ruthenium was varied between 3:1, 1:1 and 1:3 with respect to the metal content using different coating compositions for the different electrode substrates. Satisfactory electrodes were obtained.

EXAMPLE 5

The procedure according to example 4 was repeated with the modification that an application composition in the form of ruthenium chloride alone dissolved in n-butyl alcohol was used. This application composition is also heated..at.500°C.for 20 minutes, and sufficient layers was applied to provide 15 g/m of ruthenium oxide. The resulting electrode was found to have excellent resistance to loss when used as an anode in the electrolysis of salt solution in a mercury cell for the production of chlorine.

EXAMPLE 6

The procedure according to example 4 was repeated with the modification that a mixture of platinum and iridium chloride dissolved in n-butyl alcohol is used as the coating composition.

This coating composition is also heated at 500°C for 20 minutes and a sufficient number of layers are applied to provide approx. 15 g/m 2 of a mixture of platinum and iridium which adheres well to the titanium dioxide coating. An electrode prepared according to this example has excellent properties in the electrolysis of chlorine in a diaphragm cell.

EXAMPLE 7

The procedure according to example 4 was repeated with the modification that an application composition containing iridium chloride dissolved in n-butyl alcohol was used. The coating composition was also here heated at 500°C for 20 minutes, and a sufficient number of layers were applied to provide approximately 10 g/m 2 of iridium metal as a coating which adheres well to the titanium dioxide. Whether the iridium was present in metallic form or in oxide form, or as a mixture of both, was not determined. This electrode was very satisfactory, in the electrochemical production of sodium chlorate from salt solutions.

EXAMPLE' 8'

The procedure according to Example 7 was used

with the modification that the coating composition also contained some tetra-n-butyl titanate. This gives an electrode where the top layer is a mixture of oxides of titanium and iridium. The mixture of the coating composition is such that equal weight amounts of titanium dioxide and iridium oxide are formed on the assumption that the iridium will be completely oxidized. This example can be modified by replacing iridium with ruthenium.

EXAMPLE. 9

The procedure according to example 4 is repeated with the modification that the titanium electrode substrate is replaced by an electrode substrate made of tantalum. The titanium dioxide layer is applied to the tantalum substrate in the manner described, but the tantalum is mechanically roughened instead of etched. The resulting electrode proved to be highly resistant to wear when used as an anode in the electrolytic production of chlorine in a mercury cell.

EXAMPLE 10

The procedure according to example 9 is repeated, but modified to the extent that the electrically conductive coating applied to the titanium dioxide surface is that described in example 5. This also gives a highly satisfactory electrode.

EXAMPLE 11

The procedure according to example 5 is repeated with the further modification that the electrode substrate is vacuum-blasted tantalum. This also gives a very satisfactory electrode.

The tantalum was then replaced by niobium and the procedure according to this example repeated, whereby another very satisfactory electrode was obtained, although certain difficulties were encountered with regard to preventing excessive oxidation of the niobium: during the heating of the ruthenium chloride plating composition in air at 500 °C.

This procedure was again repeated using an etched zirconium electrode substrate. Thin layers of titanium dioxide were produced which adhered well to the zirconium substrate,

and which served as a satisfactory substrate for the ruthenium oxide coating composition.

This procedure was again repeated with the modification that the electrode substrate was a titanium alloy containing 6% by weight aluminium, 4% by weight vanadium, while the rest consisted of titanium. This also provided a satisfactory electrode.

In a further modification, the titanium alloy just described was replaced with a titanium alloy containing 0.2 weight percent palladium. This also provided a satisfactory electrode.

EXAMPLE 12

An electrode substrate of commercial grade titanium is vacuum-blown. The substrate is then coated with a tantalum-containing coating composition, which is dried at 250°C for 15 minutes and heated in air at 500°C for 20 minutes. The coating with the tantalum-containing coating composition and the heating were repeated as many times as necessary to provide 10 g/m 2 of tantalum oxide on the electrode substrate.

The electrode substrate was then given an electrically conductive coating by the method described in example 5.

This produced a satisfactory electrode.

EXAMPLE 13

An electrode substrate of commercial purity titanium is etched in 10% by weight oxalic acid for approx. 10 hours. On the substrate. a positive potential of 12 volts is then applied across a lead cathode, and the substrate and cathode are immersed in a 15.4. % by weight solution of phosphoric acid which contained 4.3 g/l of trivalent titanium ions. The solution is heated to and held at 90°C for approx. 7 hours. This gave a titanium dioxide coating on the electrode substrate of 13.3 g/m 2 with good adhesive strength.

The electrode substrate is then given an electrically conductive coating by the method described in Example 5. When the resulting electrode was used as an anode in the electrolysis of a salt solution in a mercury cell for the production of chlorine, it was found to be as resistant to loss as the electrode according to example 5.

EXAMPLE 14

The procedure according to example 13 is repeated except that the solution containing trivalent titanium ions is first replaced with a 10% by weight sulfamic acid solution containing 1 g/l titanium. During 5 hours at 90°C, 5.6 g/m 2 of well-adherent titanium dioxide was formed. The resulting electrode was satisfactory.

This example was repeated using 20% by weight of sulfamic acid containing 5 g/l of titanium. In the course of 7 hours, a well-adherent titanium dioxide coating of 3 g/m 2 was formed.

EXAMPLE 15

The procedure according to example 13 is repeated using a titanium-containing solution of 20% by weight oxalic acid solution which contained 5 g/l of titanium as Ti 3+ ions. With a 12 volt potential between the electrode substrate and a lead cathode and the solution boiling, a titanium dioxide coating was produced within 24 hours which adhered well to the electrode substrate and was present in an amount of approx. 35 g/m2.

EXAMPLE 16

An electrode substrate made of titanium of commercial purity is etched in 10% by weight of xoxalic acid for approx. 8 hours. The substrate is then given an electrically conductive coating using an application composition of ruthenium chloride dissolved in n-butyl alcohol. The coating is carried out on the etched titanium surface in several layers, with each layer being dried and every second layer being heated in air at 500°C for 20 min. This is continued until 10 g/m 2 of ruthenium oxide has been applied to the titanium surface.

The electrode is then given a coating of titanium dioxide by introducing it as the anode into a 7% by weight sulfuric acid solution containing 5 g/l of titanium in the trivalent state. The electrode substrate was given a positive potential of 2 volts at 90°C against a lead cathode in the solution. After 7 hours, a very well-adherent titanium dioxide layer of 12.g/m2 had formed on the ruthenium oxide surface. The resulting electrode exhibited exceptional wear resistance when used in the electrolysis of salt solution in a mercury cell for the production of chlorine.

EXAMPLE 17

An electrode substrate of commercial grade titanium was etched at 80-90°C in a 20% by weight oxalic acid solution. The etching period was 16 hours.

At the end of the etching period, the oxalic acid solution contained the desired amount of approx. 5 g/l titanium dissolved from the electrode substrate. If the solution lacks titanium after suitable etching, titanium powder or titanium oxalate can be added to bring the titanium content to approx. 5 g/l. Without removing the electrode substrate from the solution, the substrate is connected as anode to a lead cathode in the solution, and with a 12 volt potential applied to the anode, the solution is boiled. This causes precipitation of a titanium dioxide coating on the etched electrode substrate, and after 24 hours approx. 30 g/m 2 of very well-adhered titanium dioxide.

This method combines a method for etching the titanium surface with a method for depositing the titanium dioxide coating, and can be modified for use with sulfuric acid and hydrochloric acid etching solutions. The electrode is then provided with an electrically conductive coating of a mixture of iridium and ruthenium oxide as described in example 4.

One of the main advantages of electrodes manufactured according to the invention is the outstanding durability of the coating. This is always important, but is decisive in chlorine-alkali electrolysis with a mercury cathode. Electrodes produced according to the invention resist mercury and mercury amalgam in a very satisfactory manner. This resistance is believed to be due to the layer or layers of film-forming metal oxide, the method of application thereof, which provides a layer which exhibits a strong protective effect, and which also provides excellent adhesion and good electrochemical properties.

The two principle working methods according to the invention as described above comprise the chemical precipitation of an oxide of a film-forming metal according to the first method, and electrolysis of a solution of a film-forming metal according to the second method. When these are compared, it is obvious that the second method gives electrodes which have a longer life by electrolysis and a better resistance in contact with mercury and mercury amalgam (about twice as long a life) as electrodes produced by the first-mentioned method. The production is also facilitated, since oxide is deposited only on the positive electrode substrate and with a better deposition rate. It is thus preferred to precipitate the oxide of the film-forming metal by electrolysis of a solution containing this metal.

A further advantage that is achieved when a titanium oxide layer is deposited on titanium by the method according to the invention is a higher anodic breakdown voltage in chloride solutions than with uncoated titanium. This is an essential property for anodes that work in chlorine cells at elevated temperatures. The results obtained are listed in the following table.

This advantage can be exploited to a maximum by applying the layer of titanium dioxide not only to the part of the surface of the electrode substrate at which electrical conductivity is required, but also to the rest of the surface.

Claims (3)

1. Process for the production of electrodes suitable for use in electrolytic processes, where on an electrode substrate at least the surface of which consists of a film-forming metal of titanium, tantalum, niobium, zirconium and/or alloys based on at least one of these metals, an electrically conductive, electrolyte-resistant layer containing a metal from the platinum group or an oxide of a metal from the platinum group is applied, characterized in that a further layer consisting of an oxide of film-forming metal, using a solution containing metal ions of the film-forming metal, from which the layer of an oxide of the film-forming metal is separated. 2. Method as stated in claim 1, characterized in that one or more layers each consisting of an oxide of the film-forming metal are separated from the solution before application of the layer containing a metal from the platinum group or an oxide of a metal from the platinum group. 3. Method as stated in one of the preceding claims, characterized in that at least one of the layers of an oxide of film-forming metal is separated from the solution by chemical cleavage in the solution of a compound containing the film-forming metal.