Proposal is made for providing a high-performance electrode suitable for use in an oxygen-generating electrolytic process having an outstandingly low oxygen overvoltage and exhibiting high durability in a prolonged run of electrolysis. The electrode consists of an electroconductive substrate of a metal, e.g., titanium, and a multiple composite oxide coating layer thereon consisting of at least one layer of type A composed of iridium oxide and tantalum oxide in an Ir:Ta molar ratio of 40:60 to 79.9:20.1 and at least one layer of type B formed on the type A layer composed of iridium oxide and tantalum oxide in an Ir:Ta molar ratio of 80:20 to 99.9:0.1. A plural number of type A layers and a plural number of type B layers can be alternately laid one on the other so as to improve the mechanical stability of the coating layer on the substrate surface.

BACKGROUND OF THE INVENTION 
The present invention relates to a novel oxygen-generating electrode and a 
method for the preparation thereof. More particularly, the invention 
relates to an electrode having excellent durability and low oxygen 
overvoltage for generating oxygen by electrolytically oxidizing an aqueous 
solution on an anode as well as to a method for the preparation thereof. 
A type of conventional metallic electrodes widely used in the electrolytic 
industry includes those prepared by providing an overcoating layer of a 
platinum group metal or an oxide thereof on an electroconductive substrate 
made from titanium metal. 
For example, known electrodes used as the anode for producing chlorine by 
the electrolysis of brine include those prepared by providing a titanium 
substrate with an overcoating layer formed of an oxide mixture of 
ruthenium and titanium or an oxide mixture of ruthenium and tin (see, for 
example, Japanese Patent Publications 46-21884, 48-3954 and 50-11330). 
Besides the above mentioned process of electrolysis of brine in which 
chlorine is produced as the electrolytic product. various processes are 
known in the electrolytic industry in which oxygen is generated on the 
electrode. Examples of such an oxygen-generating electrolytic process 
include recovery of spent acids, alkalis or salts, electrolytic metallurgy 
of copper, zinc, etc., metal plating, cathodic protection and the like. 
These oxygen-generating electrolytic processes require electrodes quite 
different from the electrodes successfully used in the electrolytic 
processes accompanied by generation of chlorine. When an electrode for the 
chlorine-generating electrolysis, such as the above mentioned 
titanium-based electrode having a coating layer of an oxide mixture of 
ruthenium and titanium or ruthenium and tin, is used in an 
oxygen-generating electrolytic process, the electrolysis must be 
discontinued before long due to rapid corrosion of the electrode. Namely, 
the electrodes must be specialized for the particular electrolytic 
processes. The electrodes most widely used in an oxygen-generating 
electrolysis are lead-based electrodes and soluble zinc anodes although 
other known and usable electrodes include iridium oxide- and 
platinum-based electrodes, iridium oxide- and tin oxide-based electrodes, 
platinum-plated titanium electrodes and the like. 
These conventional electrodes are not always quite satisfactory due to the 
troubles which may be caused depending on the type of the 
oxygen-generating electrolytic process. When a soluble zinc anode is used 
in zinc plating, for example, the anode is consumed so rapidly that 
adjustment of the electrode distance must be performed frequently. When a 
lead-based insoluble electrode is used for the same purpose, a small 
amount of lead in the electrode is dissolved in the electrolyte solution 
to affect the quality of the plating layer. Platinum-plated titanium 
electrodes are also subject to rapid consumption when used in a process of 
a so-called high-speed zinc plating process at a high current density of 
100 A/dm.sup.2 or higher. 
Accordingly, it is an important technical problem in the technology of 
electrode manufacture to develop an electrode useful in an 
oxygen-generating electrolytic process which can be used with versatility 
in various processes without the above mentioned drawbacks. 
When an oxygen-generating electrolytic process is performed by using a 
titanium-based electrode having a coating layer thereon, on the other 
hand, it is not rare or rather usual that an intermediate layer of 
titanium oxide is formed between the substrate surface and the coating 
layer to cause a gradual increase in the anode potential or eventually to 
cause falling of the coating layer with the substrate surface being in a 
passive state. Various attempts and proposals have been made to provide an 
appropriate intermediate layer beforehand between the substrate surface 
and the coating layer in order to prevent subsequent formation of a layer 
of titanium oxide (see for example, Japanese Patent Publications 60-21232 
and 60-22074 and Japanese Patent Kokai 57-116786 and 60-184690). 
The electrode having an intermediate layer provided as mentioned above is 
not so effective as desired when the electrode is used in an electrolytic 
process at a high current density because the electroconductivity of such 
an intermediate layer is usually lower than the overcoating layer. 
It is also proposed to provide an intermediate layer formed by dispersing 
platinum in a matrix of a non-precious metal oxide (see Japanese Patent 
Kokai 60-184691) or to Provide an intermediate layer formed of an oxide of 
a valve metal, e.g., titanium, zirconium, tantalum and niobium, and a 
precious metal (see Japanese Patent Kokai 57-73193). These electrodes are 
also not quite advantageous because platinum has no very high corrosion 
resistance in itself in the former type and, in the latter type, the kind 
of the valve metal oxide and the compounding amount thereof are not 
without inherent limitations. 
Besides, Japanese Patent Kokai 56-123388 and 56-123389 disclose an 
electrode having an undercoating layer containing iridium oxide and 
tantalum oxide on an electroconductive metal substrate and an overcoating 
layer of lead dioxide. The undercoating layer in this electrode, however, 
serves to merely improve the adhesion between the substrate surface and 
the overcoating layer of lead dioxide to exhibit some effectiveness to 
prevent corrosion due to pinholes. When such an electrode is used in an 
oxygen-generating electrolytic process, disadvantages are caused because 
of the insufficient effect of preventing formation of titanium oxide and 
unavoidable contamination of the electrolyte solution with lead. 
The inventors have previously proposed an improved oxygen-generating 
electrode of which the electroconductive substrate of, for example, 
titanium metal is provided with an undercoating layer compositely 
consisting of iridium oxide and tantalum oxide in a specific molar 
proportion and an overcoating layer of iridium oxide formed thereon (see, 
Japanese Patent Kokai 63-235493). The electrode of this type having a 
bilayered coating, however, is not quite satisfactory in respect of the 
oxygen overvoltage which cannot be low enough to be desirably 400 mV or 
lower although an improvement can be obtained in the durability of the 
electrode. Further, the inventors have proposed an electrode having a 
ternary composite coating layer of iridium oxide, tantalum oxide and 
platinum metal formed on an electroconductive substrate in a specific 
molar proportion (see Japanese Patent Kokai 1-301876). The performance of 
the electrode of this type is indeed superior to the above described 
electrode with a bilayered coating and satisfactory if it is not for the 
expensiveness of the platinum metal. 
SUMMARY OF THE INVENTION 
An object of the present invention is therefore to provide a novel and 
improved electrode suitable for use in an oxygen-generating electrolytic 
process which is free from the above described problems and disadvantages 
in the prior art electrodes. More particularly, the object of the present 
invention is to provide an electrode formed of an electroconductive 
substrate of a metal such as titanium and provided with a coating layer 
basically composed of iridium oxide and tantalum oxide. 
The electrode of the present invention suitable for use in an 
oxygen-generating electrolytic process is an integral body consisting of: 
(A) an electroconductive substrate made of a metal which is preferably 
titanium; and 
(B) a multiple coating layer on the surface of the substrate, the multiple 
coating layer consisting of at least one layer of a first type essentially 
having a composite oxide composition of from 40 to 79.9% or, preferably, 
from 50 to 75% by moles as metal of iridium oxide and from 60 to 20.1% or, 
preferably, from 50 to 25% by moles as metal of tantalum oxide and at 
least one layer of a second type essentially having a composite oxide 
composition of from 80 to 99.9% or, preferably, from 80 to 95% by moles as 
metal of iridium oxide and from 20 to 0.1% or, preferably, from 20 to 5% 
by moles as metal of tantalum oxide alternately laid one on the other with 
the proviso that the undermost layer in contact with the substrate surface 
is of the first type. 
In addition to the advantages in the oxygen overvoltage and durability of 
the electrode obtained in the above defined electrode, an additional 
advantage is obtained in respect of the adhesion of the coating layer to 
the substrate surface when the multiple coating layer has at least two of 
the first type layers or each at least two of the first type layers and 
the second type layers. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As is described above, the electrode of the invention has a basic structure 
that an electroconductive substrate of a metal such as titanium is 
provided with a multiple coating layer consisting of at least one layer of 
the first type and at least one layer of the second type each having a 
specified composite oxide composition different from the other consisting 
of iridium oxide and tantalum oxide and the first type layers and the 
second type layers are laid one on the other alternately with the proviso 
that the undermost layer in contact with the substrate surface is of the 
first type. Such a multiple layered structure of the coating layer is 
advantageous in the improved electrode performance for oxygen generation 
and the increased durability of the electrode as compared with a single 
coating layer formed from iridium and tantalum oxides which is 
disadvantageous in respect of the gradual increase in the oxygen 
overvoltage when electrolysis is continued resulting in a loss of electric 
power. 
In the preparation of the inventive electrode, an electroconductive 
substrate is coated first with a coating solution for the undermost layer 
which is of the first type, referred to as the type A hereinafter, 
containing iridium and tantalum each in the form of a soluble compound 
followed by a heat treatment in an oxidizing atmosphere to effect thermal 
decomposition of the respective metal compounds into the form of an oxide 
composite of the metals composed of from 40 to 79.9% or, preferably, from 
50 to 75% by moles as metal of iridium oxide and from 60 to 20.1% or, 
preferably, from 50 to 25% by moles as metal of tantalum oxide. The 
electrode body provided with the undermost coating layer of the type A is 
then coated with another coating solution containing iridium and tantalum 
each in the form of a soluble compound in a proportion for the second 
layer which :s of the second type, referred to as the type B hereinafter. 
followed by a heat treatment in an oxidizing atmosphere to effect thermal 
decomposition of the respective metal compounds into the form of an oxide 
composite of the metals composed of from 80 to 99.9% or, preferably, from 
80 to 95% by moles as metal of iridium oxide and from 20 to 0.1% or, 
preferably, from 20 to 5% by moles as metal of tantalum oxide. The above 
described procedures of coating the surface with the coating solution for 
the type A or type B layer followed by baking to form a composite oxide 
layer can be repeated as many times as desired to form a multiple coating 
layer consisting of at least two of the type A layers and at least two of 
the type B layers alternately laid one on the other. The top layer of the 
multiple coating layer can be either of the type A or of the type B. 
The metal making the electroconductive substrate of the inventive electrode 
is selected from valve metals such as titanium, tantalum, zirconium, 
niobium and the like. These metals can be used either singly or in the 
form of an alloy of two kinds or more according to need. Titanium is 
preferred. 
The undermost layer of the multiple coating layer in contact with the 
substrate surface is of the type A of which the molar proportion of the 
iridium oxide and tantalum oxide is in the above specified range. 
Preferably, the molar proportion of iridium oxide should be relatively 
small within the range although an excessively large proportion of 
tantalum oxide may cause a disadvantageous increase in the oxygen 
overvoltage. The coating amount of this undermost layer of the first type 
composition should be in the range from 0.05 to 3.0 mg/cm.sup.2 calculated 
as iridium metal. 
The second layer provided on the above mentioned undermost layer to form 
the multiple coating layer is of the type B of which the molar proportion 
of iridium oxide and tantalum oxide is also in the above specified range. 
Preferably, the molar proportion of iridium oxide should be relatively 
large within the range although an excessively large proportion thereof 
may cause a disadvantage of a decrease in the adhesion of the coating 
layer. The coating amount of this second layer of the type B is preferably 
in the range from 0.01 to 7 mg/cm.sup.2 calculated as iridium metal. When 
the coating amount thereof is too small, consumption of the electrode in 
the electrolytic process may be unduly increased to cause a decrease in 
the durability of the electrode. 
Although the multiple coating layer basically is composed of a type A 
layer, which is the undermost layer, and a type B layer forming a 
bilayered structure, it is optional that the multiple coating layer 
consists of three or more of the layers in an alternate order of type A, 
type B, Type A, type B, and so on by repeating the coating and baking 
treatment. The topmost layer can be either of the type A or of the type B. 
Such a multiple alternate repetition of the type A and type 8 layers has 
an advantage of increasing the adhesive strength of the coating layer and 
decreasing the consumption of the electrode in the electrolytic process 
contributing to the improvement of the durability of the electrode. 
The coating solution for forming the layers of the type A and type B is 
prepared by dissolving, in a suitable solvent compounds of iridium and 
tantalum each in a specified concentration. The metal compounds should be 
soluble in the solvent and decomposed at an elevated temperature of baking 
to form an oxide of the respective metals. Examples of the metal compounds 
include chloroiridic acid H.sub.2 IrCl.sub.6. 6H.sub.2 O, iridium chloride 
IrCl, and the like as the source material of iridium oxide and tantalum 
halides, e.g., tantalum chloride TaCl.sub.5, tantalum ethoxide and the 
like as the source material of tantalum oxide. The proportion of these two 
kinds of metal compounds should be selected depending on the desired molar 
proportion of the metal oxides produced by thermal decomposition of the 
compounds to form the layer and the proportion in the coating solution can 
be about the same as in the composite oxide layer formed therefrom 
although a possible loss of certain metal compounds by vaporization in the 
course of the baking treatment, which may amount to several % of the 
content in the coating solution depending on the conditions of baking, 
should be taken into account. The electrode body coated with the coating 
solution is dried and then subjected to a heat treatment for baking in an 
oxidizing atmosphere containing oxygen such as air. The baking treatment 
is performed for 1 to 60 minutes at a temperature in the range from 400 to 
550 .degree. C so as to effect complete decomposition and oxidation of the 
metal compounds. The atmosphere for the baking treatment should be fully 
oxidizing because an incompletely oxidized coating layer may contain the 
iridium or tantalum metal in the free metallic state resulting in a 
decrease in the durability of the electrode. When a single coating 
followed by baking cannot give a layer having a desired thickness, the 
process should be repeated several times until the coating amount of the 
layer reaches a desired range. These procedures are basically the same for 
the type A coating layers and for the type B coating layers excepting that 
the formulation of the coating solutions should be different corresponding 
to the desired iridium-to-tantalum molar ratio ;n the layers of the 
composite oxide formed by the thermal decomposition. 
When adequately prepared according to the above given disclosure, the 
electrode of the invention can be used as the anode in an 
oxygen-generating electrolysis exhibiting an outstandingly long life at a 
low cell voltage or a considerably improved life at a high current density 
of 100 A/dm.sup.2 or larger with little increase in the oxygen overvoltage 
in a long run of a continued electrolytic process.

In the following, examples and comparative examples are given to illustrate 
the electrode of the invention and the method for the preparation thereof 
in more detail but not to limit the scope of the invention in any way. In 
each of the following examples and comparative examples, the electrode 
prepared was subjected to the evaluation tests for the oxygen overvoltage, 
increase of the oxygen overvoltage in the lapse of time in a continuous 
electrolysis and durability as well as for the mechanical stability of the 
coating layer in the procedures described below. 
Oxygen overvoltage 
The oxygen overvoltage was determined by the voltage scanning method at 30 
C. in a 1 M aqueous solution of sulfuric acid at a current density of 20 
A/dm.sup.2. 
Electrode durability 
Electrolysis was conducted with the electrode as the anode and a platinum 
electrode as the cathode in a 1 M aqueous solution of sulfuric acid at 
60.degree. C. at a current density of 200 A/dm.sup.2 on the anode until 
the electrolysis could no longer be continued due to an undue increase of 
the cell voltage, which was initially about 5 volts, to exceed 10 volts. 
The results are recorded in four ratings of: Excellent for the life of at 
least 3000 hours; Good for the life of 2000 to 3000 hours; Fair for the 
life of 1000 to 2000 hours; and Poor for the life of 1000 hours or 
shorter. 
Increase of oxygen overvoltage in continued electrolysis 
Electrolysis was conducted for 1000 hours under the same conditions as ;n 
the above described durability test and the electrode was subjected to the 
determination of the oxygen overvoltage to record the increase thereof 
from the initial value. The results were recorded in three ratings of: 
Good for an increase not exceeding 0.3 volt; Fair for an increase of 0.3 
to 0.7 volt; and Poor for an increase of 0.7 volt or larger. 
Mechanical stability of coating layer 
Electrolysis by using the electrode was conducted for 1000 hours in the 
same manner as in the above described durability test and then the 
electrode as dried was subjected to an ultrasonic vibration test for 5 
minutes to cause falling of the surface portion of the coating layer 
resulting in a decrease in the thickness of the layer. The decrease in the 
amount of iridium as metal per unit area of the coating layer was 
determined by the method of fluorescent X-ray analysis. The results were 
recorded in three ratings of Good, Fair and Poor when the decrease in the 
amount of iridium from the initial value was less than 5%, 5% to 10% and 
more than 10%, respectively. 
Example 1 
Experiments No. 1 to No. 12 
Several coating solutions were prepared each by dissolving chloroiridic 
acid and tantalum ethoxide in n-butyl alcohol in different molar 
proportions. The concentration of these two metal compounds in the coating 
solutions was always 80 g/liter as a total of iridium and tantalum metals. 
A titanium substrate after etching with an aqueous hot oxalic acid solution 
was brush-coated with one of the above prepared coating solutions of the 
formulation corresponding to the iridium:tantalum molar ratio in the 
composite oxide layer formed by baking as indicated in Table 1 below as 
the first type layer and then dried and baked in an electric furnace at 
500 C. for 7 minutes under a flow of air to form a composite oxide layer 
This procedure of coating with the solution, drying and baking was 
repeated several times until the coating amount at least 0.2 mg/cm.sup.2 
in Experiments No. 1 to No. 5, No. 11 and No. 12 and at least 0.4 
mg/cm.sup.2 in Experiments No. 6 to No. 10 calculated as iridium metal. 
In Experiments No. 1 to No. 5 undertaken for the invention, the thus formed 
oxide layer had a composition of iridium tantalum molar ratio in the range 
from 50:50 to 75:25 while, in Experiments No. 6 to No. 12 undertaken for 
comparative purpose, the iridium:tantalum molar ratio was varied in a 
wider range from 100:0 to 0:100 by omitting the tantalum compound or 
iridium compound in Experiments No. 6 and No. 12, respectively. 
The electrode bodies prepared in Experiments No. 6 to No. 10 provided with 
the single oxide layer of the first type formed ;n the above described 
manner were subjected as such to the evaluation tests while the electrode 
bodies prepared in Experiments No. 1 to No. 5, No. 11 and No. 12 were each 
provided with an overcoating composite oxide layer of iridium oxide and 
tantalum oxide of the second type by 7 times repetition of the coating, 
drying and baking treatment in the same manner as above excepting that the 
formulation of the coating solution was different as indicated in Table 1 
from that used for the first type coating layer. The coating amount of the 
second coating layer was about 0.4 mg/cm.sup.2 or larger calculated as 
iridium metal. 
Table 1 summarizes the iridium:tantalum (Ir;Ta) molar ratios in the oxide 
composites forming the first and the second type coating layers in each 
Experiment as well as the results of the evalation tests for the initial 
value of the oxygen overvoltage, increase of the oxygen overvoltage in the 
continued electrolysis and durability of the electrode. 
TABLE 1 
__________________________________________________________________________ 
First type 
Second type Increase of 
coating layer 
coating layer 
Oxygen oxygen overvoltage 
Experiment 
Ir:Ta in 
Ir:Ta in 
overvoltage, 
in continued 
Electrode 
No. molar ratio 
molar ratio 
mV electrolysis 
durability 
__________________________________________________________________________ 
1 50:50 85:15 385 Good Excellent 
2 60:40 85:15 385 Good Excellent 
3 60:40 90:10 390 Good Excellent 
4 70:30 90:10 395 Good Excellent 
5 75:25 90:10 395 Good Excellent 
6 100:0 430 Fair Fair 
7 70:30 410 Fair Good 
8 60:40 405 Fair Good 
9 50:50 405 Fair Fair 
10 30:70 450 Poor Poor 
11 30:70 60:40 430 Fair Fair 
12 100:0 70:30 420 Fair Fair 
__________________________________________________________________________ 
EXAMPLE 2 
Experiments No. 13 to No. 22 
The same titanium-made electrode substrate as used in Example 1 was 
provided in each of the Experiments with a multiple coating layer composed 
of at least two and up to seven coating layers of the type A and type B 
alternately laid one on the other. Table 2 below gives the iridum:tantalum 
molar ratios in the respective oxide composites forming the type A and 
type B layers in each Experiment. Table 2 also gives the total number of 
the type A and type 8 coating layers on the electrode in each of the 
Experiments. When the total number of the layers is an odd number, the 
topmost layer was of the type A and, when the total number of the layers 
is an even number, the topmost layer was of the type 8 as a matter of 
course since the undermost layer was always of the type A. 
The results of the evaluation tests undertaken with these electrodes are 
shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Type A 
Type B 
Total number Increase of 
coating 
coating 
of type A oxygen Mechanical 
layer layer and type B 
Oxygen overvoltage stability 
Experiment 
Ir:Ta in 
Ir:Ta in 
coating 
overvoltage, 
in continued 
Electrode 
of coating 
No. molar ratio 
molar ratio 
layers mV electrolysis 
durability 
layer 
__________________________________________________________________________ 
13 60:40 85:15 3 385 Good Excellent 
Good 
14 60:40 85:15 4 390 Good Excellent 
Good 
15 60:40 85:15 4 385 Good Excellent 
Good 
16 60:40 85:15 7 385 Good Excellent 
Good 
17 50:50 85:15 4 385 Good Excellent 
Good 
18 70:30 90:10 4 390 Good Excellent 
Good 
19 75:25 90:10 4 395 Good Excellent 
Good 
20 75:25 90:10 2 395 Good Excellent 
Fair 
21 30:70 60:40 4 430 Fair Fair Good 
22 70:30 100:0 2 430 Good Excellent 
Fair 
__________________________________________________________________________