EP0026482A1

EP0026482A1 - A method for producing an insoluble electrode for electroplating

Info

Publication number: EP0026482A1
Application number: EP80105832A
Authority: EP
Inventors: Hajime Nitto; Kango Sakai; Katsushi Saito
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1979-09-25
Filing date: 1980-09-25
Publication date: 1981-04-08
Also published as: AU6271880A; BR8006157A; ES495299A0; JPS5749636B2; ES8106770A1; JPS5647597A

Abstract

The present invention relates to a method for producing an insoluble durable electrode usable as an insoluble anode which comprises

electroplating a current conductive base material with a metal of the platinum group;
applying at least one compound of a metal of the platinum group to the electroplated surface and
heating the base material thus electroplated and applied with the metal compound at a temperature not higher than 600°C in a non-oxidizing atmosphere to thermally decompose the metal compound and diffuse the metal thus formed into the electroplated coating.

Description

The present invention relates to a method for preparing a durable electrode material usable as an insoluble anode usable for electrolysis in a bath containing chlorine compounds and sulfur compounds, which comprises coating a current-conductive material, such as titanium, with a metal diffusion film such as of a metal of the platinum group.
In electroplating of a continuous steel strip using a soluble electrode, the concentration of metal ions in the electroplating bath gradually increases due to the difference between the deposition efficiency of the cathode which is usually from 94% to 97% and the dissolution efficiency of the anode which is usually from 100% to 103%. In order to maintain the metal ion concentration in a proper range, it has been a common practice according to the prior art that the soluble anode is partially replaced by an insoluble anode so as to control the concentration of the metal ion, and at the same time, to cause deposition of the metal for a desired amount of electroplating so as to optionally control the metal ion concentration.
Therefore, in the prior art using an insoluble anode, it is necessary to alternately pass and cut the current and the electrode must be durable in order to perform the electroplating satisfactorily.
Also in electroplating of a continuous steel strip using a soluble anode, the amount of dissolution of the anode varies from portion to portion of the electrode, thus differing between the right side and left side of the electrode, and between the upper portion and the lower portion of the electrode. For example, this dissolution amount changes when the relative distance between the anode and the cathode changes or when the electrical resistance in the current path changes. Therefore, it is necessary to maintain a considerable distance between the cathode and the anode. However, the distance is usually 25 mm to 35 mm,' and the loss of energy by the bath resistance is large.
In order to obtain constant electroplating conditions, it is necessary to maintain the insoluble anode dimensionally stable, to maintain a shorter electrode distance so as to obtain a uniform metal deposition, and to lower the required voltage.
Further, in the electroplating using a soluble anode, for example, a tin anode, 200 to 300 cast tin anodes, each weighing about 10 kg, are used in one electroplating line.
The anodes must be supplemented to make up for the consumption (dissolution) of the anodes. Therefore, repeated heavy labour and complicated handling-in or handling-out of the anodes for such supplementation are required.
Also for eliminating the heavy labour and the complicated handling of the anodes, the anode must be insoluble.
Therefore, there is an increasing demand for insoluble anodes suitable for electroplating of continuous steel strips.
Notwithstanding only lead-based anodes have been used in commercial practice as a non-corrosive anode for electroplating in sulfuric acid baths, with some-exceptions where titanium electrodes coated with 3 to 5 micron platinum, are used, but other materials have been found to be unsuccessful in practical use due to their inferior durability and dissolution of impurities into the electrolyte.
In the case of lead-based anodes, they are undesirable for tin-plating of beverage cans, etc., from the toxicologic aspect, and have disadvantages in zinc-plating in general that the zinc coating peels off during the paint baking due to the presence of very small amounts of Pb ions and that the color and brightness of the coating are deteriorated by the chromate treatment. Therefore, electrode materials have been required which can prevent the dissolution of impurities into the electrolyte and do not deteriorate the coating quality.
Under such a situation, there is a demand for a method for the production of insoluble anodes which can meet various requirements as above.
Concerning insoluble anodes, the following prior art references are relevant:
Japanese Patent Publication No. Sho 38-10515 discloses an insoluble anode prepared by electroplating a titanium base with platinum, heating this coated material at a temperature not lower than 400°C in an inert gas or in vacuum of not higher than 10^-5 mmHg to alloy the titanium base with the platinum coating.
Japanese Patent Publication No. Sho 39-20910 discloses a method comprising electro-deposition of platinum in a thickness of 2 to 3 micron on a base material, heating in an inert gas in the temperature range from 400°C to 800°C, surface activating treatment, and further platinum plating in a thickness of 3 to 5 micron.
Japanese Patent Publication No. Sho 48-43267 (1973) discloses the preparation of non-corrosible electrodes which comprises inpinging rare gas ions on to the surface of a titanium cathode, coating the surface of the titanium cathode with a metal of the platinum group in an atmosphere substantially free from oxygen, and further applying a coating of ruthenium oxide on the platinum coating.
Japanese Patent Publication No. Sho 49-7789 discloses a method of producing anodes for electrolysis which comprises electroplating of platinum or rhodium in a thickness of 0.2 micron or more on the electrode base, further electroplating with ruthenium, and firing at a temperature not lower than 500°C in an oxidizing atmosphere.
The important technical object of these prior art anodes is to obtain better adhesion of the platinum coating and to eliminate defects of the coated film.
It has been found, however, that the base materials such as titanium are converted into nitrides and oxides when heated to 600°C or higher, so that their mechanical strength and current conductivity are lowered. On the other hand, when they are heated to temperatures lower than 600°C, the alloying between platinum and the titanium base and between the platinum coating and ruthenium is very poor and only partial.
Therefore, these prior art anodes have the defect that it is impossible to select a proper condition which can produce a desirable alloyed layer and at the same time can prevent the oxidation and nitrization of the base material.
Further, Japanese Patent Publication No.Sho 48-3954 discloses an electrode in which the oxide of a metal selected from the platinum group consisting of platinum, iridium, rhodium, ruthenium and osmium is used.
Japanese Patent Publication No. Sho 45-11014 discloses a method for electrolysis of an aqueous solution of alkali metal chlorides which comprises electrolysing the solution with an anode potential of 1.36V or lower using an anode made of an alloy of 20% to 50% platinum and 80% to 50_% palladium or an anode coated with the alloy.
The above-mentioned prior art references are directed to an anode material which is cheaper than platinum and can stabilize the oxygen overvoltage.
However, when oxides of platinum group metals or platinum group metals are to be directly coated on titanium base material and the like, no or only partial alloying with the titanium base material is obtained at the temperatures which cause the thermal decomposition of the chlorides and nitrides in the case of oxide films and platinum films, so that titanium oxide appears at non-coated portions.
The prior art insoluble anodes have been chiefly used in the chloralkali electrolysis, and although these prior art anodes can satisfactorily stand for the electrolysing conditions in a chlorine bath, such as electrolysis of sodium chloride, these prior art anodes have been found to have very little durability in a metal electroplating in a sulfuric acid bath, for example, which is performed under intermittent electrolysing conditions, because oxide films . such as TiO₂ filmsreadily dissolve. due to their reverse position in the electromotive force series.
According to the prior art for plating platinum or a metal of the platinum group on an electrode base material, such as titanium, only a thin platinum film with many pin holes can be obtained, so that the electrode base material dissolves from the coating defects (pin holes) into the electrolyte and rapidly promote the consumption of the platinum coating.
Also in the pretreatment for platinum plating, mechanical surface conditioning, such as sand blasting and shot blasting are sometime performed. In such cases, the sands or shots are blasted into the titanium base, or the surface oxide scale is forced into the base metal so that pin holes or pin hole like non-coated portions are caused in the platinum coating. These can be a vital defect to the life of the electrode.
In order to prevent these coating defects, trials have been made to give a thicker platinum coating on the base metal or to perform the electroplating in a two-step manner, but it has been almost impossible to completely eliminate the non-coated portions, because the electro-precipitation of a coating metal follows the electro-precipitation nuclei of the base metal.
Thus, the insoluble anodes to be used for continuous electroplating of steel strips, such as in tin-plating in ferrostan baths and halogen baths, and in zinc-plating in chloride baths and sulfuric acid baths must stand for various electrolysing conditions including the current passing methods, and the bath compositions. Thus the prior art anodes have been unsuccessful in electroplating of steel strips.
Therefore, the object of the present invention is to provide an insoluble anode satisfactorily usuable in continuous electroplating of steel strips and to provide a method for preparation thereof.
According to the present invention, an insoluble electrode with a coating film free from defects can be obtained.
The features of the present invention lie in that a current conductive material is coated with at least one metal of the platinum group by electroplating, then a compound or salt of the platinum group metals, such as Ir, Ru, Ph and Pt is applied on the coating previously applied and is decomposed by heating in a reducing or non-oxidizing atmosphere to make up defects of the electroplated coating. At the same time, a metal of the platinum group which is active at the thermal decomposition of the salt is diffused into the electroplated coating at a temperature not higher than 600°C to avoid nitrization of the base metal. In this way, an electrode coated with a metal of the platinum group and having a continuous, thin diffusion film is obtained.
According to a modification of the present invention, a further coating of a metal and/or oxide of the platinum group may be applied by electroplating or non-electrolytic coating on the thermally diffused coating.
Regarding the reducing atmosphere for thermal decomposition of the compound or salt of the platinum group metals in the present invention, it includes a reducing flame, and an atmosphere containing a reducing gas, such as hydrogen and carbon monoxide. While the non-oxidizing atmosphere includes an inert gas, such as nitrogen gas, desirably in the form of gas stream. Ordinary inert gases contain. a small amount of oxygen, and the thermal decomposition can be satisfactorily done in a gas stream containing up to 1000 ppm of oxygen as usually contained in a device or furnace for maintaining the inert gas atmosphere. However, it is preferable to perform the thermal decomposition under presence of not more than 100 ppm oxygen.
The platinum group metals used in the present invention include the elements belonging to the 8th group of the Periodic Table. Among these elements platinum is easiest to electroplate.
The present invention will be described by reference to the attached drawings.
Brief Explanation of the Drawings:

Fig. 1 schematically shows the cross section of an electrode according to the present invention.
Fig. 2 shows a graphical analysis of the cross. section of the electrode shown in Fig. 1 by an electron probe micro analyzer - EPMA - (scanning A).
Fig. 3 shows a graphical analysis of the cross section of the electrode shown in Fig. 1 by EPMA (scanning B).
Fig. 4(a) is a photograph showing the secondary electron image (prior to electrolysis) of the surface of the electrode electroplated with 1 micron platinum by EPMA; and Fig. 4 (b) is a photograph showing PtLa characteristics X ray.
Fig. 5(a) shows the secondary electron image of the surface of the electrode electroplated with 1 micron platinum (prior to electrolysis) by EPMA; and Fig. 5(b) shows PtLa characteristics X ray.
Figs. 6(a) - (f) show the progress of deterioration of an insoluble titanium anode electroplated with platinum (x40).
Fig. 7(a) shows the secondary electron image of 1 micron platinum coating diffused with 1 micron Ru-Rh-Ir by EPMA.
Fig. 7(b) shows the PtLa characteristics X ray of the coating in Fig. 7(a).
Fig. 8(a) shows the cross section of a titanium electrode electroplated with 1 micron platinum after 70 hours of continuous electrolysis.
Fig. 8(b) is a graphical analysis of the cross section in Fig. 8(a).
Fig. 9(a) showsthe cross section of a titanium electrode electroplated with platinum and diffused with Ru-Rh-Ir prior to electrolysis; and Fig. 9(b) shows the graphical analysis of the cross section of the same.
Fig. 10 shows the polarization of a conventional anode electroplated only with 1 micron platinum.
Fig. 11 shcws the polarization of an anode electroplated with 1 micron platinum and diffused with 1 micron Ir-Rh-Ru.
_Fig. ₁₂ shows the weight loss by time of the anode according to the present invention during electrolysis.

The following description of the present invention will be made in connection with a preferred embodiment in which platinum coating is applied by electroplating.
For preparing the insoluble anode of the present invention, the titanium base material is acid-pickled to clean the surface, coated with platinum by a conventional method.
The bath composition for the electroplating of the platinum group metals, various bath compositions may be used, such as an ammonia alkali bath containing diamine platinum nitrite. A typical bath composition and plating conditions are shown below:
The thickness of the electroplated coating must be at least 0.1 micron. In the case of the platinum coating, the coating defects still remain even if the thickness is increased as described hereinbefore. Therefore, from the . aspect of economy, it is desirable to maintain the coating thickness up to 3 micron. The most desirable range is from 0.2 to 1.0 micron.
Then a solution of chloride of Ir-Ru-Rh in ethylalcohol, butyl-alcohol or propyl-alcohl, dispersed in a reducing organic solvent such as lavender oil and/or terpene oil (oil of turpentine) is applied on the platinum coating, dried and thermally decomposed (40 cycles for 1 micron coating; each cycle comprises drying and thermal decomposition) at a temperature in the range from 380°C to 600°C preferably from 400°C to 550°C under reducing flames..
The concentration of various metal compounds may be varied within their dissolution range, but a preferable range is from 0.01% to 0.1%.
The application of the solution may be done by immersion, spraying or roll brushing. Regarding the proportions among Ir, Ru, Rh and Pt, a higher corrosion resistance can be obtained by a larger proportion of Ir and Pt. For example, 60% Ir - 20% Ru - 20% Rh (by weight) is preferable. The thickness of coating required for making up the coating defects must be 0.1 micron at least, and a preferable range is from 0.2 to 1.0 micron.
'The thermal decomposition is performed by passing the coated electrode through the reducing flames at a speed of about lm/minute. After the thermal decomposition, the electrode is heated at about 500°C to 600°C, preferably 500°C to 550°C in an oxidizing atmosphere for about one hour to take away non-decomposed chlorine. In this way, the final anode can be obtained.
As the base material for the anode according to the present invention, titanium alloys, tantalum, tantalum alloys, zirconium and zirconium alloys may be used in addition to titanium.
Other than chlorides of the platinum group metals, such as Ir, Ru and Rh, an aqueous solution of one or more of compounds of the platinum group elements, such as NO, N0₂ NOC1 compounds may be used. In short, these elements may be in any form which is soluble in water or other solvent.
Fig. 1 shows the cross section of an electrode according to the present invention. This electrode was prepared by electroplating a titanium base 3 with 1 micron platinum, then applying to this electroplated titanium base a coating of a chloride of Ir, Rh and Ru and thermally decomposing the chloride under reducing flames. The diffusion film 1 of Ir-Ru-Rh is formed at the portion at which the platinum coating is made. Meanwhile, at the coating defects, such as pin holes, the film 2 formed by the thermal decomposition of the chlorides of Ir, Ru and Rh makes up the coating defects.
As shown in Fig. 2 showing the EPMA graphical analysis of the cross section of the above anode, Ir, Rh and Ru are diffused into the platinum coating, and as shown in Fig. 3, the diffusion film of Ir-Ru-Rh formed by the thermal -decomposition is present at the portion where no platinum coating is present, namely the defect of the platinum coating.
As shown in Fig. 1, the coating on the titanium base obtained by the process of the present invention has a diffusion structure of Ir-Ru-Rh and of Pt-Ir-Ru-Rh free from pin holes.
For application of two or more metals of the platinum group on the electroplated platinum coating, two or more solutions of salts of the platinum group metals may be simultaneously applied, or a first solution of one salt of the platinum group metal is applied and heated, and then a second solution of another salt is applied and heated. In this way, a plurality of the platinum group metals can be diffused simultaneously or one after another successively to form an alloyed layer which covers the pin holes of the electroplated coating. In either way, an electrode having excellent corrosion resistance can be obtained.
Regarding the coating or film formed by the thermal decomposition of the compound or salts of the platinum group metals, it is necessary that the application of the solution, and the heating are repeated until a desired thickness of the coating or film can be obtained, because a single application of the solution and thermal decomposition gives only a very thin coating or film although it depends on the concentration of the solution. As mentioned hereinbefore, 40 cycles of application of the solution and heating are required for obtaining 1 micron coating or film.
Further, although a single cycle of electroplating . and repeated salt application and thermal decomposition can give a satisfactory corrosion resistance of the coating, when the cycle is repeated twice or more, a.very excellent surface coating extremely free from the pin holes can be obtained because the electroplating followed by the thermal decomposition coating is performed on the surface activated by the thermal decomposition.
If the electroplating and the non-electrolytic coating are repeated, better results can be obtained when they are repeated more times for the purpose of eliminating the pin holes, although there is no specific limitation therefor. Basically the more desired result can be obtained by repeating the electroplating and the non-electrolytic coating under the specified conditions. For example, an electrode having very little pin holes can be obtained a total thickness of 0.8 micron composed of by electroplating of 0.2 micron platinum, diffusion coating of 0.2 micron platinum group metal, and electroplating of 0.2 micron platinum. The electroplating coating may range from 0.1 to 1.0 micron in thickness, and the diffusion coating may range from 0.02 to 1.0 micron in thickness.
Thus, according to the present invention,-a coating of a platinum group metal diffused with a platinum group metal, which is extremely free from pin holes, can be obtained by the combination of the electroplating of a platinum group metal and the thermal decomposition of a compound or salt of a platinum group metal.
The coating according to the present invention shows very excellent corrosion resistance as compared with the conventional electroplating or spray coating of a platinum group metal alone. This excellent corrosion resistance can still be improved by the following modification of the present invention.
Thus after the electroplating of the platinum group metal, salt of the platinum group metal, such as Ir, Ru, Rh, Pt is applied on the electroplated coating and thermally decomposed in a non-oxidizing atmosphere so as to cover the defect portions of the coating, and further one or more metals of the platinum group is electroplated or coated by non-electrolytic method and/or an oxide film is formed to provide a multiple-layer coating. In this way, the corrosion resistance can be still enhanced.
When the upper-most coating is applied by electroplating, the underlying coating layer is already a pin-hole free coating of a platinum group metal, so that it is easy to obtain a continuous coating having good adhesion under . conventional electroplating conditions.
When the upper-most coating layer of a multiple coating is applied by a non-electrolytical method and thermal decomposition, compounds of the platinum group metals are applied and thermally decomposed in a reducing or substantially non-oxidizing atmosphere to deposit the metals, part or whole of which is diffused into the underlying coating. Further, the upper-most layer may be partially or wholly of oxide. In this case, also satisfactory adhesion and corrosion resistance can be obtained. The oxide can be formed by heating the metal after deposition in an oxidizing atmosphere, or by bringing the deposiaed metal into contact with an oxidizing atmosphere during the thermal decomposition step. Further, the oxide may be applied by spattering. The oxide may be an oxide of a platinum group metal, or an oxide of a metal other than the platinum group, such as titanium oxide and zirconium oxide, or a mixture of these oxides.
As described hereinabove, the most important feature of the present invention is that the defects and disadvantages of the prior art have been solved by making up the defects of the platinum coating by the formation of thermally decomposed product and diffusion at relatively low temperatures of the thermally decomposed products into the platinum coating for continuity with the platinum coating.
According to a modification of the present invention, the corrosion resistance of the coating is improved by a multiple coating layer.
Various experiments have been made with the insoluble anodes according to the present invention and the results will be explained in connection with the attached photographs of the coating surfaces, the electron probe micro analysis (EPMA) of the surfaces and the cross sections of the anodes.
In Fig. 4(a) showing the surface EPMA of the conventional titanium anode coated with platinum by electroplating and Fig. 4(b) showing the characteristic X rays (PtLa) of the platinum coating, the weakness of a single uniform film which is peculier to the platinum electroplating produces the surface defects, and the non-deposition of platinum is shown by the PtLa ccharacteristics X ray.
As shown in Fig. 5(a) showing the surface EPMA of the anode coated with platinum coating in a thickness of 7 micron, and in _Fig. 5(b) showing the PtLa characteristics X ray, even if the coating thickness is increased by 7 times, a larger film is retained so that when the electrolysis is carried out intermittently, , particularly when the current passage is stopped, the base material is dissolved and this dissolution causes the platinum coating to peel off from the base metal.
In Fig. 6 showing the progress by time of deterioration of a titanium insoluble anode coated with 3 micron platinum at the stages after 30 days, 45 days, 60 days, 75 days and 90 days of electroplating, the peelings off of the platinum coating expand from the coating defects until they connect to each other, thus increasing the non-conductive portions on the electrode.
In Fig. 7(a) showing the surface EPMA of the anode according to the present invention in which a mixture of salts of Ir, Rh and Ru is applied in an amount equivalent to 1 micron thick on 1 micron platinum coating, and thermally decomposed and diffused into the platinum coating, and in Fig. 7(b) showing the PtLa characteristics X ray, the salts of the platinum group elements diffuse into the electroplated platinum coating, while there is almost no diffusion or alloying is seen between the electroplated platinum coating and the base titanium.
Thus, as clearly shown by the attached drawings and photographs, the coating obtained by the present invention is in fact a diffused and alloyed coating which is completely free from defects and has excellent corrosion resistance.
The present invention will be better understood from the following embodiment..
The anode potentials of the titanium anode electroplated with platinum coating and diffused with the platinum group elements according to the present invention before and after electrolysis will be illustrated below.
In Fig. 10, the polarization of a conventional anode electroplated only with 1 micron platinum is shown and in Fig.11 the polarization of an anode electroplated with 1 micron platinum and further with a diffusion layer of 1 micron Ir-Rh-Ru according to the present invention.
No increase of the anode potential is seen before and after electrolysis in the case of the anode according to the present invention (Fig. 11), and no substantial increase of the anode potential is seen even at a high current density as 100 A/dm².
The loss of the anode weight during electrolysis is shown in Fig. 12. The electrolysis was done under the following conditions.
The insoluble anode and the cathode made of iron were opposingly placed in a bath containing 200 g/ℓ Na₂S0₄ with pH of 1.0 and the electrolysis was done at 50°C and 30 _A/dm². The total weight loss of the anode was measured and the weight losses by hours were shown.
When an electrode electroplated with 1 micron platinum and diffused with Ir-Ru-Rh mixture according to the present invention is used (line 1) a weight loss of only 0.5 g/m² is observed even after 70'hour electrolysis, and also when an electrode electroplated with 1 micron platinum diffused with Ir and electroplated with 0.5 micron platinum according to the present invention is used (line 2), a weight loss of only 0.2 g/m² is observed even after 70 hour electrolysis. In both cases no substantial increase in the voltage is seen.
While in the case of the conventional anode electroplated only with 1 micron platinum (line 3), a loss of 3 g/m was observed after 56 hour electrolysis, which is about 6 to 10 times larger than the loss in the case of the anode of the present invention, and a substantial voltage increase was caused after 56 hour electrolysis, thus prohibiting a further electrolysis.
Further in the case of a titanium electrode electroplated with 1 micron platinum and further coated with 3 micron platinum by three cycles of heating at 700°C in an inert gas (line 4), a weight loss of 2.5 g/m² is observed after 56 hour electrolysis, which slightly less than the weight loss in the case of the electrode electroplated with 1 micron platinum, but the voltage increases after 56 hour electrolysis, thus prohibiting further electrolysis.
In the case of the titanium anode with a thermally decomposed Ru-Rh-Ir coating directly applied to the base titanium (line 5), a loss of 10 g/m and also a voltage increase were seen after 42 hour electrolysis, thus prohibiting a further electrolysis. The weight loss of this anode is 20 to 30 times larger than the loss of the anode according to the present invention. This means a shorter life of the anode.
As understood from the forgoing descriptions of the" embodiments of the present invention, the electrode according to the present invention can be well used an insoluble anode for electroplating of steel strips, but is also applicable to ordinary electroplating and electrolysis industries.
As examples of application of the electrode according to the present invention to the electroplating of steel strips, the electrode of the present invention may be used as insoluble anodes in a vertical electrolytic tank having electrodes positioned across the steel strip, or in a horizontal electrolytic tank having electrodes positioned above and below the steel strip.
Also the electrode of the present invention can be used as an insoluble anode in a horizontal, linear-type plating tank in which the electrolyte is forcedly circulated at a high velocity between the steel strip and upper and lower in electrodes as disclosed Japanese Patent Publication No. Sho 50-8020, or in a drum-type electrolytic tank in which the electrolyte is forcedly circulated between the strip closely contacting a drum immersed in the electrolyte and the electrodes arranged around the strip as disclosed in U.S.Patent No. 3,483,113.

Claims

1. A method for producing an insoluble electrode for electroplating which comprises:

electroplating a current conductive base material with a metal of the platinum group;

applying at least one compound of a metal of the platinum group to the electroplated surface; and

heating the base material thus electroplated and applied with the metal compound at a temperature not higher than 600°C in a non-oxidizing atmosphere to thermally decompose the metal compound and diffuse the metal thus formed into the electroplated coating.

2. A method according to claim 1 in which the metal compound is a halide of a metal selected from the platinum group consisting of Ir, Ru, Rh and Pt.

3. A method according to claim 1 in which the metal compound is a NO (Nitrosyl), N0₂ (Nitrite) or NOCl (Nitryl- chloride) compound of a metal selected from the platinum group consisting of Ir, Ru, Rh and Pt.

4. A method according to claim 1 which further comprises electroplating a metal of the platinum group on the diffused coating.

5. A method according to claim 1 which further comprises non-electrolytically coating a metal of the platinum group on the diffused coating.

6. A method according to claim 1 which further comprises applying an oxide film on the diffused coating.

7. A method according to any of claims 1, 4 to 6 in which a cycle of the electroplating of the platinum group metal and the applying and heating of the metal compound are repeated twice or more.

8. A method according to any of claims 1, 4 to 7 in which a solution of two or more metal compounds of the platinum group metals is applied on the electroplated coating.

9. A method according to any of claims 1, 4 to 7 in which two or more solutions of different metal compounds of the platinum group metals are separately applied.

10. A method according to any of claims 1, 4 to 9 in which the application and heating of the metal compound are repeated untilthedesired coating thickness of a platinum group metal is obtained.

11. A method according to claim.1 which further comprises applying a film of a platinum group metal and oxide on the diffused coating.