JP2938164B2 - Electrode for electrolysis and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electrode for electrolysis used as an anode for electrolysis of an aqueous solution, particularly in an aqueous solution containing fluoride ions such as a chromium electroplating bath. The present invention relates to an anode for electrolysis that can be used as an anode and a method for producing the anode.

[Prior art] In the field of the electrochemical industry such as electrolysis and electroplating, the progress of the electrode which plays the most important role in the electrochemical reaction is remarkable. The emergence of insoluble metal electrodes, in which a coating containing a platinum group metal oxide is formed on a metal substrate called a valve metal that cannot conduct electricity, has led to the renewal of electrodes due to exhaustion in the electrochemical industry until then. What was indispensable was replaced by a long-life electrode in which the dimensions of the electrode did not change at all, and a long-term continuous operation became possible.

In the field of electroplating, which used only soluble anodes in the past, the application of insoluble metal electrodes represented by the trade name DSE allows the electrode adjustment process and plating process to be separated by insolubilizing the electrodes. As a result, plating can be performed at high speed, and a great advantage such as long-term continuous operation can be obtained.

Also in the field of electroplating, chrome plating has conventionally used an insoluble electrode made of lead or lead-containing metal instead of using chromium, which is the metal to be electroplated, for the anode.
Since chromium plating uses hexavalent chromic acid as the main component of the plating solution, the anode supplies the current required for plating and oxidizes trivalent chromium generated in the plating bath to hexavalent. Plays a role.

In chromium plating, which is carried out industrially, there are two types of baths: sulfuric acid-added catalyst and silicic-hydrofluoric acid-added as a catalytic substance, in order to increase current efficiency in the plating bath. The latter is called a hydrofluoric acid bath.

The Sargent bath is easy to use with little electrode wear and little corrosion of the plating equipment itself, but the current efficiency is about 13%
On the other hand, the hydrofluoric acid bath has a characteristic that the current efficiency is as high as about 20% and a stable and hard chromium plating layer can be obtained.

[Problems to be Solved by the Invention] However, there is a problem that the life of the anode is not sufficient when the anode of lead or lead alloy is used in chromium plating. In this case, the anode is melted by the components of the plating bath together with the dissolution of the anode during energization in the plating bath, so that there is a problem that the life of the electrode is extremely shortened.

Thus, lead dioxide or lead alloy anodes have been replaced with lead dioxide electrodes.

However, although lead dioxide is superior in corrosion resistance, it has a problem that its practical use is not easy because it is mechanically brittle as oxides generally have.

Attempts have also been made to apply electrodes formed with an oxide of a platinum group metal oxide according to the invention of DSE, but the ability to oxidize trivalent chromium to hexavalent is insufficient, and it is necessary to use it for chromium plating. Could not.

Therefore, an insoluble metal electrode in which a lead dioxide layer is formed on the electrode surface through a conductive intermediate layer using a valve metal such as titanium as a base as in the case of DSE has been put into practical use, and this has been used in chrome plating.

This electrode has a sufficient function as an anode of lead dioxide and has a sufficiently small elution, so that a long life can be expected. However, a problem arises in the case of a hydrofluoric acid bath. That is,
For unknown reasons, the use of lead dioxide-coated electrodes in a hydrofluoric acid bath, depending on the current density, can lead to small cracks in the lead dioxide layer over thousands of hours of use,
When the plating solution entering through the crack reaches the surface of titanium, which is the valve metal of the substrate, through the conductive intermediate layer,
There is a problem that the substrate is corroded and dissolved.

The conventional conductive intermediate layer is provided merely for maintaining conductivity, and has a thickness of 0.1 μm to 3 μm.
And fluoride ion resistance was not considered.

In order to solve this problem, attempts have been made to select conditions for forming lead dioxide which is less likely to cause cracking, but have not been able to obtain a material having sufficient performance.

On the other hand, even if cracks occur, it is only necessary that the base metal does not corrode, and it has strong resistance to fluoride ions.
It has been found that the purpose can be achieved by using niobium or tantalum as a substrate, and some of them have been used. However, considering that niobium and tantalum are several tens of times more expensive than titanium and have a higher specific gravity than titanium, in order to obtain an electrode of the same shape, the actual price difference is as follows: It was conceivable that it could open widely, which was a serious problem in practical use.

[Means for Solving the Problems] The present inventors have developed a lead dioxide-coated electrode having sufficient corrosion resistance and being economically sufficiently practical even when used in a bath containing fluoride ions. The present invention has been conceived by earnestly studying the provision.

That is, the present invention relates to an electrolytic solution comprising a corrosion-resistant metal as a substrate, a conductive titanium oxide layer having a thickness of 5 μm to 200 μm having substantially no through holes formed on the surface thereof, and a lead dioxide coating layer formed as a surface layer. An electrode, a method for producing an electrode for electrolysis by forming a conductive titanium oxide layer having a thickness of 5 μm to 200 μm on a corrosion-resistant metal substrate by a plasma spray method and forming a lead dioxide coating thereon by electrodeposition. is there.

The metal of the substrate used in the electrode of the present invention is not particularly limited as long as it does not touch the electrolytic solution, as long as it is a corrosion-resistant metal. A valve metal that does not easily cause elution, corrosion, and the like is preferable. Among the valve metals, niobium, tantalum, and alloys containing them have corrosion resistance to fluoride ion-containing baths, and are desirable metals, but are expensive, and are inexpensive because they are not easily available. Titanium or titanium alloy, which has a sufficient corrosion resistance for general use, is desirable.

In order to prevent the fluoride ions from directly contacting the substrate by coating the surface with conductive titanium oxide, it is necessary to perform dense conductive titanium oxide coating. Titanium oxide may be formed by any method as long as it can form a dense coating with various methods, but the most preferable one is a plasma spray method.

In other words, when coating titanium oxide whose composition is controlled by the plasma spray method, for example, magneli phase conductive titanium oxide, or coating rutile type titanium dioxide by the plasma spray method, the titanium oxide is instantaneously melted at a high temperature of 3000 ° C or more. In this state, it is fixed to the surface of the material to be treated, so that a dense coating having no through holes can be obtained even if the coating is thin. In addition, the instantaneous ultra-high temperature causes a part of oxygen of titanium oxide to be lost and voids to be generated, so that the conductivity of the conductive titanium oxide is maintained, and at the same time, the insulating titanium oxide is a rutile type titanium dioxide. However, it also has the advantage of imparting conductivity.

In order to completely impart conductivity, a so-called magneli phase titanium oxide, for example, a powder of Ti ₄ O ₇ or the like may be used, or a mixture of rutile-type titanium dioxide powder and metal titanium. It may be a raw material.

The thickness of the titanium oxide layer needs to be such that no through hole is formed in the intermediate layer. Such a thickness varies depending on the form of the oxide to be formed and the like, but may be 5 μm or more. In addition, since titanium oxide is a ceramic, if it is too thick, it may cause peeling or cracking from the base titanium, and furthermore, it is inferior in conductivity to metal, and if it is too thick, heat generation due to ohmic loss increases, so that it becomes thick. The upper limit is about 200 μm.

A lead dioxide coating layer is formed on the conductive titanium oxide intermediate layer, which may be performed according to a conventional method. However, since the base is not metal, care must be taken in its creation.

It is also considered that it is better to form the lead dioxide layer after increasing the conductivity of the titanium oxide layer and the adhesion to lead dioxide. To do so, after forming a titanium oxide layer, apply a solution containing a titanium, tantalum compound and a trace amount of a platinum compound, and thermally decompose at 450 to 550 ° C to form a more conductive coating on the surface of the titanium oxide layer. I do.

As the lead dioxide layer formed on the electrode surface, a β-lead dioxide layer may be directly formed by electrodeposition, but a thin layer of α-lead dioxide is electrodeposited on the intermediate layer, and β-lead dioxide is further deposited on this layer. If lead dioxide is electrodeposited, a uniform lead layer can be formed. Also, if necessary, a fiber or a powder having corrosion resistance and electrochemically inactivity, such as a tantalum oxide powder or a fine fibrous fluororesin, may be added to the β-lead dioxide layer to reduce the apparent electrodeposition strain. May go.

[Effect] An electrode for electrolysis, comprising: an intermediate layer made of conductive titanium oxide having a thickness of 5 μm to 200 μm on a corrosion-resistant metal base; and a lead dioxide coating layer on the electrode surface. The electrode is a highly corrosion-resistant electrolytic electrode obtained by forming an intermediate layer on a corrosion-resistant metal substrate by a plasma spray method and forming a lead dioxide coating layer on the intermediate layer by electrodeposition.

[Examples] Examples of the production of the electrode for electrolysis of the present invention will be described below, and the present invention will be described in more detail.

Example The surface of an expanded metal made of titanium having a thickness of 1.5 mm, a major axis of 20 mm, and a minor axis of 10 mm was roughened with a # 120 steel grit, and treated with 90% sulfuric acid at a concentration of 25% at 90 ° C. The surface was activated by pickling. Using titanium and rutile type titanium dioxide with a molar ratio of 10% titanium powder on the titanium surface as a coating material, the entire surface is coated with titanium oxide by a plasma spraying method using argon and hydrogen gas at a plasma forming condition of 40 KV. Formed.

The thickness of the coating was 100 μm, and no apparent through holes were found.

Next, 70% titanium, 5% tantalum, 25% platinum in molar ratio
Was prepared and applied on a titanium oxide layer and dried, and then thermally decomposed at 520 ° C. in air.

A solution obtained by dissolving lead monoxide in a 25% aqueous sodium hydroxide solution until saturation is used as an electrolyte. The electrode substrate on which the obtained intermediate layer is formed is used as an anode at 40 ° C. and a current density of 1 A / dm ^2.
The current was passed for a period of time to form a thin layer of α-lead dioxide.

Further, on the α-lead dioxide layer, a current was passed for 4 hours at a current density of 4 A / dm ² at 65 ° C. using an 800 g / l aqueous solution of lead nitrate as an electrolyte and the electrode substrate as an anode. A coating having a thickness of 600 μm was formed.

When the obtained electrode was electrolyzed as an anode at a current density of 30 A / dm ^{2 in} a chromic acid solution containing 5% silicic hydrofluoric acid, no problem occurred in the electrode even after 4000 hours.

Comparative Example An electrode manufactured by the same method as in the example except that the titanium oxide layer was not formed was subjected to electrolysis under the same conditions as in the example, and from the cracked portion of the lead dioxide layer at the lapse of 2000 hours. Dissolution of titanium in the substrate occurred, and power supply was disabled in 2500 hours.

[Effects of the Invention] The present invention provides a method for coating a corrosion-resistant metal substrate on a substrate of 5 μm to 200 μm.
It has an intermediate layer made of conductive titanium oxide with a thickness of m and a coating layer of lead dioxide on the electrode surface, and contains fluoride ions such as a fluoric acid bath using chromium plating. It is an electrode for electrolysis that consumes little even in the electrolyte solution to be used and enables stable electrolysis for a long period of time.

In addition, since corrosion resistance is high, relatively inexpensive titanium or the like such as titanium can be used for the electrode substrate.

Claims (3)

(57) [Claims] An intermediate layer made of conductive titanium oxide having a thickness of 5 μm to 200 μm and having substantially no through-holes on a substrate made of a corrosion-resistant metal, and having an α-lead dioxide layer on the intermediate layer. An electrode for electrolysis, comprising a coating layer of a β-lead dioxide layer on the α-lead dioxide layer. 2. The method according to claim 1, wherein the substrate is made of a corrosion-resistant metal and has a thickness of 5 μm to 20 μm.
An intermediate layer of 0 μm conductive titanium oxide is formed by a plasma spray method, and has an α-lead dioxide layer on the intermediate layer;
-A method for producing an electrode for electrolysis, comprising forming a coating layer of a β-lead dioxide layer on a lead dioxide layer by electrodeposition. 3. The electrode according to claim 1, wherein the corrosion-resistant metal is titanium or titanium-containing metal.