JPH0124228B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an electrolytic cathode in which a reaction to reduce hydrogen ions occurs in aqueous electrolysis. The present invention can be particularly advantageously employed in aqueous alkali chloride electrolysis for producing chlorate, hypochlorite, perchlorate, and caustic alkali-chlorine (diaphragm method and ion exchange membrane method). Hydrochloric acid aqueous electrolysis,
It is also used as a cathode for aqueous electrolysis involving reactions that reduce hydrogen ions on the electrode surface, such as water electrolysis, electrolytic oxidation and reduction in aqueous solutions, or electropolishing. An object of the present invention is to provide a long-life cathode that has low overvoltage and excellent corrosion resistance as a hydrogen ion reduction electrode. Conventionally, graphite had various drawbacks as an anode for aqueous electrolysis, so the development of an insoluble metal anode (DSA) has made dramatic progress. On the other hand, soft iron has always been used for the cathode, but since iron is a cheap and easy-to-use material, until recently there has been little direction in the development of new materials for the cathode. However, soft iron has a relatively high overvoltage as a hydrogen ion reduction cathode, and it cannot be said to have sufficient corrosion resistance against hydrogen embrittlement, dissolved chlorine, salt water, etc. In recent years, research into new cathode materials has been actively conducted from the standpoint of energy conservation, and many proposals have been made. Among these proposals, some inventions that certainly improve overvoltage characteristics can be recognized. However, in terms of corrosion resistance, all of these inventions are unsatisfactory for industrial use as they are below the iron cathode, and none of the inventions provides a solution to the problem of the active layer dissolving into the electrolyte when the electrolytic cell is stopped. Not yet. As a result of repeated research aimed at improving these defects, the present inventors succeeded in developing an industrially valuable cathode that has extremely low overvoltage and excellent corrosion resistance. That is, the present invention provides chromium plating using chrome steel or chrome-nickel steel as a base material, and on the surface thereof, one or more metal elements selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum. This is a corrosion-resistant activated cathode for aqueous solution electrolysis, which is characterized by being provided with a metal oxide layer containing an oxide as a main component. The electrode base material of the present invention is required to have high electrical conductivity, sufficient mechanical strength, good processability, and high corrosion resistance. Particularly in the case of aqueous alkaline chloride electrolysis, the cathode is exposed to a strongly reducing atmosphere during operation, but during a power outage, the atmosphere changes to a strongly oxidizing atmosphere and is attacked by dissolved chlorine in the electrolyte. As a result of various searches for materials that can withstand reducing atmospheres and are resistant to oxidation, it has been found that it is industrially advantageous to use chrome steel or chrome-nickel steel as relatively inexpensive materials. As is well known, these materials are called stainless steels. As chromium steel, so-called high chromium steel containing 15 to 30% Cr is good, and as chromium-nickel steel, Cr-containing steel is good.
15 to 25%, and one containing 6 to 22% Ni is preferable, but since those with a high nickel content are expensive, from an industrial perspective, one containing 6 to 15% Ni is preferable. It is resistant to hydrogen embrittlement and shows corrosion resistance to oxidation, and in order to provide the above characteristics as a cathode base material, it is desirable that the trace components in the base material be in the following ranges. That is, Mo; 0.1
~2.0wt%, C; 0.02~0.2wt%, Si; 0.02~1.0wt
%, P; 0.02-0.05wt%, Cu; 0.05-0.6wt%,
S; 0.005wt% or less, N; 0.01wt% or less, Mn;
2.5wt or less. The cathode shape of the present invention is (a) plate-like, (b) sheet-like, (c) plate-like with many holes, sheet-like, (d) mesh-like (including expanded metal), and (e) sagging. It is a box-shaped or cylindrical item made by welding a mesh, sudare, or punch metal to a plate, pipe, rod, or rib. The chromium layer plated on the surface of these base materials is
It adheres well to the chromium and nickel components of stainless steel, protecting the base material well against the corrosive environment, and at the same time forms a rutile type oxide with the outermost active noble metal to form a strong bond. The metal oxide layer provided on the outermost surface of the electrode must be active for the desired reaction and have high corrosion resistance, wear resistance and electrical conductivity. The metal oxide is selected from oxides of ruthenium, rhodium, palladium, osmium, iridium and platinum. These metal oxides can be used alone or as a mixed oxide of these metals, and these oxides can be provided as a single layer or a multilayer. The method for forming the metal oxide layer that constitutes the cathode surface is to pre-treat the base material, apply chrome plating, apply a salt solution of the noble metal to this surface, and heat-treat it to generate the metal oxide. and fix it at the same time. Specifically, the oxide film on the surface of the base material is removed by sand blasting, buffing, or etching to increase the unevenness and make it easier for plating to adhere. Such treatment also removes corrosive surface components of chrome steel and chrome-nickel steel, increasing corrosion resistance and providing benefits such as substantially increasing surface area and reducing overvoltage. . In the etching treatment, the substrate is immersed in a mixed aqueous solution of nitric acid, hydrochloric acid, sulfuric acid and hydrogen fluoride at a temperature of 50 to 100°C for 5 minutes to 3 hours, preferably 10 to 30 minutes, and then washed in clean water. This operation can be repeated several times as necessary. Etching agents are not particularly strict as long as they are compatible with the metal that constitutes the base material, and in addition to mineral acids of any concentration, there are two types of etching agents: hydrogen fluoride water, hydrogen fluoride and glycene, mineral acids, and hydrogen peroxide. The above mixed aqueous solution may also be used. Another method of etching is electrolytic polishing in a plating bath or other electrolyte solution. In this case, the current is 10~80A/
dm ² and the time is 5 to 60 seconds. After etching, wash thoroughly with clean water, degrease with acetone, toluene, or alcohol, and plate with chrome. The plating bath may be one commonly used, for example, a bath containing approximately 250 g of chromium sulfate steel (in terms of CrO ₃ ) and 2.5 g of sulfuric acid at a temperature of 20 to 90°C, preferably 50 to 80°C, and a current density of 10 to 100 A/dm. ² , preferably 20-60A/ ^dm2
Perform electroplating for 10 to 60 minutes. Here, in order to eliminate the formation of pinholes, the
Electrolytic polishing is performed by passing a reverse current of 10 to 80 A/dm ² for 2 seconds, and then plating is applied on top of this. This operation is repeated several times to obtain a thickness of 5 to 100 microns, preferably 10 to 50 microns. The inorganic or organic salt of the noble metal element forming the metal oxide is added to hydroxyl or an organic solvent in an amount of 0.05 to 2 g atoms per metal atom, preferably 0.1 to 2 g atom per metal atom.
Dissolve to a concentration of 0.5g atom/. Examples of organic solvents used include propanol, dimethylformamide, 2-ethylhexyl alcohol, lavender oil, and anise oil, but any solvent may be used as long as it dissolves the metal salt. In order to coat the plated electrode base material with the metal salt solvent, first, the same pretreatment as the surface of the base material before plating is performed. That is, the oxide film is removed by sandblasting, buffing, or etching to increase surface roughness. This substrate is heated in a heating furnace or on a hot plate at a temperature of 50 to 800℃, preferably 300 to 600℃.
The sample is heated in an oxygen atmosphere (generally air), removed and coated with the metal salt solution. Application methods include spraying, brushing, and immersion in a metal salt solution. 1 to 10 at the above temperature after application.
Time to sinter. This coating process can be repeated three or more times, preferably 5 to 10 times. This allows the metal oxide layer to have a thickness of 0.5 to 50 microns, preferably 1 to 10 microns. When providing a layer of two or more metal oxides, there are two methods: coating with a mixed solution of two or more metal salts and heating and baking, and a method of coating with one metal salt solution, heating and baking, and then applying another metal salt solution. There is a method in which the coating is heated and fired, and then this process is repeated alternately. It is thought that the metal oxide layer of the cathode produced in this manner acts by forming a rutile type oxide and adhering to the chromium plated on the base metal. When the cathode of the present invention is used in an electrolytic cell, it is either electrically connected to the electrolytic cell as a part of the structural material of the cell body, or electrically insulated from the cell body and used opposite to the anode. Next, the characteristics of the cathode of the present invention will be listed. (a) Overvoltage is extremely small, so power consumption is low.
Furthermore, since the cathode current density can be increased accordingly, it is possible to manufacture electrolytic cells with large production units. (b) Since the reduction reaction occurs on the surface of the metal oxide layer, the electrode base material body is protected and its lifespan is long. In particular, it has excellent corrosion resistance against dissolved chlorine during power outages, has a long life as a cathode, and is easy to maintain. (c) Since the potential gradient with respect to the current is small, a large current can be used, making it possible to use a compact electrolytic cell with a small floor space. Next, the present invention will be explained with reference to Examples, but the present invention is not limited thereto in any way. Examples 1 to 11 A large number of stainless steel, cylindrical chips with an effective area of 0.8 ^cm2 were immersed in a mixed aqueous solution of nitric acid, hydrochloric acid, sulfuric acid, and hydrogen fluoride at a temperature of 60 to 70°C for 15 minutes, and then washed with warm water. Then, it was degreased with acetone. Chromium sulfate 250g/250g using lead as an anode on this chip
(Converted to CrC ₃ ), temperature in a plating bath containing 2.5 g of sulfuric acid/
Chrome plating was applied for 20 minutes at 60° C. and a current density of 30 A/dm ² . To remove pinholes, the same amount of reverse current was applied for about 3 seconds and electrolytic etching was carried out, and then again.
I did the metsuki for 20 minutes. The thickness of the chrome plating was estimated to be 30 μm. The chrome-plated stainless steel chips produced in this way were heated to 300°C in an electric furnace, quickly taken out and brushed with a metal salt solution of 0.1 mol/concentration, and the chips immediately became dry. After repeating this operation 10 times, the temperature of the electric furnace is
The temperature was raised to 500°C and fired for 3 hours, followed by slow cooling to produce a cathode with an active surface. This cathode was incorporated into a rotating electrode, and NaCl200g/
, 25A/dm ² , 1000r with a solution at PH7.0 and temperature 40℃.
When the cathode potential at pm was measured, the results of Examples 1 to 11 in Table 1 were obtained. Note that propanol to which hydrochloric acid was added was used as the solvent for the metal salt solution.

【table】

[Table] As shown in Examples 1 to 11, the cathode of the present invention has a nobler potential of 0.23 to 0.55 volts than the conventional soft iron cathode. Examples 12 to 15 An electrolytic cell was constructed by combining a cathode obtained by the same treatment as in Examples 3, 4, 5, and 9 and an anode whose titanium surface was coated with Ru-Rh oxide. An electrolysis experiment was carried out in an electrolytic bath for soda production, and the results shown in Examples 12 to 15 in Table 2 were obtained. The composition of the electrolytic bath is NaCl200g/,
NaClO ₃ 250g/, Na ₂ Cr ₂ O ₇ 2.5g/, PH6.5,
The temperature is 70℃, the voltage is 50A/dm ² , and the distance between electrodes is 3.
This is the average value over 160 hours at m/m.

[Table] As shown in Examples 12 to 15, the cathode of the present invention can lower the cell voltage by 0.25 to 0.3 volts compared to the conventional soft iron cathode. Moreover, the corrosion resistance was also sufficient.

Claims (1)

[Claims] 1 Chrome plating is applied to a base material of chrome steel or chrome-nickel steel, and an oxide of one or more metal elements selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum is applied on the surface. A corrosion-resistant activated cathode for aqueous solution electrolysis characterized by providing a layer of metal oxide as a main component.