JPH05290880A - Material for positive electrode container of sodium-sulfur battery and manufacture thereof - Google Patents

Abstract

(57) [Summary] [Object] To provide a material for a positive electrode container of a sodium-sulfur battery having improved corrosion resistance to high temperature sulfur and sodium polysulfide, and a method for producing the same. [Structure] (1) Mainly composed of chromium carbide on the outermost surface of a base material of carbon steel having a C content of 0.55 wt% or less, or a low alloy steel containing 5 wt% or less of Mo and / or 12 wt% or less of Cr A positive electrode material for a sodium-sulfur battery having excellent corrosion resistance, which has a chromized layer having a thickness of 25 μm or more. (2) Carburizing the base material of the above composition in advance, after adjusting the carbon concentration of its surface layer to 0.7 to 1.5 wt%, then subjecting it to chromizing treatment at a temperature of 1000 to 1150 ° C. and chromium carbide on its surface. A method for producing a positive electrode material for a sodium-sulfur battery having excellent corrosion resistance, which comprises forming a coating layer as a main component. [Effect] Corrosion of the positive electrode container can be prevented, and the durability of the positive electrode container is significantly improved as compared with the conventional battery.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode material in contact with sulfur and sodium polysulfide in a sodium-sulfur battery, and more particularly to a positive electrode container material having excellent corrosion resistance to sulfur and sodium polysulfide and a method for producing the same.

[0002]

2. Description of the Related Art The battery reaction of a sodium-sulfur battery is 2
It is represented by Na + xS = Na ₂ Sx. During discharge, the molten sodium ionized and liberated from the electrons moves to the positive electrode side and reacts with the molten sulfur to become sodium polysulfide, which is stored in the positive electrode chamber. When the battery is connected to an external circuit in this process, electrons move from the negative electrode to the positive electrode and generate an electromotive force of about 2V. Since molten sulfur does not have electronic conductivity,
The positive electrode container is filled with a porous conductive material so that electrons flowing from the outside of the container can enter the inside of the positive electrode container.
On the other hand, at the time of charging, the opposite reaction to that at the time of discharging occurs and sodium and sulfur are produced. That is, a voltage equal to or higher than the electromotive force of the battery is applied between both electrodes to ionize the molten sodium forming sodium polysulfide in the positive electrode container, return it to the negative electrode side through the solid electrolyte, and receive an electron to neutralize it. .. The temperature at which the battery operates is selected to be about 300 to 350 ° C in view of the melting point of sodium polysulfide.

Carbon steel or low alloy steel is used as the positive electrode container material, and Na ₂ Sx stored in the positive electrode container is used.
Since S and S are extremely corrosive substances, carbon steel or low alloy steel may not have sufficient corrosion resistance and may cause significant sulfidation corrosion, which is a major problem in battery life and safety. .. Therefore, in order to improve the corrosion resistance of the positive electrode container as described above, the inner surface of the positive electrode container is plated with chromium (Cr) (JP-A-2-142066) and coated with a cobalt (Co) alloy (special feature). Kaihei 2-142065) has been proposed. However, these methods have the following problems.

[0004] In Cr plating, the adhesion between the base material and the plating film is weak, and at the operating temperature of the battery, there is concern that the film may peel off due to the difference in thermal expansion between the base material and the plating film. The plasma spraying method is mainly used for coating the Co alloy, but during the coating, air enters and bubbles are formed in the sprayed coating. Therefore,
Molten Na ₂ Sx may penetrate through these bubbles and corrode the base material.

On the other hand, chromizing treatment is known as a method for imparting high temperature corrosion resistance to steel materials. This method
Instead of putting a plating layer etc. on the base material,
Since the surface layer portion is made to be rich in Cr by diffusing and permeating Cr to improve the corrosion resistance, there is no problem of peeling of the film.
Therefore, an invention in which this chromizing treatment is used for manufacturing a material for a positive electrode container is proposed in British Patent No. 2017389. The invention described in the same specification uses a carbon steel as a base material and Cr-Fe-C (carbon) on the surface of which Cr exceeds 60%.
However, if the composition and thickness of this layer are not appropriate, sufficient corrosion resistance as a material for the positive electrode container cannot be obtained, as will be described later.

[0006]

In order to improve the corrosion resistance, it is effective to subject the inner surface of the positive electrode container to a chromizing treatment, but it is more effective than a mixture of molten sulfur and sodium polysulfide. The coating structure with excellent corrosion resistance and the treatment conditions for forming such a coating are unknown.
For example, if the thickness of the chromized layer is improper and the chromized layer is locally broken during use, it is expected that corrosion will accelerate. Conventionally, little consideration has been given to the corrosion resistance of chromized steel from the electrochemical viewpoint, and proper treatment conditions have been established, including problems such as the dissolution of the chromized layer when the battery is used for a long time. If not done, a material for a positive electrode container having sufficient durability cannot be obtained.

An object of the present invention is to provide, as a positive electrode container material, a material having excellent corrosion resistance to sulfur and sodium polysulfide, particularly at high temperatures.
It is to provide a material for a positive electrode container having a chromized layer having sufficient durability and a method for producing the same.

[0008]

The gist of the present invention is as follows.
There are the positive electrode container material of (1) and the manufacturing method of (2).

(1) Carbon steel having a C content of 0.55% by weight or less,
Alternatively, a low alloy steel having a C content of 0.55% by weight or less and one or both of Mo and 5% by weight or less of Cr and 12% by weight or less is used as a base material, and chromium carbide is provided on the outermost surface of the base material. Sodium with excellent corrosion resistance characterized by having a chromized layer with a thickness of 25 μm or more as a main component
Material for positive electrode container of sulfur battery.

(2) Carbon steel having a C content of 0.55% by weight or less,
Alternatively, the surface of the base material of the low alloy steel having a C content of 0.55% by weight or less and one or both of Mo of 5% by weight or less and Cr of 12% by weight or less is preliminarily carburized, After setting the carbon concentration to 0.7 to 1.5% by weight, 1000 to
For sodium-sulfur battery positive electrode container with excellent corrosion resistance, characterized by forming a chromized layer mainly composed of chromium carbide with a thickness of 25 μm or more on the outermost surface of the base material after chromizing treatment at a temperature of 1150 ° C. Material manufacturing method.

[0011]

The present inventors have conducted research to improve the corrosion resistance of a positive electrode container material for sodium-sulfur batteries by subjecting it to chromizing treatment, and have obtained the following findings in the process.

When the inner surface of the positive electrode container is subjected to chromizing treatment in order to improve the corrosion resistance, the chromized layer formed has a two-layer structure in which the outer layer is a chromium carbide layer and the inner layer is a chromium alloy layer. In this chromized layer, it is the outer chromium carbide layer that can contribute to the improvement of corrosion resistance. And the thickness of the chrome carbide layer obtained by normal chromizing treatment is at most
It is about 10 to 15 μm. However, with such a thickness of the chromium carbide layer, the corrosion resistance as the positive electrode container is not sufficient, and if this layer is locally destroyed, the corrosion progresses at an accelerated rate. That is, since the chromium carbide layer is outwardly grown (grows outward from the surface of the base material), if there are inclusions such as carbides and oxides on the surface of the base material, a uniform chromium carbide layer is present at that portion. Is not coated, leaving pinholes in the film. Further, hair cracks (fine cracks) occur in the coating film due to the difference in thermal expansion between the steel material used as the material of the positive electrode container and the chromium carbide. Due to these reasons, when the thickness of the chromium carbide layer is insufficient, it is not possible to obtain the corrosion resistance to withstand long-term use as the positive electrode container.

The material for the positive electrode container of the sodium-sulfur battery contacts Na ₂ Sx and S, which are extremely corrosive, for a long time and at high temperature. As a material for a positive electrode container having sufficient durability under such severe use conditions, the thickness of the chromium carbide layer must be 25 μm or more. With such a thickness, the chromium carbide layer also grows in the lateral direction, and even if there is an inclusion on the surface of the base material, it can be coated to cover the inclusion. Further, even if some pinholes or hair cracks are formed in the coating, the actual damage that impairs the corrosion resistance is almost eliminated. The chromium carbide layer referred to here is a layer mainly composed of Cr ₂₃ C ₆ or Cr ₇ C ₃ , or an outer layer composed of Cr ₂₃ C ₆ and an inner layer composed of Cr ₇ C ₃ .

The thickness of the chromium carbide layer becomes thicker as the amount of C in the steel increases. However, since the positive electrode container is formed into a cylindrical shape by drawing, the base material steel is also required to have excellent drawability. Therefore, the C content in the base steel cannot be made higher than 0.55% by weight.
Therefore, in the present invention, the C content of the base material steel is suppressed to 0.55% by weight or less, and only the surface layer portion thereof is carburized in advance to increase the C content, and then the chromizing treatment is performed to 25 μm.
It was decided to obtain a carbide layer having the above thickness.

Carburizing the base material in advance has the following unexpected effects. That is, the cementite layer generated in the base material surface layer portion during the carburizing treatment acts as a layer for relaxing the thermal stress generated in the cooling process after the chromizing treatment, and the occurrence of hair cracks in the chromizing layer is prevented. ..

The steel used as the base material for the positive electrode container material of the present invention has a C content of 0.55% by weight or less, preferably 0.02 to 0.50.
Wt% carbon steel, or one or two of Cr and Mo
A steel with added seeds. In addition to these, naturally steel components such as Mn and Si may be naturally contained. Mn and
The preferred Si contents are 0.2-2% by weight and
It is 0.05 to 2% by weight. The reason why the upper limit of the C content is 0.55% by weight is as described above.

The presence of Cr in the base steel accelerates the diffusion of Cr in the steel during the chromizing treatment, facilitates the formation of a chromium carbide layer, and shortens the chromizing treatment time. However, when the Cr content exceeds 12% by weight, the effect is saturated. Also, Mo in the base steel
Addition improves the corrosion resistance to Na ₂ Sx and S. However, even if the content of Mo exceeds 5% by weight, the above effect does not further increase. Therefore, one or both of Cr and Mo may be contained in the range of 12% by weight or less and 5% by weight or less, respectively, if necessary. The preferable contents of Cr and Mo are 0.5 to 12% by weight and 0.1 to
It is 5% by weight.

The base material having the above composition is previously carburized. The carburizing treatment includes solid carburizing, liquid carburizing, gas carburizing and the like, and any carburizing method is effective,
Solid carburization is cost effective.

The carburizing condition is that the carbon concentration in the surface layer is 0.7
It must be chosen to be ~ 1.5% by weight. In the case of the solid carburizing method, the above carbon concentration can be obtained by treating for 10 minutes in the temperature range from 1000 ° C to 1100 ° C. If the carbon concentration in the surface layer is less than 0.7% by weight, it is difficult to obtain a carbide layer having a thickness of 25 μm or more. On the other hand, when the carbon content of the carburized layer exceeds 1.5% by weight, carburization proceeds to the inside of the base metal and cementite is generated inside the base metal.
The impact resistance of the base material decreases.

Subsequently, the base material is subjected to a chromizing treatment. As the chromizing treatment, any of a powder pack method, a molten salt method and a gas method can be adopted, but a method of treating at a temperature of 1000 to 1150 ° C. by the powder pack method is most suitable. In the present invention, a layer mainly composed of chrome carbide has a thickness of 25 μm.
It is indispensable to generate it with a thickness of m or more (preferably 30 to 100 μm). The desirable conditions for obtaining this thickness are as follows.

As the penetrant, 65 to 95% by weight of metallic chromium or ferrochrome, 5 to 35% by weight of alumina (Al ₂ O ₃ ) which is a sintering inhibitor, and 1 to the total amount thereof are used.
~ 3 wt% flux of halide (NH ₄ Cl, NH
₄ Br, NaCl, etc.) is preferably used.
This is because if the amount of metallic chromium or ferrochrome in the penetrant is less than 65% by weight, stable formation of chromium carbide is difficult to occur, and if it exceeds 95% by weight, fusion of metallic chromium or ferrochrome occurs. .. The amount of alumina is relatively determined by the amount of metallic chromium or ferrochrome.

The processing temperature is in the range of 1000 to 1150 ° C. as described above. If the treatment temperature is less than 1000 ° C, sufficient precipitation reaction of Cr does not occur and chromium carbide does not grow. On the other hand, when the temperature exceeds 1150 ° C, chromium carbide causes thermal decomposition and does not become stable and thick.

By the above treatment, a chromium carbide layer film having a thickness of 25 μm or more is obtained. Moreover, in this treatment, the reactions of the carburizing and chromizing treatments are limited to the surface layer only, so that the mechanical properties of the base material are not adversely affected.

The positive electrode container has a cylindrical shape, and a steel pipe is used as a material. When performing this internal surface treatment, a solid carburizing method using charcoal and barium carbonate is suitable as a carburizing method, and a chromizing treatment method is an infiltration consisting of a mixed powder of metallic chromium, ammonium chloride and alumina. A powder pack method in which it is embedded in the agent and treated is suitable. That is, a carburizing agent is filled in the material steel pipe to carry out a carburizing treatment, the carburizing agent is discharged after the treatment, and then a penetrating agent is filled and a chromizing treatment is carried out. After that, the steel pipe is cut into a required length to make a positive electrode container.

[0025]

[Example] As a positive electrode container material (base material), Table 1 (1) and
A test material having the composition shown in (2) was prepared. No.10 of Table 1 (2)
Nos. 13 to 13 are examples in which the C content of the base steel is outside the range specified in the present invention, and Nos. 14 to 17 are examples in which the chromizing treatment conditions are inappropriate. No. 18 is a base metal that has not been carburized or chromized. Table 1
A corrosion test and a workability test were performed with the material of the present invention of (1). The carburizing process is 70% by weight charcoal and 30% by weight.
Was carried out at 1000 ° C. for 10 minutes by a solid carburizing method using a carburizing agent composed of barium carbonate. The chromizing process is
80 wt% Cr, 18 wt% Al ₂ O ₃ and 2 wt% NH ₄ Cl
The powder pack method using the penetrant was used.

FIG. 1 shows a schematic cross section of a corrosion test apparatus. An α-alumina Tammann tube 2 was placed in a corrosion test container 1, and a test piece 4 of the above-mentioned test material was placed in sodium polysulfide 3 inside the tube.
Was dipped, the test container 1 was heated in an electric furnace and kept at 350 ° C. for 500 hours to perform a corrosion test. Corrosion resistance is No.1 in Table 1 (2)
Evaluated by relative comparison with the corrosion amount of 8 as 100.

The workability test is carried out by a tensile test, the drawing of the fractured part is measured, and the drawing value of No. 18 is 1 as with the corrosion resistance.
The workability was evaluated by comparing the aperture values of the respective materials with each other as 00.

The test results are summarized in Tables 1 (1) and (2).

[0029]

[Table 1 (1)]

[0030]

[Table 1 (2)]

As shown in Table 1 (1) and (2), No. 1-
The present invention material No. 9 is an untreated base material No. Compared with 10, the corrosion resistance is significantly improved. Moreover, the workability (aperture value) is as high as that of the untreated base metal. On the other hand, the comparative materials No. 10 to 13 have poor workability because the C content of the base material is too high. Also N
In o.14 and 17, since the temperature of the chromizing treatment is inappropriate, the thickness of the chromium carbide layer is insufficient and the corrosion resistance is insufficiently improved.

The materials of the present invention all have a smaller amount of corrosion as compared with the conventional materials, and the number of pittings after the corrosion test is small, showing excellent characteristics. As a result of producing an actual battery using these materials of the present invention and performing an actual machine test, sufficient durability and battery performance were obtained.

[0033]

The material subjected to the chromizing treatment of the present invention is excellent in corrosion resistance against molten sulfur and sodium polysulfide and is not bad in workability. Therefore, it is possible to greatly extend the life of the battery by using it for the positive electrode container of the sodium-sulfur battery. The material of the present invention can be easily and relatively inexpensively manufactured by the method of the present invention in which a carburizing treatment and a chromizing treatment are combined.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a corrosion test device for a positive electrode container material.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshio Kudo 4-533 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Nobuo Otsuka 4-chome, Kitahama, Chuo-ku, Osaka City, Osaka Prefecture No. 5-33 Sumitomo Metal Industries Co., Ltd. (72) Inventor Sadao Mori 1-3-1, Uchiyuki-cho, Chiyoda-ku, Tokyo Tokyo Electric Power Company (72) Tadashi Maruyama 1-1-1-1, Uchiyuki-cho, Chiyoda-ku, Tokyo No. 3 in Tokyo Electric Power Company (72) Inventor Mitsuo Chizaki 4026 Kuji Town, Hitachi City, Hitachi, Ibaraki Prefecture Hitachi, Ltd. Hitachi Research Laboratory (72) Inventor Yoshiteru Chiba 4026 Kuji Town, Hitachi City, Ibaraki Hitachi Corporation Inside Hitachi Research Laboratory (72) Inventor Hisamitsu Hatto 3-1, 1-1 Sachimachi, Hitachi City, Ibaraki Hitachi Co., Ltd. Inside Hitachi Factory

Claims (2)

[Claims] 1. A carbon steel having a carbon content of 0.55 wt% or less, or a low carbon steel having a carbon content of 0.55 wt% or less and containing 5 wt% or less of molybdenum and 12 wt% or less of chromium. A material for a positive electrode container of a sodium-sulfur battery having excellent corrosion resistance, which comprises an alloy steel as a base material and has a chromized layer having a thickness of 25 μm or more, which has chromium carbide as a main component on the outermost surface of the base material. 2. A carbon steel having a carbon content of 0.55% by weight or less, or a low carbon content of 0.55% by weight or less and one or both of molybdenum of 5% by weight or less and chromium of 12% by weight or less. The surface of the base material of the alloy steel is carburized in advance, and the carbon concentration in the surface layer is adjusted to 0.7 to 1.5% by weight.
Furthermore, chromizing treatment is performed at a temperature of 1000 to 1150 ° C, and a chromized layer with a thickness of 25 μm or more, mainly composed of chromium carbide, is formed on the outermost surface of the base material. Manufacturing method of material for positive electrode container.