JP3147435B2 - Material for positive electrode container of sodium-sulfur battery and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art The battery reaction of a sodium-sulfur battery is 2
Represented by _{Na + xS = Na 2 S x} . At the time of discharge, the molten sodium ionized by releasing electrons moves to the positive electrode side through the solid electrolyte, reacts with the molten sulfur to become sodium polysulfide, receives the electrons, and becomes neutral. If connected to an external circuit during this process, electrons move from the negative electrode to the positive electrode with an electromotive force of about 2 V and operate as a battery. Since the molten sulfur does not have electron conductivity, the inside of the positive electrode container is filled with a porous conductive material so that electrons flowing from outside the container can enter the inside of the positive electrode container. On the other hand, at the time of charging, a reaction opposite to that at the time of discharging occurs, and sodium and sulfur are generated. That is, when a voltage equal to or higher than the electromotive force of the battery is applied between both electrodes,
Molten sodium forming sodium polysulfide in the positive electrode container is ionized, returns to the negative electrode side through the solid electrolyte, receives electrons, and is neutralized. The operating temperature of the battery is selected from the range of 300 to 350 ° C in view of the melting point of sodium polysulfide.

[0003] As a material for the positive electrode container, carbon steel or low alloy steel is usually used in consideration of economy.
However, Na ₂ S _x (sodium polysulfide) and S (sulfur) in the positive electrode container are extremely corrosive substances, and they cause severe sulfidation corrosion in carbon steel or low alloy steel, resulting in an increase in battery life and safety. Is a big problem.

In order to improve the corrosion resistance of the positive electrode container as described above, chromium (Cr) plating is applied to the inner surface of the positive electrode container (Japanese Patent Laid-Open No. 2-142066) and cobalt (Co) is used.
It has been proposed to coat an alloy (Japanese Patent Laid-Open No. 2-142065). However, these methods have the following problems.

[0005] In Cr plating, the adhesion between the base material and the plating film is weak, and at the operating temperature of the battery, film peeling due to the difference in thermal expansion between the base material and the plating film is concerned. The plasma spraying method is mainly used for coating the Co alloy, but air penetrates during the coating and bubbles are formed in the sprayed coating. Therefore,
Molten Na ₂ S _x may penetrate through these bubbles and corrode the base material.

On the other hand, a chromizing treatment is known as a method for imparting high-temperature corrosion resistance to steel materials. This method
Rather than placing a plating layer, etc. on the base material,
Since Cr is diffused and infiltrated to make the surface layer Cr-rich and improve corrosion resistance, there is no problem of peeling of the film.
Therefore, an invention utilizing this chromizing treatment for producing a material for a positive electrode container is proposed in British Patent Specification No. 2017389. The invention described in the specification uses a carbon steel as a base material, and Cr-Fe-C (carbon) containing more than 60% of Cr on its surface.
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 described later.

[0007]

In order to improve the corrosion resistance, it is effective to subject the inner surface of the positive electrode container to a chromizing treatment. The composition of the coating having corrosion resistance and the processing conditions for forming such a coating are unknown. For example, if the thickness of the chromized layer is inappropriate and the chromized layer is locally broken during use, corrosion proceeds at an accelerated rate.

Heretofore, there has been little study on the relationship between the composition of the chromized layer of the chromized steel and the corrosion resistance. Unless proper processing conditions are established, including problems such as elution of the chromized layer when the battery is used for a long period of time, a material for the positive electrode container with sufficient durability cannot be obtained.

In the positive electrode container material which has been subjected to the chromizing treatment proposed in the above-mentioned British Patent No. 2017389, Cr having excellent corrosion resistance is mainly contained on the inner surface side of the positive electrode container. The surface treatment layer (chromized layer) is formed, and the chromized layer has a two-layer structure in which the outer layer is a chromium carbide layer and the inner layer is a chromium alloy layer. Of the two-layered film, the outer layer of chromium carbide can contribute to the improvement of corrosion resistance, but the thickness of this layer is at most about 10 μm, and the inner layer is
The Cr concentration of the Cr diffusion layer is at most about 20% by weight. Therefore,
If the outer layer of the chromized layer is locally destroyed, the corrosion will spread at an accelerated rate. Further, the chromium carbide film has poor workability and cracks occur in the film due to slight distortion. Accordingly, there is a problem in that the chromium carbide layer is cracked by handling and strain relief processing in assembling the sodium-sulfur battery, and the corrosion resistance of the portion is significantly reduced.

[0010] An object of the present invention is to provide a material having excellent corrosion resistance to sulfur and sodium polysulfide, particularly at high temperatures, as a material for a positive electrode container.
An object of the present invention is to provide a positive electrode container material having a chromized layer having sufficient corrosion resistance and excellent workability, and a method for producing the same.

[0011]

The present invention is based on the above-mentioned prior art in that the corrosion resistance of the material for the positive electrode container is improved by a chromizing treatment.
However, for the purpose of further improving the corrosion resistance, the present inventors have studied in detail the conditions of the chromizing treatment and the chromized layer to be formed, and have found the following phenomena that have not been known before.

(A) If the Cr concentration in the steel is 25% or more, Na ₂ S
Shows sufficient corrosion resistance to Therefore, there is no need to form a chromium carbide layer on the surface.

(B) Nb and Ti are selected so as to satisfy the above expression.
The chromium carbide layer is not formed in the chromized layer of the carbon steel or the low alloy steel to which Cr is added, and only the Cr diffusion layer is formed.

(C) In addition, the Cr concentration of the diffusion layer thus formed is maintained at 25% or more, and has extremely good corrosion resistance.

(D) The above-mentioned diffusion layer has a σ phase as it is when subjected to the chromizing treatment and has poor workability.

However, by performing an appropriate post-treatment after the chromizing treatment, the diffusion layer can be transformed into the α phase having excellent workability without lowering the Cr concentration in the diffusion layer. Since this layer is rich in processability, the film does not crack even during processing during battery assembly.

The present invention based on the above findings provides the following (1)
The gist is the material of (2) and the manufacturing method of (2).

(1) Carbon steel containing at least one of Nb and Ti satisfying the following formula or Mo and 12 containing at least one of Nb and Ti satisfying the following formula and not more than 5% by weight: The base metal is a low alloy steel containing one or more types of Cr of up to 25% by weight, and the outermost surface of the base material has a Cr diffusion layer having a thickness of 25 μm or more and composed of an α phase having a Cr concentration of 25% by weight or more. A material for a positive electrode container of a sodium-sulfur battery having excellent corrosion resistance and workability, characterized by the fact that:

C ≦ (Nb / 10) + [(Ti-4N) / 6] where the element symbol indicates the content (% by weight) of the element.

(2) Carbon steel containing at least one of Nb and Ti satisfying the above formula, or Mo and 12 containing at least one of Nb and Ti satisfying the above formula and not more than 5% by weight. The base material of low alloy steel containing one or more types of Cr of less than
Immediately after applying a chromizing treatment at 1250 ° C to form a Cr diffusion layer with a Cr concentration of 25% by weight or more and a thickness of 25μm or more on the outermost surface, or immediately after furnace cooling or cooling, it is non-oxidizing 500 ℃ after reheating to 1000-1250 ℃ in atmosphere
(1) The method for producing a material for a positive electrode container of a sodium-sulfur battery according to the above (1), wherein the heat treatment is performed after cooling the temperature range up to 50 ° C./min.

[0021]

The effects of the components of the present invention and the reasons for limiting the conditions will be described below.

(I) Material for Positive Electrode Container The base material is carbon steel or low alloy steel containing Nb and / or Ti so as to satisfy the above formula. The carbon steel has a C content of about 0.02 to 0.15% by weight and does not contain alloying elements other than incidental elements such as Si and Mn. Low alloy steel is steel containing 5% by weight or less of Mo and 12% by weight or less of Cr, or both. In addition to these alloying elements, 0.02 to 0.15% by weight of C, 0.10 to 0.75% by weight of Si, 0.10 to 1.00% by weight of Mn, and about 0.50% by weight
It may contain the following Ni or the like. The contents of Cr and Mo are limited as described above for the following reasons.

The presence of Cr in the steel facilitates the formation of a chromium diffusion layer. However, when the content of Cr exceeds 12% by weight, the effect is saturated. Therefore, when adding Cr, the content is limited to 12% by weight. Mo also has the effect of preventing pitting of steel by S. However, this effect is saturated when the amount exceeds 5% by weight. Therefore, when Mo is added, the content is limited to 5% by weight. Cr and Mo are not necessarily required. These can be added alone or in combination of two or more.

The steel serving as the base material must contain Nb and / or Ti in a range satisfying the formula.
Nb and Ti combine with C in the steel to form NbC and TiC carbides that do not decompose even at the chromizing treatment temperature. Therefore, the amount of free C in the steel is reduced, and a Cr carbide layer is not formed even if Cr infiltrates by the chromizing treatment.

That is, instead of a Cr carbide layer, a Cr diffusion layer is formed on the outermost surface of the base steel. This is a major difference from that disclosed in the above-mentioned British Patent No. 2017389.

When the expression is not satisfied, that is, C> Nb / 10
In the case of + (Ti-4N) / 6, chromium carbide is generated on the outermost surface of the treated layer by the chromizing treatment, and the workability is deteriorated.

The Cr diffusion layer on the outermost surface of the base material needs to have a thickness of 25 μm or more, and the Cr concentration of the layer is 25% by weight.
Must be at least. If the thickness is less than 25μm,
If the Cr content is lower than 25% by weight, the corrosion resistance to sulfur and sodium polysulfide is insufficient.

Further, the Cr diffusion layer must be in the α phase. Usually, a σ phase is generated in a chromium-treated diffused layer in which the cooling after the treatment is furnace cooling or standing cooling, which is extremely hard and has no workability at all. In the present invention, the workability is enhanced by changing this to the α phase by the post heat treatment described below.

(II) Manufacturing Method The above-mentioned carbon steel or low-alloy steel is used as a material, formed into a predetermined shape, and then subjected to a chromizing treatment. As the chromizing treatment method, any of a powder pack method, a molten salt method, and a gas method can be adopted. In the inner surface treatment of a steel tube used as a positive electrode container material, a powder pack method in which a penetrating agent composed of chromium metal, ammonium chloride, and alumina powder is filled in the inside of the steel tube and treated is suitable. After such treatment, the steel pipe may be cut to a length required for the positive electrode container.

The temperature of the chromizing treatment is 1000 to 1250
C is appropriate. If the temperature is lower than 1000 ° C., the precipitation reaction of Cr is slow, and a long time is required for the treatment. In addition, when the temperature is higher than 1250 ° C., the material is greatly deformed.

After the chromizing treatment, the Cr diffusion phase is changed to α
The phase is changed. As shown in the well-known Cr-Fe phase diagram, in the Cr-Fe binary system, the σ phase exists below 815 ° C. Therefore, theoretically, if heating above this temperature, σ
Phase can be lost, but C, Si, Mn, and N
In the case of the material steel of the present invention containing i, Mo, etc., considering that the existence temperature of the σ phase exceeds 815 ° C., in order to completely eliminate the σ phase in the actual manufacturing process, the temperature should be 1000 ° C. or more. It is good to heat. However, at a temperature exceeding 1250 ° C., the material is greatly deformed as described above. Therefore, a suitable heating temperature for this heat treatment is 1000 to 1250 ° C.

The appropriate temperature for the subsequent heat treatment is the same as the above-mentioned temperature range for the chromizing treatment. Therefore, immediately after the chromizing process, the rapid cooling described later may be performed. However, when the material to be treated is a tube and the inside thereof is filled with a penetrant to perform the chromizing treatment, a predetermined cooling rate cannot be obtained by cooling with the penetrant filled. Therefore, after the treatment, it is necessary to perform an operation of taking out the penetrant by once cooling in a furnace or leaving it to cool, during which a σ phase is generated. In this case, after heating to 1000 to 1250 ° C again, rapid cooling described below is performed.

After the above chromizing treatment or reheating, the temperature range up to 500 ° C. is cooled at a cooling rate of 50 ° C./min or more. This is to suppress generation of the σ phase. In the case of the material steel of the present invention, the temperature at which the σ phase is formed is about 5
Since the temperature is in the range of 00 to 900 ° C., the rapid cooling of this temperature range can prevent the formation of the σ phase. The lower limit temperature of the quenching is 50 ℃ /
Minutes.

When the reheating is performed in an oxidizing atmosphere, for example, in the air, insulating Cr ₂ O ₃ is generated on the surface, and the power storage efficiency is reduced. Therefore, the heat treatment needs to be performed in a non-oxidizing atmosphere, for example, in a vacuum, an inert gas, or a reducing gas.

[0035]

Example 1 From a steel having a chemical composition shown in Table 1, a thickness of 2 mm
20mm wide and 20mm long corrosion test specimens and plate-like tensile test specimens (JI
S-Z2201 No. 13) was cut out. These were subjected to a chromizing treatment and a post heat treatment under the following conditions.

[Treatment conditions] Metal Cr: 70% by weight, Al ₂ O ₃ :
A powder packing method using a penetrant of 28% by weight and NH ₄ Cl: 2% by weight was employed. The chromizing treatment was carried out in hydrogen at 1050 ° C. for 10 hours. After forming a chromized layer having a thickness of about 60 μm, the solution was allowed to cool. The treated material is then at 1000 ° C in nitrogen
After heating for 10 minutes, it was cooled to 500 ° C at a cooling rate of 60 ° C / min.

Table 2 shows the types of the chromized layer after the treatment,
And the thickness of the layer having a Cr concentration of 25% by weight or more.

A test was performed using each of the obtained test pieces.
For the corrosion test, an apparatus whose schematic cross section is shown in FIG. 1 was used, and sodium polysulfide was filled therein, and the corrosion test piece having a tensile strain of 10% by weight in the film was immersed therein. After the whole was heated with a heater and kept at 350 ° C. for 500 hours, corrosion thinning was measured. In addition, in order to examine the workability of the chromizing layer, a 10% tensile strain was applied to the tensile test piece, and the state of cracking and peeling of the coating was investigated by the ferroxil method. The test results are also shown in Table 2.

Nos. 1 to 11 in Table 1 are those in which the chemical composition of the base material is within the range defined by the present invention, and Nos. 12 to 19 are (Nb / 10) +
[(Ti-4N) / 6] -C is negative, that is, the above formula is not satisfied. As shown in Table 2, in steels No. 1 to 11, the chromized layer is a Cr diffusion layer,
No carbide is generated. Moreover, the Cr diffusion layer is in the α phase. As a result, it shows extremely good corrosion resistance to sodium polysulfide and also has excellent workability.

On the other hand, when steels of Nos. 12 to 19 were used as materials, a Cr carbide layer having a thickness of 10 to 30 μm was formed in the outermost layer. As a result, not only the corrosion resistance to sodium polysulfide is poor, but also the workability is poor.

The corrosion resistance to sulfur of Nos. 1 to 11 and 12 to 19 was the same as the corrosion resistance to sodium polysulfide.

[0042]

Example 2 A corrosion test piece having the same size as in Example 1 was cut out from a steel having a chemical composition shown in Table 3 (a steel satisfying the conditions of the present invention), and the same conditions as in Example 1 (excluding the thickness of the chromized layer) were used. Only the processing time was changed as shown in Table 4 to change the size). Then 1000 ℃ in vacuum
And then cooled to 500 ° C. at a cooling rate of 100 ° C./min.

The corrosion test was performed in the same manner as in Example 1 for up to 3000 hours. The results are summarized in Table 4. Table 4
As can be seen from the comparative example, even if the chemical composition of the material satisfies the conditions of the present invention, if the thickness of the Cr diffusion layer is not sufficient, the corrosion resistance for a long time is insufficient. In the case of the example of the present invention in which the thickness of the chromized layer is 25 μm or more, extremely good corrosion resistance to sodium polysulfide is exhibited even when used for a long time.

[0044]

Example 3 A tensile test piece having the same size as in Example 1 was cut out from a steel having the chemical composition shown in Table 3 (a steel satisfying the conditions of the present invention), and subjected to chromizing treatment under the same conditions as in Example 1. Thereafter, post-heat treatment was performed in argon under the conditions shown in Table 5. Thereafter, a 10% tensile strain was applied to the test piece, and the test was performed in the same manner as in Example 1 to check the state of cracks in the chromized layer by the ferroxyl method. The results are also shown in Table 5.

As is evident from Table 5, when the heating temperature of the post heat treatment is too low (No. 8) and when the cooling rate is too low (Nos. 9 to 11), a σ phase is formed and workability is increased. Is bad and cracks have occurred. However, the examples of the present invention in which the heating temperature and the cooling rate are appropriate do not show any cracks, and the workability of the chromized layer is excellent.

[0046]

[Table 1 (1)]

[0047]

[Table 1 (2)]

[0048]

[Table 2]

[0049]

[Table 3]

[0050]

[Table 4]

[0051]

[Table 5]

[0052]

The material of the present invention has extremely excellent corrosion resistance to sodium polysulfide and sulfur due to the presence of a unique Cr diffusion layer on its surface. In addition, since it has excellent workability, handling and molding are easy. This material can be relatively easily produced at low cost by the method of the present invention, and greatly contributes to the improvement of durability as a material for a positive electrode container of a sodium-sulfur battery.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing an outline of a corrosion test apparatus.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-280955 (JP, A) JP-A-3-133066 (JP, A) JP-A-59-80965 (JP, A) JP-A-58-1983 152379 (JP, A) JP-A-57-57861 (JP, A) JP-A-56-73868 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 10/39

Claims (2)

(57) [Claims] 1. Carbon steel containing one or more of Nb and Ti satisfying the following formula, or Mo and 12 wt.% Containing one or more of Nb and Ti satisfying the following formula and not more than 5% by weight: %below
The base material is a low alloy steel containing one or more types of Cr, and the outermost surface of the base material is composed of an α phase having a Cr concentration of 25% by weight or more.
A material for a positive electrode container of a sodium-sulfur battery having excellent corrosion resistance and workability, characterized by having a Cr diffusion layer of 25 μm or more. C ≦ (Nb / 10) + [(Ti-4N) / 6] where the element symbol indicates the content (% by weight) of the element. 2. Carbon steel containing at least one of Nb and Ti satisfying the following formula, or Mo and 12 wt.% Containing at least one of Nb and Ti satisfying the following formula and not more than 5% by weight: %below
The base material of low alloy steel containing at least one type of Cr is 1000-1250
Chromizing treatment at ℃ to make the outermost surface have a Cr concentration of 25
Immediately after forming a Cr diffusion layer with a thickness of 25% or more by weight or more, or after being once cooled in a furnace or allowed to cool and then reheated to 1000 to 1250 ° C in a non-oxidizing atmosphere, The method for producing a positive electrode container material for a sodium-sulfur battery according to claim 1, wherein a heat treatment is performed after cooling the temperature range at a rate of 50 ° C / min or more. C ≦ (Nb / 10) + [(Ti-4N) / 6] where the element symbol indicates the content (% by weight) of the element.