JP3161269B2 - Stainless steel with excellent resistance to molten carbonate corrosion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel having excellent molten carbonate corrosion resistance used for a molten carbonate fuel cell or the like.

[0002]

2. Description of the Related Art In order to cope with environmental problems including the depletion of petroleum resources and air pollution in the 21st century, fuel cells that can use coal reformed gas as a next-generation power supply source are beginning to attract attention. Fuel cells are of a phosphoric acid type, a molten carbonate type, a solid electrolyte type, and the like, depending on the electrolyte that generates the electromotive force, and each has a different operating temperature and power generation efficiency. Among them, a molten carbonate fuel cell using LNG and coal reformed gas has been attracting attention as a large-scale centralized power supply by combined use of a distributed power supply and a gas turbine. Currently 100k
The development of the W class stack has been completed, and the development of a 1 MW class plant has been started. However, in order to realize such a large size and put it into practical use as a commercial plant, long-term stability and reliability and further cost reduction are important.

[0003] One of the major problems at present is the corrosion of metal materials by molten carbonate mixed with Li ₂ CO ₃ or K ₂ CO ₃ used as an electrolyte. In particular, 600
Separator materials and current collectors in contact with high-temperature molten carbonate at ~ 700 ° C are exposed to severe corrosive environment on the cathode side.
This is a cause of battery life deterioration.

At present, SUS316L (Fe-17C) is used as a separator material.
Although r-12Ni-2.5Mo) and SUS310S (Fe-25Cr-20Ni) are used, their corrosion resistance is not satisfactory. As a measure to improve the corrosion resistance to molten carbonate, Japanese Patent Publication No.
Japanese Unexamined Patent Publication No. 63-190143 discloses that adding 2% of Al involves adding 0.1 to 0.9% of Al and Y in a combined manner. 1-252
No. 757 discloses a stainless steel or a Ni-based alloy in which the amount of Si is regulated to 0.2% or less and Al is added.

Japanese Patent Application Laid-Open No. 64-68449 discloses a stainless steel in which the amounts of Fe, Cr and Ni are specified, and Japanese Patent Application Laid-Open No. 4-247852 specifies the amounts of Cr, Ni and Mn to improve the corrosion resistance. ing.
However, these stainless steels or Ni-based alloys are said to have improved corrosion resistance compared to conventional materials such as SUS316L, and considering long-term durability, corrosion resistance to molten carbonate is not yet sufficient. There's a problem.

Further, Japanese Patent Publication No. Hei 4-13825 proposes to add Al alone or a combination of Al and Si in order to greatly improve the corrosion resistance. However, when it is based on Al addition and is used as a battery body member,
Good electrical conductivity, which is important as battery characteristics, may be impaired.

[0007]

As described above, regarding the metal materials used in the molten carbonate fuel cell, the mechanism of corrosion in the molten carbonate has not been sufficiently elucidated, and the decisive corrosion resistance has been determined. At present, no material has been developed.

An object of the present invention is to provide a stainless steel having excellent molten carbonate corrosion resistance in an anode side environment and a cathode side environment as a metal material which can be used for a structural member of a molten carbonate type fuel cell. is there.

[0009]

Means for Solving the Problems The present inventors have developed a material for a molten carbonate fuel cell, in particular, a Fe-Cr-Ni stainless steel having excellent molten carbonate corrosion resistance even in a cathode environment. With a focus on the composition and structure of the scales generated for materials with widely varied chemical components, systematic investigations and studies were repeated, and the following findings were obtained.

A) Cr-based oxides formed on the surface of a material generally have a lower corrosion rate and are more stable than Fe-based oxides, and therefore have excellent corrosion resistance. However, in the molten carbonate, the Cr-based oxide is easily an electrolyte, Li ₂ CO ₃
To form LiCrO ₂ , resulting in poor corrosion resistance over a long period of time and loss of electrolyte.

B) The loss of electrolyte is suppressed by reducing the amount of Cr.

C) As a result of investigating elements for improving corrosion resistance against the reduction of molten carbonate corrosion resistance due to the reduction of Cr content, addition of Si and Mo is effective. Si forms SiO ₂ at the interface between the Cr-based oxide protective film and the alloy, and has the effect of increasing the adhesion of the film. Corrosion is suppressed mainly in an environment where molten carbonate exists as a gas side. However, in order to suppress corrosion to molten carbonate especially in the cathode atmosphere, a large amount of
Si amount is required.

D) Mo is not necessarily an effective element for molten carbonate corrosion resistance because it promotes elution of alloy components such as Fe and Cr into molten carbonate. However, in an alloy containing 0.8% or more of Si, Mo has a remarkable corrosion inhibitory effect, and the elution of Fe, Cr, etc. is reduced by the presence of Si. Thus, by adding appropriate amounts of Si and Mo, corrosion by molten carbonate is suppressed without adding a large amount of Si, and stable corrosion resistance is exhibited for a long time.

E) The effect of Mo in D) is further promoted by adding Cu, and particularly, it has excellent corrosion resistance in molten carbonate and is stable for a long time.

The present invention has been completed on the basis of these findings, and the gist of the invention is that "in terms of% by weight, C: 0.15% or less, Mn: 2% or less, Cr: 10 to 25%, Ni: 7%. ~ 30%, Si: 0.
8~4%, Mo: 0.1~3%, Cu: it contains 0.1% to 2%, and Si
+ 2 × Mo> 2%, and the balance substantially consists of Fe and unavoidable impurities and has excellent corrosion resistance to molten carbonate ”.

[0016]

[Action] In order to examine the effect of Si and Mo on molten carbonate corrosion resistance, corrosion tests were performed using 17% Cr-12% Ni stainless steel with various contents of Si and Mo.

FIG. 1 shows the result of the corrosion test, showing the relationship between the Si content and the thickness reduction.

17% Cr not containing Mo with varied Si content
Production of hot rolled steel sheet of -12% Ni stainless steel, then taking corrosion test specimens, suspending them with wires and Li ₂ CO ₃ : K ₂ C
O ₃ = 62: 38 (molar ratio) mixed salt at 650 ° C in semi-soaked state
After holding for 720 hours in an atmosphere gas having a gas composition of CO ₂ : air = 30: 70 (volume ratio), the corrosion loss was measured. The corrosion weight loss was obtained by subtracting the thickness of the test piece descaled after heating from the thickness of the test piece before heating. On the liquid phase side immersed in the molten carbonate, no difference in sheet thickness reduction was observed due to the increase in Si content, but on the gas phase side, which was not immersed,
It became clear that the decrease in sheet thickness became small when the Si content exceeded 2%.

FIG. 2 shows the results obtained in the corrosion test, showing the relationship between the Mo content and the thickness reduction.

17% Cr-12 containing no Si and 1% Si
The corrosion test was performed in the same manner as described above, using a% Ni stainless steel and changing the Mo content. The effect of Mo was small in the steel without Si, while the corrosion resistance of the stainless steel containing 1% Si was significantly improved by the addition of Mo.

Hereinafter, the function and effect of each component in the stainless steel of the present invention and the reason for limiting the content will be described.

C: Since C is an element effective for promoting the stabilization of the austenite structure and increasing the high-temperature strength, its content is desirably 0.01% or more.
If it exceeds 15%, hot workability is impaired, so the content is limited to 0.15% or less.

Mn: Mn is added as a deoxidizing agent at the time of melting. Further, it is effective to stabilize the austenite structure, so it is preferable to contain 0.1% or more. On the other hand, the addition of a large amount promotes corrosion of molten carbonate, so the upper limit is made.
2%.

Cr: Cr has the effect of improving the molten carbonate corrosion resistance in the cathode side environment, and its effect is exhibited at 10% or more in the alloy containing Si. However, when the Cr content increases, the Cr-based oxide generated on the surface reacts with the electrolyte Li ₂ CO ₃ to form LiCrO _2, which easily elutes into the electrolyte and reduces the stability of the film, and Cr is limited to 25% or less because of loss of electrolyte. Preferably it is 12% to 22%.

Ni: Ni is an element necessary for stabilizing the austenite structure and improving the creep strength, but if its content is less than 7%, its effect is small, while if its content exceeds 30%, it impairs hot workability. Therefore, the range should be 7% or more and 30% or less.

Si: Si is an important element in the present invention. In order to effectively exert the effect of inhibiting corrosion of Mo in molten carbonate, which will be described later, it is necessary to add 0.8% or more. Si
Is present as SiO ₂ at the interface between the Cr-based oxide and the alloy, which improves corrosion resistance mainly on the gas phase side where molten carbonate is present and enhances the adhesion of the Cr-based oxide to stabilize the protective film for a long time Has the effect of forming However, as the content increases, the workability and weldability decrease significantly, so the upper limit is 4%. Further, in order to further enhance the effect of Mo and provide stable corrosion resistance, the content is preferably 1% to 4%.

Mo: Mo is also an important element in the present invention. In alloys containing 0.8% or more of Si, 0.1% of Mo
The above content has the effect of suppressing corrosion in the molten carbonate. However, the excessive addition loses the effect of the complex addition with Si, and the elution of Fe and Cr into the molten carbonate becomes severe, thereby deteriorating the molten carbonate corrosion resistance. The upper limit is set to 3%. . Cu: Cu is added by 0.1% or more to add Si and Mo.
Promotes the effect of heat treatment and suppresses corrosion by molten carbonate.
Has the effect of causing However, adding more than 2%
The effect of not only can not be expected but also reduce the productivity
Therefore, the upper limit is set to 2%.

Si + 2 × Mo> 2%: As for the contents of Si and Mo, by satisfying the relationship of Si + 2 × Mo> 2%, corrosion by molten carbonate is suppressed, and Fe, Cr, etc. Dissolution of the alloy component in the molten carbonate.

By limiting the chemical components as described above, a stainless steel having excellent corrosion resistance in molten carbonate can be obtained.

[0030]

[0031]

EXAMPLES The present invention will be described below more specifically based on examples.

The steels of the present invention (Nos. 12 to 14) and comparative steels (Nos. 15 to 31) having the chemical compositions (% by weight, balance being Fe) shown in Tables 1 and 2
Was melted in a high-frequency electric furnace (vacuum melting) to produce a 25 kg steel ingot. This ingot was subjected to forging and hot rolling to obtain a hot-rolled steel sheet having a thickness of 5 mm. The hot-rolled sheet thus obtained was subjected to a solution treatment at 1150 ° C., and a corrosion test piece was cut out. The size of the test piece is 2 mm thick, 20 mm wide and 80 mm long.

[0033]

[Table 1]

[0034]

[Table 2]

A test salt was coated with a mixed salt of Li ₂ CO ₃ : K ₂ CO ₃ = 62: 38 (molar ratio), and an atmosphere gas having a gas composition of CO ₂ : air = 30: 70 (volume ratio) at 650 ° C. The corrosion test was performed while keeping the inside for 200 hours. The corrosion resistance was evaluated by subtracting the weight after descaling from the weight of the test piece before heating and evaluating the weight loss.

The results are shown in Tables 1 and 2. In the table, ◎ indicates a weight loss of 2.5 mg / cm ² or less, ○ indicates 2.5 to 5 mg / cm ² , and Δ indicates a value exceeding 5 7.
5 mg / cm ² and × indicate an amount exceeding 7.5 mg / cm ² , respectively.

FIG. 3 shows the results of the corrosion tests of Tables 1 and 2, in which the corrosion loss of each test material is arranged in relation to the Si content and the Mo content.

SUS3 currently used for separator materials, etc.
16L (Comparative Steel No. 31) has a very large corrosion weight loss. Also, Mo
In Comparative Steels Nos. 15 to 18 to which No. was added, the corrosion loss was reduced with respect to both the Si content, but no improvement in corrosion resistance was observed compared to SUS310S (Comparative Steel No. 30). In addition, comparative steel No.19
Nos. 24 to 24, even when the Mo content was high, no reduction in corrosion weight loss was observed, and the effect of improving the corrosion resistance of Mo was not seen because the Si content was 0.8% or less. Further, in comparative steel Nos. 25 and 26, Si
It can be seen that the value of + 2 × Mo was 2% or less, and the effect of the combined addition of Si and Mo was not exhibited. Furthermore, comparative steel No. 27
In the case of 29, the Mo content exceeds 3%, and conversely, the corrosion weight loss is large.

[0039] In the present invention steel No. 12-14, the corrosion weight loss is 2.5 mg / c
m^TwoAnd the corrosion resistance is more improved.

It can be seen that the addition of appropriate amounts of Si and Mo significantly improves the corrosion resistance against molten carbonate.

[0041]

The stainless steel of the present invention has a temperature of 600 ° C to 700 ° C.
It has excellent resistance to molten carbonate corrosion at ℃.
Therefore, when the steel of the present invention is used for a current collector plate or a separator material of a molten carbonate fuel cell, the corrosion resistance of a severe portion in contact with the molten carbonate is dramatically improved, and the loss of the molten carbonate as an electrolyte is improved. And the life of the battery can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing the influence of Si in a corrosion test in a molten carbonate.

FIG. 2 is a diagram showing the influence of Mo in a corrosion test in a molten carbonate.

FIG. 3 is a diagram showing the influence of Si and Mo in a corrosion test in a molten carbonate.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-7-188870 (JP, A) JP-A-1-252575 (JP, A) JP-A-2-54741 (JP, A) JP-A-4- 247852 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 H01M 8/02

Claims (1)

(57) [Claims] (1) In weight%, C: 0.15% or less, Mn: 2% or less,
Cr: 10 to 25%, Ni: 7 to 30%, Si: 0.8 to 4%, Mo: 0.1 to
3% or less, Cu: 0.1 to 2%, and satisfy Si + 2 × Mo> 2%, with the balance substantially consisting of Fe and unavoidable impurities. Excellent stainless steel