JPH1053890A - Stainless steel for molten salt electrolyzing electrode excellent in corrosion resistance and electric conductivity in molten salt - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a stainless steel for an electrode excellent in corrosion resistance and electric conductivity in molten salt. SOLUTION: This stainless steel has oxide coating including metallic Ni. For forming this oxide coating by natural oxidation in molten salt, the compsn, contg., by weight, <=0.10% C, <=1.0% Si, <=1.0% Mn, <=0.10% P, <=0.010% S, 12 to 40% Cr, 5 to 85% Ni, <=0.5% Mo, <=0.5% Cu and <=0.3% N, and an which the relation between the Ni content and the Cr content satisfies 100.00-(Ni+Cr)>=-18.75 (Ni/Cr)+87.50 is regulated. In this way, the inexpensive material for an electrode capable of reconciling reduction in electric resistance of the oxide coating with the improvement of corrosion resistance and usable in molten salt can be provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a stainless steel having excellent corrosion resistance in a molten salt and having an oxide film having a low electric resistance and suitable for use as an electrode in a molten salt.

[0002]

2. Description of the Related Art A molten salt is an ionic melt formed when a substance having an ionic bond represented by NaCl is heated to a temperature higher than its melting point. This is a liquid in which ions such as charged Na ⁺ and Cl ⁻ exist in a state where they can move freely. This ionic melt is
It has completely different properties from a molecular melt in which neutral molecules having no charge and consisting of a covalent bond of oxygen and hydrogen, such as water, are in a liquid state.

[0003] That is, the molten salt can dissolve a large amount of various substances. This is simply because the dissolved substance dissociates into positive and negative ions, which electrically attract the positive and negative ions that make up the ionic melt itself, creating an extremely stable state in terms of energy. It is. In addition, the dissolved ions can be electrically redoxed in the same manner as an aqueous solution, and a specific element can be electrolytically collected from a molten salt. vice versa,
As disclosed in JP-A-59-23812, when a voltage is applied to a molten metal as an anode and a molten salt as a cathode in contact with the molten metal, an element (impurity) in the metal is obtained.
Only the molten salt can be dissolved.

[0004] Further, the molten salt has the property of dissolving a large amount of gaseous species. This is called “molten salt and high temperature chemistry” (No. 3
3, pages 25-72, 1990, published by the Society of Electrochemical Molten Salts), as well as physical dissolution according to Henry's Law, as well as chemistry in which gases dissociate into ions in molten salts. This is because the target dissolution mainly occurs.
Therefore, when the molten salt is used as the electrolyte, the oxidation-reduction rate of the gas can be remarkably increased and the reaction polarization can be remarkably reduced as compared with the aqueous solution electrolysis. For example,
Fuel cells using molten carbonate have extremely high reaction efficiency and are expected as a next-generation energy source. The electrochemical reaction using a molten salt as an electrolyte is an extremely promising field.

[0005]

However, one of the factors restricting the development of the molten salt electrolysis technique is that there is no inexpensive electrode material having both corrosion resistance and electric conductivity. Stainless steel can be freely designed in an electrode shape, and can be cited as an inexpensive corrosion-resistant material in terms of material cost. However, the corrosion resistance to molten salts largely depends on the nature of the oxide film formed on the surface by natural oxidation. Generally, there is a relationship that increasing the corrosion resistance increases the electrical resistance of the oxide film. This limited the development of stainless steel for electrodes where both conductivity and high corrosion resistance were required.

In general, it has been conventionally said that it is necessary to add a large amount of Al to enhance the corrosion resistance of stainless steel in a molten salt. However, it is also known that when Al is added excessively, the electrical resistance of the oxide film increases and the battery electromotive force decreases. To improve this point, Japanese Unexamined Patent Publication No.
In Japanese Patent Application Laid-Open No. 3-190143, Al is set to 0.1 to 0.1.
A kind of compromise is disclosed that limits the corrosion resistance to 0.9% and ensures corrosion resistance by adding 0.5% or less of Y.

As described above, a technique for providing stainless steel for an electrode having excellent corrosion resistance and electric conductivity in a molten salt has not yet been developed. An object of the present invention is to provide a stainless steel for a molten salt electrolysis electrode having an oxide film having excellent electrical conductivity without impairing corrosion resistance in a molten salt.

[0008]

The present inventors investigated the relationship between corrosion resistance in a molten salt and the electrical resistance of an oxide film by varying the composition of stainless steel over a wide range, and obtained the following completely new facts. Was. When metal Ni precipitates in the oxide film, the electric resistance decreases. However, in this case, the corrosion resistance does not decrease. When the contents of Ni and Cr in the stainless steel are controlled within a certain range, the precipitation of metallic Ni occurs.

The present invention has been made on the basis of the above-mentioned findings, and is characterized by having an oxide film containing metallic Ni, which is excellent in corrosion resistance and electric conductivity in a molten salt for a molten salt electrolysis electrode. Stainless steel. And the above stainless steel is, by weight%, C: 0.10% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.01% or less, Cr: 12% or more and 40% or less, Ni: 5% or more and 85% or less, Mo: 0.5% or less, Cu: 0.5% or less, and N: 0.3% or less. When the content is 100.00− (Ni + Cr) ≧ −18.75 (Ni /
By satisfying the relationship of (Cr) +87.50, metallic Ni can be advantageously deposited on the surface.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the oxide film constituting the steel of the present invention and the reasons for limiting the range of components will be described in detail. Metal Ni is a metal excellent in molten salt corrosion resistance,
When contained in the surface oxide film, the corrosion resistance of the electrode is improved. Also, by dispersing metal Ni in the oxide,
At the time of energization, these metal Nis function as a current flow path, and exhibit the effect of reducing the electric resistance of the entire electrode.

The method of forming the oxide film containing metal Ni and the composition of the oxide are not particularly limited, but in addition to the natural oxidation of a stainless steel having a specific composition described later in a molten salt, a film formation by thermal spraying, , Or a method in which Ni oxide powder and a metal powder (such as Al) which is more easily oxidized than Ni are mixed and applied, and then a method of solid-phase reduction of Ni by heat treatment is applied. Good.

In general, electrodes are not flat, and are often processed into various shapes in order to increase the efficiency of electrolytic reaction. In such a case, it is preferable to limit the chemical composition of the stainless steel and form an oxide film containing metallic Ni by natural oxidation during electrolytic treatment. It is preferable that the component range of stainless steel for this purpose is as follows. In addition,% means weight%.

If C is contained excessively, it deteriorates the toughness of the steel and forms Cr carbide in the production process to lower the corrosion resistance. Therefore, the content is set to 0.10% or less. Si is added as a deoxidizing agent, but if added excessively, it impairs processability. Therefore, the upper limit of the addition amount is set to 1.0%.

Mn has a deoxidizing or desulfurizing action and improves the hot workability of steel. However, even if a large amount is added, not only the hot workability improvement effect commensurate with the cost increase cannot be expected, but also the hardness increases and the workability is impaired. P inhibits corrosion resistance when present in large amounts,
0.10% or less. Since S impairs hot workability, S is set to 0.01% or less.

[0015] Cr is an essential element for ensuring corrosion resistance. To ensure corrosion resistance, it is necessary to add 12% or more. However, if it is added in excess of 40%, the corrosion resistance is extremely improved, but the toughness and workability are impaired.
Therefore, Cr is set to 12% or more and 40% or less.

Ni is an essential element for ensuring corrosion resistance, and it is necessary to add 5% or more to expect its effect. However, even if it is added in excess of 85%, the effect of improving corrosion resistance that is commensurate with the cost increase cannot be obtained, resulting in excessive quality. Therefore, Ni is set to 5% or more and 85% or less.

In the present invention, in order to have excellent electric conductivity,
The quantitative relationship between Cr and Ni is defined. That is,
The relationship between the contents of Cr and Ni must satisfy the following equation. That is, 100.00− (Ni + Cr) ≧ −18.75 (Ni /
Cr) +87.50 Although the detailed reason is unknown, this relational expression indicates that Ni, which is necessary for metal Ni to precipitate in the oxide film at the time of forming the oxide film and effectively acting to lower the electric conductivity, Cr, Fe
It seems to give the content ratio. As the amount of Ni increases, the precipitation of metallic Ni also easily occurs.
The larger the ratio, the more easily this relational expression holds.

Mo and Cu each contain 0.5% or more, which impairs corrosion resistance. N
Excessively decreases the toughness of the steel. Therefore,
0.3% or less.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. A steel having a chemical composition shown in Table 1 was vacuum-melted, and hot-rolled to form a 4 mm-thick steel plate at 1150 ° C for 30 min.
After the heat treatment, a test piece was collected.

[0020]

[Table 1]

The corrosion resistance in the molten salt and the electrical resistance of the oxide film were measured in the same manner as in the potentiostatic polarization test using a three-electrode method consisting of a sample electrode, a reference electrode, and a counter electrode. It was evaluated by analyzing the harmonic current component.

The test piece was 2 mm thick, 10 mm wide and 10 mm long.
A 0.5mm diameter Au wire is spot-welded to one end of
The u wire was inserted into an alumina tube having an inner diameter of 4 mm, and the spot welded portion and the end of the alumina tube were covered with zirconia cement.
The zirconia cement was dried and solidified at room temperature for 7 days or more to obtain a sample electrode. The reference electrode has a diameter of 1 mm
mm Au wire is inserted into the alumina tube at 66.6 vol%.
A gas mixture (O ₂ / CO ₂ , Au) was used by flowing a mixed gas of oxygen-33.4 vol% carbon dioxide. An Au plate having a thickness of 0.5 mm, a width of 50 mm and a length of 50 mm was used as a counter electrode.

The three electrodes were connected to 70 vol% oxygen-30 vol.
Using a mixed gas of 1% carbon dioxide as a cover gas, the mixture was immersed in a mixed molten salt of 62 mol% lithium carbonate-38 mol% potassium carbonate heated to 650 ° C., and −0.1 V (vs O ₂
/ CO ₂ , Au) with a potentiostat for 160 hours. The current density flowing 160 hours after the start of the polarization was used as an index of corrosion resistance as an indicator of the magnitude of the dissolution rate. In addition, 1 of ± 10 mV with respect to -0.1 V
A sine wave alternating current of 00 kHz to 1 mHz was superimposed, and the impedance (AC resistance) at that time was measured. Using harmonic analysis, it was determined which frequency range of the impedance corresponded to the electrical resistance of the coating.

At the time of this harmonic analysis, a sine wave alternating current of 100 kHz to 1 mHz of ± 50 mV is superimposed on -0.1 V, and a portion where second- and third-order nonlinear components hardly appear is regarded as a film resistance. . That is, the electrochemical current-
The relationship between potentials (slope corresponds to resistance) is a non-linear Butler-
While the second and third harmonic components appear according to the Former's relationship, the harmonic components are less likely to appear in a portion where the current-potential relationship follows the linear Ohm's law. When harmonic analysis was performed on all the test pieces in Table 1, 100 mH
Since no harmonic current was observed in the frequency range below z, the film resistance was evaluated assuming that the impedance in this frequency range corresponds to the resistance component of the film. Table 1 shows the current density and the film resistance of the oxide film evaluated in this manner. From Table 1, it can be seen that No. 1 of the present invention example. 7 to 9 are excellent in corrosion resistance and electric conductivity in the molten salt.

FIG. 1 summarizes the relationship between the film resistance and the current density in Table 1. Group A (Nos. 1 to 6), which shows a correlation between the film resistance and the current density, has a current density of around 10 μA · cm−2 and a film resistance of 500Ω ·
cm ² can be divided into groups B (Nos. 7 to 9). Group A has a conventionally known relationship that the higher the corrosion resistance, the higher the film resistance. On the other hand, Group B is a group of steel grades that do not show a conventional relationship and exhibit unique performances that have not been known so far.

Therefore, Ni, Cr of these stainless steels are used.
As a result of examining the relationship between the amount and the film resistance of the oxide film, FIG.
, The boundary separating group A and group B is: 100− (Ni + Cr) = − 18.75 (Ni / Cr)
+87.50 was obtained.

FIG. 3 (a) shows the results of No. 2 of the present invention in Group B showing good electrical conductivity. 7,
FIG. The result of observing the oxide film of the test piece No. 1 after the constant potential test for 160 hours is shown. No. In No. 7, small white particles 3 were observed in the oxide film 2 formed on the surface of the base iron 1. These particles are the result of EPMA analysis
It was found to be metallic Ni. Similarly, precipitation of metallic Ni could be observed in the steel type of Group B. Thus, it can be seen that the metal Ni in the oxide film has the effect of improving the molten salt corrosion resistance of the electrode and lowering the electrical resistance of the entire electrode.

[0028]

According to the present invention, it is possible to achieve both the reduction of the electric resistance of the film and the improvement of the corrosion resistance, which have been considered to be impossible so far. It can be used and can contribute to the development of molten salt electrolysis technology.

[Brief description of the drawings]

FIG. 1 is a view showing a relationship between a film resistance of an oxide film and a current density.

FIG. 2 is a diagram showing a relationship between Ni and Cr contents and film resistance.

FIG. 3 (a) is an enlarged schematic sectional view of an oxide film after a 160-hour constant potential test of the steel of the present invention. (B) An enlarged schematic cross-sectional view of an oxide film after a 160-hour constant potential test of a comparative steel.

[Explanation of symbols]

 1: ground iron 2: oxide film 3: Ni particles

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical display location C25D 3/66 C25D 3/66 17/10 101 17/10 101 17/12 17/12 B H01M 8 / 14 H01M 8/14

Claims (2)

[Claims] 1. A stainless steel for a molten salt electrolysis electrode having an excellent corrosion resistance and electrical conductivity in a molten salt, characterized by having an oxide film containing metallic Ni. 2. In% by weight, C: 0.10% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.01% or less, Cr : 12% or more and 40% or less, Ni: 5% or more and 85% or less, Mo: 0.5% or less, Cu: 0.5% or less, N: 0.3% or less, and the content of Ni and Cr Is 100.00− (Ni + Cr) ≧ −18.75 (Ni /
2. The stainless steel for a molten salt electrolysis electrode according to claim 1, which satisfies the relationship of (Cr) +87.50, with the balance being Fe and unavoidable impurities. steel.