KR101017180B1 - Stainless steel divider and coating method - Google Patents

Abstract

Stainless steel separator of the present invention is characterized in that the coating of the electroless primary plating layer containing 0.5 to 30% by weight of phosphorus, 0.1 to 20% by weight of cobalt and 50 to 99.4% by weight of nickel. In addition, the coating method of the stainless steel separator plate of the present invention is the step of degreasing and cleaning the surface of the stainless steel separator plate and 0.5 to 30% by weight phosphorus, 0.1 to 20% by weight of cobalt and 50 to 99.4 nickel It is characterized by consisting of a step of forming an electroless primary plating layer film by immersing in a plating bath containing an electroless plating solution containing a weight%.

Stainless steel, separator, coated, plated

Description

Stainless Steel Separator and Method of Coating the Same}

The present invention relates to a stainless steel separator and a method for manufacturing the same, and more particularly, to a stainless steel separator and a method for manufacturing the same, which can increase the corrosion resistance of the stainless steel separator.

The fuel cell refers to a power generation system that directly converts the chemical reaction energy of hydrogen and oxygen contained in hydrocarbon-based materials such as methanol, ethanol, and natural gas into electrical energy.

Since molten carbonate fuel cell (MCFC) is operated at a high temperature of more than 650 ℃, the electrochemical reaction rate is fast, it is possible to use nickel instead of platinum catalyst as an electrode material. Therefore, in addition to economic advantages, carbon monoxide, which acts as a poisoning substance on the platinum electrode, may use coal gas, natural gas, methanol, and biomass due to the characteristics of the nickel electrode used as fuel through a water gas conversion reaction. Fuel selection is possible. In addition, by recovering and using high-quality waste heat as a lower cycle using a heat recovery steam generator (HRSG), the entire power generation system can improve the thermal efficiency to about 60% or more.

In addition, the high temperature operation characteristics of the MCFC provides an advantage that the internal reforming form can be adopted by simultaneously proceeding the electrochemical reaction and the fuel reforming reaction in the fuel cell stack. Since the internally reformed molten carbonate fuel cell uses the calorific value of the electrochemical reaction in a direct endothermic reforming reaction without a separate external heat exchanger, the thermal efficiency of the entire system is further increased and the system configuration is simpler than the external reforming MCFC. Has the advantage.

Typically, the MCFC consists of a repeating stacked structure of a separator plate / cathode electrode / electrolyte / anode electrode / separator plate. In this case, the separator separates the cathode portion of the lower layer cell from the anode portion of the upper layer cell in the MCFC stack and electrically connects each cell in the dust collector. The corrosion environment of the MCFC separator may be divided into a current collector in which an electrolyte moves from the electrolyte plate through the porous electrode to form a thin film, and a wet-seal part in contact with the electrolyte plate containing the electrolyte. The anode corresponds to the reducing atmosphere toward the fuel gas containing H ₂ , CO ₂ , CO, and H ₂ O, while the cathode forms the oxidation atmosphere toward the oxidizing gas comprising O ₂ , N _2, and CO ₂ . Have

Since MCFC uses highly corrosive molten carbonate as an electrolyte at high temperatures, corrosion of components is a problem. Corrosion on the current collector side of the separator is a major factor that degrades overall battery performance, such as reduced electrical conductivity due to corrosion products and pore closure of electrodes. In this atmosphere, the conditions required for the separator / current collector and wet seal of the separator should be excellent in corrosion resistance to fuel gas, oxidizing gas and electrolyte at an operating temperature of 650 ° C., and have high electrical conductivity in the current collector plate. High temperature strength is required.

Stainless steel, which is used as MCFC's separator material, is highly corrosive in high temperature molten carbonate atmosphere and is generally used to improve the corrosion resistance by coating aluminum diffusion film on the surface. 316L or 310S stainless steel, which is currently used as a separator material, is severely corroded when it comes into contact with molten carbonate, which is a major factor in reducing the life and performance of MCFC. In particular, the corrosion of the wet-sea part directly contacting the electrolyte is known to be the most serious of the various parts of the separator, and the problem of corrosion of the wet-sea part is a prerequisite for the practical application of MCFC. At present, a wet seal of MCFC is generally used to prevent corrosion by coating aluminum and forming a diffusion film.

In the oxidizing atmosphere, stainless steel forms an oxide of LiFeO ₂ on its surface when it comes into contact with a mixed salt of lithium carbonate (Li ₂ CO ₃ ) and potassium carbonate (K ₂ CO ₃ ), which is used as an electrolyte for MCFC. It is known to have a dual structure in which a mixed oxide of iron and iron is formed. However, this oxide layer does not inhibit the diffusion of oxygen into the interior of the stainless steel, so the oxidation reaction continues in the stainless steel. However, when the aluminum diffusion layer is formed on the surface of stainless steel, aluminum oxide is formed on the surface of LiAlO ₂ by reaction with molten carbonate, thereby suppressing the internal diffusion of oxygen and the external diffusion of iron, chromium and nickel which are the main components of stainless steel. It is reported to improve the corrosion resistance of stainless steel.

Currently, as a method of coating aluminum on the surface of stainless steel, ion deposition, electroplating, plasma spraying, and slurry coating are widely used, but they do not require expensive equipment and processes compared to conventional coating methods, and are coated on various shapes of supports. I need this possible way. A general slurry coating method is a method in which a thin powder is coated on a surface of a specimen with a mixture of a specific powder and a solvent, and a thin film is formed on the surface of the specimen by heat treatment. The slurry layer coated on the surface of the specimen is changed into a thin film. However, the porous aluminum layer provides an environment where the stainless steel base material can be easily exposed to the carbonate electrolyte under fuel cell operating conditions.

On the other hand, electroless plating is a method of depositing metal ions in solution as a metal layer on the surface of a base material using a reducing agent. Subsequently, the base material, which has undergone the proper catalysis, is immersed in an electroless plating bath after washing with water, and then a chemical plating reaction occurs to precipitate the metal by chemical reduction. Unlike electroplating, an electroless plating film is excellent in that a uniform plating film can be easily formed on a base material only by immersing a plated object in a plating bath. Electroless plating films in which phosphorus and boron are added to nickel have a relatively high hardness and are used for surface treatment of various mechanical parts and the like. Electroless plating films composed of nickel and phosphorus are generally suitable for formation at high speed because the plating bath is stable. However, in order to form a high hardness film, heat treatment is required at a high temperature of 280 ° C or higher. However, heat treatment at high temperatures has a problem that the toughness of the coating itself is lowered and the impact strength is also lowered. Conventional electroless plating films made of nickel and boron have a problem that it is difficult to deposit the coating at high speed because the plating bath is unstable.

In order to solve the above-mentioned problems of the prior art, the inventors of the present invention provide a stainless steel separator and its coating method which are excellent in hardness and corrosion resistance, excellent in airtightness of the plating layer, and excellent in deposition rate and adhesion even by high temperature heat treatment. To develop.

An object of the present invention is to provide a stainless steel separator and its coating method excellent in hardness and corrosion resistance even by high temperature heat treatment.

Another object of the present invention is to provide a stainless steel separator and a coating method thereof having excellent airtightness of the plating layer.

Still another object of the present invention is to provide a stainless steel separator having excellent precipitation rate and adhesion and a coating method thereof.

The above and other objects of the present invention can be achieved by the present invention described below.

Stainless steel separator of the present invention is characterized in that the coating of the electroless primary plating layer containing 0.5 to 30% by weight of phosphorus, 0.1 to 20% by weight of cobalt and 50 to 99.4% by weight of nickel.

here,

The separator may be coated with an electroless secondary plating layer including 0.5 to 30% by weight of phosphorus, 0.1 to 15% by weight of tungsten, and 55 to 99.4% by weight of nickel on the primary plating layer.

The porosity of the primary plating layer is characterized in that 0.1 to 25%.

The thickness of the primary plating layer is characterized in that 25 to 75% of the total film thickness.

The total film thickness is characterized in that 2 to 200 ㎛.

In addition, the coating method of the stainless steel separator plate of the present invention is the step of degreasing and cleaning the surface of the stainless steel separator plate and 0.5 to 30% by weight phosphorus, 0.1 to 20% by weight of cobalt and 50 to 99.4 nickel It is characterized by consisting of a step of forming an electroless primary plating layer film by immersing in a plating bath containing an electroless plating solution containing a weight%.

The coating method of the stainless steel separator plate is immersed in an electroless plating solution containing 0.5 to 30% by weight of phosphorus, 0.1 to 15% by weight of tungsten and 55 to 99.4% by weight of nickel on the primary plating layer to form an electroless secondary plating layer. It further comprises the step of coating.

The electroless plating solution is characterized in that it comprises a water-soluble nickel salt, a reducing agent and a complexing agent.

The amount of the water-soluble nickel salt, reducing agent and complexing agent is characterized in that each 0.1 g / L to 100 g / L.

The reducing agent is selected from the group consisting of hypophosphite (H ₂ PO ₂ ), hypophosphite such as sodium hypophosphite (NaH ₂ PO ₂ · H ₂ O), dimethylamine borane, trimethylamine borane and hydrazine do.

The complexing agent is characterized in that it is selected from the group consisting of carboxylic acids such as malic acid, succinic acid, lactic acid and citric acid, sodium salts of carboxylic acids and amino acids such as glycine, alanine, iminodiacetic acid, arginine and glutamic acid.

The electroless plating solution is characterized in that it further comprises a stabilizer.

The stabilizer is selected from the group consisting of water-soluble lead salts such as lead acetate and sulfur compounds such as thiodiglycolic acid.

The amount of stabilizer is characterized in that from 0.1 g / L to 100 g / L.

The temperature of the plating bath is characterized in that 30 ℃ to 95 ℃.

The present invention provides a stainless steel separator and its coating method which are excellent in hardness and corrosion resistance, excellent in airtightness of the plating layer, and excellent in deposition rate and adhesion even by high temperature heat treatment.

In order to achieve the above object, the present invention is a method of multilayer coating the wet-sealed portion of the stainless steel separator, the step of degreasing and cleaning the surface of the separator plate, and the nickel-phosphorus alloy on the surface of the separator plate It provides an electroless plating method comprising the step of sequentially forming a surface layer consisting of a base layer made of an alloy of nickel-phosphorus-tungsten.

The electroless nickel plating solution includes a water soluble nickel salt, a reducing agent and a complexing agent. Nickel sulfate and nickel chloride are typical water soluble nickel salts. The amount of nickel salt used is preferably 0.1 g / L to 100 g / L, more preferably 1 g to 50 g / L. Examples of reducing agents include hypophosphite (H ₂ PO ₂ ), hypophosphite such as sodium hypophosphite (NaH ₂ PO ₂ · H ₂ O), dimethylamineborane, trimethylamineborane and hydrazine. The amount of reducing agent used is preferably 0.1 g / L to 100 g / L, more preferably 1 g to 50 g / L. Examples of complexing agents include carboxylic acids such as malic acid, succinic acid, lactic acid and citric acid, sodium salts of carboxylic acids and amino acids such as glycine, alanine, iminodiacetic acid, arginine and glutamic acid. The amount of complexing agent used is preferably 0.1 mg / L to 100 mg / L, more preferably 1 mg to 50 mg / L.

Typically, stabilizers are added to the electroless nickel plating solution. Examples of stabilizers are water soluble lead salts such as lead acetate and sulfur compounds such as thiodiglycolic acid. The amount of stabilizer used is preferably 0.1 mg / L to 100 mg / L, more preferably 0.5 mg to 20 mg / L.

Formation of the plating film of the present invention can be formed by immersing the surface to be plated of the base material in a plating bath containing the above components for a predetermined time. The temperature of the plating bath is determined in consideration of the stability of the bath and the precipitation rate, and the like, for example, preferably in the range of 30 to 95 ° C, preferably 30 to 90 ° C. And the thickness of a plating film can be adjusted suitably by adjusting the immersion time to a plating bath. It is preferable that the thickness of the primary plating layer is 25 to 75% of the total film thickness, and the overall film thickness takes into account film hardness, toughness, sliding properties and corrosion resistance, and is in the range of 2 to 200 mm, preferably 20 to 150 mm. It is good to do.

The surface to be plated of the base material is preferably subjected to pretreatment performed in a normal plating step before being immersed in the plating bath for the purpose of improving adhesion with the plated film. Examples of such pretreatment include degreasing using a solvent or an alkaline solution, zinc substitution treatment, and acid immersion treatment.

Molten carbonate fuel cell separator base material may be used stainless steel, preferably austenitic stainless steel may be used. In the austenitic system, when 7% or more of nickel is added to the stainless steel, the austenitic structure becomes an austenite structure and has excellent corrosion resistance, oxidation resistance, and toughness. However, it is weak in acid corrosion and has a disadvantage in that its strength does not increase by heat treatment.

The present invention forms a nickel electroless plating film containing 0.5 to 30% by weight of phosphorus and 0.1 to 20% by weight of cobalt as a primary plating layer, preferably 1.0 to 20% by weight of phosphorus and 0.5 to 10% by weight of cobalt. It contains. At this time, CoCl ₂ · 7H ₂ O (cobalt chloride aqueous solution) is used to obtain cobalt metal ions. When the phosphorus content in the nickel electroless plating film is 0.5% by weight or less, plating cannot be formed in a plating solution containing phosphate as a reducing agent, the stability of the bath is poor, and the plating deposition rate is slowed. If the content of phosphorus exceeds 30% by weight, the plating film becomes amorphous and high hardness cannot be obtained even at high temperature heat treatment. When the cobalt component is 0.1% by weight or less, the precipitation hardness is low even when the heat treatment is performed, and when 10% by weight or more, the adhesion decreases. In addition, the porosity of the primary plating layer film is preferably 0.1 to 25%. When the porosity of the primary plating layer exceeds 25%, the stainless steel base material and the secondary plating layer may directly contact each other when the secondary plating layer is formed, thereby lowering the binding force of the entire plating layer.

A nickel electroless plating film containing 0.5 to 30% by weight of phosphorus and 0.1 to 15% by weight of tungsten is formed on the primary plating layer as a secondary plating layer, preferably 1.0 to 20% by weight of phosphorus and 0.5 to 10% by weight of tungsten. It contains%. In this case, sodium tungstate (Na ₂ WO ₄ ) is used to obtain tungsten metal ions, and sodium carbonate is used as a buffer for adjusting the pH of the plating bath to 9.0.

When the phosphorus content in the nickel electroless plating film is 0.5 wt% or less, as in the primary plating layer, plating cannot be formed in the plating solution containing phosphate as a reducing agent, the stability of the bath is poor, and the plating precipitation rate is also slowed. If the content of phosphorus exceeds 30% by weight, the plating film becomes amorphous and high hardness cannot be obtained even at high temperature heat treatment. When the tungsten component is 0.1% by weight or less, the airtightness of the plating layer is lowered, so that the electrolyte is easily penetrated, and when it is 15% by weight or more, the deposition rate is very slow. Since the tungsten has a property of not being plated alone due to its characteristics, it is preferable to selectively use vaccinated nickel, palladium, or the like.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the following examples are merely to illustrate the present invention, but the scope of the present invention is not limited by these examples.

Example 1

The sample specimen was a 0.3 mm thick stainless steel STS 310S steel plate polished with # 2000 SiC paper, degreased with a 10% sodium hydroxide solution for 15 minutes in an ultrasonic cleaning bath, and then completely removed with residual deionized water by flowing deionized water. Dried at room temperature. Specimens were pretreated with 1M H ₃ PO ₄ aqueous solution.

The pretreated specimen was then plated in a plating bath of the components shown in Table 1 for about 15 minutes to form a foundation plating layer. At this time, the temperature of the plating bath was maintained at 40 ℃.

Subsequently, the specimen on which the foundation plating layer was formed was subjected to intermediate plating for about 20 minutes under the conditions shown in Table 2 to form an intermediate plating layer. At this time, the temperature of the plating bath was maintained at 60 ℃.

The thickness of the plating layer formed under the conditions shown in Table 2 is formed to about 20 microns (micron), the thickness of the final plating layer can be arbitrarily adjusted by determining the corrosion resistance limit according to the use and the corrosion environment used. After forming the secondary plating layer, the specimen was heat-treated at 400 ° C. for 1 hour after washing with deionized water and drying at room temperature.

Comparative Example 1

As the sample specimen was carried out in the same manner as in Example 1 except for using the STS 310S steel sheet without a separate surface coating.

Comparative Example 2

Slurry was prepared using aluminum powder, and an aluminum layer was formed on the surface of stainless steel 316L specimen by spray painting, followed by complete drying and finally heat treatment. Aluminum powder having a particle size of 40 to 75 mm was mixed with a small amount of binder in acetone and t-butyl alcohol and uniformly dispersed by applying ultrasonic waves to prepare a slurry.

Stainless steel was used after ultrasonic cleaning of 10 x 2 cm ² specimen in CCl ₄ solution. When performing slurry painting, an injector with an internal diameter of 0.5 mm was used, and the temperature of the specimen was maintained at 50 ° C. After repeated coating so that an aluminum layer was formed on one side of the specimen to an appropriate thickness, it was completely dried, and then the specimen was mounted in a quartz tube and heat-treated in a hydrogen atmosphere. The heat treatment was performed at 600 to 800 ° C. for 3 to 12 hours, and the temperature rising and falling rate was set at 5 ° C./min. Inside the quartz tube, purified hydrogen was supplied at a rate of 10 cc / min through a palladium membrane. After the heat treatment was carried out to remove the aluminum powder remaining on the specimen and to perform a corrosion test of the specimen.

Comparative Example 3

The pre-treated specimens were carried out in the same manner as in Example 1 except that the plating was performed for about 25 minutes in the plating bath of the components shown in Table 3 to form a single alloy plating layer of nickel and phosphorus.

As described above, the separator for fuel cells of the present invention can increase the corrosion resistance.

1 is a cross-sectional view of a plating layer of a stainless steel wet seal portion according to Embodiment 1 of the present invention. As shown in FIG. 1, the electroless ternary alloy plating layer according to Example 1 shows the characteristics of the electroless plating in which the interface between the base material and the coating is not sure and is completely intimately bonded. In addition, it can be seen that there is no distinction between the interface between the primary plating layer and the secondary plating layer, thereby alloying a single layer.

2 is a cross-sectional view showing a cross-sectional structure of Al phase anodizing according to Comparative Example 2 of the present invention. As shown in FIG. 2, in the case of Comparative Example 2, unlike Example 1, since the porous structure does not have a dense cross-sectional structure, there is a disadvantage in that the penetration of the base material of the electrolyte is high.

In the electrochemical polarization test, 5 x 2 cm ² specimens were placed in 1 M aqueous solution of sulfuric acid, and a standard calomel electrode was used as a reference electrode at 1 mV / sec in the potential region of -1.2 to 0.1 V. The Tafel curves were obtained by scanning the potential at velocity, and the corrosion current and corrosion potential were determined.

3 is a graph showing a Tafel curve of the corrosion potential against the corrosion current according to Example 1 of the present invention, Figure 4 is a graph showing a Tafel curve of the corrosion potential against the corrosion current according to Comparative Example 1 of the present invention, 5 is a graph showing the Tafel curve of the corrosion potential with respect to the corrosion current according to Comparative Example 2 of the present invention. 6 is a graph showing a Tafel curve of corrosion potential against corrosion current according to Comparative Example 3 of the present invention.

As shown in Figures 3 to 6, in Example 1 of Figure 3, compared with Comparative Example 1 of Figure 4 and Comparative Example 2 of Figure 5 using conventional aluminum anodizing, significantly higher corrosion potential and significantly lower Corrosion current was shown to have very excellent characteristics. In addition, Comparative Example 3 of FIG. 6, which is a single plating layer of nickel and phosphorus alloys, was found to be vulnerable to corrosion by showing I-V characteristics of a direct relationship in a voltage range of 0 V or more, which is a battery operating condition.

The data obtained from the corrosion potential (V) and the corrosion current (V) for the standard hydrogen electrodes of Examples 1 and Comparative Examples 1 to 3 are shown in Table 4 below.

As can be seen from Table 4, the plating layer of the stainless steel wet-seal part according to the present invention showed a high corrosion potential and a low corrosion current, so that the corrosion resistance was very excellent.

1 is a cross-sectional view of a plating layer of a stainless steel wet seal portion according to Embodiment 1 of the present invention.

2 is a cross-sectional view showing a cross-sectional structure of Al phase anodizing according to Comparative Example 2 of the present invention.

3 is a graph showing the Tafel curve of the corrosion potential with respect to the corrosion current according to Example 1 of the present invention.

4 is a graph showing the Tafel curve of the corrosion potential with respect to the corrosion current according to Comparative Example 1 of the present invention.

5 is a graph showing the Tafel curve of the corrosion potential with respect to the corrosion current according to Comparative Example 2 of the present invention.

6 is a graph showing the Tafel curve of the corrosion potential with respect to the corrosion current according to Comparative Example 3 of the present invention.

Claims (15)

0.5-30 wt% phosphorus; 0.1 to 20 weight percent of cobalt; And 50 to 99.4 weight percent nickel; Stainless steel separator plate characterized in that the electroless primary plating layer coating comprising a coating. The method of claim 1, wherein the separator is on the primary plating layer coating 0.5-30 wt% phosphorus; 0.1 to 15 weight percent tungsten; And Nickel 55-99.4 weight percent; Stainless steel separator plate characterized in that the electroless secondary plating layer coating comprising a coating. The stainless steel separator according to claim 1, wherein the porosity of the primary plating layer is 0.1 to 25%. The stainless steel separator according to claim 1, wherein the thickness of the primary plating layer is 25 to 75% of the total thickness of the coating. The stainless steel separator according to claim 4, wherein the total film thickness is 2 to 200 mu m. Degreasing and cleaning the surface of the stainless steel separator; And Forming the electroless primary plating layer by immersing the cleaned separator in a plating bath containing an electroless plating solution containing 0.5 to 30% by weight of phosphorus, 0.1 to 20% by weight of cobalt, and 50 to 99.4% by weight of nickel; Coating method of the stainless steel separator, characterized in that consisting of. The method of claim 6, wherein the coating method of the stainless steel separator is immersed in an electroless plating solution containing 0.5 to 30% by weight of phosphorus, 0.1 to 15% by weight of tungsten and 55 to 99.4% by weight of nickel on the primary plating layer. Coating method of the stainless steel separator further comprises the step of coating the electroless secondary plating layer coating. 8. The method of claim 6 or 7, wherein the electroless plating solution comprises a water-soluble nickel salt, a reducing agent, and a complexing agent. The method of claim 8, wherein the amount of the water-soluble nickel salt, the reducing agent, and the complexing agent is 0.1 g / L to 100 g / L, respectively. The method of claim 8, wherein the reducing agent is selected from the group consisting of hypophosphite (H ₂ PO ₂ ), hypophosphite such as sodium hypophosphite (NaH ₂ PO ₂ · H ₂ O), dimethylamine borane, trimethylamine borane and hydrazine Coating method of a stainless steel separator, characterized in that. The complexing agent of claim 8 wherein the complexing agent is selected from the group consisting of carboxylic acids such as malic acid, succinic acid, lactic acid and citric acid, sodium salts of carboxylic acids and amino acids such as glycine, alanine, iminodiisacetic acid, arginine and glutamic acid. Coating method of the stainless steel separator, characterized in that. The method of claim 8, wherein the electroless plating solution further comprises a stabilizer. 13. The method of claim 12, wherein the stabilizer is selected from the group consisting of water-soluble lead salts such as lead acetate and sulfur compounds such as thiodiglycolic acid. The method of claim 12, wherein the amount of the stabilizer is 0.1 g / L to 100 g / L. The method of claim 6, wherein the plating bath has a temperature of 30 ° C to 95 ° C.

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