KR100639118B1 - Cr-mn-y coating layer forming method with high oxidation resistance and electrical conductivity - Google Patents

Abstract

A method for forming a Cr-Mn-Y coating layer excellent in oxidation resistance and high temperature electrical conductivity is provided to maximize utilization of an existing ferritic metal connecting material by coating oxides with excellent electrical conductivity on the surface of an existing ferritic stainless steel sheet in an oxidative atmosphere of high temperature, thereby improving oxidation resistance and high temperature electrical conductivity of the existing ferritic stainless steel sheet. A method for forming a Cr-Mn-Y coating layer excellent in oxidation resistance and high temperature electrical conductivity comprises forming a coating layer on the surface of a ferritic stainless steel sheet that is a metal connecting material for solid oxide fuel cells by electron beam deposition such that the coating layer has a composition comprising 2 to 20 wt.% of Mn, 1 wt.% or less of Y and the balance of Cr.

Description

Cr-Mn-Y COATING LAYER FORMING METHOD WITH HIGH OXIDATION RESISTANCE AND ELECTRICAL CONDUCTIVITY}

The present invention relates to a method of forming a Cr-Mn-Ｙ coating layer having excellent oxidation resistance and high temperature conductivity by coating an oxide having excellent conductivity on a surface of an existing commercialized ferritic stainless steel sheet under a high temperature oxidative component crisis.

In general, the solid oxide fuel cell connection material is basically a material that electrically connects the anode of one cell and the cathode of a neighboring cell and physically blocks air gas and fuel gas. Metal connectors have superior properties in terms of processability, economy, electrical conductivity, and thermal conductivity compared to ceramic connectors.

To date, various alloys used as solid oxide fuel cell metal interconnects include Cr-based alloys based on Cr, Ferritic Fe-Cr alloys based on Fe, and Ni-base superalloys based on Ni. .

The Cr-based alloy has been developed as a SOFC (Solid Oxide Fuel Cell) metal coupling material because it forms a stable Cr ₂ O ₃ oxide at high temperature, and its thermal expansion coefficient is similar to that of other ceramic materials constituting SOFC, and its mechanical properties at high temperature. This has an excellent advantage.

The development of Cr-based alloys has been conducted to develop alloys that can increase the adhesion of Cr ₂ O ₃ oxides and lower the growth rate, and most of the elements such as Y, La, Ce, and Zr are ODS (Oxide Dispersion Strengthening) Will be added in the form.

Representative Cr-based alloys such as Cr-5Fe-1Y ₂ O ₃ and Cr-0.4La ₂ O ₃ have been developed in this background, for example "Cr-5Fe-1Y ₂ O ₃ " is originally about 1000 ℃ It was developed to replace the ceramic interconnects of flat SOFCs operating at high temperatures, but there are limitations in their use due to problems with long term stability.

Therefore, the operating temperature should be applied at a temperature of 800 ℃ or less, but rarely use Cr-5Fe-1Y ₂ O ₃ as a metal connecting material for SOFC below 800 ℃. This is because in the temperature range (650 to 800 ° C.), Fe-based or Ni-based alloys can be applied in place of Cr-based alloys, which are poor in workability and expensive.

As the Ni-based superalloy, "Haynes 230", "Inconel 625", "Inconel 718" and the like are used, "Haynes 230" is known to show the highest electrical conductivity. This is related to the oxidation behavior of "Haynes 230" because the "Haynes 230" alloy has the smallest rate of oxidation growth.

However, "Haynes 230" alloys are also limited in their use because they do not fully meet the SOFC requirements.

On the other hand, Fe-Cr alloys are "ZMG232" and "Crofer22", ZMG232 is a Ferritic Fe-22Cr alloy containing 22% by weight of Cr, added 0.04% by weight La and 0.22% by weight Zr.

ZMG232 has a thermal expansion coefficient of 12.8 × 10 ^-6 / ℃ and is reported to show better oxidation resistance and electrical conductivity than the existing STS430 in the temperature range of 700 ~ 1000 ℃. This seems to be related to the characteristics of the oxides formed on the surface in the oxidizing atmosphere. That is, the ZMG232 alloy is said to have a structure in which the oxide structure is dense, high adhesion, and excellent electrical conductivity.

In addition, Crofer22 is a Ferritic Fe-Cr alloy originally developed for automotive APU (Auxiliary Power Unit), and contains a trace amount of 0.08% by weight of La to minimize evaporation of Cr and lower the coefficient of thermal expansion. Mn and Ti are added thereto to form an oxide structure of MnCr ₂ O ₄ in the upper layer and Cr ₂ O ₃ in the lower layer in a high temperature oxidation atmosphere. The non-insulating MnCr ₂ O ₄ oxide of the spinel structure also has a function of preventing the evaporation of Cr. Crofer22 is also known to have better electrical conductivity than ZMG232.

However, since the metal interconnect forms an oxide on the surface in the solid oxide fuel cell operating environment, the resistance is increased to decrease the electrical conductivity. In other words, the electrical conductivity of the metal connecting material depends on the electrical conductivity of the oxide formed on the surface rather than the electrical conductivity of the metal itself. Therefore, it is important to form an oxide having excellent conductivity on the surface. This is why most metal connectors for solid oxide fuel cells are designed based on Cr ₂ O ₃ -formers.

However, Cr ₂ O ₃ -former type metal interconnects form volatile Cr (VI) in the solid oxide fuel cell operating environment, and these Cr (VI) interferes with the normal electrochemical reaction of the cell and decreases battery performance. Act as a factor.

As a result, there is a difficulty in developing a metal connecting material for fuel cell due to these two contradictions.

Recently, attempts have been made to develop alloys for metal interconnects and surface treatment techniques for controlling surface characteristics.

In particular, the coating material for surface treatment is sprayed with a perovskite structure oxide such as LSM, LSC, LSCF to increase oxidation resistance and electrical conductivity of existing products and to prevent Cr evaporation. pyrosis), PVD, thermal spraying, slurry coating, etc. in various ways.

However, in the case of such a perovskite ceramic coating, the coating layer should have excellent electron conductivity, similar thermal expansion coefficient to neighboring components, and excellent adhesion, so that peeling should not occur.

In addition, it is advantageous to form oxides such as MnCr ₂ O ₄ , CoCr ₂ O ₃ , and CoFe ₂ O _{4 having} a spinel structure at the interface between the coating layer and the substrate after prolonged exposure at high temperature. Because these oxides are non-insulating, they do not significantly increase the contact resistance at the interface. On the other hand, if an oxide such as SrCrO ₄ or La ₂ O ₃ is formed as a reactant, it should not be overlooked that the electrical conductivity of the coating layer is greatly reduced.

Furthermore, the texture of the coating layer should be dense. This is because it is possible to prevent the movement of oxygen diffused inward through the coating layer from the outside, and to prevent the external diffusion of the Cr component from the substrate. Therefore, even if the coating layer has the same perovskite phase, it may have different properties depending on the components of the coating layer, the structure and composition of the coating layer.

Therefore, since the control of the coating layer has a close relationship with the coating process, it is required to find an appropriate coating method having an economical coating layer having excellent properties.

The present invention has been made in view of the limitations of the prior art as described above. The present invention has been made to solve this problem by coating an oxide having excellent conductivity in a high temperature oxidation atmosphere on a surface of an existing ferritic stainless steel sheet used for a solid oxide fuel cell. The main object of the present invention is to provide a method of forming a Cr-Mn- 방법 coating layer having excellent oxidation resistance and high temperature conductivity by oxidizing and improving high temperature conductivity, thereby maximizing the utilization of existing ferrite-based metal connecting materials.

The present invention, to achieve the above technical problem, to have a composition consisting of Mn: 2 to 20% by weight, Y: 1% by weight or less, Cr on the surface of a ferritic stainless steel sheet which is a substrate for a solid oxide fuel cell metal connecting material. The present invention provides a method for forming a Cr-Mn coating having an excellent oxidation resistance and high temperature conductivity, which is charged into an alumina crucible and deposited to have a thickness of 5 µm using an electron beam deposition method.

Hereinafter, a preferred embodiment according to the present invention will be described in more detail.

First, if a conventional ferritic stainless steel sheet such as "SUS444" is used without coating, MnCr ₂ O ₄ having a spinel structure and Cr ₂ O ₃ oxide having a corundum structure are formed on the surface thereof.

Comparing only the phases of these oxides, it is the same as the "Crofer22APU" alloy or "ZMG232" developed as the solid oxide fuel cell metal coupling material.

However, ferritic stainless steel plates such as "SUS444" have a lower electrical conductivity than "Crofer22APU" alloy or "ZMG232" developed as a solid oxide fuel cell metal coupling material because the concentration of Cr and Mn is small and does not contain rare earth metal components such as La. Falls.

In other words, when ASR (Area Specific Resistance) is measured after oxidizing under the same conditions in a high temperature oxidizing atmosphere with materials "SUS444" and "Crofer22APU", the ASR value of the "Crofer22APU" alloy is small.

This means that the rho (Resistivity) value of the oxide formed on the surface of the "Crofe22APU" material is much smaller than the oxide formed on "SUS444". This is because the "Crofer22APU" alloy is designed to form an oxide layer having excellent electrical conductivity on the surface layer by appropriately adding Mn and Cr with a small amount of La.

According to the present invention, oxides such as Y and MnCr ₂ O ₄ having high conductivity and spinel structure, which not only increase the adhesion of oxides formed on the surface in a high temperature oxidizing atmosphere but also reduce the growth rate of the scale, reduce the ASR value of the material. Focusing on Cr and Mn which can be formed, these are coated on a commercially available ferritic stainless steel sheet to form a coating layer.

In addition, in the present invention, the conventional commercialized ferritic stainless steel sheet "SUS444" was used as a coating substrate, an electron beam evaporation (EB evaporation) was used to manufacture a Cr-Mn-Y coating layer having a variety of alloy compositions, alloys For coating, chromium (Cr), manganese (Mn) and yttrium (Y) were charged in a ratio in one alumina crucible and then coated using an electron beam.

At this time, the Mn is 2 to 20% by weight, Y is 0.1% by weight or less, the depositing object consisting of the remaining Cr was charged to deposit in an alumina crucible, the reason for limiting the numerical value so as to have such a composition.

That is, MnCr ₂ O ₃ which is excellent in conductivity is not formed when MnC is added in an amount of 2% by weight or less, and when it is added in 20% by weight or more, Mn ₂ O _{3 which} is not dense and has low conductivity is formed on the surface. It is preferable to add in the composition range as described above.

In addition, Y is added in order to increase oxidation resistance and increase high temperature conductivity at the same time in a high temperature oxidizing atmosphere. When the amount of Y added is large, insulating Y ₂ O ₃ is formed on the surface, resulting in poor thermal conductivity and thermal shock. When the Y ₂ O ₃ scale peels easily, oxidation resistance is also reduced. Therefore, the amount of Y added is preferably added at 0.1% by weight or less.

Finally, Cr basically forms Cr ₂ O ₃ on the surface, but can work with Mn added together to make MnCr ₂ O ₃ with excellent conductivity. That is, Mn is added in an amount of 2 to 20% by weight, and the balance is added by Cr. In other words, the ratio can be adjusted according to the component content of Mn.

EXAMPLE

Hereinafter, a preferred embodiment according to the present invention will be described.

After depositing the deposition object in the alumina crucible to have the composition according to the present invention described above, the temperature of the substrate was fixed at 350 ° C. under the coating conditions during vacuum deposition, and the vacuum degree was maintained at 5 × 10 ⁻⁵ torr or less, and the coating thickness was 5 Fixed to μm.

Thereafter, the coating was made to test pieces, and the oxidation resistance and high temperature conductivity of the test pieces were evaluated.

At this time, the oxidation resistance was exposed for 1000 hours in an air atmosphere at 700 ℃, and the weight increase of the test piece was measured, the evaluation criteria are ◎, if the weight increase (mg / cm ² ) is less than 0.2, ◎, if 0.5 to 0.5, ○, 0.5 It was set as x as it was above.

In addition, the conductivity of the coating layer was evaluated by measuring the ASR value after exposure for 1000 hours in an air atmosphere at 700 ° C., for example, the evaluation criterion was: ◎ when the ASR value was 0.01 m 2 or less; If it is 0.01-0.015 mm <2>, it is ○; It was set as x if it was 0.015 cm <2> or more.

First, within the range of the composition of the present invention, Mn: 2, 2, 2, 20, 3% by weight, and correspondingly changed to Y: 0.1, 0.2, 1, 0.2, 0% by weight, and the balance Cr The deposition target was prepared as the invention examples 1, 2, 3, 4 and 5, respectively, and charged into an alumina crucible, and then deposited on the surface of SUS444 by electron beam deposition at a thickness of 5 μm in the above vacuum and temperature ranges to form a coating layer. It was.

And, after evaluating each oxidation resistance and high temperature conductivity by the above-mentioned evaluation method through a specimen having such a coating layer is shown in Table 1 below.

On the other hand, for comparison with the present invention, the deposition object consisting of only Cr without adding Mn and Y at all as Comparative Example 1, Mn: 30% by weight, Y: 0% by weight, Cr: 70% by weight Cr In Comparative Example 2, Mn: 2% by weight, Y: 2% by weight, Cr: 96% by weight of the sample to prepare a specimen through the same process as in Examples 1 to 5 to Comparative Example 3 In the same manner, the oxidation resistance and the high temperature conductivity of each specimen were evaluated and shown in Table 1 below.

As shown in Table 1, in the preparation of the Cr-Mn-Y alloy coating layer, the amount of Mn added is 2 to 20% by weight, the amount of Y is added to 1% by weight or less, and the remainder is a Cr-deposited object. Through electron beam deposition, it was confirmed that the oxidation resistance and conductivity of the conventional commercialized ferritic stainless steel sheet could be increased.

That is, it was confirmed through the present invention that by selectively coating Cr-Mn-Y on an existing ferritic stainless steel sheet without producing a new alloy, it is possible to form an oxide having excellent conductivity on the surface in a high temperature oxidation atmosphere.

As described above in detail, according to the present invention, a solid oxide having excellent oxidation resistance and high temperature conductivity by forming a coating layer formed by appropriately mixing manganese, yttrium, and chromium by using an electron beam evaporation method on a surface of a ferritic stainless steel sheet already manufactured. Fuel cell interconnects can be manufactured.

Claims (1)

On the surface of ferritic stainless steel sheet, which is a substrate for solid oxide fuel cell metal connecting material, Formation of Cr-Mn coating having excellent oxidation resistance and high temperature conductivity, wherein the coating layer is formed to have a component composition consisting of Mn: 2 to 20% by weight, Y: 1% by weight or less, and Cr by using an electron beam deposition method. Way.