KR100887831B1 - Surface reforming method of metal bipolar plate for fuel cell - Google Patents

Abstract

A surface modification method of a metal bipolar plate for a fuel cell is provided to realize high conductivity and anti-corrosive property with low costs by forming a CrNx layer of nano-sized thickness on the surface of a bipolar plate. A surface modification method of a metal bipolar plate for a fuel cell comprises a step for manufacturing a metal bipolar plate and a step for surface-modifying the surface of the bipolar plate with a chromium nitride layer(11) by implanting Cr ion and N ion on the surface of the metal bipolar plate, to improve conductivity and anti-corrosive property of the metal bipolar plate.

Description

Surface reforming method of metal bipolar plate for fuel cell

The present invention relates to a method for surface modification of a metal-based separator plate for fuel cells, and more particularly, to improve the high conductivity and corrosion resistance of the metal-based separator plate for fuel cells using Cr (chromium) ions and N (nitrogen). The present invention relates to a method for surface modification of a metal-based separator plate for a fuel cell in which ions are implanted into a stainless steel separator plate to form a nano-thick CrN _x (0.1 ≦ _x ≦ 1) layer.

Polymer Electrolyte Membrane Fuel Cell (PEMFC) is a membrane electrode assembly (MEA) with an electrode / catalyst layer on both sides of the membrane, which conducts electrochemical reactions on both sides of the polymer solid electrolyte membrane where hydrogen ions move. And a gas diffusion layer (GDL) that distributes the reactors evenly and delivers generated electricity, gaskets and fastening mechanisms for maintaining the tightness and proper clamping pressure of the reactors and the cooling water. And an energy conversion device composed of a bipolar plate through which the reactants and the cooling water move, and generate electric current by cell reaction when hydrogen and oxygen (air) are injected.

In the polymer solid electrolyte fuel cell, hydrogen is supplied to an anode (also referred to as a “fuel electrode”), and oxygen (air) is supplied to a cathode (also referred to as a cathode, “air electrode” or “oxygen electrode”).

The hydrogen supplied to the anode is a hydrogen-ion (Proton, H ⁺⁾ by the electrode catalyst constructed on both sides of the electrolyte membrane and the electron (Electron, e ^-) to be decomposed, of hydrogen ions (Proton, H ⁺⁾ only selectively cation is delivered to the cathode through the electrolyte membrane-exchange membrane, at the same time e (electron, e ^-) is transferred to the cathode through a conductive gas diffusion layer and the bipolar plate.

In the cathode, the hydrogen ions supplied through the electrolyte membrane and the electrons delivered through the separator meet with oxygen in the air supplied to the cathode by the air supply to generate water.

At this time, current is generated by the flow of electrons through the external conductor generated due to the movement of hydrogen ions, and heat is incidentally generated in the water generation reaction.

The electrode reaction of the polymer solid electrolyte fuel cell is shown in the following reaction formula.

[Reaction at the fuel ^{_{electrode] 2H 2 → 4H + + 4e}} -

[Reaction in the air ^{_{electrode] O 2 + 4H + + 4e}} - → 2H 2 O

[Total Reaction] 2H ₂ + O ₂ → 2H ₂ O + Electrical + Thermal

The separator is a fuel cell core component along with the membrane electrode assembly, which is structural support of the membrane electrode assembly and the gas diffusion layer, collection and transfer of generated current, transport of reaction gas, transport and removal of reaction product, and transport of cooling water for removal of reaction heat. It is a fuel cell core part that plays various roles such as.

Accordingly, the separator, which is a key component, requires special material properties such as excellent electrical conductivity, thermal conductivity, gas tightness, and chemical stability.

Conventional separators have been manufactured using graphite-based materials having excellent electrical conductivity and chemical stability, and composite graphite materials mixed with resin and graphite.

However, the graphite-based separator plate has low mechanical stiffness and hermeticity compared to the metal-based material, and has a big problem of high process cost and low mass production since the graphite separator is processed manually without using a machine due to the risk of breakage of graphite easily during processing.

Therefore, research to replace this with a metal-based separator is being actively conducted.

If the metal-based separator is applied, the volume of the fuel cell stack can be reduced and lightened by reducing the thickness of the separator (previous graphite plate is 0.2mm or more, and the metal-based separator plate is 0.1mm), and stamping is used. It is possible to manufacture and have the advantage of ensuring mass productivity.

However, when the fuel cell is used, metal-based separators gradually generate metal corrosion, which is not present in the graphite separator, which causes contamination of the membrane electrode assembly, which causes the deterioration of the performance of the fuel cell stack.

In addition, due to such corrosion, the oxide film grows on the metal surface, and the oxide film thickens gradually over long periods of time, increasing the internal resistance of the fuel cell, thereby shortening the lifespan.

Therefore, in order to minimize the above problems, it is necessary to use a metal material having good corrosion resistance, and stainless steel, titanium alloys, aluminum alloys, nickel alloys, and the like have been considered as candidate materials for metal-based separators.

Among them, stainless steel has received a lot of attention as a separator material due to the advantages of relatively inexpensive material cost and excellent corrosion resistance, but there are still many problems to commercialize because it still exhibits low characteristics in terms of corrosion resistance and electrical conductivity.

Therefore, in order to manufacture a high performance metal-based separator plate, a surface forming technology using a material having high electrical conductivity while maintaining excellent corrosion resistance is required.

Japanese Patent Laid-Open No. 11-162478, Japanese Patent Laid-Open No. 10-228914, U.S. Patent No. 6637762 B1, 6967065 B1, and European Patent No. 1,5111,105 A1 are plated with precious metals such as gold on the surface or Although a method of coating the oxide of the platinum group metal to secure corrosion resistance and electrical conductivity has been disclosed, manufacturing costs are high and defects such as pinholes occur, and thus there is a limit to the practical step.

Japanese Patent Laid-Open Nos. 2003-276249 and 2003-272653 disclose a method of applying a very thin gold plating for cost reduction, but due to the low hardness of the gold plating, damage such as scratches when the metal separator contacts the electrode And the likelihood of occurrence of corrosion can be excluded.

Japanese Patent Laid-Open No. 2003-277133 discloses a method of improving electrical conductivity by dispersing relatively inexpensive carbon powder on the surface of a metal separator plate, but there is a possibility that the carbon particles may peel off when a physical shock or vibration is applied. . In addition, when the proper pretreatment is not performed, the contact resistance of the stainless steel, which is the raw material (base material), is high.

In addition, metal nitrides or carbides are known to have excellent corrosion resistance and electrical conductivity, and thus developments for coating applications thereof have been made. In particular, chromium nitride (Chromium Nitride) is excellent in corrosion resistance and chemical stability under the fuel cell environment and excellent in inhibiting the surface oxide growth.

U.S. Patent No. 6893765 B1 discloses a method of coating CrN on the surface of a base material by using a plasma PVD method, but the PVD method is a layer on a material having a complex shape such as a metal separator plate compared to electroplating and other vapor deposition. The step coverage property is bad. Also, due to the linearity of the coated particles, PVD coating must rotate the coating material in order to apply the coating on the entire surface, and it is necessary to eliminate the obstruction of the field of view between the target and the coating material, so the loading amount of the coating material is limited. It shows the inferior property.

U.S. Patent Nos. 2003/0190515 A1, 2005/0238873 A1, 6893765 B1, 2005/0202302 A1, 6893765 B1, and 2006/0204818 A1 provide a protective layer on the surface through nitriding of stainless steel. As a method of forming a metal nitride layer such as chromium nitride is disclosed.

U.S. Pat.Nos. 6893765 B1, 2005/0202302 A1 and 2006/0204818 A1 include M ₄ N (M = metal) or (Fe _{100 -xy- z} Cr _x Ni _y Mo _z ) through stainless steel nitriding. A method of forming ₄ N metal nitride or the like is disclosed, but the composition of the metal nitride is very limited (M: N = 4: 1), and corrosion resistance when a precipitate such as CrN is formed during coating layer preparation This rather reduced process control is very difficult.

U.S. Patent Nos. 2003/0190515 A1 and 2005/0238873 A1 disclose a method of forming a chromium nitride layer or precipitate as a protective layer on the surface of a base material through nitriding.

Japanese Patent Laid-Open No. 2000-353531 discloses forming a chromium nitride made of CrN, Cr ₂ N, CrN ₂ , Cr (N ₃ ) _3, etc. by coating chromium on a surface of a base material and then performing nitriding. Reduction of nitriding temperature and time is required to secure the properties and reduce the process cost. In forming the chromium nitride layer as a protective layer, it is difficult to secure the target corrosion resistance when reducing the temperature and time of the nitriding treatment.

Accordingly, the present invention has been invented in view of the above, and is a fuel cell metal system capable of mass-producing at a low cost a high-performance metal-based separator plate having excellent high electrical conductivity and corrosion resistance and light weight and thin thickness. It is an object of the present invention to provide a method for surface modification of a separator.

In addition, the manufacturing cost is lower than that of the conventional coating method, and since it does not coat the CrN _x phase separately, there is no peeling phenomenon of the manufactured separator, and the surface modification method of the metal-based separator plate for fuel cell can shorten the manufacturing time. The purpose is to provide.

It is also an object of the present invention to provide a method for surface modification of a metal-based separator plate for fuel cell, in which there is no dimensional deformation of the manufactured separator plate, and stiffness such as hardness is improved, so that defects caused by stack fastening can be largely eliminated.

In order to achieve the above object, the present invention, in order to improve the corrosion resistance and electrical conductivity of the metal-based separator plate for fuel cells, after producing a metal-based separator plate in the ion implantation equipment Cr ion and N on the surface of the metal separator plate Provided is a method for surface modification of a metal-based separator plate for fuel cells, characterized in that the ion implantation, the surface of the separator is surface-modified with a chromium nitride (Chromium Nitride) layer.

Preferably, the metal-based separator is ion-implanted to the metal-based separator manufactured by using a ferritic stainless steel plate containing Cr 12-16 wt% as the material, characterized in that the surface modification.

In addition, the metal-based separator is characterized in that the surface of the metal-based separator prepared by using austenitic stainless steel plate containing Cr 16 ~ 25 wt% and Ni 6 ~ 14 wt% as a material.

In addition, the ion-implanted Cr and N ions on the surface of the metal separator using ion energy of 150 to 200 keV.

In addition, the surface depth region of 10 to 40 nm on the surface of the metal-based separator is characterized in that the surface modification to the CrN _x (0.1≤x≤1).

In addition, after the ion implantation of the Cr ions and N ions, a process for heat treatment for 10 seconds to 60 seconds at a temperature of 700 ~ 1000 ℃ for homogenization.

According to the surface modification method of the metal-based separator plate for fuel cell of the present invention as described above, in order to improve the high conductivity and corrosion resistance of the stainless steel metal separator plate, Cr (chromium) ions and N (nitrogen) ions are formed by using ion implantation. By implanting the nano-thick CrN _x (0.1≤x≤1) layer on the surface of the separator, it is very inexpensive, excellent in electrical conductivity, corrosion resistance, light weight, and thin, high-performance metal-based separator in comparison with the existing expensive graphite separator. It can be provided.

In particular, it is possible to manufacture a metal-based separator plate for high reliability fuel cells capable of mass production at a lower cost than the graphite separator plate.

In addition, the manufacturing cost is lower than that of the conventional PVD coating method, and since the coating of CrN _x phase is not performed separately, there is no peeling phenomenon of the manufactured separator, and the production time is shortened due to the nano-grade surface modification using strong energy. Excellent in sex

In addition, there is no dimensional deformation of the manufactured separator, and the rigidity, such as hardness, is improved, so that defects caused by stack fastening can be largely eliminated, and in the case of mass production, a relatively short process time and a large area of the homogeneous separator can be modified. There are advantages of low cost and easy quality control.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

1 is a view showing an example of a metal-based separation plate to which the surface modification method according to the present invention can be applied, and FIG. 2 is a view for explaining surface layer formation by surface modification according to the present invention.

The present invention proposes a surface modification method using a material that maintains excellent corrosion resistance and has high electrical conductivity for the manufacture of a high performance metal-based separator.

That is, the present invention is to form a CrN _x layer on the surface of the metal-based separator 10 in contact with hydrogen and water to improve the corrosion resistance and electrical conductivity after manufacturing the metal-based separator plate using a stainless steel plate. As shown in FIG. 2, high-performance metal-based separation for fuel cells in which Cr (chromium) ions and N (nitrogen) ions are implanted by ion implantation to modify the surface of the separator into a CrN _x layer 11. The manufacturing technology of the plate.

Particularly, in the present invention, a metal separator is manufactured by using a ferritic stainless steel sheet containing 12 to 16 wt% Cr or an austenitic stainless steel sheet containing 16 to 25 wt% Cr and 6 to 14 wt% Ni. Then, Cr and N ions are injected to each surface of the separator in contact with hydrogen and water with a strong energy of 150 to 200 keV by ion implantation, thereby CrN _x (0.1≤x ≤ 1) to the layer 11, wherein the surface-modified layer 11 is formed to 10 ~ 40 nm as the core content.

In the present invention, the CrN _x (0.1 ≦ _x ≦ 1) layer 11 formed by ion implantation on the surface of the metal-based separator 10 is not coated with a dissimilar material by using conventional PVD or CVD method, but a stainless steel base material. It is formed by the surface modification method in which only the outermost surface of is transformed into CrN _x (0.1≤x≤1) layer by using ions, which can be processed at low temperature, and does not cause deformation of the base material due to no thermal expansion or coating layer, and peeling phenomenon. There is also an advantage that does not accompany.

This eliminates distortion of dimensions during fastening the stack of the fuel cell divider plate, thereby providing an effect of significantly reducing defects caused during assembly.

In addition, since the rigidity and corrosion resistance can be improved by improving the surface hardness due to surface modification, there is an advantage of ensuring the reliability of the product.

The present inventors maintain a high electrical conductivity (about 15% improvement) as well as excellent corrosion resistance (about 20% improvement) of a metal-based separator surface-modified with a CrN _x (0.1≤x≤1) layer than an unmodified separator. Confirmed that the present invention was completed.

Metal-based separator material used in the present invention is 0.1 ~ 0.2 mm commercial stainless steel (ferritic stainless steel containing Cr 12 ~ 16 wt%, or Cr 16 ~ 25 wt% and Ni 6 ~ 14 wt% Austenitic stainless steel), which is significantly cheaper than expensive graphite separators and is easily applicable to mass production processes.

The reason for using stainless steel as a metal-based separator material is that stainless steel is a relatively inexpensive and rigid material, and has good corrosion resistance and electrical conductivity, and thus is widely used as a conventional metal-based separator material for fuel cells.

Generally, in the case of stainless steel, ferritic stainless steel containing 11 to 30 wt% Cr and austenitic stainless steel containing 16 to 25 wt% Cr and 6 to 14 wt% Ni in order to further improve corrosion resistance. It is commercially used.

In the present invention, not only the ferritic stainless steel sheet but also austenitic stainless steel sheet having better corrosion resistance due to the addition of Ni, and the content of Cr and Ni, which is a key component in improving the corrosion resistance of stainless steel, is regulated as follows. .

Cr is known as an element that improves the corrosion resistance. In the case of ferritic stainless steel of 12 wt% or less, the corrosion resistance effect due to Cr is slightly weakened, so durability cannot be guaranteed. However, as the material price is high, in the present invention, a ferritic stainless steel having a Cr content of 12 to 16 wt% having sufficient corrosion resistance is used as a separator plate material.

Further, when Ni is added to the stainless steel, the corrosion resistance is further improved. Therefore, the austenitic stainless steel containing 16 to 25 wt% Cr and 6 to 14 wt% Ni may also be used as a separator material.

Cr. 16.wt% or less and Ni 6 wt% or less have a weak problem because corrosion resistance is insufficient and may exist as an unstable phase at room temperature. Cr 25.wt% or more and Ni 14wt% or more have a sharp material price. As it becomes expensive, it is difficult to use in mass production.

In order to manufacture the metal-based separator plate according to the present invention, first, the above-described stainless steel sheet is molded through a normal press stamping to manufacture a metal-based separator plate, and then before using the ion implantation equipment, which is a vacuum equipment, The surface is cleaned to remove foreign substances from the surface.

As a surface cleaning operation, chemical cleaning using DI water and an ultrasonic cleaner is performed, and then separated into ion implantation equipment (200keV class) as shown in FIG. After the plate is mounted, vacuum cleaning is performed to remove foreign substances by sputtering the separator for 10 minutes using Ar ions at a high vacuum of 10 ^-6 torr.

Next, as an ion implantation, ion implantation is performed to accelerate Cr ions and N ions to the separator plate while applying a high voltage of about 150 to 200 keV at a high vacuum of 10 ⁻⁶ torr.

At this time, the implantation amount of each ion is adjusted to 1 × 10 ¹⁸ / ㎠ or more, and undergoes a homogenization operation for heat treatment for 10 seconds to 60 seconds at a temperature of about 700 ~ 1000 ℃ to stabilize the surface of the separator after ion implantation. .

During the ion implantation and homogenization, phase transformation occurs on the surface of the stainless steel at about 40 nm with CrN _x (0.1 ≦ _x ≦ 1).

In general, the CrN layer is known as an excellent corrosion resistance and abrasion resistant material having high hardness (Hv 1500 or more), and has a dense Cr ₂ N phase formed at the time of forming the CrN layer, which is a columnar structure, to have corrosion resistance and low electrical conductivity. Will be.

In the present invention, the chemical structure of the surface-modified CrN _x layer can be seen through the attached depth profile of FIG. 4, which has a depth of 30 nm (surface depth, denoted as 'thickness' in FIG. 4) at the surface. The CrN phase and the Cr ₂ N phase were formed in the region of about 30 to 40 nm.

If the surface modification layer is formed at 10 nm or less, no uniform CrN _x phase is formed and thus surface modification effects such as poor corrosion characteristics are not seen. Also, the surface modification layer having a thickness of 40 nm or more has sufficient corrosion resistance and electrical conductivity, but is ion implantation process. Because of the long time required and high cost, the present invention defines the surface modification to form a CrN _x (0 ≦ _x ≦ 1) layer structure having a thickness of 10 to 40 nm on the surface.

In order to obtain the optimal effect of ion implantation, the ion energy applied to the sample is important, but the present invention stipulates that the ion energy is 150 to 200 keV.

If the energy is 150keV or less, heavy metal-based ions such as Cr may not be sufficiently injected. If the energy is 200keV or more, CrN may be sputtered or a relatively light N ion deeply injected into the CrN due to high energy. _It becomes difficult to form the image of _x .

Therefore, in the present invention, the ion energy for injecting Cr ions and N ions for optimal surface modification is defined as 150 to 200 keV.

The CrN _x layer thus formed has excellent corrosion resistance against chemical reactions caused by hydrogen, oxygen, and water reacting during operation of the fuel cell separator, and does not cause a drop in electrical conductivity due to corrosion. It is most suitable as a method of surface modification of the separator.

Hereinafter, the difference between the prior art and the present invention will be described.

In the case of the conventional graphite separator, the thickness is usually made thicker than 2.0mm due to the weak mechanical stiffness of the problem easily broken, and thus there was a problem of weight increase and very vulnerable to stiffness.

In addition, the manufacturing cost is very expensive because the separation plate is manufactured by a manual process, and because it is heavy, it is very difficult to achieve the desired performance when the fuel cell vehicle is manufactured, and the maintenance is also difficult.

However, this problem can be largely solved when fabricating a metal-based separator plate, which can use a thin plate having a thickness of 1.0 mm or less, which can reduce the volume and weight, and can be applied to mass-produce a high-strength separator at low cost. There is an advantage.

In particular, the ion implantation technology used in the present invention is a low-temperature technology, there is almost no deformation of the object can implement the desired properties on the surface without affecting the physical properties of the base material stainless steel.

In other words, the ion implantation method using Cr and N ions to form CrN _x (0.1≤x≤1) phase on the outermost surface layer provides reliable separation surface modification to improve corrosion resistance and maintain electrical conductivity of the surface of the separator. It is suitable as a method.

In addition, the manufacturing cost is lower than that of the conventional PVD coating method, and since the CrN _x phase is not coated separately, there is no peeling phenomenon.

In addition, the nano-grade surface modification using a strong energy is shortened the manufacturing time is excellent in mass productivity, there is no dimensional deformation and the rigidity, such as hardness is improved, there is an advantage that can remove a considerable amount of defects when stacking.

In addition, it is possible to modify the surface of the homogeneous separator plate of relatively short process time and large area when mass production is applied, which has the advantage of low cost and easy quality control.

The present inventors conducted a contact resistance test and a corrosion test to find the effect of the present invention.

The contact resistance test is an experiment for measuring the contact resistance of the surface of the separator plate using an apparatus configured as shown in FIG.

According to the present invention, the carbon paper was placed on the surface of the 25 cm 2 separation plate (Example) to improve the adhesion, and the copper plates were closely attached to each other to apply pressure and current, and the voltage and contact resistance were measured at this time.

As a comparative material, a conventional graphite separator (Comparative Example 1) and a stainless steel separator (Comparative Example 2) without ion implantation were used, and the evaluation results of Examples and Comparative Examples 1 and 2 are shown in FIG. 6. It was.

Contact resistance evaluation equipment and conditions are shown in Table 1 below.

According to the US Department of Energy (DOE), the contact resistance can be used as a separator only when the contact resistance satisfies about 25 mΩ ㎠ or less. As a result of the contact resistance test, the separator of Example 1 and the graphite separator of Comparative Example 1 were each 13.9 mΩ. It has been found to have excellent initial contact resistance of ㎠, 11.4mΩ ㎠ and stable results with little increase with time.

However, the separator of Comparative Example 2 showed a result that the contact resistance continuously increased over time in the initial 72mΩ ㎠.

Next, the corrosion test was carried out under the conditions of Table 2, and the corrosion current was measured over time. Experimental results are shown in FIG.

In the corrosion test, first, the surface-modified separator of 1cm 2 was precipitated in 0.1N sulfuric acid and 2ppm hydrofluoric acid solution, and then maintained at 80 ° C and air bubbling. Then, the current density was measured using potentiostat. Done.

According to the US Department of Energy (DOE), the corrosion current should satisfy about 1 mA / cm 2 or less. As a result of the experiment, the graphite separator of Example 1 and the graphite separator of Comparative Example 1 had excellent characteristics of 0.9 mA / cm 2 and 0.45 mA / cm 2, respectively. With the initial current value, no corrosion occurred over time.

However, the stainless steel separator of Comparative Example 2, which was not ion implanted, showed a large initial corrosion current as well as an increase in corrosion current as the corrosion progressed over time.

1 is a view showing an example of a metal-based separator plate to which the surface modification method according to the present invention can be applied,

2 is a view for explaining the surface layer formation by surface modification according to the present invention,

3 is a view showing the ion implantation equipment available in the present invention,

4 is a view showing the chemical structure of the surface-modified CrN _x layer according to the surface depth in the present invention,

5 is a view for explaining an experimental method for measuring the contact resistance of the surface of the separator,

6 is a view showing the results of measuring contact resistance of Examples and Comparative Examples of the present invention,

7 is a view showing a corrosion current measurement results of Examples and Comparative Examples of the present invention.

<Explanation of symbols for the main parts of the drawings>

10 metal separator 11 CrN _x layer (surface modified layer)

Claims (6)

In order to improve the corrosion resistance and electrical conductivity of the metal-based separator plate for fuel cell, after manufacturing a metal-based separator plate ion implantation equipment by ion implanting Cr ions and N ions to the surface of the metal-based separator plate, the surface of the separator plate Surface modification method of a metal-based separator plate for fuel cells, characterized in that the surface modification with a chromium nitride (Chromium Nitride) layer. The method according to claim 1, The surface modification method of the metal-based separator plate for fuel cells, characterized in that the surface of the metal-based separator plate by ion implantation of the metal-based separator plate manufactured using a ferritic stainless steel plate containing Cr 12 ~ 16 wt% as a material. The method according to claim 1, For the fuel cell, characterized in that the metal-based separator is ion-implanted a metal-based separator prepared by using austenitic stainless steel sheet containing 16 to 25 wt% Cr and 6 to 14 wt% Ni as a material. Surface modification method of the metal-based separator plate. The method according to claim 1, Cr ion and N ions to the surface of the metal separator using ion energy of 150 ~ 200 keV ion surface modification method of the metal-based separator plate for a fuel cell. The method according to any one of claims 1 to 4, Surface modification method of the surface depth region of 10 ~ 40 nm on the surface of the metal-based separator plate CrN _x (0.1≤x≤1) on the surface modification method of a metal-based separator plate for a fuel cell. The method according to claim 1, Method for surface modification of a metal-based separator plate for a fuel cell, characterized in that the heat treatment for 10 seconds to 60 seconds at a temperature of 700 ~ 1000 ℃ for homogenization after the ion implantation of the Cr ions and N ions.

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