Chromium Upgrade Method for Ferrite Steel Interconnects for Solid Oxide Cell Stack Applications

Abstract

A method for chromium upgrading of ferritic steel interconnects for use in solid oxide cell stacks, comprising the steps of shaping the interconnect, depositing a Cr-containing coating on at least one surface of the shaped interconnect, and performing one or more heat treatments at temperatures less than 1000°C, resulting in a Cr concentration near the surface of the interconnect that is greater than the Cr concentration in the ferritic steel prior to shaping. Specifically, the average Cr concentration of the shaped interconnect is increased to 26 wt% Cr or greater.

Description

Technical Field

[0001] The present invention relates to a method for chromium upgrading of a ferrite steel material, and more particularly to a method for chromium upgrading of a ferrite steel interconnect used in a solid oxide cell (SOC) stack.

Background Art

[0002] Stainless steel is an iron alloy with a minimum Cr content of 10.5 wt% and a maximum carbon content of 1.2 wt%.

[0003] Stainless steels are divided into different families based on their crystal structures: austenitic, ferritic, duplex, and martensitic structures. The largest group of stainless steels is the austenitic type. Austenitic stainless steels can be further divided into the following five subgroups: Cr-Mn type, Cr-Ni type, Cr-Ni-Mo type, high-performance type, and high-temperature type. The most common austenitic steel is Cr-Ni, which contains 8-10 wt% Ni, 17-18 wt% Cr, and the balance Fe, and is often referred to as 18-8 type stainless steel. The Ni in the steel is required to stabilize the austenite phase (γ-Fe) having a face-centered cubic (FCC) crystal structure that remains in a stable state at room temperature. Austenitic grades are classified as non-magnetic materials with good weldability and formability.

[0004] Ferritic stainless steels are the second most widely used group of stainless steels and are often referred to as the "nickel-free" alternative to austenitic steels. Ferritic steels mainly contain Fe and Cr, and the Cr content can vary widely (10.5 - 29 wt%) depending on the application. Ferritic steels can be further subdivided into five different groups. Groups 1 - 3 have the widest range of applications and, therefore, also have the largest production volume among ferritic steels. Steels in Groups 1 - 3 are often also referred to as "standard ferritic grades". Group 1 has the lowest Cr content (in the range of 10.5 - 14 wt%), while Groups 2 - 3 contain Cr in the range of 14 - 18 wt%. Group 2 is the most widely used family among ferritic stainless steels. AISI 430 is a particularly widely used type among Group 2 stainless steels and is superior to the austenitic alternative AISI 304 in many indoor applications where the importance of corrosion resistance is relatively low, but price stability (due to the Ni-free formulation) is desirable. Group 3 differs from Group 2 in that it contains additional stabilizing elements such as Ti, Nb, and Zr that tie up carbon and nitrogen and leave a completely ferritic crystal structure at all temperatures. Therefore, the Group 3 family generally exhibits better weldability and resistance to sensitization than other groups. Group 4 contains 10.5 - 18 wt% Cr and is alloyed with Mo for additional corrosion resistance. The ferritic steels of Group 5 have more than 18 wt% Cr alloyed or do not belong to the other groups. Typically, Group 5 ferritic steels have very high corrosion resistance but low weldability and are also sensitive to embrittlement. Grades of Group 5 with high Cr and high Mo are referred to as "super ferritic" and are designed to replace titanium in applications where corrosion resistance is of utmost importance. Ferritic stainless steels have a body-centered cubic lattice (BCC) crystal structure (α-Fe), are magnetic, and have a lower coefficient of thermal expansion than austenitic steels.

[0005] The duplex system is a different group of stainless steels. Duplex steel is basically a mixture of a ferrite phase and an austenite phase with an approximate phase balance of 50% ferrite and 50% austenite. Duplex stainless steels are characterized by a high Cr content (20.1 - 25.4 wt% Cr) but a rather low Ni content (1.4 - 7 wt% Ni). In duplex steel, many beneficial properties from both austenitic and ferritic steels are combined. Duplex grades are magnetic because they contain ferrite.

[0006] Martensitic stainless steels are the smallest group of stainless steels. Martensitic steels typically contain 12 - 17 wt% Cr and Ni in the range of 0 - 5 wt%. This is a combination of the alloy composition and a high cooling rate during quenching that transforms the microstructure into martensite with a body-centered tetragonal (BCT) crystal structure. Martensitic steels are hardenable and magnetic.

[0007] A solid oxide cell (SOC) can operate as a solid oxide fuel cell (SOFC) or as a solid oxide electrolysis cell (SOEC), or reversibly, i.e., by switching between SOFC and SOEC modes.

[0008] A solid oxide fuel cell includes an oxygen ion-conducting electrolyte, an oxygen electrode (cathode) where oxygen is reduced, and a fuel electrode (anode) where fuel (e.g., hydrogen, methane, or natural gas) is oxidized. The overall reaction in an SOFC is a reaction in which the fuel and oxygen used react electrochemically to produce electricity, heat, and oxidized chemical species. The oxidized chemical species is water when hydrogen is used as fuel, carbon dioxide when carbon monoxide is used as fuel, and a mixture of water and carbon dioxide in the case of hydrocarbon fuels.

[0009] A solid oxide electrolysis cell includes an oxygen ion conductive electrolyte, a fuel electrode (cathode) where oxidized species (e.g., water or carbon dioxide or both) are reduced by an externally applied electric field, and an oxygen electrode (anode) where oxygen ions are oxidized to molecular oxygen. The overall reaction in the SOEC is one where the oxidized species are electrochemically converted, using electricity and heat, to the reduced species. When the oxidized species supplied into the stack is water, hydrogen is produced on the fuel electrode. When the oxidized species is carbon dioxide, carbon monoxide is produced on the fuel electrode. When the oxidized species is a mixture of water and carbon dioxide, a mixture of carbon monoxide and hydrogen (also known as syngas) is produced.

[0010] The SOC operates in a temperature range of about 500 °C to about 1100 °C. The elevated operating temperature is necessary to ensure sufficiently high oxygen ion conductivity in the electrolyte. Commonly used electrolyte materials for the SOC include, but are not limited to, yttrium stabilized zirconia (YSZ) and gadolinia doped ceria (CGO).

[0011] In an SOC stack, a plurality of cells, each including a fuel electrode, an electrolyte, an oxygen electrode, and optionally additional layers, are connected in series by intervening interconnect plates (or "interconnects" or "interconnectors") between each pair of cells. The role of the interconnect is to provide electrical contact from one cell to the next, to assist in the distribution of gases across the cells, and in some designs, to avoid mixing of gases between the anode chamber and the cathode chamber.

[0012] The interconnect can be made of ceramic materials such as doped lanthanum or yttrium chromite, or it can be made of metals such as stainless steel. The advantages of metal interconnects over ceramic interconnects include: 1) lower material and manufacturing costs, 2) easier and less complex shaping, 3) higher electrical and thermal conductivity, and 4) ductility. Therefore, for SOCs operating at temperatures below 850°C, metal interconnects are preferred.

[0013] Suitable materials for metal SO C interconnects need to be oxidation-resistant at elevated operating temperatures against the gases supplied to both the oxygen electrode and the fuel electrode, and they also need to exhibit a coefficient of thermal expansion (CTE) that matches that of the ceramic components of the cell. Furthermore, the protective oxide barrier formed on the surface of the steel at high temperatures needs to be electrically conductive. From the perspective of these requirements, ferritic alloys that form a chromium oxide surface layer (e.g., chromia-forming ferritic steels) are particularly suitable for use as interconnects in SO C stack applications. Examples of such high-chromium ferritic steels include, but are not limited to, AISI441, AISI444, AISI430, AISI446, Crofer 22H, Crofer 22APU, ZMG G10, E-brite, Plansee ITM, etc. Other materials used for metal SO C interconnects include, for example, Plansee CFY (an alloy based on <95 wt% Cr, 5 wt% Fe, and Y).

[0014] For example, US6,936,217B2 (Patent Document 1) discloses a high-temperature material made of a chromium oxide-forming ferroalloy containing a) 12 to 28% by weight of Cr, b) 0.01 to 0.4% by weight of La, c) 0.2 to 1.0% by weight of Mn, d) 0.05 to 0.4% by weight of Ti, e) less than 0.2% by weight of Si, and f) less than 0.2% by weight of Al. The high-temperature material can form an MnCr₂O₄ spinel phase at a temperature of 700°C to 950°C. The ferritic stainless steel included above is commercialized under the trade name Crofer 22APU. The CTE of Crofer 22APU between 20°C and 800°C is 11.9 ppmK -1 is.

[0015] WO2008 / 013498A1 (Patent Document 2) by the present applicant discusses a ferritic chromium stainless steel containing a) 20 to 25% by weight of Cr, b) 0.5 to 2% by weight of Mo, c) 0.3 to 1.5% by weight of Nb, d) up to 0.1% by weight of C, e) up to 0.6% by weight of Mn, f) up to 2% by weight of Ni, g) up to 0.5% by weight of Ti, h) up to 0.5% by weight of Zr, i) up to 0.1% by weight of Al, j) up to 0.07% by weight of N, k) up to 0.3% by weight of rare earth metals, and l) the balance of Fe and normally occurring impurities. Here, the content of Zr + Ti is at least 0.20% by weight. Further, the most preferred embodiment is a steel having an approximate composition (the values are in weight%) of 0.2 of Si, 0.3 of Mn, 22 of Cr, 1 of Mo, 0.4 of Nb, 0.3 of Zr, 0.05 of Ti, the balance of Fe and normally occurring impurities. The above steel is suitable for use as an interconnect in fuel cells such as solid oxide fuel cells due to the good adhesion of the oxide formed on the surface of the material and the low electrical contact resistance when tested in contact with (La, Sr)MnO₃ at 750°C in air.

[0016] The corrosion rate of ferritic stainless steel depends greatly on the Cr content in the steel. For example, I.G. Wight in Metals Handbook, 9 thEdition, Vol. 13 Corrosion (1987) (Non-Patent Document 1) teaches that the parabolic rate constant of corrosion in a Fr-Cr alloy decreases by more than four digits at 1000 °C as the Cr content in the alloy increases from 0 to 20 wt% Cr. At a Cr content of less than approximately 28 wt%, the oxide scale formed on the surface of the alloy consists of a layer of Fe- or Fe-Cr mixed oxides, resulting in incomplete protection of the steel. At a Cr content of more than approximately 28 wt%, the oxide scale formed on the surface of the alloy consists of pure and continuous Cr oxide, providing more complete protection of the steel (i.e., minimum corrosion rate). Therefore, for SOC applications, it is desirable to use ferritic stainless steels with a Cr content of more than 28 wt%. If the SOC stack is operated at a temperature below 1000 °C, a slightly lower Cr content (e.g., 26 wt%) may be sufficient. Unfortunately, the Cr content of the most widely used ferritic steels is not high enough to withstand long-term exposure to SOC conditions.

[0017] Problems associated with the introduction of ferritic stainless steels of Groups 1 to 3 (17 - 18 wt%) for SOC interconnects are usually addressed with high-temperature oxidation-resistant coatings. For example, J.G. Grolig et al. in Journal of Power Sources, 248 (2014) 1007 - 1013 (Non-Patent Document 2) demonstrated that the corrosion rate of AISI 441 with a chromium content of 17.83 wt% when exposed to SOFC cathode conditions at 850 °C in air with a moisture content of 3% can be reduced by a protective coating containing cerium or lanthanum, or by a double-layer coating of cerium or lanthanum combined with cobalt. These coatings are applied by physical vapor deposition. The main drawback of such coatings is that they do not provide protection against corrosion when the coating is damaged, for example, by defects, cracks, pinholes, poor adhesion, etc. If the coating does not work well, the steel will probably undergo severe iron oxidation due to the low Cr level, and the SOC stack will fail. Furthermore, the shaping of the steel after coating damages the conformal nature of the coating, resulting in incomplete corrosion protection.

[0018] The thermal expansion coefficient (CTE) of Fe-Cr alloys also depends on the Cr content of the alloy. Generally, the CTE of the alloy decreases as the Cr content increases. For example, the CTE of AISI 430 (16 - 18 wt% Cr) measured between 25 °C and 727 °C is 12.94 ppm / K. The CTE of Crofer 22 APU (20 - 24 wt% Cr) measured between 20 °C and 800 °C is 11.9 ppm / K. The CTE of Plansee ITM (26 wt% Cr) measured between room temperature and 800 °C is 11.6 ppm / K. The CTE of CFY (95 wt% Cr) measured between room temperature and 800 °C is 10.5 ppm / K. The optimal CTE value for compatibility with the CTE of the 40 vol% Ni - 60 vol% 8YSZ (8 mol% yttria-stabilized zirconia) support layer in SOC would be 12.5 ppm / K (F. Tietz, Ionics, 5 (1999) 129 (Non-Patent Document 3)).

[0019] The shaping of metals in manufacturing can be divided into two main categories, namely, the material retaining processes and the material removing processes. The material retaining processes are usually classified as forming or deformation processes, and are processes in which the material undergoes plastic deformation in the generation of the shape. Formability is, in many cases, a term used in the manufacture of metals in the material retaining process category. The term "formability" represents the ability of a metal to undergo plastic deformation into the desired shape without damaging the workpiece. Examples of damage during plastic deformation include the formation of cracks or fissures. Examples of forming processes include, but are not limited to, punching, forging, rolling, extrusion, roll forming, and hydroforming. The material removing processes are described by processes that shape metals by removing material from the workpiece, and are generally referred to as machining. Machining encompasses a wide variety of different processes and is divided into three different categories, namely, mechanical, chemical, and thermal machining. In mechanical machining, the tool removes material by cutting or abrasion. Chemical machining and / or electrochemical machining are defined as processes that remove material by etching it away from the workpiece to obtain the desired shape. Thermal machining often uses electrical energy to evaporate the material from the workpiece. Therefore, the term "machinability" is a very broad term because it encompasses many different processes. However, the meaning of this term is the ability of the material that can be removed from the workpiece.

[0020] For example, the applicant's US8,663,863B2 (Patent Document 3) describes an interconnect for a fuel cell made of a metal sheet having a protruding contact area. The protrusions can be made by shaping the metal sheet by any known method such as punching, pressing, slicing, deep drawing, etc.

[0021] US7,718,295 B2 (Patent Document 4) describes a method including shaping an interconnect for a planar solid oxide fuel cell by etching. Suitable methods include, among others, photochemical and electrochemical etching, and laser cutting.

[0022] In US9,472,816 B2 (Patent Document 5), a member formed by powder metallurgy is produced from a powder composed of 95 wt% Cr and 5 wt% FeY master alloy (an alloy containing 0.5 wt% Y). 1 wt% of a pressure aid (wax) is added to this powder batch. Then, this powder batch is mixed in a tumble mixer for 15 minutes. Using a pressing tool, the powder is pressed into a green compact, which is pre-sintered at 1100 °C for 20 minutes in a hydrogen atmosphere in a continuous belt furnace for the purpose of dewaxing. Thereafter, for the purpose of further densification and alloy formation, the member is subjected to high-temperature sintering at 1400 °C for 7 hours in a hydrogen atmosphere. Then, in order to close the residual pores that may exist until the permeability of the material becomes sufficiently low, the member is pre-oxidized at 950 °C for 10 to 30 hours. Finally, the oxide layer is removed from all sides of the surface of the member by sandblasting. The described example includes many high-temperature sintering steps, some of which are carried out under a hydrogen atmosphere, and further includes using metal powders with strict requirements regarding particle size and shape, which makes the process very costly. Furthermore, the size of the interconnect plate manufactured by powder metallurgy is limited by the size of the mold and the pressing force of the press.

[0023] US2008 / 0269495 A1 (Patent Document 6) describes a method for manufacturing a metal interconnect for a fuel cell stack, including providing a sheet metal blank and forming this sheet metal blank by a plastic forming process. The main drawback of this method is that extremely high pressure (1000 kN / cm 2 or 10000 bar) is required to emboss the sheet metal blank, which severely limits the size of the interconnect plate that can be manufactured using this method.

[0024] Generally, the formability of ferritic stainless steel decreases as the Cr content in the steel decreases. For example, Design Guidelines for the Selection and Use of Stainless Steel (Nickel Development Institute, A Designers’ handbook Series No.9014) (Non-Patent Document 4) teaches that AISI 430 steel (16-18 wt% Cr) is rated "excellent" in terms of the ease of coining, embossing, and roll forming, while on the other hand, AISI 446 steel (23-27 wt% Cr) is rated "good". AISI 430 steel is further rated "excellent" in terms of the ease of cold forging and spinning, while on the other hand, AISI 446 is rated "fair".

[0025] In order to quantitatively describe the formability of steel from various viewpoints, several different parameters can be used. The parameters used include, but are not limited to, the work hardening index, the ratio of tensile strength to yield strength, total elongation, uniform elongation, and r-value, etc. The work hardening index represents the ductility of the steel, the total elongation characterizes the bendability of the steel, the uniform elongation correlates with the sheet stretching ability of the steel, and the r-value correlates with the deep drawing ability of the steel. For example, the elongation (A5) of AISI 430 steel (16-18 wt% Cr) is ≧20-28%, while on the other hand, the elongation (A5) of AISI 446 steel (23-27 wt% Cr) is ≧10%, suggesting that the higher the Cr content, the higher the formability.

[0026] For example, US2016 / 0281184A1 (Patent Document 7) relates to a ferritic stainless steel having excellent corrosion and sheet forming properties. This steel contains 20-24 wt% Cr and has a uniform elongation (Ag) between 17.0% and 19.1% and an r-value between 1.81 and 2.55.

[0027] A ferritic stainless steel sheet excellent in press formability and workability is described in EP1452616B1 (Patent Document 8). The Cr content in this steel sheet is 10 to 19% by weight, and this steel sheet has a lubricating film on one or both surfaces. The main drawback of this invention is that due to the low Cr content, this steel does not provide good sufficient corrosion protection under SOC conditions.

[0028] Therefore, for SOC applications, it is desirable to use a stainless steel having the following characteristics: 1) high oxidation resistance in both reducing and oxidizing atmospheres, 2) a thermal expansion coefficient compatible with the thermal expansion coefficient (CTE) of SOC, 3) the ability to form an electrically conductive oxide scale, 4) easy formability or machinability, 5) low cost, and 6) wide availability (e.g., a wide range of suppliers).

[0029] In particular, ferritic stainless steels developed for SOC applications, such as Plansee ITM (26% Cr by weight), provide excellent oxidation resistance due to their high Cr content. ITM steels further form a scale based on Cr oxides, which is more conductive than scales based on alumina or silica. The main drawback of high-Cr steels is related to the difficulty of shaping the material; for example, interconnects made of Plansee ITM are produced by powder metallurgy. Due to the high shaping process cost and low production volume of these steels, interconnects made of such steels are very expensive. Furthermore, the limited availability of this steel is also a problem. Finally, the CTE of such steels is not optimal: the CTE of Plansee ITM is 11.6 ppm / K, while on the other hand, the optimal CTE value compatible with the CTE of 40 vol% Ni - 60 vol% 8YSZ used in SOC would be 12.5 ppm / K (F. Tietz, Ionics, 5 (1999) 129 (Non-Patent Document 3)).

[0030] On the other hand, although standard ferritic stainless steels, such as group 2 ferritic stainless steels, are simple in shape, widely available, produced in large quantities, and inexpensive, they have a low Cr content (16 - 18 wt% in AISI 430). This relatively low Cr content gives the material poor corrosion resistance, which shortens the life of the SOC stack to an unacceptable low level. The CTE of commonly used ferritic stainless steels has some variations, but AISI 430, for example, has a CTE of 12.94 ppm / K, which is slightly too high to ideally match the CTE of 40 vol% - 60 vol% 8YSZ.

Prior Art Documents

Patent Documents

[0031]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Non-Patent Documents

[0032]

Non-Patent Document 1

Non-Patent Document 2

[0033] Therefore, an object of the present invention is to provide a method for manufacturing a metallic SOC interconnect that combines the advantages of standard ferritic stainless steels (i.e., low cost, wide availability, and ease of shaping) with excellent oxidation resistance. Furthermore, it is also an object of the present invention to provide an interconnect made of ferritic steel for use in a solid oxide cell stack that is low - cost, widely available, easy to shape, and has excellent oxidation resistance. [Means for Solving the Problems]

[0034] According to the present invention, the above object is achieved by a method for chromium upgrading of a ferritic - steel interconnect used in a solid oxide cell stack, comprising the following steps: - a step of shaping the interconnect, - a step of depositing a coating containing Cr on at least one surface of the shaped interconnect, and - a step of performing heat treatment one or more times at a temperature lower than 1000 °C, wherein the method achieves a Cr concentration generated near the surface of the interconnect that is higher than the Cr concentration in the ferritic steel before shaping. ​

Best Mode for Carrying Out the Invention

[0035] Here, the description of "chromium upgrade" refers to means for increasing the Cr content rate in the material. The description of "shaping of the interconnect" refers to forming or machining the interconnect into a desired shape. The description of "shaped interconnect" refers to an interconnect that has been formed or machined into a desired shape.

[0036] Advantageously, the average Cr concentration of the shaped interconnect can be increased to 26 wt% Cr or more.

[0037] Advantageously, the ferritic steel is a ferritic steel of Group 1, a ferritic steel of Group 2, a ferritic steel of Group 3, a ferritic steel of Group 4, or one of the following steels, namely Crofer22APU, Crofer22H or ZMG G10.

[0038] Advantageously, the ferritic steel is a ferritic steel of Group 2, such as AISI430.

[0039] At this time, the method according to the present invention enables the production of SOC interconnects from ferritic stainless steels that are, for example, low-cost, easy to shape, and widely available, while at the same time achieving excellent corrosion resistance. This is achieved by first shaping the interconnect into a desired shape, thereby taking advantage of the ease of shaping of a steel with a relatively low Cr content rate. After shaping, the Cr content rate of the shaped interconnect is increased, thereby taking advantage of the relatively high corrosion resistance of a steel with a relatively high Cr content rate. Ferrite exhibits a very high solubility of chromium and a very low carbon content rate, and thus solubility and carbide formation do not pose a problem with respect to the increase in the Cr content rate when using the method of the present invention.

[0040] Advantageously, the deposition step can be characterized as hard chromium plating.

[0041] Methods for electroplating chromium can be divided into two categories: hard chromium plating and bright chromium plating. Although important for this application, the main difference between hard chromium plating and bright chromium plating is the layer thickness of the coating. Hard chromium plating provides a relatively thick coating with a thickness of 1 μm to 1000 μm, and is generally used as an abrasion-resistant and corrosion-resistant coating for industrial purposes. Bright chromium plating provides a layer thickness in the range of 0.25 μm to 1 μm, and therefore these are generally used to improve the surface appearance for decorative purposes. The terms "industrial hard chromium" and "decorative bright chromium" are also often used to represent the differences between these coatings. This sub-classification of these two chromium plating processes is carried out even though the electroplating bath compositions are similar. Conventional sulfate-catalyzed chromium electrolytes are in principle similar in composition, and as a result, these can be used for both hard chromium coating and bright chromium coating plating (see Table 1 below). The main difference between these two bath compositions is that hard chromium plating can be carried out at a significantly higher current density, enabling a faster deposition rate compared to the bright chromium plating process.

[0042]

Table 1

[0043] The conventional sulfate catalyst process, also known as the standard 100:1 sulfate bath (i.e., the ratio between chromium trioxide and sulfate is 100:1), has historically been the most prevalent bath used for chromium electroplating. However, industrially, efforts have been made to further optimize the catalyst used in the hard chromium process because of the fact that chromium cannot be reduced from hexavalent chromium (Cr(VI)) to its metallic state (Cr) without the presence of one or more catalysts. The development of this catalyst has not only led to an electroplating bath with significantly higher current efficiency but also improved the properties of the hard chromium coating. Examples of such improved properties are higher hardness, more crack-free deposits, and also less substrate etching.

[0044] As described above, industrial standards for hard chromium electrolytes have been developed and are currently subdivided into three different groups shown in Table 2 below. All electrolytes are based on chromium trioxide and sulfate.

[0045]

Table 2

[0046] Advantageously, the thickness of the coating deposited by hard chromium plating on the shaped interconnect is at least 1 micron and less than 1 millimeter.

[0047] Advantageously, the deposition step can be characterized as a chromizing method.

[0048] Chromizing is a thermochemical process mainly involving saturating alloy iron of steel with chromium through diffusion. This is done to extend the service life of tools and components exposed to wear and corrosion (including gas corrosion) at temperatures up to 900 °C. For chromizing, a source metal powder (in this case, Cr), an activator (e.g., halide), and a diluent (an inert powder that prevents sintering of packed powder particles with each other, e.g., Al2O3) are used, and this method is often referred to as "pack cementation". Ferritic steels of Groups 1 - 3 have a very low carbon content and are characterized by a high solubility of Cr. Therefore, the chromizing process particularly promotes the diffusion of metallic Cr into the ferritic crystal structure. The activator keeps the interface oxide - free and enables the diffusion of the source metal. Chromizing is classified into two types, corrosion resistance and surface hardening, depending on its application.

[0049] US6,387,194B1 (Patent Document 9) describes a method for chromizing parts made of 400 - series stainless steel, especially 430 stainless steel. A diffusion coating composition for use in this method is also described.

[0050] Advantageously, the shaping of the interconnect is performed by forming processes.

[0051] Advantageously, the forming processes are performed by punching, pressing, forging, rolling, coining, embossing, extrusion, roll forming, hydroforming, or deep drawing.

[0052] Advantageously, the pressing force used for the forming process of the interconnect is less than 500 bar, preferably less than 200 bar.

[0053] Advantageously, the shaping of the interconnect is performed by machining processes.

[0054] Advantageously, the machining is performed by drilling, milling, photochemical etching, electrochemical etching, dry etching, or laser cutting.

[0055] Advantageously, the coefficient of thermal expansion of the interconnect after chromium upgrade is greater than 12 ppm / K but less than 13 ppm / K.

[0056] Thereby, the method of the present invention enables the SOC interconnect to be manufactured, for example, from a ferritic stainless steel such as AISI 430 having a relatively low chromium content, a CTE higher than an optimum value of approximately 12.5 ppm / K, but whose CTE is lowered near the optimum value by chromium upgrade.

[0057] Hereinafter, the present invention will be described in more detail with reference to the drawings.

Brief Description of the Drawings

[0058]

Figure 1

Figure 2

Figure 3

[0059] [Detailed Description of the Drawings] Figure 1 illustrates one possible embodiment of the present invention. Ferritic stainless steel (101) is first shaped (Process A) into a shaped SOC interconnect (104). Thereafter, a Cr-containing coating (105) is deposited by hard chromium plating on at least one surface of the shaped interconnect (104) (Process B) to obtain a coated SOC interconnect (106). Next, one or more heat treatments (Process C) are performed at a temperature below 1000 °C to obtain a chromium-upgraded SOC interconnect (102). The Cr concentration generated near the surface of the chromium-upgraded interconnect (102) is higher than the Cr concentration in the ferritic steel (101) before shaping.

[0060] Figure 2 illustrates one possible embodiment of the present invention. Ferritic stainless steel (101) is first shaped (Process A) into a shaped SOC interconnect (104). Thereafter, a Cr-containing coating (107) is deposited by a chromizing process on at least one surface of the shaped interconnect (104) (Process D) to obtain a coated SOC interconnect (108). Next, one or more heat treatments (Process E) are performed at a temperature below 1000 °C to obtain a chromium-upgraded SOC interconnect (103). The Cr concentration generated near the surface of the chromium-upgraded interconnect (103) is higher than the Cr concentration in the ferritic steel (101) before shaping.

[0061] Figure 3 shows the Fe and Cr contents near the surface of the Crofer22APU sheet after chromium upgrade. The elemental composition was determined by energy-dispersive X-ray spectroscopy (EDX) point analysis performed at various depths (i.e., distance from the surface of the sheet), indicated as "X" in Figure 3, on the cross-section of the Crofer22APU sheet after chromium upgrade. The Cr and Fe contents in the steel are expressed in units of weight% (indicated as "%" in Figure 3). The original Crofer22APU sheet had a thickness of 300 microns and a chromium content of 22 wt%. Based on the EDX data, after chromium upgrade by the chromizing process (Process D) and heat treatment (Process E), the chromium concentration near the surface of the chromium-upgraded metal sheet was higher than the Cr concentration in the as-formed ferritic steel (101) before shaping. More specifically, the Cr content in the steel was ≧26 wt% up to a depth of approximately 25 microns from the surface of the sheet. This application relates to the invention described in the claims, but the disclosure of this application also includes the following: 1. A method for chromium upgrading of a ferrite steel interconnect used in a solid oxide cell stack, comprising the following steps: - A step of shaping the interconnect, - A step of depositing a coating containing Cr on at least one surface of the shaped interconnect, - A step of performing heat treatment one or more times at a temperature lower than 1000 °C, including, provided that the Cr concentration generated near the surface of the interconnect is higher than the Cr concentration in the ferrite steel before shaping, said method. 2. The method according to 1. above, wherein the average Cr concentration of the shaped interconnect is increased to 26 wt% Cr or more. 3. The method according to 1. or 2. above, wherein the ferrite steel is ferrite steel of Group 1, ferrite steel of Group 2, ferrite steel of Group 3, ferrite steel of Group 4, or one of Crofer22APU, Crofer22H, and ZMG G10 steel. 4. The method according to 3. above, wherein the ferrite steel is ferrite steel of Group 2, for example, AISI430. 5. The method according to any one of 1. to 4. above, wherein the deposition step can be characterized as hard chromium plating. 6. The method according to 5. above, wherein the thickness of the deposited coating is at least 1 micron and less than 1 millimeter. 7. The method according to 1., 2., 3., or 4. above, wherein the deposition step is characterized as a chromizing process. 8. The method according to any one of 1. to 7. above, wherein the shaping of the interconnect is performed by forming. 9. The method according to 8. above, wherein the forming is performed by punching, pressing, forging, rolling, coining, embossing, extrusion, roll forming, hydroforming, or deep drawing. 10. The method according to 8. or 9. above, wherein the pressure used for the forming of the interconnect is less than 500 bar, preferably less than 200 bar. 11. The method according to any one of 1. to 7. above, wherein the shaping of the interconnect is performed by machining. 12. The method according to claim 11, wherein the machining is performed by drilling, milling, photochemical etching, electrochemical etching, dry etching, or laser cutting. 13. The method according to any one of claims 1 to 12, wherein the coefficient of thermal expansion of the interconnect after the chromium upgrade is greater than 12 ppm / K but less than 13 ppm / K. 14. A ferrite steel interconnect for use in a solid oxide cell stack, produced by the method according to any one of claims 1 to 13.

Claims

1. A method for chromium upgrading of a ferrite steel interconnect used in a solid oxide cell stack, comprising the following steps: - a step of shaping the interconnect, - a step of depositing a Cr-containing coating on at least one surface of the shaped interconnect by hard chromium plating or chromizing, - a step of performing heat treatment one or more times at a temperature lower than 1000 °C, wherein, the Cr concentration generated near the surface of the interconnect chromium-upgraded through the shaping step, deposition step and heat treatment step is higher than the Cr concentration in the ferrite steel before shaping, and the ferrite steel before shaping contains Cr in the range of 10.5 to 18% by weight, the method.

2. The method according to claim 1, wherein the deposition step can be characterized as hard chromium plating.

3. The method according to claim 2, wherein the thickness of the deposited coating is at least 1 micron and less than 1 millimeter.

4. A method for chromium upgrading of a ferrite steel interconnect used in a solid oxide cell stack, comprising the following steps: - a step of shaping the interconnect, - a step of depositing a Cr-containing coating on at least one surface of the shaped interconnect by hard chromium plating, - a step of performing heat treatment one or more times at a temperature lower than 1000 °C, wherein, the Cr concentration generated near the surface of the interconnect chromium-upgraded through the shaping step, deposition step and heat treatment step is higher than the Cr concentration in the ferrite steel before shaping, the method.

5. The method according to claim 4, wherein the thickness of the deposited coating is at least 1 micron and less than 1 millimeter.

6. The method according to any one of claims 1 to 5, wherein the average Cr concentration of the shaped interconnect is increased to 26% by weight Cr or more.

7. The method according to any one of claims 1 to 6, wherein the ferrite steel contains 14 to 18% by weight of Cr.

8. The method according to any one of claims 1 to 7, wherein the shaping of the interconnect is performed by forming.

9. The method according to claim 8, wherein the forming process is performed by punching, pressing, forging, rolling, coining, embossing, extrusion, roll forming, hydroforming or deep drawing.

10. The method according to claim 8 or 9, wherein the pressing force used for the forming process of the interconnect is less than 500 bar.

11. The method according to any one of claims 1 to 7, wherein the shaping of the interconnect is performed by machining.

12. The method according to claim 11, wherein the machining is performed by drilling, milling, photochemical etching, electrochemical etching, dry etching, or laser cutting.

13. The method according to any one of claims 1 to 12, wherein the coefficient of thermal expansion of the interconnect after chromium upgrade is greater than 12 ppm / K but less than 13 ppm / K.

14. The method according to any one of claims 1 to 13, wherein the Cr concentration in the surface region up to a depth of 25 microns from the surface of the interconnect chromium-upgraded through the shaping step, deposition step and heat treatment step is higher than the Cr concentration in the ferrite steel before shaping.

15. A ferrite steel interconnect for use in a solid oxide cell stack, manufactured by the method according to any one of claims 1 to 14.

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