Method of chromium upgrading ferritic steel interconnects for solid oxide cell stack applications

Abstract

A method of chromium upgrading interconnects made of ferritic steel for a solid oxide cell stack comprises the steps of: forming the interconnects; depositing a coating comprising Cr on at least one surface of the shaped interconnect; one or more heat treatments are carried out at a temperature of 1000 DEG C or less in which the resulting Cr concentration in the vicinity of the surface of the interconnection is higher than the Cr concentration in the ferritic steel prior to molding. In particular, the average Cr concentration of the shaped interconnect increases to 26 wt% Cr or more.

Description

[0001] This application is a divisional application of an invention patent application with an application date of July 13, 2020, application number: 202080050715.1, and name: “Method for chromium upgrading of ferritic steel interconnects for solid oxide battery stack applications”. Technical Field

[0002] The present invention relates to chromium upgrading of ferritic steel materials, and more particularly to chromium upgrading of ferritic steel interconnects for use in solid oxide cell (SOC) stacks. background

[0004] Stainless steel is an iron alloy with a minimum Cr content of 10.5 wt% and a maximum carbon content of 1.2 wt%.

[0005] Stainless steels are classified into different families based on their crystal structure: austenitic, ferritic, duplex and martensitic. The largest class of stainless steel is austenitic. Austenitic stainless steels can be further divided into five subclasses: Cr-Mn, Cr-Ni, Cr-Ni-Mo, high performance and high temperature. The most common austenitic steel is Cr-Ni, which contains 8-10wt% Ni and 17-18wt% Cr, with the remainder being Fe, which is commonly referred to as 18-8 type stainless steel. Ni is needed in the steel to stabilize the austenitic phase (γ-Fe) with a face-centered cubic (FCC) crystal structure, which remains stable at room temperature. Austenitic grades are classified as non-magnetic and have good weldability and formability.

[0006] Ferritic stainless steels are the second most commonly used type of stainless steel and are often referred to as the "nickel-free" alternative to austenitic steels. Ferritic steels contain primarily Fe and Cr, and the Cr content can vary over a wide range (from 10.5 to 29 wt%), depending on the application. Ferritic steels can be further subdivided into five different classes. Classes 1-3 have the widest application and, therefore, the largest production of ferritic steels. Steels in classes 1-3 are often referred to as "standard ferritic grades". Class 1 has the lowest Cr content (in the range of 10.5 to 14 wt%), while classes 2-3 have Cr contents in the range of 14 to 18 wt%. Class 2 is the most widely used family of ferritic stainless steels. AISI 430 is a particularly widely used class 2 stainless steel, which has surpassed the austenitic alternative AISI 304 in many indoor applications where corrosion resistance is less important but smaller price fluctuations (due to the nickel-free formulation) are desired. Class 3 differs from Class 2 in that Class 3 contains additional stabilizing elements such as Ti, Nb, and Zr, which, combined with both carbon and nitrogen, leave a completely ferritic crystal structure at all temperatures. Therefore, Class 3 materials generally exhibit better weldability and resistance to sensitization than the other classes. Class 4 has 10.5 to 18 wt% Cr and is alloyed with Mo for additional corrosion resistance. Class 5 ferrites have more than 18 wt% alloyed Cr or do not belong to the other classes. Generally, Class 5 ferritic steels have very high corrosion resistance, but low weldability, and they are also sensitive to embrittlement. Grades in Class 5 that contain both high Cr and Mo are called "super ferritics" and are used to replace titanium in applications where corrosion is considered extreme. Ferritic stainless steels have a body-centered cubic (BCC) crystal structure (α-Fe), are magnetic, and have a lower coefficient of thermal expansion than austenitic steels.

[0007] Duplex stainless steel is another class of stainless steel. Duplex steel is essentially a mixture of ferrite and austenite phases with an approximate phase balance of 50% ferrite and 50% austenite. Duplex stainless steels are characterized by a high Cr content (20.1 to 25.4 wt% Cr) but a fairly low Ni content (1.4 to 7 wt% Ni). In duplex steels, many of the beneficial properties of austenitic and ferritic steels are combined. Due to the ferrite content, duplex grades are magnetic.

[0008] Martensitic stainless steel is the smallest type of stainless steel. Martensitic steels typically contain 12-17wt% Cr and 0-5wt% Ni. It is the combination of the alloy composition and the 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.

[0009] Solid oxide cells (SOCs) can operate as solid oxide fuel cells (SOFCs), solid oxide electrolyzers (SOECs) or be reversible, ie switch between SOFC and SOEC modes.

[0010] Solid oxide fuel cells include an oxygen ion conducting electrolyte, an oxygen electrode (cathode, where oxygen is reduced) and a fuel electrode (anode, where fuel (such as hydrogen, methane or natural gas) is oxidized). The overall reaction in a SOFC is an electrochemical reaction of the fuel used and oxygen to produce electricity, heat and an oxidized substance. If hydrogen is used as the fuel, the oxidized substance is water, if carbon monoxide is used as the fuel, the oxidized substance is carbon dioxide, and for hydrocarbon fuels, the oxidized substance is a mixture of water and carbon dioxide.

[0011] A solid oxide electrolyzer comprises an oxygen ion conducting electrolyte, a fuel electrode (cathode, where an oxidized species (e.g., water or carbon dioxide or both) is reduced with the aid of an applied electric field) and an oxygen electrode (anode, where oxygen ions are oxidized to molecular oxygen). The overall reaction in a SOEC is to electrochemically convert the oxidized species into a reduced species using electricity and heat. If the oxidized species fed into the stack is water, hydrogen is formed at the fuel electrode. If the oxidized species is carbon dioxide, carbon monoxide is formed at the fuel electrode. If the oxidized species is a mixture of water and carbon dioxide, a mixture of carbon monoxide and hydrogen (also called syngas) is produced.

[0012] SOCs operate at a temperature range of about 500°C to about 1100°C. The operating temperature needs to be increased to ensure sufficiently high oxygen ion conductivity in the electrolyte. Common electrolyte materials for SOCs include, but are not limited to, yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (CGO).

[0013] In a SOC stack, multiple cells, each comprising a fuel electrode, an electrolyte, an oxygen electrode, and optional additional layers, are connected in series by inserting an interconnecting plate (or "interconnector" or "interconnector") between each pair of cells. The role of the interconnector is to provide electrical contact from one cell to the next, to help distribute the gas throughout the cell, and in some designs to avoid mixing of gases between the anode and cathode chambers.

[0014] Interconnects can be made of ceramic materials (such as doped lanthanum or yttrium chromate) or metals (such as stainless steel). The advantages of metal interconnects over ceramic interconnects include: 1) lower material and manufacturing costs; 2) simpler and less complex shaping; 3) higher electrical and thermal conductivity; 4) ductility. Therefore, for SOCs operating at temperatures below 850°C, metal interconnects are preferred.

[0015] Materials suitable for metal SOC interconnects need to be resistant to oxidation of gases supplied to the oxygen electrode and the fuel electrode at elevated operating temperatures, and they must also exhibit a CTE that matches the coefficient of thermal expansion (CTE) of the ceramic components of the cell. In addition, the protective oxide barrier formed on the steel surface at high temperatures must be conductive. In view of these requirements, ferritic alloys that form a chromium oxide surface layer (e.g., ferritic steels that form chromium oxide) are particularly suitable for use as interconnects in SOC stack applications. Examples of such high-chromium ferritic steels include, but are not limited to, AISI 441, AISI 444, AISI430, AISI 446, Crofer 22H, Crofer 22APU, ZMG G10, E-brite, Plansee ITM, etc. Other materials for metal SOC interconnects include Plansee CFY (an alloy based on <95wt% Cr, 5wt% Fe and Y).

[0016] For example, US 6.936.217 B2 describes a high temperature material consisting of chromium oxide forming an iron alloy comprising: a) 12 to 28 wt% Cr, b) 0.01 to 0.4 wt% La, c) 0.2 to 1.0 wt% Mn, d) 0.05 to 0.4 wt% Ti, e) less than 0.2 wt% Si, f) less than 0.2 wt% Al, wherein the high temperature material is capable of forming a MnCr2O4 spinel phase at a temperature of 700°C to 950°C. The ferritic stainless steel covered by the above description has been commercialized under the trade name Crofer 22APU. Crofer 22APU has a CTE of 11.9 ppm K between 20°C and 800°C. -1 .

[0017] WO 2008 / 013498 A1 belonging to the applicant relates to a ferritic chromium stainless steel comprising: a) 20 to 25 wt% Cr, b) 0.5 to 2 wt% Mo, c) 0.3 to 1.5 wt% Nb, d) up to 0.1 wt% C, e) up to 0.6 wt% Mn, f) up to 2 wt% Ni, g) up to 0.5 wt% Ti, h) up to 0.5 wt% Zr, i) up to 0.1 wt% Al, j) up to 0.07 wt% N, k) up to 0.3 wt% rare earth metals, l) the remainder being Fe and commonly present impurities, wherein the content of Zr+Ti is at least 0.20 wt%. Furthermore, the most preferred embodiment is a steel having the following approximate composition (in weight percent): Si-0.2, Mn-0.3, Cr-22, Mo-1, Nb-0.4, Zr-0.3, Ti-0.05, the balance being Fe and impurities that are usually present. The described steel is suitable for use as an interconnect in a fuel cell (e.g., a solid oxide fuel cell) because the oxide formed on the surface of the material has good adhesion and the material has a low contact resistance when tested in contact with a (La, Sr)MnO3 plate in air at 750°C.

[0018] The corrosion rate of ferritic stainless steel depends largely on the Cr content in the steel. For example, IGWight in Metals Handbook, 9 th Edition, Vol. 13 Corrosion (1987) teaches that the parabolic rate constant of Fe-Cr alloy corrosion at 1000°C decreases by more than four orders of magnitude as the Cr content in the alloy increases from 0 to 20 wt% Cr. When the Cr content is below about 28 wt%, the oxide scale formed on the surface of the alloy consists of a layer of Fe-oxide or Fe-Cr mixed oxide, resulting in incomplete protection of the steel. When the Cr content is above about 28 wt%, the oxide scale formed on the surface of the alloy consists of pure and continuous Cr oxide, providing more complete protection (i.e., the lowest corrosion rate) for the steel. Therefore, for SOC applications, it is desirable to use ferritic stainless steels with a Cr content above 28 wt%. When the SOC stack is operated at temperatures below 1000°C, a slightly reduced Cr content (e.g., 26 wt%) may be sufficient. Unfortunately, the Cr content in most widely used ferritic steels is not high enough to withstand long-term exposure to SOC conditions.

[0019] The problem of using Class 1-3 ferritic stainless steels (17-18 wt%) for SOC interconnects is usually solved by high temperature oxidation resistant coatings. For example, JG Grolig et al. in Journal of Power Sources, 248 (2014) 1007-1013 showed that the corrosion rate of AISI 441 with a chromium content of 17.83 wt% when exposed to SOFC cathode conditions at 850 ° C and 3% water content in air can be reduced by a protective coating containing cerium or lanthanum, or by a double layer coating of a combination of cerium or lanthanum and cobalt. The coating is applied by physical vapor deposition. The main disadvantage of such coatings is that they do not provide corrosion protection in the event of damage to the coating, such as due to defects, cracks, pinholes, poor adhesion, etc. If the coating fails, then due to the low Cr content, the steel is likely to undergo severe iron oxidation, resulting in failure of the SOC stack. In addition, the forming of the steel after coating will destroy the conformality of the coating, resulting in incomplete corrosion protection.

[0020] The coefficient of thermal expansion (CTE) of Fe-Cr alloys also depends on the Cr content of the alloy. In general, the CTE of an alloy decreases as the Cr content increases. For example, the CTE of AISI 430 (16-18wt% Cr) measured between 25°C and 727°C is 12.94ppm / K. The CTE of Crofer 22APU (20-24wt% Cr) measured between 20°C and 800°C is 11.9ppm / K. The CTE of Plansee ITM (26wt% Cr) measured between room temperature and 800°C is 11.6ppm / K. The CTE of CFY (95wt% Cr) measured between room temperature and 800°C is 10.5ppm / K. The best CTE value that matches the CTE of a 40 vol% Ni-60 vol% 8YSZ (8 mol% yttria-stabilized zirconia) support layer in a SOC is 12.5 ppm / K (F. Tietz, Ionics, 5 (1999) 129).

[0021] Metal forming in manufacturing can be divided into two major categories: material retaining processes and material removing processes. Material retaining processes are usually divided into forming or deformation processes, which are processes in which the material undergoes plastic deformation while producing a shape. Formability is a term often used in metal manufacturing in the category of material retaining processes. The term "formability" describes the ability of a metal to be plastically deformed into a desired shape without damaging the workpiece. Examples of damage during plastic deformation include tearing or fracture formation. Examples of forming processes include, but are not limited to: stamping, forging, rolling, extrusion, flow forming, and hydroforming. Material removal processes are explained by the process of removing material from the workpiece to shape the metal, which is generally referred to as machining. Machining includes a variety of different processes and is divided into three different categories: machine, chemical, and thermomechanical processing. In machine 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. Thermomechanical processing generally uses electrical energy to evaporate material from the workpiece. Therefore, the term "machinability" is very broad as it covers many different processes. However, the meaning of the term is the ability of material to be removed from the workpiece.

[0022] For example, US 8.663.863 B2 belonging to the applicant describes an interconnect for a fuel cell made of a metal sheet with protruding contact areas. The protrusions can be made by shaping the metal sheet by any known process such as stamping, pressing, milling, deep drawing, etc.

[0023] US 7.718.295 B2 describes a method involving shaping interconnects of planar solid oxide fuel cells by etching. Suitable processes include photochemical and electrochemical etching and laser cutting, among others.

[0024] In US 9.472.816B2, a powder metallurgy formed component is made of a powder consisting of 95wt% Cr and 5wt% FeY master alloy (an alloy containing 0.5wt% Y). 1wt% of a pressing aid (wax) is added to the powder. Then, the powder is mixed in a drum mixer for 15 minutes. The powder is pressed into a compact using a pressing tool, which is pre-sintered at 1100°C for 20 minutes in a continuous belt furnace in a hydrogen atmosphere to achieve the purpose of dewaxing. The component is then sintered at a high temperature of 1400°C for 7 hours in a hydrogen atmosphere to further compact and form an alloy. The component is then pre-oxidized at 950°C for 10 to 30 hours to close any residual pores that may exist to a degree that makes the permeability of the material sufficiently low. Finally, the oxide layer on all sides of the component surface is removed by a sandblasting process. The examples described involve many high temperature sintering steps, some in a hydrogen atmosphere, and further involve the use of metal powders with stringent particle size and shape requirements, making the process very expensive. In addition, the size of the interconnect sheet produced by powder metallurgy is limited by the die size and press force.

[0025] US2008 / 0269495 A1 describes a method for producing a metal interconnect for a fuel cell stack, which comprises providing a metal sheet blank and shaping the metal sheet blank by a molding process. The main disadvantage of this method is that it requires extremely high pressure (1000 kN / cm 2 or 10,000 bar) to stamp the metal sheet blank, which severely limits the size of the interconnect panels that can be produced using this method.

[0026] Generally, the formability of ferritic stainless steel deteriorates with increasing Cr content in the steel. For example, Design Guidelines for the Selection and Use of Stainless Steel (Nickel Development Institute, A Designers' handbook Series No. 9014) teaches that AISI 430 steel (16-18 wt% Cr) is rated "excellent" in terms of ease of stamping, embossing and roll forming, while AISI 446 steel (23-27 wt% Cr) is rated "good". AISI 430 steel is also rated "excellent" in terms of ease of cold heading and spinning, while AISI 446 steel is rated "fair".

[0027] Several different parameters can be used to quantitatively describe different aspects of the formability of a steel. 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. The work hardening index describes the stretchability of the steel, the total elongation characterizes the bendability of the steel, the uniform elongation is related to the sheet drawing ability of the steel, and the r-value is related to the deep drawing ability of the steel. For example, the elongation (A5) of AISI 430 steel (16-18wt% Cr) is ≥20-28%, while the elongation (A5) of AISI446 steel (23-27wt% Cr) is ≥10%, indicating that the formability decreases with increasing Cr content.

[0028] For example, US2016 / 0281184 A1 relates to a ferritic stainless steel having excellent corrosion resistance and sheet forming properties. The steel contains 20-24wt% Cr, uniform elongation (A g ) ranged from 17.0% to 19.1%, and the r values ​​ranged from 1.81 to 2.55.

[0029] EP 1 452 616 B1 describes a ferritic stainless steel sheet having excellent stamping formability and workability. The Cr content in the steel sheet is 10-19 wt%, and the steel sheet has one or more lubricating films on one or both surfaces. The main disadvantage of this invention is that due to the low Cr content, the steel does not provide sufficiently good corrosion protection under SOC conditions.

[0030] Therefore, for SOC applications, it is desirable to use a stainless steel having the following properties: 1) high oxidation resistance in both reducing and oxidizing atmospheres, 2) a CTE that matches that of the SOC, 3) the ability to form a conductive oxide scale, 4) ease of forming or machining, 5) low cost, and 6) wide availability (i.e., a wide range of suppliers).

[0031] Ferritic stainless steels developed specifically for SOC applications, such as Plansee ITM (26 wt% Cr), have excellent oxidation resistance due to their high Cr content. ITM steels further form a skin based on Cr oxide, which is more conductive than skins based on alumina or silica. The main disadvantage of high-Cr steels is related to the difficulty in forming the material: for example, interconnects made of Plansee ITM are manufactured by powder metallurgy. Due to the expensive forming process and due to the low production volume of these steels, interconnects made of this steel are very expensive. In addition, the limited supply of 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 the best CTE value that matches the CTE of 40 vol% Ni-60 vol% 8YSZ used in SOCs is 12.5 ppm / K (F. Tietz, Ionics, 5 (1999) 129).

[0032] On the other hand, standard ferritic stainless steels, such as Class 2 ferritic stainless steels, are easy to form, widely available, mass-produced, and inexpensive, but have a lower Cr content (16-18 wt% Cr for AISI 430). The lower Cr content makes the material less corrosion resistant, which in turn reduces the lifetime of the SOC stack to unacceptably low levels. The CTE of commonly used ferritic stainless steels varies slightly, but for example AISI 430 has a CTE of 12.94 ppm / K, which is slightly higher and does not ideally match the CTE of 40 vol% Ni-60 vol% 8YSZ.

[0033] It is therefore an object of the present invention to provide a method for preparing a metallic SOC interconnect which combines the advantages of standard ferritic stainless steels (i.e. low cost, wide availability, easy forming) with excellent oxidation resistance. In addition, it is an object of the present invention to provide a ferritic steel interconnect for use in a solid oxide cell stack which is low cost, widely available, easy to form and has excellent oxidation resistance. Summary of the invention

[0034] According to the present invention, this object is achieved by a method for chromium upgrading of interconnects made of ferritic steel for solid oxide battery stacks, the method comprising the following steps

[0035] - shaping the interconnects,

[0036] - depositing a coating comprising Cr on at least one surface of the formed interconnect,

[0037] - one or more heat treatments at temperatures below 1000°C,

[0038] The resulting Cr concentration near the interconnect surface is higher than the Cr concentration in the ferritic steel before forming.

[0039] Here, the term "chromium upgrading" refers to the means of increasing the Cr content in the material. The term "forming of the interconnect" refers to forming or machining the interconnect into a desired shape. The term "formed interconnect" refers to an interconnect that has been formed or machined into a desired shape.

[0040] Advantageously, the average Cr concentration of the formed interconnect is increased to 26 wt % Cr or higher.

[0041] Advantageously, the ferritic steel is a Class 1 ferritic steel, a Class 2 ferritic steel, a Class 3 ferritic steel, a Class 4 ferritic steel, or one of the following steels: Crofer 22APU, Crofer 22H, ZMG G10.

[0042] Advantageously, the ferritic steel is a Class 2 ferritic steel, such as AISI 430.

[0043] Thus, the method of the present invention allows SOC interconnects to be made of, for example, low cost, easily formable, widely available ferritic stainless steels, while achieving excellent corrosion resistance. This is achieved by first forming the interconnect into the desired shape, thereby taking advantage of the easy formability of steels with relatively low Cr content. Forming is followed by increasing the Cr content of the formed interconnect, thereby taking advantage of the higher corrosion resistance of steels with relatively high Cr content. Ferrite exhibits very high chromium solubility and very low carbon content, so neither solubility nor carbide formation is a problem for increasing the chromium content using the method of the present invention.

[0044] Advantageously, the depositing step is characterized as hard chrome plating.

[0045] The methods of electroplating chrome can be divided into the following two major categories: hard chrome plating and bright chrome plating. The main difference between hard chrome plating and bright chrome plating, and important for the application, is the layer thickness of the coating. Hard chrome plating provides relatively thick coatings with thicknesses ranging from 1 to 1000 μm and is mainly used as wear-resistant and corrosion-resistant coatings for technical purposes. Bright chrome plating provides layer thicknesses in the range of 0.25-1 μm, so they are mainly used to improve the appearance of the surface for decorative purposes. The terms "technical hard chrome" and "decorative bright chrome" are also often used to describe the difference between the coatings. Despite the similarities in the composition of the plating baths, the two chrome plating processes are still subdivided. Conventional sulfate-catalyzed chromium electrolytes are in principle so close in composition that they can be used for both hard chrome plating and bright chrome plating coatings (see Table 1 below). The main difference between the two bath compositions is that hard chrome plating can be operated at much higher current densities than bright chrome plating processes, which can achieve faster deposition rates.

[0046] Table 1. Composition of hard chrome plating and bright chrome plating baths based on conventional sulfuric acid catalysis process

[0047] Hard Chrome Bright Chrome <![CDATA[CrO3[g / L]]]> 250 400 <![CDATA[H2SO4[g / L]]]> 2.5 4.0 <![CDATA[Current density [A / dm 2 > 15-70 10-20 Temperature ℃ 45-60 35-45

[0048] The conventional sulfate catalyzed process, also known as the standard 100:1 sulfate bath (i.e., a 100:1 ratio between chromium trioxide and sulfate, respectively), has historically been the most widely used bath for chromium electroplating. However, 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 has prompted the industry to further optimize the catalysts used in the hard chrome process. This catalyst development has not only resulted in baths with much higher current efficiencies, but has also improved the properties of the hard chrome coating. Examples of such improved properties are higher hardness, less cracked deposits, and low substrate etching.

[0049] As mentioned above, industry standards for hard chrome electrolytes have been developed and are now divided into three different categories, listed below in Table 2. All electrolytes are based on chromium trioxide and sulfate.

[0050] Table 2. Description of the main types of hard chrome processes used in the hard chrome plating industry today.

[0051]

[0052] Advantageously, the thickness of the coating deposited by hard chrome plating on the shaped interconnect is at least 1 micrometer and less than 1 millimeter.

[0053] Advantageously, the deposition step may be characterized as a chromization process.

[0054] Chromizing is a thermochemical process that involves saturating ferrous alloys (mainly steels) with chromium by means of diffusion. The purpose is to extend the service life of tools and components exposed to wear and corrosion (including gas corrosion) at temperatures up to 900°C. Chromizing involves a source metal powder (Cr in this case), an activator (such as a halide) and a diluent (an inert powder, such as Al2O3, which prevents the accumulated powder particles from sintering together), and the process is often referred to as "packing". Ferritic steels of groups 1-3 have very low carbon contents and are characterized by a high solubility of Cr, so the chromizing process is particularly favorable for the diffusion of metallic Cr into the ferrite crystal structure. The activator keeps the interface free of oxides and allows the diffusion of the source metal. Chromizing is divided into two categories according to its use: corrosion protection and surface hardening.

[0055] In US 6.387.194 B1, a method for chromizing 400 series, in particular 430, stainless steel parts is described. A diffusion coating composition for use in the method is also described.

[0056] Advantageously, shaping of the interconnection is performed by forming.

[0057] Advantageously, the shaping is carried out by stamping, pressing, forging, rolling, coining, embossing, extruding, flow forming, hydroforming or deep drawing.

[0058] Advantageously, the pressing power used to shape the interconnection is less than 500 bar, preferably less than 200 bar.

[0059] Advantageously, the shaping of the interconnection element is performed by machining.

[0060] Advantageously, the machining is performed by drilling, milling, photochemical etching, electrochemical etching, dry etching or laser cutting.

[0061] Advantageously, after the chromium upgrade, the coefficient of thermal expansion of the interconnect is above 12 ppm / K but below 13 ppm / K.

[0062] Thus, the method of the present invention allows SOC interconnects to be made from, for example, a relatively low chromium ferritic stainless steel (such as AISI 430) having a CTE above the optimum value of about 12.5 ppm / K, and by chromium upgrading, the CTE of the steel is reduced to closer to the optimum value. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Hereinafter, the present invention will be explained in more detail with reference to the accompanying drawings.

[0065] Figure 1 is a schematic diagram of a method according to one embodiment of the present invention, wherein the depositing step can be characterized as hard chrome plating.

[0066] Figure 2 is a schematic diagram of a method according to one embodiment of the present invention, wherein the deposition step can be characterized as a chromization process.

[0067] Figure 3 is a graph showing the Fe and Cr contents near the surface of Crofer 22APU sheet after chromium upgrading.

[0068] Detailed description with drawings

[0069] Figure 1A possible embodiment of the present invention is described. First, a ferritic stainless steel (101) is formed (process A) into a formed SOC interconnect (104). Thereafter, a coating (105) comprising Cr is deposited on at least one surface of the formed interconnect (104) by hard chrome plating (process B), thereby obtaining a coated SOC interconnect (106). Then, one or more heat treatments are performed at a temperature below 1000°C (process C), thereby obtaining a chromium-upgraded SOC interconnect (102). The resulting Cr concentration near the surface of the chromium-upgraded interconnect (102) is higher than the Cr concentration in the ferritic steel (101) before forming.

[0070] Figure 2 A possible embodiment of the present invention is described. First, a ferritic stainless steel (101) is formed (process A) into a formed SOC interconnect (104). Thereafter, a coating (107) comprising Cr is deposited on at least one surface of the formed interconnect (104) by a chromization process (process D), thereby obtaining a coated SOC interconnect (108). Then, one or more heat treatments are performed at a temperature below 1000°C (process E), thereby obtaining a chromium-upgraded SOC interconnect (103). The resulting Cr concentration near the surface of the chromium-upgraded interconnect (103) is higher than the Cr concentration in the ferritic steel (101) before forming.

[0071] Figure 3 The Fe and Cr contents near the surface of Crofer 22APU sheets after chromium upgrading are shown. The elemental composition was determined by energy dispersive X-ray spectroscopy (EDX) point analysis of cross sections of Crofer 22APU sheets after chromium upgrading at different depths, i.e. different distances from the sheet surface (at Figure 3 The content of Cr and Fe in steel is expressed in wt% (in Figure 3 The initial Crofer 22APU sheet thickness was 300 μm and the chromium content was 22 wt%. According to EDX data, after chromium upgrading by chromization process (process D) and heat treatment (process E), the chromium concentration near the surface of the chromium upgraded metal sheet was higher than the chromium concentration in the ferritic steel (101) before forming. More specifically, the Cr content in the steel was ≥ 26 wt% up to a depth of about 25 μm from the sheet surface.

Claims

1. A method for upgrading chromium of an interconnect made of ferritic steel for a solid oxide battery stack, the method comprising the following steps - shaping the interconnects, - depositing a coating comprising Cr on at least one surface of the formed interconnect, and then - one or more heat treatments at temperatures below 1000°C, Thereby a chromium upgraded interconnect is obtained, wherein the resulting Cr concentration near the surface of the chromium upgraded interconnect is higher than the Cr concentration in the ferritic steel before forming.

2. The method of claim 1, wherein the average Cr concentration in the chromium upgraded interconnect is increased to 26 wt% Cr or higher up to a depth of about 25 microns from the sheet surface.

3. The method according to claim 1 or 2, wherein the ferritic steel is a Class 1 ferritic steel, a Class 2 ferritic steel, a Class 3 ferritic steel, a Class 4 ferritic steel, or one of the following steels: Crofer22APU, Crofer22H, ZMGG10.

4. The method according to claim 3, wherein the ferritic steel is a Class 2 ferritic steel, such as AISI 430.

5. The method according to any one of the preceding claims, wherein the depositing step is characterized as hard chrome plating.

6. The method of claim 5, wherein the deposited coating has a thickness of at least 1 micrometer and less than 1 millimeter.

7. The method of claim 1, 2, 3 or 4, wherein the depositing step is characterized as a chromization process.

8. A method according to any one of the preceding claims, wherein shaping of the interconnect is performed by forming.

9. The method according to claim 8, wherein the forming is performed by stamping, pressing, forging, rolling, coining, embossing, extruding, flow forming, hydroforming or deep drawing.

10. Method according to claim 8 or 9, wherein the pressing power for forming the interconnection is less than 500 bar, preferably less than 200 bar.

11. The method according to any one of claims 1 to 7, wherein the forming 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. A method according to any one of the preceding claims, wherein after chromium upgrading the coefficient of thermal expansion of the interconnect is above 12 ppm / K but below 13 ppm / K.

14. A ferritic steel interconnect for a solid oxide cell stack, wherein the interconnect is prepared by a method according to any one of the preceding claims.

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