Coated component

Abstract

A coated component for a high-temperature device is disclosed. The component comprises a chromium-containing substrate, a first protective coating on a surface of the substrate, and a second protective coating comprising at least one layer comprising a rare earth containing material on the first portion of the substrate. The rare-earth containing material may comprise a praseodymium-containing material, a lanthanum-containing material and / or a terbium-containing material. Also disclosed is a method for producing such a coated component.

Description

[0001] COATED COMPONENT

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to coated components, especially to coated components for use in devices and to electrochemical devices comprising coated components. The invention also relates to methods for producing such coated components.

[0004] BACKGROUND OF THE INVENTION

[0005] Coatings based on rare earth compounds have been used to reduce corrosion of, and to provide specific surface properties to, substrates.

[0006] Yan Yan et al. (Surface and Coatings Technology, Vol. 272, (2015), pp. 415-427) discloses fabrication of lanthanum, yttrium and cerium oxide coatings on porous stainless steel for use in metal-supported solid oxide fuel cells. The paper explains that the suitability of rare earth oxides as coatings is highly dependent on which rare earth oxide is used.

[0007] US-A-2004 / 186201 discloses corrosion resistant coatings (including carbon pigments) for substrates that may contain rare earths. US-A-2005 / 0061664 discloses electrolysers and, more specifically, interconnect supported electrolyser assemblies, preforms and methods of fabrication. ES 2359550 discloses vitreous coating compositions and methods for obtaining vitreous coatings, obtained by means of a sol-gel process, which are stable at temperatures of up to 250 °C. KR 1020200131553 discloses an interconnector for a solid oxide battery having a PrxO2x-5 coating. KR 1020200132500 discloses a battery module. WO-A-2023 / 175353 discloses a method of coating a chromium-containing component, comprising providing a coating mixture of a source of praseodymium and / or a source of terbium.

[0008] WO-A-05 / 071021 discloses corrosion inhibiting coating compositions as self-priming topcoats that contain fluorinated resins and may include corrosion-inhibiting rare earth compounds. US-A-2013 / 251942 and WO-A-2012 / 021822 each disclose a substrate with a hydrophobic coating of a rare earth element material (oxide, carbide, nitride, fluoride and / or boride) having a dynamic water contact angle of at least about 90 degrees. Fontana et al. (Journal of Power Sources 171 (2007) pp. 652-662) disclose corrosion resistance and conductivity evaluation studies on alloys using La, Y and Nd oxide coatings deposited by metal organic chemical vapour deposition.

[0009] Electrochemical cells formed of oxide layers (often known as solid oxide cells: SOC) that may include rare earth oxide layers may be used as fuel cells or electrolyser cells.

[0010] SOC fuel cell units produce electricity using an electrochemical conversion process that oxidises fuel. SOC fuel cell units can also, or instead, operate as regenerative fuel cells (or reverse fuel cells) units, often known as solid oxide electrolyser fuel cell units, for example to separate hydrogen and oxygen from water, or carbon monoxide and oxygen from carbon dioxide.

[0011] A solid oxide fuel cell (SOFC) generates electrical energy through the electrochemical oxidation of a fuel gas (usually hydrogen-based) and the device is generally ceramic-based, using an oxygen-ion conducting metal-oxide containing ceramic as its electrolyte. Many ceramic oxygen ion conductors (for instance, doped zirconium oxide or doped cerium oxide) have useful ion conductivities at temperatures in excess of 450 °C or 500 °C (for ceriumoxide based electrolytes) or 650 °C (for zirconium oxide-based ceramics), so SOFCs tend to operate at elevated temperatures. The fuel electrode, electrolyte and air electrode of an SOC may each be formed of one or more layers to optimise operation.

[0012] In operation, the electrolyte of the SOFC conducts oxygen ions from a cathode to an anode located on opposite sides of the electrolyte. A fuel contacts the anode (usually known as the “fuel electrode”) and an oxidant, such as air or an oxygen rich fluid, contacts the cathode (usually known as the “air electrode”). Conventional ceramic-supported (e.g. anodesupported) SOFCs have low mechanical strength and are vulnerable to fracture. Hence, metal-supported SOFCs have since been developed which have the active fuel cell component layers supported on a metal substrate. In these cells, the ceramic layers can be very thin since they only perform an electrochemical function: that is to say, the ceramic layers are not self-supporting but rather are thin coatings / films laid down on and supported by the metal substrate. Such metal supported SOFC stacks are more robust, lower cost, have better thermal properties than ceramic-supported SOFCs and can be sealed using conventional metal welding techniques.

[0013] Applicant’s WO-A-2015 / 136295 discloses metal-supported SOFCs in which the metal support plate has a porous region surrounded by a non-porous region with the active layers being deposited upon the porous region so that gases may pass through the pores from one side of the metal support plate to the opposite side to access the active layers coated thereon. The porous region comprises small apertures (holes drilled through the metal foil substrate) extending through the support plate.

[0014] WO-A-2016 / 128721 discloses an interconnect for a low temperature solid oxide fuel cell, in particular to an interconnect comprising a chromium oxide layer (chromium (III) oxide / chromia).

[0015] Components including metal supports and interconnects and other components in SOFCs (and other devices) may be formed of SOFC-specific materials including steels (such as SS441). There is a desire to use materials containing chromium, that may be lower cost or may have other beneficial properties. It has, however, been observed that such materials (for example metal alloys) may exhibit chromium volatility. Volatile chromium compounds may cause problems during manufacture and may poison SOFC electrodes during operation. There have been attempts to coat stainless steel interconnects for use in an SOC stack on one or both sides to prevent corrosion and / or chromium evaporation. One coating process involves the deposition of a 10-30 nm layer of cerium and around 600 nm of cobalt on the side of the steel which will be exposed to air during stack operation using a continuous reel- to-reel PVD process on steel strip prior to forming the interconnect plates. In use, either during stack operation or through a thermal treatment prior to stack assembly, the cobalt and cerium metals oxidise to form a metal oxide coating on the steel which both protects the steel from corrosion and prevents evaporation of chromium from the oxide scale on the steel.

[0016] Unfortunately, during the forming processes the cobalt coating can crack in regions where the steel is stretched, such as around convex or concave formed regions, reducing the protection for the steel.

[0017] Robert Berger et al, “Self Healing of Precoated AISI 441 for Solid Oxide Fuel Cell Interconnects”, 2015 ECS Trans. 68 1649 reported that upon extended exposure to conventional SOC operating temperatures (168h at 800°C) cobalt oxide on the surface can diffuse over cracked regions and allow cracks to self-heal. In contrast, at lower operating temperatures of 650°C very limited self-healing of cracks was observed after 168h exposure.

[0018] Furthermore, Claudia Goebel et al, “The Influence of Different Factors on the Dual Atmosphere Effect Observed for AISI 441 Interconnects Used in Solid Oxide Fuel Cells”, 2019 ECS Trans. 91 2261 reported that at low temperatures (500-650°C) accelerated initiation of breakaway corrosion may occur in regions where the coating is cracked exposing the underlying steel where the other side of the steel is exposed to hydrogen. There is, therefore, a risk at lower temperature of operation or heat treatment that breakaway corrosion will occur in regions of cracked coating before the coating has a chance to self heal, and once that happens the formation of iron-oxide nodules prevents any further healing of the coating.

[0019] There is a need, therefore, to reduce or prevent problems associated with the use of chromium containing components, especially deriving from forming operations on the component.

[0020] It is an aim of the present invention to address this need.

[0021] SUMMARY OF THE INVENTION

[0022] The present invention accordingly provides, in a first aspect, a coated component for a high- temperature device, the coated component comprising: a chromium-containing substrate, a first protective coating on at least a first portion of a surface of the substrate, and a second protective coating at least on the first portion of the substrate, the second protective coating comprising at least one layer comprising a rare earth containing material.

[0023] Optionally the rare earth containing material of the second protective coating does not contain Ce.

[0024] Surprisingly, the inventors have discovered that the invention provides the advantages that the second protective coating acts to adhere cracked parts of the first protective coating especially at shoulders / dimples (which arise during forming processes) and may improve self-healing of the first protective coating even at relatively lower temperature. It appears (without wishing to be bound) that the rare earth containing material of the second protective coating may react with components of the first protective coating and / or chromium to form electrically conductive coatings (e.g. RECoCh, where RE indicates rare earth). Furthermore the rare earth containing material of the second protective coating acts as protective coating and getter of Cr, and also (as mentioned above) the rare earth containing material of the second protective coating tends to react with any Cr on the surface of the component to form stable oxides (e.g. perovskite, RECrCh). The substrate / component may comprise a metallic alloy comprising chromium. Optionally, the substrate / component may comprise a ferrous alloy comprising chromium. The substrate may be a ferritic stainless steel.

[0025] The substrate may be selected from a Ni and / or Co superalloy. Often, the substrate may be stainless steel, thus the component may comprise stainless steel.

[0026] The alloy may comprise 11% wt Cr or greater; optionally 15%wt Cr or greater; optionally 17%wt Cr or greater; optionally 19%wt Cr or greater.

[0027] Examples of suitable alloys include stainless steel with 23% Cr (a stainless steel high in La developed specifically for use in SOFCs), ferritic stainless steel with 22% Cr, SS441 and SS316.

[0028] The Cr compositions of such stainless steels may be:

[0029] Crofer 22 H: Chromium 23%; carbon 0.007%; nitrogen 0.02%; Mn 0.45%; Si 0.25%; Al <0.05%; tungsten 2%; niobium 0.5%; Ti 0.06%; La 0.1%, S < 0.002%, Fe balance.

[0030] SS441: Chromium 17.5 - 18.5%; carbon 0.03%; Si 1.00%; Mn 1.00%; P 0.04%; S 0.015%; Ti 0.1 to 0.6%; niobium (3*C+0.3) to 1.00%; Fe balance.

[0031] Ferritic stainless steel with 22% Cr: Chromium 22.1%.

[0032] SS316 Chromium 18.2%; molybdenum 2.2%; carbon 0.05%; nickel 10%

[0033] The device is suitable for and may be for operation at a temperature of 450 °C or higher, optionally 500 °C or higher.

[0034] The first protective coating may be a self-healing coating. The first protective coating may comprise a cobalt-containing material, a manganese containing material, and / or a copper containing material. Usually, the first protective coating may comprise a rare-earth metal containing material.

[0035] Thus, the first protective coating may comprise a cerium-containing material. The first protective coating may comprise a material comprising cobalt and cerium. The first protective coating may comprise a material comprising cobalt and manganese. The first protective coating may comprise a material comprising cobalt and / or copper. The first protective coating may comprise a layer (e.g. a first layer) of cerium containing material and a layer (e.g. a second layer) of cobalt-containing material, a copper-containing material and / or a manganese-containing material.

[0036] Usually, the first layer of cerium containing material may have a thickness above 5 nm. Optionally, the first layer of cerium-containing material may have a thickness in the range 5 to 100 nm, optionally 5 nm to 75 nm, optionally 10 nm to 70 nm, optionally 15 nm to 60 nm, optionally 20 nm to 50 nm, optionally 20 nm to 40 nm.

[0037] The second layer of cobalt-containing material may have a thickness in the range 400 to 800 nm, optionally 450 nm to 750 nm, optionally 500 nm to 700 nm, optionally 550 nm to 650 nm.

[0038] Usually, the first protective coating will be a physical vapour deposited (PVD) coating that may be deposited using a continuous reel-to-reel PVD process on steel strip prior to forming the interconnect plates.

[0039] The second protective coating may be electrically conductive.

[0040] The second protective coating may have a thickness in the range 50 nm to 1 pm, optionally the coating may have a thickness in the range 50 nm to 500 nm.

[0041] The second protective coating may have one layer, or two or more layers, each layer comprising a rare earth containing material.

[0042] The second protective coating may comprise at least one layer comprising the rare earth containing material and cobalt.

[0043] In embodiments of the invention, the substrate may comprise a formed substrate, optionally wherein the formed substrate comprises at least one formed portion that is raised or lowered out of a plane of the substrate. Forming may be by known methods used on steel, for example stamping, hydroforming, fine-blanking or a similar technique for forming shaped parts from steel strip. The formed portions or portions on the substrate may comprise portions where the substrate has undergone mechanical deformation. Such deformation may involve tensile and / or compressive forming. Forming may involve, for example, indentation forming. The formed portion may be formed into various shapes, for example, indentations such as dimples and / or channels. Dimples may be, for example, of generally round (e.g. generally circular) shapes on the plane of the substrate, having an indentation on one surface and a raised curved generally spheroidal cap / dome on the other surface. Channels may comprise elongate indentations on one surface and raised elongate portions on the other surface. Channels may be, for example, parallel, single serpentine, and / or multi-serpentine.

[0044] Because of the surprising advantages, in particular beneficial interaction with the first protective layer, the invention is particularly applicable to e.g. interconnect dimples which previously have posed problems as discussed herein.

[0045] The device will usually comprise an electrochemical cell. Examples of such cells are an electrolysis cell, an oxygen separator, a sensor or a fuel cell, preferably a SOFC.

[0046] The electrochemical cell may further comprise an electrolyte, a first electrode, and a second electrode.

[0047] The device may comprise a system comprising a stack of electrochemical cells.

[0048] The first protective coating and the second protective coating may be located on at least the surface of the component that is on the air side of the electrochemical (e.g. SOC) device.

[0049] The component may comprise an interconnect, a spacer, a metal plate, a substrate, pipe fitting, pipe, heat exchanger, or a valve component.

[0050] Thus, the coated component may comprise a coated component for use in a device, optionally for operation at a temperature of 450°C or higher.

[0051] This, in a second aspect the present invention provides, a high temperature device comprising at least one electrochemical cell, the device comprising a coated component according to the first aspect.

[0052] Preferably, the device is for operation at a temperature of 450 °C or higher.

[0053] Optionally, the rare-earth containing material comprises a praseodymium-containing material, a lanthanum-containing material and / or a terbium-containing material.

[0054] In a third aspect, the present invention provides a method for producing a coated component, the method comprising: a) providing a component comprising a chromium-containing substrate having a first protective coating on at least a first portion of a surface thereof, b) providing a coating mixture comprising at least one solvent, and a source of a rare earth containing material, c) contacting at least the first portion of the surface of the substrate with the coating mixture, d) optionally drying the component, and e) heating the component in an oxidising atmosphere at a temperature of 450 °C or higher.

[0055] Optionally the rare earth containing material of the second protective coating does not contain Ce.

[0056] Optionally, the rare-earth containing material e.g. of the second protective coating comprises a praseodymium-containing material, a lanthanum-containing material and / or a terbium- containing material.

[0057] The source of rare earth containing material may be a rare-earth salt which is optionally soluble and thermally decomposes to the oxide.

[0058] The source of rare earth containing material may be a source of rare earth (III). The source of rare earth containing material may comprises a rare earth salt, optionally rare earth (III) nitrate, and / or the source rare earth (III) may comprise chelated rare earth (III), optionally rare earth (III) (acetyl acetonate).

[0059] The method, if required, may further comprise repeating steps a) to c), and optionally d) and e), thereby producing a coated component having a coating comprising a plurality of layers comprising rare earth containing material.

[0060] The method may comprise (a) providing a component comprising a chromium-containing substrate having a first protective coating on at least a first portion of a surface thereof, wherein the chromium-containing substrate having the first protective coating comprises at least one formed portion that is raised or lowered out of a plane of the substrate.

[0061] Optionally, the method may further comprise after step a): al) causing the chromium- containing substrate having a first protective coating to undergo at least one forming step to form at least one formed portion that is raised or lowered out of a plane of the substrate.

[0062] The coating mixture may comprise a coating solution, comprising a solvent. The solvent may comprise a low viscosity solvent, optionally comprising a Ci to Ce alcohol, an ester, water containing one or more surfactants, and / or a water and Ci to Ce alcohol mixture. Thus, the solvent may be selected from one or more of ethanol, propanol, ethyl acetate and methoxypropanol. The low viscosity solvent may comprise a Ci to Cs alcohol, optionally a Ci to C4 alcohol.

[0063] In some embodiments, contacting at least one surface of the component with the coating mixture may comprise dip coating the component in the coating mixture and / or spray coating the component with the coating mixture.

[0064] If the method comprises dip coating, dip coating the component may be performed at a controlled dip rate to aid control of the thickness and other characteristics of the coating.

[0065] If the method is spraying, the method may comprise atomised spraying, optionally using a sonic atomiser or an ultrasonic atomiser.

[0066] Contacting at least one surface of the component with the coating mixture may be undertaken at a component temperature in the range 10°C to 100°C, optionally at 12 °C to 50 °C, optionally at around room temperature.

[0067] The surface of the component may be cleaned / polished before contacting the surface of the component with the coating mixture.

[0068] Heating the component in an oxidising atmosphere may comprise heating the component to a temperature of 480°C or higher, optionally 500°C or higher, optionally to a temperature of 520°C or higher, optionally 550°C or higher.

[0069] Heating the component in an oxidising atmosphere may comprise heating the component to a temperature of 580°C, 610°C, 650°C, 670°C, 720°C, 770°C, 820°C, 870°C, 900°C, 910°C or higher.

[0070] The oxidising atmosphere may comprise oxygen and may usually comprise air.

[0071] In some embodiments, the component or substrate may be subject to a heat treatment at higher temperature than the normal operating temperature of the SOC stack prior to the assembly of the stack, to oxidise the metallic coatings to their respective oxides, and to form a protective sub-scale of chromium oxide on the steel underneath the coating.

[0072] If an interconnect which has been additionally coated as described herein is subjected to such a heat treatment, at a temperature in excess of 200°C or so the film of rare earth salts may start to decompose to form a film of the rare earth oxide. At higher temperatures the rare earth oxide of the second protective coating may react with the components (e.g. cobalt oxide) of the first protective coating to form a mixed oxide (e.g. a perovskite such as RECoCh) which may be stable and electrically conductive. Furthermore, it may also react with the chromium oxide forming on exposed areas of the steel to form a stable mixed oxide, for example a perovskite RECrCh.

[0073] In addition, the inventors have surprisingly discovered that the rare earth containing material of the second protective coating may act as an adhesive between the first protective coating and the underlying steel in regions where the first protective coating has been cracked or damaged due to e.g forming, as there are often regions where the first protective coating has started to debond from the steel adjacent to cracks.

[0074] Under these circumstances, as the first protective coating expands on oxidation the resulting stresses can cause the first protective coating to peel away from the steel, and this inhibits self healing of cracks in the first protective coating. As the rare earth containing material of the second protective coating may react with both chromium oxide and materials in the first protective coating (e.g. cobalt oxide) it may act to bond the coating with the oxide scale on the steel, improving coating adhesion around defects which may include both inherent defects on the surface and intentional deformations.

[0075] The second protective layer is particularly advantageous when the substrate and first protective coating are formed into the appropriate shape for the component, e.g. formed into an interconnect because this may induce cracking or other damage to the first protective coating.

[0076] The invention in its various aspects is advantageous because it may protect substrates, components and devices (including electrochemical cells) from contamination by chromium that may otherwise evaporate from components (including stainless steel components) at higher temperature and which may otherwise react to form a stable chromate phase over the active surface of the components (e.g. over electrodes in electrochemical cells).

[0077] Definitions

[0078] In this specification, the term “source of’ an element, compound or other material refers to a material comprising the element, compound or other material whether or not chemically bonded in the source. The source of the element, compound or other material may be an elemental source (e.g. Pr, Tb or O2) or may be in the form of a compound or mixture comprising the element, compound or other material including one or more of those elements, compounds or materials.

[0079] In this specification, the terms “rare earth” or “rare earth metal” refer to metals selected from Y, Sc, and lanthanoid. References to rare earth containing material or rare earth containing metal are to be construed accordingly.

[0080] “Lanthanoid”, “lanthanide” and “Ln” are used interchangeably and mean the metallic chemical elements with atomic numbers 57-71.

[0081] The term "dopant" as used herein is not intended to be restricted to a maximum percentage of elements, ions or compounds added to chemical structures. Similarly, the term "doping" is intended to mean the addition of a certain amount of elements, ions or compounds to a material. It is not limited to a maximum quantity of material, after which, further addition of material no longer constitutes doping.

[0082] The term “solid oxide cell” (SOC) is intended to encompass both solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs).

[0083] In this specification references to electrochemical cell, SOC, SOFC and SOEC may refer to tubular or planar cells. Electrochemical cell units may be tubular or planar in configuration. Planar fuel cell units may be arranged overlying one another in a stack arrangement, for example 100-400 fuel cell units in a stack, with the individual fuel cell units arranged electrically in series.

[0084] Electrochemical cells may be fuel cells, reversible fuel cells or electrolyser cells. Generally, these cells may have the same structure and reference to electrochemical cells may refer (unless the context suggests otherwise) to any of these types of cell. The cell may be based upon a solid oxide electrolyte, optionally a metal-supported solid oxide cell. In fuel cell mode, a fuel contacts the anode (fuel electrode) and an oxidant, such as air or an oxygen-rich fluid, contacts the cathode (air electrode), so in fuel cell mode operation, the air electrode will be the cathode. A solid oxide electrolyser cell (SOEC) may have the same structure as an SOFC, but is essentially the SOFC operating in reverse, or in a regenerative mode, to achieve the electrolysis of water and / or carbon dioxide by using the solid oxide electrolyte to produce hydrogen gas and / or carbon monoxide and oxygen.

[0085] In this specification, Area Specific Resistance (ASR; units of Qcm2or m cm2when not normalised) refers to the internal resistance of an electrochemical cell normalised to the cell active area (commonly used to allow direct comparison of cells of different active areas). ASR is equivalent to the voltage drop due to the internal cell resistance when multiplied by the current density (in Acm'2)

[0086] Secant ASR is derived from the voltage change from open circuit to the cell operating voltage divided by the applied current density (so a cell of lower ASR will have a higher operating voltage and power output at any given applied current density).

[0087] Ohmic / series resistance (Rs) is the component of the cell internal resistance with no associated capacitance (resistance of the oxide scales of steel components may be described by Rs).

[0088] The term “fluid flow path” is used to define fluid flow paths between various components, and thus it is also to be understood that those components are in fluid flow communication with one another.

[0089] The various features of aspects of the disclosure as described herein may be used in combination with any other feature in the same or other aspect of the disclosure, if needed with appropriate modification, as would be understood by the person skilled in the art.

[0090] Furthermore, although all aspects of the invention or disclosure preferably “comprise” the features described in relation to that aspect, it is specifically envisaged that they may “consist” or “consist essentially” of those features outlined in the claims.

[0091] The invention will now be described with reference to the accompanying figures and examples.

[0092] BRIEF DESCRIPTION OF THE FIGURES

[0093] Figure 1 shows optical micrographs of formed concave dimples of interconnects from Example 1 after test.

[0094] Figure 2 shows optical micrographs of formed convex dimples of interconnects from Example 1 after test.

[0095] Figure 3 shows SEM micrographs of formed convex dimples of interconnects from Example 1 after test. Figure 4 shows a SEM micrograph of formed convex dimples of interconnects from Example 1 after test.

[0096] Figure 5 shows top-down SEM micrographs of convex dimple shoulders of interconnects from Example 2 after heat treatment.

[0097] Figure 6 shows top-down high-resolution SEM micrographs of coating damage of interconnects from Example 2 after heat treatment.

[0098] Figure 7 is a schematic flow diagram of the method.

[0099] Figure 8 shows a schematic of a fuel cell system illustrating fluid flow paths.

[0100] Figure 9 shows a schematic (not to scale) cross section of a coated component.

[0101] Figure 10 shows a top-down SEM micrograph of convex dimple shoulders of interconnects from Example 3 after heat treatment.

[0102] Figure 11 shows a top-down high-resolution SEM micrograph of coating damage of interconnects from Example 3 after heat treatment.

[0103] Figure 12 is a graph of Normalised SecantASR rate of change for the stack of cells from Example 4

[0104] DETAILED DESCRIPTION OF THE INVENTION

[0105] Figure 7 shows a schematic flow diagram illustrating the method of providing the coating on a component as in the Examples. In Figure 7, dashed arrows indicate optional steps.

[0106] A coating mixture 320 containing solvent, and a source of Pr, such as, for example, a mixture of Pr nitrate and Pr acac is provided. The coating mixture 320 is applied in step 330 to the component (which may be of stainless steel having a Co / Ce coating on a surface, the Ce / Co coating having been applied using PVD) 310. After applying 330 the coating mixture 320, the component is optionally dried 340 and then heated 350 in an oxidising atmosphere at 450°C or above. Optionally, the coating steps are repeated 360 after the step 330, after the optional drying step 340 or after the heating step 350 to form further layers and a thicker coating.

[0107] Chromium contamination may affect electrochemically active components in fuel cell systems, especially where chromium-containing components are in fluid flow communication with electrochemically active components. Coatings as described herein may form barrier coatings on chromium-containing components reducing or preventing chromium volatility and acting to protect components (e.g. that are in fluid communication with the chromium- containing components) that may be susceptible to chromium contamination.

[0108] Thus, referring to Figure 8, fuel cell system 15 is an intermediate-temperature solid oxide fuel cell (IT-SOFC) system. Fuel cell stack 20 is a metal-supported IT-SOFC fuel cell stack, as taught in WO-A-2015 / 004419. Fuel cell system 15 has a steady state IkW electric output from fuel cell stack 20, and comprises 121 metal-supported IT-SOFC fuel cells 30. Each fuel cell 30 has an anode side 40, electrolyte layer 50, and cathode side 60. Each fuel cell layer in the fuel cell stack is separated by an electrically conducting gas impermeable metal interconnect plate (interconnector) (not shown). Fuel cell stack endplates and compression means (not shown) are also provided.

[0109] Reference herein to fuel cell 30 is to the full set of 121 fuel cells 30.

[0110] Electrical load L is placed across fuel cell 30.

[0111] Fuel cell stack anode inlet 41 is in fluid flow communication with fuel cell anode inlet 41 A for the flow of anode inlet gas to the anode side 40 of fuel cell 30. Fuel cell anode outlet 42A is in fluid flow communication with fuel cell stack anode off-gas outlet 42 for the flow of anode off-gas.

[0112] Fuel cell stack cathode inlet 61 is in fluid flow communication with fuel cell cathode inlet 61 A for the flow of cathode inlet gas to the cathode side 60 of fuel cell 30. Fuel cell cathode outlet 62A is in fluid flow communication with fuel cell stack cathode off-gas outlet 62 for the flow of cathode off-gas.

[0113] Steam reformer 70 comprises reformer inlet 71 for anode inlet gas and reformer outlet 72 for exhausting anode inlet gas.

[0114] Tail-gas burner 80 is in fluid flow communication with fuel cell stack anode and cathode offgas outlets 42, 62 and has a tail gas burner exhaust 81, anode off-gas inlet 82 and cathode offgas inlet 83. Tail-gas burner 80 defines a fluid flow path from fuel cell stack anode and cathode off-gas outlets 42, 62 to tail-gas burner exhaust 81, and is configured for burning anode and cathode off-gases and producing a tail-gas burner off-gas. An anode inlet gas fluid flow path A is defined from fuel source 90 to evaporator 100 to steam reformer 70 to fuel cell stack anode inlet 41 to fuel cell anode inlet 41 A, i.e. the components are in fluid flow communication with one another.

[0115] An anode off-gas fluid flow path B is defined from fuel cell anode outlet 42A to fuel cell stack anode off-gas outlet 42 to anode off-gas heat exchanger 110 (HX-AOG) to condenser heat exchanger 120 to separator 130 to anode off-gas inlet 82 of tail-gas burner 80.

[0116] Main cathode inlet gas flow path 230 and air bypass inlet gas flow path 240 have a number of common components and share a common flow path in a number of places, marked as cathode inlet gas fluid flow path C.

[0117] Main cathode inlet gas flow path 230 is defined from oxidant inlet 140 to blower 210 to valve / separator 220 to anode off-gas heat exchanger 110 to air pre-heater heat exchanger 150 (HX-APH) to reformer heat exchanger 160 (HX-Ref) to fuel cell stack cathode inlet 61 to fuel cell cathode inlet 61 A.

[0118] Air bypass inlet gas flow path 240 is defined from oxidant inlet 140 to blower 210 to valve / separator 220 to air bypass inlet 190 to reformer heat exchanger 160 to fuel cell stack cathode inlet 61 to fuel cell cathode inlet 61 A.

[0119] Valve / separator 220 is controlled by control means 200 so as to split the flow of inlet air between main cathode inlet gas flow path 230 and air bypass inlet gas flow path 240.

[0120] Thus, the air bypass inlet gas flow path 240 bypasses anode off-gas heat exchanger 110 and air pre-heater heat exchanger 150.

[0121] In this embodiment, the common parts of gas flow paths 230 and 240 (cathode inlet gas fluid flow path C) are therefore (a) oxidant inlet 140 to blower 210 to valve / separator 220, and (b) reformer heat exchanger 160 to fuel cell stack cathode inlet 61 to fuel cell cathode inlet 61 A.

[0122] A cathode off-gas fluid flow path D is defined from fuel cell cathode outlet 62A to fuel cell stack cathode off-gas outlet 62 to cathode off-gas inlet 83 of tail-gas burner 80.

[0123] A tail-gas burner off-gas fluid flow path E is defined from tail gas burner exhaust 81 to air pre-heater heat exchanger 150 to evaporator heat exchanger 170 (HX-Evap) to fuel cell system exhaust 180.

[0124] Anode off-gas heat exchanger 110 is in fluid flow communication with (i) fuel cell stack anode off-gas outlet 42 (i.e. with fuel cell anode outlet 42A) and tail-gas burner anode off-gas inlet 82, and (ii) oxidant inlet 140 and fuel cell stack cathode inlet 61 (i.e. with fuel cell cathode inlet 61 A), and is arranged for exchanging heat between anode off- gas from fuel cell stack 20 and cathode inlet gas to fuel cell stack 20.

[0125] Air pre-heater heat exchanger 150 is in fluid flow communication with (i) tail-gas burner exhaust 81 and fuel cell system exhaust 180, and (ii) oxidant inlet 140 and fuel cell stack cathode inlet 61 (i.e. with fuel cell cathode inlet 61A), and is arranged for exchanging heat between tail-gas burner 81 off-gas and cathode inlet gas to fuel cell stack 20.

[0126] Reformer heat exchanger 160 is a parallel-flow heat exchanger and is in fluid flow communication with (i) oxidant inlet 140 and fuel cell stack cathode inlet 61 (i.e. with fuel cell cathode inlet 61 A), and (ii) fuel source 90 and fuel cell stack anode inlet 41 (i.e. with fuel cell anode inlet 41 A), and is arranged for exchanging heat between cathode inlet gas and anode inlet gas.

[0127] Evaporator 100 has a fuel inlet 101 for anode inlet gas from fuel source 90, a water inlet 102 for water from water supply 103, and an evaporator exhaust 104 for exhausting anode inlet gas from evaporator 100, and is located in the anode inlet gas fluid flow path between fuel source 90 and steam reformer 70. Evaporator 100 additionally comprises evaporator heat exchanger 170 located in the tail-gas burner off- gas fluid flow path E between air pre-heater heat exchanger 150 and fuel cell system exhaust 180.

[0128] Evaporator heat exchanger 170 is in fluid flow communication with (i) tail-gas burner exhaust 81 and fuel cell system exhaust 180, and (ii) fuel source 90 and water supply 103 and fuel cell stack anode inlet 41 (i.e. with fuel cell anode inlet 41 A), and is arranged to exchange heat between tail-gas burner off-gas and anode inlet gas and water, generating a steam fuel mix for the anode inlet gas to steam reformer 70.

[0129] Condenser heat exchanger 120 is in fluid flow communication with (i) fuel cell stack anode off-gas outlet 42 (i.e. with fuel cell anode outlet 42A) and tail-gas burner anode off-gas inlet 82, and (ii) cooling circuit 121, and is arranged for exchanging heat between anode off-gas from fuel cell stack 20 and a cooling fluid in cooling circuit 121.

[0130] Separator 130 is located in the anode off-gas fluid flow path between condenser heat exchanger 120 and tail-gas burner 80, and has a separator condensate outlet 131, and is adapted to separate condensate from the anode off-gas fluid flow path, and exhaust the condensate via the condensate outlet 131. Control means 200 is connected to fuel cell stack cathode inlet gas temperature sensor Tl, fuel cell stack cathode off-gas temperature sensor T2, blower 210 and valve / separator 220. Control means 200 is configured to maintain the temperature determined by temperature sensors Tl and T2 at or about a desired temperature during steady-state operation of the fuel cell system.

[0131] Control means 200 is adapted to operate two independent control loops which operate upon the cathode inlet gas passing through the cathode inlet gas fluid flow path C.

[0132] In the first control loop, the heating of cathode inlet gas is controlled. In the second control loop, the mass flow rate of cathode inlet gas is controlled.

[0133] In use, fuel cell system 10 goes through three phases: start-up, steady state, and shutdown. In the start-up phase, fuel cell stack 20 is cold (or at least below its steady-state operational temperature), and therefore must be heated in order to achieve an operational state. In the steady state phase, fuel cell stack 20 is maintained at operational temperature, as determined by the sensors Tl and T2. Electricity is generated and used by load L across fuel cell 30. Temperatures detected by temperature sensors Tl and T2 will vary, and control means 200 varies the inlet air mass flow rate and the splitting of air between flow paths 230 and 240 accordingly. In the shutdown phase, electrical power is no longer required from fuel cell system 10, and a controlled shutdown sequence is initiated. Power demand from fuel cell stack 20 is reduced to zero and the temperature set point for fuel cell stack air inlet Tl is reduced, while the air flow rate from the blower 210 is increased.

[0134] Figure 9 shows a schematic cross section through a coated component. Component 402 of stainless steel (containing chromium) which may be an interconnect, substrate or other metal component has on at least one surface a Ce / Co coating (deposited by PVD) 403. A coating 402 comprising Pr is located on the Ce / Co coating 403 on the surface of the component deposited using a method as detailed below.

[0135] Examples

[0136] Example 1

[0137] Formed interconnect plates of stainless steel having a Co / Ce PVD deposited coating on the surface were dip coated in a solution with a total concentration of Pr3+ions of 0.04M, consisting of 80mol% praseodymium (III) acetylacetonate and 20mol% praseodymium (III) nitrate, in a solvent consisting of 90% ethanol and 10% methoxypropanol by volume.

[0138] The interconnect plates were then dried in an oven at 120°C and then placed in a furnace in flowing air and heated to 850°C for 2h to oxidise the coatings and grow an oxide scale on the underlying steel.

[0139] The interconnects then had a suitable electrical contact paste deposited on the tops of formed dimples where the plate makes contact with the air electrodes od adjacent cells.

[0140] Finally the interconnect plates were built into an SOFC stack and operated for 1 lOOh with air containing 1.5% water vapour flowing over the interconnect plates and air electrodes at a temperature of 610°C.

[0141] For comparison, interconnects without the additional praseodymium coating were also included in the same stack to compare corrosion.

[0142] The conditions above are known to promote breakaway corrosion rapidly in regions of the interconnect where the coating is damaged.

[0143] Example 2

[0144] Formed interconnect plates of stainless steel having a Co / Ce PVD deposited coating on the surface were dip coated in a solution with a total concentration of Pr3+ions of 0.04M, consisting entirely of Praseodymium (III) nitrate, in a solvent consisting of 90% ethanol and 10% methoxypropanol by volume.

[0145] The interconnect plates were then dried in an oven at 120°C and then placed in a furnace in flowing air and heated to 870°C for 2h to oxidise the coatings and grow an oxide scale on the underlying steel.

[0146] Interconnects without the additional coating were placed in the same furnace to compare the appearance of as-manufactured but not operated interconnects.

[0147] Example 3 Formed interconnect plates of stainless steel having a Co / Ce PVD deposited coating on the surface were dip coated in a solution with a total concentration of La3+ions of 0.04M, consisting entirely of Lanthanum (III) nitrate, in a solvent consisting of isopropanol.

[0148] The interconnect plates were then dried in an oven at 120°C and then placed in a furnace in flowing air and heated to 870°C for 2h to oxidise the coatings and grow an oxide scale on the underlying steel.

[0149] Example 4

[0150] Formed interconnect plates of stainless steel having a Co / Ce PVD deposited coating on the surface were dip coated in a solution with a total concentration of La3+ions of 0.04M, consisting entirely of Lanthanum (III) nitrate, in a solvent consisting of 90% isopropanol and 10% methoxypropanol by volume.

[0151] The interconnect plates were then dried in an oven at 120°C and then placed in a furnace in flowing air and heated to 870°C for 2h to oxidise the coatings and grow an oxide scale on the underlying steel.

[0152] Finally the interconnect plates were built into an SOFC stack and operated for 1500h at a temperature of 610°C.

[0153] For comparison, interconnects without the additional coating were also included in the same stack to compare corrosion.

[0154] Results

[0155] The interconnects as discussed above were examined using optical microscopy and scanning electron microscopy (SEM).

[0156] Figure 1 shows optical micrographs of formed concave dimples of interconnects from Example 1 after test. Concave formed dimples from praseodymium coated interconnect (left) and standard interconnect with no additional coating (right) showing (1) centre of dimple, (2) convex rim of dimple where the coating is cracked during forming, (3) sporadic localised iron oxide nodules on praseodymium coated part, and (4) iron oxide nodules from breakaway corrosion surrounding most of the rim of the dimple for the standard interconnect. Figure 2 shows optical micrographs of formed convex dimples of interconnects from Example 1 after test. Concave formed dimples from Praseodymium coated interconnect (left) and standard interconnect with no additional coating (right) showing (5) centre of dimple coated in black contact material, (6) convex rim of dimple where the coating is cracked during forming, (7) sporadic localised iron oxide nodules on praseodymium-coated part, (8) iron oxide nodules from breakaway corrosion surrounding the entire rim of the dimple for the standard interconnect.

[0157] Figure 3 shows SEM micrographs of formed convex dimples of interconnects from Example 1 after test. SEM cross section of praseodymium-coated dimple from example 1 after test, typical (left) and small area of corrosion (right). The reference numerals refer to (9) steel, (10) contact paste, and 11) small area of breakaway corrosion.

[0158] Figure 4 shows a SEM micrograph of formed convex dimples of interconnects from Example 1 after test without addition praseodymium coating showing extensive area of breakaway corrosion (12) in the convex shoulder region where the coating was damaged after forming.

[0159] Figure 5 shows a top-down SEM micrographs of convex dimple shoulders of interconnects from example 2 after heat treatment. Low magnification (2.83kX) SEM micrograph of the convex dimple shoulder of a standard interconnect (left) and a praseodymium coated interconnect (right) after 870°C heat treatment showing cracking of the coating. Note lack of large coating defects in the praseodymium-coated image even where the coating has peeled away from the surface during oxidation.

[0160] Figure 6 shows top-down high resolution SEM micrographs of coating damage of interconnects from Example 2 after heat treatment. High magnification (28.3kX) SEM images of a coating defect in a standard interconnect (left) and a Pr-coated interconnect (right). (13) Predominantly CO3O4 spinel coating. (14) CrMnCU spinel native oxide scale on the exposed steel. (15) Iron oxide nodule- initiation of breakaway corrosion. (16) Void under the coating where it has debonded from the steel during oxidation. (17) Predominantly CO3O4 spinel coating with an outer layer of PrCo mixed oxide. (18) Pr,Co,Cr mixed oxide coating the exposed steel. Also note cobalt oxide coating has not debonded from the steel in this image.

[0161] Figures 10 and 11 show SEM micrographs of formed convex dimples of interconnects from Example 3 after heat treatment, showing that the cracks in the darker CO3O4 spinel coating (20) have been filled with the La containing coating to form a La,Co,Cr mixed oxide coating (22) on the exposed steel.

[0162] Figure 12 and Table 1 below show the normalized SecantASR rate of change for a stack of cells from Example 3. Odd numbered cells had no additional coating, and even numbered cells had the Lanthanum based coating. As can be seen, the cells with lanthanum based coatings had on average a 40% lower SecASR rate of increase per 1000 hours than the cells without the lanthanum based coating, indicating that the cells with the lanthanum based coating has a lower degradation rate and improved performance to the cells without the coating. Table 1

[0163] Reference signs in Figures 7 to 9: - fuel cell system - fuel cell stack - fuel cell - anode side fuel cell stack anode inlet A - fuel cell anode inlet - fuel cell stack anode off-gas outletA - fuel cell anode outlet - electrolyte layer - cathode side - fuel cell stack cathode inlet A - fuel cell cathode inlet - fuel cell stack cathode off-gas outletA - fuel cell cathode outlet - steam reformer - reformer inlet - reformer outlet - tail-gas burner - tail-gas burner exhaust - anode off-gas inlet - cathode off-gas inlet - fuel source 0 - evaporator 1 - fuel inlet 2 - water inlet 103 - water supply

[0164] 104 - evaporator exhaust

[0165] 110 - anode off-gas heat exchanger

[0166] 120 - condenser heat exchanger

[0167] 121 - cooling circuit

[0168] 130 - separator

[0169] 131 - separator condensate outlet

[0170] 140 - oxidant inlet

[0171] 150 - air pre-heater heat exchanger

[0172] 160 - reformer heat exchanger

[0173] 161 - reformer heat exchanger oxidant inlet

[0174] 162 - reformer heat exchanger oxidant outlet

[0175] 170 - evaporator heat exchanger

[0176] 180 - fuel cell system exhaust

[0177] 190 - air bypass inlet

[0178] 200 - control means

[0179] 210 - blower

[0180] 220 - valve / separator

[0181] 230 - main cathode inlet gas flow path

[0182] 240 - air bypass inlet gas flow path

[0183] 250 - fuel source

[0184] 260 - air bypass inlet gas flow path

[0185] A - anode inlet gas fluid flow path B - anode off-gas fluid flow path C - cathode inlet gas fluid flow path D - cathode off-gas fluid flow path

[0186] E - tail-gas burner off-gas fluid flow path

[0187] G - reformer cathode off-gas fluid flow path L - electrical load

[0188] T1 - fuel cell stack cathode inlet gas temperature sensor

[0189] T2 - fuel cell stack cathode off-gas temperature sensor

[0190] T3 - fuel cell stack anode inlet gas temperature sensor

[0191] 310 - stainless steel component

[0192] 320 - coating mixture

[0193] 330 - apply coating mixture

[0194] 340 - (optionally) dry

[0195] 350 - heat

[0196] 360 - (optional) repeat of steps

[0197] 401 - second protective coating

[0198] 402 - stainless steel component

[0199] 403 - first protective coating of Ce / Co

[0200] All publications mentioned in the above specification are herein incorporated by reference. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be performed therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Claims

Claims1. A coated component for a high-temperature device, the coated component comprising: a chromium-containing substrate, a first protective coating on at least a first portion of a surface of the substrate, and a second protective coating on at least on the first portion of the substrate, the second protective coating comprising at least one layer comprising a rare earth containing material.

2. A coated component as claimed in claim 1, wherein the component comprises a metallic alloy comprising chromium.

3. A coated component as claimed in either claim 1 or claim 2, wherein the component comprises a ferrous alloy comprising chromium.

4. A coated component as claimed in claim 3, wherein the component comprises stainless steel.

5. A coated component as claimed in any one of the preceding claims, wherein the alloy comprises 11% wt Cr or greater; optionally 15%wt Cr or greater; optionally 17%wt Cr or greater; optionally 19%wt Cr or greater.

6. A coated component as claimed in any one of the preceding claims, wherein the device is for operation at a temperature of 450 °C or higher, optionally 500 °C or higher.

7. A coated component as claimed in any one of the preceding claims, wherein the first protective coating is a self-healing coating.

8. A coated component as claimed in any one of the preceding claims, wherein the first protective coating comprises a cobalt-containing material, a manganese containing material, and / or a copper containing material.

9. A coated component as claimed in any one of the preceding claims, wherein the first protective coating comprises a rare-earth metal containing material10. A coated component as claimed in claim 9, wherein the first protective coating comprises a cerium-containing material.

11. A coated component as claimed in any one of the preceding claims, wherein the first protective coating comprises a material comprising cobalt and cerium, cobalt and manganese, and / or cobalt and copper.

12. A coated component as claimed in any one of the preceding claims, wherein the first protective coating comprises a first layer of cerium containing material and a second layer of cobalt-containing material.

13. A coated component as claimed in claim 12, wherein the first layer of cerium containing material has a thickness in the range 5 to 100 nm.

14. A coated component as claimed in either claim 12 or claim 13, wherein the second layer of cobalt-containing material has a thickness in the range 400 to 800 nm.

15. A coated component as claimed in any one of the preceding claims, wherein the first protective coating is a physical vapour deposited (PVD) coating.

16. A coated component as claimed in any one of the preceding claims, wherein the second protective coating is electrically conductive.

17. A coated component as claimed in any one of the preceding claims, wherein the second protective coating has a thickness in the range 50 nm to 1 pm, preferably the coating has a thickness in the range 50 nm to 500 nm.

18. A coated component as claimed in any one of the preceding claims, wherein the second protective coating has two or more layers each comprising a rare earth containing material.

19. A coated component as claimed in any one of the preceding claims, wherein the second protective coating comprises at least one layer comprising the rare earth containing material and cobalt.

20. A coated component as claimed in any one of the preceding claims, wherein the substrate comprises a formed substrate, optionally wherein the formed substrate comprises formed portions that are raised or lowered out of a plane of the substrate.

21. A coated component as claimed in any one of the preceding claims, wherein the rare- earth containing material comprises a praseodymium-containing material, a lanthanum- containing material and / or a terbium-containing material.

22. A coated component for a device as claimed in any one of the preceding claims, wherein the device comprises an electrochemical cell.

23. A coated component for a device as claimed in claim 22, wherein the component comprises an interconnect, a spacer, a metal plate, or a substrate.

24. A coated component for a device as claimed in any one of the preceding claims, wherein the device comprises a system comprising a stack of electrochemical cells.

25. A coated component for a device as claimed in claim 24, wherein the component comprises an interconnect, a substrate, pipe fitting, pipe, heat exchanger, or a valve component.

26. A method for producing a coated component, the method comprising a) providing a component comprising a chromium-containing substrate having a first protective coating on at least a first portion of a surface thereof, b) providing a coating mixture comprising at least one solvent, and a source of a rare earth containing material, c) contacting at least the first portion of the surface of the substrate with the coating mixture, d) optionally drying the component, and e) heating the component in an oxidising atmosphere at a temperature of 450 °C or higher.

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