SE2350332A1 - Method for forming a chromium oxide layer on substrate comprising chromium - Google Patents

Abstract

A method for forming a chromium oxide layer (420, 520) on a substrate (430, 530) comprising chromium, wherein the substrate (430, 530) is placed in a furnace chamber (600), and a heat exchanger (110) comprising one or more conduits (214), wherein the one or more conduits comprise a substrate (430, 530) oxidized according to the method. The method comprises introducing (S1) a gas composition into the furnace chamber (600), the gas composition comprising an inert gas, hydrogen gas, and water vapor; and heating (S3) the furnace chamber (600) with the introduced gas composition to a temperature of at least 600 degrees Celsius.

Description

TECHNICAL FIELD The present disclosure relates to methods for forming a chromium oxide layer on a substrate comprising chromium for reducing hydrogen permeation rate, and to heat exchangers comprising one or more conduits provided with the chromium oxide layer.

BACKGROUND Thermal energy can be converted into electrical energy in several ways. Some systems use Stirling engines as a generator to convert thermal energy to electrical energy. Stirling engines are closed-cycle engines that use an external heat source to expand a working fluid (often a working gas) which drives one or more pistons. Although many types of fluids can be used as the working fluid, hydrogen gas is often preferred since its heat transfer characteristics yield a high power output. ln the heat transfer system comprising the Stirling engine, the hydrogen gas is at one point heated via a heat exchanger. This heat exchanger can also be called a gas heater. The heat exchanger typically comprises a plurality of conduits through which the working fluid is transported. The hydrogen gas is heated via the conduits by a source fluid flowing across an outside of the conduits.

A problem with such conduits is that the hydrogen gas may permeate through the conduits, which is undesired. This problem can be alleviated by providing a protective layer/coating on the inside of the conduits and/or on the outside of the conduits. One example of such a protective layer is aluminum oxide. A protective layer of aluminum oxide may be provided on a substrate by first depositing the substrate with aluminum (e.g. via chemical vapor deposition, CVD), and thereafter exposing the deposited aluminum at high temperatures for several hours to convert the deposited aluminum to aluminum oxide. This oxidation treatment at high temperatures, however, could damage the substrate. ln addition, CVD is very costly when it comes to the coating of large components as well as small batches.

Thus, there is a need for improved ways of providing a protective layer that reduces hydrogen permeation. Such improved protective layers are needed in heat exchangers for Stirling engines and for any system/apparatus in general that transports or contains hydrogen gas.

SUMMARY lt is an object of the present disclosure to provide improved protective layers that reduce hydrogen permeation. This object is at least in part achieved by a method for forming a chromium oxide layer on a substrate comprising chromium, wherein the substrate is placed in a furnace chamber. The method comprises introducing a gas composition into the furnace chamber. The gas composition comprises an inert gas, hydrogen gas, and water vapor. The method further comprises heating the furnace chamber with the introduced gas composition to a temperature of at least 600 degrees Celsius. ln contrast to e.g. providing a protective layer of aluminum oxide, which may be a two-step process, the disclosed method only requires a single step. This is advantageous since the method is less complex and less labor intensive. ln particular, there is no need for vacuum furnace used in CVD, and there is no need to move the substrate from a CVD chamber to another separate oxidation furnace.

The disclosed method only requires heating the substrate to 600 degrees Celsius, which is a relatively low temperature compared to known oxidation processes. This relatively low temperature advantageously puts less strain on the substrate. Higher temperatures may e.g. degrade heat treatments (hardening) of the substrate.

The disclosed method does not require a line of sight to any of the surfaces that will be oxidized, and is therefore suitable for oxidizing complex geometries.

The disclosed method enables high control of oxidization of the substrate, in particular by selecting the amount of water vapour in the chamber. Furthermore, the gas composition reduces any undesired general oxidation of other materials that may be composed in su bstrate.

The method provides a chromium oxide layer that is continuous, and denser and more homogeneous compare to most known oxide layers. The disclosed method therefore provides a protective layer with improved reduction of hydrogen permeation.

The chromium oxide layer obtained from the disclosed method only needs to be in the order of 1 micron thick or so to obtain a desired reduction in hydrogen permeation. Consequently, the disclosed method is suitable for small structures such as conduits of a heat exchanger for a Stirling engine.

According to aspects, the introducing of the gas composition comprises continuously flushing the furnace chamber with the gas composition, wherein the flushing is continued during the heating of the furnace chamber. This reduces the amount of other undesired gases in the furnace Chamber and also provides a constant supply of water vapor as an oxidant agent to oxidize chromium.

According to aspects, the amount of the hydrogen gas is between 5 and 5 x 104 times larger than the amount of the water vapor, and wherein the temperature is 600-800 degrees Celsius. According to some other aspects, the amount of the hydrogen gas is between 3 and 6 x104 times larger than the amount of the water vapor, and wherein the temperature is 800-1000 degrees Celsius. According to yet some other aspect, the amount of the hydrogen gas is between 2 and 2 X 103 times larger than the amount of the water vapor, and wherein the temperature is 1000-1100 degrees Celsius. These ranges have been found sufficient to promote the formation of chromium oxide and suppress the formation of other undesirable oxides. Furthermore, the hydrogen will react with any oxygen that may have leaked into the furnace chamber. ln addition, with this volume ratio, the water activity in the system is not enough for the formation of any undesirable oxides during heating and cooling of the substrate.

According to some aspects, the method further comprises waiting a time period after the gas composition has first been introduced into the furnace chamber and before the heating the furnace chamber. ln this way, any oxygen content in the chamber is reduced. The time period may e.g. be 1-12 hours. Such a time period has shown to reduce any oxygen content in the chamber to tolerable amounts. The time period may be selected based on the size of the chamber and/or the flow rate of the gas composition if it is introduced by flushing.

According to some aspects, the inert gas is argon gas, which is an inexpensive and readily available inert gas.

According to some aspects, the inert gas constitutes 1-99% of the total amount of all constituents of the gas composition. However higher content of inert gas leads to a lower amount of water vapor available in the system, which decreases the reaction rate of chromia formation.

According to some aspects, the amount of hydrogen gas constitutes 0.1-7 % of the total amount of all constituents of the gas composition.

According to some aspects, the introduced gas composition provides a pressure of 0.5-2 bar in the furnace chamber. Preferably, the pressure is about 1 bar, which is easy to achieve in a controlled environment. The range of 0.5-2 bar is also relatively easy to achieve.

According to some aspects, the substrate comprises at least 58% nickel by weight and 20- 23% chromium by weight. ln this way, the substrate is suitable for the operational temperatures of conduits for a working fluid in a Stirling engine.

According to some aspects, the substrate comprises the alloy according to the unified numbering system, UNS, designation N06625. This alloy is particularly suitable for the operational temperatures of conduits for a working fluid in a Stirling engine.

According to some aspects, the furnace chamber is held at the temperature for at least 24 hours. This provides a thickness of chromium oxide that is about 0.5-10 microns. Such thickness has been found to provide good protection from hydrogen permeation.

According to some aspects, the furnace chamber is heated to the temperature by a heating rate of 3-9 degrees Celsius per minute, and thereafter cooled by a cooling rate of 3-9 degrees Celsius per minute. Although theformation of the chromium oxide is not sensitive to the heating and cooling rates, rates of 3-9 degrees Celsius per minute have been found to be suitable choices that do not strain the substrate.

There is also disclosed herein a heat exchanger for transferring heat between a source and a working fluid. The heat exchanger comprises one or more conduits for the working fluid, wherein the one or more conduits comprise a substrate comprising chromium oxidized according to the discussions above. The chromium oxide layer may be formed on an inward- facing surface and/or on an outward facing surface of the one or more conduits. This heat exchanger with the chromium oxide layer is associated with the above-discussed advantages.

There is also disclosed herein control units, computer programs, computer readable media, computer program products, associated with the above-discussed advantages.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. Further features of, and advantages with, the present disclosure will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present disclosure may be combined to create embodiments other than those described in the following, without departing from the scope of the present disclosure.

BREEF ÛESCRÉPTlÛN GF THE ÜRAVVÉNKÉE With reference to the appended drawings, below follows a more detailed description of embodiments of the present disclosure cited as examples. ln the drawings: Figure 1 shows a heat transfer system; Figure 2 shows a heat exchanger; Figure 3 shows a cross sectional image of an oxidized substrate; Figure 4 shows a cross sectional image of an oxidized substrate; Figure 5 shows a schematic illustration of an oxidized substrate; Figure 6 shows a schematic illustration of a furnace chamber; Figure 7 is a flow chart illustrating a method; and Figure 8 schematically illustrates a control unit.

DETAILED DESCRIPTION The present disclosure is described more fully below with reference to the accompanying drawings, in which certain aspects of the present disclosure are shown. The present disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments and aspects set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. Like numbers refer to like elements throughout the description. lt is to be understood that the present disclosure is not limited to the embodiments described herein and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.

Figure 1 shows a heat transfer system 100 comprising a heat exchanger 110, and Figure 2 shows details of the heat exchanger 110. ln Figure 2, it can be seen that the heat exchanger 110 comprises a plurality of conduits 214 through which the working fluid is transported. Portions of an outer cover ofthe heat exchanger 110 are transparent in Figure 2 to make the conduits visible. The working fluid is heated via the conduits 214 by a source fluid flowing across an outside of the conduits. The heat exchanger110 comprises a working fluid inlet 210 and a working fluid outlet 211 which are in fluid communication with the conduits. The heat exchanger 110 further comprises a source fluid inlet 212 and a source fluid outlet 213 that are fluidly isolated from the inside of conduits.

Thus, the working fluid is fluidly isolated from the source fluid. ln Figure 1, the source fluid is transported from a first heat transfer fluid pipe 140 into the source fluid inlet 212 of the heat exchanger 110 and is thereafter transported into a second heat transfer fluid pipe 150 via the source fluid outlet 213. Similarly, the working fluid is transported from a first working fluid guide tube 120 into the working fluid inlet 210 of the heat exchanger 110 and is thereafter transported into a second working fluid guide tube (not shown in Figure 1)via the working fluid outlet211. Figure 1 also shows a working fluid expansion top 130 in fluid communication with the first working fluid guide tube 120.

On a mass basis, hydrogen has nearly three times the energy content of gasoline. While hydrogen has high energy density per unit mass, it has low-volumetric energy density at room conditions and has an ability to permeate metal-based materials, which can present operational and safety constraints. This makes transporting and containing hydrogen a challenge because it requires high pressures, low temperatures, or chemical processes to be stored compactly.

Figure 3 shows a cross-sectional image of an oxidized substrate 300. This oxidized substrate can e.g. be used as a protective layer inside the conduits 214 of the heat exchanger to reduce hydrogen permeation. The substrate 330 is a metallic substrate comprising chromium. More specifically, the substrate 330 is an alloy according to the unified numbering system, UNS, designation N06625. The figure also shows an epoxy cold mounting resin 310 for enhancing the grip on the component during grinding and polishing for microstructural analysis and for providing protection to the specimen's edges. The substrate 330 has been heated and oxidized for 24 hours in air at 900 degrees Celsius to form an oxide layer 320 that comprises chromium oxide mixed with other oxides. This oxide layer 320 is, however, discontinuous, inhomogeneous, and partially porous, which are undesired properties since they degrade the permeation reduction of the oxide layer.

Thus, there is a need for improved protective layers that reduce hydrogen permeation. Such improved protective layers are needed in heat exchangers for Stirling engines and for any system/apparatus in general that transports or contains hydrogen gas.

Figure 4 shows a cross-sectional image of an oxidized substrate 400 with an improved protective layer 400, which has been obtained from the method disclosed herein. The substrate 430 is of the same type as in Figure 3, i.e. the substrate 430 is a metallic substrate comprising chromium, particularly the alloy N06625. The figure also shows a cold mounting resin 410. The substrate 330 has been oxidized according to the disclosed method to form a chromium oxide layer 420. ln particular, in a first step, a sample of the substrate was placed in a furnace chamber 12 hours at room temperature, where the furnace chamber was filled with a gas composition of 5% by volume hydrogen gas, 1% by volume water vapor, and 94% by volume argon gas. During this time, the mentioned gas composition is constantly flowing into the furnace chamber to make sure that the whole volume of the furnace chamber is filled with the gas composition, and such that the amount of other gases in the chamber is as small as possible. ln a second step, the furnace chamber, filled with the same gas composition which is constantly flowing into the furnace Chamber, was heated to 900 degrees Celsius, and was held at 900 degrees Celsius for 24 hours. After that, the furnace chamber was cooled to ambient temperature, while containing the gas composition. The substrate was heated from an ambient temperature and cooled back to the ambient temperature with a heating and cooling rate, respectively, of 6 degrees Celsius per minute. The gas composition was exposed onto the surface of the substrate 430 with a pressure of 1 bar and flow rate of 200 ml/min. lt can be seen that the chromium oxide layer 420 is continuous, denser, and more homogeneous compared to the oxide layer 320 of Figure 3.

Figure 5 shows a schematic illustration of an oxidized substrate 500 with an improved protective layer, which has been obtained from the method disclosed herein. ln the figure, a chromium oxide layer 520 is formed on a substrate 530, which is a substrate comprising chromium. This oxidized substrate 500 can e.g. be used as a protective layer inside the conduits 214 of the heat exchanger. ln that case, side 510 may be the inwards facing side and side 540. The side 510 may also, or alternatively, be an out\Nards facing side of a conduit.

Figure 6 shows a schematic illustration of a furnace chamber 600. The furnace chamber 600 may also be called a chamber furnace, a box furnace, and a muffle furnace. The furnace chamber 600 is typically a cubic or box shaped chamber. The furnace chamber 600 is a chamber arranged to receive the substrate 430, 530 or a component comprising the substrate 430, 530. The furnace chamber 600 is a chamber further arranged to receive and contain the gas composition. The furnace chamber 600 is also arranged to heat the substrate 430, 530 and the gas composition contained in the furnace chamber 600.

With reference to Figure 7, there is disclosed herein a method for forming a chromium oxide layer 420, 520 on a substrate 430, 530 comprising chromium, wherein the substrate 430, 530 is placed in a furnace chamber 600. The substrate can also be called a chromia-forming alloy. The method comprises introducing S1 a gas composition into the furnace chamber 600. The gas composition comprises an inert gas, hydrogen gas, and water vapor.

The gas composition is preferably introduced into the furnace chamber such that air or other gas previously occupying the chamber will be purged from the furnace chamber. For example, the introduction of the gas composition may comprising continuously flushing the furnace chamber with the gas composition, i.e., having a continuous input and output of the gas composition. ln other words, in the method, the introducing of the gas composition may comprise continuously flushing S11 the furnace chamber 600 with the gas composition. The flushing is continued during heating of the furnace chamber 600. The heating is discussed in more detail below. The gas composition may be introduced at ambient temperatures, e.g. room temperature. The amount of gas composition needed for flushing may be in the order of ten times the volume of the furnace Chamber. The flushing may be done using relatively high flow rates to save time, such as and flow rate of 0.5 to 5 liters per minute. Other flow rates are also possible. Different valves may be set in such a way as to allow the gas composition to flow through the system. ln an example, the partial pressure of oxygen of an exhaust of the furnace chamber during flushing can be monitored. ln that case, a threshold value of the partial pressure of oxygen can be selected to indicate when the furnace chamber is ready to be heated.

The method further comprises heating S3 the furnace chamber 600 with the introduced gas composition to a temperature of at least 600 degrees Celsius. ln other words, the furnace chamber is configured to heat the substrate 430, 530 to at least 600 degrees Celsius while exposing a surface of the substrate 430, 530 with the gas composition. However, the furnace chamber 600 may be heated to more than 600 degrees Celsius to increase the reaction rate of the oxidization. ln contrast to e.g. providing a protective layer of aluminum oxide, which is a two-step process, the disclosed method only requires a single step. This is advantageous since the method is less complex and less labor intensive. ln particular, there is no need for vacuum furnace used in CVD, and there is no need to move the substrate from a CVD chamber to another separate oxidation furnace. Furthermore, the disclosed method does not require a line of sight to any of the surfaces that are oxidized, and is therefore suitable for oxidizing complex geometries. The water vapor is used to oxidize the chromium. According to the following reaction: 3H2O + 2Cl' = CF2Û3 + 3H2 Here the oxidant agent is water. Water vapor has been chosen as oxidant agent instead of oxygen. ln this process, the amount of oxygen should be kept as minimum as possible in order not to promote the formation of other undesirable oxides. lf oxygen is present in the gas environment around the substrate when the substrate is heated, that oxygen may oxidize other metals than chromium that may be present in the substrate. For example, if the substrate comprises iron, oxygen may form iron oxide. Most oxides other than the chromium oxide are generally undesired since the resulting oxidization layer will likely have reduced protection from hydrogen permeation. The hydrogen gas present in the gas composition reacts with any oxygen that potentially is present. More specifically, the hydrogen gas will react with the oxygen to form water. The presence of the hydrogen, therefore, improves the oxidation layer. ln general, when the temperature is more than 600 degrees Celsius, the amount of the hydrogen gas is preferably between 3 and 106 times larger than the amount of the water vapor. ln an example, the amount of the hydrogen gas is between 5 and 5 x 104 times larger than the amount of the water vapor, and wherein the temperature is 600-800 degrees Celsius. ln another example, the amount of the hydrogen gas is bet\Neen 3 and 6 x 104 times larger than the amount of the water vapor, and wherein the temperature is 800-1000 degrees Celsius. ln yet another example, the amount of the hydrogen gas is between 2 and 2 x 103 times larger than the amount of the water vapor, and wherein the temperature is 1000-1100 degrees Celsius. These ranges have been found sufficient to promote the formation of chromium oxide and suppress the formation of other undesirable oxides. Furthermore, the hydrogen will react with any oxygen that may have leaked into the furnace chamber. ln addition, with this ratios, the oxygen activity in the system is not enough for the formation of any unstable oxides during heating and cooling of the substrate. Oxygen activity is a measure of the chemical potential of oxygen in a system. lt indicates the relative abundance of oxygen in the gas mixture. At higher temperatures, the reaction rate would be higher. However, higher temperature impose more stresses on the sample and may deteriorate heat treatment of the substrate and thus reduces tensile strength and other mechanical properties. Having an amount of the hydrogen gas between 3 and 6 x 104 times larger than the amount of the water vapor, and having the temperature at 800-1000 degrees Celsius strikes a good balance. lf the substrate 430, 530 comprises nickel and iron, a volume ratio of hydrogen to water should preferably be kept within the range specified above depending on the temperature so that the formation of nickel oxide and iron oxide would be prevented. Consequently, the formed oxide will be mostly, preferably completely, be pure chromium oxide.

The amount of a substance in a gas mixture can expressed in the number of moles of that substance in the gas mixture. A mole fraction or molar fraction is the amount of a constituent (which is expressed in moles) over the total amount of all constituents in a mixture (which is also expressed in moles). The total amount, i.e., the sum of all mole fractions, of a gas mixture is equal to 1. The deal gas equation for a gas can be expressed as PV=nRT Where P is the pressure of the gas, V is the volume of the gas, n is the amount of substance of gas, R is the universal, gas constant. As an example, consider two gases, A (with partial volume VA and amount nA) and B (with partial volume VB and amount nB), at the same temperature and pressure in a gas mixture. lf the two gases are at the same temperature and pressure, the V/n ratio for each gas must be the same. So, the following ratio can be expressed E = E "A "B Which can be rewritten as E = "_A VB "B Thus, in a gas mixture, two constituents can be expressed as a ratio of their amount or ratio of their weight%, which is equal to a ratio of the partial volumes of the two constituents. According to the ideal gas law, such ratio of partial volumes is independent of pressure and temperature. Consequently, an amount of the hydrogen gas being between 5 and 5 x 104 times larger than the amount of the water vapor is equivalent to saying that the partial volume of the hydrogen gas is between 5 and 5 x 104 times larger than the partial volume of the water vapor when the gas composition is sea|ed in the furnace chamber.

The gas composition is preferably an oxygen-free composition. However, some oxygen may leak into the furnace chamber. Preferably, the maximum amount of oxygen leaking into the furnace chamber is less than the concentration of hydrogen gas divided by tvvo. Therefore, the method may further comprise waiting S2 a time period after the gas composition has first been introduced into the furnace chamber 600 and before the heating the furnace chamber 600. ln this way, any oxygen content in the chamber is reduced. The time period may e.g. be 1-12 hours. Such a time period has shown to reduce any oxygen content in the chamber to tolerable amounts.

Any inert gas may be used. However, argon gas is a suitable choice since it is inexpensive and readily available. Helium is another examples of a suitable inert gas. Any combination of different inert gases are also suitable.

The inert gas may constitute the remainder of the gas composition in addition to the hydrogen gas and the water vapor. Generally, the amount of inert gas may constitute 1-99% of the total amount of all constituents of the gas composition. However higher content of inert gas leads to a lower amount of water vapor available in the system, which decreases the reaction rate of chromia formation.

Preferably, however, the amount of inert gas constitutes 92-96% of the total amount of all constituents of the gas composition. The reason is that a lower amount of inert gas demands higher amount of hydrogen in the mixture which will raise safety issues. ln an example, the amount of hydrogen gas constitutes 0.1-7 % of the total amount of all constituents of the gas composition.

As mentioned, the gas composition comprises inert gas, hydrogen gas, and water vapor. For example, the amount of argon gas is 92-96% of the total amount of all constituents of the gas composition, wherein the gas composition further comprises or consists of hydrogen gas and water vapor with corresponding amount ratios as discussed above. According to another example, the amount of argon gas is 93.5-94.5%, the amount of hydrogen gas is 3.5-4.5%, 11 and the amount of water vapor is 0.8-1.2% of the total amount of all constituents of the gas composition.

The gas composition may consist of inert gas, hydrogen gas, and water vapor. As an example, the amount of argon is 94%, the amount of hydrogen gas is 5%, and the amount of water vapor is 1% of the total amount of all constituents of the gas composition.

Gas mixtures consisting of about 95% argon gas and about 5% hydrogen gas are readily available, which is an advantage. Thus, the gas composition herein may be based on such a gas mixture, where water vapor is subsequently added.

The method provides a chromium oxide layer that is continuous, and denser and more homogeneous compared to other known oxide layers. This is due to a relatively low oxygen activity of the environment around the surface of the substrate. As the result, other undesired oxides, such as iron oxide and nickel oxide if the substrate comprises iron and nickel, cannot form. The disclosed method therefore provides a protective layer with improved reduction of hydrogen permeation.

The chromium oxide layer obtained from the disclosed method only needs, in some example, to be in the order of 1 micron or so to obtain the desired protection against hydrogen permeation since the chromium oxide layer is continuous, dense, and homogeneous. The disclosed method is thus suitable for small structures such as conduits of a heat exchanger for a Stirling engine.

The disclosed method only requires heating the substrate to 600 degrees Celsius. The reason is that from a thermodynamic and kinetic point of view, temperatures 600 Celsius or more is required to achieve a continuous and homogeneous chromium oxide that could effectively protect the substrate against hydrogen permeation. However, the substrate is preferably heated to 900 degrees Celsius to increase the reaction rate. A temperature within 600 to around 900 degrees Celsius is a relatively low temperature compared to the temperatures used in other known oxidation processes. This relatively low temperature advantageously puts less strain on the substrate. Higher temperatures may e.g. degrade heat treatments (e.g. hardening) of the substrate. According to some embodiments, the furnace chamber 600 is heated to at most 1100 degrees Celsius. This way, the substrate is not unnecessarily strained. ln general, the upper temperature (i.e., maximum allowable temperature) may set by a requirement to not damage the substrate.

According to some examples, the furnace chamber 600 is held S31 at the temperature (e.g. 600 degrees Celsius) for at least 24 hours. This provides a thickness of chromium oxide that is about 0.5-10 microns. Such thickness has been found to provide good protection from hydrogen permeation. 12 The furnace Chamber 600 may be heated S32 to the temperature (e.g. 600 degrees Celsius) by a heating rate of 3-9 degrees Celsius per minute, and thereafter cooled by a cooling rate of 3-9 degrees Celsius per minute. Although the formation of the chromium oxide is not sensitive to the heating and cooling rates, rates of 3-9 degrees Celsius per minute have been found to be suitable choices that do not strain the substrate.

The introduced gas composition may provide a pressure of 0.5-2 bar in the furnace chamber 600. Preferably, the pressure is about 1 bar, which easy to achieve in a controlled environment.

The range of 0.5-2 bar is also relatively easy to achieve.

When it comes to metallic materials to be used at high temperatures (above 600 degrees Celsius), generally there are two types, i) alumina-forming alloys and ii) chromia-forming alloys. Such alloys are typically stainless steels or nickel-based alloys. ln the disclosed method, the substrate is a chromia-forming alloy. The substrate comprises an amount of chromium to form chromium oxide at high temperatures (above 600 degrees Celsius). Other materials in the substrate, such as nickel or other elements, do not play a role as long as the alloy has an amount of chromium in it to form chromium oxide at high temperatures. The minimally preferred chromium concentration for the formation of a protective chromium oxide layer depends on the type of alloy. For example, for ferritic stainless steel, the minimum chromium content should preferably be about 9-15% by weight. For nickel base alloys, the minimum chromium content should preferably be about 9-15% by weight. For austenitic steels, the minimum chromium content should preferably be about 11-17% by weight.

As an example, the substrate 430, 530 may comprise at least 58% nickel by weight and 20- 23% chromium by weight. ln this way, the substrate is suitable for the operational temperatures of conduits for a working fluid in a Stirling engine.

Preferably, the amount of aluminum in the substrate 430, 530 should be low enough so that the substrate cannot form an external aluminum oxide layer. As an example, the amount of aluminum is less than 2.5 % by weight.

The substrate 430, 530 may comprise the alloy according to the unified numbering system, UNS, designation N06625. This alloy is particularly suitable for the operational temperatures of conduits for a working fluid in a Stirling engine.

There is also disclosed herein a heat exchanger 110 for transferring heat between a source and a working fluid, as is shown in the example of Figure 2. The heat exchanger 1 10 comprises one or more conduits 214 for the working fluid. The one or more conduits comprise a substrate 430, 530 comprising chromium that is oxidized according to the disclosed method.

A substrate oxidized according to the disclosed method may be used in any system/apparatus in general that transports or contains hydrogen gas. For example, in applications involving a 13 hydrogen-based energy economy, the oxidized substrate may be used in hydrogen pipeline network, hydrogen storage tanks, hydrogen fuel cell vehicles (H FCVs), and high-temperature steam reforming and other techniques to create pure hydrogen. Furthermore, the oxidized substrate may be used in extreme high vacuum (XHV) applications, where the achievement of very low pressure normally is hindered by permanent hydrogen outgassing from all inner surfaces of stainless steel walls.

Figure 8 schematically illustrates, in terms of a number of functional units, the components of a control unit 800 according to embodiments of the discussions herein. Processing circuitry 810 is provided using any combination of one or more of a suitable central processing unit CPU, multiprocessor, microcontroller, digital signal processor DSP, etc., capable of executing software instructions stored in a computer program product, e.g. in the form of a storage medium 830. The processing circuitry 810 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

Particularly, the processing circuitry 810 is configured to cause the control unit 800 to perform a set of operations, or steps, such as the methods discussed in connection to Figure 7. For example, the storage medium 830 may store the set of operations, and the processing circuitry 810 may be configured to retrieve the set of operations from the storage medium 830 to cause the control unit 800 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuitry 810 is thereby arranged to execute methods as herein disclosed.

The storage medium 830 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid-state memory or even remotely mounted memory.

The control unit 800 may further comprise an interface 820 for communications with at least one external device. As such, the interface 820 may comprise one or more transmitters and receivers, comprising analogue and digital components and a suitable number of ports for wireline or wireless communication.

The processing circuitry 810 controls the general operation of the control unit 800, e.g., by sending data and control signals to the interface 820 and the storage medium 830, by receiving data and reports from the interface 820, and by retrieving data and instructions from the storage medium 830. Other components, as well as the related functionality, of the control unit 800 are omitted in order not to obscure the concepts presented herein.

There is also disclosed herein a computer readable medium carrying a computer program comprising program code means for performing the methods illustrated in Figure 7, when said 14 program product is run on a control unit. The computer readable medium and the code means may together form a computer program product.

Claims (16)

1. A method for forming a chromium oXide layer (420, 520) on a substrate (430, 530) comprising chromium, wherein the substrate (430, 530) is placed in a furnace Chamber (600), the method comprising: introducing (S1) a gas composition into the furnace chamber (600), the gas composition comprising an inert gas, hydrogen gas, and water vapor, and heating (S3) the furnace chamber (600) with the introduced gas composition to a temperature of at least 600 degrees Celsius.

2. The method according to claim 1, wherein the introducing of the gas composition comprises continuously flushing (S11) the furnace chamber (600) with the gas composition, wherein the flushing is continued during the heating of the furnace chamber (600).

3. The method according to claim 1 or 2, wherein the amount of the hydrogen gas is between 5 and 5 X 104 times larger than the amount of the water vapor, and wherein the temperature is 600-800 degrees Celsius.

4. The method according to claim 1, wherein the amount of the hydrogen gas is between 3 and 6 X 104times larger than the amount of the water vapor, and wherein the temperature is 800-1000 degrees Celsius.

5. The method according to claim 1, wherein the amount of the hydrogen gas is between 2 and 2 X 103times larger than the amount of the water vapor, and wherein the temperature is 1000-1100 degrees Celsius.

6. The method according to any previous claim, further comprising waiting (S2) a time period after the gas composition has been first introduced into the furnace chamber (600) and before the heating the furnace chamber (600).

7. The method according to claim 5, wherein the time period is 1-12 hours.

8. The method according to any previous claim, wherein the inert gas is argon gas.

9. The method according to any previous claim, wherein the amount of inert gas constitutes 1-99% of the total amount of all constituents of the gas composition.

10. The method according to any previous claim, wherein the amount of hydrogen gas constitutes 0.1-7 % of the total amount of all constituents of the gas composition.

11. The method according to any previous claim, wherein the introduced gas composition provides a pressure of 0.5-2 bar in the furnace chamber (600).

12. The method according to any previous claim, where the substrate (430, 530) comprises at least 58% nickel by weight and 20-23% chromium by weight.

13. The method according to any previous claim, wherein the substrate (430, 530) comprises the alloy according to the unified numbering system, UNS, designation N

14. The method according to any previous claim, wherein the furnace chamber (600) is held (S31) at the temperature for at least 24 hours.

15. The method according to any previous claim, wherein the furnace chamber (600) is heated (S32) to the temperature by a heating rate of 3-9 degrees Celsius per minute, and thereafter cooled by a cooling rate of 3-9 degrees Celsius per minute.

16. A heat exchanger (110) for transferring heat between a source and a Working fluid, wherein the heat exchanger comprises one or more conduits (214) for the working fluid, wherein the one or more conduits comprise a substrate (430, 530) oxidized according to the method of any of claims 1-15.