DE102021209592A1 - Process for reducing the electrical contact resistance of components made of corrosion-resistant steels, component made of corrosion-resistant material - Google Patents

Abstract

The invention relates to a method for reducing the electrical contact resistance of components made of corrosion-resistant steels, in particular bipolar plates or porous bodies, in which a surface of the component is subjected to a thermochemical heat treatment, comprising the steps a) activating the surface, for example by pre-oxidizing the surface with synthetic air,- by ions generated in the plasma, or- by pickling and/or acids, andb) enriching the surface with nitrogen, in particular by nitrating or nitrocarburizing the surface, with the penetration depth of the nitrogen diffusing into the surface being controlled by the process parameters temperature (T) and/or time (t) depending on the base material of the component.The invention also relates to a component made of corrosion-resistant steel, in particular a bipolar plate or a porous body.

Description

The invention relates to a method for reducing the electrical contact resistance of components made of corrosion-resistant steel. The components can in particular be bipolar plates or porous bodies for fuel cells, preferably for PEM (polymer electrolyte membrane) fuel cells. Furthermore, a component, in particular a bipolar plate or a porous body, made of corrosion-resistant steel is proposed.

State of the art

Surface modification allows the surface properties of components to be specifically adjusted for the respective component function. Depending on the material and the requirements for the component, for example requirements for wear behavior and/or electrical or thermal conductivity, different methods are used for surface treatment.

For example, bipolar plates that are used in fuel cells and are usually made of thin-walled corrosion-resistant steels must have good electrical conductivity on their outside to ensure good electrical contact with the so-called membrane-electrode assembly during fuel cell operation. However, since corrosion-resistant steels form a passive film on the surface, which is responsible for the good corrosion properties on the one hand and is associated with high electrical contact resistance on the other hand, low contact resistances (< 10 mΩcm ² ) with very good corrosion properties at the same time are usually achieved by means of a conductive coating . Gold coatings are known, for example, which are produced by vapor deposition using physical vapor deposition (PVD) and are very durable over the long term, but at the same time are also very expensive. Other coating solutions for bipolar plate materials investigated to date have either not proven to be sufficiently long-term robust and/or have not been able to achieve the required cost targets either. The surface coating of porous materials or bodies, which are used in fuel cells as an alternative to the structured bipolar plate on the water side, can only be implemented with great effort. Investigations into materials in combination with various coating processes for use in fuel cells are therefore the subject of current research.

Thermochemical heat treatment processes for the surface modification of components made of corrosion-resistant steels represent a promising alternative. In order to reduce the electrical contact resistance of bipolar plates made of corrosion-resistant austenitic steels, thermochemical processes such as nitriding or nitrocarburizing have already been investigated. A reduction in the contact resistance was demonstrated as a result of the formation of a surface layer of "expanded austenite" enriched with nitrogen or with nitrogen and carbon. In industrial practice, nitrocarburizing processes with a treatment temperature between 400° C. and 480° C. and process durations of up to 48 hours are used to produce such a layer.

The present invention is concerned with the task of specifying a robust and at the same time automatable method that enables the formation of an intrinsic surface layer for reducing the electrical contact resistance for components made of corrosion-resistant steels, in particular for bipolar plates or porous bodies.

To solve the problem, the method with the features of claim 1 is specified. Advantageous embodiments can be found in the dependent claims. In addition, a component made of corrosion-resistant steel, in particular a bipolar plate or a porous body, is specified.

Disclosure of Invention

1. a) Aktivieren der Oberfläche, beispielsweise
   • - durch Voroxidieren der Oberfläche mit synthetischer Luft,
   • - durch im Plasma erzeugte Ionen oder
   • - durch Beizen und/oder Säuren,
   und
2. b) Anreichern der Oberfläche mit Stickstoff, insbesondere durch Nitrieren oder Nitrocarburieren der Oberfläche, wobei die Eindringtiefe des in die Oberfläche diffundierenden Stickstoffs über die Prozessparameter Temperatur T und/oder Zeit t in Abhängigkeit vom Grundwerkstoff des Bauteils eingestellt wird.

In the proposed method for reducing the electrical contact resistance of components made of corrosion-resistant steels, in particular of bipolar plates or porous bodies, a surface of the component is subjected to a thermochemical heat treatment. The procedure includes the steps:

1. a) Activating the surface, for example
   • - by pre-oxidizing the surface with synthetic air,
   • - by ions generated in the plasma or
   • - by pickling and/or acids,
   and
2. b) Enriching the surface with nitrogen, in particular by nitriding or nitrocarburizing the surface, the penetration depth of the nitrogen diffusing into the surface being adjusted via the process parameters temperature T and/or time t depending on the base material of the component.

In the proposed method, the actual thermochemical heat treatment in step b) is preceded by a step a), which of the Activation of the surface is used. As a result of the pretreatment of the component surface in step a), the nitrogen can penetrate the surface more easily in the subsequent step b). This has a particularly advantageous effect on the process parameter of the time t.

In step b), an intrinsic surface layer is then formed by nitriding or nitrocarburizing the component surface to reduce the contact resistance of the component. A material-specific optimum for the contact resistance can be set on the basis of defined process parameters, in particular due to a temperature-time regime dependent on the base material of the component.

Atomic nitrogen N ₁ diffuses during nitriding, atomic nitrogen N ₁ and atomic carbon C ₁ diffuse into the component surface during nitrocarburizing. The process parameters temperature T and time t determine the amount of nitrogen or nitrogen and carbon that penetrates the component surface to a certain depth. Depending on the process control and base material of the component, different layer thicknesses of an intrinsic surface layer made of "expanded austenite (S-phase)" can be adjusted in this way. The base material is still present in the underlying layers, in which there is no accumulation of nitrogen or nitrogen and carbon. The properties of the intrinsic surface layer can thus be determined with the aid of the process parameters temperature T and time t. The formation of a Cr ₂ N and/or CrN phase should be avoided, since this reduces or reduce the corrosion resistance of the component.

Compared to a conventional coating method, the proposed method has the advantage that the surface of the component itself is modified so that an “intrinsic” surface layer is formed. This offers a high degree of flexibility for subsequent process steps and is very robust. The process can also be used on porous materials such as metallic foams.

In a development of the invention, it is proposed that the process parameter time t be less than 30 minutes, regardless of the respective base material of the component. This means that in all cases in step b) a process duration of less than 30 minutes is observed. Correspondingly, step b) can be referred to as short-term nitrating or short-term nitrocarburizing. The short process times are also associated with minimal distortion of the components.

The further process parameter temperature T is set depending on the respective base material of the component, since the component - depending on the area of application - can be made of different corrosion-resistant steels.

In the case of a base material from the group of ferrites or martensites, the temperature T is preferably 350-650.degree. C., more preferably 400-520.degree.

In the case of a base material from the austenite group, the temperature T is preferably 350-650.degree. C., more preferably 510-590.degree.

In the case of a duplex base material, the temperature T is preferably 350-650.degree. C., more preferably 400-590.degree.

In this way, the contact resistance can be optimized for each material in such a way that the corrosion resistance of the component is retained. In particular, intrinsic surface layers can be formed to reduce the contact resistance, which have a thickness of about 3-20 μm and/or a contact resistance below 10 mΩcm ² .

When carrying out the method according to the invention, a continuous furnace with two chambers is advantageously used, step a) being carried out in a first chamber and step b) being carried out in a second chamber. In this way, large batches of components can be nitrided or nitrocarburized in short cycle times, which leads to a considerable cost advantage, especially compared to the coating processes usually used in the production of bipolar plates.

Alternatively or additionally, it is proposed that at least step b) of the method is carried out in a high-pressure vacuum furnace.

The proposed component made of corrosion-resistant steel, which can in particular be a bipolar plate or a porous body, has an intrinsic surface layer to reduce the contact resistance, which has been produced by thermochemical heat treatment, preferably using one of the preceding methods. Accordingly, the intrinsic surface layer has preferably been produced by nitriding or nitrocarburizing. This means that atomic nitrogen N ₁ or atomic nitrogen N ₁ and atomic carbon C ₁ is or are embedded in the surface and modify it in such a way that an intrinsic surface layer is formed. In contrast to a coating, the intrinsic surface layer is formed by the base material of the component itself.

The use of the above-described method according to the invention for producing the intrinsic surface layer enables short process times with at the same time minimal distortion of the component. Furthermore, it can also be used with porous materials such as metallic foams.

The intrinsic surface layer of the proposed component is preferably a few micrometers thick, in particular 3-20 μm. Accordingly, the component can also be a thin metal sheet for forming a bipolar plate.

The electrical contact resistance of the intrinsic surface layer is preferably <10 mΩcm ² . The component thus has a sufficiently low contact resistance so that it can be used in a fuel cell, for example as a bipolar plate or as a porous body.

• 1 ein Diagramm (schematisch) zur Darstellung der Stickstoff- und Kohlenstoffmenge bei definierter Temperatur T und Zeit t,
• 2 ein Phasendiagramm,
• 3 ein Diagramm zur Darstellung des Verlaufs des Kontaktwiderstands über die Zeit t und
• 4 ein Diagramm zur Darstellung des Verlaufs des Gitterparameters über den Gehalt an interstitiell eingelagerten Stickstoff- und Kohlenstoffatomen sowie dessen Auswirkung auf die magnetische Ordnung.

The invention is explained in more detail below with reference to the accompanying drawings. These show:

• 1 a diagram (schematic) to show the amount of nitrogen and carbon at a defined temperature T and time t,
• 2 a phase diagram,
• 3 a diagram showing the course of the contact resistance over time t and
• 4 a diagram showing the course of the lattice parameter over the content of interstitially embedded nitrogen and carbon atoms and its effect on the magnetic order.

Detailed description of the drawings

The method according to the invention for reducing the electrical contact resistance of components made of corrosion-resistant steels provides for a thermochemical heat treatment by means of nitriding or nitrocarburizing of the surface of the component. Before nitriding or nitrocarburizing in step b) of the process, the surface is activated in step a). The activation of the surface facilitates the penetration of nitrogen or nitrogen and carbon during the nitriding or nitrocarburizing in step b).

As exemplified in the 1 shown, the method is carried out in a nitrogen or nitrogen and carbon-emitting atmosphere at a temperature T ₁ in a defined time t ₁ . Atomic nitrogen N ₁ or atomic nitrogen N ₁ and atomic carbon C ₁ diffuse into the surface, so that an intrinsic surface layer is created with a nitrogen content or nitrogen and carbon content that decreases with increasing depth into the interior of the component. The parameters temperature T and time t determine the amount of nitrogen or the amount of nitrogen and carbon and thus the properties of the intrinsic surface layer made of "expanded austenite".

The phase diagram of 2 shows the relationship between the nitrogen content N, the temperature T and the time t as well as the areas in which a Cr ₂ N and/or CrN phase is formed, which should be avoided since this reduces the corrosion resistance of the component. Depending on the process control and base material of the component, different layer thicknesses of "expanded austenite" can be set.

The process parameters temperature T and time t are selected depending on the respective base material of the component, so that the optimum for the desired reduction in electrical contact resistance can be achieved for each material. The time t, which refers to the duration of the process, is always less than 30 minutes.

3 shows an example of the parameter field to be selected for an austenitic stainless steel and the course of the contact resistance resulting from the set parameters.

During the diffusion of nitrogen, the N atoms are stored in interstitial lattice sites, which results in a distortion of the original lattice and - as can be seen from the 4 apparent - is accompanied by a change in the lattice parameters. Depending on the amount of embedded N atoms or N and C atoms and thus depending on the size of the resulting lattice parameter Y _N or Y _N and Yc, this can lead to a change in the magnetic order from originally paramagnetic to ferromagnetic.

Optimum electrical surface properties with minimum contact resistance at the same time are achieved with targeted process management using short-time nitriding or short-time nitrocarburizing by providing a closed intrinsic surface layer of "expanded austenite", whose N content or N and C content and thus that caused by the The state of distortion in the surface caused by the interstitially embedded N atoms or N and C atoms is so great that the material is just about ferromagnetic. The lowest contact resistance is then to be expected for the lattice parameter Y _{N,ferro_max} . If the surface layer of "expanded austenite" is not closed and the N content or N and C content is too low, or the layer is closed but the N content or the N and C content is too high, this is the case - as exemplified in the 4 - enters a paramagnetic state, resulting in significantly poorer contact resistance values.

Claims (10)

Method for reducing the electrical contact resistance of components made of corrosion-resistant steels, in particular of bipolar plates or porous bodies, in which a surface of the component is subjected to a thermochemical heat treatment, comprising the steps a) Activating the surface, for example - by pre-oxidizing the surface with synthetic air, - by ions generated in the plasma or - by pickling and/or acids, and b) Enriching the surface with nitrogen, in particular by nitriding or nitrocarburizing the surface, the penetration depth of the nitrogen diffusing into the surface being set via the process parameters temperature (T) and/or time (t) depending on the base material of the component. procedure after claim 1 , characterized in that the process parameter time (t) is less than 30 minutes. procedure after claim 1 or 2 , characterized in that with a base material from the group of ferrites and martensites, the temperature (T) is 350-650 °C, preferably 400-520 °C. procedure after claim 1 or 2 , characterized in that in the case of a base material from the austenite group, the temperature (T) is 350-650 °C, preferably 510-590 °C. procedure after claim 1 or 2 , characterized in that with a duplex base material the temperature (T) is 350-650 °C, preferably 400-590 °C. Method according to one of the preceding claims, characterized in that when carrying out the method a continuous furnace with two chambers is used, step a) being carried out in a first chamber and step b) being carried out in a second chamber. Method according to one of the preceding claims, characterized in that at least step b) is carried out in a high-pressure vacuum furnace. Component made of corrosion-resistant steel, in particular a bipolar plate or porous body, which has an intrinsic surface layer to reduce the contact resistance, which has been produced by thermochemical heat treatment, preferably using one of the preceding methods. component after claim 8 , characterized in that the intrinsic surface layer is 3-20 µm thick. component after claim 8 or 9 , characterized in that the electrical contact resistance of the intrinsic surface layer is < 10 mΩcm ² .

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