Multi-layer coating

Abstract

A multi-layer coating for protection of metals and alloys against oxidation at high temperatures is provided. The invention utilizes a multi-layer ceramic coating on metals or alloys for increased oxidation-resistance, comprising at least two layers, wherein the first layer (3) and the second layer (4) both comprise an oxide, and wherein the first layer (3) has a tracer diffusion coefficient for cations Mm+, where M is the scale forming element of the alloy, and the second layer (4) has a tracer diffusion coefficient for oxygen ions O2− satisfying the following formula:∫lnp(O2)inlnp(O2)ex(DO+m2DM)lnp(O2)<5·10-13cm2 / swherein p(O2)in, p(O2)ex, DM, and DO are as defined herein. The coating may be used in high temperature devices, particularly for coating interconnect materials in solid oxide electrolytic devices, including solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs).

Description

FIELD OF THE INVENTION[0001]The present invention relates to a multi-layer coating for the protection of metals and metal alloys against oxidation at high temperatures. The coating may be used in high temperature devices, particularly for coating interconnect materials in solid oxide electrolytic devices, including solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs).BACKGROUND ART[0002]Applications of solid oxide electrolytic devices include the power generation by SOFCs and production of fuel gases by SOECs. In both devices, SOFCs and SOECs, individual cells are stacked together, with interconnect plates separating the cells, so as to obtain a higher energy output by electricity or by fuel gases, respectively. The interconnect plates separate the fuel gas from the oxidant, which is typically air, and furthermore function as the electrical connection between individual cells in a stack.[0003]Hence, the requirements for an interconnect plate include long term du...

AI Technical Summary

Benefits of technology

This patent aims to solve problems with existing coatings for metal surfaces in high-temperature applications such as SOECs and SOFCs. The invention provides a long-term durable coating that ensures the longevity of metallic interconnects. Additionally, the patent also introduces a method for producing this coating.

Problems solved by technology

However, during aging under operation conditions, oxides form on both sides of the metallic interconnect.

The growth of said oxides disadvantageously leads to an increased electrical resistance across the interconnect plate and, thus, increased power loss.

The growth rate of chromia at operation temperatures >800° C. is however too high, which in turn results in the electrical resistance across the interconnect plate reaching unacceptable high values due to the low conductivity of chromia.

A further problem when using chromia-forming alloys as interconnects is the evaporation of chromium containing oxides and oxy-hydroxides on the air side of the interconnect during operation.

Said evaporation leads to deposition of chromium-containing oxides at the air-electrode-electrolyte interface, which decreases the electrode performance in the long term.

However, the chromium containing spinel still evaporates chromium containing species, and thus a sufficient protection cannot be realized.

Moreover, Cr-diffusion is in fact faster in the spinel than in the chromia and thus the formation of a dublex scale leads to an increased rate of the corrosion, thereby reducing the overall lifetime of the device.

The proposed coating, however, does not stop chromium containing species from diffusing further outwards from the alloy.

Therefore, metallic coatings forming a chromium containing oxide do not act as an effective diffusion barrier towards chromium diffusion.

Instead, the metal coating merely impedes the chromium diffusion during the initial stages of the oxidation.

Furthermore, the metallic coating does not solve the problem regarding chromium poisoning.

However, the proposed coating does also not act as a sufficient diffusion barrier for chromium from the interconnect.

Furthermore, Cr-poisoning will occur during long term operation, since the formed spinel becomes itself chromium rich and the respective oxides evaporate therefrom into the air-electrode-electrolyte interface.

However, this coating nevertheless suffers from the above described problems, since the chromium transport from the alloy is not entirely stopped and the reaction layer, although being initially free from chromium, will eventually contain chromium.

Thus, Cr-poisoning and increasing electrical resistance will be the result during long term operation.

Said coating is, thus, not suitable for applications requiring a very long durability of SOFC and SOEC stacks.

However, platinum is undesirably expensive, making a commercialization of SOFC and SOEC technology cumbersome.

Said layer is however insufficient to prevent the further growth of the oxide layer during operation.

Furthermore, if a chromium-containing metallic material is employed either as the metallic base material or as a component of the metallic layer, chromium-poisoning will still occur.

However, the formed oxide layer is insufficient to prevent the further growth of the oxide layer during operation of the fuel component.

If furthermore a chromium-containing metallic material is employed as the metallic base material or metallic coating layer, chromium-poisoning will still occur.

The long term durability of the interconnects described in the prior art up to date is not sufficient for many applications.

The use of specifically designed alloys for interconnect materials does not eliminate the problem of oxide growth on the interconnect, considerably resulting in an insufficient life time when the interconnects are used in solid oxide cells or the like.

Moreover, if chromium-containing metallic materials are employed, which are so far the most preferred materials for interconnects, chromium poisoning of the electrode will still occur; the use of the so far proposed coatings on said alloys cannot eliminate the undesired oxide growth, and does not prevent chromium poisoning.

Further, the use of expensive metals, such as platinum, although leading to better results, is not feasible for the commercial potential of solid state devices, such as SOFCs and SOECs, due to the high price.

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