System and process for aluminization of metal-containing substrates

Abstract

A system and method are detailed for aluminizing surfaces of metallic substrates, parts, and components with a protective alumina layer in-situ. Aluminum (Al) foil sandwiched between the metallic components and a refractory material when heated in an oxidizing gas under a compression load at a selected temperature forms the protective alumina coating on the surface of the metallic components. The alumina coating minimizes evaporation of volatile metals from the metallic substrates, parts, and components in assembled devices that can degrade performance during operation at high temperature.

Claims

1. A system for aluminizing a surface of metal-containing substrates, the system comprises: a compression assembly comprising at least one compression component configured to compress an aluminum foil of a selected thickness between one or more metal-containing substrates and a refractory material of a selected thickness under a selected compression load in an oxidizing gas at a selected temperature for a time sufficient to form an aluminum oxide coating layer on the surface of the one or more metal-containing substrates.

2. The system of claim 1, wherein the at least one compression component is a compression plate comprising a high-temperature refractory ceramic or a refractory metal that delivers the compression load uniformly through the aluminum foil to the one or more metal-containing substrates.

3. The system of claim 1, wherein the at least one compression component couples with a compression disc or a compression spring to deliver the compression load to the one or more metal-containing substrates.

4. The system of claim 1, wherein the refractory material is a high-temperature ceramic or metal.

5. The system of claim 1, wherein the at least one compression component is configured to apply a compression load at the selected temperature of greater than or equal to about 6800 Newtons/m.sup.2.

6. The system of claim 1, wherein the at least one compression component is configured to apply the compression load at the selected temperature for a time greater than or equal to about 5 minutes.

7. The system of claim 1, wherein the selected temperature is a temperature about 900° C.

8. The system of claim 1, wherein the selected temperature is a temperature between about 660° C. and about 1200° C.

9. The system of claim 1, wherein the aluminum foil includes a thickness selected between about 0.001 mm and about 0.5 mm.

10. The system of claim 1, wherein the aluminum foil includes a thickness less than about 25 μm.

11. The system of claim 1, wherein the refractory material is a mica sheet.

12. The system of claim 1, wherein the refractory material includes a thickness of about 0.3 mm.

13. The system of claim 1, wherein the system is configured to aluminize one or more metal-containing substrates prior to assembly in a device.

14. The system of claim 1, wherein the metal-containing substrate includes a flat surface.

15. The system of claim 1, wherein the metal-containing substrate includes a surface that is other than a flat surface.

16. The system of claim 1, wherein the metal-containing substrates are selected from interconnects, cell frames, or interconnects and cell frames of an electrochemical device.

17. The system of claim 1, wherein the system is configured to aluminize one or more metal-containing substrates within an assembled device in-situ.

18. The system of claim 1, wherein the metal-containing substrates are present in an assembled device selected from a solid oxide fuel cell, a solid oxide electrolyzer cell, or an oxygen membrane.

19. The system of claim 1, wherein the metal-containing substrates are within a stack assembly of an electrochemical device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a system for aluminizing metal-containing parts and components, according to one embodiment of the present invention.

(2) FIG. 2 is a cross-sectional view of a system for aluminizing metal-containing substrates of an assembled device in-situ, according to an embodiment of the present invention.

(3) FIG. 3 is an electromicrograph showing an alumina coating layer formed on a surface of a metal-containing substrate in accordance with the present invention.

(4) FIG. 4 is an electromicrograph showing an alumina coating layer formed on a surface of another metal-containing substrate in accordance with the present invention.

DETAILED DESCRIPTION

(5) A system and method are detailed for coating metallic parts and metal-containing substrates with a protective alumina layer that eliminates need for post-coating treatments including, e.g., post-coating firing or other post treatment steps. The process is simple, efficient, and economical compared with conventional aluminization processes. The following description includes a preferred best mode of one embodiment of the present invention. While the invention is susceptible of various modifications and alternative constructions, it should be understood, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims. Therefore the present description should be seen as illustrative and not limiting.

(6) FIG. 1 shows a compression assembly 100 for aluminizing metal-containing parts and components. “Aluminization” as the term is used herein means to coat metallic surfaces of metal-containing substrates with an aluminum oxide (alumina) coating. Compression assembly 100 includes aluminum foil 4 of a selected thickness that is sandwiched between a metal-containing substrate 2 composed of a selected metal or metal alloy, and a refractory 6 composed, e.g., of a sheet of mica. Thickness of mica sheet 6 and aluminum foil 4 are not limited. In some embodiments, substrate 2 may include, or be composed of, e.g., ferritic stainless steel (e.g., grade AISI441 ferritic stainless steel) or another suitable metal. Compression assembly 100 may include a compression plate 8 that maintains contact between aluminum foil 4 and the surface of substrate 2 and further delivers a compression load 10 directly and uniformly through aluminum foil 4 that facilitates aluminization of the surface of substrate 2. The aluminum (Al) foil may include selected thicknesses. In various embodiments, the aluminum foil may include a thickness of between about 0.001 mm and about 0.5 mm. In some embodiments, the aluminum foil may include a thickness of less than about 25 μm (microns).

(7) Compression assembly 100 can deliver a compression load required for aluminization of the components in electrochemical devices (e.g., SOFC devices) and simultaneously serve as a hybrid compression seal for operation of the electrochemical devices. In some embodiments, the compression load may be greater than or equal to about 6800 Newtons/m.sup.2. Compression loads may be provided by components in the electrochemical device assembly such as high-temperature compression discs, or via such devices as external high-load compression springs. However, all compression devices as will be implemented by those of ordinary skill in the art in view of the disclosure are within the scope of the invention. No limitations are intended. In some embodiments, the compression load may be applied for a time greater than or equal to about 5 minutes at the selected temperature.

(8) In an exemplary embodiment, the mica sheet 6 may include a thickness of about 0.012 inches (0.3 mm). But, thickness dimensions are not intended to be limited. When assembled into the electrochemical device assembly, mica sheet 6 may be heated at a temperature of, e.g., 550° C. for a time sufficient to burn off organic binders in the mica sheet that may be detrimental to operation of the electrochemical device in which it is introduced. Times for removing binders are not limited. Typical times for removing organic binders may be up to about 2 hours, but times and temperatures are not intended to be limited. Alternatively, mica sheets free of organic binders may be employed.

(9) Compression assembly 100 may be heated, e.g., in a furnace or heater 12 in an oxidative gas such as air at the selected aluminization temperature for a time sufficient to aluminize the surface of the substrate 2. Temperatures for aluminization are not limited. In some embodiments, aluminization temperature may be, e.g., about. 900° C. In various embodiments, aluminization temperature may be between about 660° C. and about 1000° C. In some embodiments, heating the substrate (including parts or components) to the aluminization temperature may include a heating rate of between about 1° C. per minute and about 10° C. per minute.

(10) Times to effect aluminization are also not limited. In some embodiments, aluminization can be completed by heating substrate for a time greater than or equal to about 5 minutes at the selected aluminization temperature. In some embodiments, aluminization can be completed by heating the substrate at the aluminization temperature for about 2 hours on average.

(11) Aluminum oxide coatings on aluminized metallic substrates obtained in concert with the present invention minimize release of volatile metal species including chromium (Cr) from metallic substrates in assembled electrochemical devices during operation at elevated temperatures that can poison and degrade performance in these electrochemical devices and cells.

In-situ Aluminization in Assembled Devices

(12) The present invention can be used to aluminize metal-containing substrates in assembled devices in-situ including metallic parts that contain stainless steel or other metal alloys comprising chromium (Cr) in high temperature electrochemical devices. Devices are not limited. Exemplary devices include high temperature electrochemical devices. High temperature electrochemical devices include, but are not limited to, e.g., solid oxide fuel cells (SOFCs), solid oxide electrolyzer cells (SOECs), oxygen membranes, and other devices. While aluminization of flat substrates, parts, and components is described hereafter, the invention is not limited to flat substrates, parts, and components only. FIG. 2 shows a cross-sectional view of a compression assembly 100 configured for aluminizing metallic components of an exemplary planar solid oxide fuel cell (SOFC) 50 in-situ, in accordance with the present invention. The instant configuration also thermalizes device components in preparation for thermal cycling during operation of SOFC 50. As shown in the figure, aluminum (Al) foil 4 may be applied to surfaces of cathode interconnect 22, anode interconnect 24, window (stack) frame 26, and/or other metallic components of SOFC 50. In the instant embodiment, mica paper 6 may be positioned between respective sheets or layers of aluminum foil 4 positioned on the cathode side of ceramic (electrochemical) cell 28 to maintain good contact between the layers of aluminum foil 4 on an inner side of interconnect 22 and window frame 26 to maintain good contact between the layers of aluminum foil 4, and on a side of interconnect 24 and window frame 26 on the anode side of ceramic (electrochemical) cell 28, respectively. Compression (load carrying) plates 8 may be positioned on an exterior side of cathode interconnect 22 and an exterior side of anode interconnect 24 that delivers compression load 10 to respective sides of electrochemical device 50 mounted in compression assembly 100. Metallic components of electrochemical device 50 may be aluminized by introducing an oxidizing gas through a gas manifold 30 that is coupled to compression block 8 on the cathode interconnect 22 side of ceramic cell 28 and/or on the anode interconnect 24 side of ceramic cell 28. Gas manifold 30 delivers oxidizing gas to metallic components and aluminum foil 4 in electrochemical (e.g., SOFC) device 50 during heating of compression assembly 100 in furnace 12 at conditions that form the protective alumina coating layer on the surfaces of components in the electrochemical device 50.

(13) FIG. 3 is a scanning electron micrograph (SEM) showing an exemplary alumina coating (protection) layer 14 formed on an exemplary alloy (AISI441) substrate 2 in accordance with the present invention. In the figure, three spots (Spot #1-1) 60, (Spot #2-1) 62, (Spot #3-1) 64 of substrate 2 were assayed and subjected to chemical analysis by Energy Dispersion Spectroscopy (EDS). TABLE 1 lists results of the chemical analyses.

(14) TABLE-US-00001 TABLE 1 Chemical Analysis Results for Selected Spots of an aluminized surface analyzed by Energy Dispersion Spectroscopy (EDS) in accordance with the present invention. Spot #1-1 Spot #2-1 Spot #3-1 Element (Atom %) (Atom %) (Atom %) O 59.73 62.03  57.72  Mg 0.73 — — Al 29.14 32.64  37.46  Si 5.27 0.52 0.50 K 0.15 — — Cr 0.74 1.24 1.14 Fe 4.24 3.57 3.19

(15) FIG. 4 is a scanning electron micrograph (SEM) showing an exemplary alumina protection layer 12 formed on another (AISI441) alloy substrate 2 in accordance with the present invention. Three spots (Spot #2-1) 66, (Spot #2-2) 68, (Spot #3-2) 70 of substrate 2 shown in the figure were assayed and subjected to chemical analysis by Energy Dispersion Spectroscopy (EDS). TABLE 2 lists results.

(16) TABLE-US-00002 TABLE 2 Chemical Analysis Results for Selected Spots of an aluminized surface analyzed by Energy Dispersion Spectroscopy (EDS) in accordance with the present invention. Spot #1-2 Spot #2-2 Spot #3-2 Element (Atom %) (Atom %) (Atom %) O 54.88 55.61 44.51 Al 37.55 34.58 32.97 Si 0.62 0.39 0.56 Nb — — 0.36 Cr 2.38 2.00 3.84 Fe 4.57 7.13 17.76

(17) Spot analyses show that the protective alumina layer is formed on the surface of the metal substrates. Other metal species (e.g., Cr) from the metal substrate are also present. In general, the alumina protection layers exhibit various morphologies and different penetration depths on the surface of the metal (e.g., AISI441) substrates. The alumina protection layer adheres to the metal substrate and does not spall during thermal cycling and operation.

(18) The present invention can also be used to aluminize metal substrates, parts, and components in other than in-situ applications, e.g., where continuous and protective alumina coatings are needed. Metallic substrates, parts, and components can be of any shape as long as aluminum foil can be applied to the surfaces of interest and a compression load can be delivered through an inert medium such as mica paper. Sources of aluminum can include foils as described herein, or layers deposited by various processes such as, e.g., electroplating.

Applications

(19) The present invention has numerous applications in the production of high-temperature electrochemical devices, as well as applications in manufacturing of coated items with uses in a wide range of high-tech industrial manufacturing processes.

EXAMPLE

(20) The following Example provides a further understanding of the invention.

Example 1

In-situ Aluminization of Alloy Substrates

(21) The compression assembly of FIG. 1 was used. Aluminum foil of a thickness less than 25 um (microns) was sandwiched between a flat metal plate composed of, e.g., AISI441 steel (dimensions: 2″×2″×0.04″) and a sheet of mica (e.g., 0.012″ thickness). The compression assembly was first fired in air to 550° C. for 2 hours to burn off organic binders in the mica sheet. Then, the compression assembly was heated to an elevated temperature of 900° C. for 2 hours to aluminize the metal substrate.

(22) While exemplary embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its true scope and broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the scope of the invention.