Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Patent
1995-07-10
1996-10-15
Yee, Deborah
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
420 62, 148327, 148325, 148609, C22C 3806, C22C 3840, C21D 802
Patent
active
055651674
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a stainless steel excellent in fused-salt corrosion resistance and more particularly to a stainless steel for the separator of a fused carbonate fuel cell.
BACKGROUND ART
The material used for the separator of a fused carbonate fuel cell is required to exhibit a degree of corrosion resistance sufficient to be safe from fused-salt corrosion by the fused carbonate electrolyte of the cell. The practice has therefore been to use a high-grade stainless steel such as SUS316L or SUS310S as the material for the separator of a fused carbonate fuel cell.
Since the fused-salt corrosion resistance of a stainless steel is known to be generally proportional to its Cr content, high Cr content stainless steels such as SUS310S are the most frequently used. Stainless steels with high Cr content also include a large amount of Ni for stabilizing the austenite phase.
Such steels have also been used as separator material after being imparted with high corrosion resistance by coating with alumina.
Technologies for improving the resistance of stainless steels to corrosion by fused carbonate are taught by, for example, JPA-62-294153 (Separator Material for Fuel Cells) and JPA-1-252757 (Fe-base Alloy Excellent in Fused-Carbonate Corrosion Resistance). The first of these, JPA-62-294153, teaches improvement of fused-carbonate corrosion resistance by addition of Al to the steel.
When a large amount of Al is added to a stainless steel for increasing resistance to fused salt, however, the hot workability of the steel is markedly degraded and large cracks tend to occur during hot rolling for production of steel sheet. Since this restricts the size of the steel sheet that can be produced, lower product yield and higher production cost are unavoidable. For coping with this hot workability problem of austenitic stainless steel containing Al, JPB-55-43498 teaches a method of improving the hot workability of a stainless steel such as by causing precipitation of a small amount of .delta. ferrite during solidification or by addition of a rare earth element such as La or Ce, while JPA-60-262945 proposes a method of preventing cracking by conducting hot rolling at a temperature in the range of 1000.degree.-1200.degree. C.
OBJECT OF THE INVENTION
Since the corrosive activity of the fused carbonate of a fuel cell is extremely strong, even use of a high-grade stainless steel such as SUS316L or SUS310S as the separator material does not provide adequate corrosion resistance. The problems of high raw material cost and poor hot workability are particularly severe in the case of SUS310S owing to its high Cr and Ni content. From the viewpoint of product cost, it is important to reduce Cr and Ni content, especially Ni content, as far as possible. In this regard, JPA-62-294153 (Separator Material for Fuel Cells), with its basic 25% Cr--20% Ni composition, is high in raw material cost and also not very good in workability.
The method of improving the corrosion resistance of the separator material by coating the surface of the stainless steel with alumina has the drawback of making the overall production process complex because it requires additional steps such as for coating and heat treatment.
The Fe-base alloy with excellent fused-carbonate corrosion resistance described in JPA-1-252757 has to have a particularly low Si content. Since this makes it necessary to take special precautions during steel production, the production cost becomes high.
While JPB-55-43498 and JPA-60-262945 disclose Al-containing steels with excellent heat resistance and oxidation resistance, they include no teaching regarding improvement of corrosion resistance in a fused-salt environment involving high temperature, extremely strong corrosivity and a complex corrosive mechanism.
The object of this invention is therefore to develop an inexpensive stainless steel separator material for fuel cells which exhibits excellent resistance to fused-carbonate corrosion as well as excellent workability and hot workability, notwithstanding that i
REFERENCES:
patent: 4102225 (1978-07-01), Michels
patent: 4802894 (1989-02-01), Usami et al.
Fujimoto Nobukazu
Hiramatsu Naoto
Uematsu Yoshihiro
Nisshin Steel Co. Ltd.
Yee Deborah
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