Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Reexamination Certificate
2000-07-14
2001-09-25
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C148S331000, C148S325000
Reexamination Certificate
active
06294131
ABSTRACT:
The present invention relates to a heat resistant steel, and is particularly concerned with such a steel for use in components of solid oxide fuel cells.
The operating conditions in a solid oxide fuel cell are very severe on most metals, causing them to degrade via loss of mechanical strength, oxidation or other form of corrosion, distortion, erosion or creep. Various heat resistant metals have been developed to cope with many of these forms of degradation. Most such metals are alloys based on iron or nickel with substantial additions of chromium, silicon and/or aluminium, plus, in some alloys, more expensive elements such as cobalt, molybdenum and tungsten. Chromium based metals are also available.
These alloys are either expensive to make and fabricate, unsuitable for long term use in certain components in fuel cells, or both. A relatively cheap iron based steel which is sufficiently suitable for critical fuel cell components has not hitherto been developed.
The significant feature of all heat resistant steels is the oxide layer, particularly its type and nature, which is formed when the steel is exposed to mildly and strongly oxidising conditions at elevated temperatures. Heat resisting steels form tight, adherent, dense oxide layers which prevent further oxidation of the underlying metal. These oxide layers are composed of chromium, aluminium or silicon oxides or some combination of these. These oxide layers are very effective in providing a built-in resistance to degradation due to high temperature oxidation.
However, while this feature is used to advantage in many applications, the presence of this oxide layer inhibits the use of these steels in key components of solid oxide fuel cells. The oxides, especially those of silicon and aluminium, are electrically insulating at all temperatures, and this is a major problem for those components in a fuel cell which must act as electrical current collectors. Of all the heat resisting steels available, those based on the iron-chromium binary systems are the best in this regard, but they too have severe limitations.
The currently available steels contain additional elements which have been found to affect the nature of the oxide layer when it forms. These elements are present in small quantities, either as deliberate additions to assist with the control of oxygen during steelmaking, or as residual impurities inherited from the raw materials used in making the steel, i.e. tramp elements. Many of these minor elements have a profound effect on the type and thickness of the oxide layer which forms on the surface of the steel when it is subjected to oxidation at elevated temperatures. For example, manganese is deliberately added to most steels to assist with the deoxidation of the iron during melting and to eliminate iron sulphides from the steel. This is beneficial for most applications of heat resistant steels, but not when the steel is used as an interconnect or connector plate in a solid oxide fuel cell.
In a paper in “Nature” Feb. 13, 1965, vol.205, p.609, Caplan and Cohen reported that in tests of high temperature oxidation rates on Fe-26Cr alloys, those with manganese levels around 0.003 to 0.004 weight percent oxidised slower than those with levels of 0.75 to 1.00 percent manganese. The applicant has now found that the presence of manganese in quantities above 0.10 percent by weight modifies the form of the oxide layer as it begins to grow, giving rise to a rather loose and wavy layer. This results in a particularly poor electrical conductivity through the layer, at the stage of formation and at a later stage when the composition may have shifted to one of the other more stable oxides such as chromium oxide. The applicant has also found that the beneficial properties of low oxidation rates can be achieved at manganese levels much higher than the 0.003 to 0.004 weight percent levels of Caplan and Cohen, providing the manganese level is kept below 0.10 weight percent and providing the inclusion of certain other elements is also limited. This higher permissible manganese level allows the production of commercial tonnages of steel at a reasonable price.
Another example is the effect which the element silicon has on the formation of oxide layers at the surface of the steel. Silicon and aluminium are used as cheap and effective additives to control the oxidation of iron during the steel smelting process. Small amounts of silicon, e.g. 0.5 weight percent, in an iron chromium heat resisting steel lead to the formation of a subsurface layer of silica which, if fully formed, has a very high electrical resistivity. For most applications this feature is not deleterious, but for a connector plate in a solid oxide fuel cell it completely negates a prime purpose of the component.
For these heat resisting steels to be useful for electrical conducting components in fuel cells, it is important that the aforementioned disadvantages be alleviated.
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Brezner David J.
Ceramic Fuel Cells Limited
Flehr Hohbach Test Albritton & Herbert LLP
Yee Deborah
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