Patent Publication Number: US-2007122304-A1

Title: Alloys for intermediate temperature applications, methods for maufacturing thereof and articles comprising the same

Description:
BACKGROUND  
      This disclosure is related to ferritic stainless steels for high temperature applications, methods for manufacturing thereof and articles comprising the same.  
      Solid oxide fuel cells (SOFCs) are devices that produce energy, usually electricity, from a variety of fuels using an electrochemical reaction. Oxygen transfer through the electrolyte, which improves the efficiency of energy conversion, is greatly accelerated at temperatures above 700° C. The overall fuel to electricity conversion efficiency in SOFCs can be as high as 90% and is not limited by classical thermodynamics for heat engines (Carnot cycle). Due to their high exhaust gas temperature, SOFCs have the ability to cogenerate heat and electricity. Hybrid power generation systems integrating the SOFCs and turbines can have very high overall system efficiencies.  
      SOFCs may be tubular or planar in assembly. The key components of an SOFC are an anode, a cathode, an electrolyte, interconnects, a manifold and seals. The cathode is largely exposed to a hot, oxidant environment, and is generally called the air or oxygen electrode. The temperature of the cathode feed gas is usually about 400° C. or higher. Similarly, the anode is exposed to the fuel and is called the fuel electrode. The interconnects interface with the anode on the fuel side and with the cathode on the air side and are usually made using oxidation resistant, heat resistant materials such as lanthanum chromite, lanthanum strontium chromite, ferritic stainless steels and chromium base alloys.  
      Highly oxidizing conditions prevail at the cathode at temperatures of greater than or equal to about 850° C. and high oxygen partial pressures. These, along with humidity and atmospheric moisture may oxidize chromium present in interconnects to chromium oxides or hydroxide or oxyhydroxide that grow as cathode scales and can vaporize to poison or deactivate the cathode. Cathode scales may grow to a thickness of tens of microns after exposure for thousands of hours in the SOFC environment in an intermediate temperature range of about 800° C. Chromium hydroxide and oxyhydroxide are particularly volatile and may degrade the cathode. To enhance life expectancy and operational efficiency of the SOFC cathode it is desirable to reduce or eliminate cathode degradation.  
      Current methods for minimizing cathode degradation in SOFCs are not adequately developed and limit the useful operating life of the SOFCs. The problem may be reduced or eliminated by frequent maintenance or cathode scale removal. This may result in cell stoppage and induce a significant energy penalty associated with the power generation cycle.  
      Alternatively, non-chromium containing alloys and ceramic materials with non-volatile chromium have been employed in interconnects. However, these materials are expensive, brittle, weak under tensile forces, or have high resistive losses making them unsuitable for interconnect applications. Many SOFC stacks employ interconnects and components made from alloys containing chromium and few suitable replacement materials are available. The problem of high cathode degradation rates has not been solved.  
      It is therefore desirable to use ferritic stainless steels that can facilitate a reduction in the cathode degradation rates in SOFC&#39;s that operate at temperatures of about 800° C.  
     SUMMARY  
      Disclosed herein is a composition comprising iron; about 18 to about 30 wt % chromium; up to about 7 wt % tungsten; up to about 1.5 wt % manganese; up to about 1 wt % aluminum; about 0.02 to about 0.1 wt % of a rare earth metal and/or yttrium; wherein the weight percents are based on the total weight of the composition.  
      Disclosed herein too is a method comprising melting together a composition comprising iron; about 18 to about 30 wt % chromium; up to about 7 wt % tungsten; up to about 1.5 wt % manganese; up to about 1 wt % aluminum; about 0.02 to about 0.1 wt % of a rare earth metal and/or yttrium; wherein the weight percents are based on the total weight of the composition; casting the composition; and rolling the composition.  
      Disclosed herein too are articles manufactured from the composition. 
    
    
     DETAILED DESCRIPTION OF FIGURES  
      With reference now to the figures, wherein like elements are numbered alike:  
       FIG. 1  is a schematic depicting one exemplary embodiment of a solid oxide fuel cell (SOFC);  
       FIG. 2  is a schematic depicting the sandwich that is used for the ASR measurements;  
       FIG. 3  is a depiction of the test set-up for measuring the ASR of the ferritic stainless steels; and  
       FIG. 4  depicts the electrical set-up for the platinum foils that is used for determining the ASR of the ferritic stainless steels. 
    
    
     DETAILED DESCRIPTION  
      In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top”, “bottom”, “outward”, “inward”, and the like are words of convenience and are not to be construed as limiting terms. It is to be noted that the terms “first,” “second,” and the like as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes the degree of error associated with measurement of the particular quantity).  
      Disclosed herein are ferritic stainless steels that reduce oxidation and improve chemical compatibility of the metal interconnect in solid oxide fuel cells (SOFCs) and other high temperature applications. The ferritic stainless steels can be advantageously used as interconnects in a SOFC environment while reducing degradation due to corrosion. The ferritic stainless steels display a low oxide growth rate, can be advantageously used for coefficient of thermal expansion (CTE) matching and have a low total area specific resistivity (ASR) of about 5 to about 40 milliohm-square centimeter (measured at 750° C.) when subjected to oxidation at about 750° C. for about 1,500 hours. The ferritic stainless steels advantageously comprise chromium, aluminum, tungsten, manganese, rare earth elements and/or yttrium, with the balance being iron.  
      With reference now to the  FIG. 1 , an exemplary fuel cell system  200  comprises a fuel cell  30  having an anode  40 , an electrolyte  60 , a cathode  80 , an interconnect  100  and a seal  105 . The cathode  80  and the interconnect  100  are in intimate electrical communication via contact  90 . A fuel cell stack is obtained by repeated stacking of repeating unit  180  that comprises an anode  40 , electrolyte  60 , cathode  80 , cathode-interconnect contact  90  and interconnect  100 . The fuel cell is encased between the end plates  120   
      As can be seen from the  FIG. 1 , the interconnect, connects one cell to another electrically when multiple SOFCs are used in a stack to generate electricity. Interconnects also serve as separators for the anode and cathode gases in addition to providing mechanical stability to the SOFC stack. Since electrical connectivity of SOFCs is the function of interconnects, the electrical conductivity of the materials in the interconnect has to be high and should stay high at the operating temperature under the cell conditions for the entire life of the SOFC. Further, the interconnect is in physical communication with the other components of the cell such as the cathode and anode. Seals are used to make the fuel cell gas-tight to avoid the intermixing of fuel and oxidant gases and the interconnects can be in physical communication with the seals too. Thus, it is desirable for the interconnects to be chemically inert and to have matching coefficient of thermal expansion with the other cell components. Even if there is reaction between the interconnect and the electrodes, the reaction product should be a good electrical conductor.  
      In one embodiment, the ferritic stainless steel used in the interconnect comprises chromium in an amount of greater than or equal to about 18 weight percent (wt %), based on the weight of the ferritic stainless steel. In another embodiment, the ferritic stainless steel comprises chromium in an amount of about 18 wt % to about 30 wt %, based on the weight of the ferritic stainless steel. In yet another embodiment, the ferritic stainless steel comprises chromium in an amount of about 20 wt % to about 29 wt %, based on the weight of the ferritic stainless steel. In yet another embodiment, the ferritic stainless steel comprises chromium in an amount of about 21 wt % to about 28 wt %, based on the weight of the ferritic stainless steel. An exemplary amount of chromium is about 20 to about 25 wt %, based on the weight of the ferritic stainless steel. If less than 18 wt % of chromium is added, then a continuous protective layer of chromium oxide may not be formed. This protective layer of chromium oxide minimizes the rate of degradation of the ferritic stainless steel. If the chromium is added in amounts of greater than or equal to about 30 wt %, then the ASR will increase. There is also a risk of increased volatilization if chromium is added in amounts of greater than or equal to about 30 wt %, based on the weight of the ferritic stainless steel.  
      The aluminum can be present in amounts of up to about 1 wt %, based on the weight of the ferritic stainless steel. In one embodiment, the aluminum can be present in amounts of about 0.5 to about 0.9 wt %, based on the weight of the ferritic stainless steel. In another embodiment, the aluminum can be present in amounts of about 0.55 to about 0.85 wt %, based on the weight of the ferritic stainless steel. In yet another embodiment, the aluminum can be present in amounts of about 0.5 to about 0.80 wt %, based on the weight of the ferritic stainless steel. An exemplary amount of aluminum is about 0.75 wt %, based on the weight of the ferritic stainless steel. If aluminum is added in amounts of greater than or equal to about 1.0 wt %, then too much alumina may be formed in the ferritic stainless steel thereby increasing the surface resistance.  
      Tungsten facilitates a reduction in the coefficient of thermal expansion (CTE) of the ferritic stainless steel. The amount of tungsten can be varied to facilitate CTE matching between the interconnect and those components of the SOFC that it is physical communication with. The tungsten can be present in amounts of up to about 7 wt %, based on the weight of the ferritic stainless steel. In one embodiment, the tungsten can be present in amounts of about 5 to about 6.8 wt %, based on the weight of the ferritic stainless steel. In another embodiment, the tungsten can be present in amounts of about 5.5 to about 6.5 wt %, based on the weight of the ferritic stainless steel. An exemplary amount of tungsten is about 5 to about 7 wt %, based on the weight of the ferritic stainless steel.  
      The presence of manganese in the ferritic stainless steel facilitates the formation of a spinel phase upon oxidation. The presence of manganese reduces the volatilization of the chromium-containing oxides and/or hydroxides. The manganese can be present in amounts of up to about 1.5 wt %, based on the weight of the ferritic stainless steel. In one embodiment, the manganese can be present in amounts of about 0.5 to about 1.35 wt %, based on the weight of the ferritic stainless steel. In another embodiment, the manganese can be present in amounts of about 0.6 to about 1.25 wt %, based on the weight of the ferritic stainless steel. In yet another embodiment, the manganese can be present in amounts of about 0.7 to about 1.2 wt %, based on the weight of the ferritic stainless steel. An exemplary amount of manganese is about 0.75 wt %, based on the weight of the ferritic stainless steel.  
      The rare earth elements are effective in controlling oxidation as they effectively block the grain boundary diffusion of chromium. An exemplary rare earth element is lanthanum. Other rare earth metals from the lanthanide and actinide series of rare earth metals may be added to lanthanum if desired. Examples of such rare earth metals are cerium, praseodymium, neodymium, samarium, europium, gadolinium, uranium, neptunium, plutonium, or the like, or a combination comprising at least one of the foregoing rare earth metals.  
      It is generally desirable to add the rare earth metals in amounts of about 0.02 wt % to about 0.1 wt %, based on the total weight of the ferritic stainless steel. In one embodiment, the rare earth metals can be added in amounts of about 0.05 wt % to about 0.08 wt %, based on the total weight of the ferritic stainless steel. In another embodiment, the rare earth metals can be added in amounts of about 0.06 wt % to about 0.075 wt %, based on the total weight of the ferritic stainless steel. If the rare earth metals are added in an amount of greater than or equal to about 0.1 wt %, then the cost of processing the ferritic stainless steel increases.  
      As noted above, the ferritic stainless steels can also comprise yttrium in addition to or in lieu of the rare earth metals. In one embodiment, yttrium can be added with the rare earth metals to the ferritic stainless steels. In another embodiment, the yttrium can be used to replace the rare earth metals in the ferritic stainless steels.  
      In one embodiment, the rare earth metals and the yttrium can be added in amounts of about 0.0001 wt % to about 0.1 wt %, based on the total weight of the ferritic stainless steel. In one embodiment, the rare earth metals and the yttrium can be added in amounts of about 0.005 wt % to about 0.08 wt %, based on the total weight of the ferritic stainless steel. In another embodiment, the rare earth metals and the yttrium can be added in amounts of about 0.007 wt % to about 0.06 wt %, based on the total weight of the ferritic stainless steel. In yet another embodiment, the rare earth metals and the yttrium can be added in amounts of about 0.008 wt % to about 0.05 wt %, based on the total weight of the ferritic stainless steel.  
      In one embodiment, in one method of manufacturing the ferritic stainless steel, the iron, chromium, aluminum, tungsten, manganese, rare earth elements and/or yttrium are vacuum arc melted followed by casting, forging and rolling into the final sheet form. In another embodiment, the ferritic stainless steel can be manufactured into a desired shape by other powder metallurgy based methods including, hot pressing, hot isostatic pressing, sintering, hot vacuum compaction, or the like. An exemplary method of manufacturing the ferritic stainless steel is by vacuum arc melting followed by casting forging and rolling into final sheet form.  
      After vacuum arc melting the material is then cast into an ingot. The ingot may then be forged and rolled into final sheet form. In one embodiment, the ingot can be hot rolled at a temperature of about 1000° C., followed by cold rolling to a thickness of less than or equal to about 2.54 millimeters. During the process of reduction in the thickness of the cross-sectional area, periodic annealing may be performed on the ferritic stainless steels.  
      The ferritic stainless steels advantageously display an area specific resistivity (ASR) of about 5 to about 40 milliohm-square centimeter (mohm-cm 2 ) when used in an alloy sandwiches that are oxidized at 750° C. for 1,500 hours and an ASR of about 20 to about 120 mohm-cm 2  when used in an alloy sandwiches that are oxidized at 850° C. for 1,500 hours. The aforementioned ASR values are measured at a test temperature of 750° C. As detailed below, the alloy sandwiches contain a layer of lanthanum strontium manganate disposed between two ferritic stainless steel plates.  
      The ferritic stainless steels also advantageously display a coefficient of thermal expansion (CTE) of about 11 to about 12.75 parts per million per degree centigrade (ppm/° C.). In one embodiment, the ferritic stainless steels display a coefficient of thermal expansion (CTE) of about 11.75 to about 12.50 ppm/° C. In another embodiment, the ferritic stainless steels display a coefficient of thermal expansion (CTE) of about 11.85 to about 12.25 ppm/° C. The ferritic stainless steels advantageously have a thermal expansion coefficient to match to that of the electrolyte material that is used in commercially available SOFC&#39;s i.e., 8% yttria stabilized zirconia (YSZ), which is about 11 ppm/° C. in the temperature range of about 20 to about 800° C.  
      The present disclosure is illustrated by the following non-limiting example.  
     EXAMPLE  
      This example was performed to determine the area specific resistivity (ASR), the coefficient of thermal expansion (CTE) and the thickness of an oxidation layer formed on the ferritic stainless steel in a solid oxide fuel cell environment. To measure the ASR, a sandwich of an LSM (lanthanum strontium material) and the ferritic stainless steel was created. As shown in the  FIG. 2 , this Sandwich Configuration comprises a layer of LSM disposed between two ferritic stainless steel plates. The whole assembly shown in the  FIG. 2  was oxidized at high temperatures for a certain duration of time. The temperatures chosen were 750 and 850° C. respectively and the duration of time was 1,500 hours.  
      In order to sandwich the LSM between the ferritic stainless steel plates, 10 wt % polyvinyl alcohol (PVA) is dissolved in hot water to make a PVA solution. LSM paste was prepared with 30 wt % of this PVA solution, that is 70 grams of LSM was mixed with 30 grams of PVA solution. The LSM paste was then applied to one surface of a ferritic stainless steel plate and another ferritic stainless steel plate was pressed on it. These alloy sandwiches were then oxidized at 750° C. and 850° C. respectively for 1,500 hours. These oxidation temperatures were chosen because they are similar to the operating temperature of a SOFC.  
      In order to measure the ASR, after oxidizing the sandwiches, the top and bottom surfaces of the sandwich were polished off to remove the oxide that is formed on the bare surfaces of the ferritic stainless steel plates. Then the sandwich is introduced into the measuring equipment between the platinum foils, as shown in  FIG. 3 . As can be seen in the  FIG. 3 , the platinum foils each having two leads are in intimate contact with the outer surfaces of the sandwich. This is depicted clearly in the  FIG. 4  where two of the leads are connected to the top platinum foil and the other two to the bottom platinum foil. One of the leads on top and one from bottom are used for passing a constant current and the other pair for measuring the voltage drop across the sandwich.  
      The advantages of this configuration are a) after polishing off the oxide on the top and bottom surfaces of the sandwich, the platinum foils make direct contact with the alloys and b) The total ASR measured is across two ferritic stainless steel-LSM interfaces thereby increasing the accuracy of measurement.  
      A Keithley programmable constant current source (model 2400) and Keithley Nanovoltmeter (model 2182) were used for passing the constant current and measuring voltage drop across the sample, respectively. The voltage drop was also measured by reversing the polarity of the constant current and the average of the two readings was taken. This way any thermoelectric effects that may be present because of temperature gradients in the furnace are also annulled. The temperature was increased at a rate of 5 degrees centigrade per minute and the data was collected at an interval of 20 degrees both during heating and cooling.  
      The compositions along with the ASR results for these compositions are shown in the Table 1 below. In addition to the ASR measurements, CTE measurements were also made using a Netzsch DIL 402C dilatometer having temperature capability from 25 to 1500° C. CTE results are also shown in the Table 1 below.  
      In addition samples were oxidized to determine the oxide thickness. Ferritic stainless steel pieces were coated with LSM slurry and were oxidized at 750 and 850° C. for 1,500 hours. The oxidized alloys were mounted edge-on to determine the oxide thickness. In order to ensure perpendicularity, a couple of metallic clips were used. The samples supported by the clips were inserted in the plastic cylindrical mold of 1 inch diameter. Low viscosity epoxy resin was prepared by mixing 3 parts resin and 1 part hardener. The cylindrical molds were half filled with the resin and kept in the vacuum desiccators. The desiccator was evacuated using a rotary pump until the epoxy started frothing and reached to the rim of the mold. The vacuum was broken so that resin sank again. The process described above was repeated once again. Finally, the mold was filled with the resin fully. The resin was allowed to cure overnight at room temperature.  
      The cold mounted samples were metallographically polished. In order to provide a leakage path for the electrical current developed during electron microscopy, a silver contact was provided between sample and bottom of the molded plastic. The mounted samples along with the plastic were degassed in the oven at 105° C. for 4 to 5 hours. The degassed mounted samples were coated with gold by DC sputtering. The thickness of the gold layer was 150 to 200 Angstroms. The oxide thickness was measured in a scanning electron microscope (SEM) at a magnification of 3000 to 5000. Often EDS was used as an aid for thickness measurement, wherever the boundaries of oxides were poorly defined. Thickness was measured at a minimum of 5 locations. Oxide thickness results are also shown in the Table 1 below.  
                                   TABLE 1                                       Oxide   Oxide                       Thickness   Thickness                       (μm)   (μm) (LSM                   ASR @   (bare)   coated)               CTE   750° C.   (Oxidized at   (Oxidized at               (775-100° C.)   (mohm-   750° C. for   750° C. for       Sample #   Composition   (ppm/° C.)   cm 2 )   1,500 hours)   1,500 hours)                                                        Sample #1   Fe—25Cr—0.75Mn—0.05(La + Y)—7W   11.78       2.4 ± 0.4   1.6 ± 0.3       Sample #2   Fe—25Cr—0.75Mn—0.05(La + Y)—1Al   12.57   12   3.3 ± 1.1   2.1 ± 0.3       Sample #3   Fe—25Cr—0.75Mn—0.1(La + Y)   12.23   11   2.2 ± 0.5   1.9 ± 0.4       Sample #4   Fe—25Cr—0.75Mn—0.1(La + Y)—7W—1Al   12.29   12   4.4 ± 1.1   1.9 ± 0.5                  
 
      From the Table 1, it can be sent that the ferritic stainless steels have CTE&#39;s that are about 11.75 to about 12.6 ppm/° C. These CTE values permit closer thermal expansion match to electrolyte materials that are suitable for use in commercially available SOFC&#39;s.  
      From the Table 1, it may also be seen that the ASR for the disclosed compositions is about 11 to about 12 mohm-cm 2 . These values of ASR render the ferritic stainless steels useful for solid oxide fuel cells that operate at temperatures of about 800 to about 850° C. The average value of the oxide thickness layer for the LSM coated samples is about 1.9 micrometers when oxidized at 750° C. for 1,500 hours.  
      Thus from the examples it may be seen that the ferritic stainless steels can be advantageously used in interconnects and other high temperature applications. They can be advantageously used at temperatures of up to 850° C. They display good oxidation resistance leading to increased stability of the LSM-ferritic stainless steel interface. The ferritic stainless steel also comprises elements that permit oxidation resistance as well as chemical compatibility with other components of a SOFC.  
      While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention.