FeCr alloys

However, the maximum operating temperature is lower compared with some ceramic materials, cf. Table 1. Nevertheless, they can be used in combustor designs where the catalyst temperature is limited, like in the hybrid combustor described in Section 7.2.3. In that case, the catalyst temperature is limited to 900-1000°C. 

The coating of metallic monolith, which was more problematic than for ceramics, has been extensively studied and improved. Zwinkels et have shown that special treatment of metallic alloy generates an alumina whisker-covered surface. Those alumina whiskers act as an anchor and facilitate the stable washcoating with ceramics. 

Among the different metallic alloys, FeCr Alloy is the most commonly used in catalytic combustion. It contains 15% Cr and 5% Al, traces of rare earth metals, and balance Fe. 

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Figure 2.25 Product composition vs. product gas temperature for partial oxidation of propane in an Rh/AI203/FeCr alloy reactor [55] (by courtesy of ACS).

GIXS was combined with XPS to characterise thin oxide films formed by heating at 873 K in air FeCr alloys with different concentrations [146]. The main crystallographie stmcture of the oxide film was found of comndum type stmctures (Fe203 and Cr203) although it depends on the bulk chromium concentration. 

J. Camra, E. Bielanska, A. Bemasik, K. Kowalski, M. Zimowska, A. Bialas, M. Najbar, Role of A1 segregation and high affinity to oxygen in formation of adhesive alumina layers on FeCr alloy support, Catal. Today 105 (2005) 629. 

FeCr alloy 15% Cr, 5% Al, Y traces, balance Fe 1250-1350 11 Excellent thermal shock resistance, low cost 165. 166... 

Cl has been found in the outer parts of a film only. Usually, passive films have at least a bilayer structure. Usually the outer part is a hydroxide film. This hydroxide part on passive Ni incorporates chloride. Chloride has been found in the iimer layer only when the passive layer has been formed in a solution containing chloride. Similar results were obtained for prepassivated FeCr alloys [49,50]. Another possibility is its incorporation after long waiting periods in chloride-containing solutions with continuous breakdown and repair events of the passive layer, which... 

Figure 10.9 Potential dependence of the partial current densities of iron (spheres) and chromium (rectangles) dissolution from FeCr alloys in sulfuric acid (a) FeCrg gj, Tafel slope 40 mV and (b) FeCr j, Tafel slope 100 mV. (Reproduced with permission from Ref. [17], 1980, Elsevier.)...

Another example is the active dissolution of FeCr alloy (Figure 10.9). At low chromium content the observed Tafel slopes are similar to pure iron (40 mV). At higher chromium content the Tafel slopes are similar to pure chromium (100 mV). The surface layer changes from an iron rich to a chromium rich phase. 

Fig. 49. Fatigue life curves for some lilamenis of glassy metals. Full curves indicate ribbons and broken curves indicate wires. For further details and references see Table 7. The curves for the wires and the FeCr alloy have been measured in the bending mode with imposed surface strain. In order to represent the.se on the same stress scale this strain has been multiplied by their Young modulus. The bulk amorphous alloy has also been measured in the bending mode but with imposed bending stress.

Rh/Al203/FeCr alloy microchannel monohths were compared with Rh/alumina foams for the production of hydrogen from propane. Temperature profiles obtained along the central axis were valuable in understanding the different behaviors of the reactor systems [58],... 

Several materials have demonstrated acceptable corrosion resistance in liquid lead. Fecr-alloy (Fe,0.2 C,13 Cr,4 Al) presented no visible signs of attack after 551 days in 700 C liquid Pb[9]. 

In catalytic channels, the flat plate surface temperature in Eq. (3.32) is attained at the channel entry (x O). As the catalytic channel is not amenable to analytical solutions, simulations are provided next for the channel geometry shown in Fig. 3.3. A planar channel is considered in Fig. 3.3, with a length L = 75 mm, height 21) = 1.2 mm, and a wall thickness 5s = 50 pm. A 2D steady model for the gas and solid (described in Section 3.3) is used. The sohd thermal conductivity is k = 6W/m/K referring to FeCr alloy, a common material for catalytic honeycomb reactors in power generation (Carroni et al., 2003). Surface radiation heat transfer was accounted for, with an emissivity = 0.6 for each discretized catalytic surface element, while the inlet and outlet sections were treated as black bodies ( = 1.0). To illustrate differences between the surface temperatures of fuel-lean and fuel-rich hydrogen/air catalytic combustion, computed axial temperature profiles at the gas—wall interface y=h in Fig. 3.3) are shown in Fig. 3.4 for a lean (cp = 0.3) and a rich cp = 6.9) equivalence ratio, p = 1 bar, inlet temperature, and velocity Tj = 300 K and Uin = 10 m/s, respectively. The two selected equivalence ratios have the same adiabatic equilibrium temperature, T d=1189 K. 

SIMS has also been used to study the air stability of oxide films formed on FeCr alloys in H O-enriched solutions [48,49]. At first glance, the results are somewhat surprising and suggest that the oxides are less stable to air exposure than those formed on nickel and iron. Indeed, the extent of this air instability increases Copyright 2002 Marcel Dekker, Inc. 

Fig. 7.22 Interfacial regions between G18 sealing glass and a Nicrofer 6025, b FeCr alloy after 750°C for 4 h in air (Yang and coworkers 2003)...

AG for reaction between Cr and H2O is —222 kJ/mol suggesting that reaction (7.23) is highly favourable with A1 and Y. Figure 7.22b also indicates large pore formation when G18 interacts with FeCr alloy. Evaporation of alkali metals may also lead to the pore formation... 

The reaction with M = Y and A1 (the major elements of FeCr alloy) are highly favourable whereas those with Fe, Cr, and Ni are unfavourable. Hence, it could be concluded that the extent of glass reaction with alloys depends upon the nature of the alloys used and exposure conditions as well. 

The simple model of a homogenous passive layer of Figure 5.6 becomes more complicated if a second alloy component is present as shown in Figure 5.30. The composition of the passive layer is then determined by the oxidation rates of the components A and B at the metal surface, fheir transfer rates through the film, and their transfer across the passive layer-electrolyte interface, i.e., their individual corrosion rates in the passive state. The reaction rates at both interfaces may be decisive for the layer composition. One example is the preferential dissolution of Fe " ions due to the extremely slow cation transfer of Cr " ions at the surface of the film, which leads to an accumulation of Cr(lll) wifhin the film for FeCr alloys. Another example is the preferential oxidation of A1 of an A1 alloy containing 1% Cu. Cu does not enter the film and is accumulated at the metal surface while an AI2O3 film is formed. These examples are discussed in defail in the following. 

XPS studies of the reduction of a film formed at E = 0.96 V in phthalate buffer pH 5.0 on Fe-15A1 show characteristic compositional changes. Galvanostatic reduction with i = -20 pA cm yields a decrease of Fe(III) with an increase of Fe(II) to a maximum after 20 s and a decrease to a constant value of 12% after 40 s [117]. The Al(III) content stays constant till 40 s and drops afterwards to a constant value of 15%. Apparently, the Al(III) oxide remaining at the surface protects the remaining Fe(II) oxide against dissolution. The A1 enrichment in the center of the passive layer is displaced to the surface due to dissolution of iron after its reduction to Fe(II). The oxidation of Fe(II)-to-Fe(III) and its reduction with appropriate changes of the potential remain the same compared to that of the passive layers formed on pure iron and FeCr alloys as described above. 

Time-resolved studies are also important for a better understanding. It is a rule that the lower valent species are formed first and will be oxidized later. The layer structure develops within milliseconds or hours. This time scale depends on potential and thus on the driving force for the electrochemical reactions but also on the system. The duplex film on iron forms within a few seconds and it takes ca. 100s to get the final structure, which gets faster with increasing potentials. On the contrary, the accumulation Cr within passive layers on FeCr alloys and its change from a hydroxide to an oxide will get to its final stage after hours or days. 

The honeycomb catalytic burner tested in this work was a subscale version of the actual catalytic combustor proposed for the micro-gas-turbine-based power unit, and was embedded inside the high-pressure vessel (see Fig. 2.4). It comprised a 35 mm inner-diameter, 75 mm long (L) and 1.5 mm thick steel tube, wherein alternating flat and corrugated FeCr-alloy foils (with thickness d = 50 pm) were rolled up forming a honeycomb structure with a channel density of 400 cpsi. 

The cross section of each channel was triangular with rounded corners and the equivalent hydraulic radius was — 0.507 mm. The FeCr-alloy foils were coated with a porous 5%-wt Pt/AbOa washcoat of 15 pm thickness. Details on the... 

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