Chromium-Aluminum-Iron Alloys

Twenty percent chromium-nickel and various chromium-aluminum-iron alloys are in common use as furnace windings. To achieve higher temperatures in air, a 10% Rh-Pt alloy can be used up to at least 1400°C (2550 °F). This alloy performs better than pure platinum because of higher strength and a lower rate of grain growth. A single crystal of the same dimensions as the resistance wire cross section tends to shear easily and cause failure. 

Another option are iron-chromium-aluminum (FeCrAl) alloys [24,55,57], which are mechanically stable up to very high temperatures exceeding 1 200 °C. In addition, this alloy type offers the opportunity of a unique way of generating catalyst coatings, which will be discussed below (see Section 2.10). 

The most efficient alloying elements for improving oxidation resistance of iron in air are chromium and aluminum. Use of these elements with additional alloyed nickel and silicon is especially effective. An 8% Al-Fe alloy is reported to have the same oxidation resistance as a 20% Cr-80% Ni alloy [51]. Unfortunately, the poor mechanical properties of aluminum-iron alloys, the sensitivity of their protective oxide scales to damage, and the tendency to form aluminum nitride that causes embrittlement have combined to limit their application as oxidation-resistant materials. In combination with chromium, some of these drawbacks of aluminum-iron alloys are overcome. 

A series of nickel—chromium—iron alloys based on the soHd solution Inconel 600 alloy (see Table 4) was developed, initially depending on aluminum ... 

Specialized alloys are used for high temperature appHcations on turbine blades, furnace parts, thermocouples, etc. These coatings can be as simple as iron—silicon—chromium or as exotic as chromium—aluminum—hafnium (36,41,52). 

Iron (Fe), 74 490-529. See also Fe entries Ferr- entries Iron compounds Ironmaking processes Manganese ferroalloys MoFe protein Nickel-chromium—iron alloys Nickel—iron-aluminum catalyst Ni-Fe-base alloys VFe protein... 

Nickel hydroxides, 17 111 Nickel—iron alloys, 17 101 Nickel—iron—aluminum catalyst, 17 121 Nickel—iron cells, 3 491—493 Nickel—iron—chromium alloy 825 in galvanic series, 7 805t Nickel—iron—chromium alloys, 17 102—103 Nickel—iron plating, 9 821 Nickel itch, 12 691, 701 Nickel—matrix composites, 17 104 Nickel metal, forms of, 17 95—99 Nickel metal hydride cells, 3 431, 471, 509-512... 

A similar reaction occurs during pitting corrosion of iron and its alloys. Partial hydrolysis, leading to the formation of Al(OH) and Al(OH) may also occur, but all such reactions lead to the formation of acid, making the solution inside the pit much more aggressive than outside. Measurement of the pH inside a pit is not an easy matter, but estimates based on various calculations and on measurements in model pits lead to values as low as 1-2 for chromium-containing ferrous alloys and about 3.5 for aluminum-based alloys, depending on experimental conditions. 

Most often, it is the anodic polarization behavior that is useful in understanding alloy systems in various environments. Anodic polarization tests can be conducted with relatively simple equipment and the scans themselves can be done in a short period of time. They are extremely useful in studying the active-passive behavior that many materials exhibit. As the name suggests, these materials can exhibit both a highly corrosion-resistant behavior or that of a material that corrodes actively, while in the same corrodent. Metals that commonly exhibit this type of behavior include iron, titanium, aluminum, chromium, and nickel. Alloys of these materials are also subject to this type of behavior. 

The lattice of vanadium expands approximately linearly with the addition of aluminum [64]. The aluminum intermetallic compound, V3AI (V-25 atom% Al), expands the lattice by about 1% from 0.3025 nm in unalloyed vanadium to 0.3054 nm [64]. Molybdenum, cobalt and titanium also expand the lattice of vanadium, whereas elements such as chromium and iron cause the lattice to contract [83]. Addition of these elements can increase the mechanical strength of alloys relative to unalloyed vanadium [85]. For niobium and tantalum, mechanical properties can also be improved by alloying [86]. Buxbaum has patented a number of alloys of niobium, tantalum and vanadium for membrane use, including Ta-W, V-Co, V-Pd, V-Au, V-Cu, V-Al, Nb-Ag, Nb-Pt, Nb-Pd, V-Ni-Co, V-Ni-Pd, V-Nb-Pt, and V-Pd-Au [45]. 

Design of cathodic protection for marine structures in both fresh and salt water require special techniques. Galvanic systems usually employ zinc or aluminum alloy anodes. Impressed current systems frequently use high silicon, chromium bearing iron, platinized niobium, or mixed-metal oxide/titanium anodes. The structure being protected affects the design. Stationary facihties such as bulkheads and support piles require different techniques from ship hulls [55]. 

Compounds Formed in Alloy Layer of Diffusion Coatings Formed by Aluminum, Zinc, and Chromium on Iron... 

Intergranular corrosion can also occur in other metals and alloys in connection with grain boundary precipitation, for example, in aluminum magnesium (AlMg) 5 alloy, nickel chromium 30 iron (NiCr30Fe), or Ti, and in methanol with traces of water and HCl. Solution annealed austenitic chromium-nickel steels can suffer intergranular corrosion from strong oxidants such as chromium... 

Improvement in oxidation resistance of iron by alloying with aluminum or chromium probably results from a marked enrichment of the innermost oxide scale with respect to aluminum or chromium. The middle oxide scales are known, from chemical analysis, to be so enriched, and electron-microprobe analyses confirm marked enrichment of chromium in the oxide adjacent to the metal phase in the case of chromium-iron alloys [52]. These inner oxides resist ion and electron migration better than does FeO. For chromium-iron alloys, the enriched oxide scale is accompanied by depletion of chromium in the alloy surface immediately below the scale. This situation accounts for occasional rusting and otherwise poor corrosion resistance of hot-rolled stainless steels that have not been adequately pickled following high-temperature oxidation. 

ASTM B 78, Method for Accelerated Life Test of Iron-Chromium-Aluminum Alloys for Electrical Heating... 

This practice covers the determination of the resistance to oxidation of iron-chromium-aluminum alloys for electrical heating alloys at elevated temperatures under intermittent heating using a constant temperature cycle test. This test is used for internal comparative purposes only. 

Selective leaching or dealloying is the selective removal of one element from an alloy by corrosion processes. The most common example is the selective removal (dezincification) of zinc in brass alloys. Dezincification may either be plug-type or uniform. In other alloy systems, aluminum, iron, cobalt, nickel, chromium, and other elements may be selectively removed [49]. Little work has been done in differentiating susceptibility of selective leaching of alloys in synthetic and naturtil seawater [6]. 

Quantitative gas chromatographic schemes now exist for the determination of beryllium in blood, urine, and tissue,chromium in serum," aluminum in uranium, aluminum, gallium, and indium, in aqueous solu-tions," iron in ore, chromium in steel, titanium in bauxite, aluminum, iron, and copper in alloys,uranium, tungsten and molybdenum in alloys and ores, " and the list continues to grow rapidly. In the ultratrace analysis of beryllium the lower limit of detectability is ca. 10 g. The gas... 

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