Ni -Cr base alloys

Fe-Ni-Cr base alloys FCC 15-20 Good High Readily Fairly expensive... 

Other categories of chromia forming alloys—including Ni(-Fe)-Cr base and Fe(-Ni)-Cr base alloys (e.g., austenitic stainless steels)—have a face-centered cubic (FCC) substrate structure. In comparison to the FSS, the FCC base alloys, in particular the Ni(-Fe)-Cr base alloys, are generally much stronger and potentially more oxidation resistant in the SOFC interconnect operating environment [6, 123-129], However, the FCC Ni(-Fe)-Cr base alloys with sufficient Cr for an appropriate... 

Nevertheless, Ni(-Fe)-Cr base alloys may find application as interconnect materials through the use of innovative SOFC stack and seal designs and novel interconnect structures. For example, a cladding approach has been applied to fabricate a stable composite interconnect structure consisting of FCC Ni-Cr base alloy claddings on a BCC FSS substrate [134,135], The clad structure appeared to be stable over 1000 hours at 800°C in air and exhibited a linear CTE close to that of the FSS, but needs further long-term stability evaluation before its commercial use. 

Ni-Cr base alloys with Cr-nitride surface coating provide excellent corrosion resistance and low interfacial contact resistance. However, Ni-Cr bipolar plates are too expansive compared to aluminum, carbon-steel, and stainless steel bipolar plates. 

Alloys exposed to SCW at temperatures up to 700°C after in-reactor irradiation (not in SCW) [56] showed no increase in general corrosion however, the effects of coolant irradiation (water radiolysis) could not be assessed in this work. Zr—l%Nb and Ni—Cr-based alloys irradiated by a 9.76 MeV/6.23 kWe electron beam for 497 h in water in the vicinity of the pseudocritical point showed enhanced oxidation rates under irradiation [29,30,57]. Tests of AUoy 800 in superheated steam at temperatures up to 738°C under irradiation [58] showed generally uniform corrosion penetration consistent with out-of-reactor data, but higher than predicted oxide loss. 

Thermal expansion coefficients For high-temperature utilization, Ni-Cr-based alloys are excellent from the anticorrosion point of view. Even so, such Ni-Cr alloys have high thermal expansion coefficients dictating a larger match with YSZ. Cr-based alloys developed by Siemens/Plansee have an essentially similar thermal expansion coefficient as YSZ. This alloy was utilized by Sulzer Hexis. Alternatively, Fe-Cr ferritic alloys are frequently utilized. Although complete matching in thermal expansion coefficient with YSZ is not obtained, Ni-Cr alloys provide considerable improvement. 

Another type of nickel alloy with which problems of intergranular corrosion may be encountered is that based on Ni-Cr-Mo containing about 15% Cr and 15% Mo. In this type of alloy the nature of the grain boundary precipitation responsible for the phenomenon is more complex than in Ni-Cr-Fe alloys, and the precipitates that may form during unfavourable heat treatment are not confined to carbides but include at least one inter-metallic phase in addition. The phenomenon has been extensively studied in recent years . The grain boundary precipitates responsible are molybdenum-rich M C carbide and non-stoichiometric intermetallic ix... 

Replacing some of the nickel with iron produces a family of alltws with intermediate corrosion resistance between stainless steels and the Ni-Cr-Mo alloys. Alloys such as Incoloy 825 and Hastelloy G-3 and G-30 are in this family. Incoloy 825 has 40 percent Ni, 21 percent Cr, 3 percent Mo, and 2.25 percent Cu. Hastelloy G-3 contains 44 percent Ni, 22 percent Cr, 6.5 percent Mo, and 0.05 percent C maximum. These alloys have extensive applications in sulfuric acid systems. Because of their increased nickel and molybdenum contents they are more tolerant of chloride-ion contamination than are standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking molybdenum improves resistance to crevice corrosion and pitting. Many of the nickel-based alloys are proprietary and are coverecf by the following specifications ... 

Ni(-Fe)-Cr base alloys FCC 14-19 Good High Readily Expensive... 

Early works were typically carried out using single atmosphere exposure conditions, either air (or moist air) representing the cathode side environment [124-129, 139, 142,144-162] or a reducing atmosphere simulating the anode side environment [124, 125, 127-129, 144, 145], Lately, studies have been also performed to determine the oxidation/corrosion behavior of metal and alloys under dual-atmosphere exposure conditions that closely simulate the interconnect exposure conditions during SOFC operation [154-159], The alloys studied include both Fe-Cr base FSSs and Ni or Ni-Cr base heat-resistant alloys, as well as Cr or Cr base alloys. 

Based on the chemical composition, corrosion-resistant nickel-based alloys consist of commercially pure nickel. Ni-Cu alloys, Ni-Mo alloys, Ni-Cr-Mo alloys, and Ni-Cr-Fe alloys.63 The cast versions of the nickel-based alloys do not have the same corrosion resistance as the corresponding wrought products, mainly due to the higher carbon and silicon contents and the anisotropic microstructure of the cast products. (Rebak)5... 

The presence of water vapour adversely affects the selective oxidation of elements such as aluminium and chromium from iron-base and nickel-base ° alloys. This is illustrated in Figure 7.2 for a Ni-Cr-Al alloy. Exposure in dry air results in a continuous external alumina film while exposure in moist air results in profuse internal oxidation of the Al. It has also been found that water vapour affects the selective oxidation of aluminium from TiA1.2i... 

X. Shan, H. Ha, J.H. Payer, Comparison of crevice corrosion ofPe-based amorphous metal and crystalline Ni-Cr-Mo alloy, in Metallurgical and Materials Transactions, vol. 40, Springer, Boston, 2009,... 

HfB2> NbB2, and TaB2 and Ni-Co-Cr based alloys. The refractory and extraordinarily hard MB2 phases are hence in equilibrium with a comparatively soft and brittle ternary compounds. 

Keywords Intermetallics, Ni-based alloys, Cr-based alloys, Metallothermic process... 

Inconel = referred to Ni-Cr based super-alloy support. 

Apart from the well-tried Fe—Ni—Cr—(Be, Til-based elinvar-type alloys, Fe—Ni—Mo-based alloys have gained technical application. Addition of Mo improves the elastic properties, lowers the Curie temperature, and increases the resistance to corrosion. While in Europe and the USA, Fe—Ni-based elinvar-type alloys mainly were developed, in Japan Co—Fe-based elinvar alloys were discovered. Ternary Co—Fe—Cr alloys and quaternary alloys containing Ni (Co-elinvar) attained significant technical relevance. Distinguishing... 

---