Materials Science Forum Vol. 696

Abstract: A Ni aluminide layer containing Zr or Hf was formed on a Ni specimen by the simultaneous electrodeposition of Al and Zr or Al and Hf using a molten-salt bath. When the simultaneous electrodeposition of Al and Zr was carried out using molten NaCl-KCl containing 3.5 mol%AlF₃ and 0.05 mol%ZrF₄, the electrodeposited layers were formed in the order of Ni₂Al₃, NiAl₃ and Al from the Ni substrate side. The ZrAl₃ particles were uniformly formed in the surface region of the NiAl₃ and Al layers. On the other hand, when the simultaneous electrodeposition of Al and Hf was carried out using molten NaCl-KCl containing 3.5 mol%AlF₃ and 0.05 mol%HfF₄, the electrodeposited layer consisted of Ni₂Al₃ as the inner layer and NiAl₃ as the outer layer were formed with HfAl₃ particles uniformly formed in the surface region of the NiAl₃ layer. For the sample treated with the simultaneous electrodeposition of Al and Zr, no significant change in the mass gain was observed during the cyclic-oxidation test at 1423 K, suggesting that the sample had a high cyclic-oxidation resistance. Similarly, the sample treated by the simultaneous electrodeposition of Al and Hf had a high cyclic-oxidation resistance. An adhesive scale, having localized inward penetrations consisting of Al₂O₃ containing ZrO₂ or HfO₂, was formed on the samples having the high cyclic-oxidation resistance.

Abstract: Crack-healing effectiveness was investigated on 5 vol% nano-Ni dispersed Al₂O₃ hybrid materials. Influence of the Y or Si doping or SiC co-dispersion was also studied on the crack healing behavior. Cracks were introduced by a Vickers indentation to be a crack length of approximately 60 μm. Cracks of nano-Ni/Al₂O₃ were completely disappeared, for example, by oxidation at 1200°C for 6 h in air, Y/Si doped one and SiC co-dispersed one have similar performance of crack disappearance. Bending strength of crack-disappeared samples showed about 550 MPa and was comparable or improved with that of as-sintered one. Mechanism of crack healing was considered as filling up of cracks by NiAl₂O₄ oxidation product which is developed by outward diffusion of cations at grain boundary of Al₂O₃ matrix. Nano-Ni/Al₂O₃ with Y or Si doping or SiC co-dispersion are realized to have crack-healing effectiveness with improved high-temperature oxidation resistance.

Abstract: Pure Ni was aluminized with an Ni₃Al, NaCl and Al₂O₃ pack powder mixture at 1100°C for 12 h. The Ni(Al) solid solution formed in aluminized Ni was internally oxidized at 1000°C for 2 to 12 h with a Co/CoO buffer. The internal oxidation zone consisted of 2 regions: granular precipitate zone near the surface and rod-like zone near the oxidation front. The growth of internal oxidation zone deviated from the parabolic law due to the non-constant Al profile developed by the aluminizing process. The granular zone grows linearly from the surface of the alloy.

Abstract: Isothermal oxidation of two g-TiAl-based intermetallic alloys: Ti48Al and Ti46Al8Nb alloys was studied in synthetic air at 1073-1223 K for up to 240 hrs. Mass change per unit area for the oxidized samples followed approximately the parabolic rate law. The k_p values for the studied temperature interval were in the range from 7.2×10^-13 to 1.8×10^-11 g²cm^-4s^-1. The activation energy for oxidation of Ti48Al alloy in air at 1073-1223 K was E_a = 165±12 kJ/mol. Niobium addition to Ti48Al alloy in the amount of 8% increased its oxidation resistance. Structure and chemical composition of the oxidation products, and morphology of the oxidized samples were investigated using XRD, SEM-EDS, and TEM. The oxide scales formed on Ti48Al and Ti46Al8Nb alloys were well adherent to the metallic substrates and exhibited a multilayer structure. Depending on the oxidation temperature and the alloy composition, the scale consisted of variable amounts of TiO₂ and Al₂O₃. Additionally TiN, and niobium rich particles were also identified in the appropriate oxide scale. To understand the growth mechanism of oxide scale formed on Ti46Al8Nb alloy, two stage oxidation experiment was performed using ¹⁶O₂/¹⁸O₂, followed by SNMS and TEM-EDS. Particular attention was paid to the use of TEM in order to precisely characterize the reaction products on the Ti46Al8Nb alloy.

Abstract: The oxidation behavior of β-SiAlONs (Si_6-zAl_zO_zN_8-z, z = 1, 3, and 4) was investigated at temperature ranging from 1473 to 1673 K in a (N₂-3%O₂)-20%H₂O atmosphere. Oxidation kinetics was followed on the basis of the mass gains, and the oxidized specimens were characterized by FE-SEM, XRD, and EPMA. The mass gain was found to clearly increase with temperature and the z value. For oxidation at 1473 and 1573 K, the gain in mass was observed to be small. During oxidation at 1573 K, mass loss occurred. This loss may be because of the formation of volatile SiO(OH)₂. The changes in mass depended on the z value. On the other hand, the oxidation rates at 1673 K were found to be higher than those at and below 1573 K by more than one order of magnitude. The oxidation kinetics at 1673 K followed an almost linear rate law. The XRD and EPMA results showed that the oxide formed on β-SiAlON (z = 1) was composed of a mixture of amorphous aluminosilicate and mullite, whereas that on β-SiAlONs (z = 3 and 4) was composed only of mullite.

Abstract: The present paper is a review on the oxidation kinetics, electrical properties, chromium vaporization rate and microstructure investigations of oxide products formed on an uncoated Crofer 22 APU alloy and 1.4762 steel and coated by means of pulsed laser deposition and screen-printing methods with films of (La,Sr)CoO₃, (La,Ca)CrO₃, (La,Sr)CrO₃, (La,Sr)(Co,Fe)O₃ and MnCo₂O₄ in air and Ar-H₂-H₂O and Ar-CH₄-H₂O gas mixtures at 1023-1173 K for up to 1200 hrs. Microstructure investigations using SEM-EDS showed the influence of the reaction products formed in different atmospheres at the steel/coating interface on the electrical properties and Cr vaporization rate of these composite materials, which are used for construction of SOFC interconnects.

Abstract: Manganese-cobaltite spinel coatings were produced on Fe-Cr alloys for the improvement of the chemical and mechanical stability of solid oxide fuel cell (SOFC) interconnects. It was found by thermal investigation, i.e., the microscopy of various samples that were heat-treated in air at 800 °C, that the screen-printed coating more effectively inhibited oxide scale growth than the sputtered coating. The reason why the manufacturing method of the spinel coating affected the oxide scale growth rate was investigated. It was demonstrated that the oxide scale in both the samples after annealing in air at 800 °C for 5000 h comprised MnCr₂O₄ and Cr₂O₃ with no difference in composition. However, the interface between the alloy and the oxide scale was deeper and rougher and had a larger grain size because of the high oxygen diffusivity in the sputtered coating. In contrast, in the screen-printed sample, the dense spinel layer above the oxide scale blocked oxygen diffusion into the alloy, so the oxide scale growth rate was lower and the interface between the alloy and oxide scale remained flat even after thermal treatment. Introducing a reduction treatment in the manufacturing process made the Mn-Co spinel layer denser and further inhibited the oxide scale growth. Moreover, the addition of Li as a sintering aid into the Mn-Co spinel was found to even more effectively inhibit the oxide scale growth.

Abstract: Two important degradation mechanisms in Solid Oxide Fuel Cells (SOFC) are directly related to the metallic interconnects. The formation of volatile chromium oxides from metallic interconnects commonly causes fast degradation in cell performance due to poisoning the cathode. Secondly is the ability of the metallic interconnect to form a thin protective oxide one of the most important lifetime limiting factors for SOFC. Chromium volatilization of various uncoated steels is studied as a function of temperature by a recently developed denuder technique which allows time resolved quantification of volatile chromium species. The inhibition of Cr evaporation by Co thin film coatings (800nm) is investigated; it will be shown that these coatings are more effective than much thicker ceramic coatings that are commonly used for this purpose. In order to increase the lifetime of the metallic components in SOFC nano-coatings of reactive elements (RE) have been investigated as well. The application of such coatings can reduce the corrosion rates substantially and thus increase the lifetime of the fuel cell stack. It will be shown that it is possible to combine the positive effects of RE with the beneficial effects of a Co coating and thus to obtain an interconnect material with low Cr evaporation and increased oxidation resistance.

Abstract: Ferritic stainless steels have nowadays been used as materials for interconnectors in solid oxide fuel cells (SOFCs) at intermediate temperatures (800°C). Their degradation in contact with dry synthetic biogas used instead of other fuel gas has already been studied. In such biogas atmosphere, humidity may play an important role. The objective of this study is therefore to understand the effect of H₂O on the corrosion kinetics of the ferritic stainless steels type AISI441 (18CrTiNb) under synthetic biogas (70%CH₄ and 30%CO₂) mixed with 3%H₂O. The thermodynamic analysis by FactSage was used to determine the partial pressure of oxygen and the activity of carbon in the humid biogas. The results showed that the partial pressure of oxygen is in the range 10^–24.8 to 10^–21.2 bar for temperatures between 600-800°C and that the formation of solid carbon can occur in these conditions. This was not different compared with the conditions in dry biogas. These conditions lead to the stability of some important oxides such as Cr₂O₃ and Cr-Mn spinel and to carbon deposition and/or carbide formation. The surface morphology of 441 subjected to humid biogas showed oxide scale composed mainly of Cr₂O₃ topped with Cr-Mn spinel. Some carbide such as Cr₇C₃ was found in chromia scale. Kinetic experiments under both dry and humid biogas at temperatures between 600 and 800°C showed linear weight changes. Arrhenius law was followed and the rate-determining steps were identified as parallel oxidation and carburization limited by oxide-gas interface reactions.