Papers by Keyword: Reactive Element (RE)

Abstract: Alumina-forming alloys have been studied for over 50 years and are now needed for high efficiency power generation applications operating at higher temperatures. Especially in the presence of water vapor, alumina-forming alloys outperform conventional chromia-forming alloys above 1000°C. However, alloy mechanical behavior is a significant issue and alumina-forming alloy development has been limited. The opportunity for alloy development is discussed as well as thefactors that limit oxidation resistance, including alloy thermal expansion and optimizing reactive element additions. Finally, lifetime modeling is discussed for thick section components together with the need to address performance in more complex environments.

Abstract: This small review deals mainly with three issues regarding the nature and protectiveness of alumina scales grown during high-temperature oxidation: (1) sequences of phase transportation of alumina scales formed on Fe-Cr-Al and NiAl alloys, and a few aluminides, (2) combined additions of reactive element (RE) and (3) convolution of α-Al₂O₃ scales. Though the general phase transformation sequence of alumina scales is γ to θ to α phases at intermediate temperatures, variations have been reported. Directional growth of transient aluminas such as γ-Al₂O₃ and θ-Al₂O₃ is discussed with a particular emphasis on its driving force. Parabolic rate constants for the growth of α-Al₂O₃ scales are smaller when the period of transient alumina is longer because of larger α-Al₂O₃ grains. The effect of RE in slowing the parabolic oxidation saturates at a certain concentration, however combined addition further decreases the oxidation rate. The α-Al₂O₃ scales on Fe-Cr-Al alloys without RE are highly convoluted, however those on NiAl and other aluminides are not so convoluted. The α-Al₂O₃ layer beneath the outer NiO layer or NiAl₂O₄ layer is flat in the oxidation of Ni₃Al. Directions for future work are proposed.

Abstract: The “reactive element effect”, modified from its earlier representation of the “rare earth effect”, is a well known term within the oxidation community. It describes several beneficial outcomes on the oxidation behavior of alumina and chromia forming alloys. Any element can be considered “reactive” if it is more oxygen active than the scale forming element, namely that of Al or Cr. However, the relative effectiveness of each element can be quite different. Numerous scientific studies have been carried out on this topic since its discovery more than 70 years ago to gain understanding of the manifestations of and reasons for these effects. This paper gives an overview that summarizes current understandings on this effect and points to issues that warrant further studies.

Abstract: Cathodic electrodeposition was used to generate a rare earth (RE)-containing deposit on a single crystal Ni-based superalloy. The deposition parameters were optimised in order to get a RE oxy-hydroxide coating with a “well-fitted” dry-mud like morphology, i.e. presenting a multi-cracks network. A further thermal treatment was applied to dehydrate the deposit to obtain a well crystallised oxide coating (RE_xO_y). The uncoated and RE_xO_y-coated substrates were then submitted to cyclic oxidation tests at 1100°C in laboratory air. They demonstrated the efficiency of the coating as uncoated samples severely spalled after a few cycles whereas the coated ones did not lose their protective oxide layer even after 2000 cycles. This result was attributed to the formation of a duplex oxide scale very similar to that obtained on g/g’ coatings, to the presence of nanograins at the RE_xO_y/scale interface and to the Hf-rich oxide pegs at the scale/substrate interface.

Abstract: It is known that additions of reactive elements such as Ce, La or Y improve the properties of protective oxide-scales on Ni and Fe based alloys [ - ] by increasing oxide adhesion, decreasing the transient time until a continuous Cr2O3 layer is formed and decreasing the parabolic rate constant. Nevertheless, the precise roles played by these reactive elements to improve scales and the precise mechanisms by which they are incorporated into the scale during the surface treatment processes are unknown. Although they are believed to be associated with transport properties in the scale, it is not clear how this occurs or why it improves oxidation resistance. This project is aimed to gain understanding of the scale evolution in Fe-22 wt.% Cr alloys at 800 oC in dry air during the transient stage after 15 minutes of oxidation. The effect of La (120 and 290 ppm) and Ce (270 and 610 ppm) additions added during melt-stage processing are investigated. The surface oxidation process was imaged in-situ through a Confocal Scanning Laser Microscope (CSLM) and the results were correlated with post-experiment characterization through FEG-SEM and FIB-SEM combined with 3D reconstruction. The roles of rare-earth oxide particles on nucleation of Cr2O3 and blockage of short-circuit diffusion paths in the oxide scale and underlying metal are discussed.

Abstract: K38 nanocrystalline coatings with various amounts of yttrium addition were deposited by magnetron sputtering. Cyclic oxidation tests were conducted at 800-1000oC in air in order to reveal the effect of reactive elements on the oxidation behavior of nanocrystalline material. The results indicated that the influence of yttrium was not observable at 800oC. At 900 and 1000oC, addition of 0.1 wt.% Y decreased the growth rate of scale, while 0.5 wt.% Y addition significantly increased the oxidation rate of nanocrystalline coating.

Abstract: The effect of reactive element additions (external doping as an yttrium-oxide coating on the metal) on the oxidation behaviour of a commercial FeCrAl alloy (Kanthal A1) has been investigated during isothermal exposures in air at 1373K. The scale growth kinetics of the bare alloy obey a parabolic rate law during the whole oxidation test whereas the kinetic curves of the yttrium-bearing specimen exhibit an initial transient stage during the first hours, followed by a parabolic regime. The yttrium addition to the bare alloy does not give the beneficial effect usually ascribed to the reactive elements. No significant oxidation rate improvement of the alloy is observed, the parabolic rate constants values obtained are roughly similar for the both specimens. In situ X-ray diffraction reveals a marked influence of the reactive element on the composition of the oxide scale. The oxide layer formed on the yttrium-bearing specimen revealed, in addition to α- alumina which is the main oxide also identified on the bare specimen, the presence of yttrium aluminates (YAlO3, Y3Al5O12) located in the outermost part of the layer.

Abstract: Due to the reduction of operating temperature from 1000°C to 800°C, chromia forming alloys are the best candidates for interconnects in Solid Oxide Fuel Cells (SOFCs). These interconnects have to be operational in service conditions, at 800°C in air (cathode side) and in humidified hydrogen (anode side). The performance of the interconnect stainless steels is limited by the oxide scale formation (chromia), the low electronic conductivity of this scale and the possible volatility of chromium oxides. In the field of high temperature oxidation of metals, it is well known that the addition of a nanometric layer made of reactive element oxide such as, La2O3, Nd2O3 and Y2O3 by MOCVD (Metal Organic Chemical Vapor Deposition) on alloy surface resulted in an important improvement in the high temperature oxidation resistance. These coatings are made on metallic alloys in order to form perovskite oxides such as LaCrO3, NdCrO3 and YCrO3, which are expected to present a good conductivity at 800°C in air. However, this temperature looks somewhat too low to guarantee the formation of perovskite oxides and thus to improve the oxidation resistance and electrical conductivity. In fact, XRD analyses revealed that for Y2O3 coatings, perovskite oxides were not formed after oxidation in air at 800°C for 100 hours. The goal of this study is to perform pre-oxidation at 1000°C for 2 hours in air under atmospheric pressure on coated Crofer22APU to pre-form perovskite phases. The so-prepared perovskite were tested in a thermobalance in air. Experiments performed in H2/10%H2O under 150 mbar at 800°C validated the coating influence from the anode side as well as the cathode side. The corrosion products were analyzed after 100 hours ageing at 800°C by SEM, EDX, and XRD. ASR (Area Specific Resistance) was measured for the same times and temperature in air.

Abstract: Many high-temperature coatings rely on the formation of a continuous and adherent thermally grown oxide (TGO) scale of α-Al2O3 for extended resistance to degradation. For instance, the durability and reliability of thermal barrier coating (TBC) systems in gas turbines are critically linked to the oxidation behavior and stability of an alumina-forming β-NiAl-based bond coat. This study focuses primarily on the development of unique Pt+Hf-modified γ′-Ni3Al+γ-Ni coating compositions that form highly adherent, slow-growing TGO scales during both isothermal and cyclic oxidation at high temperature. Recent findings on the isothermal and cyclic oxidation behavior of γ′+γ alloys and coatings will be discussed, with particular emphasis on the effects of Pt, Al and Hf contents and distributions. Inferred reasons for the observed “Pt effect” will also be presented.