Abstract: Wagner’s 1959 diffusion model of the internal oxidation process provided a method of predicting the rate at which a binary alloy was penetrated by dissolved oxygen as it precipitated the more reactive (but dilute) alloy component. Parabolic kinetics were predicted to depend on oxygen permeability in the unreacted alloy solvent and also, in cases where the reactive component was sufficiently mobile, the diffusion coefficient of the latter. The model has proven very successful, but is restricted to single oxidant-binary alloy systems, in which the precipitated oxide has extremely low solubility. This paper reviews recent results on a number of internal precipitation processes which cannot be described with the Wagner theory. These include formation of low stability carbides and nitrades; internal precipitation driven by multiple oxidants; the templating effects of prior precipitates on subsequently formed corrosion products; cellular precipitation morphologies; internal interface diffusion effects; volume changes in the reaction zone and the effects upon them of simultaneous external scaling.
Abstract: The concept of the diffusion barrier coating system (DBC system) is summarized and the latest results are presented. The DBC system is comprised of alloy substrate/diffusion barrier/Al-reservoir/an external scale. Diffusion flux (JAl) of Al through the barrier layer will be given approximately by JAl = DAl x SAl x (d CAl/d x), where DAl and SAl are the diffusion coefficient and solubility limit of Al in the barrier layer, respectively as well as d CAl / d x is driving force given by the concentration difference across the barrier (d CAl) divided by the thickness of the barrier layer (d x). A slow diffusion flux can be obtained by using low values of DAl, SAl, or (d CAl /d x). Accordingly, a selection of a barrier layer with lower DAl and SAl is essential. A low driving force is also an important factor, and can be achieved by using lower CAl with a constant barrier layer thickness dx. At higher temperatures, however, the barrier layer can react with the alloy substrate and Al-reservoir layer, resulting in gradual degradation of the barrier layer. This means that the thickness dx of the barrier layer tends to decrease and may finally disappear. With decreasing thickness of the diffusion barrier layer, the driving force (dCAl/dx) will increase, and the effectiveness of the barrier layer will be eliminated. Therefore, it is essential to maintain a constant thickness of the barrier layer for long exposure time. Several types of the DBC system are proposed, a single barrier layer and triple-layers with g + g’ and g’ inserted among these barrier layers.
Abstract: In oxides which are exposed to thermodynamic potential gradients, transport processes of the mobile components occur. These transport processes and the coupling between different processes are not only of fundamental interest, but are also the origin of degradation processes, such as kinetic demixing, kinetic decomposition, and changes in the morphology of the material. The kinetics of high temperature oxidation processes of metals can be studied in situ by X-ray absorption spectroscopy (XAS), optical microscopy and X-ray diffraction (XRD) at elevated temperatures and defined oxygen partial pressures. As an example, the in situ XAS investigation of the oxidation of cobalt, forming layers of CoO and Co3O4, will be discussed.
Abstract: Void formation in a duplex scale formed on Fe-5Cr alloy at 773 K has been elucidated by oxygen chemical potential distribution, the flux of oxide ion and its divergence. The calculation predicts that voids preferentially form at the interface between inner and outer scales in the low oxygen partial pressure in which the predominant defect of iron is interstitial ion. The flux of oxide ion changes discontinuously at this interface and the divergence of the flux gives voids. Calculated volume fraction of voids at this interface is in good agreement with that has been measured.
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: Laboratory metal dusting test of several Ni binary alloys containing the representative element was conducted in a simulated syngas atmosphere at 650°C for 100h. The Ni alloys containing element belonging to Group 14 and 15 in the periodic series exhibited excellent metal dusting resistance, while those containing Group 13 did not. This behavior was able to be reasonably interpreted from the Blyholder mechanism and the concept of Pauling’s electronegativity.
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: 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: Very thin Fe-coatings, ~50nm, were found to suppress metastable Al2O3 formation on Fe-50Al and Ni-50Al alloys in our previous study. The authors proposed a mechanism whereby α-Al2O3 precipitates from the Al-saturated Fe2O3, which was formed during initial oxidation, since α-Al2O3 and α-Fe2O3 have isomorphous structures. In order to confirm the proposed mechanism, in-situ measurements were made of structural changes in the oxide scales formed on FeAl with and without Fe coating during heating and subsequent isothermal high temperature oxidation by synchrotron radiation with a two-dimensional X-ray detector. Diffraction peaks from Fe2O3 were initially observed at around 350°C on Fe-coated samples. The lattice parameter of the Fe2O3 initially increased linearly due to thermal expansion, but then rapidly decreased due to the formation of a solid solution of Fe2O3-Al2O3. α-Al2O3 started to appear at around 800°C, but no peaks from metastable Al2O3 were observed. The diffraction peaks from the α-Al2O3 on Fe-coated samples consisted of two distinct peaks, indicating that the α-Al2O3 had two different lattice parameters. These results suggest that the α-Al2O3 was formed not only by precipitation from the Al-saturated Fe2O3, but also by oxidation of Al in the substrate.