Effect of the Third Element Cr on Filming Behaviors of Fe-xCr-10Al (at.%) Alloys during Initial Stage Oxidation at High Temperatures

RHEED study of the initial stages of crystallization and oxidation of lead and tin

The kinetics of the initial stages of the reaction of uranium with oxygen

Summary. — The initial stages of oxide nucleation and surface oxide formation are hot topics at the moment due to the possible application of these materials in many fields of science and technology. The understanding of the parameters controlling these processes is therefore pivotal not only for the fundamental knowledge of the physical phenomenon but also for enabling the growth of better quality oxide phases, with a higher degree of order and/or a lower density of contaminants. Here I will summarize the main results obtained by a collaboration between experimental groups in Genova and Osaka and between the experimentalists in Genova and theoretical groups in Trieste and Ljubljana, on the initial oxidation of the noble metals Ag and Cu. I will show that the local morphology of surface defects and/or the dosing conditions are essential elements to determine the nature of the oxide form which starts to nucleate upon exposure to O2. On stepped Ag we find that, under vacuum conditions, the stoichiometry of the initial oxide nuclei is tuned by the atomic geometry at the low coordination site, while on Cu(410) the oxidation efficiency comes out to be highly enhanced both by the presence of steps and by exposure to hyperthermal oxygen. The relative amount of cuprous and cupric oxide formed depends on oxidation temperature.

The microstructure and chemical composition of oxide films formed during the initial stages of high temperature oxidation have long been recognised as having a major effect on subsequent scaling behaviour. For example, the development of thick (>lμm) duplex scales on Fe-Cr alloys is assumed to occur via a mechanism of pore and fissure formation in the initial oxide (Atkinson, Tomlinson and Cory.) However, this process has rarely been observed using electron microscopy.The investigation of the oxidation behaviour of Fe-9Cr alloys in high temperature steam gives an insight into these scale breakdown processes. Within 3 minutes of the onset of oxidation, significant breakdown of the initially formed thin film occurs due to the development of readily visible fissures and pores at oxide grain boundaries (figure 1). This leads to the ready ingress of oxidant and the rapid development of a thick duplex Fe3O4/(Fe,Cr)3O4 oxide with a thin outer layer of α-Fe2O3 (figure 2). The extremely porous nature of this scale allows further inward diffusion of molecular oxidant, thereby instituting the continued growth of (Fe,Cr)3O4 at the scale/metal interface.

Evolution of surface chemistry and morphology of oxide scale formed during initial stage oxidation of modified 9Cr–1Mo steel

This study investigates the influence of Pt addition on the oxidation behavior of a Cr2O3‐forming superalloy. Inconel 718 (IN718) alloys with varying Pt content were prepared and subjected to isothermal oxidation tests. The results demonstrate that Pt significantly enhances the oxidation resistance of IN718, as evidenced by reduced weight gain, thinner oxide layers, and smaller oxide particles. Pt addition also increases the activation energy for both initial interface oxidation and ion diffusion during long‐term oxidation. Furthermore, Pt promotes the formation of a Cr2O3 layer while suppressing the formation of other undesirable oxides, resulting in a more cohesive and stable oxide layer. The improved oxidation resistance is attributed to two key factors: during the initial oxidation stage, Pt, as a noble element, reduces the activity of the primary oxide‐forming element Cr to oxidative environments, thereby lowering its susceptibility to initial oxidation at the metal–oxidant interface. During long‐term oxidation, Pt preferentially substitutes for Ni in major phases such as γ‐Ni(Cr,Fe) and γ′‐Ni3(Al,Ti), locally increasing the Cr composition. This promotes Cr oxidation, effectively suppressing the oxidation of Ni or Fe. These findings suggest that Pt addition is a promising approach for enhancing oxidation resistance in alloy design.

As the size of an electronic element shrinks to nanoscale, trench design of Si strongly influences the performance of related semiconductor devices. By reactive force field molecular dynamics (ReaxFF MD) simulation, the initial stage oxidation on nano-trenched Si(1 0 0) angled 60°, 90°, 120°, 150° under temperatures from 300 K to 1200 K has been studied. Inhomogeneous oxidation at the convex–concave corners of the Si surface was observed. In general, the initial oxidation process on the Si surface was that, firstly, the O atoms ballistically transported into surface, then a high O concentration induced compressive stress at the surface layers, which prevented further oxidation. Compared to the concave corner, the convex one contacted a larger volume of oxygen at the very beginning stage, leading an anisotropic absorption of O atoms. Afterwards, a critical compression was produced at both the convex and concave corners to limit the oxidation. As a result, an inhomogeneous oxide film grew on nano-trenched Si. Meanwhile, due to enhanced O transport and compression relaxation by increasing temperature, the inhomogeneous oxidation was more obvious under 1200 K. These present results explained the observed experimental phenomena on the oxidation of non-planar Si and provided an aspect on the design of nano-trenched electronic components in the semiconductor field.

Ultrathin PdO-TiO(2) composite films have been prepared for the first time by thermal decomposition of the multilayer Langmuir-Blodgett (LB) films of octadecyl amine-metal (Ti and/or Pd) ion complexes. The composite oxide film has been characterized by various spectroscopic techniques and compared with the pure ultrathin TiO(2) film. The results of X-ray diffraction (XRD) and Raman spectroscopy reveal the formation of a mostly crystalline anatase phase in the pure TiO(2) thin film whereas separate phases of PdO and TiO(2) in the composite thin film. Crystallite sizes of 4-7 nm have been estimated from the XRD line broadening. Atomic force microscopy images also reveal oriented aggregates of nanocrystallites in the ultrathin films. The results of absorption spectroscopy have shown allowed direct transitions in both films. Pure TiO(2) and the composite films have been compared for their ability to act as photocatalysts in hydrogen generation from a methanol-water mixture. It is found that the composite film has a uniform hydrogen generation rate for a long period of time and shows drastic enhancement in hydrogen production as compared to pure TiO(2) film. This is because Pd in the composite film acts as an electron trapping centre and thereby decreases the recombination process in the oxide catalyst. The present study demonstrates the potential of the LB technique to fabricate high quality composite metal oxide films useful for photocatalytic hydrogen generation.

The short term oxidation behaviour of an Fe,Al,Si alloy, oxidised at 1000°C in unpurified air, has been characterised using a range of techniques including transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was observed that this alloy formed a flat adherent alumina scale, even though analysis did not detect any conventional reactive elements. The research showed that the scale developed during the early stages of oxidation was similar to that formed at longer times (up to 24 hours) and that the tendency for the scale to remain flat and adherent was due to the presence of an impurity, uranium. Upon initial oxidation, uranium was found to segregate strongly to the metal/oxide interface, but was not detected at the oxide grain boundaries until oxidation times of approximately 20 minutes. Sulphur was observed within the scale during the initial stages of oxidation but was not associated with the uranium, the oxide grain boundaries or the metal/oxide interface. Therefore, uranium is not acting as a sulphur getter, reducing sulphur activity in the scale, but instead it appears to either block the sites at the oxide/metal interface where sulphur would normally segregate or change the chemistry of the interface in such a way as to make the segregation of sulphur thermodynamically unfavourable.

Effects of Si concentration and surface polishing conditions on the structures and chemical compositions of the initial oxide films formed on 18-8 and 18Cr stainless steels in high temperature water were investigated by means of electron microscopy, transmission electron diffraction and X-ray microanalysis.Main results obtained are as follows:(1) Polishing conditions had effects on structures and compositions of oxide films produced on low-Si containing 18-8 stainless steel (0.02 wt%Si); namely corundum type oxides formed on the mechanically polished surface consisted of Fe and Cr, but spinel type oxides produced on the chemically (or electrolytically) polished surface consisted of Fe, Cr and Ni.(2) The spinel type oxides, on the contrary, were formed on three differently polished surfaces of 18-8 stainless steels bearing 2.29 wt%Si. However, chemical compositions of these spinel type oxides were affected by the polishing conditions; Fe, Cr and Si were detected from the oxides formed on the mechanically polished surface, while the oxides produced on the electrolytically (or chemically) polished surface consisted of Fe, Cr, Ni and Si.(3) No effects of polishing conditions were noticed on structures and chemical compositions of the oxide films formed on variously polished surfaces of the both low-Si (0.17 wt%Si) and high-Si (2.24 wt%Si) containing 18Cr stainless steels. The corundum type oxides consisting of Fe and Cr were formed on 18Cr-0.17Si stainless steels and spinel type oxides of Fe and Cr with amorphous silicon oxides were produced on 18Cr-2.24Si stainless steels.(4) For both 18-8 and 18Cr stainless steels, the corundum type oxides formed in high temperature water changed to the spinel type oxides with increasing Si concentration.

The aim of this work was to perform thermal characterization of commercially pure titanium in dry air to determine its oxidation kinetics and the structure of the oxide. The oxidation kinetics were determined thermogravimetrically under isothermal conditions in the temperature range 300 to 750 ºC for 48 hours and the structure of the oxides was determined by differential thermal analyses and X-ray diffraction in the temperature range room temperature - 1000ºC. The oxidation rate of titanium increased with increase in temperature. It was high in the initial stages of oxidation and then decreased rapidly with time, especially up to 600 ºC. The kinetic laws varied between inverse logarithmic at the lower temperatures (300 and 400 ºC) and parabolic at the higher temperatures (650, 700 and 750 ºC). Evidences from X-ray diffraction and differential thermal analyses data revealed that the passive oxide film formed at room temperature crystallized into anatase at about 276 ºC. The crystallized oxide formed in the range 276 - 457 ºC consisted of anatase, in the range 457 - 718 ºC consisted of anatase and rutile sublayers, and at temperatures beyond 718 ºC consisted of a layer of pure rutile. Scanning electron microscopy observations reveled that the oxidized surfaces were crack-free and the surface roughness increased steadily with oxidation temperature.

Initial oxidation of Ni-based superalloy and its dynamic microscopic mechanisms: The interface junction initiated outwards oxidation

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We investigate the initial and secondary oxidation products on the Si(111)-(7 × 7) surface at room-temperature using atomic force microscopy (AFM) and density functional theory calculations. At the initial oxidation stages, we find that there are two types of bright spots in AFM images. One of them is identified as a Si adatom with one O atom inserted into one of the backbonds, while the other is ascribed to a Si adatom with two inserted O atoms. We observe that the latter one turns into the secondary oxidation product by a further coming O2 molecule, which appears as a more protruded bright spot. The atomic configuration of this product is identified as Si adatom whose top and all three backbonds make bonds with O atoms. The appearances of initial and secondary oxidation products are imaged as bright and dark sites by scanning tunneling microscopy, respectively. It is revealed that AFM gives us the topographic information close to the real atomic corrugation of adsorbed structures on the semiconductor surfaces.

Fe - 9 to 12%Cr alloys are a material for the thick sections boiler components and steam lines of a power plant. The role Fe - 9 to 12%Cr alloys is becoming more prominent in the development of a new generation of Ultra-Supercritical (USC) Power Plant due to the target operating temperature is reaching 620 °C (893 K), in 100% steam condition as well as pressure in excess of 300 bar (30 × 106 Pa). In such condition, the integrity of Fe - 9 to 12%Cr alloys relies on the oxide scale formed during the time of exposure. However due to the high temperature and water vapor condition, it is a well known fact that, the formation of oxide scale is accelerated thus depleting the structural integrity of the Fe - 9 to 12%Cr alloys over the time. Studies show that not only the formation of protective oxide scale was suppressed but the formation of non-protective oxide scale was accelerated instead. Decades of studies done by various groups around the globe has yet to have consensual on the exact mechanism of this phenomenon. Initial stage oxidation of these alloys plays great roles in hope to understand the formation of oxide scale in water vapor condition at high temperature. This paper reviews previous research works to understand the initial stage oxidation of Fe - 9 to 12%Cr alloys at high temperature in water vapor condition.