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

It is well-known that Cr addition to Al2O3 forming alloys and coatings are beneficial to decrease the critical Al content for an external Al2O3 scale formation. This beneficial Cr effect is known as a Third Element Effect, TEE. The TEE explains that initial Cr2O3 scale formation in the transient oxidation period reduces the oxygen potential at the surface of alloy, promoting the exclusive Al2O3 scale formation. However, our previous study of oxidation of Fe-low Cr-Al alloys found that an Al2O3 scale was formed on alloys with lower Al content regardless initial Cr2O3 scale formation, which suggests that decreased oxygen potential due to initial Cr2O3 scale formation is not the essential reason for decreasing the critical Al content for an external Al2O3 scale formation. In this study, we investigated the initial oxidation behavior of Fe-(6~10)Al, Fe-(4~24)Cr, and Fe-(4~24)Cr-(6~10)Al (in at.%) model alloys and tried to understand the role of not only Cr but also Fe and Al on development of the Al2O3 scale. Particular attention was paid for the effect of the initial transient oxide scale on development of an external Al2O3 scale. The formation of initially formed transient oxide(s) and its transition to α-Al2O3 during heating, followed by isothermal oxidation up to 10~60min at 1000°C were characterized by in-situ high-temperature X-ray diffraction by means of Synchrotron radiation. The oxide scale formed at different temperatures during heating was observed by TEM. The distribution of each element in the oxide scale was measured by STEM-EDS. The oxide scale initially observed on all alloys by XRD was α-Fe2O3. The transition from initial transient oxide, Fe2O3, to Cr-rich Cr2O3-Fe2O3 solid-solution during heating on the alloys with higher Cr content was confirmed by an in-situ high-temperature XRD experiments. Beneath this transient oxide scale, Al-rich amorphous oxide scale was found to develop on the alloys with Al addition. No Cr effect was observed in terms of the amorphous layer formation. This Al-rich amorphous layer prevented the growth of outer Fe-rich transient oxide. Therefore, the oxidation mass gain of alloys with Al at relatively lower temperatures, ~700°C was much smaller than that of binary Fe-Cr alloys. Break down of the amorphous layer resulted in rapid oxidation of alloys. The time for break down of the amorphous layer became longer with increasing alloy Cr content, which indicates that Cr helped to maintain the Al-rich amorphous layer for a longer oxidation time at lower temperatures, ~700°C. The amorphous layer crystalized to α-Al2O3 on the alloys with higher Al content, when the sample temperature became about 900 to 1000°C. The crystallization to α-Al2O3 occurred continuously from the interface between an outer transient/amorphous layers to the amorphous/alloy interface. However, on the alloy with lower Al content, amorphous internal Al2O3 precipitates were developed and the amorphous layer disappeared accompanied by formation of internal precipitates. Although the volume fraction of internal precipitates increased with increasing alloy Cr content, higher Cr addition did not prevent internal oxidation of Al. During heating, amorphous internal precipitates transformed to α-Al2O3. After the transformation to α-Al2O3, the internal precipitate grew larger and connected each other, forming an α-Al2O3 scale below the transient oxide scale. No apparent effect of the transient oxide scale on formation of the amorphous internal oxide and its transition to an α-Al2O3 layer was observed on alloys with lower Al content. The Al-rich amorphous layer was found to have important role for preventing Fe-rich oxide scale formation for all alloys with Al. The Fe- or Cr-rich transient oxide, which formed in the initial oxidation stage acted as the template for the transformation from amorphous to α-Al2O3 for the alloys with higher Al content, but it did not have apparent role for formation of the α-Al2O3 scale on the alloys with lower Al content.

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.

Natural source of Cr(VI) in soil: The anoxic oxidation of Cr(III) by Mn oxides

TEM study of the initial oxide scales of Ti 2AlC

Characteristics of oxide scale formed on Cu-bearing austenitic stainless steel during early stages of high temperature oxidation

Marine microbial Mn(II) oxidation mediates Cr(III) oxidation and isotope fractionation

Hydrogen have an effect on initial steam oxidation behavior of heat resistant steels and alloys. This effect can be observed clearly in dual atmosphere oxidation. Initial oxidation test in the dual atmosphere gives useful knowledge to clarify the hydrogen effect in steam oxidation. However, dual atmosphere oxidation tests are inapplicable to initial oxidation and include some technical difficulties in assembling and sealing. In contrast, in-situ observation using microscopes is one of the useful techniques to clarify particularly initial oxidation, whereas this technique is used only for small samples. This study aims to fabricate a small sample with hydrogen charged palladium and simulate dual atmosphere oxidation using this sample. A disk-shaped sample of Fe-9mass%Cr alloy with blind holes was prepared. Several pieces of hydrogen charged palladium were placed in the blind holes, then the blind holes were sealed with cupper bars. After preparing the sample, oxidation test was conducted at 923 K for 1.8 ks in ambient air. A sample without hydrogen charged palladium was also oxidized as a reference sample. For the sample with hydrogen charged palladium, magnetite mainly formed on the sample after oxidation. This acceleration in oxidation is caused by hydrogen permeation from inside the sample during oxidation. Overall, this method is useful for in-situ observation during dual atmosphere oxidation.

Rapid fabrication of Nb-Si based alloy by selective laser melting: Microstructure, hardness and initial oxidation behavior

Oxidation of alloy often involves chemical partition and injection of vacancies. Chemical partition is the consequence of selective oxidation, while injection of vacancies is associated with the differences of diffusivity of cations and anions. It is far from clear as how the injected vacancies behave during oxidation of metal. Using in-situ transmission electron microscopy, we captured unprecedented details on the collective behavior of injected vacancies during oxidation of metal, featuring an initial multi-site oxide nucleation, vacancy supersaturation, nucleation of a single cavity, sinking of vacancies into the cavity and accelerated oxidation of the particle. High sensitive energy dispersive x-ray spectroscopy mapping reveals that Cr is preferentially oxidized even at the initial oxidation, leading to a structure that Cr oxide is sandwiched near the inner wall of the hollow particle. The work provides a general guidance on tailoring of nanostructured materials involving multi-ion exchange such as core-shell structured composite nanoparticles.

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.

Manganese (Mn) oxides, which are generally considered biogenic in origin within natural systems, are the only oxidants of Cr(iii) under typical environmental conditions. Yet the influence of Mn biooxide mineral structural evolution on Cr(iii) oxidation under varying geochemical conditions is unknown. In this study we examined the role of light, organic carbon, pH, and the structure of biogenic Mn oxides on Cr(iii) oxidation. Aging of Mn oxides produced by a marine bacterium within the widespread Roseobacter clade resulted in structural ripening from a colloidal hexagonal to a particulate triclinic birnessite phase. The structurally diverse Mn oxides were then reacted with aqueous Cr(iii) within artificial seawater in the presence or absence of carbon and light. Here we found that Cr(iii) oxidation capacity was highest at near neutral pH and in the combined presence of carbon and light. Mn oxide ripening from a hexagonal to a triclinic birnessite phase led to decreased Cr(iii) oxidation in the presence of carbon and light, whereas no change in reactivity was observed in the absence of carbon and/or in the dark. As only minimal Cr(iii) oxidation was observed in the absence of Mn oxides, these results strongly point to coupled Mn oxide- and photo-induced generation of organic and/or oxygen radicals involved in Cr(iii) oxidation. Based on Mn oxide concentration and structural trends, we postulate that Mn(ii) produced from the oxidation of Cr(iii) by the primary Mn oxide is recycled in the presence of organics and light conditions, (re)generating secondary hexagonal birnessite and thereby allowing for continuous oxidation of Cr(iii). In the absence of this Mn oxide regeneration, Cr(iii) induced structural ripening of the hexagonal birnessite precludes further Cr(iii) oxidation. These results highlight the complexity of reactions involved in Mn oxide mediated Cr(iii) oxidation and suggest that photochemical carbon reactions are requisite for sustained Cr(iii) oxidation and persistence of reactive Mn oxides.

Effect of various factors on the metastable to stable phase transformation behavior of a thermally grown oxide scale of Al2O3 was reviewed and discussed on the basis of the results obtained from our recent studies. In this paper, among the factors affecting this phase transformation, particular attention has focused on the effect of Fe in order to understand the different phase transformation behavior of Al2O3 scale formed on different alloy systems in different oxidation environments. In order to explore the effect of Fe on the phase transformation, the oxidation behavior of various Al2O3 forming Fe-based model alloys was systematically investigated. The development of different oxide scales on different alloys during a high-temperature oxidation, including the initial transient stage of oxidation followed by isothermal oxidation in different oxidation atmospheres, was examined by in-situ X-ray diffraction by means of the Synchrotron radiation. Fe was found to accelerate the phase transformation to α-Al2O3. This effect can be considered to be due to an initial formation of Fe2O3, which is an isomorphous crystal structure with α-Al2O3. Further contributing factor of Fe on the transformation can be a doping of Fe3+ ion, which formed during an initial transient oxidation stage, in the Al2O3 scale. The oxidation atmospheres are considered to indirectly affect the phase transformation by changing in the formation behavior of Fe2O3 and (Al, Fe)2O3 solid solution during an initial transient oxidation stage.

Dependence of scale thickness on the breaking behavior of the initial oxide on plasma spray bond coat surface during vacuum pre-treatment

Cr(III) oxidation to Cr(VI) significantly increases Cr mobility and toxicity and thus its environmental risks. Manganese (Mn) oxides may serve as the potential oxidants of Cr(III) in environment. Natural Mn oxides in the environment are believed to be derived from bacterial oxidation. The objective of this study was to examine the Cr(III) oxidation capacity of biogenic Mn oxide and the role of Mn-oxidizing bacteria in Cr(III) oxidation. Batch experiments were conducted to investigate the capacities of Cr(III) oxidation by chemically synthetic Mn oxides and biogenic Mn oxide. Biogenic Mn oxide was formed by Bacillus sp. WH4, a Mn-oxidizing bacterium isolated from Fe–Mn nodules of a Chinese soil. Various Cr(III) and Mn(II) were added to the growth medium of Bacillus sp. WH4 to evaluate Cr(III) oxidation coupled with Mn(II) bacterial oxidation. The Cr(III) oxidation capacity of biogenic Mn oxide was 0.24 mmol g−1 and higher than three chemically synthetic Mn oxides. No Mn(III) intermediate was detected during Mn(II) bacterial oxidation. Bacillus sp. WH4 could promote Cr(III) oxidation through oxidizing Mn(II), although it could not oxidize Cr(III) directly. The participation of Mn-oxidizing bacteria makes Cr(III) oxidation more complicated in environment. These findings illustrate the need to consider bacterial activity and the Mn(II) level when predicting the fate of Cr and the potential applications of Mn oxides in the remediation of pollutants in environment.

Substrate temperature-induced changeover from homogeneous to step nucleated growth modes for the initial thermal oxidation of Si(001) by O 2