Microstructural investigation of the breakdown of the protective oxide scale on a 304 steel in the presence of oxygen and water vapour at 600°C (original) (raw)
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Microstructural investigation of the oxide scale on low carbon steel
There has been a long history of studies on the oxidation behaviour of carbon steel at high temperatures. The oxidation behaviour and the scale structure developed are complex. The microstructure and the ratio of the three classical layers of wüstite, magnetite and haematite may be difficult to identified by conventional techniques such as optical, standard Scanning Electron Microscopy (SEM) or X-ray diffraction. An unambiguous characterisation of the scale and the correct identification of the phases within the scale are difficult unless the crystallographic structure for each phase in the scale is considered and a microstructuretexture analysis is carried out. Throughout this work, Electron Backscatter Diffraction (EBSD) has proved to be a powerful technique for identifying the individual phases in the oxide scale accurately in order to obtain a better understanding of the mechanism of the oxide scale growth on steel and how scale microstructures develop. The results have shown how oxidation conditions such as temperature and time of exposure effect the grain size and microstructural development of the oxide phases.
The Use of EBSD to Study the Microstructural Development of Oxide Scales on 316 Stainless Steel
316 stainless steel has been oxidised at 1200 degrees C in air for varying times and with different cooling rates. The resulting scales were examined using optical and electron microscopy techniques including electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS). It was found that the scales on a sample oxidised for 4 hours consists of three layers; the lowest layer is a fine equiaxed region which has a uniform distribution of chromium which is similar to the base metal, followed by a larger equiaxed layer with very little chromium content but a high iron content, with a final layer of columnar grains of which some are rich in nickel. With a slower cooling rate a large amount of internal oxidation within the metallic substrate was observed which showed a chromium content higher than the oxidised metal.
The Use of EBSD to Characterise the High Temperature Oxides Formed on Low Alloy and Stainless Steels
Materials Science and Technology, 2006
Exposure of steel to high temperatures in air leads to the formation of an oxide scale, the composition and structure of which depends sensitively on the oxidation conditions and the alloying elements contained within the steel. In this paper the oxide scale structures formed on low alloy and stainless steels are characterised using Electron Backscatter Diffraction (EBSD) in the SEM. In low alloy steels this crystallographic information enables both the phases within the scale (i.e. haematite, magnetite and wüstite) and orientation relationships between them to be established. This showed that both strong preferred growth within the phase layers, and orientational relationships between phase layers, can occur depending on the composition and oxidation conditions. For the scales on stainless steels the technique enabled the two crystallographic structures that form; corundum and spinel to be isolated. These structures can be easily differentiated using the EBSD data alone, but the individual phases within them can only be distinguished by using the chemical data, collected simultaneously with the EBSD data, because of their crystallographic similarity. This technique revealed two discrete phases for each structure within the oxide scales. For the spinel structure this consisted of a predominantly chromium and iron containing layer adjacent to the substrate below a coarse grained phase composed of nickel and iron. Meanwhile an iron rich (haematite) layer at the upper scale surface and a thin chromium rich phase that exists within the fine-grained lower scale both possessed the corundum structure.
Study of the morphology of oxide scale formedon hot-rolled steel
2014
Mechanism of oxide scales formation on steel during hot rolling process is delicately determined and their structures are extremely complex. This work is part of larger studies made to understand the oxide scale behavior. Therefore, the morphology of oxides is determined by optical microscopy. Identification of the mechanical properties of oxide scales is achieved by micro-hardness measurement. The work has revealed a variation of microstructure in several layers of oxide. It was obtained that the oxide scales consisting mainly of wustite FeO, magnetite Fe3O4 and hematite Fe2O3 owing to the formation of voids and cracks in the scales, especially on the outer layer where it is high porous. The intermediate layers is thicker than others oxide layers. The outer layer has a lowest hardness and highest porosity.
Oxidation of Metals
The high-temperature corrosion of low-alloyed steels and stainless steels in the presence of KCl(s) has been studied extensively in the last decades by several authors. The effect of KCl(s) on the initial corrosion attack has retained extra focus. However, the mechanisms behind the long-term behavior, e.g., when an oxide scale has already formed, in the presence of KCl(s) are still unclear. The aim of this study was to investigate the effect of the microstructure of a pre-formed oxide scale on low-alloyed steel (Fe-2.25Cr-1Mo) when exposed to small amounts of KCl(s). The pre-oxidation exposures were performed at different temperatures and durations in order to create oxide scales with different microstructures but with similar thicknesses. After detailed characterization, the pre-oxidized samples were exposed to 5%O 2 + 20%H 2 O + 75%N 2 (+KCl(s)) at 400 °C for 24, 48, and 168 h and analyzed with scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and focused ion beam. The microstructural investigation indicated that Clinduced corrosion is a combination of oxide thickness and microstructure, and the breakaway mechanism in the presence of KCl(s) is diffusion-controlled as porosity changes prior to breakaway oxidation were observed.
The corrosion behavior of three stainless steels (316L, Alkrothal 720 and Kanthal-APM) in static LBE with oxygen concentration of 10 À 5 wt% at temperatures of 700°C for 230 h and 800°C for 360 h was studied. The steel surface morphology of the oxide scales formed was investigated by SEM, XRD and Raman spectroscopy and the cross-section by SEM and EDX. A transitional inner-layer of Fe-Cr-Al oxides was found at the substrate interface and an Al-oxide outer layer on the Fe-Cr-Al alloys. Fast growing nonprotective oxide scale with underlying dissolution was observed on the 316L alloy.
Journal of Nuclear Materials, 2007
Oxide dispersion strengthened ferritic steels are being considered for a number of advanced nuclear reactor applications because of their high strength and potential for high temperature application. Since these properties are attributed to the presence of a high density of very small (nanometer-sized) oxide clusters, there is interest in examining the radiation stability of such clusters. A novel experiment has been carried out to examine oxide nanocluster stability in a mechanically alloyed, oxide dispersion strengthened ferritic steel designated 12YWT. Pre-polished specimens were ion irradiated and the resulting microstructure was examined by atom probe tomography. After ion irradiation to 0.7dpawith150keVFeionsat300°C,ahighnumberdensityof0.7 dpa with 150 keV Fe ions at 300°C, a high number density of 0.7dpawith150keVFeionsat300°C,ahighnumberdensityof4 nm-diameter nanoclusters was observed in the ferritic matrix. The nanoclusters are enriched in yttrium, titanium and oxygen, depleted in tungsten and chromium, and have a stoichiometry close to (Ti + Y):O. A similar cluster population was observed in the unirradiated materials, indicating that the ultrafine oxide nanoclusters are resistant to coarsening and dissolution under displacement cascade damage for the ion irradiation conditions used.
Protective and non-protective oxide formation on 304 stainless steel
Applications of Surface Science, 1981
Parameters controlling the formation of protective and non-protective oxides on 304 stainless steel were examined by• using Auger electron spectroscopy to monitor oxides formed in the vacuum chamber. Variables found to influence the oxide formation include: oxygen partial pressure,