Sead Canovic

Microstructural investigation of the effect of water vapour on the oxidation of alloy 353 MA in oxygen at 700 and 900°C

Abstract The objective of this work was to study the impact of water vapour on the corrosion behaviour of the austenitic stainless steel 353MA at 700 and 900°C through a detailed microstructural characterisation of the oxide scales formed, after 168 hours, in O2 and O2 with 40% H2O. The oxidized samples were investigated by scanning electron microscope/energy dispersive X-ray, focused ion beam and transmission electron microscope/energy dispersive X-ray. At 700°C 353MA forms a Cr-rich protective (Fe,Cr)2O3 oxide scale, with some silica at the oxide/metal interface. Breakaway oxidation occurs in H2O/O2 mixtures because the oxide scale is depleted in Cr due to the formation of CrO2(OH)2(g). However, the microstructural investigation indicated that a healing Cr-rich oxide layer formed beneath the Fe-rich oxide after some time. This could be a result of the high Cr/Fe ratio of 353MA. The behaviour at 900°C was different. In spite of the loss of Cr from the oxide scale, breakaway oxidation did not occur, i.e. the oxide scale remained protective. The microstructural investigation showed a thick, almost continuous silica layer at the oxide/metal interface, which may act as a diffusion barrier at the higher temperature.

Oxidation of FeCrAl foils at 500–900°C in dry O2 and O2 with 40% H2O

Abstract High temperature resistant FeCrAl alloys are frequently used in high temperature applications such as heating elements and metal based catalytic converter bodies. When exposed to high temperatures an adherent, slowly growing, dense aluminium oxide layer forms on the surface, which protects the underlying alloy from severe degradation. The composition, structure and properties of the formed oxide layer are strongly dependent on the alloy composition, temperature and oxidation environment. In this study, the Sandvik 0C404 FeCrAl alloy, in the form of 50 μm thick foils, was exposed isothermally in the temperature range 500–900°C for 168 hours in dry O2 and in O2 with 40 vol.% H2O. The surface morphology, composition and microstructure of the grown oxide scales were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), grazing incidence X-ray diffraction (GIXRD), Auger electron spectroscopy (AES), and time of flight secondary ion mass spectrometry (TOF-SIMS). The oxidation process was faster at 900°C than at 500 and 700°C. At 500°C a thin (10–20 nm) mixed oxide of Fe, Cr and Al was formed. Exposure at 700°C resulted in a similar (40–50 nm) duplex oxide, in both dry O2 and in O2 with 40 vol.% H2O. These oxide scales consisted of an inner and an outer relatively pure alumina separated by a Cr-rich band. This type of duplex oxide scale also formed at 900°C with a thin inward growing α–Al2O3 at the oxide/metal interface and an outward growing layer outside a Cr-rich band. However, at 900°C the outward growing layer showed two types of oxide morphologies; a thin smooth base oxide and a much thicker nodular oxide grown on top of substrate ridges. In dry O2 atmosphere, the main part of this outward growing layer had transformed to α–Al2O3. Only in the outer part of the thick oxide nodules, metastable alumina was found. When exposed in the presence of water vapour the main part of the metastable alumina remained untransformed.

Microstructural Investigation of the Initial Oxidation of the FeCrAlRE Alloy Kanthal AF in Dry and Wet O2 at 600 and 800 ° C

The FeCrAlRE (where RE is reactive element) alloy Kanthal AF was exposed isothermally at 600 and 800 degrees C for 72 h in dry O-2 and in O-2 with 10 vol % H2O. The mass gains were 3-5 times higher at the higher temperature. The presence of water vapor increased the oxidation rate at 800 degrees C, while no significant effect was observed at 600 degrees C. A thin two-layered oxide formed at 600 degrees C: an outer (Fe, Cr)(2)O-3 corundum-type oxide, containing some Al, and an inner, probably amorphous, Al-rich oxide. At 800 degrees C a two-layered oxide formed in both environments. The inner layer consisted of inward grown alpha-Al2O3. In dry O-2 the originally formed outward grown gamma-Al2O3 had transformed to alpha-Al2O3 after 72 h. Water vapor stabilized the outward grown gamma-Al2O3 and hence no transformation occurred after 72 h in humid environment. RE-rich oxide particles with varying composition (Y, Zr, and Ti) were distributed in the base oxide at both temperatures and in both environments. The RE-rich particles were separated from the alloy substrate by a layer of Al-rich oxide. At 800 degrees C the Y-rich RE particles were surrounded by thick oxide patches in both dry and humid O-2

Grain contrast imaging in FIB and SEM

Grain contrast imaging can be performed with several techniques. In order to be able to choose the most suitable one it is important to know which techniques are available and also to be aware of the strengths and weaknesses of each technique. In this work, the grain contrast imaging is performed with secondary electrons, backscattered electrons, forward scattered electrons, transmitted electrons in the scanning electron microscope, and with secondary electrons in the focused ion beam instrument. The advantages and disadvantages of each method are discussed in order to make it easier to choose the most appropriate technique for grain contrast imaging.

CVD TiC/alumina multilayer coatings grown on sapphire single crystals.

Multilayers of TiC/α-Al(2)O(3) consisting of three (1 μm thick) alumina layers separated by thin (∼ 10 nm) oxidized TiC layers have been deposited onto c-, a- and r-surfaces of single crystals of α-Al(2)O(3) by chemical vapour deposition (CVD). The aim of this paper is to describe and compare the detailed microstructure of the different multilayer coatings by using transmission electron microscopy (TEM). The general microstructure of the alumina layers is very different when deposited onto different surfaces of α-Al(2)O(3) single crystal substrates. On the c- and a-surfaces the alumina layers grow evenly resulting in growth of single crystal layers of TiC and alumina throughout the coating. However, when deposited on the r-surface the alumina layers generally grow unevenly. No pores are observed within the alumina layers, while a small number of pores are found at the interfaces below the TiC layers. The TiC and alumina layers grow epitaxially on the c- and a-surface substrates. On the r-surface, epitaxy is present only at some rare locations. The TiC layers were oxidized in situ for 2 min in CO(2)/H(2) prior to the alumina layer deposition. For all three samples chemical analyses show that the whole TiC layer is oxidized. On the c- and a-surfaces the TiC layer was oxidized to an fcc TiCO phase. On the r-surface the oxidation stage resulted in a transformation of the initially deposited fcc TiC to a monoclinic TiCO phase, which appears to be a modified TiO structure with a high carbon content.

Influence of H2O ( g ) on the Oxide Microstructure of the Stainless Steel 353MA at 900 ° C in Oxygen

This work investigates the impact of water vapor on the corrosion behavior of the austenitic Si-containing FeCrNi steel 353MA at 900°C through a detailed microstructural characterization of the oxide scales formed after 168 h in O2 and in O2 with 40% H2 O. The oxidized samples were investigated by focused ion beam and transmission electron microscopy in combination with energy dispersive X-ray analysis. The microstructural investigation showed that the oxide scales were affected by the presence of water vapor. However, there was no significant difference in scale thickness. In both atmospheres a continuous chromia [Cr-rich (Fe,Cr)2 O3] layer was present beneath the spinel oxides. The influence of water vapor on scale composition is attributed to chromia evaporation by the formation of Cr O2 (OH)2 (g). The ability of the alloy to maintain a continuous chromia layer in spite of chromia evaporation and to avoid breakaway oxidation is attributed to several factors. First, the supply of chromium to the scale by diffusion in the alloy must be rapid. Second, the presence of spinel oxides at the oxide/gas interface may decrease chromia evaporation. Third, the high CrFe ratio in the alloy is suggested to make it difficult to convert the protective chromia to poorly protective hematite.

Thin foil analysis in the SEM

This paper explores the possibilities for imaging and chemical analysis of thin foil specimens in the SEM. Bright field and dark field imaging provide high resolution imaging with crystallographic information within the grains. In multiphase materials with varying electron transmission the dark field images generally provide a more even contrast in all phases. It is possible to obtain high-quality quantitative EDX data with high spatial resolution.

Comparing depth profiling of oxide scale on SOFC interconnect-materials using ToF-SIMS with 69Ga+, Bi3+/Cs+ and C60+/C602+ as primary and sputter ions

Abstract Oxide scale cross-sections of CeO2 coated FeCr based solid oxide fuel cell interconnect materials were examined using secondary ion mass spectrometry (SIMS) depth profiling. A duplex spinel∶chromia scale was formed after 1 h at 850°C. Ti and ceria were observed between these layers. Additionally, minor concentrations of Mn, Si and Nb were observed at the oxide/metal interface. Furthermore, Al and Ti were concentrated primarily in the metal surface close to the oxide/metal interface. Secondary ion mass spectrometry sputter depth profiles using different ion sources; 69Ga+, Bi3+/Cs+ and C60+/C602+ were compared with TEM oxide scale cross-section and field emission gun–Auger electron spectroscopy depth profiling. Secondary ion mass spectrometry depth profiling with 69Ga+, Bi3+/Cs+ showed decreased secondary ion yields in the metallic matrix. This decrease could be avoided using oxygen flooding. The C60 cluster ion depth profiles were less sensitive to type of matrix and gave the best correspondence to the TEM cross-section. However, the impact energy has to be high enough to avoid carbon deposition.

Cyclic oxidation of two FeCrAlRE foils at 1100 oC - The influence of the concentration of minor alloying elements on scale microstructure

Two FeCrAlRE alloys, a commercial, 0C404, and a model alloy in the form of thin foils, with different Mn, Nb, Mo and Ti concentrations were subjected to cyclic oxidation in lab air at 1100°C. The oxidized samples were studied by gravimetry, Grazing-Incidence X-ray Diffraction (GI-XRD), Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive X-ray (EDX) analysis. The two FeCrAl alloys exhibit different oxidation kinetics; however, both alloys have the same weight gain after 500 hours exposure. During the early stages the scale consists mainly of α-Al2O3 together with some oxide particles containing Mn, Al, Fe and Cr formed on the alloys. After 500 hours the 0C404 scale locally also consists of larger polycrystalline regions of Mn-Cr-Al spinel. In addition, Si-rich oxide, chromia and Al-Cr oxide could be observed at the metal/oxide interface.