Torbjörn Jonsson

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.

Microstructural investigation of the KCl-induced corrosion of the austenitic alloy Sanicro 28 (35Fe27Cr31Ni) at 600°C

Abstract High-temperature corrosion of stainless steel is important, particularly in bio-fuelled boiler applications. The flue gas is rich in water vapour and alkali salts, which accelerate the corrosion of the boiler material. The focus of this paper is on the breakdown of the protective oxide scale formed on Sanicro 28, which is a highly alloyed austenitic stainless steel (35Fe27Cr31Ni), in the presence of KCl(s). Laboratory exposures were carried out at 600°C in 5% O2+40% H2O for 1, 24 and 168 hours. The samples were coated with 0.10 mg/cm2 KCl prior to exposure and uncoated samples were exposed for reference. The aim was to link the observed mass gains and microstructure to oxidation mechanisms. The oxidized samples were analyzed by XRD, SEM/EDX, FIB and TEM/EDX. The exposures results in a very complex corrosion chemistry, including, e.g. the formation of potassium chromate and the rapid transport of iron species on the surface resulting in accumulation of oxide on the former KCl particles. However, the consumption of chromium and the presence of chloride on the surface does not result in the breakdown of the protective oxide. The ability of the alloy to withstand this harsh environment is tentatively attributed to the high Cr/Fe ratio.

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.

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.

Microstructural investigation of the initial oxidation at 1450°C and 1500°C of a Mo(Si,Al)2-based composite

Abstract The initial oxidation behaviour of a Mo(Si,Al)2-based composite (Kanthal Super ER) was investigated. The microstructures of unexposed reference specimens and samples oxidized isothermally in air at 1450°C for 5 and 50 minutes and also at 1500°C for 5 minutes were evaluated. Thin foil specimens were prepared using a focused ion beam/scanning electron microscope and analysed in an SEM with scanning transmission electron microscopy detectors and an energy dispersive X-ray system for high-resolution imaging and chemical analysis. The samples formed relatively pure α-Al2O3 scales during the oxidation. The Al supply to the growing alumina scale is mainly provided by the Mo(Si,Al)2 phase in the bulk material. The outward Al transport is counterbalanced by inward diffusion of Si into Mo(Si,Al)2. Therefore an Al poor Mo5(Si,Al)3 phase forms directly below the scale. The Al concentration in the subjacent Mo(Si,Al)2 decreased to 20 at.%, corresponding to Mo(Si1.4,Al0.6).

The Influence of H2O on Iron Oxidation at 600°C: A Microstructural Study

The oxidation of iron in dry O2 and in wet O2 (40% H2O) has been studied at 600°C. The oxide microstructure was investigated by SEM/EDX, FIB and XRD. Iron forms a layered scale in dry and wet oxygen at 600°C. The scale consists of a top hematite layer, a middle magnetite layer and a wüstite layer close to the scale metal interface. All three layers grow with time, but with different growth rates, the overall growth being approximately parabolic. The presence of water vapour increases the rate of oxidation and affects the evolution of the oxide microstructure. The higher rate of oxidation in the presence of water vapour is due to an increased growth rate of the magnetite layer and, especially, of the hematite layer, while the growth of the wüstite layer is not affected. It is suggested that water vapour influences grain boundary transport in the hematite layer.

The effect of Ce on the high temperature oxidation properties of a Fe–22%Cr steel: microstructural investigation and EELS analysis

Abstract The effect of a 10 nm Ce coating layer on long term oxidation behaviour (up to 3000 hours) of a Fe–22%Cr ferritic stainless steel for solid oxide fuel cell (SOFC) interconnect application is investigated. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) analysis showed segregation of Ce to the grain boundaries of Cr,Mn spinel layer adjacent to the scale-gas interface. The findings in this study are in line with the grain boundary blocking model for the Ce effect mechanism. However, segregation of Ce was observed at the grain boundaries of the (Cr,Mn) spinel in the vicinity of the scale-gas interface. No evidence of Ce segregation was found in the grain boundaries of the chromia layer, neither any Ce rich particle was observed in the chromia layer after longer exposure times.

Microstructural investigations of pure nickel exposed to KCl induced high temperature corrosion

Abstract Oxidation of 99·99% pure nickel was studied with and without 0·10 mg cm−2 KCl(s) in an environment containing 5 vol.-%O2, 40 vol.-%H2O and 55 vol.-%N2 at 600°C for up to 168 h. Oxide microstructure was investigated by X-ray diffraction (XRD), focused ion beam (FIB), broad ion beam (BIB) and SEM/EDX. Oxidised nickel shows an approximately parabolic oxide growth rate. The oxide scale is dense with some pores at the oxide/metal interface. Adding small amounts of KCl does not result in a faster corrosion rate of nickel. However, the surface morphology changes and small oxide crusts were observed in the vicinity of former KCl particles. This is proposed to be the result of a NiCl2–KCl eutectic on top of the oxide scale formed above 514°C. The oxide scale formed in the presence of KCl contains more and differently distributed voids than the scale formed without KCl.

Characterization of pack cemented Ni2Al3 coating exposed to KCl(s) induced corrosion at 600 °C

Abstract Oxidation of Ni2Al3 produced by pack aluminizing of pure nickel was studied with and without 0.10 mg cm−2 KCl(s) deposit in an environment containing 5% O2, 40% H2O and 55% N2 at 600 °C for up to 168 h. Oxide microstructure and composition was investigated by SEM/EDX, BIB, TEM and GDOES. Oxidised Ni2Al3 shows minimal weight gain, while adding KCl results in a small weight loss consistent with evaporation of KCl. On the surface of samples exposed to the gas environment only, a 30 nm oxide of Al oxide was present, but where KCl was present as deposit, 50–250 nm thick nodules form that are enriched in K, O and Al.

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.