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

Oxidation of iron at 400-600 °C in dry and wet O2

The oxidation of iron in dry and wet O-2 at 400-600 degrees C has been re-investigated using gravimetry, SEM/EDX, XRD and FIB. In the presence of O-2, water vapour accelerates iron oxidation at 500 and 600 degrees C. At 400 and 500 degrees C the magnetite layer is duplex and exposure to water vapour results in the formation of blades on top of a fine-grained hematite layer. At 600 degrees C it results in a surface without needles and blades. The increased oxidation rate at 500 and 600 degrees C is attributed to a smaller grain size in the hematite layer resulting in faster ion transport

Microstructural Investigation of the HCl-Induced Corrosion of the Austenitic Alloy 310S (52Fe26Cr19Ni) at 500 °C

This paper investigates the influence of 500 ppm HCl in a 5 %O2-95 %N2 atmosphere on the oxidation of the austenitic stainless steel AISI 310S at 500 °C. Laboratory exposures were made for one, 24, 72 and 168 h and the samples were analysed with XRD, SEM/EDX, FIB and TEM/EDX. When exposed in oxygen a thin and protective chromium-rich oxide scale forms. Addition of HCl causes significantly accelerated corrosion. Within the first hour of exposure, accumulations of FeCl2, CrCl2 and NiCl2 forms below the chromium-rich oxide, especially at steel grain boundaries. The chlorine-induced corrosion is suggested to occur through an electrochemical reaction, in which the dissociation of HCl to form chloride ions at the scale surface is coupled to the oxidation of the metal surface beneath the scale by an outwards electronic current and inwards diffusion of chloride ions along oxide grain boundarie

High Temperature Oxidation of the Austenitic (35Fe27Cr31Ni) Alloy Sanicro 28 in O-2 + H2O Environment

The present study investigates the high temperature oxidation of alloy Sanicro 28 (35Fe27Cr31Ni) in 5% O-2 and in 5% O-2 + 40% H2O. Polished steel coupons were isothermally exposed in a tube furnace at 600, 700 and 800 A degrees C for up to 168 h. The samples were investigated by gravimetry, grazing angle X-ray diffraction (XRD), Auger electron spectroscopy (AES), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy/energy dispersive X-rays (STEM/EDX). The results show that the material forms a protective scale in both environments. The scale is duplex. The inner part of the scale consists of corundum type chromium-rich (Cr (x) Fe1-x )(2)O-3, and the outer layer consists of spinel type oxide. Chromia is lost from the protective oxide by vaporization of CrO2(OH)(2) in O-2 + H2O environment. The capacity of Sanicro 28 to suffer chromia vaporization without forming a rapidly growing iron-rich oxide is attributed to its high Cr/Fe ratio. The spinel formed at the oxide/gas interface could in addition be beneficial for oxidation behavior in wet oxygen because it may slow down chromia evaporation

Oxidation of Binary FeCr Alloys (Fe–2.25Cr, Fe–10Cr, Fe–18Cr and Fe–25Cr) in O2 and in O2 + H2O Environment at 600 °C

The oxidation behaviour of the binary alloys Fe-2.25Cr, Fe-10Cr, Fe-18Cr and Fe-25Cr (wt%) in dry and wet O 2 at 600 °C is investigated by isothermal exposures of carefully polished samples for up to 168 h. The oxidized samples are investigated gravimetrically and the oxides formed are studied by X-ray diffraction. X-ray photoelectron spectroscopy is used for depth profiling of the thin oxides. The scale surface is imaged by SEM. Cross-sections through the scale are analyzed by SEM/EDX for imaging and for measuring the chemical composition. The oxidation behavior of the four FeCr alloys is intermediate between those of iron and chromium. Fe-2.25Cr oxidizes in a way similar to iron in both environments, forming a poorly protective scale consisting of FeCr spinel at the bottom, magnetite in the middle and a hematite cap layer. In dry O 2, Fe-10Cr, Fe-18Cr and Fe-25Cr form a thin and protective (Fe,Cr) 2O 3 oxide similar to the chromia film formed on pure chromium. In wet O 2, Fe-10Cr, Fe-18Cr and Fe-25Cr initially form the same kind of protective oxide film as in dry conditions. After an incubation time that depends on alloy chromium content, all three alloys go into breakaway oxidation and form thick, poorly protective scales similar to those formed on Fe-2.25Cr. Breakaway oxidation in wet O 2 is triggered by the evaporation of CrO 2(OH) 2 from the protective (Fe,Cr) 2O 3 oxide

Paralinear Oxidation of Chromium in O2 + H2O Environment at 600–700 °C

The oxidation of chromium in dry O2 and in O2 + 10%H2O at 600 and 700 °C is studied. Scale morphology is investigated by several methods, including scanning electron microscopy (SEM) of cross sections prepared by focussed ion beam milling (FIB). In O2 + H2O at 600 and 700 °C, chromium forms a duplex scale consisting of an inner barrier oxide and a discontinuous outer oxide made up of blade-shaped crystals. Thermogravimetric (TG) measurements show that water vapour influences chromium oxidation by causing vaporization of the protective oxide, resulting in paralinear oxidation kinetics. An extension of the original treatment by Tedmon is deduced, which allows for the determination of the evaporation rate constant k s and the parabolic oxidation rate constant k d from TG data acquired during short exposures. The results show that k d is the same in dry O2 and in O2 + 10%H2O. Equivalently, the transport properties of chromia are the same in the two environments. The equilibrium constant of CrO2(OH)2 formation from chromia is reported. The activation enthalpy of the vaporization reaction is determined