Henrik Asteman

Influence of Water Vapor and Flow Rate on the High-Temperature Oxidation of 304L; Effect of Chromium Oxide Hydroxide Evaporation

The effect of roman PH2O and flow rate on the oxidation of 304Lat 873 K in oxygen is reported. High concentrations of water vapor and highflow rates result in breakaway corrosion. The mass gain after 168 hrincreased by four to five times, compared to oxidation in dry O2. Inthe presence of H2O, the corrosion products consisted of arelatively thin (Cr,Fe)2O3 oxide plus large oxide islandsconsisting mainly of Fe2O3. A mechanism explaining theeffect of water vapor on marginal chromia formers is proposed.

Indication of Chromium Oxide Hydroxide Evaporation During Oxidation of 304L at 873 K in the Presence of 10% Water Vapor

The oxidation of type 304L stainless steel wasinvestigated at 873 K in the presence of O2and O2 + 10% H2O. Oxidation timevaried between 1 and 672 hr. The oxidized samples wereinvestigated by a number of surface-analytical techniques includinggrazing-angle XRD, SEM/EDX, auger spectroscopy, SIMS andXPS. Oxidation in dry oxygen results in the formation acorundum-type oxide (Me2O3) withadditional formation of spinel oxides after prolonged exposure. Theoxide layer contained mainly chromium, with smalleramounts of Fe and Mn. Oxidation in the presence of watervapor results in an oxide that contains more Fe and less Cr, the outer part of the oxide beingdepleted in Cr. In the presence of water vapor, a massloss is detected after prolonged exposure. We show thatthe mass loss is caused by chromium evaporation. The volatile species is suggested to beCrO2(OH)2.

Evidence for chromium evaporation influencing the oxidation of 304L: The effect of temperature and flow rate

The influence of temperature and flow rate on the oxidation of 304L steel in O2/H2O mixtures was investigated. Polished samples were isothermally exposed to dry O2 and O2+40% H2O at 500–800°C at 0.02–13 cm/sec flow velocity, for 168 hr. The samples were analyzed by gravimetry, XRD, ESEM/EDX, and AES depth profiling. The oxidation of 304L in water vapor/oxygen mixtures at 500–800°C is strongly influenced by chromium evaporation. The loss of chromium tends to convert the protective chromia-rich oxide initially formed into a poorly protective, iron-rich oxide. The rate of oxidation depends on flow rate; high flow rates result in an early breakdown of the protective oxide. The most rapid breakdown of the protective oxide occurs at the highest temperature (800°C) and the highest gas flow (4000 ml/min=13 cm/sec). The oxide formed close to grain boundaries in the metal is more protective, while other parts, grain surfaces suffer breakaway corrosion. The protective oxide consists of a Cr-rich 50–200-nm thick M2O3 film, while the parts experiencing breakaway corrosion form a 10–30-μm thick Fe-rich M2O3/M3O4 scale. The results show that chromium evaporation is a key process affecting the oxidation resistance of chromia formers and marginal chromia formers in O2/H2O mixtures.

Oxidation of 310 steel in H2O/O2 mixtures at 600 °C: the effect of water-vapour-enhanced chromium evaporation

Abstract The oxidation of type 310 stainless steel was investigated at 600 °C in the presence of O 2 and O 2 +10% and 40% H 2 O. The effect of gas velocity was studied. The oxidized samples were investigated by grazing angle X-ray diffraction, SEM/EDX and SAM. The addition of H 2 O to O 2 resulted in a change of oxidation behaviour. A strong dependence on flow rate was observed in O 2 /H 2 O mixtures. At low flow rates a thin (30–50 nm) protective α-(Cr,Fe) 2 O 3 formed, the outer part being depleted in chromium. When the flow rate was increased beyond a critical value the protective oxide failed. Under these conditions ⩾5 μm thick α-Fe 2 O 3 /(Cr,Fe) 3 O 4 , oxide islands formed on the part of the surface corresponding to the centre of the alloy grains. The effect of water vapour is attributed to the water-vapour-assisted evaporation of chromium from the oxide, in the form of a chromium oxide hydroxide, probably CrO 2 (OH) 2 . The oxidation behaviour is rationalized using a qualitative mechanism proposed previously and parallels that of the 304L alloy.

Effect of Water-Vapor-Induced Cr Vaporization on the Oxidation of Austenitic Stainless Steels at 700 and 900°C Influence of Cr/Fe Ratio in Alloy and Ce Additions

The oxidation of 153MA, 310 and 353MA austenitic stainless steels was investigated at 700°C and 310 and 353MA at 900°C in O 2 and in O 2 + 40% H 2 O. 153MA was not studied at 900°C because it suffered excessive corrosion. The influence of gas velocity was studied. The oxidized samples were investigated by grazing angle X-ray diffraction, scanning electron microscopy/energy dispersive analysis by X-rays Auger-electron spectroscopy, and glow discharge optical emission spectroscopy. In the presence of water vapor, gas velocity strongly influenced oxidation. This effect is attributed to chromium evaporation in the form of CrO 2 (OH) 2 . Exposure in O 2 + 40% H 2 O at 700°C using high flow rates caused breakaway corrosion on all alloys. At 900°C, only the 310 and 353MA alloys were investigated. In O 2 + H 2 O environment, both alloys showed a mass loss at low flow rates due to chromium evaporation. At high flow rates, alloy 310 suffered breakaway corrosion while 353MA did not. The corrosion products consisted of a relatively thin Cr-rich (Cr,Fe) 2 O 3 oxide plus thick, iron-rich oxide islands. The greater corrosion resistance of 310 and 353MA steels in environments where chromium evaporation is a factor is attributed to the high Cr/Fe ratio. In contrast, the presence of Ce and Si in the MA grade steels appears to have little beneficial effect on breakaway corrosion triggered by Cr evaporation.

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.

A pilot plant study of the effect of alkali salts on initial stages of the high temperature corrosion of alloy 304L

Alloy 304L was exposed for between 15 min to 12 hr in the 12MW CFB research boiler at the Chalmers university of technology using an air-cooled probe. The base fuel consisted of a mixture of 67% wood chips and 33% pellets. In addition to the base fuel experiment, a number of exposures were performed where S and Cl was added to the fuel in the form Of SO2(g) and HCl(aq) in order to control the flue gas chemistry in the superheater region. After the exposures the samples were analysed by ESEM/EDX, XRD and SAM. Burning a mixture of woodchips/pellets without adding sulphur or chlorine results in the formation of K2SO4 deposits on the corrosion probes. When HCl is added to the fuel KCl deposits form. The simultaneous addition of HCl and SO, results in a deposit consisting of a mixture of KCl and K2SO4. In all environments studied an oxide in the 100nm range forms. With time, the oxide becomes covered by ash deposits. After exposure to the biomass flue gas environment, the oxide is enriched in K, especially the outer part. Chlorine is not present in the oxide even when the KCl(s) forms on the surface. It is suggested that potassium chromate formation occurs by the reaction of potassium chloride with chromium oxide.

Investigation of FIB-thinned TEM cross-sections of oxide scales formed on type 310 steel at 600°C in water vapour-containing oxygen atmospheres

AbstractThe effect of water vapour on the oxidation of the type 310 (25Cr19Ni) austenitic stainless steel was studied through the detailed microstructure characterisation of the oxide scales formed at 600°C in dry O2, O2 containing 40% H2O at 0.5 cm/s, and O2 containing 40% H2O at 5 cm/s. FIB equipment was utilised for imaging and for producing thin foils containing the cross-section of oxide features for detailed analysis using TEM/EDS. Thin protective Cr-rich (Cr,Fe)2O3 was maintained in dry O2 atmosphere. Cr-loss through CrO2(OH)2 evaporation in H2O-containing O2 atmosphere from the oxide scale could, at sufficiently high flow rate, trigger local breakaway oxidation after 168 hours of exposure. These breakaway sites featured large oxide nodules, each consisting of an outward growing oxide island and an inward growing oxide crater. The local post-breakaway oxidation behaviour was discussed based on the composition and distribution of the various phases in these oxide nodules.

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.

Microstructure of Oxidised 304L Steel and the Effects of Surface Roughness on Oxidation Behaviour

This report deals with the microstructure of the oxide formed on 304L steel exposed to dry oxygen at 600°C. The specimen surfaces were pre-treated by either mechanical grinding or polishing and oxidised for 24, 168 or 672 hours. It is observed using SEM and TEM that the oxide is composed of two types of oxide species, i.e. the sub-micron spinel particles on the outer regions and the finer corundum-type base oxide grains that are formed first on the steel surfaces. The spinel particles contain roughly 40 cation % Mn, 45 cation % Cr, 8 cation % Cu and 7 cation% Fe. Such a result shows a significant Mn, Cr and Cu enrichment compared to the composition of the steel. Individual particles are homogeneous while the composition varies substantially from one particle to another. Also, more spinel particles form on specimens with ground surfaces. The base oxide grains, measuring ≃ 10 nm, are of the corundum-type oxides (Cr,Fe) 2 O 3 with varying Cr/Fe ratios. EDX line scans results from the specimen cross-section indicate that the oxide closest to the steel is most often Cr-rich and that Cr and Mn are depleted from the steel immediately below the oxide.