Willem J. Quadakkers

Differences in growth mechanisms of oxide scales formed on ODS and conventional wrought alloys

The oxidation behavior in air of two ODS alloys, MA 754, a chromia former, and MA 956 an alumina-scale former, has been compared with that of conventional wrought model alloys with similar compositions. The main effects on scale properties of both oxide types due to oxide dispersions were found to be improved adherence, decreased growth rates, and enhanced selective oxidation. In addition to metallography, X-ray diffraction, energy dispersive X-ray analysis of the scales, and studies of scale morphology, the detailed growth mechanisms of the oxide layers were studied using an18O tracer technique. The results show that the oxides on the conventional alloys grow by both metal and oxygen transport, and that the addition of oxide dispersions suppresses the outward scale growth. This change in growth mechanism is a possible explanation for the observed improved scale adherence, decreased growth, and enhanced selective oxidation in the yttria-containing alloys.

Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

Abstract In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO2 and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO2–30% H2O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. It was found that in the CO2- and/or H2O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide formation in the CO2-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO2(–H2O)-rich gases appeared to be permeable to CO2 molecules resulting in substantial carburization of the steel.

The effect of microstructure on the oxidation behaviour of TiAl-based intermetallics

Abstract The oxidation behaviour of three TiAl-based intermetallics, Ti47Al2Cr, Ti48Al5Cr and Ti48Al5Ni, in Ar20%O 2 at 900°C was investigated. The main emphasis was placed on the effect of alloy microstructure on the oxidation behaviour. It was found, that, depending on the exact composition, the oxidation resistance of TiAl-based intermetallics is strongly affected by the amount and distribution of the various phases which are present in the alloy. The oxide scale growth rates depends on the actual manufacturing process and subsequent heat treatment procedures. The differences imparted by these factors can be much larger than the effects imparted by ternary or quaternary alloying additions.

The oxidation behaviour of niobium containing γ-TiAl based intermetallics in air and argon/oxygen

The oxidation behaviour of γ-TiAl based alloys with different Nb contents (2–10 At.%) was investigated in air and in argon-20% oxygen at 900 °C using thermogravimetric analysis. The oxide scales were characterized by a combination of optical microscopy, SEM/EDX and X-ray diffraction analyses. Although in all studied cases the presence of niobium improves the oxidation resistance of γ-TiAl, the oxidation kinetics, scale morphology and composition in air differed strongly from that in argon-oxygen. In air the oxidation resistance increases with increasing niobium-content. In Ar/O2 the niobium dependence is far more complex because internal oxidation occurs which is favoured by the presence of niobium. SNMS analysis revealed that the differences in behaviour in the two atmospheres are related to the formation of Ti-rich nitride at the scale/alloy interface during air oxidation. The positive effect of niobium on the oxidation resistance of γ-TiAl is mainly caused by a decrease of the transport processes in the heterogeneous TiO2/Al2O3-surface scale. Nitride formation and/or niobium enrichment at the scale/alloy interface also affect the oxidation behaviour, however these factors are believed to be the result of the decreased transport processes rather than the main reason for the niobium effect.

Composition, structure and protective properties of alumina scales on iron-based oxide dispersion strengthened alloys

The oxidation behaviour of a number of iron/chromium/aluminium based oxide dispersion strengthened (ODS) alloys of the type MA 956 was investigated at temperatures between 900 and 1200°C. Apart from studies on long-time cyclic oxidation resistance, the main emphasis was placed on the effect of alloy yttria content on scale composition and structure as a function of time and temperature. The detailed scale analyses were made by using Rutherford backscattering analysis in addition to electron-optical methods. It was found that during short-term oxidation the scale growth rate at 900°C showed a deviation from the Arrhenius-type temperature dependence observed at higher temperatures. This could be attributed to formation of a metastable alumina modification in the initial stages of oxidation at 900°C. Studies of a series of alloys with variations of alloy yttria content in the range 0.17−0.7% showed an increase in the oxide growth rate with increasing yttria content. The reason for this technologically Import...

Thermodynamic and Kinetic Aspects of the Corrosion of High-Temperature Alloys in High-Temperature Gas-Cooled Reactor Helium

In corrosion tests with iron- and nickel-based alloys in simulated cooling gases of the primary circuit of high-temperature gas-cooled reactors (HTGR helium), different effects have been found. The materials may be carburized or decarburized, depending on gas composition, gas supply rate, and test temperature. The surface scales may be composed of oxides and spinels, of mixed oxide/carbide layers, or of carbides, and internal oxidation may become significant. The basic corrosion mechanism could not be explained by the simple use of thermodynamics, but a significant step forward is possible if the kinetics of the different oxidation and carburization reactions are taken into account. The classical stability diagram for chromium, the most important alloying element in these alloys, can then be used for the prediction of the corrosion effects and the corrosion products. Besides the usual description of reaction rates, the kinetics must include the changes in the oxidizing and carburizing potentials at the metallic surface caused by surface scale formation. The influence of some additional alloying elements present in commercial high-temperature alloys can be estimated by comparing their stability with the stability of chromium.

Surface analytical investigations on the oxidation behaviour of TiAl-base intermetallics

SummaryThe oxidation behaviour of TiAl-base intermetallics has been investigated at 800°C in air. The main emphasis was placed on the mechanism by which niobium additions decrease the oxidation rate of titanium aluminides. For this purpose specimens of Ti50Al and Ti45Al10Nb were oxidized in a two-stage oxidation technique using an 18O-tracer. The scale formed during this oxidation process was analyzed by SNMS. It was found that niobium is mainly incorporated in the titanium dioxide and the nitride scale which forms beneath the initially formed alumina layer. The effect of this behaviour of the niobium on the oxide scale growth rates is discussed.

Parameters affecting transient oxide formation on FeCrAl based foil and fibre materials

The effects of oxidation temperature and atmosphere on the formation of alumina scales on two commercial FeCrAl foil materials have been investigated. The oxidized specimens were characterized using a range of surface analysis techniques including SEM, XRD, laser induced optical spectrometry (LIOS), AES and XPS. During oxidation at temperatures exceeding 1000°C, the protective oxide scales formed on FeCrAl-alloys consist mainly of alpha alumina. At lower temperatures, however, formation of transient alumina modifications, has been observed. Although after longer oxidation times transformation into the stable alpha alumina occurs, the high initial growth rate of the metastable oxide phases could lead to a critical depletion of the Al-reservoir in thin walled (e.g. 20 (m) components, resulting in early breakaway failure. The occurrence of metastable oxides cannot simply be correlated with alloy composition.

Analysis and modelling of transport processes in alumina scales on high temperature alloys

SummaryThe oxidation behaviour of a number of FeCrAl-based oxide dispersion strengthened (ODS) alloys of the type MA 956 was investigated at temperatures between 1100°C and 1200°C. The main emphasis was placed on the effect of alloy yttria content on scale composition and structure. Studies of three model alloys with various alloy yttria contents in the range 0.02–0.7% showed an increase in the oxide growth rate with increasing yttria content. Alumina scales on alloys with 0.17% and 0.7% yttria grow almost exclusively by oxygen diffusion, whereas the scale on the alloy with 0.02% yttria exhibits a significant contribution from cation diffusion. By using SIMS depth-profiling and SIMS-imaging it was found that the oxygen diffuses through the alumina scale via oxide grain boundaries. TEM-studies revealed that the yttria is incorporated in the scale grain boundaries as precipitations and as a segregation layer. This behaviour of the yttrium is believed to be responsible for its influence on scale growth mechanisms. A mathematical model has been developed which describes the alumina scale growth by oxygen diffusion through grain boundaries. The model accounts for the observed increase in aluminium oxide grain size in scale growth direction.

The oxidation behaviour of the 9 % Cr steel P92in CO2- and H2O-rich gases relevant to oxyfuel environments

Abstract In oxyfuel plants metallic heat exchanging components will be subjected to service environments containing high amounts of CO2 and water vapour. In the present paper, the oxidation behaviour of the ferritic/martensitic 9 % Cr steel P92 was studied in a model gas mixture containing 70 % CO2-30 % H2O in the temperature range 550 – 650 °C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. In the CO2- and/or H2O-rich gases, the steel formed iron-rich oxide scales which possess substantially higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich oxide scales are formed as a result of a decreased flux of chromium in the bulk alloy toward the surface. This is the result of enhanced internal oxidation of chromium in the H2O-containing gases and carburisation in the CO2 gases. The oxide scales allow molecular transport of CO2 towards the metallic surface, resulting in carburisation of the alloy. The presence of water vapour induced buckling in the outer haematite layer, apparently as a result of compressive oxide growth stresses. Buckling did not occur in the H2O-free gas. This has been discussed in terms of the potential for H2O to increase growth stresses and accelerate crack propagation. The oxidation rates in CO2–H2O do not seem to be higher than those observed in flue gases of conventional fossil fuel fired power plants.