Materials Science Forum Vols. 595-598

Abstract: The present study is concerned with the influence of sputter-coatings CaO on the oxidation behavior of Ni polycrystals. The experiments were performed in air, in the temperature range 800°-1200°C. Below 1200°C, CaO coatings reduce the oxidation rate, while this beneficial effect disappears at 1200 °C. The oxidized specimens were examined by SEM and X-Ray diffraction, but also by EPMA depth profiling to evaluate the scale composition. Furthermore, electrical conductivity measurements and kinetic demixing studies were carried out on Ca-doped NiO single crystals, to get a better insight regarding the transport processes involved during oxidation. These last results show that the key features allowing to explain the effect of CaOcoatings on the oxidation rate of Ni are the influence of calcium on the increase of the dissociation pressure of NiO, which delays the oxidation of nickel, the kinetic demixing of the cations, which controls the distribution of CaO precipitates in the scale responsible for blocking effects, and the increase of the diffusion coefficient of both the cations and the cationic vacancies, which play a decisive role at high temperature, when the scale growth is dominated by lattice diffusion.

Abstract: Long-term cyclic oxidation behavior was compared for commercial FeCrAl alloys and model Fe-Al and FeCrAl alloys, and their coefficients of thermal expansion (CTE) were measured. For single-phase disordered (ferritic) Fe(Al) alloys, the CTE increased only slightly with Al content and was similar to that of FeCrAl alloys. More significant CTE increases were observed at ≥20%Al, as intermetallic phases, Fe3Al and FeAl, formed. As expected, the intermetallic compositions showed increased oxide spallation rates during cyclic oxidation at 1100° and 1200°C. However, after extensive spallation and loss of Al from the substrate, the compositions of Fe3Al and FeAl specimens entered the ferritic phase field, and the amount of scale spallation decreased. Among commercial oxide dispersion strengthened (ODS) FeCrAl alloys, a composition containing Mo (ODM751) exhibited the lowest thermal expansion and showed the slowest degradation rate in long-term testing at 1100°C. The concept of minimizing CTE as a route for alloy development was investigated.

Abstract: The oxidation behaviour and the subsequent chemical failure of a Fe-20Cr-5Al based alloy were studied at 1200°C in laboratory air, at times of up to 700 hours. Tests on 70 micron thick foils, showed void formation at the metal/oxide interface soon after the aluminium content in the alloy dropped below a critical level (≤0.5 wt%). At this stage, the alloy could no longer sustain alumina scale formation and resulted in the initiation and development of a Cr-rich sub-layer. This chromia layer was found to be continuous and of a uniform thickness. As the sub-layer formed, voids were also observed at the metal/oxide interface. The voids were found to fill with chromia after further exposure. It is thought that the change in oxide growth mechanism from alumina to chromia growth is responsible for the void formation. This also explains the lack of void formation during the sustainable growth of the alumina scale. The introduction of silicon to the Fe-20Cr-5Al based alloy via a diffusion couple was found to significantly influence the oxidation behaviour of the thin foils. Void formation was observed directly beneath the alumina scale and filled voids were now found to contain silicon oxide rather than chromia. The void filling mechanism also appeared to be different. With chromia filled voids, the filling commenced from the underside of the oxide, with the oxide growing inwards, while silica containing voids were filled by silica growing outwards into the void from the substrate. Throughout the study, optical and scanning electron microscopes were used to analyse all stages of oxidation and the subsequent failure of the thin foil samples. EBSD was also used to generate a more comprehensive analysis of selected locations.

Abstract: The paper describes an examination of the effect of the addition of zirconium as a third element on the heat-resisting properties and explains the high temperature oxidation mechanism of Fe3Al intermetallic compounds. The Fe3Al and Fe3Al-0,05Zr specimens have been isothermally oxidized in the temperature range of 1173-1473 K in synthetic air for 100 hrs. The formed oxide layer, about 1,5-2 μm thick, was Al2O3. An examination of the cross-sectioned scales by SEM-EDS showed that the alumina layer consisted of a small inner columnar layer and an outer equiaxed grain layer. Additionally, very fine (50-150 nm) oxide grains rich in Zr, further identified as ZrO2, were found across the alumina scales. To understand the role of Zr on the growth mechanism of α–Al2O3 oxide scale on Fe3Al materials, two-stage oxidation experiments were performed (16O2/18O2), followed by SIMS and TEM-SAD observations. Particular attention was paid to the use of TEM in order to precisely characterize the products on samples prepared using the FIB (Focused Ion Beam) method. A combination of analytical techniques revealed that ZrO2 particles, most of which were formed along alumina grain boundaries, enhanced oxygen diffusion along grain boundaries due to oxygen-deficient composition of zirconium oxide (ZrO2-y).

Abstract: For a number of chromia and alumina forming high temperature alloys and coatings, recent studies revealed, that in some cases the specimen/component or coating thickness may substantially affect the growth rates of the surface oxides. For the alumina formers the thickness dependence is mainly governed by depletion of oxygen active elements such as Y, Zr, Hf, Mg which are either intentionally added alloying elements or manufacturing related alloy impurities. In the case of the chromia forming materials, which tend to exhibit a more substantial dependence of oxidation rate on specimen/component thickness, depletion of minor alloying additions is also an important factor to be considered. However, for these alloys relaxation of oxide growth stresses by plastic deformation of the metallic substrates seems to be the dominant parameter which governs the observed behaviour.

Abstract: The determination of the corrosion processes of metallic materials in glass melts is of great interest for glass makers. Our attention has been specifically focussed on Cr-bearing alloys that form Cr2O3 layers when they are immersed in a silicate melt and offer a good resistance to melt corrosion. The comprehension of the corrosion processes has been extensively studied in the last 10 years using stationary electrochemical techniques. Results relative to the thermodynamic state were described. Complex impedance spectroscopy offers the possibility to determine the reaction kinetics in terms of limiting processes. Three chromium rods were directly immersed in molten glass at T=1050°C, maintained respectively in the active, passive and transpassive state and studied using electrochemical techniques. The results that are reported in this paper show a good agreement between complex impedance spectroscopy data, scanning electron microphotographies of the glass/metal interface and previous results obtained using stationary electrochemical techniques.

Abstract: Chromia-forming steels are excellent candidates to resist to high temperature oxidizing atmospheres because they form protective oxide scales. The oxide scale growth mechanisms are studied by exposing AISI 304 stainless steel to high temperature conditions in air, and the analyses were carried out by means of thermogravimetry and in situ X-rays diffraction. The in situ XRD analyses carried out during high temperature AISI 304 steel oxidation in air reveals the accelerated growth of iron-containing oxides such as hematite Fe2O3 and iron-chromite FeCr2O4, when the initial germination of the oxide layer contains the presence of a manganese-containing spinel compound (1000°C). When the initial growth shows the only chromia formation (800°C), hematite formation appears differed in time. Protection against corrosion is thus increased when the initial germination of manganese-containing spinel oxide is inhibited in the oxide scale.

Abstract: Based on a coupled numerical and experimental approach, the design and implementation of an in situ thermal-gradient-controlled cyclic oxidation test, dedicated to the investigation of the TGO and TBC spallation, is presented. The influence of the specimen through thickness thermal gradient as well as the benefits of the video real-time monitoring of the cooling phase of an oxidation test is discussed in the case of the spallation of an alumina scale grown on FeCrAl alloys.

Abstract: Interdiffusion in Fe-Ni-Cr (fcc γ phase) alloys with small additions of Si and Ge at 900°C was studied using solid-to-solid diffusion couples. Alloy rods of Fe-24 at.%Ni, Fe-24 at.%Ni- 22at.%Cr, Fe-24 at.%Ni-22at.%Cr-4at.%Si and Fe-24 at.%Ni-22at.%Cr-1.7at.%Ge were cast using arc-melt, and homogenized at 900°C for 168 hours. Sectioned alloy disks from the rods were polished, and diffusion couples were assembled with in Invar steel jig, encapsulated in Argon after several hydrogen flushes, and annealed atz 900°C for 168 hours. Polished cross-sections of the diffusion couples were characterized to determine experimental concentration profiles using electron probe microanalysis with pure elemental standards. Interdiffusion fluxes of individual components were calculated directly from the experimental concentration profiles, and the moments of interdiffusion flux profiles were examined to determine the average ternary and quaternary interdiffusion coefficients. Effects of alloying additions on the interdiffusional behavior of Fe-Ni- Cr-X alloys at 900°C are presented with due consideration for the formation of protective Cr2O3 scale.

Abstract: This work compares the oxidation behaviour of three iron-based substrates (C40E, Invar and 304L) in CO2 industrial gas, in order to determine the conditions for producing wüstite (Fe1-xO), on the basis of kinetic and morphologic studies. For the three alloys at the beginning of the reaction, wüstite formed under 105 Pa of CO2 following a rate law          RT 220000 10.2 P e dt d( m/S) CO2 . For 304L, formation of a spinel phase chromite briefly preceded it during a first step. Magnetite appeared for long times of experiment, in the case of Invar and 304L. This can be explained by the stopping of iron outer diffusion. These results are discussed according to the literature and thermodynamic data. They open new fields for coating these alloys by plasma spray processes.