Browsing by Author "Huntz, Anne Marie"

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    Comparative study of high temperature oxidation behaviour in AISI 304 and AISI 439 stainless steels.
    (2003) Sabioni, Antônio Claret Soares; Huntz, Anne Marie; Luz, Elizete Conceição da; Mantel, Marc; Haut, Christian
    This work deals with a comparison of high temperature oxidation behaviour in AISI 304 austenitic and AISI 439 ferritic stainless steels. The oxidation experiments were performed between 850 and 950 °C, in oxygen and Ar (100 vpm H2). In most cases, it was formed a Cr2O3 protective scale, whose growth kinetics follows a parabolic law. The exception was for the the AISI 304 steel, at 950 °C, in oxygen atmosphere, which forms an iron oxide external layer. The oxidation resistance of the AISI 439 does not depend on the atmosphere. The AISI 304 has the same oxidation resistance in both atmospheres, at 850 °C, but at higher temperatures, its oxidation rate strongly increases in oxygen atmosphere. Concerning the performance of these steels under oxidation, our results show that the AISI 439 steel has higher oxidation resistance in oxidizing atmosphere, above 850 °C, while, in low pO2 atmosphere, the AISI 304 steel has higher oxidation resistance than the AISI 439, in all the temperature range investigated.
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    Diffusion of iron in Cr2O3 : polycrystals and thin films.
    (2005) Sabioni, Antônio Claret Soares; Huntz, Anne Marie; Silva, Firmino da; Jomard, François
    Chromia protective layers are formed on many industrial alloys to prevent corrosion by oxidation. The role of such layers is to limit the inward diffusion of oxygen and the outward diffusion of cations. A number of chromia-forming alloys contain iron as a major component, such as the stainless steels. To check if chromia is a barrier to the outward diffusion of iron in these alloys, iron diffusion in chromia was studied in both polycrystals and oxide films formed by oxidation of Ni–30Cr alloy in the temperature range 700–1100 ◦C at an oxygen pressure equal to 10−4 atm. An iron film of about 80 nm thick was deposited on the chromia surface, and after the diffusing treatment, iron depth profiles were established by secondary ion mass spectrometry (SIMS). Two diffusion domains appear whatever the nature of the chromia material, polycrystals or films. In the first domain, using a solution of the Fick’s second law for diffusion from a thick film, effective or bulk diffusion coefficients were determined. With the second domain, Le Claire’s and Hart’s models allowed both the bulk diffusion coefficient and the grain-boundary diffusion parameter (αDgbδ) to be obtained. Iron bulk and grain-boundary diffusion does not vary significantly according to the microstructure of chromia. The activation energy of grain-boundary diffusion is at least equal or even greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Iron diffusion was compared to cationic self-diffusion in the bulk and along grain boundaries and related to the protective character of chromia.
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    High temperature oxidation behavior of the AISI 430A and AISI 430E stainless steels in Ar/H2/H2O atmosphere.
    (2008) Salgado, Maria de Fátima; Sabioni, Antônio Claret Soares; Huntz, Anne Marie; Rossi, Édson Hugo
    The high temperature oxidation behavior of two ferritic stainless steels type AISI 430A and AISI 430E is examined at low oxygen pressure and high temperatures. The AISI 430A steel is ferritic up to 860 °C. Above this temperature, this steel is bi-phased: presence of austenite and ferrite phases. The 430E steel is stabilized with Nb, and is ferritic at all temperatures. The oxidation experiments were performed in a thermobalance SETARAM TGDTA 92, in the range of 850-950 °C, in Ar/H2/H2O atmosphere, under oxygen partial pressures lower than 1.3 x 10–18 atm. The microstructure and the composition of the oxide scales were analysed by scanning electronic microscopy (SEM) and energy dispersive spectroscopy (EDS). Different oxidation behaviors in AISI 430A and AISI 430E stainless steels were observed. At 850 °C, the oxidation of the 430A steel is greater than that of the 430E steel, but above 900 °C the oxidation of the 430A steels is lower than that of the 430E steel. The oxidation rate of the 430A steel shows low dependence on temperature, while the oxidation of the 430E follows an Arrhenius law, with an activation energy corresponding to the chromia scale growth.
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    Measurement of iron self-diffusion in hematite single crystals by sims and comparison of cation self-diffusion in corundum-structure oxides.
    (2005) Sabioni, Antônio Claret Soares; Huntz, Anne Marie; Daniel, Antônio Márcio J. Mucci; Macedo, Waldemar Augusto de Almeida
    Iron bulk self-diffusion coefficients were measured in Fe2O3 single crystals using an original methodology based on the utilization of 57Fe stable isotope as iron tracer, and depth profiling by secondary ion mass spectrometry (SIMS). The iron self-diffusion coefficients were measured along and perpendicular to c-axis direction, between 900 and 1100o C, in oxygen atmosphere. Along caxis they can be described by D//c (cm2/s)= 5.2x106 exp [-510 (kJ/mol)/RT], and are close to reliable data available in the literature, obtained by means of radioactive techniques. Perpendicular to c-axis, D⊥c (cm2/s)= 83 exp [-430 (kJ/mol)/RT], and the coefficients are smaller than coefficients along caxis. The results are compared with previously obtained results of cation bulk self-diffusion in Cr2O3 and in Al2O3 single crystals.
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    Oxidation of AISI 304 and AISI 439 stainless steels.
    (2007) Huntz, Anne Marie; Reckmann, A.; Haut, Christian; Sévérac, C.; Herbst, M.; Resende, Fernando César Teixeira; Sabioni, Antônio Claret Soares
    The oxidation behaviour of AISI 304 and AISI 439 stainless steels was studied at high temperatures, under various oxygen pressures and in the presence or not of water vapour. Thermogravimetric analyses were conducted in isothermal conditions from 850 to 950 ◦C for 50 h and microstructural and chemical analyses of the oxide films grown by oxidation were performed by SEM and EDX. The oxide films were also analysed by grazing X-ray diffraction and by X photoelectron spectroscopy (XPS). The AISI 439 steel has higher oxidation resistance than AISI 304, above 850 ◦C, under high oxygen pressures. On the other hand, the AISI 304 steel has higher oxidation resistance under low oxygen pressures in the whole temperature range. In order to check whether the growth kinetics of Cr2O3 formed by the oxidation of stainless steels was controlled by oxygen or/and chromium diffusion through the oxide film, the oxidation constants were theoretically calculated on the basis of diffusion data using Wagner’s Theory. The calculated values of the oxidation constants agree reasonably well with experimental values, therefore confirming the role of diffusion processes on the growth of chromia on stainless steels.
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    Oxygen diffusion in Bi2O3-doped ZnO.
    (2008) Sabioni, Antônio Claret Soares; Daniel, Antônio Márcio J. Mucci; Ferraz, Wilmar Barbosa; Pais, Rafael Witter Dias; Huntz, Anne Marie; Jomard, François
    In order to clarify the influence of Bi-doping on oxygen diffusion in ZnO, the bulk and grain boundary oxygen diffusion coefficients were measured in Bi2O3-doped ZnO polycrystals by means of the gas-solid exchange method using the isotope 18O as the oxygen tracer. The experiments were performed on ZnO sintered samples containing 0.1, 0.3 and 0.5 mol% Bi2O3. The diffusion annealings were performed at 942, 1000 and 1092 °C, in an Ar+18O2 atmosphere under an oxygen partial pressure of 0.2 atm. After the diffusion annealings, the 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). The results show an increase in the oxygen diffusion in the Bi2O3-doped ZnO, when compared to the oxygen diffusion in the undoped ZnO polycrystal under the same experimental conditions, both in bulk and in grain-boundaries. Moreover, it was observed that the higher the Bi2O3 concentration, the higher the oxygen diffusion. These results suggest that the incorporation of Bi2O3 increases the interstitial oxygen concentration which agrees with an interstitial diffusion mechanism both in bulk and in grain-boundaries.
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    Relation between the oxidation growth rate of chromia scales and self-diffusion in Cr203.
    (1992) Sabioni, Antônio Claret Soares; Huntz, Anne Marie; Philibert, J.; Lesage, B.
    In most cases, chromia scales are assumed to grow by predominant chromium diffusion. However, results of Atkinson and Taylor indicated that chromium bulk diffusion could not account for the growth rate of chromia scales. Moreover, recent results of Park et aL showed that oxygen diffusion in chromia was faster than chromium diffusion. So, at this date, the controlling process of the growth of chromia scales is not elucidated. To interpret such a phenomenon, oxygen and chromium self-diffusion coefficients in Cr203 single crystals and polycrystals were determined in the same materials and in the same experimental conditions, thus allowing a direct comparison. Tracers were introduced by ion implantation, thick film methods, and isotopic exchange, using the 54Cr, ~~ and 180 isotopes. Depth profiling was made by secondary ion mass spectroscopy (SIMS). The bulk diffusion coefficients were computed by using a general solution of the Fick's law taking into account evaporation and exchange at the surface. Grain-boundary diffusion coefficients were computed by using the Whipple-Le Claire equation for type B intergranular diffusion. Lattice and grain boundary self-diffusien coefficients were determined as a function of temperature and oxygen pressure. The diffusion coefficients are lower than results given in the literature and do not depend on the oxygen pressure. Moreover, it is found that oxygen diffusion is faster than chromium diffusion. These results are compared to the oxidation constants of chromia-forming alloys and it is shown that neither lattice self-diffusion, nor grain-boundary self-diffusion can justify the growth rate of chromia scales. Such a situation is compared to NiO case, for which authors found important differences in grain-boundary diffusivity, according to the elaboration mode of NiO (thermal oxidation or growth from the melt).
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    Roughness and oxidation : application to NiO growth on Ni at 800C
    (2000) Huntz, Anne Marie; Lefevre, B.; Cassino, Flávio Sandro Lays
    The changes with oxidation time of the oxide outer surface roughness, the oxide grain size and the oxidation rate constant kc for NiO growth on Ni were studied at 800°C in 1 atm oxygen. The roughness was determined by optical interferometry establishing 2D-profiles and 3D-images. Correlations were found between the variations of the oxide outer surface roughness or the kc values and the oxide grain size. These are attributed to the decrease of the oxide grain boundary density with time, which induces a decrease of the effective diffusion coefficient.