Papers by Author: Frédéric Riffard

Abstract: In isothermal oxidation condition, water vapour has little effect on the oxidation rate and scale composition of a nickel-based SY 625 alloy oxidized at 1100°C. The scale is composed of an outer Cr₂O₃ and an internal CrNbO₄ scale. The oxide scale morphology differs between dry and wet conditions. Under dry conditions the oxide scale appears to be compact and chromia pegs are observed at the internal interface. Under wet conditions, porosities are observed spread inside the scale and the chromia grain size is smaller. At this temperature some scale spallation is observed under dry and wet conditions. Under cyclic oxidation conditions the oxide scale adherence is slightly improved in wet environment. The chromia scale is adherent during the 4 first oxidation cycles. In dry air, spallation occured after the first cycle. In dry and wet conditions, after the chromia scale spallation has started, NiO and NiCr₂O₄ form first. NiMoO₄ forms later on the alloy surface during the cycling test. The best resistance of the alloy under thermal cycling conditions under wet conditions is related to the presence of a more plastic and adherent scale owing to a higher scale porosity and smaller chromia grain size compared to dry conditions.

Abstract: Water vapour has little effect on the oxidation rate and scale composition of a nickel-based SY 625 alloy oxidized between 900 and 1100°C. At 900 and 1000°C, the outer scale is composed of Cr₂O₃ and a continuous NbNi₄ - Ni₃Mo subscale is found at the oxide/alloy interface. At 1100°C the scale is composed of an outer chromia scale and an internal CrNbO₄ subscale. The oxide scale morphology differs between dry and wet conditions. Under dry conditions the oxide scale appears to be compact and chromia pegs are observed at the internal interface. Under wet conditions, porosities are observed inside the scale. At 1100°C some scale spallation is observed under dry and wet conditions.

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: The present study focuses on the high temperature oxidation of a AISI 304 chromiaforming nitrided alloy. Isothermal oxidations were performed in air, at 800°C. The effect of nitridation on the steel surface depends on the temperature of the treatment. It leads whether to a γN solid solution formation or to CrN formation. In situ X-ray diffraction has been used to follow the oxides formation. Results show the concomitant growth of CrN and Fe2O3 at the beginning of the test. Then, Cr2O3 quickly appears which leads to the formation of a protective oxide scale (a parabolic rate law is observed). Our conclusions suggest that nitridation increases the high temperature oxidation resistance of 304 steels at 800°C.

Abstract: The effect of reactive element additions (external doping as an yttrium-oxide coating on the metal) on the oxidation behaviour of a commercial FeCrAl alloy (Kanthal A1) has been investigated during isothermal exposures in air at 1373K. The scale growth kinetics of the bare alloy obey a parabolic rate law during the whole oxidation test whereas the kinetic curves of the yttrium-bearing specimen exhibit an initial transient stage during the first hours, followed by a parabolic regime. The yttrium addition to the bare alloy does not give the beneficial effect usually ascribed to the reactive elements. No significant oxidation rate improvement of the alloy is observed, the parabolic rate constants values obtained are roughly similar for the both specimens. In situ X-ray diffraction reveals a marked influence of the reactive element on the composition of the oxide scale. The oxide layer formed on the yttrium-bearing specimen revealed, in addition to α- alumina which is the main oxide also identified on the bare specimen, the presence of yttrium aluminates (YAlO3, Y3Al5O12) located in the outermost part of the layer.

Abstract: Manganese addition and subsequent yttrium implantation effects on extra low carbon steel were studied by Rutherford Backscattering Spectrometry (RBS), Reflection High Energy Electron Diffraction (RHEED), X-ray Diffraction (XRD) and Glancing Angle X-ray Diffraction (GAXRD). Thermogravimetry and in situ X-Ray Diffraction at 700°C and PO2=0.04 Pa for 24h were used to determine the manganese alloying addition and subsequent yttrium implantation effects on reference steel oxidation resistance at high temperatures. This study clearly shows the combined effect of manganese alloying addition and subsequent yttrium implantation which promotes the formation of several yttrium mixed oxides seem to be responsible for the improved reference steel oxidation resistance at high temperatures.

Abstract: The aim of the present work is to investigate the effect of Lanthanum surface addition on the oxidation behaviour of the AISI 304 stainless steel, in air, at 1000°C. The in situ X-ray diffraction (XRD) analyses on the blank steel reveal that after the first 10h oxidation, a change in the structural composition of the oxide scale occurs. During the first ten hours oxidation an initial growth of chromia and Mn1,5Cr1,5O4 is observed. After 10 h oxidation, chromia is not detected anymore and iron-containing oxides such as hematite (Fe2O3) and iron chromite (FeCr2O4) are observed in the outer part of the scale. With blank AISI 304 specimens, the iron-containing oxides are generally not very protective and show severe spallation during cooling to room temperature due to thermal stresses. They do not allow a good adherence of the corrosion layer under thermal cycling. On the Lanthanum coated AISI 304 Stainless Steel the oxidation rate is 10 times lower. In situ XRD analyses show the absence of iron containing oxides. It reveals the formation of a fine convoluted Cr2O3 layer associated with the formation of the mixed oxides Mn1,5Cr1,5O4 and LaCrO3. LaCrO3 is found to be located at the oxide/steel interface. Our results show that, even though the scale formed under isothermal conditions is not composed of iron containing oxides, Lanthanum sol-gel coating does not prevent spallation during thermal cycling at 1000°C.