Raphaël Rolland

Influence of water vapour on the zircaloy-4 oxidation at high temperature - Stress determination by in situ X-Ray diffraction

A key parameter for the understanding the effect of water vapour on the oxidation mechanism is the determination of the stress level in the monoclinic zirconia scale and in the Zircaloy-4 (Zy-4) substrate at high temperature. In order to provide an accurate description of the microstructure of the oxide layers, X-ray diffraction (XRD) analyses have been performed in situ under dry and wet oxidizing environments at high temperature on Zy-4 flat sheet samples. The aim of the present work is to show the influence of water vapour on the stress developed at high temperature in the oxide scale as well as inside the alloy. The stress evolution during cooling to room temperature has also been determined.

Cyclic oxidation of AISI 316L stainless steel – influence of water vapour between 800 and 1000°C

Abstract At 800 and 900°C, 10 vol.-%H2O in air has little effect on the AISI 316L stainless steel oxidation under isothermal and cyclic conditions. The oxide scale is composed of Cr2O3 with a small amount of Mn1·5Cr1·5O4 at the external interface. Results show that water molecules or protons can modify the diffusion process in the scale and lower the oxidation rate. At 1000°C, a deleterious effect of water vapour on the scale structure is observed. In situ X-ray diffraction was used to analyse the oxide formation on AISI 316L specimens during isothermal oxidation at 1000°C in moist air. Results show that the breakaway oxidation is due to the iron oxidation starting after 31 h oxidation. This leads to an external Fe2O3 scale growth and an internal multilayered FeCr2O4 scale formation. In wet air, thermal cycling conditions lead to continuous weight losses at 1000°C, whereas the scale remains adherent in dry air.

Influence of Water Vapour on a Nickel-Based Alloy Oxidation

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.

Influence of Water Vapour on Isothermal and Thermal Cyclic Oxidation Conditions of a Nickel-Based SY625 Steel at 1100°C

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.