Papers by Author: Yves Wouters

Abstract: AISI 441 ferritic stainless steel is a good candidate for metallic interconnects in solid oxide fuel cells (SOFCs). The minor elements Ti and Nb are used to stabilize the ferritic matrix and also to reduce creep by a combination of solid solution strengthening and precipitation of intermetallic Laves phase particles along the grain boundaries. However their influence on the oxidation behavior is not well understood. This study focuses on the early stages oxidation (from 4 to 24 h) at 800 °C of AISI 441 under 5% H₂O in O₂. A relatively smooth micro-crystallized oxide scale and Ti, Nb containing nodules are observed. The internal microstructure of these objects is studied by FIB tomography which allows computing cross sectional views in any direction of interest. FIB study reveals a complex microstructure and a development strongly linked to the presence of niobium and/or titanium in the substrate.

Abstract: Nickel based alloys are well considered materials for uses in high temperature applications. Inconel 690 is the one of outstanding candidate with the superior properties. The aim of this report is to present the influence of water vapor on thermal oxide film. Inconel 690 was oxidized under dry and wet atmosphere at 900°C for 30 hours. The oxidized samples were characterized by SEM/EDS, Raman spectroscopy, and photoelectrochemical technique. The results illustrated that typical thermal oxides grown on alloy composed of Cr, Fe, Ni, in forms of Cr₂O₃, NiFe₂O₄, NiCr₂O₄, Fe₂O₃, Fe_2–xCr_xO₃ and/or solid solution of NiFe₂O₄−NiCr₂O₄. The presence of water vapor affected on oxide morphology, its stoichiometry, and also semiconducting behavior. Oxide film grown under water vapor atmosphere show the homogeneity. Water vapor promoted the predominated oxide of Cr₂O₃ with n−type semiconducting. Moreover the characterization revealed the effect of surface orientation on oxidation mechanism in case of sample which oxidized in oxygen atmosphere.

Abstract: For economical and environmental reasons, hydrogen is considered as a major energetic vector for the future. Hydrogen production via high temperature water vapour electrolysis (HTE) is a promising technology. A major technical difficulty related to high temperature water vapour electrolysis is the development of interconnects working efficiently for a long period. Working temperature of 800°C enables the use of metallic materials as interconnects. Chromia forming alloys are among the best candidates. The interconnect material chosen in the present study is a ferritic stainless steel with 18% chromium content. High temperature corrosion resistance and electrical conductivity of the alloy was tested in both cathode (H₂/H₂O) and anode (O₂/H₂O) atmospheres. Corrosion products were then characterized by SEM-EDX and XRD. Moreover chromium evaporation measurements were carried out under anode atmosphere.

Abstract: The electronic properties of chromia scales grown between 800°C and 900°C on chromium metal and chromia-forming ferritic stainless steels were determined using room temperature PhotoElectroChemistry (PEC) experiments and the relative importance of the n- and p-character of the scales could be assessed. According to the thermodynamic previsions of defects structures, the external part of all the scales grown in oxygen exhibits band gap energy around 3.5 eV, with a marked p-type character on chromium and a possibly n-type behaviour on stainless steels. On the contrary, the internal part of the scales is always n-type, with predominant interstitial chromium defects. A major change appears when chromium or stainless steels are oxidised in water vapour-argon mixtures, where the absence of a p‑type semiconductor in the scales could be evidenced. Hydrogen defects are thought to be responsible of this particular behaviour which leads to a strong reduction of residual stresses due to increased high temperature relaxation. Moreover, the inversion of the growth direction resulting from high mobility of the OH defects makes the chromia scales grown in water vapour more adherent than when grown in oxygen.

Abstract: Chromia scales grown on several chromia-forming metallic substrates in various conditions were characterised by photoelectrochemistry (PEC), highlighting the presence of two semiconductor phases signed by their respective bandgaps (3.0 and 3.5 eV) with variations of semiconduction type (n, p and insulator, more or less doped). The protective character of the scale was clearly demonstrated when the highest bandgap phase (3.5 eV), identified as the external subscale was close to an insulator.

Abstract: The tensile test, accompanied by the corresponding theoretical model, has been developed to quantify the mechanical adhesion energy of the oxide scale on metallic substrate in our previous works. The method to quantify the adhesion energy took into account the effect of residual stress. The effect of the variation of the measured residual stress on the quantified adhesion energy is assessed in this paper. For the scales failed at strains initiating the spallation of 0.018 and 0.011 followed by the transverse crack, it was found that the quantified adhesion energy of the oxide is not sensitive to the variation of the residual stress measured in the range from 0.5 to 2.0 GPa. This is due to the compensation of the decrease in stored energy due to the stress applied in the loading direction (x direction) and the increase in stored energy due to the stress applied in the direction perpendicular to the loading direction (y direction) when the residual stress increases. For the scale failed by the transverse crack followed by the spallation, the quantified adhesion energy tends to be sensitive to the variation of the measured residual stress. The assumption of energy relaxation during the tensile test is alternatively proposed. It is assumed that the energy stored due to the stress in x and y directions is totally released at the first crack. The energy stored due to the stress in y direction from strain initiating the crack to strain initiating the spallation is used in the quantification of the adhesion energy. The scatter of the adhesion energy values quantified by this method and those measured by the inverted-blister test is reduced comparing to the results reported in the previous work.

Abstract: Two ferritic AISI 430 and AISI 441 and two austenitic AISI 304 and AISI 316L stainless steels were submitted to short term oxidation in a complex atmosphere 3% O2, 16% H2O, 8% CO2, 73% N2 to simulate phenomena occurring during the rapid furnace annealing taking place after the final cold rolling. This thermal sequence is devoted to metallurgical aims but generates undesirable oxides which have to be further pickled. Temperature of the furnace was set to the values used in industrial practice: 900°C for 430, 1060°C for 441 and 1120°C for both austenitics. Six different oxidation durations were used between 30 and 300 s. For the shortest times, sample temperature was not constant and heating rate depended on sample thickness. Oxide thickness measured by GDOS was shown to increase monotonically for all grades whereas mass change measurements exhibited initial mass losses for the austenitic grades. XRD and Raman spectroscopy were used for phase characterisation and confirmed the increase of the ratio chromia/haematite with increasing annealing time. Enrichment of manganese (MnCr2O4), silicon (SiO2) and boron (B-containing oxide) at the external (Mn) and internal (Si, B) interfaces was observed on the GDOS profiles (boron for austenitic grades only). Manganese spinel was responsible for blocking chromium (VI) volatilisation after a certain time, and interface oxides for hindering chromium transfer from the steel to the oxide scale. Ferritic grades behaved the same, except that no boron enrichment was detected. Besides, stabilised 441 exhibited Ti and Nb enrichments as oxides at both internal and external interfaces. External TiO2-NbO2 solid solution was assumed to be hardly dissolved in acidic pickling baths. All these results were consistent with the different pickling behaviours of the materials.

Abstract: The oxidation of γ-Zr(Fe,Cr)2 intermetallic particles during the thermal exposition of Zircaloy-4 at 470°C in oxygen was investigated with PhotoElectroChemical techniques (PEC). Via the measurement of bandgap, haematite Fe2O3 (2.2 eV), rhomboedric solid solution (FexCr1-x)2O3 (2.6 eV) and chromia Cr2O3 (3.0 eV) phases were identified as components of oxidised particles. Evolution of size, lateral distribution and density of these particles was studied in function of zirconia scale thickness. During the first stage of oxidation, the density of oxidised particles increased with thickness but decreased during a second stage, highlighting in an innovative way the phenomenon of haematite and chromia dissolution in the zirconia matrix. It is concluded that PEC techniques represent a sensitive and powerful way to locally analyse the various semiconductor phases in an oxide scale at the micron scale.

Abstract: Adhesion energy values for thermal oxide scales cyclically grown at 850 and 950°C in air on ferritic and austenitic stainless steels were obtained using an inverted blister test and a tensile test working in the SEM chamber. The blister test used water pressure for debonding the metalscale interface, whereas the tensile test led to transverse compression generating scale spallation by buckling. Adhesion energy, defined by energy for interface crack propagation by unit area, was shown to be in the range 10 to 650 J.m–2 for the chromia-rich scales with thickness in the micrometer range. Ferritic grades gave less adherent scales than austenitic ones, and a great influence of titanium was evidenced, greatly increasing scale adhesion; niobium was less operative. Adhesion was well connected with nature and morphology of Ti and/or Nb-containing precipitates at the metal-scale interface.