Application of the microtensile testing to investigate the adhesion of thermal oxide scales grow on AlSl 441 stainless steel sheet oxidised in air and water vapour

International audienceA micro-tensile testing has been developed to investigate the adhesion behaviour of the oxide scale thermally grown on AISI 441 stainless steel sheet oxidised at 800 °C in different atmospheres-synthetic air and water vapour. In the test, a sample was placed in a tensile testing machine sitting in the chamber of a scanning electron microscope at room temperature. Evolution of the failure of the oxide scale was monitored in function of the imposed strain. It was found that the scale formed on steel oxidised in synthetic air exhibited the drastically lower spallation ratio in function of strain comparing to the scale on steel oxidised in 20 %v/v H2O/N2. For the sample oxidised in water vapour, it was clearly observed that the scale was primarily failed by the crack perpendicular to the tensile loading direction, followed by the spallation due to the compressive stress generated by the Poisson effect. After the test, precipitates rich in Nb, Si, and possibly Ti were observed at the internal interface between scale and steel substrate. For the oxidised samples that the final polishing direction paralleled to the main sample axis, the strain provoking the first spallation of the samples oxidised in synthetic air and 20%H2O/N2 were 6.23 and 3.52 % respectively. The theoretical model was developed in our previous work to quantify the mechanical adhesion energy. These values were 357 and 68 J.m2 for the steels oxidised in synthetic air and 20%H2O/N2 respectively

Oxidation of simulated recycled steels with 0.23 and 1.03wt.% Si in Ar–20%H2O at 900°C

International audienceSimulated recycled steels containing 0.002, 0.23 and 1.03 wt.% Si were oxidised in Ar-20%H2O at 900 degrees C. The oxidation kinetics of the steels was linear-parabolic. Higher Si content in the steel resulted in prolonging the period of linear oxidation and increasing Si content from 0.23 to 1.03 wt.% enhanced the parabolic oxidation rate constant. The integrated framework using thermodynamic analysis in static and dynamic modes together with the stoichiometric one was proposed for analysing the global reaction mechanisms. The role of hydroxyl ion on the oxidation behaviour of the steels was discussed

Behaviour of ferritic stainless steels subjected to dry biogas atmospheres at high temperatures

International audienceThe objective of this study is to understand the high temperature corrosion behaviour of the ferritic stainless steel type AISI 441 (18CrTiNb), a candidate for SOFC interconnectors, under dry synthetic fermentation biogas (CH(4) + CO(2) mixtures), possibly used at the anode side of the cell. Thermodynamic analysis showed that, in such mixtures, the partial pressure of oxygen lies in the range of 10(-23) to 10(-20) bar for temperature between 700 and 900 degrees C and that the formation of solid carbon may take place in several conditions. XRD results confirmed the formation of Cr(2)O(3) and Mn-Cr spinel, with a mixture of internal carbides. In this temperature range, kinetic experiments showed linear mass change. Comparing with the linear rate constants of 441 oxidised in pure CO(2), corrosion in biogas was larger and increased with increasing the methane content in the biogas. The surface morphology of the corroded specimens showed a dense oxide scale at temperatures less than 800 degrees C, serving as an efficient barrier to carbon penetration. However, when the temperature reaches 900 degrees C, cracks and pores appear in the oxide scale, carbon can precipitate and diffuse easier than at 800 degrees C and may lead to internal carbide formation. In such biogas atmospheres, 800 degrees C seems the maximum operating temperature of devices containing this ferritic stainless steel

Possible connection between nodule development and presence of niobium and/or titanium during short time thermal oxidation of AISI 441 stainless steel in wet atmosphere

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Adhesion of Thermal Oxide Scales on Hot-Rolled Conventional and Recycled Steels

International audienceThe mechanical adhesion of thermally-formed oxide scales formed on industrial hot-rolled low carbon steel strips produced through the blast-furnace route (conventional steel) or the electric-arc-furnace route (recycled steel) was studied. A new macro-tensile test was compared to a micro-tensile test previously used. It was observed that spallation of scales during straining increased with increasing the tensile strain rate. A higher strain rate resulted in a lower strain inducing the first spallation. As a result, the mechanical adhesion energy of scales actually formed on the recycled steel was in the range 300-700 J m(-2). Comparison at the same strain rate of the conventional and recycled steels showed higher scale adhesion for the recycled steel due to the presence of high amounts of interfacial silica

Oxidation Kinetics of AISI 441 Ferritic Stainless Steel at High Temperatures in CO2 Atmosphere

International audienceFerritic stainless steels used as interconnectors in SOFC stacks are subjected to air and fuel atmospheres at 800 A degrees C. The use of hydrogen as fuel gas may be substituted by fermentative biogas consisting of mainly CO2 and CH4. In this gas mixture, carbon dioxide leads to steel oxidation whereas methane induces carburization. The objective of this study was to investigate the oxidation kinetics of the AISI 441 ferritic stainless steel under pure CO2 in order to understand oxidation mechanisms. The results show that the kinetic behaviour is linear at low temperatures (800-900 A degrees C) and initially linear then parabolic at higher temperatures (925-1,000 A degrees C). Oxide scale consisted of major Cr2O3-rich oxide, topped with MnCr2O4 and a dispersion of TiO2. The chromium-rich oxide was analysed by using the photoelectrochemical method. It exhibits N-type semi-conductor. Oxidation kinetics is modelled by the mixed surface and oxide-diffusion limited step

Effect of Humidity on the Corrosion Kinetics of Ferritic Stainless Steels Subjected to Synthetic Biogas.

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Comments on the Quantification of Mechanical Adhesion Energy of Thermal Oxide Scale on Metallic Substrate Using Tensile Test

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