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

Ferritic stainless steels used as interconnectors in SOFC stacks are subjected to air and fuel atmospheres at 800 °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 °C) and initially linear then parabolic at higher temperatures (925–1,000 °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 steps.     

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

The 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.     

Effects of Process Parameters on Mechanical Adhesion of Thermal Oxide Scales on Hot-Rolled Low Carbon Steels

The present work investigated the effects of process parameters in a hot-rolling line, finishing and coiling temperatures, on mechanical adhesion of scale on low carbon steel substrate using a tensile test. Modification of our previous model to quantify mechanical adhesion energy was proposed for a system consisting of a cracked scale on a metallic substrate by introducing a distribution function of stress in scale. When a linear distribution was assumed, the quantified mechanical adhesion energy lay in the range of 40–890 J m^?2. Higher finishing temperature had a prominent role on increasing final scale thickness and weakening scale adhesion. For scale with similar thickness, the mechanical adhesion energy was lowered for the sample subjected to higher temperature gradient between finishing and coiling temperatures. This was considered to be from the increased water vapour in atmosphere due to the higher amount of water used to cool down the steel strip. The mechanical adhesion test was further conducted to attest this assumption. It was found that humidified atmosphere during oxidation weakened the scale adhesion to low carbon steel substrate measured at room temperature.     

Adhesion of oxide scales grown on ferritic stainless steels in solid oxide fuel cells temperature and atmosphere conditions

Adhesion of thermal oxide scales grown at 800 °C on ferritic stainless steels F18TNb (AISI 441) and F18MT (AISI 444) proposed as interconnectors in solid oxide fuel cells (SOFCs) was investigated. The effect of oxidising atmosphere – synthetic air or 2% H₂O in H₂ as the representative cathode and anode atmosphere respectively – was considered. Using a room temperature tensile test sitting in the SEM chamber, thermally grown oxide scales were forced to spall and their adhesion energy was derived. Adhesion energy, considered as the elastic energy per unit area stored in oxide was determined at the strain of first spallation or at the strain where the derivative of spallation versus strain was maximum. Adhesion energies were shown to lie in the range 10–100 J cm⁻². Adhesion values exhibited decreasing values with increasing oxide thickness, with higher values for oxidation in 2% H₂O/H₂ compared to oxidation in synthetic air. The adhesion energy of scales on F18MT was lower than that on F18TNb due to the presence of Mo-containing intermetallic compounds at the metal/scale interface.     

High Temperature Oxidation of Micro-alloyed Steel and Its Scale Adhesion

The present work investigated the high temperature oxidation behaviour of the micro-alloyed steel and the adhesion of thermal oxide scale to its steel substrate. Oxidation testing was conducted at 815 °C in oxygen without and with 17.9% v/v water vapour. The oxidation kinetics in the two atmospheres were parabolic with similar rate constants, i.e. 1.13 × 10^?9 and 1.17 × 10^?9 g² cm^?4 s^?1 for the sample oxidised in oxygen without and with water vapour, respectively. The XRD peaks for wustite, magnetite, Ti-doped magnetite and titanium carbide were detected for the sample oxidised in oxygen. For the sample oxidised in the humidified atmosphere, Ti-doped magnetite was dominantly observed, additionally with titanium carbide. A tensile testing machine equipped with an optical lens was used to monitor scale failure during straining. For the sample oxidised for 1 min, the strain initiating the first spallation of the steel oxidised in the humidified oxygen was 1.74 ± 0.14%. This strain was higher than the strain initiating the first spallation of the steel oxidised in oxygen which was 1.00 ± 0.04%, indicating the improved adhesion of scale formed in the atmosphere containing water vapour. Mechanisms of water vapour effect on scale adhesion are discussed.