Oxidación de aceros ferríticos con vapor de agua: cinética y microestructura

Abstract

The ferritic 2.25Cr-1Mo steel has been subjected to isothermal and non-isothermal oxidation treatments in water steam at several temperatures ranging from 550 to 700 °C for up to 56 days. Under isothermal conditions this steel follows a parabolic oxidation kinetics, with an activation energy of 324 kJ mol -1 . This value corresponds to an apparent activation energy for the global process, which includes both outward diffusion of Fe cations and inward diffusion of oxygen. The oxidation products present in the oxide scales, which were characterised by X-ray diffraction and SEM, are in total agreement with the Fe-O phase diagram. Thus, magnetite is the most stable oxide at low temperatures and wustite starts to form above 570 °C. Further studies of the effect of cooling rate have shown that wustite formed at 700 °C transforms into magnetite during a slow cooling, whereas a rapid cooling inhibits this transformation to a certain extent. For non-isothermal oxidation treatments consisting of a holding period at 550 °C followed by a single or double 4 hours exposure at 700 °C, the oxidation process seems to occur in sequence, thus presenting an additive effect of the oxidation treatments carried out at each temperature. This effect was observed both, for the type of oxide grown, and for the kinetics of the process. Microscopic observations of the oxide scales formed after the various oxidation treatments revealed that the oxide scales are constituted by sublayers of distinct microstructure and chemical composition changing from their surface to the substrate interface. Keywords: Ferritic steel, isothermal and non-isothermal oxidation, kinetics, diffusion mechanism. Oxidación de aceros ferríticos con vapor de agua: cinética y microestructuraSe han realizado tratamientos de oxidación isotermos y no isotermos a un acero ferrítico 2,25Cr-1Mo en vapor de agua, a temperaturas comprendidas entre 550 y 700 °C y tiempos de hasta 56 días. En condiciones isotermas, este acero tiene una cinética de oxidación parabólica, con una energía de activación de 324 kJ mol -1 . Este valor corresponde a una energía de activación aparente del proceso global, que incluye tanto la difusión hacia el exterior de cationes de Fe, como la difusión de oxígeno hacia el interior. Los productos de oxidación presentes en la capa de óxido, caracterizados por difracción de rayos-X y SEM, están en total concordancia con el diagrama de fases del sistema Fe-O. La magnetita es el óxido más estable a bajas temperaturas, y la wustita comienza a formarse por encima de 570 °C. Estudiando el efecto de la velocidad de enfriamiento se ha comprobado que la wustita formada a 700 °C se transforma en magnetita en un enfriamiento lento, mientras que el enfriamiento rápido inhibe en parte esta transformación. En los tratamientos de oxidación no isoterma, consistentes en un periodo de mantenimiento a 550 °C seguido de exposición simple o doble a 700 °C, el proceso de oxidación parece producirse secuencialmente, presentando así un efecto aditivo de los tratamientos de oxidac...

Acoustic emission analysis on tensile failure of steam-side oxide scales formed on T22 alloy superheater tubes

“…10 Lots of research efforts have, thus, been performed on steam-side oxide scales. However, the fact is that, although the high temperature steam oxidation mechanism has been extensively investigated in the past years, [11][12][13][14][15] there are still lots of issues associated with the mechanical integrity of the steam-side oxide scales that need more research attention. Among these issues, of special interest are the critical failure resistance and the damage mechanism of the oxide scales.…”