The behaviour of aluminium-chromium ferritic steels in simulated coal gasification atmospheres at 723 K

Two aluminium-chromium ferritic steels were exposed to simulated coal gasifier atmospheres for times of up to 900 h at 723 K. High-resolution analytical transmission electron microscopy was used to study the development of the corrosion product films as a function of alloy composition and time. The effect of pre-oxidation prior to exposure to these atmospheres was also investigated. An oxide film with a spinel structure was formed initially, and after 15 min a chromium-rich thiospinel formed as an outer layer. In the case of Fecralloy, with continued exposure, aluminium enrichment of the oxide restricted further growth of the thiospinel, and pre-oxidation leading to the formation of an aluminium-rich spinel was shown to inhibit sulphide (thiospinel) formation. Fecralloy was shown to provide long-term protection despite the early formation of sulphides.