Secondary ion mass spectrometry and multireflection infrared analyses of conversion coatings on Fe–Cr–Al stainless steels for catalysis: study of thermal stability

The thermal stability of three stainless steel conversion coatings for high temperature applications (e.g. photothermal conversion catalysis) are investigated. The thermal oxidation in air up to 1000°C of Fe–17Cr, Fe–18Cr–1·3Al, and Fe–22Cr–5Al coatings (all wt-%) are compared. This study has revealed a critical temperature below which the coating thickness is preserved; the critical temperature increases and the thermal oxidation of the conversion coating decreases with higher chromium and aluminium content. This is attributed to the difference in the substitution ratio of γ lacunar phase (additionally oxidised substituted magnetite), which is the main component of the conversion coatings. The thermal stability of this phase is higher when it is richer in chromium or aluminium. Higher contents of these elements raise the temperature of formation of chromite (FeCr2O4) and alumina, the occurrence of which causes thickening of the coating during thermal treatment.MST/1891