Small-angle neutron scattering study of the wet and dry high-temperature oxidation of alumina- and chromia-forming stainless steels

Foils of T347 stainless steel and a developmental alumina-forming austenitic (AFA) stainless steel were oxidized at 800 C in dry air, air with 10% H2O, and air with 10% D2O. The T347 foils exhibited a transition to rapid Fe-base oxide formation between 24 and 72 h of exposure in H2O and D2O, but exhibited protective Cr-rich oxide formation in dry air. In contrast, only thin, protective Al-rich oxide surfaces were observed for the AFA alloy foils under all conditions studied. Changes in the small angle neutron scattering (SANS) signal were observed for the T347 stainless steel as a function of oxidation time in dry air, attributed to oxide grain growth and porosity formation/partial scale detachment associated with spinel phase at the scale/gas interface. For the AFA alloy, only minor changes in scattering as a result of oxidation time were observed. For both T347 and AFA, similar scattering was observed in dry and wet air (H2O and D2O) exposure. This finding indicates that water vapor exposure did not induce significant morphological changes in the oxide scales (such as increased porosity) in the 5-300 nm size regime accessed by SANS.