Early stage oxidation behavior of Al- and Si-alloyed stainless steels as well as Ni-based alloys in air at elevated temperatures

Several alloys including Al-alloyed austenitic and ferritic stainless steels, Ni-based alloy and Si-alloyed ferritic stainless steel were annealed at 550, 750, and 950 °C for up to 700 h. AISI 347HFG and 10CrMo910 were used as reference materials. Weight gain was measured and the samples were analyzed with SEM/EDS and XRD. Materials having high enough Al-content performed best due to ability to form a continuous protective Al₂O₃-oxide layer on their surface at elevated temperatures. Lower Al-content materials were not able to form a continuous protective Al-oxide on their surface and suffered from accelerated internal Al-oxide formation due to alloying elements. Si-alloying improves the oxidation resistance of the studied materials and their performance was practically in the same level as that of the best Al-alloyed materials. In Al-alloyed materials the crystallinity of Al₂O₃ on ferritic stainless steel weld metal was higher than that on welded and wrought austenitic stainless steels.