TEM Study on the Internal Oxidation and Nitriding of Dual Phase Fe–Mn–Al–C Alloy AfterNaCl-Induced Corrosion

A dual-phase Fe–Mn–Al–C alloy was oxidized at 750°C in air with 2 initial mg/cm2 NaCl deposits. After 9 hr of exposure a fine-void zone was observed in the middle subscale and a coarse-void layer in the inner subscale. The fine-void zone formed due mainly to the interaction between selective oxidation of manganese and the oxidation of metal chlorides, while the formation of the coarse-void layer was caused by chlorination. The product remaining in the fine-void zone was mostly Al2O3, and the structure of the substrate in this zone is oxidation-induced ferrite, where nitriding of aluminum occurs forming AlN. Fine Fe3O4 particles fill in the coarse voids and the structure of the substrate in this layer is secondary austenite. The mechanism of the formation and growth of the internal oxidation and nitriding in the void zones of the subscale are discussed.