The Development And Corrosion Behaviour Of High Temperature Oxidation Mn Depletion Of Fe-Mn Base Alloys

The oxidation behaviors of Fe24Mn4Al5Cr, Fe30Mn5Al and Fe32Mn3A17Cr2Si austenitic alloys were studied at high temperature in air. The phase microstructure and surface morphology of oxide scale were characterized by X-ray diffraction and scanning electron microscopy; The concentration-depth of alloys surface layer element were investigated by electron probe and auger electron spectroscopy. The corrosion resistance behaviour of oxidation-induced Mn depletion layer in1mol l-1Na2SO4solution were evaluated by the measurement of anodic polarization curves and electrochemical impedance spectroscopy. The results indicate that:The oxide scales of Fe24Mn4A15Cr austenitic alloy oxidized at800℃in air for160h were mainly consisted of outer Mn2O3, Fe2O3,(Cr, Fe)2O3, FeMn2O4and inner Al2O3, MnAl2O4. Because of that MnAl2O4has an amorphous structure, the oxidation process becomes selective for manganese. A layer of the metal near the scale-metal interface was induced to transform to ferrite and become enriched in Fe (78at.%), and depletion in Mn (15at.%), cpmpare with the matrix. In lmol l-1Na2SO4solution, the self-corrosion potential Ecorr of oxidation-induced Mn depletion layer of Fe24Mn4A15Cr austenitic alloy oxidized at800℃in air for160h increased from-710mV to57mV, and the passive current density ip decreased from the3.3μA/cm2to0.9μA/cm2, relative to Fe24Mn4A15Cr austenitic alloy. With Mn2O3and Fe2O3spalling, the oxide layer was mainly consisted of FeMn2O4,(Cr, Fe)2O3and little MnAl2O4, Al2O3as the temperatuer up to950℃. Meanwhile, there formed a ferrite near the scale-metal interface, which was discontinuity as atom diffusibility increase and performed worse corrosion resistance than exposure at800℃in air for160h.Fe30Mn5Al and Fe32Mn3Al7Cr2Si austenitic alloys were oxidized at800℃in air for160h. There was a ferrite layer near the scale-metal interface15μm thickness enriched in Fe (83at.%), depletion in Mn (8at.%) and20μm thickness enriched in Fe (72at.%), Cr (14at.%), depletion in Mn (11at.%), respectively, with selective for manganese. In lmol l-1Na2SO4solution, the self-corrosion potential Ecorr of oxidation-induced Mn depletion layer increased from-750mV to-130mV, and the passive current density ip decreased from the310μA/cm2to29μA/cm2, relative to Fe3OMn5Al austenitic alloy. The EIS of the Mn depletion layer has the larger diameter of capacitive arc, the higher impedance modulus|Z|, and the wider phase degree range. Using an equivalent electric circuit of RS-(RX//CPE), the Mn depletion layer resistant Rt increased to2.7kΩ·cm2from9.9kΩ·cm2, relative to Fe30Mn5Al austenitic alloy. the self-corrosion potential Ecorr of oxidation-induced Mn depletion layer increased from-463mV to248mV, and the passive current density ip decreased from the2.9μA/cm2to0.4 μA/cm2, relative to Fe32Mn3A17Cr2Si austenitic alloy. The EIS of the Mn depletion layer has the larger diameter of capacitive arc, the higher impedance modulus|Z|, and the wider phase degree range. Using an equivalent electric circuit of Rs-(Rt//CPE), the Mn depletion layer resistant Rt increased to69.7kΩ·cm2from15.5kΩ·cm2and Brug et al’capacitance decreased to10.3μF/cm2from16.6μF/cm2, relative to Fe32Mn3A17Cr2Si austenitic alloy. corrosion resistance of the alloy were both improved significantly.