Unveiling the reasons for corrosion resistance difference between two phases of the Fe40Cr40Ni20 duplex MEA in the transpassive region

The passive film structure and corrosion behavior of the MEA are characterized using electrochemical tests, XPS, ToF-SIMS and TEM. The obtained results indicate that the MEA is consisted of the Cr enriched BCC phase (Fe₃₇Cr₅₂Ni₁₁) and Ni enriched FCC phase (Fe₄₂Cr₃₂Ni₂₆). The passive film of the MEA displays a bilayer structure, where Fe, Ni oxides are accumulated in outer layer, while Cr oxides are concentrated in inner layer. The Cr content in passive film decreases with the increasing applied potential, resulting in the degradation of film at high potentials. The presence of NiCr₂O₄ in the passive film is confirmed by XPS, and its fraction in the Cr species increases with the applied potential, which suggests that NiCr₂O₄ is more stable than Cr₂O₃ at high potentials. Since high content of Ni is beneficial for the formation of NiCr₂O₄, the passive film on FCC phase in transpassive region is more stable than that on BCC phase. This is responsible for the preferential dissolution of BCC local film in transpassive region.