The microstructural characterization and pitting corrosion behavior of Z3CN20.09M duplex stainless steel were investigated by TEM, potentiodynamic polarization, and electrochemical impedance spectroscopy after thermal aging at 475 °C for 0, 500, 1000, 2000, and 3000 h. The results showed that the ferrite in the steel decomposes into coherent Cr‐rich, Fe‐rich, and G phases. No any precipitates were found in the austenite during the thermal aging process. It is revealed that the pitting resistance of this steel has a negative relationship with increasing aging time and the priority nucleation position changes before and after the thermal aging process. The composition fluctuation region adjacent to the G phase and the Cr‐depleted region around the Cr‐rich phase are preferential to form pits. Therefore the G phase precipitation and the formation of Cr‐depleted region are harmful to the pitting resistance of the steel.
The protectiveness and characterization of the passive film formed on Z3CN20.09M duplex stainless steel aged for different times were investigated by using electrochemical measurements and surface analysis method. The thermal aging causes the spinodal decomposition of the ferrite phase in the steel, resulting in the decrease of localized corrosion resistance. The increased aging time causes a thicker, dispersive, and lower resistance passive film formed on the surface of the steel based on EIS and Mott–Schottky (MS) analysis. The Auger electron spectroscopy (AES) and X‐ray photoelectron spectroscopy (XPS) results indicate that Cr is enriched in the inner layer of the passive films with a reduced content, decreasing the compactness and homogeneity of the passive films.
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