Characterization of the Native Passive Film on Ferrite and Austenite Phases of Sensitized 2205 Duplex Stainless Steel

Abstract

The influence of isothermal sensitization of 2205 duplex stainless steel (DSS) in 850°C was quantified using double-loop electrochemical potentiodynamic reactivation test. We assessed the microstructure, Volta potential distribution, and semiconductive properties of the native passive film on individual austenite and ferrite phases of solution annealed and sensitized DSS, using optical and scanning electron microscopy, scanning Kelvin probe force microscopy, and scanning tunneling spectroscopy. Beside conventionally used line profile scans, we used a multi-modal Gaussian distribution method for analyzing the Volta potential distribution over the entire scanned area. The Volta potential distribution evaluations indicated that the driving force for preferential corrosion at the ferrite/austenite interface in DSS increases after sensitization. Scanning tunneling spectroscopy revealed that the formation of intermetallic phases and depleted zones in DSS after sensitization, decrease the surface energy bandgap in both austenite and ferrite phases, whereas the density of electronic states in these phases increases, compared to those of solution annealed DSS. The multi-analytical approach that is used in this research provides complementary evidence for the detrimental effects of sensitization on corrosion resistance of DSS.