Characterization of 475◦C embrittlement of duplex stainless steel microstructure via scanning Kelvin probe force microscopy and magnetic force microscopy

Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure of 22Cr-5Ni duplex stainless steel have been correlated to microstructure development with aging treatments at 475◦C. Magnetic force microscopy (MFM) was employed to differentiate crystallographic phases to provide complementary information. The absolute Volta potentials of both ferrite and austenite increased after 5 hours of aging, indicating electrochemical ennoblement of the entire microstructure. Longer aging resulted in a gradual decrease of measured Volta potentials in both phases. The microstructure showed after 255 hours aging up to 2.5-times larger potential differences than in the as-received condition, indicating impaired electrochemical nobility. In the as-received microstructure, the ferrite phase was less noble than the austenite, whereas after 5 hours aging both phases had similar, balanced Volta potentials which indicated a balanced nobility of ferrite and austenite. Longer aging treatment caused severe loss of nobility for the entire microstructure, with ferrite showing larger changes in Volta potential than the austenite. Spinodal microstructure decomposition and associated phase reactions of the ferrite, with elemental redistribution in the austenite, are the reason for the observed changes in microstructure nobility.