Metastable pitting in 25 Cr duplex stainless steel

Evidence of metastable pitting in conventional austenitic stainless steels at room temperature is now common. Because of the small currents associated with individual events, most studies have been carried out on micro electrodes (typically 50 mm diameter). Here in contrast, with high chromium duplex stainless steels (DSS), metastable pitting is clearly observed on larger electrodes in tests at elevated temperatures in neutral NaCl solutions. Below the Critical Pitting Temperature (CPT), large metastable current peaks are observed across a range of low potentials, with less activity at higher potentials. Above the CPT, stable pits initiate at the same low potentials where metastability occurs below the CPT, usually preceded by one or two metastable events. There are evident differences between the growth of metastable pits—and the transition from metastable to stable pitting—in high-chromium DSS from that in single phase austenitic steels. A proposed model to explain the metastability in these highly alloyed duplex steels is described.