Role of potential in stable pit growth of austenitic stainless steels

The pitting behaviour of AISI type 316 stainless steel in 3.5% NaCI solution was investigated. using two electrochemical polarization methods: i) PD tests-samples were potentiodynamically polarized up to the pitting potential; ii) PDPS tests - initially samples were potentiodynamically polarized up to the metastable potential range and then the mode was changed from potentiodynamic to potentiostatic at a potential with metastable potential region. The first observation is that stable pit growth did not occur when the polarization mode was changed from PD to PS at a potential in the metastable potential region. By analyzing the current spikes corresponding to the metastably growing pits, the pit radii, pit current densities, and the pit stability product (x·i) were calculated and compared between the PD and PDPS tests. The pit radii and the values of xi increased with the potential in the PD tests while they showed no noticeable changes with time in the PDPS tests. The pit current densities in the PD tests showed an increasing trend with the potential, which is thought to be leading to the stable pitting since in the PDPS tests they were decreasing with time in the PS mode.