Analysis of current transients and morphology of metastable and stable pitting on stainless steel above pitting potential

In order to record metastable pit current transients, a 904L austenitic stainless steel was polarized potentiostatically by applying 750 mV (Ag/AgCl) in 1 M NaCl solution at 45, 49 and 54degreesC, knowing that the alloy CPT is 56degreesC. Stable pit current transients were recorded employing the same potential and test solution but at 62degreesC. It was found that the metastable pit current, I, follows a power law relation ship of I proportional to t(n) during the initiation and growth prior to repassivation. The value of n was found to vary with temperature, however, values of 0.5 to 1.5 were obtained in all cases. The results of Scanning Electron Microscopy (SEM) studies on these transients showed two types of pit morphology: small pits associated with the small transients, whose interior surface were not visible, and open pit cavities with polished surface for larger transients, recorded at temperature closed to the CPT. Current transients from the stable pits showed an initial shape similar to that of stable pit. The stable pit currents rises approximately in proportion to t(3/2), but only for a few seconds and then continues to increase proportion to t(1/2) without repassivation. The SEM studies also revealed that stable pits are covered with a lace-like pattern with polished interior surface.