Dissolution and Repassivation of Freshly Bared Austenitic Stainless Steel Surfaces in Nitrate Solution

This presentation will focus on repassivation transients generated from freshly bared stainless steel (SS) 304 L surfaces in nitrate (NO₃^-) and chloride (Cl^-) solutions. To evaluate the role of NO₃^- in pitting inhibition, transient peak current as a function of potential as well as transient decay rate were analyzed.  At equivalent concentrations bare surface dissolution rates were lower in nitrate solutions as compared to chloride (e.g. Tafel slopes from bare surface polarization curves were lower).  With respect to decay rates, the slope of log-log plots of current vs. time were solution dependent, with NO₃^- containing solutions having faster decay rates as compared to Cl^- alone.  Similar trends were also observed for shorter decay times.  To quantify these differences a mathematical expression for fitting the decay portion of transient at short times was developed.  The resulting fitting parameters were used to generate plots of the anodic dissolution, passivation current as well as the fraction of passive film coverage with time.  For NO₃^- containing solutions, anodic dissolution fell to zero more quickly, the passivation current peaked sooner and correspondingly, the fraction of oxide film coverage was higher at any given time as compared to Cl^- alone.  From these results, it was concluded that NO₃^- inhibits pitting corrosion by promoting film growth.