The Influence of Nitrate on Pit Stability In Austenitic Stainless Steel

In this work we have examined the role of nitrate on metastable pitting and scratch test behavior at elevated temperature. It is shown that nitrate inhibition of localized corrosion in stainless steel 304L at 55o C does not occur below a threshold nitrate to chloride ratio of 7:3. In potentiostatic metastable pitting experiments two types of current transients were observed, sharp rise / sharp decay and slow rise / sharp decay. The nucleation frequency of slow rise transients in solutions containing both nitrate and chloride is strictly ratio dependent: in solutions below the threshold nitrate to chloride ratio of 7:3, slow rise transients occur at about the same nucleation frequency as they do in solutions containing only chloride. In solutions above the 7:3 ratio, the nucleation frequency of slow rise transients goes to zero. Finally, a mathematical expression for fitting transient decay is presented that accounts for both dissolution and repassiviation. It is demonstrated that there is a direct correlation between the repassivation phenomena that occurs during a scratch test and that during a sharp rise / sharp decay current transient.