Potential and Chloride Concentration as Thresholds for Crevice Corrosion Initiation in SS 316L

This work investigated the relationship between crevice chloride concentration ([Cl − ]) and the potential necessary for initiation in stainless steel 316L (SS 316L). Real-time fluorescence measurements were used to measure [Cl − ] inside SS 316L crevice assemblies as a function of bulk [Cl − ] and potential. Crevice initiation at potentials slightly less than or equal to the bulk repassivation potential was associated with chloride transport, owing to passive dissolution, and an increase in the crevice [Cl − ]. Once the [Cl − ] within the crevice increased to the concentration necessary to initiate SS 316L pitting at the applied potential, crevice corrosion was initiated. For example, in a bulk solution of 0.1 M NaCl and at an applied potential of 0.05 V SCE , crevice initiation occurred once the crevice [Cl − ] increased to 1.2 M. This is the critical [Cl − ] for pitting at 0.05 V SCE as measured by polarization curves in metal chloride solution. As expected, when the [Cl − ] within the crevice remained below the threshold for pitting at the applied potential, initiation did not occur even when the [Cl − ] reached 2 M. However, at potentials greater than the bulk repassivation potential, crevice initiation occurred rapidly, well below the threshold [Cl − ]. With respect to propagation, fluorescence measurements found that the crevice [Cl − ] during early times (far from the crevice mouth) was on the order of 2–3 M Cl − , well below the saturation threshold, only achieving saturation at later times and distances closer to the crevice mouth.