Atmospheric corrosion of austenitic stainless steels

Atmospheric corrosion was investigated using electrochemical and droplet studies. The effects of changes in bulk solution concentration and local pit chemistry on pit propagation and repassivation of 304L and 316L stainless steels were investigated using \(in\) \(situ\) synchrotron X-radiation and electrochemical techniques. Radiography and zig-zag electrochemical sweeps showed that in dilute chloride solutions, partial passivation was observed to initiate locally and propagate across the corroding surface. This caused repassivation gradually rather than as a uniform event. In concentrated chloride solutions, repassivation did not show a sudden drop in current but rather a gradual decrease as potential swept down. Pitting behaviour was also affected by solution concentration. Dilute solutions showed metastable pitting followed by a sharp breakdown (pitting) potential. Concentrated solutions however showed no metastability and a gradual increase in current when pitting. To determine the cause of current oscillations in 304L artificial pits in NaCl:NaNO\(_3\) solutions near the repassivation potential, the salt layers were scanned \(in\) \(situ\) using XRD. The salt layer was confirmed to be FeCl\(_2\).4H\(_2\)O and no nitrate salt was found. A mechanism was suggested to explain the current oscillations in terms of partial passivation being undercut by the advancing corrosion front