In situ imaging of corrosion events in austenitic steels by scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) was used to study the corrosion behavior of 300 Series austenitic steels. Using the substrate generation / tip collection (SG/TC) mode, the I^- ion was used as both the aggressive species and the method of detecting oxide breakdown. With the microelectrode tuned to detection of the oxidized product (I₃ ^-), it was brought to within 10 μm of the sample surface. Mapping of the surface showed that current was isolated to discrete locations where pitting corrosion was observed. The SECM maps of the surface followed the expected trend of stability: 316 ≫ 304L > 304. The 316L sample did not pit in I-solution unless Cl^- was added. As a test of the technique, the corrosion of a cross-sectioned 316 weld was imaged. The expected behavior was pitting surrounding the weld due to heat sensitization. The results showed that the base 316 material was contaminated with non-metallic inclusions and showed high activity compared to the filler material which showed no activity. The potential of SECM in studying corrosion was discussed.