Acoustic Emission During Pitting and Transgranular Crack Initiation in Type 304 Stainless Steel

The acoustic emission (AE) response of a low-carbon type 304 stainless steel (SS) (UNS S30400) during pitting and transgranular stress corrosion cracking (TGSCC) has been measured. Tests were conducted in 0.01 and 1 M NaCl with the pH adjusted to 1 with HCl at potentials of –380 mV and 0 mV (SCE) for no applied stress and with a stress equal to 75% of the yield strength of the material. Acoustic emission signals were detected using piezoelectric sensors attached to each end of cylindrical samples. The two detectors allowed the discrimination between signals generated within the sample gauge section from those generated elsewhere. The AE activity during pitting corrosion was significant; it was concluded that the AE signals did not emanate from cracking or dislocation activity. Applied stress exhibited an inconsistent effect on the AE rate, but it did shorten the transition time from low to high AE rates. The AE rate increased slightly with increasing sample current in 0.01 M NaCl and with increasing corrosion rate, resulting from an increase in salt concentration from 0.01 to 1 M NaCl. Hydrogen and oxygen gas bubble formation was not observed on the samples, and the electrochemical conditions were not consistent with their formation; therefore, AE from bubble formation was ruled out. It was suggested that rupture of an oxide or salt cap during pit growth is a possible source of the AE. If such a process did occur, the total AE/pit ratio suggests that only 1 to 3% of the pits were involved. Considering the larger pits are more apt to produce an oxide cap, the pit size distribution suggests pits larger than approximately 10 μm were acoustically active.