Analysis of Potential Noise and Acoustic Emission Daring Chloride Stress Corrosion Cracking of SUS304 Steel
Shinya Fujimoto, Mikio Takemoto
Both the potential noises (PNs) and acoustic emissions (AEs) during inter-granular (IG) and trans-granular (TG) chloride stress corrosion cracking (SCC) of Type 304 austenitic stainless steel sheet in a 38 mass% MgCl2 solution were analyzed. Both AEs and PNs were observed for IG-SCC but only PNs for TG-SCC.
Pitting corrosion, crevice corrosion and IG-SCC via pitting were produced by SCC test under combined thermal and applied tensile stresses. These corrosion damages produced characteristic ENs. In addition to the RD-type PNs (rapid drop to active potential) by pitting corrosion in the early stage (Zone I), RR-type PNs (rapid rise to noble potential) were observed in the middle stage (Zone II). AEs from fast inter-granular cracking were detected during the later stage (Zone III) when small-amplitude RD-type PNs with low frequency component (<0.07Hz) were observed. Timings of AEs and PNs during zone-III were classified into two types: RD-type potential shifts just after the AEs suggested an anodic current from a new-borne grain boundary. RD-type potential shifts just before the AEs were supposed to be due to the IG cracking at pit bottom.
Trans-granular (TG)-SCC did not produce any AEs but produced RD-type PNs with frequency component less than 0.016Hz. Potential shift rate (0.08mV/s) of TG-SCC is 6% that (1.4mV/s) of IG-SCC.