Detection and Analysis of Crevice Corrosion-SCC Process by the Use of AE Technique

Abstract

Crevice corrosion and crevice corrosion-SCC processes of SUS 304 stainless steel have been detected and analyzed in a 3% NaCl solution at 308 K by the use of AE (Acoustic Emission) technique. The AE detected during the crevice corrosion induced by the polarization scan is shown to arise from hydrogen gas which is evolved inside the artificial crevice. The hydrogen gas evolution is thought to occur when the pH value of the solution contained in the crevice becomes sufficiently low and the potential falls below E°(H₂). It can provide AE, amplitude of which may reach some hundreds μV, measured by the piezoelectric AE sensor that has the maximum sensitivity "6000 V/(m/s)". The SCC test using DCB specimen with the artificial crevice shows, on the other hand, that AE with higher energy than mV level is accompanied with the SCC microcrack initiation which takes place successively after the crevice corrosion. The following three facts suggest that hydrogen embrittlement may be involved in this SCC process ; 1) H₂ gas evolves inside the crevice during the crevice corrosion process, 2) High energy emissions are emanated with the SCC microcrack initiation under plane strain condition, 3) Dissolution of metal does not provide sufficiently high AE energy to detect : Practical methods to apply AE technique to fundamental studies in laboratory and continuous monitoring in engineering structures are also discussed in detail.