CORRELATION BETWEEN ACOUSTIC-EMISSION SIGNALS AND HYDROGEN PERMEATIONIN HIGH-STRENGTH, LOW-ALLOY STEEL CRACKING IN WET H2S

Plates of high strength, low alloy (HSLA) steel (JIS-G-3115 SPV-50Q) w ere found to be susceptible to sulfide stress cracking and hydrogen-in duced cracking (HIC) in wet H2S. The diffusion coefficient for the hyd rogen atom in the SPV-50Q steel was determined at 25-60 degrees C and the activation energy for diffusion was 24.7 kJ/mol. The HSLA tensile specimens were immersed in the H2S-saturated NACE solution (0.5% aceti c acid + 5% NaCl) at ambient temperature and their tensile properties were examined after hydrogen charging. The results suggest that the fa ilure behavior is controlled by the barrier effect of the dense iron s ulfide film on the steel surface and is not caused by the diffusion of hydrogen atoms in the steel. This suggestion is supported by the resu lts of hydrogen permeation measurements, scanning electron microscopy examination, and acoustic emission (AE) tests. Results of potentiodyna mic scans and hydrogen permeation tests indicate that the AE signals o bserved in the H2S environment may come from the formation or rupture of an iron sulfide film on the metal surface. The energy of AE signals in the H2S environment is much higher than that under cathodic chargi ng. When HIC occurs in H2S-saturated NACE solution, AE energy values a re often higher than 500 dB x mu s.