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Plates of high strength, low alloy (HSLA) steel (JIS-G-3 115 SPV-50Q) were found to be susceptible to sulfide stress cracking and hydrogen-induced cracking (HIC) in wet H2S. The diffusion coefficient for the hydrogen atom in the SPy-50Q steel was determined at 25-60°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% acetic acid + 5% NaCl) at ambient temperature and their tensile properties were examined after hydrogen charging. The results suggest that the failure behavior is controlled by the barrier effect of the dense iron sulfide film on the steel surface and is not caused by the diffusion of hydrogen atoms in the steel. This suggestion is supported by the results of hydrogen permeation measurements, scanning electron microscopy examination, and acoustic emission (AE) tests. Results of potentiodynamic scans and hydrogen permeation tests indicate that the AE signals observed 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 charging. When HIC occurs in H2S-saturated NACE solution, AE energy values are often higher than 500 dB X p.s.

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