Acoustic emission as a technique to study hydrogen induced cracking behavior of low-carbon steel

The strength loss resulting from hydrogen induced cracking (HIC) has been known to cause several serious failures in refineries and other plants. The study of this cracking mode using only electrochemical techniques is not fully efficient for the detection and control on line of this phenomenon. The technique using acoustic emission (AE) to detect defects in pressure vessels on line has been significantly developed in recent years. In order to investigate the effect of HIC degree on AE behavior of steel used in pressure vessel, specimens made of low-carbon steel was immersed in the H₂S-saturated solution at 313k temperature for different time and their metallographic structure were examined after hydrogen charging. The AE signals showed that the corrosion process can be detected through AE testing and with the increase of the HIC developing, the totally AE activity increased during hydrogen charging. The AE signals of HIC can be divided into gestation stages and cracking stages, which can be either from the FeS film forming and breaking, or the development and evolution of hydrogen bubbles formed inside blisters during H₂S corrosion, and the micro cracks on grain boundaries. The exact feature of AE sources within hydrogen blisters is not identified clearly, but the results demonstrate that an acoustic emission technique can be used to detect and even real time monitor the occurrence of such phenomena.