Abstract
Hydrogen Induced Cracking (HIC) is a major issue for line pipe steels exposed to sour environments. In general, 5.0 wt% NaCl and 0.5 wt% CH2COOH solution with 0.1 MPa H2S provided by NACE TM0284 as solution A is used to evaluate the HIC resistance of steel products. However, in many cases, the test condition is too severe compared to the actual field conditions. Therefore, establishment of an appropriate HIC evaluation method under mildly sour conditions has been the subject of considerable investigations in recent years. In this study, the influences of balance gas (N2 and CO2) and specific solution volume on pH stability and sour corrosion behavior of carbon steel were investigated in order to recommend an appropriate test solution for mildly sour HIC evaluation in a specified pH condition. The remarkable pH shift during a long term HIC test was observed by using 0.05N acetate buffer solution, even though an appropriate balance gas and a large specific solution volume were selected. In contrast, 0.93N acetate buffer solution showed excellent pH stability during long term HIC tests even in low pH and low H2S partial pressure conditions regardless of balance gas selection and specific solution volume. The pH stability can contribute to stable hydrogen entry to the material and the correct HIC evaluation. Therefore, from the viewpoints of convenience and effectiveness for evaluating the resistance of steels to HIC in a specified pH condition, an acetate buffer solution including high total amount of acetic acid and sodium acetate is desirable.
