Corrosion and hydrogen permeation of low alloy steel in H2S-containing environments : the effect of test buffer solution chemistry

International audienceH 2 S-containing (sour) service environments present a considerable risk of hydrogen induced cracking (HIC) and sulfide stress cracking (SSC) to steel line pipe, pressure vessel and tubular components during upstream oil and gas production, through the ability of H 2 S to corrode and promote hydrogen entry into the material bulk via a cathodic reaction process. Materials selection for sour service is made via standard test methods such as NACE TM0284 and NACE TM0177. A commonly used test solution (NACE TM0177 solution A) comprises sodium chloride (5.0%) + acetic acid (0.5%), to work in a range between pH 2.8-4.0. When pH stability is essential over long testing periods, solutions that are buffered by acetic acid with sodium acetate are proposed. NACE TM0177 solution B (5.0% NaCl + 0.4% sodium acetate + 2.5% acetic acid) presents an initial pH of 3.4-3.6, specified not to exceed pH 4.0 over the testing duration. Newer, alternative solutions from the high-strength line pipe (HLP) research committee from the Iron and Steel Institute of Japan (ISIJ) propose higher acetic acid/acetate concentrations for enhanced buffering capacity. This may offer practical testing advantages, although material corrosion rates and hydrogen uptake are possibly affected. In this conference proceeding, we report on the corrosion and hydrogen uptake performance of a sour-grade X65 steel exposed to NACE Solutions A and B, and an HLP solution (at the same pH as NACE B solution, i.e. pH 3.5) under continuous H 2 S purging (0.1 MPa, T = 24°C) over 720 hours. Electrochemical methods measure electrochemical impedance at the entry face of, and hydrogen permeation across, the X65 membrane. Overall, the differences we note are linked to the different weak acid/conjugate base concentration. Keywords Hydrogen permeation, acetic acid, hydrogen sulfide, X65 steel