Micrographic Acceptance Criteria for SSC Testing

Abstract

Abstract The oil and gas industry has increasingly been pursuing development of resources containing high concentrations of H2S. While the negative effects of severe sour environments on pipeline steels and girth welds can be mitigated through modern metallurgical designs, some gaps in understanding still remain. In particular, environments of low pH and moderate amounts of H2S within Region 3 of the NACE Diagram can lead to challenges in materials selection via conventional approaches to evaluate sulfide stress cracking susceptibility. In these environments, pitting corrosion and cracking driven by hydrogen embrittlement can simultaneously damage the surface of the steel test coupon. Subsequent coupon analysis and a pass/fail determination can be problematic if the surface damage features have characteristics of both pits and cracks. Misidentification of these surface damage features, sometimes referred to in the literature as stress grooves, could lead to inappropriate materials selection (or welding procedures) and thus significant risks to safety and environment. The NACE TM0316 standard provides little guidance on classification of crack-like features that develop only partially through the test coupon. The work presented in this article is intended to provide a more objective criteria that can be used instead of engineering judgment. A variety of base metal and weld metal coupons were tested in NACE Region 3 sour environments and then analyzed from both cross-sectional and surface perspectives. Precise dimensional measurements of surface damage features were gathered in order to support quantitative criteria where possible. For more qualitative judgments, the authors have proposed a micrograph library to facilitate engineering judgment. The resulting acceptance criteria flowchart encourages a more detailed analysis, thus optimizing materials selection and welding practices for sour service pipelines.