From Pits to Cracks: A Review of 45-Years of Published Research on Surface Trenches in Low Alloy Steels Exposed to H₂S-Containing Environments

Surface trenches are described as elongated pits or blunt cracks with a large depth (d) to width (a) ratio that occur as the result of an environmentally and stress-assisted damage. Trenches may represent a transition from pits to cracks in stressed carbon and low alloy steels specimens when exposed to H₂S-containing solutions under certain experimental conditions. In the literature, surface trenches are also known as deep or sharp pits, small blunt cracks, stress-induced microgrooves, fissures, cracklets, or microcracks. Since these features were first presented in 1977 by Dunlop, many authors have typically reported their depth while others have included the shape aspect ratio (d/a) for a more consistent characterization. In 2000, Pargeter published a flowchart to distinguish pits from cracks based on the microscopy analysis of the cross-sections of tested samples. In the absence of detrimental phases such as hard microstructures, and for indications with a depth higher than 250 μm, Pargeter classified cracks as features that presented sharp tips and parallel sides. However, indications with depths below the 250 μm limit reported in the literature remain unclassified according to Pargeter's guidelines. In recent years, efforts have been made to clarify the limits between trenches with respect to pits and cracks. Nonetheless, there is still a lack of precise guidelines to characterize the appearance of surface trenches and what is more relevant, conducting only a dimensional analysis of these features could be insufficient to evaluate their influence in crack initiation. Additionally, the variables that control trench formation, such as the threshold stress for trench initiation, are not clearly defined or controlled in tests. This review compiles previous findings and explores the effect of the factors that control trench nucleation and growth, including chemical composition, thermomechanical history, microstructure, and mechanical properties of steels, surface preparation, chemical composition of the environment (e.g., H₂S concentration, pH, the presence of organic acids, and level of chlorides), temperature, electrochemical potential, time of exposure, and applied loads.