Characteristics of Near-Neutral-pH Stress Corrosion Cracks in an X-65 Pipeline Available to Purchase

Two patches of near-neutral-pH stress corrosion cracks (SCC), as developed on a pipeline steel during service, have been characterized. The two patches can be characterized as containing either dense cracks or sparse cracks, based on their spatial separation. Cracks from both patches were categorized as either linked cracks or independent cracks; the former were distinguished by multiple peaks in the depth direction, considered to be a result of crack coalescence, while the latter had a single peak and a smooth depth profile. The linked cracks were usually <500 μm in depth with little increase in depth as the crack length extended beyond ~2 mm. In contrast, the independent cracks exhibited a linear relationship between crack length and crack depth. The linked cracks, although dormant in the depth direction, appeared to be actively corroding in the width direction near the crack root and possibly at the overlapped region between two joined cracks. For the independent cracks, corrosion in the width direction became faster as the crack deepened, implying that the growing cracks were evolving toward a pit. This also suggests that alternative mechanisms (e.g., possibly hydrogen embrittlement, as evidenced in this study) must become operative in the process of rupture following the initial dissolution mechanism for crack initiation and initial growth.