What is the primary mechanism by which hydrogen induced cracking (HIC) degrades pipeline steel?

Hydrogen induced cracking (HIC) primarily degrades pipeline steel through a stepwise cracking mechanism initiated by the diffusion of atomic hydrogen into the steel microstructure. Atomic hydrogen, generated from corrosion reactions (especially in sour environments containing hydrogen sulfide), enters the steel and diffuses towards areas of high stress concentration or microstructural discontinuities. These discontinuities often include inclusions like manganese sulfides (MnS) or elongated grain boundaries. The hydrogen atoms then combine to form molecular hydrogen (H2), creating internal pressure within these microstructural voids. This pressure builds up, exceeding the cohesive strength of the steel, causing the void to expand and eventually crack. Over time, these small, isolated cracks link together in a stepwise manner, perpendicular to the direction of the applied stress, leading to a significant reduction in the steel's mechanical properties and ultimately, pipeline failure. The presence of hydrogen sulfide accelerates this process by poisoning the hydrogen recombination reaction on the steel surface, promoting the entry of atomic hydrogen into the metal. The severity of HIC is also influenced by the steel's composition, microstructure (e.g., grain size, hardness), and the applied stress level.