Experimental and In-Service Observations of HIC Nucleation and Growth in Pipeline Steel

This paper presents the results of experimental and in-service observations of the nucleation and growth of hydrogen-induced cracking (HIC) in hydrocarbon transport pipelines made of type API 5L steel. The experimental work was done by inducing HIC on steel plates by electrochemical cathodic hydrogen charging and using a straight beam ultrasonic inspection technique to observe the crack growth behavior. Scanning electron microscopy was also used to observe the crack nucleation and propagation mechanisms. The study was complemented by the fractographic analysis of a pipe segment removed from a sour gas pipeline after an in-service rupture caused by HIC, so the pipe segment contained a significant group of blisters and laminations caused by HIC. The results of the cathodic charging indicated that HIC cracks nucleated in less than one hour of hydrogen charging at specific non-metallic inclusions and not necessarily the largest ones as commonly thought. It is observed that the HIC cracks propagated by a quasi-cleavage mechanism in transgranular paths, linking to other cracks by ductile tearing. However, after a few hours of hydrogen charging, the crack growth rate dropped to almost zero, and the overall HIC growth was due almost solely to the interconnection of previously formed individual cracks. The examination of the in-service failed pipe showed similar fractographic and growth characteristics as compared to the laboratory-induced ones. It showed that HIC was little affected by the primary stresses and the proximity of other defects and structural discontinuities.