Strain rate induced stress corrosion cracking in buried pipelines

Abstract

Abstract Transgranular stress corrosion cracking (TGSCC) has been observed in buried pipelines where the metal is in contact with a dilute solution under a disbonded coating. It is already known that such cracking involves the evolution and permeation of hydrogen at the crack tip, as previously observed in nuclear pressure vessel steels and other low alloy steels under variable loading. This cracking mechanism is much more influenced by loading conditions (crack tip strain rate) than by the solution chemistry. With reference to pipeline steels, several authors claim that it is difficult to reproduce in the laboratory the actual cracking morphology observed in the field. The present paper presents and discusses the results of tests where the fracture surface appearance observed in pipes taken from service has been reproduced. In this work, attention has been focused on mechanical factors rather than chemical parameters. It is proposed that the term low pH stress corrosion cracking, normally used to define the cracking phenomenon, is inappropriate and can be misleading. If greater attention is paid to strain rate and loading parameters, TGSCC in buried pipelines can be reproduced in laboratory tests (with no major influence of solution pH) and the phenomenon can be better described. Through a comparison with data in the literature data describing the behaviour of landslides, the hypothesis that crack growth occurs only during short intervals, followed by long periods where only generalised corrosion takes place, is proposed. This is consistent with features observed on the fracture surfaces of pipelines that failed in service.