Electrochemical Fracture Analysis of In-service Natural Gas Pipeline Steels

Long-term operation of natural gas transit pipelines implies aging, hydrogen-induced and stress corrosion cracking and it causes hydrogen embrittlement of steels, degradation of mechanical properties associated to a safe serviceability of pipelines, and failure risk increase. The implementation of effective diagnostic measures of pipelines steels degradation would allow planning actions in order to reduce a risk of fracture. In this paper, a new scientific and methodical approach based on the electrochemical analysis of fracture surface for evaluation of in-service degradation of operated pipeline steels was developed. It was suggested that carbon diffusion to grain boundaries and to defects inside grains, intensified by hydrogen, under long-term operation led to formation of nanoparticles of carbides, which resulted in intergranular cracking of operated pipeline steels under service and their transgranular cracking under impact toughness testing. Therefore, fracture surface was enriched by carbon compounds, and electrochemical characteristics were sensitive to this. In-service degradation of ferrite-pearlite pipeline steels was accompanied by a sharp shift in open-circuit potential of the fracture surface (brittle fracture) of specimens after impact toughness tests compared with that of polished steel surfaces. A significant difference between potentials of the fracture surface and the polished steel surface (over 60 mV in 0.3% NaCl solution) of specimens made of ferrite-pearlite pipeline steels observed after their long-term operation was evidently due to the increased content of carbon compounds on the fracture surface. Mechanism of ferrite-pearlite pipeline steels embrittlement under operation consisted in carbides enrichment not only grain boundaries, but also intragranular defects, has been revealed, as it is indicated by an increase of carbon content on transgranular fracture surfaces determined electrochemically.