Evaluation of impact toughness of gas pipeline steels under operation using electrochemical method

Abstract

Natural gas transmission pipelines are an important component of worldwide energy supply. However, many pipelines are near the end of their design life. Long-term operation of pipelines leads not only to an appearance of macro defects but also to in-bulk damaging of pipeline steels at nano- and microscales. A decrease in characteristics of brittle fracture resistance of pipeline steels under long-time operation increases significantly a failure risk. Deterioration of pipelines under operation calls for effective methods for current condition evaluation.In this paper, a non-destructive electrochemical method for an evaluation of in-service degradation of brittle fracture resistance of pipeline steels under operation was developed. The low-carbon low-alloyed ferrite-pearlite pipeline steels of gas transit pipelines operated during 25-53 years were investigated. It was shown that mechanical and electrochemical properties of the pipeline steels were deteriorated due to long-term operation. It was found that one of the most sensitive parameter to in-bulk steel degradation among electrochemical properties was polarisation resistance. An acceptable correlation between relative changes in impact toughness and polarisation resistance of steels caused by long-term service was revealed. It was concluded that impact toughness changes of pipeline steels caused by degradation under operation can be evaluated by measurements of changes in their polarisation resistance. Having initial properties of the steel, its current properties can be predicted. The method enables non-destructive in-service assessment of degradation degree of pipeline steels.