Non-destructive Electrochemical Evaluation of Pipeline Degradation

Long-term operation of natural gas transmission pipelines leads not only to the appearance of macro defects but also to in-bulk damaging of pipeline steels at nano- and micro-scales. In-bulk steel degradation and a decrease in characteristics of brittle fracture resistance of pipeline steels under long-time operation increase significantly a failure risk. Therefore, deterioration of pipelines under operation calls for effective methods for current condition evaluation. The paper is aimed to the development of a prediction method of degradation of pipeline steel in operating conditions based on electrochemical correlation. The low-carbon ferrite-pearlite steels with different strength of gas transit pipelines after long-term operation 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 parameters to in-bulk steel degradation among electrochemical properties was polarization resistance. An acceptable correlation between relative changes in polarization resistance and impact toughness of steels caused by long-term service was revealed. It was concluded that mechanical properties changes of pipeline steels caused by degradation under operation can be evaluated by measurements of changes in their electrochemical characteristics. Having initial properties of the steel, its current properties can be predicted. The method enables non-destructive in-service assessment of degradation degree of brittle fracture resistance of pipeline steels. The verification studies of prediction method of pipeline steel degradation were carried out on damaged and operated pipeline steels.