Assessment of operational degradation of pipeline steels

This paper summarizes a series of the authors’ research in the field of assessing theoperational degradation of oil and gas transit pipeline steels. Both mechanical and electrochemicalproperties of steels are deteriorated after operation, as is their resistance to environmentally-assistedcracking. The characteristics of resistance to brittle fracture and stress corrosion cracking decreasemost intensively, which is associated with a development of in-bulk dissipated microdamages ofthe material. The most sensitive indicators of changes in the material’s state caused by degradationare impact toughness and fracture toughness by the J-integral method. The degradation degree ofpipeline steels can also be evaluated nondestructively based on in-service changes in their polarizationresistance and potential of the fracture surface. Attention is drawn to hydrogenation of a pipe wallfrom inside as a result of the electrochemical interaction of pipe metal with condensed moisture,which facilitates operational degradation of steel due to the combined action of operating stresses andhydrogen. The development of microdamages along steel texture was evidenced metallographicallyas a trend to the selective etching of boundaries between adjacent bands of ferrite and pearlite andfractographically by revealing brittle fracture elements on the fracture surfaces, namely delaminationand cleavage, indicating the sites of cohesion weakening between ferrite and pearlite bands. Thestate of the X52 steel in its initial state and after use for 30 years was assessed based on the numericalsimulation method.