Specific features of the electrochemical behavior of steels under corrosion cracking of pipelines

We analyze the role of external and internal factors in stimulation of processes of stress cracking of pipelines. We identify the number of successive stages that cause fracture of pipelines. We experimentally established that passivation of a pipe steel in electrolyte under the coating and formation of a passive film consisting of oxides and iron carbonates is the main stage of corrosion cracking. Under the action of activating factors, the breakdown of a passive film occurs and local sites of corrosion appear. This stage is determined by the composition of the electrolyte under the insulated coating, the state of the surface of the metal, and its nature. Then pittings are transformed into microcracks in the field of tensile stresses. At the final stage, microcracks form colonies, which join together into the main crack. We established the similarity of cracks formed in actual gas pipelines and cracks initiated under laboratory conditions. We investigate the corrosion-electrochemical behavior of 09G2BT steel in NaHCO3 solutions of various concentrations. We established that a decrease in the concentration of a solution leads to an increase in the density and depth of pittings. We draw a conclusion about the probabilistic character of initiation of pittings on the surface of steel, which is described by the Poisson law. We propose to use the density of pittings and the current in a single pitting for the early diagnostics of fracture of pipelines.