Methodical Approach and a Criterion for the Evaluation of the Susceptibility of Pipe Steel to Corrosion Cracking
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METHODICAL APPROACH AND A CRITERION FOR THE EVALUATION OF THE SUSCEPTIBILITY OF PIPE STEEL TO CORROSION CRACKING L. І. Nyrkova,1,2 S. О. Osadchuk,1 А. О. Rybakov,1 and S. L. Mel’nychuk1 Specific features of the stress-corrosion fracture of specimens of Kh70 pipe steel are analyzed under the conditions simulating the influence of various combinations of stress-corrosion factors. For the evaluation of the susceptibility of steels to stress-corrosion cracking (SCC), we propose to use a dimensionless coefficient K s equal to the ratio of the relative narrowing of the specimen in air to its relative narrowing in a solution. We introduce the following evaluation criterion: steel is susceptible to SCC if the coefficient K s is equal to or greater than 1.6. Keywords: Kh70 pipe steel, criterion of susceptibility to stress-corrosion cracking, deformation at low rates, potentiometry, stress-corrosion cracking
Introduction The use of underground pipelines is the most efficient and safest method for the long-distance transportation of oil and gas [1, 2]. For their protection, it is customary to use anticorrosion insulation and electrochemical protection but it is impossible to completely suppress the influence of the ambient medium [1, 3]. Thus, the phenomenon of stress-corrosion cracking (SCC) proves to be one of potential threats for their safe operation [2–5]. In the course of long-term operation under the indicated conditions, irreversible processes run in the metal even in the case where all technological requirements and conditions are satisfied. This lowers the possibility of subsequent safe operation of the pipeline [6]. Stress corrosion cracking develops under the simultaneous action of mechanical stresses and the complex influence of the ambient medium and metallurgical factors [2]. At present, extensive investigations of this phenomenon are carried out in different countries. In particular, the influence of stresses, hydrogen, and their synergism on the dissolution of steel at the crack tip in media whose pH values are close to neutral was quantitatively evaluated in [7–13], the effect of cathodic polarization on the mechanism of SCC was analyzed in [14, 15], and a conceptual model for the description of SCC under the conditions of cathodic protection in solutions whose pH values are close to neutral was proposed in [16]. The role of carbon dioxide and sulfate-reducing bacteria in the initiation of SCC was clarified in [17–19]. It was emphasized that there are no evident relationships between the failures of pipelines caused by SCC for the pH values close to neutral (or higher) and the chemical compositions or microstructures of the pipes [17]. At the same time, it was discovered [9] that, in solutions with almost neutral рН values, the pipes made of Kh70 steel with bainitic microstructure are highly susceptible to SCC, whereas the pipes made of Kh70 steel with ferritic matrix are weakly susceptible to SCC. 1 2
Paton Institute of Electric Welding, Ukrainian National Academy of Science, Kyiv, Ukraine. Corresponding au
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