Corrosion Cracking of Kh70 Pipe Steel under the Conditions of Cathodic Protection
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CORROSION CRACKING OF Kh70 PIPE STEEL UNDER THE CONDITIONS OF CATHODIC PROTECTION L. I. Nyrkova
UDC 620.194.22
We study the mechanism of corrosion cracking of Kh70 steel in a neutral soil electrolyte under the conditions of cathodic protection. We distinguished three areas of potentials in which the processes of corrosion cracking of steel are governed by different mechanisms. Thus, the process of local anodic dissolution proves to be predominant under potentials more positive than – 0.75 V, the mechanism of hydrogen embrittlement is predominant under potentials more negative than – 1.05 V; at the same time, both these mechanisms act within the range of potentials from – 0.75 to – 1.05 V. Keywords: Kh70 pipe steel, neutral soil medium, cathodic polarization, corrosion cracking, slow-rate strains.
Introduction The underground pipelines are protected against corrosion by polymeric protective coatings in combination with cathodic protection. In the course of long-term operation under the complex action of external and internal factors, the protective coatings can exfoliate from the pipe surface and, as a result, the bare metal comes in contact with the environment. The experience of operation shows that the processes of corrosion cracking (CC) may develop in welded pipelines at the sites of defects and exfoliations in the coatings even if the protective potential is kept within the specified limits. In the course of overhead technical diagnostics of pipelines, it is difficult to measure the actual values of protective potentials under the exfoliated coating. Therefore, the local values of potentials on the surface of the pipes inside the defects and in the zones with intact coating may significantly differ from each other [1–3]. Note that the contact of the pipe wall with corrosive media may lead to the formation of hydrogen on the metal surface, which is typical, e.g., of acidic and anaerobic conditions [4–6]. On the other hand, hydrogen can be also formed in neutral media under the conditions of cathodic polarization [4–10]. The mechanism of corrosion processes running under the conditions of cathodic protection has its own specific features. The results of experimental investigations confirm the existence of a certain critical concentration of hydrogen, which causes a significant loss of the local fracture resistance of the material [11]. It is also necessary to mention the results on the existence of a critical potential range in which the pipe steel is in the nonequilibrium electrochemical state and the process of anodic dissolution may occur under the conditions of cathodic polarization [12–14]. The aim of the present work is to study the mechanism of corrosion cracking of Kh70 pipe steel under various cathodic potentials whose values belong to the range specified by the standards. E. Paton Institute of Electric Welding, National Academy of Sciences of Ukraine, Kyiv; e-mail: [email protected]. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 56, No. 2, pp. 124–129, March–April, 2020. Original articl
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