PSI - Issue 26

Myroslava Hredil et al. / Procedia Structural Integrity 26 (2020) 409–416 Hredil et al. / Structural Integrity Procedia 00 (2020) 000 – 000

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a b Fig. 5. Fracture surfaces of the long-term operated steel X70 after the tests in air (a) and NS4 solution (b).

The scheme of the experimental setup is presented in Fig. 6a. As depicted on the scheme, a specimen is only partially immersed in an electrolyte in such a way that the crack tip is always kept above the level of an environment. Therefore, the only way for hydrogen to reach the crack tip is diffusion. A moderate cathodic current density 5 mA/cm 2 was chosen basing on the previous experience of Nykyforchyn 1 et al. (2019). This experiment simulates conditions which are typical for operation of gas main pipelines. Hredil et al. (2010) shoved that hydrogenation is possible during pipeline operation as a result of corrosion processes at the inner surface of a pipe. However, it is well known that stress corrosion cracking usually occurs at the external pipe surface initiating from the pits in the places of a damaged insulative coating (Fig. 6b). The method is intended to evaluate the possible impact of hydrogen absorbed and accumulated inside the pipe wall on the crack growth in steel from the pipe outer surface.

a b Fig. 6. Experimental setup for fatigue testing under hydrogen charging (a): 1 – specimen; 2 – grips; 3 – Pt wire (auxiliary electrode); 4 – electrochemical chamber; 5 – power supply) and (b) scheme of SCC at the pipe external surface promoted by hydrogen diffused from the pipe inner surface (b). Using the proposed method of hydrogen charging of the prefracture zone in the vicinity of the crack tip without its direct contact with an electrolyte allows indication the accelerated crack growth at the Paris region of the diagram d a /d N – Δ K even in the case of 17H1S steel testing in the as-received state. It is found that hydrogen absorbed by the metal under the testing conditions caused a leap of crack growth rate in the Paris region of the fatigue crack growth curve (Fig. 7, red dots). As a result, a plateau is formed in a certain range of  K values where crack growth rate is independent on applied stresses. The observed plateau is typical for environmentally assisted cracking under the  K level approaching K scc . This value corresponds to the threshold level of SIF for SCC under static stresses. Since metal in the crack tip did not contact with corrosive medium in the considered case, thus the obtained accelerating effect could be concerned with hydrogen embrittlement of the metal prefracture zone. It should also be noted that there was no acceleration of crack growth under the testing of this steel in NS4 solution without application of cathodic current (blue dots in Fig. 7). Thus, NS4 solution itself doesn’t have a sufficient hydrogenating capability to accelerate crack growth in the as-received 17H1S steel under the proposed testing conditions.