PSI - Issue 59

Myroslava Hredil et al. / Procedia Structural Integrity 59 (2024) 151–157 Myroslava Hredil, Oleksandr Tsyrulnyk, Ivan Shtoyko, Olha Zvirko / Structural Integrity Procedia (2024)

155 5

along the working surface of the specimen, and it caused a drop in polarization resistance compared to the previous stages (for the operated steel, polarization resistance value is 3.5 times less than that at the beginning of the experiment), which confirms the essential role of hydrogen in the electrochemical activation of the steel. It is noticeable that polarization resistance of the operated steel did not recover even some time after the experiment (it reached only 34% from its initial value 90 min after hydrogen bubbling termination), obviously indicating irreversible damage to the steel.

X70 as-received

X70 operated

Mixing

2

.

0,6

R p , kOhm сm

Hydrogen bubbling

2

1

3

0,4

0,2

0,0

90 120 150 180 210 240 270 300 330

 min

Fig. 4. Polarization resi stance changes for Х70 steel in the model solution under a combined effect of mixing of the environment and hydrogen bubbling at various intensities: under the current densities of 14.3 mA/cm 2 ( 1 ); 57.1 mA/cm 2 ( 2 ); and 142.9 mA/cm 2 ( 3 ). A thorough observation of the specimen after the tests revealed some cracks on its working surface oriented in the rolling direction of the steel (Figure 5). This is considered clear evidence of hydrogen-induced cracking (HIC) caused by high internal stresses in the metal due to hydrogen uptake. This mechanism implies intensive surface hydrogenation. Ohaeri et al. (2018) stated that only a tiny part of hydrogen, formed at the steel surface due to corrosion, penetrates the metal, while almost all hydrogen recombines in molecules and releases as a gas; however, hydrogen amount absorbed by the metal essentially depends on time (Dmyrtakh 2 et al. (2021)).

Fig. 5. Working surface of the operated steel X70 after the test (sequence 2 ) revealing hydrogen-induced cracks.