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)

154 4

Another experiment (Sequence II) involved measurements under stirring by a magnet stirrer to distinguish the effect of hydrogen itself and mechanical mixing of the corrosion environment. Hydrogen evolution was controlled by applying three current densities to the platinum cathode (14.3 mA/cm 2 ; 57.1 mA/cm 2 ; and 142.9 mA/cm 2 ) , providing corresponding hydrogen bubbling intensities. 3. Results and discussion The results showed that the steel in the as-received state revealed lower corrosion activity (higher polarization resistance) at all stages of the experiment following sequence I (Figure 3). In particular, in stationary conditions (before hydrogen bubbling), polarization resistance of the steel is 0.89 kOm∙cm 2 and 0.71 kOm∙cm 2 in the as received and operated states, respectively. Rising the intensity of hydrogen bubbling reduced polarization resistance of both steel states, but it is more notable for the operated one (almost three times). Moreover, the aftereffect of hydrogen was much more pronounced for this steel – even 90 min after hydrogen bubbling termination, the polarization resistance recovered only to 65% from its initial value, whereas, for the as-received state, it reached 87%. It should be noted that just the mixing of the environment also accelerates electrochemical processes on the steel due to a higher diffusion rate of depolarizers towards the electrodes, but this effect is less pronounced (a decrease in 1.5 – 1.7 times, the red line level in Figure 3).

X70 as-received

X70 operated

0,8 Rp , kOhm сm 2

Hydrogen bubbling

2 3

1

0,6

0,4

0,2

0,0

90 120 150 180 210 240 270 300

 , min

Fig. 3. Polarization resistance changes for Х70 steel in a model solution under 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 red line corresponds to the R p value for the as-received steel in a mixing environment without hydrogen bubbling. The experiment on sequence II aimed at shedding light on two aspects: the prehistory of the previous test (effect of preliminary hydrogen bubbling of the specimen on its corrosion activity) and the contribution of each factor (mechanical mixing and hydrogen) in the overall process. It was noted that polarization resistance in the stationary conditions for both steels was lower than previously measured (Figure 4). It means that the preliminary exposure to hydrogen caused some corrosive activation of steels, and it is more significant for the operated one, both in stationary conditions and under mechanical mixing (polarization resistance is two times lower for the as-received steel and 2.4 times lower for the operated one). Dmytrakh 1 et al. (2021) in turn noted that preliminary hydrogenation – dehydrogenation of low-alloy steel enhanced its ability to further hydrogen absorption. Concerning the impact of hydrogen on the corrosion activity of the specimens under mixing, it should be noted that low-intensity hydrogen bubbling ( 1 and 2 in Figure 4) insignificantly affected polarization resistance due to spreading the hydrogen flux in the bulk of the solution by intensive diffusion caused by mixing, so here, the mechanical factor prevailed. Under the highest intensity of hydrogen generation ( 3 ), a steady hydrogen flux formed