PSI - Issue 59

Liubomyr Poberezhnyi et al. / Procedia Structural Integrity 59 (2024) 158–166 Liubomyr Poberezhnyi et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Table 2. Electrochemical characteristics from potentiodynamic data Electrochemical parameter BM 190 min in 3.5% NaCl solution

WM 18h in 3.5% NaCl solution

E cor vs Ag/AgCl, V

-0.476

-0.492

J cor ,  A/cm 2 V cor, mm/year

26

58.8

0.306 0.398

0.692 0.634 0.138

b c , V/dec b a , V/dec

0.0636

The next stage of the corrosion tests scrutinizing the corroded areas ’ surface using a metallographic microscope Leica DM4 P (Figs. 5, 6). The localization of corrosion processes on the surface of the base metal is less pronounced compared to that on the surface of the welded metal. Nonetheless, a few notable deep corrosion defects are discernible, marked by white arrows. Conversely, the corrosion on the welded metal surface is predominantly ulcerative in nature. This corrosion behavior can be explained by the presence of localized defects on the working area surface. Upon examination surfaces of welded joint parts, numerous surfaces defects were found (Fig. 6, marked by white arrows). These defects are mainly concentrated near the boundary of the heat affected zone with the base metal and weld metal. Within the WM zone, a macrodefect was also discovered (Fig. 6, f) in the form of an unfilled cavity, likely a gas bubble that didn’t escape during welding. Such defects are quite dangerous as they act as stress concentrators and, when exposed to corrosive environment, become places where cracks are likely to initiate. Since the electrochemical parameters for welded joint parts were both measured and calculated, they serve as initial values in the COMSOL model. Initially, the COMSOL model incorporated only the measured Open Circuit Potential (OCP) values. OCP was measured for all welded joint parts over a 24 h period (Fig. 3) and at the beginning of each potentiodynamic test. The values showed some differences, and the COMSOL model was calculated for two scenarios: specimens without protective paint and those with paint defects. The modelling results are shown in Fig. 7 and have significant differences. The calculation for the sample without protective paint based on OCP after 24 h in a 3.5% NaCl solution (Fig. 7, a) showed the highest current densities at the HAZ-WM interface, while the calculation using OCP measured before the potentiodynamic tests showed the highest value at the HAZ-BM boundary. Comparison of the absolute values of current densities showed that in the case of using 24 hour OCP measurement, the current density is approximately 1.6-1.8 times higher (Fig. 7, a, b).

Fig. 5. Corrosion of welded joint: (a) BM; (b) WM.