PSI - Issue 42

V.K. Yadav et al. / Procedia Structural Integrity 42 (2022) 594–601 V.K. Yadav et al., 2022/ Structural Integrity Procedia 00 (2019) 000 – 000

598

5

potential values were -633.5 mV SCE and -669.3 mV SCE for weld nugget zone and as-received material conditions, respectively. This potential-time behavior does not reveal any corrosion mechanism of the material under investigation, but will be useful to analyze underlying corrosion mechanism using electrochemical impedance. After stabilizing the specimen’s surface in the salt water, EIS measurements of weld nugget zone and as -received material were carried out. The EIS spectra were taken at a frequency range of 0.01 Hz to 100 kHz, using a 10 mV (rms) sinusoidal perturbation. After measurements, EIS spectra are presented as Nyquist plot, as shown in the Fig. 4. The Nyquist results confirm that the weld nugget zone has better corrosion resistance than the as-received material due to the refined and stable grain structure. Weld nugget zone due to the stable and recrystallized grains provide more coherent passivation layer and also acts as more barrier against crystallographic pitting corrosion. To get detailed analysis of corrosion phenomena, the obtained results were fitted using the combination of capacitances and resistances. Although, the fitted line has some deviations compared to EIS data in low frequency range, but fitted results are in good agreement. To measure the corrosion resistance and durability of welded samples, pre-corroded (pre-corrosion in saltwater for 10, 24, 48, 100, and 200 hours) samples in saltwater were also examined and studied using EIS spectra, and are presented in the Fig. 5a. Figure revealed that with the increase in the immersion time, charge transfer resistance i.e., corrosion resistance decreases up to 100 hours of immersion, afterward no significant change was observed. Probably due to the formation of passive oxide layer over the surface that protect it from the further oxidation. Thus, similar saturation effect on the fatigue crack behavior was expected and investigated after 100 hrs of immersion in the 3.5% NaCl solution.

Fig. 3 . Potential-time behavior of as-received and weld nugget zone.

Other than that, of OCP and EIS-spectra, potentiodynamic polarization tests were performed at an initial delay period of 10 min at OCP, as shown in the Fig. 5b. Tests were carried out within the range of ±250 mV and at a scan rate of 0.1mV/s. The figure depicts that polarization potential of weld nugget zone is higher than as-received material. 3.3. Fatigue Crack Growth (FCG) Analysis The fatigue crack growth rate of the pre-corroded welded specimens was investigated at the two different load ratios of 0.1 and 0.5, and compared with the without corroded specimen results, are shown in Fig 6. The observed data points were fitted using second-order polynomial. The long crack growth rate (da/dN, where a is the crack length and