5 Figure 2. Photomicrographs of NNpHSCC crack cross-section features on the R-T surface cycled in NNpH solution sparged with 5% CO2/ balance N2 gas mixture (a) in soil solution under load condition A [19]; (b) in C2 solution under load condition A [20]. 1200 1000 800 600 400 200 80 70 60 50 40 30 20 10 0 Crack depth, micron Number of cracks (a) 1400 1200 1000 800 600 400 200 0 Crack depth, micron (b) Figure 3. Distribution of crack depth on the sample cyclically loaded at condition B1 in C2 solution; (a) number of cracks versus crack depth; (b) box plot of crack depth. 3.2. BLUNT CRACK GROWTH After blunt cracks had initiated, they might grow under benign conditions. However, the majority of the cracks would not grow to the critical depth size of 0.5 to 0.6 mm. Once the blunt crack growth rate was higher than the pit growth rate, the stress facilitated corrosion dissolution rate would be dominant and thus, dissolution would be localized and localized corrosion around the corrosion pits would be accelerated and blunt cracks would grow. In the end, dissolution formed blunt crack would survive and might grow under intermittent conditions. It was sometimes seen that non-metallic inclusions (mainly MnS as revealed by EDS) were located ahead of the cracks on the crack advancement plane, as shown in Figure 4. When the blunt cracks grew and reached these non-metallic inclusions, the solution in the blunt crack enclaves could get to these inclusions and electrochemically react with the inclusions [5-11]. The inclusions would then (a) Loading direction (b)
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