Issue 50

N. Alexopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 342-353; DOI: 10.3221/IGF-ESIS.50.29 345 (a) (b) (c) (d) (e) (f) Figure 2: Typical photographs of AA2024-T3 pre-corroded tensile specimens exposed to the EXCO solution for (a)-(c) 2 h, 4 h and 24 h of EXCO solution and (d) – (f) 6 h, 168 h and 720 h of 3.5 wt. % NaCl solution, respectively. the width and the length of the exposed area of the specimens, respectively. For short exposure times to exfoliation corrosion solution and up to 2 h, pitting formation on the corroded surfaces remains rather limited. With increasing exposure time to EXCO solution, an increase in the pitting density is evident. Corrosion damage initiates in the form of surface corrosion pits and evolves to the formation of micro-cracks and exfoliation areas due to the presence of intergranular corrosion. Regarding the corrosion environment of 3.5 wt. % NaCl solution, it is evident that for the short exposure times the surface deterioration remains limited since pits were not identified after 6 h of exposure. However, the pitting density and size tend to increase with increasing exposure time; corrosion damage in the form of pits was observed after 168 h of exposure while more pits of higher diameter as well as pit coalescence are evident after 720 h exposure. Typical tensile curves of pre - corroded specimens Typical nominal tensile stress-strain curves for the investigated exposure times of AA2024-T3 to EXCO and 3.5 wt. % NaCl solutions can be seen in Figs.3(a,b), respectively. The nominal stress calculation was based on the nominal cross-section of the tensile specimens, namely width x thickness = 12.5 mm x 3.2 mm. It can be noticed that for the short corrosion exposure times to EXCO solution and up to 2 h, the values of axial nominal stress are not essentially influenced by the corrosion exposure while for higher exposure times, e.g. after 4 h, a significant stress drop was noticed [33]. This stress drop was attributed to the decrease of the specimen’s cross-section – due to the corrosion-induced micro-cracks formation - that withstand the applied mechanical loading, so called as “effective thickness” [29]. On the contrary, an essential decrease of tensile ductility is evident even for the very short exposure times, e.g. 0.5 h that can be attributed to the hydrogen embrittlement phenomenon. For even higher exposure times, both the axial nominal stress and axial nominal strain are essentially decreased. Regarding corrosion exposure to 3.5 wt. % NaCl solution, no essential stress decrease was noticed even for the highest exposure time, e.g. 4200 h. However, elongation at fracture exhibited a significant degradation even for the very short exposure times such as 6 h. It is worth mentioning that higher ductility degradation was observed at the time range between 6 and 168 h, where pitting incubation takes place, as well as in the time range of 720 and 2184 h, probably because of the change in the degradation mechanism, e.g. pit growth and coalescence. Typical nominal tensile curves for the specimens of AA2024-T3 with machined surface notches can be seen in Fig.3c; the surface notch depth is a varying parameter. In the same figure, a reference tensile curve without any notches (black circles) was added for comparison. As can be seen, the surface notches act as stress concentrators and tend to decrease the tensile mechanical properties of the alloy and especially elongation at fracture that decreases in higher rates than the yield stress. An essential decrease of the axial nominal strain was noticed even for the low-depth notches; nevertheless, this was not the case for the strength properties that seem to be almost unaffected even for the notch depth of 0.30 mm. For instance, the tensile curves of 0.10 and 0.15 mm notch depths showed quite the same behavior with the reference one with the exception of the significant loss in ductility. By increasing the notch depth, up to 0.50 mm in this work, a continuous elongation at fracture decrease was noticed (magenta circles) exhibiting extremely low tensile elongation at fracture values. Characterization of the fracture mechanism Stereoscopical examination on the fractured, pre-corroded specimens of AA2024-T3 (Fig.4) revealed that the pitting corrosion mechanism is extremely limited for the very short corrosion exposure times, i.e., up to 2 h to EXCO solution. It is evident that with increasing exposure time to EXCO solution, the pitting density tends to increase. A ductile fracture mechanism is evident from the 45˚ slope of the fracture surface as can be seen in Figs.4(a,b). For the higher exposure times, e.g. 24 h in Fig.4c, it seems that the fracture paths follow a non-linear pattern and definitely from pit to pit. For the specimens exposed to 3.5 wt. % NaCl solution, pitting was not as evident as observed in the respective EXCO specimens. Pitting density for relatively short exposure times of 6 h (Fig.4d) and 168 h (Fig.4e) was extremely small, while the pits substantially increased for higher exposure times, e.g. 720 h (Fig.4f). However, the 45˚ slope of the fracture surface remains even after 720 h of exposure revealing a ductile fracture mechanism.