Issue 50

N. Alexopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 342-353; DOI: 10.3221/IGF-ESIS.50.29 350 Figure 8: Correlation of the elongation at fracture A f decrease due to exposure to the 3.5 wt. % NaCl solution as well as to the presence of the artificial surface notches. Fig.9 shows the correlation results of the effect of the two corrosive environments, i.e. EXCO and 3.5 wt. % NaCl solutions, with regard to the same corrosion-induced tensile ductility decrease. To this end, for the conversion of exposure times to EXCO solution to the respective times of exposure to NaCl solution, the n coefficient in [-] is formulated as: n EXCO to NaCl = ୣ୶୮୭ୱ୳୰ୣ ୲୧୫ୣ ୲୭ ୒ୟେ୪ ሺ୦ሻ ୣ୶୮୭ୱ୳୰ୣ ୲୧୫ୣ ୲୭ ୉ଡ଼େ୓ ሺ୦ሻ . (4) The best calculation results were found by using the value n = 92 for the empirical coefficient and for the simulation of the tensile ductility decrease curve of the investigated specimens for short exposure times regime. The same rate decrease is evident between the two different corrosive solutions at the short exposure times where slight pitting formation is the dominant degradation mechanism. By using this coefficient value, it is obvious that the results of the exfoliation corrosion are very close to the experimental values of exposure to 3.5 wt. % NaCl up to 1 h of exposure and 14 % elongation at fracture. Thus, it can be concluded that 1 h of exposure to EXCO solution is equivalent to 92 h of exposure to 3.5 wt. % NaCl solution regarding the A f degradation. For higher exposure times, there is no correlation between the corrosive environments regarding the ductility decrease since the corrosion-induced degradation mechanism is different. By exploiting this empirical tool, the design engineer could estimate the equivalent surface notches problem (fictitious notch) with the true problem of corrosion exposure of AA2024-T3. This might be a very useful tool as through experimental data or through finite element calculations, the design engineer could estimate the residual mechanical properties of the sheet alloy for maintenance and repair actions. It is obvious that the proposed empirical correlation for 2024-T3 gives reliable results for short exposure duration in exfoli- ation corrosion solution. In short corrosion exposure times, the degradation mechanism of ductility has been correlated in the open literature with the hydrogen embrittlement. To this end, the above-mentioned empirical correlation is suggested to be exploited for exposure times higher than 2 h, where the first surface pits and sub-sequent micro-cracks are beginning to be formed in the investigated cross-section of the AA2024-T3. C ONCLUSIONS ummarizing the findings of the present experimental study, it could be concluded that surface pitting corrosion remains limited for the short exposure times to both corrosive environments, e.g. up to 2 h in EXCO and 168 h in NaCl solution; a substantial increase in pitting density and size was observed with increasing exposure times for both investigated solutions. Moreover, all investigated tensile mechanical properties of AA2024-T3 are exponentially decreasing with increasing exposure time to corrosive solutions as well as surface notch depth; tensile ductility decreases with higher 0 1000 2000 3000 4000 5000 6000 7000 8000 0 2 4 6 8 10 12 14 16 18 20 Elongation at fracture A f [%] Exposure time to NaCl times solution [hours] Aluminium alloy 2024-T3, t = 3.2 mm Exposure to 3.5 % NaCl solution Experimental results of exposure to 3.5 % NaCl solution Results of the empirical correlation total depth of the notches with exposure time to 3.5 % NaCl solution m = 15000 m = 500 S