506

Tensile Behavior after Exposure to Various Corrosion Tests Figures 9.99.13 show the variation of the tensile properties degradation for the investigated accelerated corrosion tests as compared to the tensile properties of the reference material; Figs. 9.9- 9.13 stand for the alloys 2024, 2024 "ref. 2," 6013, 8090, and 2091, respectively. In all figures the values of the residual tensile properties are given in percent of the respective properties of the reference material. Referring to the data in Figs. 9.5 and 9.7, it is evident that the corrosion attack causes a decrease of ultimate tensile stress and yield stress. The effect is small for most corrosion processes considered and becomes appreciable with increasing aggressiveness of the corrosive solutions. This observation does not apply for material 6013; the loss of yield and ultimate tensile stress for this material following exfoliation corrosion was remarkable. On the other hand a dramatic drop has always been determined on elongation to fracture and energy density. Tensile ductility reduction was found to be appreciable even for the 2024-T3 "ref. 2" specimens, which, prior to corrosion tests, were subjected to anodizing and sealing. Particularly strong was the influence of exfoliation corrosion; it leads to extremely low values of remaining tensile ductility. The metallographic characterization of the materials has shown serious corrosion attack in this aggressive environment, and the measured depth of attack has reached values up to 0.49 mm for the worst case of the 6013 alloy. For the 2024 alloy the measured depth of attack was under 0.33 mm. Mechanical degradation has been associated with the initiation of corrosion defects that grow and lead to early failure of the material [29, 30]. The results of the performed metallographic evaluation may explain the determined degradation of yield and ultimate tensile stress but not the dramatic embrittlement of all materials investigated. Even more difficult to explain is the measured appreciable tensile ductility drop after a short outdoor exposure time. For the same outdoor exposure conditions, the yield and ultimate tensile stress, practically, do not decrease. Notice that stereoscopic analysis of the respective tensile specimens (performed after the outdoor exposure and classical metallographic characterization following the tensile tests) could not prove the occurrence of significant corrosion. Yet, the exposure of the materials during the corrosion processes in hydrogen-rich environments

Ultimate Stress

Yield Stress I I Elongation to failure Energ density

100 n

0 0

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