利用三點彎曲試驗探討Mg-5wt.%Sn合金在不同NaCl濃度下蝕孔與應力腐蝕裂縫關係之研究

In this study, we investigated the qualitative analysis of stress corrosion cracks for Mg-5wt.%Sn alloy by employing 3-point bending test which was different from slow strain rate test regarded as quantitative measurement. We focused on analyzing the correlation between pittings and stress corrosion cracks by using the method mentioned above as well as the discussion of the crack propagation path of Mg-5wt.%Sn under different concentration of Cl- (1,3.5,7wt.%). The result indicated that the route of stress corrosion crack of Mg-5wt.%Sn was obvious intergranular type. We suspected that this behavior could be attributed to the second phase Mg2Sn which was 5~10μm and distributed along the grain boundary after heat treatment. As a result, with the galvanic effect of Mg2Sn and the high chemical activity of grain boundary , we suggested that the prior stress corrosion path of Mg-5wt.%Sn could be grain boundary. The phenomenon that number of pittings for Mg-5wt.%Sn increased with increasing Cl- concentration could be observed through Scanning Electron Microscope (SEM). Besides, Cl- would promote the growth of pittings and accelerate the generation of cracks. The cracks of Mg-5wt.%Sn alloy would appeared with immersion in 7wt.%,3.5wt.% and 1wt.% NaCl solution for 3 hours,5hours and more than 8 hours, respectively. This outcome expressed that the incubation period of crack initiation with immersion in higher Cl- concentration could be sooner than that with immersion in lower Cl- concentration. Furthermore, the propagation rate of cracks under higher Cl- concentration was faster than that under lower Cl- concentration. It could be concluded that the number and depth of pittings, the incubation time of cracks and the propagation rate of cracks were highly sensitive to the concentration of Cl-. These results confirmed the influence of Cl- concentration on the surface passive film which was mainly composed of MgO and Mg(OH)2.