13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- considered that the SCC behavior is dominated by the anodic dissolution in the corrosion region and by the hydrogen embrittlement in the immunity region. According to the Pourbaix diagram, the cathodic potential of -1.4 V corresponds to the corrosion region under which the SCC behavior is dominated by the anodic dissolution. The cathodic potentials of -3.0 V and -4.0 V correspond to the immunity region under which the SCC behavior is dominated by the hydrogen embrittlement. The cathodic potential of -2.5 V corresponds to the boundary between the corrosion and immunity regions. The SCC test conditions of cathodic potentials and electrochemical phases for each material are summarized in Table 3. Figure 5. Pourbaix diagram for magnesium Table 3. SCC test condition Material Cathodic potential Corrosion region of Mg Immunity region of Mg 0V -1.4V -2.5V -3.0V -4.0V AZ31 Tested Tested Tested AZ61 Tested Tested Tested Tested AZ61-T5 Tested Tested 3. Experimental results 3.1. Crack growth behavior Fig. 6 shows the relationship between the crack length, a, and the testing time, t, after the crack initiated to propagate in a stable manner. The SCC behavior depends on the material, i.e., AZ31 specimens fractured more rapidly than AZ61 and AZ61-T5 specimens, and AZ61-T5 specimens fractured more rapidly than AZ61 specimens. In all materials, the final crack lengths were shorter and specimens fractured more rapidly under the cathodic potentials of -3.0 V and -4.0 V which correspond to the immunity region than under the cathodic potentials of 0 V and -1.4 V which Test conditions Mg2+ Corrosion Mg Immunity -4 0 -1 -2 E (V) pH 7 0 14 -3 Mg(OH)2 Passivation
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