13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- In AZ31 and AZ61, KISCC is the highest under the cathodic potential of -2.5 V in the boundary between the corrosion and immunity regions compared with those under higher and lower cathodic potentials. It is considered that under the higher cathodic potential, the corrosion is dominant and under the lower cathodic potential, the hydrogen embrittlement is dominant. The anodic dissolution is dependent on the cathodic potential, i.e., as the cathodic potential becomes lower, the anodic dissolution becomes decreased since the Mg phase becomes more stable than the Mg2+ phase as shown in Fig. 5 of the Pourbaix diagram, which is the reason that KISCC under the cathodic potential of -2.5 V is higher than that under the cathodic potential of -1.4 V. The generation of hydrogen molecules, H2, should be also dependent on the cathodic potential, i.e., as the cathodic potential becomes lower, more H2 are generated. Thus the lower cathodic potentials accelerated the hydrogen charging and KISCC becomes lower as the cathodic potential is lower than -2.5 V. 5. Conclusions In the present study, the SCC tests were performed using the wrought magnesium alloys, AZ31, AZ61 and AZ61-T5 under the controlled cathodic potential and the effects of the anodic dissolution and the hydrogen embrittlement on the SCC behavior of Mg alloys were investigated. From the experimental results and the discussion, the following conclusions could be made: (1) The crack growth rate of AZ31 was faster and KISCC of AZ31 was lower than those of AZ61 and AZ61-T5 under the same cathodic potential since the added Al acts to enhance the strength and to decrease the anodic dissolution. (2) The crack growth rate of AZ61-T5 was faster and KISCC of AZ61-T5 was lower than those of AZ61 under the same cathodic potential since the precipitated phases in AZ61-T5 are sensitive to hydrogen embrittlement. (3) The crack growth rate depends on KI. However in AZ31 the crack growth rate is not so sensitive to KI. Especially the crack growth rate is insensitive to KI under the cathodic potential of -3.0 V. (4) In AZ31 and AZ61, KISCC under the cathodic potential of -2.5 V where the boundary between the corrosion and immunity regions is higher than KISCC under the higher cathodic potentials in the corrosion region where the anodic dissolution is dominant and KISCC under the lower cathodic potentials in the immunity region where the hydrogen embrittlement is dominant. (5) In all materials, KISCC under the cathodic potential of -1.4 V was lower than that under the cathodic potentials of -3.0 V and -4.0 V. (6) The amount of corrosion products depends on the cathodic potential and the exposure time to corrosive environment. Less corrosion products were observed on the fracture surfaces under the cathodic potentials of -3.0 V and -4.0 V than on the fracture surfaces under the cathodic potentials of 0 V and -1.4 V. A part of the fracture surfaces of AZ31 and almost all of the fracture surfaces of AZ61 and AZ61-T5 were covered with corrosion products where the exposure time was long. References [1] Y. Uematsu, T. Kakiuchi, M. Nakajima, Stress corrosion cracking of the wrouthg magnesium alloy AZ31 under controlled cathodic potentials, Materials Science and Engineering A, 531 (2012) 171–177. [2] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, London, 1966. [3] K. Tokaji, M. Nakajima, Y. Uematsu, Fatigue crack propagation and fracture mechanisms of wrought magnesium alloys in differenct environment, International Journal of Fatigue, 31 (2009) 1137–1143.
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