This review aims to provide a foundation for the safe and effective use of magnesium (Mg) alloys, including practical guidelines for the service use of Mg alloys in the atmosphere and/or in contact with aqueous solutions. This is to provide support for the rapidly increasing use of Mg in industrial applications, particularly in the automobile industry. These guidelines should be firmly based on a critical analysis of our knowledge of SCC based on (1) service experience, (2) laboratory testing and (3) understanding of the mechanism of SCC, as well as based on an understanding of the Mg corrosion mechanism.

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Mechanisms for the stress corrosion cracking (SCC) of Mg-Al alloys have been investigated by scanning electron microscopy (SEM) of the fracture surfaces, for the two-phase alloy AZ91 and the single-phase alloys AZ31 and AM30 in distilled water. The mechanism for crack initiation in AZ31 and AM30 involves localized dissolution. The mechanisms for crack propagation in AZ31 and AM30 involve microvoid coalescence and cleavage, respectively. The mechanism for crack initiation in AZ91 is unclear, but may involve the fracture of b particles near the surface. The mechanism for crack propagation at moderate strain rates in AZ91 is similar to that in AZ31, with b particles acting as sources of H for mobile dislocations. The fracture surface for AZ91 tested at the strain rate 3 · 10 -8 s -1 was similar to that for specimens precharged in gaseous H 2 . This fracture surface is the result of (1) the nucleation and growth of MgH 2 particles, (2) sudden fracture through the MgH 2 particles at some critical stress, and (3) decomposition of the MgH 2 particles after fracture.

Transgranular stress corrosion cracking (TGSCC) of the Mg alloy AZ91 in distilled water and 5 g/L NaCl solution has been evaluated using the Linearly Increasing Stress Test (LIST) and the Constant Extension Rate Test (CERT). The differences between these techniques, with respect to fractography and the measurement of SCC parameters, are discussed. The LIST and CERT techniques are both useful in identifying the occurrence of SCC and, when coupled with a technique for characterizing crack extension, measuring the threshold stress and crack velocity. During a LIST, fast fracture ensues a relatively short time after the threshold stress is attained, whereas during CERT crack growth over a much longer time period is facilitated by a reduction in stress. Consequently, the LIST is typically 30-50 % shorter in duration, whereas the CERT produces a larger SCC fracture surface.

This paper evaluates the important elements of delayed hydride cracking (DHC) for transgranular stress corrosion cracking (TGSCC) of Mg alloys. A DHC model was formulated with the following components: (i) transient H diffusion towards the crack tip driven by stress and H concentration gradients; (ii) hydride precipitation when the H solvus is exceeded; and (iii) crack propagation through the extent of the hydride when it reaches a critical size of ~0.8 µm. The stress corrosion crack velocity, V c, was calculated from the time for the hydride to reach the critical size. The model was implemented using a finite element script developed in MATLAB. The input parameters were chosen, based on the information available, to determine the highest possible value for V c . Values for V c of ~10 -7 m/s were predicted by this DHC model. These predictions are consistent with measured values for Vc for Mg alloys in distilled water but cannot explain values for V c of ~10 -4 m/s measured in other aqueous environments. Insights for understanding Mg TGSCC are drawn. A key outcome is that the assumed initial condition for the DHC models is unlikely to be correct. During steady state stress corrosion crack propagation of Mg in aqueous solutions, a high dynamic hydrogen concentration would be expected to build up immediately behind the crack tip. Stress corrosion crack velocities ~ 10 -4 m/s, typical for Mg alloys in aqueous solutions, might be predicted using a DHC model for Mg based on the time to reach a critical hydride size in steady state, with a significant residual hydrogen concentration from the previous crack advance step.