The influence of chloride-chromate solution composition on the stress corrosion cracking of a MgAl alloy

Characterisation of stress corrosion cracking (SCC) of Mg–Al alloys

“…Table 2 shows that the stress corrosion crack velocities for AM30 (V c = 3.6 × 10 −10 to 9.3 × 10 −10 m/s) are much slower than for AZ91 (V c = 1.6 × 10 −9 to 1.2 × 10 −8 m/s) and AZ31 (V c = 1.2 × 10 −9 to 6.7 × 10 −9 m/s). These results are consistent with the crack velocity measured by Speidel et al [61] for the Mg alloy ZK50A-T5 in distilled water, but much slower than those measured for Mg alloys in other aqueous environments [13,14,36,38,39,49,61] and predicted using our numerical model for DHC (V c ≈ 10 −7 m/s), although this result was based on a speculative diffusion coefficient (D = 10 −9 m 2 /s) since no data exists for Mg alloys at room temperature [15]. The different crack velocities for AZ91, AZ31 and AM30 may be due to a difference in the predominant mechanism for SCC propagation.…”

“…It follows that, at these strain rates, the influence of repassivation at the crack tip is negligible, and the influence of strain rate is related to the mechanism for crack propagation. In contrast with the present work, previous workers [13,14,36,37,12,38,39,49] used strongly passivating solutions containing K 2 CrO 4 . This indicates that the occurrence of maximum SCC susceptibility at intermediate strain rates observed for Mg alloys in strongly passivating solutions is a characteristic of these environments rather than a characteristic of Mg alloys.…”

“…The influence of strain rate on the susceptibility of Mg alloys to SCC has previously been ascribed to, at low strain rates, the balance between repassivation and mechanical film rupture at the crack tip and, at high strain rates, the propensity for the inert fracture mechanism to overwhelm the SCC fracture mechanism [14,38,39]. In the present work, decreasing strain rate is associated with a continuous increase in SCC susceptibility (characterised by: (i) an increasing difference between σ SCC and the UTS; (ii) a decreasing elongation-to-failure; and (iii) a decrease in σ SCC for AZ91 and AZ31).…”

“…The influence of strain rate on SCC initiation is associated with the propensity for mechanical film rupture at the surface, which causes localised corrosion and H ingress. The influence of strain rate on SCC propagation is associated with: (i) the propensity for repassivation at the crack tip; (ii) the transition to the inert fracture mode (at high strain rates); and (iii) the time required for embrittlement and fracture of the region ahead of the crack tip [14,[16][17][18][38][39][40][41]. It should also be noted that the propensity for surface film breakdown and repassivation at the crack tip is also dependent on the interaction between the alloy and environment.…”

Stress Corrosion Cracking of Magnesium Alloys

“…In distilled water, the influence of repassivation at the crack tip is negligible, and the influence of strain rate is related to the mechanism for crack propagation. In contrast, Ebtehaj et al [34] used strongly passivating solutions. This indicates that the occurrence of maximum SCC susceptibility at intermediate strain rates for Mg alloys in strongly passivating solutions is a characteristic of these environments rather than a characteristic of Mg alloys.…”