Stress corrosion cracking SCC of magnesium Mg alloys

The chapter builds on our critical reviews on Mg corrosion [1-4] and Mg SCC [5,6]. SCC [5-8] involves (1) a stress, (2) a susceptible alloy and (3) an environment. SCC is related to hydrogen embrittlement (HE). HE is SCC that is caused by hydrogen (H), which can be gaseous, can come from corrosion, or can be internal from prior processing. HE is often postulated as the SCC mechanism. SCC can be extremely dangerous. Under safe loading conditions, SCC causes slow crack growth. Fast fracture occurs when the crack reaches a critical size. SCC, for any alloy + environment combination, can be characterised by [7,8] the threshold stress, ctscc threshold stress intensity factor, iscc the stress corrosion crack velocity. 

1 Model for transgranular stress corrosion cracking (TGSCC) [9]. 

Pure Mg is susceptible to SCC [60,98-102]. Winzer et al. [102] found transgranular stress corrosion cracking (TGSCC) for pure Mg in 5g/L NaCl. Meletis and Hochman [99] reported crystallographic TGSCC for 99.9% pure Mg in a chloride-chromate solution. Fracture was cleavage-like. 

Lynch and Trevena [98] studied SGC of cast 99.99% pure Mg in aqueous 

2 Load vs. elongation at a strain rate of 5.7 x 10 s for commercial purity Mg, with anodic polarisation to -1.16V stressed in air , X stressed in Na2S04 solution A pre-exposed to Na2S04 solution, stored for 24h and stressed in air [101]. 

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