Stress Corrosion Cracking of wrought Mg-Zn-Mn-Si alloys

The stress corrosion cracking (SCC) of high strength and ductility Mg-Zn-Mn alloys containing Si was studied using the slow strain rate test (SSRT) technique in air and in 3.5 wt% NaCl solution saturated with Mg(OH)₂. All alloys were susceptible to SCC to some extent. The fractography was consistent with a significant component of intergranular SCC (IGSCC). The TGSCC fracture path in ZSM620 is consistent with a mechanism involving hydrogen. In each case, the IGSCC appeared to be associated with the second-phase particles along grain boundaries. For the IGSCC of the ZSM6X0 alloys, the fractography was consistent with micro-galvanic acceleration of the corrosion of α-magnesium by the second-phase particles, whereas it appeared that the second-phase particles themselves had corroded. The study suggests that Si addition to Mg-Zn-Mn alloys can significantly improve SCC resistance as observed in the case of ZSM620. However, the SCC resistance also depends on the other critical alloying elements such as zinc and the microstructure.