1. Virtually any Al-containing Mg alloy consists of two main phases (the very important influence of minor additives is ignored at this stage):
α-phase: essentially pure magnesium
β-phase: Mg17Al12 (calculated 55.7wt% Mg)
2. Considering the equilibrium phase diagram for the system Al-Mg, β-phase should be more exactly referred to as γ-phase, because a congruent compound is found at ~ 50% Al:
Likewise, the α-phase is not necessarily the pure magnesium; rather it may contain up to 17% of Al solute. The "true" β-phase must contain about 35-37% Al and have the nominal stoichiometry Al8Mg5. Indeed, such a phase is sporadically mentioned in the literature, however, it is never referred to as β-phase. We here will also refer to the Mg17Al12 as β-phase in accordance with the mainstream.
3. These three phases have the following free corrosion potentials (1. O. Lunder, K. Nisancioglu and R. S. Hansen, SAE Technical Paper Series, 1993, 930755; 2. K Mutombo, M Du Toit, "Corrosion fatigue behaviour of aluminium 5083-H111 welded using gas metal arc welding method, in ARC WELDING", InTech OPEN ACCESS (www.interchopen.com) 2011 1, 177-218):
|phase||Ecor, V vs. SHE|
|β-phase: Mg17Al12||- 1.201|
As is seen from the table, both intermetallic phases are strongly cathodic with respect to the α-phase. Therefore, they should cause fast oxidation of the latter. However, in practice, the corrosion rate strongly depends on some additional circumstances (Song, G., Atrens, A., "Understanding Magnesium Corrosion—A Framework for Improved Alloy Performance," Advanced Engineering Materials 2003, 5(12) 837-858):
1. If the amount (volume fraction) of the β-phase is small, then it enhances the corrosion of magnesium. However, as the volume fraction of the β-phase is high enough, then this phase can serve as a protective coating and thus retard the corrosion process.
2. For the same volume fraction of the β-phase, an alloy having finer grain structure will be better protected from corrosion that the same alloy having coarser grains. This may be explained by the fact that finer grains of the α-phase are coated by practically continuous thin network of the β-phase, which protects them. Therefore, die cast alloys corrode more slowly than sand cast.
3. The peripheria of an ingot corrodes more slowly (sometimes by the factor of 10) than the core. The explanation is that grains are typically coarser in the core and finer in the periphery.