Relation Between Zn Additions, Microstructure and Corrosion Behavior of New Wrought Mg-5Al Alloys

In the present study, the effect of varying Zn additions (0.93–3.16 wt%) on the corrosion resistance of wrought Mg-5Al alloys was studied and related to the microstructure changes. Three secondary phases were found: Al–Mn, β-Mg17Al12 enriched with Zn, and Φ-Mg21(Al, Zn)17; the latter observed only in alloys with relatively high Zn content. The corrosion behavior in short periods of immersion is related to pitting corrosion, and strongly influenced by micro-galvanic effects between the secondary phase particles and the α-Mg matrix. In long periods of immersion, a protective layer is formed, and filiform corrosion observed for alloys with Zn content above 1 wt%. However, the micro-galvanic effects still play a crucial role in deterioration of corrosion resistance in alloys with relatively high Zn content. These effects depend on the cathodic behavior of the particles, and on the total number of the particles presented on the alloy’s surface.