Distribution of phases, morphologies of discharge products, and concentration of Al and Zn elements in discharge products affect corrosion, anodic efficiency, and discharge performance of AZ61, AZ63, AZ101, and AZ103 alloys. The AZ101 alloy has a low overpotential for the dissolution of the Mg, which provided the highest OCP and the lowest discharge potential (−1.474 V vs SCE). The detachment of a large number of the β-Mg17Al12 phase particles and the loose discharge products with deep cracks on the AZ101 alloy surface provided the highest self-corrosion rate (2.225 g cm−2 year−1), the lowest anodic efficiency (41.76 ± 1.43%) and the lowest overpotential for hydrogen gas evolution. Fine β-Mg17Al12 phase particles, low concentration of Al and Zn within α-Mg, and condensed discharge products of the AZ63 alloy contribute to uniform corrosion. Despite offering a higher potential by the AZ63 alloy (−1.432 V), it has a lower chemical corrosion rate (0.476 g cm−2 year−1) and the highest anodic efficiency (56.07 ± 1.48). It suggests that AZ63 alloy is a good candidate as an anode for long-term seawater batteries.