The Volta potential change and corrosion behaviors of alloys Al-0.63Mg-0.28Si and Al-0.52Mg-0.63Si under MgCl2 drops with the initial concentration of 0.1–1.0 M in environments with relative humidity (RH) 33% were studied using a Kelvin probe (KP). For the alloy Al-0.63Mg-0.28Si, the Volta potential curve was mainly featured with many potential transients in the form of a sudden drop followed by an immediate recovery along the potential baseline at a higher potential value of −0.35 to −0.5 VSHE, which were associated with initiation and repassivation of metastable pits with diameter less than 3 μm. For the alloy Al-0.52Mg-0.63Si containing excess Si particles, the potential curve was featured with potential change in the form of slow increase followed by a sudden drop along the potential baseline at a lower potential value of about −0.7 VSHE. The sudden potential drop and stabilizing at the lower potential level were associated with corrosion initiation and growth, respectively. In the case of lower initial concentration and higher Si content leading to excess Si particles, the corrosion rating was higher. However, the excess Si particles showed a greater impact than the initial concentration.