The corrosion behaviors of 1420 Al–Li alloy under 4 μl MgCl2 drops with initial concentration of 0.1–1.0 M in relative humidity (RH) 33% were studied using a Kelvin Probe. In all cases, the equilibrium MgCl2 concentration was 4.9 M. The corrosion was highly dependent on the initial MgCl2 concentration. After equilibration with the environment RH, the whole available cathodic limiting current associated with oxygen reduction was decreased with increasing the initial MgCl2 concentration. Accordingly, the corrosion rate was decreased. Under the MgCl2 drops with initial 0.1 and 0.3 M concentrations, the main corrosion form was filiform-like corrosion. While, as the initial concentration was increased to 1.0 M, the main corrosion form was meta-stable pitting corrosion. In the case of 0.1 and 0.3 M initial concentrations, along the Volta potential baseline at −0.7 to −0.8 VSHE, there existed many potential transients in the form of rapid increase followed by immediate drop, which should be associated with the filiform-like corrosion growth stop and initiation. In the case of 1.0 M initial concentration, much more potential transients appeared in the form of sudden drop followed by a recovery, which was associated with the initiation and re-passivation of meta-stable pits.