AZ91 alloy with 1.0 wt.% (La, Ce) mischmetal (MM) addition was solution treated at 593 K for 4 h, 653 K for 2 h and 693 K for 8 h (T4), which was subsequently aged at 493 K for 5 h, 8 h and 11 h, respectively. Solid solution and aging treatment mainly led to the change of number of β-Mg17Al12 phases and the size of mischmetal (MM) intermetallics. The relative potential difference is an index for local corrosion of alloys because the corrosion is driven by micro-galvanic coupling of the microstructure. In the present work, the relative Volta potential difference between the dominated intermetallic phase and Mg-matrix phase in the AZ91 alloy with (La, Ce) MM addition after heat treatment was measured by scanning Kelvin probe force microscopy (SKPFM). Then correlative changes of relative Volta potentials of intermetallic phases with solid-solution-treatment and subsequently aging treatment (T6) were inferred. The relative Volta potential difference of mischmetal intermetallic phases increased with the solid solution treatment. With the increase of aging time, the relative Volta potential difference of mischmetal intermetallic phases decreased. The relative Volta potential difference of intermetallic phases containing rare earth was ranked as: T6-11 h <T6- 8 h<T6-5 h< as-received < T4.