This work assesses the role of micro-level yttrium (0.2 wt.%) in governing the corrosion characteristics of a low-alloyed Mg–0.5Zn–0.2Ca (ZX) system. The Y addition transforms the cathodic CaMgSi phase into a newly identified, less noble Y₂MgSi₂ phase, reducing the VPD from ~170 mV to ~108 mV and diminishing the micro-galvanic corrosion and pitting initiation. Y also facilitates the formation of a more compact and stable oxide film, thus serving as an effective barrier against the intrusion of corrosive ions. Consequently, the Mg–0.5Zn–0.2Ca-0.2Y (ZX-Y) alloy attains a corrosion rate of 0.29 mm y⁻¹, corresponding to about a 47% decrease relative to the ZX alloy, despite its extremely low total alloying level (0.83 wt.%). Moreover, the alloy exhibits a transition toward more homogeneous surface dissolution, highlighting the substantial improvement in overall corrosion resistance. These findings demonstrate that Y microalloying provides an efficient, cost-effective strategy for mitigating the detrimental effects of Si impurities and enhancing the surface film protectiveness in dilute Mg alloys. This work provides fresh perspectives for tailoring lightweight Mg alloys toward enhanced corrosion resistance.