<p>Corrosion-resistant phases are key to designing stainless Mg alloys; yet, their identification remains limited. In this work, extruded Mg-2Y and Mg-2Gd alloys were investigated to elucidate the role of rare earth elements in corrosion resistance. The Mg₂₄Y₅ phase in Mg-2Y and Mg₅Gd in Mg-2Gd was identified as protective phases that act as sacrificial anode sites, thereby reducing microgalvanic corrosion of the Mg matrix. To understand the mechanism, the average distance, average number density, and average equivalent diameter of secondary phases were quantified, and the corrosion products were measured using XPS. It demonstrates that the presence of Mg(OH)₂, MgO, MgCO₃, and rare earth oxides forms the protective layer, inhibiting further corrosion reaction. Although the Pilling-Bedworth ratio (PBR) of the Mg matrix is only 0.81, which loosely covers the surface, the secondary Mg₂₄Y₅ particles can actually increase this PBR and sacrifice themselves as an anode, resulting in stainless properties for the alloy. Therefore, the observation provides fundamental understanding of key corrosion-resistant phases in Mg alloys, enlightening future stainless phases by design philosophy.</p> Graphical Abstract <p></p>