<p>Enhancing corrosion resistance in cast alloys using straightforward and cost-effective micro-alloying techniques has emerged as a key area of investigation in materials science. The challenge lies in applying this technique to further enhance the already excellent properties of CoCrNi medium-entropy alloys (MEAs) for casting applications. A micro-alloying approach was proposed to improve the corrosion resistance of as-cast CoCrNi MEAs by incorporating cerium (Ce). The corrosion resistance of CoCrNi MEAs firstly increases and then decreases as the Ce content increases in a 3.5wt.% NaCl solution. At a Ce content of 0.02at.%, the passivation current density reaches its minimum value (26.383 µA·cm⁻²), while the breakdown potential reaches its maximum (0.471 V_SCE), imparting exceptional corrosion resistance. The results indicate that the enhanced corrosion resistance is primary due to Ce micro-alloying, which affects inclusions by forming a non-conductive precipitated phase and modifying the passivation film. Ce micro-alloying presents a promising strategy for enhancing the corrosion resistance of as-cast CoCrNi MEAs.</p>