Corrosion behavior and mechanisms of selective laser melted AlCoCrFeNi_2.1 eutectic high-entropy alloy in supercritical CO₂

The effect of selective laser melting (SLM) processing on the corrosion resistance of AlCoCrFeNi_2.1 eutectic high-entropy alloy in supercritical CO₂ (500 °C, 25 MPa) was systematically investigated and compared with that of the as-cast alloy. Long-term exposure tests up to 1600 h were conducted, and corrosion weight gain, corrosion kinetics, and corrosion scale morphology and structure were characterized by gravimetric analysis, SEM/EDS, XRD, and TEM. The as-cast alloy exhibited pronounced phase-selective oxidation between FCC and BCC regions, resulting in fluctuating weight gain accompanied by local scale instability and spallation. The Cr-depleted BCC phase failed to form a continuous Cr₂O₃ protective layer, leading to discontinuous surface scales, enhanced outward diffusion of Fe, and the formation of a thick internal Fe-Al amorphous layer. In contrast, the SLM-built alloy showed lower and more stable weight gain, associated with a uniform and continuous corrosion scale consisting of an Al₂O₃-rich inner layer and a Cr₂O₃-rich outer layer. The refined lamellar microstructure and homogeneous elemental distribution introduced by SLM promoted rapid scale continuity and effectively suppressed phase-selective oxidation. Consequently, distinct amorphous layer evolution mechanisms were observed: the amorphous layer propagated inward as an internal oxidation front in the as-cast alloy, whereas it remained thin and confined near the surface in the SLM-built alloy.