Effect of Si Content on Phase Structure, Microstructure, and Corrosion Resistance of FeCrNiAl0.7Cu0.3Six High-Entropy Alloys in 3.5% NaCl Solution

This study examines the microstructure and corrosion resistance of FeCrNiAl0.7Cu0.3Six (x = 0, 0.1, 0.3, and 0.5) high-entropy alloys (HEAs) in a 3.5% NaCl solution. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electrochemical testing were employed to systematically analyze the alloys’ microstructures and corrosion behavior. The XRD results indicate that the addition of Si affects the phase structure of the alloy. At Si = 0, the alloy exhibits a single BCC phase. By increasing the Si content to 0.1 and 0.3, a BCC2 phase appears. At Si = 0.5, Si-containing intermetallic compounds form. SEM observations reveal that as the Si content increases, the alloy develops a distinct dendritic structure. Polarization tests in the 3.5% NaCl solution show that the corrosion current density first decreases and then increases with increasing Si content. At Si contents of 0.1, 0.3, and 0.5, the corrosion current densities are 4.275 × 10−6 A·cm−2, 4.841 × 10−7 A·cm−2, and 2.137 × 10−6 A·cm−2, respectively. FeCrNiAl0.7Cu0.3S0.3 HEA exhibits the lowest corrosion current density, indicating a lower corrosion rate. Electrochemical impedance spectroscopy (EIS) tests show that at Si = 0.3, the alloy has the largest capacitive arc radius. The charge-transfer resistance (RCT) for the alloys with the Si contents of 0.1, 0.3, and 0.5 are 2.532 × 105 Ω·cm2, 4.088 × 105 Ω·cm2, 4.484 × 105 Ω·cm2, and 2.083 × 105 Ω·cm2, respectively. FeCrNiAl0.7Cu0.3Si0.3 HEA has the highest RCT, which indicates a more stable passivation film and better resistance to chloride ion intrusion. The corrosion morphology observed after polarization testing shows that all alloys exhibit intergranular corrosion characteristics. The Si content alters the distribution of passivation film-forming elements, Cr and Ni. Compared to other alloys, the corrosion morphology of FeCrNiAl0.7Cu0.3Si0.3 HEA is more complete. Combining the polarization, EIS, and corrosion morphology results, it can be concluded that FeCrNiAl0.7Cu0.3Si0.3 HEA exhibits the best corrosion resistance in the 3.5% NaCl solution.