Contribution of a Corrosion-Induced Multicomponent Passive Film to the Marine Corrosion Resistance of High-Entropy Carbide Coatings Fabricated Using Microwave-Synthesized Powders

High-entropy carbide (HEC) coatings are promising for marine corrosion protection because of their complex composition and stable ceramic structure. However, the roles of coating compactness and the corrosion-induced surface film are still not fully separated. In this work, single-phase rock-salt (Zr 0.2 Mo 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )C powders were synthesized by microwave-assisted carbothermal reduction at 1350 °C and deposited on Ti-6Al-4V substrates by atmospheric plasma spraying. Coatings were prepared at 30, 33, 36, and 39 kW. Among them, HEC-36 had the lowest apparent cross-sectional defect area fraction and the best electrochemical performance. Its apparent corrosion current density was 1.01 × 10 -8 A cm -2, compared with 1.09 × 10 -5 A cm -2 for bare Ti. During 0-6 h immersion, the impedance modulus of HEC-36 at 0.01 Hz was approximately 70-127 times higher than that of bare Ti. No definite breakdown was detected before 0.60 V vs. SCE during cyclic polarization. The coating also showed a stable current response during 7200 s of potentiostatic polarization. XPS results showed increased metal-oxygen contributions after corrosion, indicating the formation of a surface oxide film containing oxides of Zr, Mo, Ti, Nb, and Ta. Mott-Schottky analysis showed n-type behavior and similar apparent donor densities for the surface films on bare Ti and the HEC coating. The results show that the dense ceramic coating provides the main corrosion barrier, while the multicomponent oxide film contributes to interfacial stabilization.