Acid corrosion of high-entropy ceramics: Insights from an entropy-stabilized oxide model system

High-entropy engineering has emerged as a promising strategy for designing compositionally complex materials with enhanced structural stability. However, it remains unclear whether high-entropy structures intrinsically improve corrosion resistance. Herein, an entropy-stabilized rock-salt (MgCoNiCuZn)O (HEO) high-entropy oxide was selected as a model system and compared with a mechanically mixed oxide (MO) consisting of MgO, Co3O4, NiO, CuO, and ZnO mixed at equimolar metal-cation ratios. Despite the same nominal cation composition, MO consists of independent oxide phases, whereas HEO forms a single-phase rock-salt solid solution. Hydrochloric-acid immersion experiments reveal distinctly different corrosion pathways. MO undergoes pronounced elemental dissolution selectivity, with Mg-, Zn-, and Cu-containing phases preferentially removed while Ni- and Co-containing phases are relatively retained. In contrast, HEO exhibits substantially reduced elemental dissolution selectivity, consistent with coupled multielement dissolution within the common lattice. Moreover, the corrosion response of HEO strongly depends on acid concentration. HEO exhibits lower net mass loss than MO in 1 M HCl, whereas its degradation becomes substantially more severe in 6 M HCl and the net mass loss exceeds that of MO. Residual HEO retains the multielement rock-salt structure while developing interfacial stratification and localized Cu/Cl-containing corrosion-derived species. These results show that high-entropy solid-solution formation modifies corrosion pathways rather than universally enhancing corrosion resistance, highlighting the coupled roles of phase constitution, constituent chemistry, and environmental aggressiveness in designing corrosion-resistant ceramics.