Preparation and molten salts corrosion behavior of high-entropy (Ca,Sr,Ba)TMO3 (TM= Zr, Hf and Ta): Promising protecting materials of reference electrode for metal smelting through fused salts electrolysis

Abstract The protecting materials of reference electrode with good corrosion resistance to molten salts play vital roles in realizing smelting of refractory and rare earth metals through fused salts electrolysis. BaZrO3 has been considered as a candidate protecting material of reference electrode for its good chemical stability and high-temperature corrosion resistance. However, the unsatisfactory corrosion resistance to molten salts hinders its practical application. In order to enhance the corrosion resistance to molten salts, three perovskite structured high-entropy oxides HE (Ca,Sr,Ba)TMO3 including (Ca,Sr,Ba)(Zr,Hf)O3, (Ca,Sr,Ba)(Zr,Ta)O3 and (Ca,Sr,Ba)(Zr,Hf,Ta)O3, were designed and prepared in this work. The as-prepared HE (Ca,Sr,Ba)(Zr,Hf)O3 and (Ca,Sr,Ba)(Zr,Hf,Ta)O3 ceramics are single-phase, while the as-prepared HE (Ca,Sr,Ba)(Zr,Ta)O3 ceramic contains 11.6 vol.% of (Ca,Sr,Ba)2Ta2O7. These HE (Ca,Sr,Ba)TMO3 ceramics possess better corrosion resistance to molten NaCl-KCl salts than BaZrO3, among which (Ca,Sr,Ba)(Zr,Hf,Ta)O3 shows the best corrosion resistance to molten NaCl-KCl salts. The mass loss per unite area of HE (Ca,Sr,Ba)(Zr,Hf,Ta)O3 after corrosion by molten NaCl-KCl salts at 800°C for 8 h is mere 2.6☓10−6 g/mm2 which is ca. 1/60 of that of BaZrO3 (∼1.7☓10−4 g/mm2), and its surface after corrosion is plain and smooth without any new phase detectable after corrosion. It also exhibits better corrosion resistance to molten NaF-KF salts than BaZrO3. The mass loss per unite area after corrosion by molten NaF-KF salts at 800°C for 8h is only 2.9☓10−5 g/mm2 which is smaller than that of BaZrO3 (3.6☓10−5 g/mm2), the corrosion caused by molten NaF-KF salts only occurs on the surface and the thickness of the corrosion layer is mere ∼8 μm, being less than 1/10 of that of BaZrO3 (∼100 μm). These results indicate that the three HE (Ca,Sr,Ba)TMO3, especially (Ca,Sr,Ba)(Zr,Hf,Ta)O3, are promising as protecting materials of reference electrode for the smelting of refractory and rare earth metals through fused salts electrolysis.