*2.3. TiB<sup>2</sup> Coating Characterization*

The TiB<sup>2</sup> coating prepared via plasma spraying was peeled off from the base carbon block and then ground. The phase composition of the prepared TiB<sup>2</sup> coating powder was analyzed using the X'pert 3 powder-type diffractometer produced by PANalytical (Malvern, UK). The parameters were as follows: Cu target, acceleration voltage of 40 kV, current of 40 mA, and sweep speed of 8◦ min−<sup>1</sup> . After the TiB<sup>2</sup> cathode coating subjected to different treatments was polished with sandpaper, the coating surface was observed under a QUANTA600 scanning electron microscope to detect the microscopic internal structure of the coating. Moreover, energy spectrum analysis was performed using the NORAN SYSTEM SIX energy spectrometer (SelectScience, Bath, UK).

#### *2.4. Method for Wettability Measurement*

In this study, the wetting angle between the plasma-sprayed TiB<sup>2</sup> inert cathode coating and molten aluminum was not directly measured; only the wettability between the TiB<sup>2</sup> inert cathode coating and molten aluminum was qualitatively characterized. First, an appropriate number of aluminum ingots were placed in the silicon carbide crucible, and the silicon carbide crucible was placed in a resistance furnace, model RF-15-10, produced by Changsha Gongtai Experimental Electric Furnace Co., Ltd (Changsha, China), and the heating system was set to 750 ◦C. When the aluminum ingot in the silicon carbide crucible became molten, the plasma-sprayed TiB<sup>2</sup> cathode coating sample and the aluminum electrolytic cathode carbon block preheated at the same temperature were simultaneously immersed in the molten aluminum; the immersion time was 5 s. Subsequently, the sample was taken out from the molten aluminum and placed on a steel plate at 45 ◦C with the horizontal ground. After the molten aluminum on the surface of the sample was cooled, aluminum was found to occur on the TiB<sup>2</sup> inert cathode coating and the carbon block. The spreading situations on the cathode and the wettability of the TiB<sup>2</sup> coating and carbon block were qualitatively evaluated.

#### *2.5. Corrosion Resistance Test Method*

Determination of the dissolution loss of TiB<sup>2</sup> coating in molten aluminum: The TiB<sup>2</sup> coating sample was soaked in high-temperature liquid aluminum at 960 ◦C for 48 h. After the molten aluminum sample was cooled, the change in the Ti content in the molten aluminum was analyzed, and then the dissolution loss of the TiB<sup>2</sup> coating in the molten aluminum was evaluated. The ARL841OX XRF was used to determine the titanium content in the liquid aluminum.

Static molten salt corrosion test of TiB<sup>2</sup> coating: The TiB<sup>2</sup> coating sample was placed on the bottom of a 0.5 L graphite crucible, and the prepared electrolyte (including Na3AlF6, NaF, Al2O3, and CaF<sup>2</sup> in mass percentages of 71.5%, 14.5%, 9.0%, and 5.0%, respectively) was added into a graphite crucible. The crucible was placed into a 960 ◦C atmosphere furnace and kept for 100 h. After the corrosion was over, the crucible was cooled to room temperature. Then, the TiB<sup>2</sup> coating sample at the bottom of the electrolytic cell was removed, and the residual electrolyte on the surface was cleaned. The sample was then cut along the central axis. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to characterize the TiB<sup>2</sup> coating after static corrosion.

Static molten salt corrosion test of TiB<sup>2</sup> coating: The TiB<sup>2</sup> coating sample was placed on the bottom of a 0.5 L graphite crucible, and the prepared electrolyte (including Na3AlF6, NaF, Al2O3, and CaF<sup>2</sup> in mass percentages of 71.5%, 14.5%, 9.0%, and 5.0%, respectively) was added into the graphite crucible. The crucible was then placed into a 980 ◦C pit furnace. The graphite anode embedded in cast iron was inserted into the electrolytic cell. The area of the graphite anode was 28 cm<sup>2</sup> . After the electrolyte was melted, the current density was kept at 0.7 A·cm−<sup>2</sup> , and electrolysis was conducted for 4 h. The electrolyte was added to the electrolytic cell every 15 min. The subsequent treatment after electrolysis was the same as that in the static test.

#### **3. Results and Discussion**
