*3.5. Analysis of Static Corrosion Results of TiB<sup>2</sup> Coating*

Resistance to Na penetration is an important indicator of wettable cathode materials. Figure 7a is a low-power-SEM image of the surface of the sample after the static corrosion of the carbon block, and Figure 7b is a low-power-SEM image of the TiB<sup>2</sup> coating surface after static corrosion. The figure shows that after cryolite corrosion under the same conditions, the surface of the carbon block cathode was loose and porous, the area of the carbon block was significantly reduced, and the electrolyte was more likely to penetrate into the carbon block. Figure 8a is the surface micro-topography image of the TiB<sup>2</sup> coating after corrosion, and Figure 8b is the cross-sectional micro-topography image of the TiB<sup>2</sup> coating after corrosion. Figure 8a shows that after the coating surface was corroded, the TiB<sup>2</sup> particles still maintained the original crystal structure state and were not corroded by cryolite melt.

Figure 8b shows that the TiB<sup>2</sup> coating cross section also maintained the original complete structure; the coating structure was still dense. After 100 h of static corrosion, the TiB<sup>2</sup> coating internal structure was dense, and the corrosion resistance was good. Figure 9 shows the EDS spectrum of the TiB<sup>2</sup> coating after corrosion. The figure shows that the main component element in the coating was Ti, and the metal Na<sup>+</sup> ions did not penetrate into the coating. *Minerals* **2022**, *12*, x FOR PEER REVIEW 7 of 11 **Figure 6.** XRD analysis results of TiB2 coatings before and after erosion in liquid aluminum for

*Minerals* **2022**, *12*, x FOR PEER REVIEW 7 of 11

**Figure 6.** XRD analysis results of TiB2 coatings before and after erosion in liquid aluminum for around 120 min. (**a**) Before corrosion. (**b**) After corrosion. **Figure 6.** XRD analysis results of TiB<sup>2</sup> coatings before and after erosion in liquid aluminum for around 120 min. (**a**) Before corrosion. (**b**) After corrosion. 9 shows the EDS spectrum of the TiB2 coating after corrosion. The figure shows that the main component element in the coating was Ti, and the metal Na+ ions did not penetrate into the coating.

TiB2 coating internal structure was dense, and the corrosion resistance was good. Figure

9 shows the EDS spectrum of the TiB2 coating after corrosion. The figure shows that the main component element in the coating was Ti, and the metal Na+ ions did not penetrate into the coating. **Figure 7.** Charcoal surface and the coating surface after etching contrast. (**a**) Low magnification SEM image of sample surface after carbon block static corrosion, and (**b**) low magnification SEM image of TiB2 coating surface after static corrosion. **Figure 7.** Charcoal surface and the coating surface after etching contrast. (**a**) Low magnification SEM image of sample surface after carbon block static corrosion, and (**b**) low magnification SEM image of TiB<sup>2</sup> coating surface after static corrosion. *Minerals* **2022**, *12*, x FOR PEER REVIEW 8 of 11

**Figure 9.** Energy‐dispersive X‐ray spectroscopy (EDS) analysis after corrosion of TiB2 coating sur‐

**Figure 10.** The section after the corrosion of the TiB2 coating.(**a**) Low‐magnification figure and (**b**)

and the EDS energy spectrum of points taken near the TiB2 coating surface.

Figure 10 is a cross‐sectional view of the TiB2 coating after dynamic corrosion, and Figure 11 is the cross section of the TiB2 coating after the dynamic corrosion experiment

**Figure 8.** (**a**) Surface and (**b**) cross section after TiB2 coating corrosion. **Figure 8.** (**a**) Surface and (**b**) cross section after TiB<sup>2</sup> coating corrosion.

of TiB2 coating surface after static corrosion.

face.

Micro‐topography

*3.6. Analysis of Dynamic Corrosion Results of TiB2 Coating*

face.

face.

bon elements.

**4. Conclusions** 

nent in the coating.

to Na penetration.

**Figure 8.** (**a**) Surface and (**b**) cross section after TiB2 coating corrosion.

**Figure 8.** (**a**) Surface and (**b**) cross section after TiB2 coating corrosion.

*Minerals* **2022**, *12*, x FOR PEER REVIEW 8 of 11

**Figure 7.** Charcoal surface and the coating surface after etching contrast. (**a**) Low magnification SEM image of sample surface after carbon block static corrosion, and (**b**) low magnification SEM image

of TiB2 coating surface after static corrosion.

**Figure 9.** Energy-dispersive X-ray spectroscopy (EDS) analysis after corrosion of TiB2 coating sur-**Figure 9.** Energy-dispersive X-ray spectroscopy (EDS) analysis after corrosion of TiB<sup>2</sup> coating surface.

#### *3.6. Analysis of Dynamic Corrosion Results of TiB<sup>2</sup> Coating 3.6. Analysis of Dynamic Corrosion Results of TiB2 Coating*

*3.6. Analysis of Dynamic Corrosion Results of TiB2 Coating*  Figure 10 is a cross-sectional view of the TiB2 coating after dynamic corrosion, and Figure 11 is the cross section of the TiB2 coating after the dynamic corrosion experiment Figure 10 is a cross-sectional view of the TiB<sup>2</sup> coating after dynamic corrosion, and Figure 11 is the cross section of the TiB<sup>2</sup> coating after the dynamic corrosion experiment and the EDS energy spectrum of points taken near the TiB<sup>2</sup> coating surface. Figure 10 is a cross‐sectional view of the TiB2 coating after dynamic corrosion, and Figure 11 is the cross section of the TiB2 coating after the dynamic corrosion experiment and the EDS energy spectrum of points taken near the TiB2 coating surface.

 **Figure 10.** The section after the corrosion of the TiB2 coating.(**a**) Low-magnification figure and (**b**) **Figure 10.** The section after the corrosion of the TiB2 coating.(**a**) Low‐magnification figure and (**b**) Micro‐topography **Figure 10.** The section after the corrosion of the TiB<sup>2</sup> coating.(**a**) Low-magnification figure and (**b**) Micro-topography. *Minerals* **2021**, *11*, x FOR PEER REVIEW 9 of 11

**Figure 11.** Cross section and EDS spectrum of coating after corrosion. **Figure 11.** Cross section and EDS spectrum of coating after corrosion.

Figure 10a shows that the TiB2 coating after electrolysis maintained a compact struc-

this area. The main component of the coating was Ti; this indicates that a small amount of cryolite penetrated the coating surface after the electrolysis experiment; the penetration depth was not deep, indicating that the TiB2 coating obtained via plasma spraying has improved resistance to cryolite corrosion. The TiB2 wettable cathode coating prepared through plasma spraying technology does not contain carbon elements, which avoids the defect that the TiB2/C carbon glue coating is easily corroded by the electrolyte due to car-

In this study, under optimal parameters and the optimal particle size, a TiB2 coating was prepared on a carbon cathode surface using plasma-spraying equipment, and the phase and microstructure of the coating were analyzed via SEM, XRD, and other characterization methods. After the analysis, the wettability and corrosion resistance of the coat-

The TiB2 coating prepared via APS had a smooth surface free of peeling and cracking. The TiB2 coating internal structure was dense and uniform. The large particles were in a semi-melted state, forming a disc-shaped structure embedded in the coating, and the small particles were completely melted; they connected the large particles to fill the pores between the large particles. In the plasma-spraying process, the air drawn into the plasma jet contacted the molten TiB2 particles, causing the TiB2 particles to be oxidized and form oxidation products such as TiO2 and B2O3. TiB2 is still the most important phase compo-

The wettability between the TiB2 wettable cathode coating and molten aluminum was significantly better than that between graphite cathode carbon block and molten aluminum. Through static corrosion experiments, the abilities of the TiB2 coating and graphite cathode carbon block to resist Na penetration and to prevent molten cryolite corrosion were compared. The TiB2 coating resistance to Na penetration and corrosion resistance to molten cryolite were better than those of the graphite cathode carbon block. Moreover, after the TiB2 coating was dynamically corroded for 4 h, only a small amount of F, Na, and Al penetrated into the TiB2 coating inner surface. Given the results, the TiB2 coating prepared via plasma spraying has good liquid aluminum wetting ability and good resistance

ing were measured, and the following conclusions were drawn:

Figure 10a shows that the TiB<sup>2</sup> coating after electrolysis maintained a compact structure. Figure 10b shows that the bonding surfaces of the TiB<sup>2</sup> coating and the carbon block substrate were in good contact, without peeling or porosity of the coating. The TiB<sup>2</sup> coating had good bonding strength with the carbon block substrate. From the analysis of the EDS results, a small amount of cryolite components such as F, Na, Al, and Ca occurred in this area. The main component of the coating was Ti; this indicates that a small amount of cryolite penetrated the coating surface after the electrolysis experiment; the penetration depth was not deep, indicating that the TiB<sup>2</sup> coating obtained via plasma spraying has improved resistance to cryolite corrosion. The TiB<sup>2</sup> wettable cathode coating prepared through plasma spraying technology does not contain carbon elements, which avoids the defect that the TiB2/C carbon glue coating is easily corroded by the electrolyte due to carbon elements.

#### **4. Conclusions**

In this study, under optimal parameters and the optimal particle size, a TiB<sup>2</sup> coating was prepared on a carbon cathode surface using plasma-spraying equipment, and the phase and microstructure of the coating were analyzed via SEM, XRD, and other characterization methods. After the analysis, the wettability and corrosion resistance of the coating were measured, and the following conclusions were drawn:

The TiB<sup>2</sup> coating prepared via APS had a smooth surface free of peeling and cracking. The TiB<sup>2</sup> coating internal structure was dense and uniform. The large particles were in a semi-melted state, forming a disc-shaped structure embedded in the coating, and the small particles were completely melted; they connected the large particles to fill the pores between the large particles. In the plasma-spraying process, the air drawn into the plasma jet contacted the molten TiB<sup>2</sup> particles, causing the TiB<sup>2</sup> particles to be oxidized and form oxidation products such as TiO<sup>2</sup> and B2O3. TiB<sup>2</sup> is still the most important phase component in the coating.

The wettability between the TiB<sup>2</sup> wettable cathode coating and molten aluminum was significantly better than that between graphite cathode carbon block and molten aluminum. Through static corrosion experiments, the abilities of the TiB<sup>2</sup> coating and graphite cathode carbon block to resist Na penetration and to prevent molten cryolite corrosion were compared. The TiB<sup>2</sup> coating resistance to Na penetration and corrosion resistance to molten cryolite were better than those of the graphite cathode carbon block. Moreover, after the TiB<sup>2</sup> coating was dynamically corroded for 4 h, only a small amount of F, Na, and Al penetrated into the TiB<sup>2</sup> coating inner surface. Given the results, the TiB<sup>2</sup> coating prepared via plasma spraying has good liquid aluminum wetting ability and good resistance to Na penetration.

#### **5. Future Prospective**

The oxidation behavior of TiB2 in the plasma spraying process is very complex, the process is very much influenced by the transfer (momentum transfer, heat transfer and mass transfer), especially the transfer phenomenon of gas phase B2O3 under high temperature conditions determines the TiB2 oxidation behavior, and the plasma spraying TiB2 coating is carried out under high temperature conditions, and metallurgical bonding is observed through experiments, however, the bonding mechanism and the effect on TiB2 coating properties are still unclear; finally the introduction of multi-component TiB2 base can further improve the properties of the coating such as corrosion resistance and wettability. Therefore, on the basis of this thesis study, the following studies are proposed to be carried out in the future:

(1) Study the momentum transfer, heat transfer and mass transfer behaviors of TiB2 during the movement to the surface of carbon block during the spraying process, analyze the time-varying laws of flow field and temperature field, and reveal the oxidation behavior of TiB2 and the symmetry mechanism of multi-physical field; and study in depth the mechanism of pore formation of TiB2 coating under high temperature conditions and study the transfer phenomenon of gas phase B2O3.


**Author Contributions:** For research articles with several authors, Conceptualization, B.Y. and Y.H.; methodology, R.P.; validation, N.Y. and D.Z.; formal analysis, B.Y. and R.P.; investigation, Y.H.; resources, G.X.; writing—original draft preparation, B.Y.; writing—review and editing, B.Y.; supervision, Y.H.; funding acquisition, Y.H. and G.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Data sharing not applicable. No new data were created or analyzed in this study. Data sharing is not applicable to this article.

**Acknowledgments:** This work was financially supported by the National Natural Science Foundation of China (22168019, 52074141). The authors are grateful to NSFC, China for their support.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

