Abstract
The fully-coated TiB2-SiC coating was prepared by supersonic atmospheric plasma spraying (SAPS). The oxidation performance of TiB2-SiC coating at 400 and 800 °C was studied and the oxidation mechanism was investigated. The corrosion resistance of TiB2-SiC coating to aluminum melting salt at 900 °C was studied, and the anti-corrosion mechanism of molten salt was discussed. The results show that the TiB2-SiC coating prepared by SAPS has good anti-oxidation performance. The oxidation rate constant at 400 °C is 1.92×1
Science Press
In the process of industrial aluminum electrolysis, the surface of the cathode will be electrolyzed as aluminum and sodium metal at the same tim
| Al+3NaF=3Na+AlF3 | (1) |
The generated Na and Al diffuse into the cathode, promoting the wettability and penetration of the molten salt electrolyte to the cathode. Under the condition of aluminum electrolysis, although the precipitation of aluminum and sodium is restricted, the precipitated sodium and aluminum melt will penetrate into the cathode material through the pores, causing the aluminum electrolytic cathode material to expand and to break, shortening the service life of the cathode materia
At present, there are some researches on the oxidation properties of TiB2-SiC composite ceramic material
The TiB2 powder and SiC powder were mixed with an appropriate amount of deionized water, followed by milling for 3 h in the planetary ball mill, and then polyvinyl alcohol was added into the slurry to continuously mill them for 3 h; finally the composite powder slurry was prepared for spray granulation. Spray granulation was made by mobile minor spray dryer produced by Gea Niro of Germany. The optimum technological parameters for spray granulation of TiB2-SiC composite powder include 50wt% solid, 5wt% polyvinyl alcohol binder, and 10wt% SiC. The TiB2-SiC powder after spray granulation was vacuum calcined. The organic binder was removed by holding at 300 and 500 °C for 2 h. The TiB2-SiC powder was calcined at 1400 °C for 2 h to connect the large and small particles of TiB2-SiC to improve the strength of TiB2-SiC powder. With TiB2-SiC powder after spray granu-lation and vacuum calcination as the sprayed powder, the surface of the graphite substrate was fully coated by the 100 HE supersonic plasma spraying system. Based on the previous research of our group, the spraying parameters shown in
The oxidation performances of TiB2-SiC coating at 400 and 800 °C were studied. The fully coated TiB2-SiC coating was initially weighed prior to the oxidation experiment and recorded as m1. The fully coated TiB2-SiC coating was placed in a high temperature resistance furnace at corresponding temperature and oxidized for 3, 6, 9, 12 and 15 h, and then cooled to room temperature naturally. The oxidized TiB2-SiC coating was weighed again and recorded as m2. Through these, the mass change rate Δw of the TiB2-SiC coating can be calculated, and the calculation formula is as follow
| (2) |
The corrosion resistance of TiB2-SiC coating at 900 °C was studied. The fully coated TiB2-SiC coating was prepared by SAPS. The fully coated TiB2-SiC coating was placed in a conventional electrolyte for aluminum electrolysis, and it was etched at 900 °C for 8 h to investigate its resistance to molten salt corrosion and electrolyte penetration. The electrolyte composition was 90wt% Na3AlF6, 5wt% CaF2, and 5wt%A12O3.
The morphology of the coating was observed by nova-nano-sem-430 (with EDS analysis) field emission scanning electron microscope (SEM). The phase structure of powder and coating was analyzed by D8 advance X-ray diffractometer (Bruker, Germany). The Kα-ray of Cu target was used as diff-racted source, the scanning step was 0.02°/s, and the scanning range was 2θ from 10° to 90°. TG-DSC test was carried out on SETSYS 18/24 integrated thermal analysis system produced by Setaram Instrumentation Company in France.
According to the thermogravimetric curve, the oxidation reaction has not occurred at 400 °C, and it is basically completed at 800 °C. To study the oxidation kinetics before and after oxidation reaction, 400 and 800 °C are chosen as the oxidation temperatures. The morphologies of TiB2-SiC coating oxidized at 400 °C for 3, 9 and 15 h are shown in Fig.1. It can be seen that after the oxidation of TiB2-SiC coating at 400 °C for 3 h, the coating has almost no change, and the surface is smooth. There is a trace of oxidized particle distribution, and the coating structure is dense, as indicated in Fig.1a and 1d. With the increase of oxidation time, when the TiB2-SiC coating is oxidized at 400 °C for 9 h, a large amount of particulate matter is distributed on the surface of the coating, and the small particles are oxidized, as indicated by EDS analysis in Fig.1g. The cross section shows a slight amount of crack generation, as shown in Fig.1b and 1e. When the TiB2-SiC coating is oxidized at 400 °C for 15 h, more TiO2 oxide particles are still distributed on the surface of the coatings. The TiO2 oxide particles on the surface of the coatings are covered by glassy materials, which is confirmed as the mixture of SiO2 and B2O3 by EDS analysis (as shown in Fig.1h). The cross-section morphology shows that the coating density increases without cracking, as shown in Fig.1c and 1f.
The TiB2-SiC coating is slightly oxidized at 400 °C for 3 h, and the coating morphology does not change much. With the increase of oxidation time, TiO2 is formed in the TiB2-SiC coating after oxidation at 400 °C for 9 h, which leads to the formation of cracks in the coating. When the TiB2-SiC coating is oxidized at 400 °C for 15 h, a small amount of glassy oxidation products of B2O3 and SiO2 are formed, which seal the cracks, increase the density of the coating, and hinder the further oxidation of the coating.
XRD results of the TiB2-SiC coating oxidized at 400 °C for 0, 3, 9, and 15 h are shown in Fig.2. The TiB2-SiC coating after SAPS is mainly composed of TiB2 phase and SiC phase. SiC phase cannot be seen in the XRD pattern because its content is small, as shown in the curve of 0 h. When the TiB2-SiC coating is oxidized at 400 °C for 3 h, the phase of the coating does not change significantly, but the peak of the (101) crystal plane diffraction peak of TiB2 decreases significantly. After oxidation at 400 °C for 9 h, new phase of TiO2 is produced, but the crystallinity is small. When TiB2-SiC is oxidized at 400 °C for 15 h, the diffraction peaks of TiO2 (101) crystal plane and (211) crystal plane are enhanced, and the diffraction peak of the (111) crystal plane appears. It can be seen from the curve of 9 h that the oxidation products SiO2 and B2O3 are also produced, but they cannot be detected through XRD due to their small amount, as shown in the curve of 15 h.
In summary, with the increase of oxidation time of TiB2-SiC coating at 400 °C, the TiB2-SiC coating will be slightly oxi-dized to form oxidized protective film of TiO2, B2O3 and SiO2, which can effectively isolate oxygen diffusion from the inside of the coating and retard the oxidation process of the coating, giving the coating good oxidation resistance.
The microstructures of TiB2-SiC coating oxidized at 800 °C for 3, 9 and 15 h are shown in Fig.3. It can be seen that the surface of the TiB2-SiC coating is flat and dense after the oxidation at 800 °C for 3 h, a small amount of oxides appear, and the oxidized product of TiO2 is evenly distributed. Though a few cracks are generated in the coating, the coating is still relatively dense because of the protection of the dense glassy oxidation product SiO2, as shown in Fig.3a and Fig.3d. When the TiB2-SiC coating is oxidized at 800 °C for 9 h, there is a large amount of sheet-like products stack on the surface of the coating, which is analyzed by EDS as an oxidation product of B2O3 (as shown in Fig.3g). Part of the coating surface shows granular morphology, wrapped by glassy SiO2. The coating still has a few cracks and holes, as shown in Fig.3b and 3e. After the TiB2-SiC coating is oxidized at 800 °C for 15 h, the sheet-like products disappear, and the spinous oxidation product B2O3 is produced. The granular product TiO2 is evenly distributed on the surface of the coating, and the coating has a large number of pores and cracks, as shown in Fig.3c and 3f.
TiB2-SiC coating oxidizes to form TiO2, B2O3 and SiO2 at 800 °C. Thermal residual stress is generated during the oxida-tion process, and the glassy SiO2 seals the cracks and holes, which reduces further oxidation of the coating. As the oxidation continues, B2O3 and SiO2 are insufficient to fill the cracks and pores generated by oxidation, which increases the contact area between oxygen and TiB2-SiC coating and reduces the oxidation resistance of the coating.
Fig.4 is the XRD results of the TiB2-SiC coating oxidized at 800 °C for 0, 3, 9, and 15 h. When the TiB2-SiC coating is oxidized at 800 °C for 3 h, the diffraction peaks and peaks of the TiB2 (101) crystal plane and the (100) crystal plane signifi-cantly decrease, and the oxidized product TiO2 forms. After oxidation at 800 °C for 9 h, the diffraction peak of TiB2 con-tinuously decreases, and the diffraction peaks of (112), (200), (201) and (112) planes of TiB2 disappear. At the same time, many diffraction peaks of TiO2 appear according to the curve of 9 h. At 3 h, the formation of B2O3 phase can be deduced, but it cannot be detected at 9 h because B2O3 is amorphous. When the TiB2-SiC coating is oxidized at 800 °C for 15 h, the oxidation phase of the TiB2-SiC coating tends to be stable, and does not change much compared with that at 9 h.
In summary, it can be seen that the TiB2-SiC coating is firstly oxidized at 800 °C to form a granular TiO2 phase, and the oxygen diffusion rate of SiC is low, which alleviates the oxidation process of TiB2-SiC coating. As the oxidation proceeds, a flaky B2O3 phase and a glassy SiO2 phase are formed, which seal the cracks and pores generated by the oxidation, thereby effectively alleviating the further diffusion of oxygen into the interior of the coating. However, with the prolongation of oxidation time, the amount of B2O3 and SiO2 are not enough to fill the cracks and pores generated by oxi-dation; thus the oxidation of TiB2-SiC coating is accelerated.
The oxidation kinetics curve of TiB2-SiC coating at 400 and 800 °C for 15 h is shown in Fig.5. It can be seen from Fig.5 that the TiB2-SiC coating has a uniform oxidation mass gain trend at different temperatures, which is consistent with the pa-rabolic law. When the TiB2-SiC coating is oxidized at 400 °C, the mass gain rate is faster in the initial stage. As the oxidation time increases, the growth rate gradually becomes gentle and enters into the steady state. When the TiB2-SiC coating is oxidized at 800 °C, the oxidation rate is fast with uniform acceleration of mass gain in the initial stage. As the oxidation goes on, the surface of the coating is covered by the oxidized protective films TiO2, B2O3 and SiO2, and the oxidation rate is gradually reduced.
The oxidation mass gain curve of the TiB2-SiC coating conforms to the parabolic equation, i.e., it satisfies
To further illustrate the oxidation resistance mechanism of TiB2-SiC coating, the thermogravimetric weight of TiB2 and TiB2-SiC coating from room temperature to 1400 °C was ana-lyzed. Fig.6 shows the TG-DSC curves of TiB2 and TiB2-SiC. It can be seen that the oxidation trends of TiB2 and TiB2-SiC are basically the same.
The DSC curve in Fig.6a shows that two endothermic peaks appear at 501.2 and 717.6 °C for pure TiB2, which is caused by the reaction of TiB2 with O2 in the air to form TiO2 and B2O3. The TG curve indicates that TiB2 has two distinct mass gain steps in the oxidation process, corresponding to the two endothermic peaks in the DSC curve.
The DSC curve in Fig.6b shows that two endothermic peaks appear at 516.6 and 663.6 °C for TiB2-SiC coating, which is caused by the reaction of TiB2-SiC with O2 in air to form TiO2, B2O3 and SiO2. The TG curve indicates that TiB2-SiC has two distinct mass gain steps in the oxidation process, corresponding to the two endothermic peaks in the DSC curve.
According to the analysis of Fig.6, the oxidation of TiB2 and TiB2-SiC can be roughly divided into four stages, as shown in
The oxidation process of TiB2-SiC is as follows. (1) When the temperature is between room temperature and 568.8 °C, the mass gain rate is 8.22%. At this time, TiB2-SiC is slightly oxidized
Combined with the previous analysis, it can be concluded that the following chemical reactions occur in the TiB2-SiC coating from room temperature to 1400 °C:
| (3) |
| (4) |
| (5) |
| (6) |
When the temperature is lower than 500 °C, the TiB2-SiC coating undergoes a slight oxidation reaction to form a solid phase oxidized protective film TiO2 and B2O3 to avoid internal oxidation of the coating material. As the temperature increases, the oxidation time increases, TiB2-SiC coating produces dense glassy oxidation product B2O3 and SiO2, which can seal pores and cracks generated by oxidation, and effectively alleviate further oxidation of TiB2-SiC coating. When the temperature reaches 1082.4 °C, some liquid B2O3 begins to volatilize, resulting in pores in the coating, which causes a larger contact area of the TiB2-SiC coating with air and accelerates the oxidation of the coating.
During the oxidation process, the oxidized product TiO2 acts as a hard “skeleton” for the TiB2-SiC coating, supporting the entire coating syste
Since 900 °C is close to the practical application tempe-rature, it is chosen for the molten salt corrosion resistance research for TiB2-SiC coating. The microscopic morphologies of the TiB2-SiC coating melted in aluminum solution for 8 h are shown in Fig.7. After the TiB2-SiC coating prepared by SAPS is etched in the molten salt, the surface is covered with a dense glassy substance without obvious molten salt adhesion and fine cracks on the surface, as shown in Fig.7a. This is because the TiB2-SiC coating prepared by SAPS is very dense, and the molten salt is difficult to adhere to the surface of the TiB2-SiC coating. At the same time, the cooling process of oxidized product of B2O3 generates thermal residual stress and causes cracks.
It can be seen from Fig.7b that the cross section morpho-logy of the TiB2-SiC coating after molten salt corrosion can be divided into four layers: a molten salt layer, an outer etching layer, an inner etching layer and a substrate laye
In order to further understand the corrosion of TiB2-SiC coating by aluminum electrolyte, the element distribution of TiB2-SiC coating after molten salt corrosion was analyzed.
Fig.8 SEM image (a) and element distribution (b~h) of TiB2-SiC coating prepared by SAPS after molten saft corrosion
At present, there are two main viewpoints on the corrosion infiltration mechanism of aluminum electrolytic molten salt to cathode materials: one is the sodium vapor migration mechanism proposed by Del
Since the temperature of the molten salt experiment is higher than the boiling point of sodium, there is also a corrosion of the TiB2-SiC coating by sodium vapor during the diffusion of the molten salt. The TiB2-SiC coating has good wettability with aluminum liquid and can form a dense aluminum liquid protective layer on the surface of TiB2-SiC coating. When the molten salt electrolyte penetrates into the substrate and the cathode material expands and breaks, it must pass through the aluminum liquid layer on the surface of the TiB2-SiC coating. As a barrier layer of the cathode, the TiB2-SiC coating prevents the penetration of the molten salt electrolyte into the substrate and to suppress the generation of Na. The higher the TiB2 content in the TiB2-SiC coating, the better the wettability with the aluminum liquid and the stronger the cathodic protectio
1) The TiB2-SiC coating prepared by SAPS has good oxidation resistance, and there is no obvious ablation at 400 and 800 °C. The oxidation kinetic curves of TiB2-SiC coating at 400 and 800 °C conform parabolic law. The oxidation rate constants of TiB2-SiC coating at 400 and 800 °C are 1.92×1
2) When the temperature is low, the TiB2-SiC coating is slightly oxidized to form oxidized protective films TiO2 and B2O3 to avoid further oxidation of the coating. When the temperature is high, the formation of dense glassy protective film SiO2 can seal the pores and cracks generated by oxidation. When the temperature reaches about 1082.4 °C, B2O3 evaporates and pores remain inside the coating to increase the contact area of the coating with air, accelerating oxidation of the coating.
3) The TiB2-SiC coating prepared by SAPS has good resistance to molten salt corrosion. TiB2-SiC coating maintain a dense structure after molten salt corrosion, and no cracking and peeling of the coating occur. TiB2-SiC coating has good wettability with aluminum liquid, and forms a tight aluminum liquid barrier layer during electrolysis, which effectively prevents the penetration of molten salt electrolyte into the substrate, and further reduces the penetration and corrosion of electrolyte into the cathode material.
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