The Oxidation Resistance of the B₄C-SiO₂-Albite Coating Influenced by the In Situ Formation and Self-Healing Ability of Borosilicate Glass at 1173 K

Abstract

The electrolytic aluminum industry is facing severe challenges, such as excessive carbon consumption, resulting in more cost and environmental pollution due to the oxidation of carbon anodes. The isothermal oxidation resistance of B₄C-SiO₂-Albite (BSA) composite coating influenced by the in situ formation behavior and self-healing ability of the borosilicate glass at 1173 K was investigated through XRD, TG-DSC, Raman, FTIR spectroscopy, and SEM/EDS in this paper. The results show that the composite coating with 20 wt% B₄C has a relatively low mass gain rate of −0.082% after 24 h at 1173 K. It depends on the in situ formation of the amorphous borosilicate phase layer that can effectively protect the carbon anode from oxidation, which depends on the content of B₄C. The amorphous borosilicate glass forms from the reaction between the SiO₂ and the B₂O_3, from the oxidation of B₄C during exposure. More B₄C promotes the formation and volatilization of B₂O₃, which improves the viscosity and stability of the borosilicate glass by changing the glass network coupled with Na⁺ and Al³⁺ from Albite. It is a feasible strategy for designing durable coatings with appropriate B₄C addition for high-temperature applications.

1. Introduction

Aluminum alloys serve as one of the key non-ferrous metals in modern industrial society, as a rich resource with excellent performance, extensive application, and recoverable-reusable materials. Currently, global electrolytic aluminum production widely employs the cryolite alumina molten salt electrolysis process system [1,2,3]. The anode carbon materials are continuously consumed and release substantial greenhouse gases during the production process [4]. The gas given off by electrolytic cell is dominated by carbon dioxide, accompanied by characteristic pollutants such as fluorinated gases and fluorinated dust [5,6]. This renders the electrolytic aluminum industry the sector with the highest carbon emission intensity among non-ferrous metal industries [7]. The surface coating protection method is considered a promising approach for protecting the carbon materials from oxidation [8]. Currently, antioxidant systems for carbon materials mainly include glass coating systems and ceramic coating systems [9,10,11,12,13,14]. The glass coating systems primarily include phosphate [15,16], borate [17,18,19,20], and silicate glasses [21].

The B₄C content plays important role in the antioxidation performance of borosilicate glass coatings prepared by the slurry brushing method [22]. The results indicated that the coating containing 25 wt% B₄C exhibited excellent self-healing property and antioxidant performance under high-temperature conditions. A SiO₂-B₄C-glass coating for Al₂O₃-C refractory materials achieved a weight gain of only −1.721% after duration for 100 h in an oxidative environment at 800 °C [23]. Modification of the HfB₂-SiC coating with B₄C successfully broadened the application range of this ceramic coating in intermediate- and low-temperature environments [24]. The ZrB₂-MoSi₂-SiC gradient coating displayed good oxidation resistance performance due to a triple mechanism of oxidative consumption glass phase formation stable phase generation [25]. It is indicated that `SiO₂ with ZrO₂ forms ZrSiO₄, reducing the volatilization rate of SiO₂ and enhancing the high-temperature stability of the oxide layer.

Suppressing the high-temperature oxidation of carbon anodes is of great significance for both enhancing the economic benefits of the aluminum electrolysis industry and achieving its environmental protection goals [26,27]. Carbon anodes function within a temperature field of 1203 K–823 K in electrolytic cells. The submerged bottom region where electrochemical reactions occur reaches 1203 K, while the unsubmerged middle section remains in a high-temperature environment of about 1173 K [28]. The isothermally high temperature oxidation resistance performance of the B₄C-SiO₂-Albite (BSA) composite coating at 1173 K were investigated in this paper. The effect of the B₄C addition, Na⁺ and Al³⁺ from albite on the in situ formation behavior and self-healing ability of the borosilicate glass, volatilization of B₂O₃, viscosity and stability of the borosilicate glass were discussed in detail. The research innovatively utilizes the high oxygen barrier property of amorphous materials to protect carbon anodes. The prepared coating forms a flowable amorphous melt in situ at 1173 K, shielding the carbon anode from oxygen attack, while the fluidity of the amorphous phase also provides the coating with a self-healing capability.

2. Materials and Methods

2.1. Sample Preparation

The preparation process of BSA coated carbon anodes is illustrated in Figure 1. Carboxymethyl cellulose (CMC, Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China) was first dissolved in deionized water under stirring for 10 min to form an aqueous solution. B₄C, SiO₂ (Shanghai Macklin Biochemical Co., Ltd., Shanghai, China) and Albite (Na₂O·Al₂O₃·6SiO₂, Henan Borun Foundry Materials Co., Ltd., Zhengzhou, China) powders were subsequently added, and the mixture was stirred for another 2 h to obtain a homogeneous BSA coating slurry. The slurry was uniformly coated onto the cylindrical carbon anode (50 × 50 mm) and dried to obtain the sample bearing the BSA coating. The final coating thickness was controlled within 240 ± 20 μm by measuring dimensional changes post-drying. Insufficiently thick coatings were recoated, while excessively thick ones were slightly polished using 2000 grit SiC paper. The mass percentage of the components in the BSA coating are presented in

Table 1. The chemical composition of the components in BSA coating (wt%).

Coatings	B4C	SiO2	Albite
15BSA	15	40	45
20BSA	20	40	40
25BSA	25	40	35
30BSA	30	40	30

. The B₄C content ranges from 15 wt% to 30 wt% for the samples. The BSA coatings coated samples were labeled as 15BSA, 20BSA, 25BSA, and 30BSA according to the B₄C content, respectively.

2.2. Evaluation of Oxidation Property

The antioxidant test refers to placing a specimen of carbon anode material with a composite coating prepared on its surface into a muffle furnace for high-temperature oxidation testing at a set temperature. The composite coating’s oxidation resistance can be quantitatively evaluated through the mass gain rate of the coating found on the carbon block substrate before and after oxidation. The initial mass of the prepared specimen was measured using an electronic analytical balance with a precision of ±0.1 mg. Subsequently, the specimens were placed in a muffle furnace for 100 h at 1173 K (KSL-1700 X, Hefei Kejing Material Technology Co, Ltd., Hefei, China). The specimens were heated to the test temperature at a heating rate of 5 °C/min, maintained at that temperature for a specified duration, and subsequently furnace-cooled to room temperature. Finally, the specimens were reweighed, and the oxidation mass gain rate was calculated using Equation (1) to assess the antioxidant property of the coating.

$$\Delta \mathrm{w}=({\mathrm{m}}_1-{\mathrm{m}}_0)/{\mathrm{m}}_0\times 100\%$$

(1)

Here Δw is the mass gain rate of the coated sample after oxidation, %; m₀ is the mass of the coated sample before oxidation, g; m₁ is the mass of the coated sample after oxidation for 100 h at 1173 K, g.

2.3. Characterization

An X-ray diffraction (XRD) technique was employed to characterize the phase composition of materials (XRD, D8 Advance, Bruker, Karlsruhe, Germany). The TG/DTG simultaneous thermal analyzer characterized the temperature-dependent weight variations associated with chemical reactions in the coating powder mixtures (TG-DSC, STA449 F5, Netzsch, Selb, Germany). The furnace was heated from room temperature to 1273 K at a rate of 10 K/min in an air atmosphere. Raman spectroscopy was utilized to obtain molecular structure information by analyzing characteristic peak positions (Raman, inVia Reflex, Renishaw, Wotton-under-Edge, UK). Molecular bonds in the amorphous phase formed after thermal exposure were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet iS 50, TMO, Waltham, MA, USA). Morphological evaluation of the composite coating was conducted using SEM on both surface and cross-section, with simultaneous elemental mapping via EDS (SEM, S-3400 N, Hitachi, Tokyo, Japan). This aimed to investigate the coating densification, thereby analyzing the microstructure and underlying mechanisms of the material phenomena.

3. Results and Discussion

3.1. Analysis of Oxidation Property

The isothermal oxidation performance of the B₄C-SiO₂-Albite (BSA) composite coating coated with carbon block at 1173 K is shown in

Table 2. The mass gain rate of samples after thermal exposure.

Exposure Time (h) Mass Gain Rate (%)	BSA15	BSA20	BSA25	BSA30	C
0	0	0	0	0	0
1	−0.010	0.001	0.015	0.046	−20.726
2	−0.032	−0.002	−0.001	0.026	−36.486
3	−0.059	−0.006	−0.022	0.017	−52.953
4	−0.087	−0.012	−0.052	−0.002	−66.580
8	−0.323	−0.029	−0.080	−0.069	−80.899
12	−0.550	−0.057	−0.114	−0.121	−90.283
20	−1.330	−0.070	−0.134	−0.321	−94.380
24	−2.101	−0.082	−0.152	−0.510	−96.340

and Figure 2. The pure carbon substrate exhibits severe mass gain, with a rapid reduction of −20.726 wt% within 1 h and a near complete gain of −96.340 wt% after 24 h exposure. Coatings of 15BSA, 20BSA, 25BSA, and 30BSA applied on carbon blocks effectively protect the substrate, reducing the mass gain rates to −0.2101%, −0.082%, −0.152%, and −0.510%, respectively, after 24 h of exposure. The BSA composite coating shows good anti-oxidation property during exposure when the B₄C addition is 20 wt%. The coating with relatively less B₄C such as 15BSA exhibits negative mass gain during high temperature, and it is about −0.010% after 1 h of exposure. They display positive gains for 20BSA, 25BSA, and 30BSA samples, which are 0.001%, 0.015%, and 0.046%, respectively. The oxidation of B₄C drives the experimentally observed mass gain via the retention of excess oxygen atoms during B₂O₃ formation, with the resultant molten B₂O₃ reacting with SiO₂ to form a dense, self-healing glassy phase [29]. The masses of the samples begin to decrease after 1 h, and the 30BSA sample shows the most significant drop from 0.046% at 1 h to −0.002% after 4 h due to the volatilization of B₂O₃ at high temperature.

The 15BSA underwent a pronounced mass loss over the same exposure period, albeit through a mechanistic pathway fundamentally different from that of 30BSA. Portions of the carbon blocks were directly exposed to the ambient atmosphere and underwent accelerated oxidation, driven by the contraction and curling of the 15BSA in situ-formed amorphous phase at elevated temperatures. Thermally induced structural deformation arose from the marked increase in high-temperature viscosity and surface tension of the amorphous matrix, induced by the elevated fraction of defect-induced tetrahedral [BO₄] units derived from B₂O₃ in the phase. The 20BSA and 25BSA coatings exhibited the most superior oxidation resistance in thermal exposure performance tests. This exceptional high-temperature performance stems from the moderate B₄C loading in these two formulations, which endowed the in situ-formed amorphous phase with well-tailored flowability and structural stability. The amorphous phase can both heal micro-pores and cracks induced by gas erosion via viscous flow during high-temperature service [19], and retain the full architectural integrity of the coating under sustained thermal flux with this optimized viscoelastic property.

Figure 2b–f shows the macroscopic morphologies of BSA-coated specimens with different B₄C contents and the uncoated carbon negative electrode specimen after exposure for 24 h at 1173 K. The 15BSA-coated specimen has a smooth and dense surface, but vitreous shrinkage creates large exposed areas and oxidizes the carbon substrate beneath coating gaps (Figure 2b). This results from the excessive surface tension of the glass at high temperatures caused by insufficient boron carbide content, inducing curling and shrinkage that damages coating integrity. Smooth and dense surfaces are exhibited by 20BSA, 25BSA, and 30BSA specimens with increasing B₄C content. No bubbles or cracks are observed, indicating a dense glassy phase formed to shield the carbon substrate against oxidation (Figure 2c,d). Numerous micro-pits appear on the surface of the 30BSA-coated specimen after gas pore healing. Excellent self-healing ability is demonstrated, enabling the rapid repair of gas pores during oxidation (Figure 2e). The high B₄C content endowed the 30BSA-coated specimen with self-healing properties but also caused damage. Excess B₂O₃ generated from B₄C continuously volatilized, creating numerous tiny pores in the glass phase [30]. Numerous micro-pits remained after gas pore healing, as high temperature impairs rapid and complete coating self-repair. These micro-pits are more likely to become channels for oxygen diffusion in subsequent oxidation. The uncoated carbon anode specimens exhibit a mass gain of −96.340% after exposure at 1173 K for 24 h, approaching complete oxidation (Figure 2f). It is indicated that the BSA composite coating shows good oxidation resistance to high temperature exposure, consistent with that of Figure 2a.

3.2. Analysis of Microstructure Evolution for the Coating

BSA-coated specimen before oxidation, distinct diffraction peaks of B₄C, SiO₂, and Albite were detected. Among these, B₄C diffraction peaks show high intensity, which increases with its content (Figure 3a). BSA coatings prepared by slurry brushing possess sufficient thickness to fully cover the carbon cathode substrate, and no carbon characteristic peaks are detected in any coatings. In the XRD pattern of the BSA specimen after exposure, weak diffraction peaks of SiO₂ and Albite can be observed. Compared with Figure 3a, the diffraction peaks of B₄C disappear completely in Figure 3b and are replaced by the characteristic broad peaks of the amorphous phase. The in situ formed B₂O₃ and SiO₂ combine to form a borosilicate amorphous phase, resulting in the absence of sharp characteristic diffraction peaks and only the presence of broad amorphous diffuse humps in the XRD patterns. The amorphization degree of oxidized BSA samples shows a clear positive correlation with boron carbide content, and the 30BSA sample displays more pronounced amorphous characteristic peaks in its oxidized XRD pattern. Such results demonstrate the full oxidation of B₄C to B₂O₃ on coating surfaces, with BSA specimens existing as amorphous glass phases at 1173 K.

The microscopic morphology images of the BSA coating specimens are shown in Figure 4a. No obvious bubbles or large cracks were observed on the coating surface, indicating that the coating prepared by the slurry brushing method is relatively dense. However, fine gaps exist between the coating particles. The organic binder CMC forms a temporary bonding network to immobilize particles and preserve coating morphology via van der Waals forces, hydrogen bonding and capillary effects during slurry drying. This stage involves physical bonding and cannot fully form a dense coating. The optimal thickness for superior coating performance is approximately 240 ± 20 µm. Cross-sectional morphologies of the BSA-coated specimens reveal no distinct gaps at the coating substrate interface, indicating excellent interfacial bonding stability (Figure 4b). The prepared coating exhibits a uniform thickness and is free of critical defects including through-thickness cracks and large-sized internal pores. Only uniformly distributed microporous structures are observed within the coating, with a measured porosity of 21.754%. The pore structure provides sufficient space for the flow-induced healing of the glass phase at elevated temperatures. The bonding interface between the coating and the carbon substrate is smooth and dense without obvious gaps or delamination, which effectively prevents oxygen from rapidly diffusing along the interface to erode the substrate during high-temperature oxidation, and provides a core structural basis for the long-term anti-oxidation protection of the coating.

BSA coatings form a dense glass layer after oxidation at 1173 K for 24 h as shown in Figure 5, and exhibit a significant reduction in porosity from the as-prepared 21.754% to approximately 10%. At this stage, the residual porosity in the coatings is dominated by the gaseous volatilization of borides. BSA coating forms a flowing amorphous phase that adheres tightly to the substrate under the effect of fluidity at elevated temperatures. Due to the beneficial densification effect caused by the flowing amorphous phase, the coating did not develop through-thickness cracks but contained a small number of pores. Among the samples, 15BSA and 25BSA exhibit more fine pores, while 20BSA show relatively fewer (Figure 5a–c). The number of internal pores within the as-prepared coatings decrease with increasing B₄C content. Surface defects of the coatings increase with rising B₄C content. The opposing trends are attributed to volatilization behavior induced by increased B₂O₃ content. The 15BSA coating form unfilled voids in its central region at elevated temperatures. The relatively low boron carbide loading in the 15BSA coating result in the insufficient formation of boron trioxide, thereby hindering its effective fluxing role for SiO₂. Consequently, the system exhibit high viscosity and low flowability at elevated temperatures, which promote the formation of unfilled voids in the central region of the coating. Increased B₄C content in the 20BSA coating induce a general downward trend in melt viscosity, and the internal porosity of the coatings drop continuously across the formulation gradient from 11.712% for 15BSA to 8.357% for 30BSA. The 25BSA and 30BSA coatings exhibit rapidly reduced viscosity and enhanced flowability, and achieve the rapid self-healing of inherent coating defects. Extensive boride volatilization in 30BSA leave residual pores within the glass phase, and the low viscosity of the amorphous melt enable the rapid migration of these pores to the coating surface, ultimately resulting in a loose surface structure with abundant surface voids in the 30BSA coating (Figure 5b–d). These depressions reduce the thickness of the dense layer, making it a weak link in the oxygen barrier, to a certain extent.

The coating is dense and uniform after exposure (Figure 6a). Si elemental distribution analysis reveals a tightly bonded interface between the coating and substrate, indicating effective adhesion and surface protection provided by the coating. It blocks oxygen permeation. It also inhibits chemical reactions between oxygen and the substrate (Figure 6b). Meanwhile, the Si element is uniformly distributed in the coating, as it serves as the matrix of the glass network. Na element is partially dispersed into the glass components (Figure 6c), as Na⁺ acts as a glass network modifier that breaks Si-O bonds and promotes glass formation. However, most Na elements aggregate with Al elements (Figure 6d). This occurs because Al³⁺ can substitute for Si⁴⁺ to enter tetrahedral coordination [AlO₄], but the [AlO₄] tetrahedron carries a net negative charge. As per the charge compensation mechanism, this negative charge must be neutralized by a neighboring positively charged network modifier cation Na⁺ to maintain electrical neutrality and stability. The charge compensation effect of sodium enables aluminum to enter the network in tetrahedral form, partially repairing the network breakage caused by the introduction of network modifiers with Na₂O. The modification process significantly enhances the chemical stability, mechanical strength, and viscosity of the glass. Simultaneously, the process inhibits the separation of B enriched and SiO₂ enriched phases to a certain extent, thereby maintaining coating stability.

3.3. Analysis of the In Situ Formed Amorphous Borosilicate

BSA coatings with different B₄C additions exhibited smooth and dense surfaces after oxidation at 1173 K for 1 h (Figure 7). Originating from the high-temperature oxidation of boron carbide to B₂O₃, the latter further reacts with silica to form borosilicate glass. By acting as a glass network modifier, the Na⁺ in Albite disrupts the Si-O bonds in silica, reduces melt viscosity and melting point, thus allowing the glassy phase to form at a relatively lower temperature meanwhile. The limited oxidation period led to the incomplete dissolution of SiO₂ and a failure in glass component formation, with consequent observation of SiO₂ particles on the glass surface. Notably, a relatively smaller amount of SiO₂ was detected in BSA30, indicating that more B₄C promoted the formation of sufficient borosilicate glass [22]. The coating rapidly and successfully formed a continuous amorphous glass layer within 1 h, effectively inhibiting oxygen diffusion and protecting the substrate from oxidation, consistent with the XRD results shown in Figure 3.

SiO₂ possesses characteristics such as a high melting point, excellent thermal stability, chemical inertness, and an amorphous nature, making it a core raw material for preparing silicate glasses. Albite (Na₂O·Al₂O₃·6SiO₂) is an important flux in glass preparation; adding Albite can reduce the melting point of raw materials and the viscosity of the melt, forming a low-melting-point liquid phase melt at lower temperatures. B₄C exerts a critical role in the high-temperature oxidation resistance of the coatings, and undergoes oxidation to form B₂O₃ at an exposure temperature of 823 K [31]. The generated B₂O₃ undergoes further reaction with silica within the system to yield an in situ-formed amorphous borosilicate glass phase. The low temperature-formed amorphous phase rapidly constructs a dense barrier layer to effectively inhibit the oxidation reaction between oxygen and the carbon substrate. The flowing amorphous phase at elevated temperatures diffuses to fill microcracks within the coating and completes the self-healing process of the protective layer upon the mechanical damage of the coating [22]. The Raman spectra in Figure 8a and peak assignment analysis in

Table 3. Raman peak assignments and structural characteristics of the BSA coating after exposure at 1173 K for 1 h.

Band Position (cm−1)	Assignment	Reference
463	Si-O-Si bond	[32]
686–752	[BO4] stretching	[32]
805	B–O–Si bond	[33]
1085	Q3 silicon–oxygen units	[33]
1250–1400	[BO3] stretching	[33]

indicate that the BSA coating forms an amorphous borosilicate glass after exposure at 1173 K for 1 h.

Specifically, the diffraction peak at 463 cm⁻¹ is attributed to the bending vibration of Si-O-Si bonds, indicating that amorphous silicates exist in a network form constituting the glass matrix skeleton. The broad peak in the range of 686–752 cm⁻¹ originates from the vibration of [BO₄] tetrahedra, reflecting the partial conversion of B₂O₃ into tetra-coordinated boron. The broad peak at 1350 cm⁻¹ stems from the asymmetric stretching vibration of [BO₃], confirming that part of B₂O₃ exists in the glass network as tri-coordinated boron. Subsequently, the diffraction peak at 805 cm⁻¹ is assigned to the mixed vibration of [SiO₄] and [BO₄] tetrahedra, directly proving that boron is covalently incorporated into the silicon oxygen network. Critically, the diffraction peak at 1085 cm⁻¹ corresponds to the stretching vibration of Q³ silicon oxygen units, which feature a structure of [SiO₄] tetrahedra containing Si-O-Si bridging bonds and Si-O non-bridging bond. This peak confirms Na⁺ in Albite as a network modifier, breaking Si-O-Si bonds to form non-bridging oxygen (Si-O⁻·Na⁺) and inducing network depolymerization. Together, the characteristic Raman peaks and XRD broad diffuse peaks confirm the system’s amorphous nature. Meanwhile, the infrared characteristic peaks of [BO₃] and [BO₄] are shown in

Table 4. FTIR peak assignments and structural characteristics of the BSA coating after exposure at 1173 K for 1 h.

Band Position (cm−1)	Assignment	Reference
800–1200	[BO4] stretching	[34]
1200–1600	[BO3] stretching	[34]

. FTIR analysis further reveals that B₂O₃ ultimately exists stably in the glass network in the form of [BO₄] tetrahedra network formers and [BO₃] triangles isolated structural units (Figure 8b). The peak intensity of [BO₄] units greatly exceeds that of [BO₃] units in FITR spectra, and B₂O₃ exists dominantly as [BO₄] in amorphous BSA after 1 h thermal exposure.

The internal B₄C is oxidized at high temperatures to generate B₂O₃, which reacts with SiO₂ to contribute to the formation of the amorphous borosilicate glass. The B₂O₃ exists as [BO₃] and [BO₄] units in the borosilicate glass after 1 h exposure of the BSA coating at 1173 K. The viscosity of the glass melt continuously decreases and its fluidity significantly improves with the increase in the [BO₃]/[BO₄] ratio [35]. The [BO₃] units adopt layered or cyclic structures, while [BO₄] units exhibit a three-dimensional network structure [36]. [BO₄] tetrahedra form a three-dimensional network structure to enhance the glass network crosslinking and thus increase surface tension. [BO₃] units adopt a loose two-dimensional layered structure to reduce the surface tension of the glass phase. However, [BO₄] possesses a three-dimensional crosslinked network, forming strong Si-O-B covalent bonds with silicate units to densify the structure and enhance rigidity, continuity, and deformation resistance.

Figure 9 presents B 1s spectra for as-prepared BSA coatings after 1 h of isothermal exposure at 1173 K. [BO₃] structural units show a binding energy of 191.90 eV, with [BO₄] units at 192.60 eV [37]. Both [BO₃] and [BO₄] structural units coexist in the BSA amorphous phase, as all B 1s peaks exhibit binding energies above 193 eV after isothermal exposure [38]. The fraction of [BO₃] increases from 2.91% to 9.91% as B₄C loading rises from 15 wt% to 30 wt%. The observed trend confirms that rising B₄C loading increases the proportion of [BO₃] structural units in the amorphous phase, which results in a gradual improvement in high-temperature flowability. Albite introduces Na⁺ and Al³⁺ into the borosilicate system synchronously after high-temperature melting, with the two ions synergistically regulating borate structure and comprehensive properties of the glass phase. On one hand, the low melting point of Albite significantly reduces the melting temperature of the system, realizes uniform melting of SiO₂ and B₂O₃ at 1173 K, and ensures the formation of a continuous amorphous glass phase in the coatings. On the other hand, the Na⁺ and Al³⁺ acts preferentially as a charge compensator to promote the conversion of neutral planar triangular [BO₃] units into negatively charged tetrahedral [BO₄] units [39,40], further enhancing the crosslinking and the surface tension of the glass network.

The 15BSA delivers the highest surface tension, with the highest Albite and SiO₂ content across the gradient. Sufficient Na+ maximizes [BO₄] tetrahedra fraction and glass network crosslinking degree. The coupling of high surface tension and high flowability leads to severe high-temperature shrinkage and deformation, failing to form a uniform flat protective layer. 20BSA exhibits the optimal deformation resistance and structural stability, with moderate Albite and SiO₂ content. Moderate Na⁺ content enables balanced [BO₃] to [BO₄] conversion, endowing the glass phase with suitable crosslinking degree and surface tension. The precise balance between viscosity and flowability prevents both excessive shrinkage and spreading failure at elevated temperatures. 25BSA with further reduced Albite and SiO₂ content shows excellent film-forming ability with no obvious high-temperature shrinkage. The 30BSA presents the lowest surface tension across the gradient, with the lowest Albite and SiO₂ content. Insufficient Na⁺ leads to a [BO₃] dominated structure, and abundant B₂O₃ gives extremely high flowability. Lower surface tension effectively inhibits high-temperature shrinkage and enables the formation of a continuous amorphous protective layer. Excessively low crosslinking degree causes a slight decline in high-temperature structural stability. Gaseous volatilization of B₂O₃ simultaneously exerts a more pronounced destructive effect on the amorphous structure. These results are consistent with the macroscopic morphology analysis of BSA coatings after 1173 K exposure in Figure 2b–e.

3.4. Analysis of the Antioxidant Mechanism

The composite coating exhibits excellent oxidation resistance, a property attributed to the rapid formation of an amorphous glass layer and its exceptional oxygen diffusion resistance. All three components in the BSA layer exhibit melting points higher than approximately 1100 °C. Nevertheless, the mass of the mixed powder increases when heated beyond 579.1 °C, owing to chemical reactions within the mixture. The reaction peak temperature is 747.8 °C, and the end temperature of significant mass increase is 874.1 °C (Figure 10). During the initial heating process, the volatilization of crystalline water in the raw material powder leads to a decrease in sample mass. According to Li’s study [41], B₄C undergoes a chemical reaction in air as the temperature increases, as shown in Equation (2). Additional oxygen incorporation during the oxidation of boron carbide accounts for the mass gain of the sample, starting at 580 °C.

B₄C + 4O₂ = 2B₂O₃ (l) + CO₂

(2)

B₂O₃ (l) = B₂O₃ (g)

(3)

Meanwhile, the Gibbs free energy of the oxidation of B₄C to B₂O₃ is negative at 423 °C, indicating that the reaction proceeds spontaneously at this temperature. Large-scale generation of B₂O₃ occurs between 600 °C and 850 °C, with the sample exhibiting a significant mass increase and a distinct exothermic peak. The spontaneity of the reaction diminishes with rising temperatures, as reflected by the ΔG shift from −2.5 (kJmol⁻¹) × 10⁻³ at 423 °C to −2.25 (kJmol⁻¹) × 10⁻³ at 1023 °C [41]. The TG curve shows an inflection point at 870 °C, attributed to the volatilization of B₂O₃ at high temperatures (Equation (3)). According to thermodynamic calculations, its volatilization rate is still much lower than the generation rate of B₂O₃ [42]. Therefore, oxidative weight gain still dominates despite volatilization beginning, as the sample’s mass rises but its rate slows due to volatilization loss. At this stage, the sample still retains a large amount of B₂O₃ and is in a mass-increasing state.

B₂O₃ remains solid below 438 °C but forms a liquid phase with silica via eutectic melting above 500 °C, as the phase diagram [32]. Elevated B₄C content induces a proportional increase in both the liquid phase fraction and the eutectic phase of B₂O₃ and SiO₂ within the coating structure. This microstructural evolution directly enhances the coating softening and promotes the formation of an extensive glass liquid phase. Within this binary system, inverse relationship between B₂O₃ content and liquidus temperature is pronounced, exhibiting a distinct negative dependence. A higher boron content leads to a lower liquid phase transition temperature. Such thermodynamic behavior gives rise to a self-regulating mechanism wherein elevated B₂O₃ generation intrinsically reduces the phase transition temperature, thereby facilitating enhanced glass formation under reduced thermal budgets [43]. By adjusting the B₄C addition amount to control the generation of B₂O₃, the fluidity of BSA glass is ultimately regulated to achieve optimal antioxidant performance.

Before exposure, the internal structure of the coating is relatively dense and uniform. B₄C particles in the coating react rapidly with O₂ to form B₂O₃ during exposure to a high-temperature oxidation atmosphere at 1173 K. The generated B₂O₃ undergoes eutectic melting with SiO₂ to form amorphous glass with a low melting point and superior fluidity, effectively filling defects produced in the coating during exposure (Figure 11). Na⁺ introduced by Albite effectively lowers the temperature of glass-forming reactions, while Al³⁺ stabilizes the glass network structure and improves its stability. A BSA glass layer can form rapidly and serve as an outstanding oxygen diffusion barrier to effectively suppress O₂ penetration under the synergistic effect of Na and B. Long-term coating degradation primarily results from continuous B₂O₃ volatilization, which induces porosity development and thickness reduction within the protective layer. These structural changes create oxygen diffusion pathways that progressively compromise barrier effectiveness. When the formation of irreversible macrocracks and pores exceeds the self-repair capacity of the glass phase, the coating ultimately fails to maintain its protective function. Therefore, the glass coating exhibits good self-healing properties and antioxidant performance, representing a promising strategy to protect the substrate carbon blocks from oxidation under high-temperature exposure.

4. Conclusions

The oxidation behavior and the oxidation mass gain rate of the B₄C-SiO₂-Albite (BSA) coating were deeply discussed in this work. The result shows that it is a promising coating candidate, coating material with the appropriate B₄C addition for the electrolytic aluminum industry due to its strong oxidation resistance.

(1) The oxidation resistance of B₄C-SiO₂-Albite composite coating greatly depends on the B4C content, and the coating with 20 wt% B₄C exhibits the best oxidation property among the samples for the minimum mass gain rate of only—0.082% after 24 h exposure at 1173 K.

(2) Increased B₄C promotes the formation of liquid B₂O₃ and amorphous borosilicate phase, while Na⁺ and Al³⁺ from Albite markedly improve the viscosity and stability of the borosilicate glass. Synergistic control of both components dictates the integrity of the BSA coating during service.

(3) The continuity and completeness of the in situ formed amorphous borosilicate covered layer shows the long-term protection from oxidation for the carbon substrate due to the self-healing ability, influenced by the volatilization of B₂O₃.