Corrosion Mechanisms and Protective Technologies for Furnace Linings Under Chlorine/Fluorine-Containing Atmospheres

In modern high-temperature reaction processes—such as fine chemical processing, hazardous waste incineration, precious metal refining, and semiconductor raw material smelting (e.g., silicon tetrachloride and nitrogen trifluoride production)—the generation of high concentrations of halogen gases (e.g., $\text{Cl}_2, \text{HCl}, \text{F}_2$, $\text{HF}$) within the system is often unavoidable. At elevated temperatures ranging from $800^\circ\text{C}$ to $1500^\circ\text{C}$, these highly reactive and corrosive gases exhibit aggressive chemical attack against the furnace lining surfaces. This severely compromises traditional alumina ($\text{Al}_2\text{O}_3$) or silicon carbide ($\text{SiC})$ specialty refractories, leading to structural loosening, spalling, and even catastrophic burn-through.

Based on thermodynamic degradation mechanisms, this article delivers an in-depth analysis of the corrosive nature of halogen gases on mainstream refractories and introduces cutting-edge protective technologies utilizing Rare Earth Oxide (REO) coatings.

I. Corrosion Mechanisms of Halogen Gases on Mainstream Refractories

Under highly reactive atmospheres, the degradation of refractory materials is rarely a consequence of simple physical erosion. Instead, it is driven by gas-solid interfacial chemical reactions that induce inorganic salt volatilization and volumetric destruction.

1. “Gas-Phase Volatilization” Mechanism of Alumina-Based Refractories ($\text{Al}_2\text{O}_3$ System)

High-purity corundum ($\alpha\text{-Al}_2\text{O}_3$) exhibits exceptional stability under conventional oxidizing or reducing atmospheres. However, its chemical inertness is completely shattered when exposed to high-temperature atmospheres containing chlorine or fluorine.

• Chlorination Attack: In reducing environments or in the presence of reducing agents (such as carbon or carbon monoxide), $\text{Al}_2\text{O}_3$ readily undergoes high-temperature chlorination:$\text{Al}_2\text{O}_3\text{(s)} + 3\text{Cl}_2\text{(g)} + 3\text{CO(g)} \rightarrow 2\text{AlCl}_3\text{(g)} + 3\text{CO}_2\text{(g)}$
• The resulting aluminum trichloride ($\text{AlCl}_3$) has a sublimation point of only $178^\circ\text{C}$ and volatilizes directly as a gas phase under the furnace’s high operating temperatures. This process relentlessly consumes grain boundaries, leading to a progressive loss of the corundum skeletal framework until the entire structure collapses.
• Fluorination Attack: Fluorine possesses a higher electronegativity than chlorine, making its destructive impact on $\text{Al}_2\text{O}_3$ even more devastating:
• $\text{Al}_2\text{O}_3\text{(s)} + 6\text{HF(g)} \rightarrow 2\text{AlF}_3\text{(g)} + 3\text{H}_2\text{O(g)}$
• The generated aluminum trifluoride ($\text{AlF}_3$) likewise exhibits high vapor pressure and volatilizes readily at elevated temperatures. Furthermore, during thermal fluctuations, $\text{AlF}_3$ reacts with the substrate to form low-melting-point eutectics, promoting premature vitrification and subsequent spalling of the lining surface.

2. “Passive-to-Active Oxidation and Protective Layer Failure” of Silicon Carbide Refractories ($\text{SiC}$)

Silicon carbide materials are widely deployed in severe corrosion zones due to their high thermal conductivity, low thermal expansion, and excellent erosion resistance. Their oxidation resistance fundamentally relies on a spontaneously formed silicon dioxide ($\text{SiO}_2$) passivation film on the surface. However, halogen gases possess an extraordinary capacity to penetrate and strip away this passive layer.

• Direct Halogenation of the Matrix: Once chlorine/fluorine-containing gases penetrate the micro-pores or micro-cracks within the passivation layer, they react directly with the underlying $\text{SiC}$ matrix:
• $\text{SiC(s)} + 2\text{Cl}_2\text{(g)} \rightarrow \text{SiCl}_4\text{(g)} + \text{C(s)}$
• $\text{SiC(s)} + 4\text{HF(g)} \rightarrow \text{SiF}_4\text{(g)} + \text{CH}_4\text{(g)}$
• The generated silicon tetrachloride ($\text{SiCl}_4$, boiling point $57.6^\circ\text{C}$) and silicon tetrafluoride ($\text{SiF}_4$, gas phase) escape instantaneously into the gas stream. This leaves behind only a loose, porous carbonaceous skeleton where a dense network of $\text{SiC}$ grains once existed, causing a total loss of mechanical integrity and erosion resistance.

II. Protective Coating Technology: The Integration of Rare Earth Oxides

Because modifying the bulk refractory substrate alone cannot simultaneously optimize thermal shock resistance and absolute chemical inertness, constructing high-temperature physical barrier coatings—specifically Thermal Environmental Barrier Coatings (TEBCs)—on the refractory surface has emerged as a vital strategy to interrupt halogen gas attack. In recent years, Rare Earth Oxides (REOs) have become a focal point of research due to their unique electronic structures and superior high-temperature thermodynamic stability.

1. Thermodynamic Advantages of Rare Earth Oxides

Compared to $\text{Al}_2\text{O}_3$ and $\text{SiO}_2$, rare earth oxides such as yttria ($\text{Y}_2\text{O}_3$), ceria ($\text{CeO}_2$), and lanthana ($\text{La}_2\text{O}_3$) display a much more negative Gibbs free energy of reaction with halogens. Consequently, their corresponding halides exhibit significantly lower vapor pressures at elevated temperatures.

In simpler terms, under identical chlorine/fluorine-containing high-temperature atmospheres, rare earth oxides are highly resistant to halogenation-driven volatilization, enabling them to maintain a reliable, long-term solid barrier.

2. Design and Implementation of Protective Coating Systems

• Composite Coating Architectures: A prominent design involves composite coating systems pairing Yttria-Stabilized Zirconia (YSZ) with rare-earth zirconates (such as $\text{La}_2\text{Zr}_2\text{O}_7$). Utilizing Atmospheric Plasma Spraying (APS) or slurry sintering, a dense rare-earth zirconate topcoat is deposited onto the $\text{Al}_2\text{O}_3$ or $\text{SiC}$ substrate. These materials are not only chemically inert to halogen gases, but their coefficients of thermal expansion (CTE) are also highly compatible with corundum matrices (typically within $9 \times 10^{-6} \sim 11 \times 10^{-6}/\text{K}$), effectively mitigating the common issue of coating cracking or delamination during severe thermal cycling.
• Microstructural Barrier and “Self-Healing” Mechanisms: The dense REO coating cuts off gas-phase diffusion pathways, preventing $\text{Cl}_2/\text{F}_2$ molecules from infiltrating the underlying refractory via open porosity. Furthermore, certain micro-doped, ceria-based coatings can leverage lattice oxygen migration at high temperatures to subtly adjust non-stoichiometric structures. This enables a degree of “self-healing” for internal micro-cracks generated over time, ensuring sustained, long-term barrier performance.

III. Critical Engineering Considerations for Industrial Applications

When evaluating refractory engineering solutions for the design or retrofitting of actual chemical reactors, several critical boundary conditions must be analyzed realistically:

1. Dynamically Alternating Atmospheres: Many reactor vessels experience complex, cyclic processes involving both chlorination and minor carbonization. When selecting coatings and refractory linings, rigorous thermodynamic phase-equilibrium calculations must be performed to guarantee the avoidance of low-melting-point eutectic phases.
2. Feasibility of Coating Application: Executing large-scale, on-site thermal spraying of specialty rare earth coatings inside non-standard industrial reactors is frequently constrained by field conditions, which can compromise bonding strength and coating uniformity. Consequently, a modular construction strategy utilizing “high-purity precast shapes + factory-precoated surfaces” typically delivers substantially higher reliability than on-site monolithic spraying.

Epilogue

Mitigating halogen-driven corrosion on high-temperature furnace linings is an imperative step toward eliminating equipment bottlenecks in specialized chemical processing hardware. By addressing the degradation at its chemical root and introducing high-performance shielding barriers like rare earth oxides, engineers can drastically extend the service life of core components. This provides a robust asset-integrity foundation for the green, highly efficient manufacturing of ultra-pure chemicals.

If you are interested in high-temperature corrosion-resistant materials and specialty industrial furnace design, stay tuned for our upcoming insights. In our next feature, we will explore methods to combat the complex, multi-component degradation challenges presented by sulfur- and alkali-rich atmospheres.