Why is quartz the only heating material stable in high-concentration fluoride acid?

# Why is quartz the sole heating material that remains stable in high-concentration fluoride acid? Fluoride acid is a common ingredient in special strain fermentation and raw material purification processes. However, it poses a significant corrosive threat to the majority of metal and polymer heating equipment. Under high-concentration fluoride acid, 316 stainless steel, Grade 2 titanium, and PFA coated heaters all experience varying degrees of structural damage. Conversely, high-purity fused quartz heating tubes maintain a stable chemical inertness that enables long-term continuous operation. This exceptional stability is the result of the unique silicon dioxide crystal structure. The following table compares the material's performance under fluoride acid working conditions. | Heating Tube Material | Chemical Reaction with High-Concentration Fluoride Acid | Visible Surface Damage | Maximum Continuous Service Cycle | Applicable Fluoride Acid Scenario | | ---- | ---- | ---- | ---- | ---- | | 316 Stainless Steel | Rapid pitting electrochemical corrosion | Dense rust pits on welds | Less than 1 week | Only trace fluoride below 10 ppm | | Grade 2 Titanium | Complete dissolution of TiO₂ passivation film | Milky white uniform etching layer | Less than 3 days | Strictly zero fluoride environment only | | PFA Coated Heater | Slow fluoroplastic swelling and permeation | Coating dullness and tiny blisters | 1–2 months | Low-concentration intermittent fluoride use | | High-Purity Quartz | No spontaneous chemical reaction at 20–90℃ | Smooth intact surface without marks | More than 1 year continuous operation | High-concentration fluoride acid long-cycle production | The core component of qualified industrial quartz tubes is silicon dioxide with purity exceeding 99.99%, which forms a tight three-dimensional network via silicon-oxygen covalent bonds. Fluoride ions are unable to disrupt stable Si-O bonds within the temperature range of conventional fermentation. Consequently, no soluble metal fluoride byproducts will be produced. Quartz's entire crystal matrix is inherently anti-fluoride, which eliminates the possibility of protective layer dissolution or failure, in contrast to metal heating tubes that depend on thin surface passivation films for protection. The exposed inner crystal remains inert to fluoride acid, preventing accelerated corrosion to occur, even if solid particulates scratch the tube surface. Metallic heating tubes are susceptible to fatal defects when exposed to fluoride acid. The entire tube wall thins uniformly within a few days as a result of the violent reaction between titanium's titanium dioxide passivation film and fluoride ions, resulting in the formation of soluble titanium fluoride. Concentrated pitting corrosion is a common occurrence in 316 stainless steel at weld positions, which can result in medium waste and abrupt leakage. The long-term soaking of high-concentration fluoride acid will permeate micro gaps inside the coating, resulting in blistering and separation from the carbon steel substrate. PFA fluoropolymer can only temporarily resist low-concentration fluoride; isolation protection will progressively be lost. Nevertheless, quartz's exceptional fluoride resistance is accompanied by an apparent drawback: it is incapable of interacting with alkaline cleaning liquid. Hydroxide ions disrupt the covalent bonds between silicon and oxygen, resulting in the formation of soluble silicate. This process results in the formation of irreversible frosted layers on the tube wall. The structural strength of quartz declines significantly upon the appearance of frosting, and the expansion of internal microcracks and the subsequent rupture of the tube will result from cold-heat alternation during startup and closure. Consequently, in order to completely isolate alkali circulation loops, quartz heating pipelines must implement mechanical hard interlocks. Additionally, acid and alkali cleaning pipelines require independent marking and control programs. In laboratory and small-scale fermentation lines that necessitate high-concentration fluoride acid treatment, other heating materials are incapable of satisfying long-term production requirements and can only be employed as temporary transition equipment. Quartz tubes become the sole dependable option. Operators are required to regulate the heating rate to below 0.4℃ per minute in order to mitigate thermal shock stress and to conduct bi-weekly light transmission inspections to detect incidental alkali frosting. Quartz heating tubes can fully exploit their exclusive anti-fluoride advantage by rigorously separating acid and alkali media, thereby reducing the frequency of equipment replacement costs and preventing medium contamination risks caused by the corrosion failure of metal or polymer heaters.