How long can glass-based sensors survive in acidic and alkaline environments?
The service life of glass sensors (with optical fibers made of high-purity silicon dioxide \text{SiO}_2 as the core) in acidic and alkaline environments is a systems engineering problem involving material thermodynamics, kinetic reaction rates, and external encapsulation processes.
Without external physical protection, bare glass exhibits fundamental physical and chemical differences in its theoretical durability in acidic and alkaline media. The following provides a detailed technical analysis and lifespan estimation:
I. Chemical Durability of High-Purity Silicon Dioxide Glass in Acids/Alkalis (Bare Fiber State)
1. Theoretical Lifespan in Acidic Environments: Long (Except Hydrofluoric Acid)
Silicon dioxide (\text{SiO}_2) possesses extremely strong resistance to acid corrosion. At ambient and moderate temperatures, it reacts with most common inorganic acids (such as sulfuric acid, hydrochloric acid, nitric acid, etc.) negligibly.
- Acidic Limit: Hydrofluoric Acid (\text{HF})
Hydrofluoric acid rapidly attacks silicon dioxide, with the reaction equation as follows:
\text{SiO}_2 + 4\text{HF} \rightarrow \text{SiF}_4\uparrow + 2\text{H}_2\text{O}
In environments containing hydrofluoric acid, bare glass sensors will dissolve and fracture completely within minutes to hours. - Lifespan in Static Acid Environments:
In conventional acidic solutions other than hydrofluoric acid, if the bare optical fiber is in a static state without external tensile stress, its chemical structure can remain intact for years at room temperature without significant degradation. - Stress Corrosion Cracking (SCC):
If the glass fiber is subjected to tensile stress in an acidic medium, water molecules (\text{H}_2\text{O}) in the acid solution, under stress induction, will accelerate the attack on the siloxane bonds (\text{Si-O-Si}) on the fiber surface, leading to subcritical crack growth. In such cases, the mechanical lifespan of bare glass in acidic environments may drastically reduce from years to several months or even days.
2. Theoretical Lifespan in Alkaline Environments: Short
As silicon dioxide is an acidic oxide, it is highly susceptible to chemical erosion in alkaline environments (especially in strong alkalis with $\text{pH}