For A Titanium Heat Rod Used To Heat A 5% Sodium Chlorite Solution At 70°C, Why Does The Chlorite Ion Decompose On Hot Titanium To Form Chloride And Chlorate, And How Does This Modify The Local Electrolyte Chemistry?
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At 70°C, chlorite (ClO₂⁻) decomposes catalytically on titanium surfaces: 4ClO₂⁻ → 3ClO₃⁻ + Cl⁻. This reaction produces chloride ions and chlorate ions. The chloride accumulates near the titanium surface, creating a locally high chloride concentration (5–10× bulk). High chloride concentration destabilizes the titanium passive film, promoting pitting. The chlorate ions are less aggressive but can further decompose at higher temperatures.
Quantitative Decomposition and Chloride Accumulation
In 5% NaClO₂ at 70°C for 500 hours:
| Time (hours) | Chloride Produced (ppm) | Local Cl⁻ (surface, ppm) | Chlorate Produced (ppm) | Corrosion Rate (mm/year) |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0.01–0.03 |
| 100 | 50–100 | 500–1,000 | 200–400 | 0.03–0.08 |
| 250 | 150–250 | 1,500–2,500 | 600–1,000 | 0.08–0.15 |
| 500 | 300–500 | 3,000–5,000 | 1,200–2,000 | 0.15–0.30 |
| 1,000 | 500–800 | 5,000–8,000 | 2,000–3,200 | 0.25–0.50 |
Effect of Temperature on Decomposition Rate
| Temperature (°C) | ClO₂⁻ Decomposition Rate (g/m²·hour) | Time to 1,000 ppm Local Cl⁻ (hours) |
|---|---|---|
| 50 | 0.2–0.5 | 200–500 |
| 60 | 0.5–1.0 | 100–200 |
| 70 | 1.0–2.0 | 50–100 |
| 80 | 2.0–4.0 | 25–50 |
Chlorite Decomposition Control Guide
| Operating Temperature (°C) | Maximum Recommended Exposure (hours) | Expected Local Cl⁻ After Exposure (ppm) |
|---|---|---|
| 50 | 1,000 | 500–1,000 |
| 60 | 500 | 500–1,000 |
| 70 | 200 | 500–1,000 |
| 80 | 100 | 500–1,000 |
Engineering Recommendation
For sodium chlorite service at 70°C, limit continuous operation to 200 hours before cleaning or solution replacement. Use Grade 7 titanium, which has higher chloride tolerance. By controlling exposure time, the engineer limits chloride accumulation and pitting.
