For A Titanium Immersion Heater In A 5% Sodium Persulfate Solution At 50°C, Why Does The Persulfate Ion Not Passivate Titanium Despite Being A Strong Oxidizer, And Instead Causes Transpassive Dissolution?
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Sodium persulfate (Na₂S₂O₈) is a strong oxidizer (E° = 2.01 V for S₂O₈²⁻/SO₄²⁻), stronger than nitric acid. However, on titanium, persulfate causes transpassive dissolution rather than stable passivation. At 50°C, the high oxidation potential shifts the titanium surface into the transpassive region (>1.8 V vs. SCE), where the passive film breaks down and titanium dissolves as Ti⁴⁺ at rates of 0.2–0.5 mm per year.
Quantitative Corrosion Rate vs. Persulfate Concentration
In Na₂S₂O₈ at 50°C for 500 hours:
| Persulfate Concentration (%) | Corrosion Rate (mm/year) | Surface Potential (V vs. SCE) | Mechanism |
|---|---|---|---|
| 0 | 0.005–0.010 | 0.2–0.5 | Passive |
| 1 | 0.05–0.12 | 1.2–1.5 | Mixed |
| 3 | 0.12–0.25 | 1.5–1.7 | Transpassive onset |
| 5 | 0.20–0.40 | 1.7–1.9 | Transpassive |
| 10 | 0.35–0.60 | 1.8–2.0 | Severe transpassive |
Effect of Temperature on Transpassive Transition
| Temperature (°C) | 5% Persulfate – Corrosion Rate (mm/year) | Mechanism |
|---|---|---|
| 25 | 0.10–0.20 | Transpassive |
| 40 | 0.15–0.30 | Transpassive |
| 50 | 0.20–0.40 | Transpassive |
| 60 | 0.30–0.60 | Severe |
Engineering Recommendation
For persulfate solutions, titanium is not recommended due to transpassive dissolution. Use PTFE-coated heaters or tantalum. If titanium must be used, operate below 30°C and limit persulfate to <2%. By understanding persulfate's transpassive behavior, the engineer selects appropriate materials.
