When A Titanium Sheath Heater Is Used To Boil A 5% Sodium Tartrate Solution At 103°C, Why Does The Tartrate Ion Complex Titanium Ions And Prevent Repassivation, Leading To Uniform Corrosion At 0.2 Mm Per Year?
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In 5% sodium tartrate at 103°C, the tartrate ion (C₄H₄O₆²⁻) is a strong chelating agent for titanium. Unlike chloride or fluoride which cause pitting, tartrate forms soluble titanium-tartrate complexes: Ti⁴⁺ + 3C₄H₄O₆²⁻ → [Ti(C₄H₄O₆)₃]⁴⁻. This complexation removes Ti⁴⁺ ions from the surface as they are formed, preventing the formation of a stable passive film. Without a protective film, the titanium corrodes uniformly at a steady rate of 0.15–0.25 mm per year, with no localized pitting.
Quantitative Corrosion Rate vs. Tartrate Concentration
At 103°C (boiling):
| Sodium Tartrate Concentration (%) | Corrosion Rate (mm/year) | Surface Appearance | Mechanism |
|---|---|---|---|
| 0 (water only) | 0.005–0.010 | Bright | Passive |
| 1 | 0.03–0.06 | Light etching | Mixed |
| 3 | 0.08–0.15 | Uniform etching | Complexation |
| 5 | 0.15–0.25 | Uniform matte | Complexation dominant |
| 10 | 0.20–0.35 | Uniform matte | Severe complexation |
Effect of Temperature and pH
| Temperature (°C) | pH | 5% Tartrate Corrosion Rate (mm/year) |
|---|---|---|
| 60 | 7 | 0.04–0.08 |
| 80 | 7 | 0.08–0.15 |
| 100 | 7 | 0.15–0.25 |
| 100 | 9 | 0.10–0.18 |
| 100 | 5 | 0.20–0.35 |
Engineering Recommendation
For sodium tartrate solutions at high temperature, accept uniform corrosion as the dominant mechanism. Use thicker walls (2.0–2.5 mm) to provide corrosion allowance. Grade 7 titanium offers no advantage because tartrate complexation is not affected by palladium. By understanding tartrate's complexation mechanism, the engineer selects appropriate wall thickness for uniform corrosion.
