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?

In 5% sodium tartrate, the tartrate ion (C₄H₄O₆²⁻) is a potent chelating agent for titanium at 103°C (boiling). Pitting attack is caused by chloride and fluoride, but tartrate leads to the formation of soluble titanium tartrate complexes: Ti⁴⁺ + 3C₄H₄O₆²⁻ → [Ti(C₄H₄O₆)₃]⁴⁻. The complexation removes Ti 4+ ions from the surface as they are formed and inhibits formation of a stable passive film. Without a protective covering, titanium will erode uniformly at a constant rate of 0.15 – 0.25 mm per year with no localized pitting. Corrosion rate is not a function of time after an initial transitory phase.

Mechanism of Tartrate-Induced Uniform Corrosion

The tartrate ion has several hydroxyl and carboxyl groups capable of forming stable five membered chelate rings with Ti 4+ . log K~20).The formation constant for the Ti-tartrate complex is exceptionally high This means that any Ti⁴⁺ released from the metal surface is complexed and promptly taken into solution. The passive film cannot grow, because the solubility product of TiO2 is only exceeded at very high local concentrations of Ti4+. The corrosion process is based on the direct dissolving of the metal: Ti → Ti⁴⁺ + 4e⁻, followed by fast complexation. Because there is no passive film to locally break down, the process is uniform .

Quantitative Corrosion Rate versus Tartrate Concentration

Controlled tests in boiling sodium tartrate solutions (103 °C) for 500 hr have established the following for Grade 2 titanium. At 0% tartrate (pure water) the corrosion rate is 0.005–0.010 mm/yr, the surface is bright and the mechanism is passive. At 1% tartrate, the rate is 0.03–0.06 mm per year, the surface shows light etching and the mechanism is mixed. At 3% tartrate the rate is 0.08–0.15 mm/year, the surface shows uniform etching and complexation is the prevalent process. With 5% tartrate, the rate is 0.15–0.25 mm/yr, the surface is uniform matte finish, and the process is complexation dominant. 0.20-0.35 mm/yr at 10% tartrate, surface evenly matte, mechanism severe complexation.

Effect of pH and Temperature on Complexation Rate

The rate of complexation and corrosion rate are a function of pH and temperature. Corrosion rate at 60 C, pH 7 and 5% tartrate is 0.04-0.08 mm/yr. The rate at 80°C and pH 7 is 0.08-0.15 mm/yr. The rate is 0.15 to 0.25 mm/year at 100 °C and pH 7. The rate is 0.10–0.18 mm per year at 100°C and pH 9 (higher pH reduces corrosion). Rate at 100°C, pH 5, 0.20–0.35 mm/year (lower pH increases corrosion rate). The activation energy is around 50 kJ/mol.

Control of Tartrate Concentration and pH in Titanium Heaters

The following table gives the recommended operating conditions for Grade 2 and Grade 7 titanium heaters in sodium tartrate solutions.

Sodium Tartrate Concentration (%) Temperature (°C) pH Grade 2 Corrosion Rate (mm/yr)Corrosion Rate (mm/year) Grade 7Recommendation <1 100 7 0.03–0.06 0.02–0.04 Acceptable 1–3 80 7 0.05–0.10 0.03–0.06 Acceptable 3–5 80 7 0.08–0.15 0.05–0.10 Use Grade 7 3–5 100 7 0.15–0.25 0.10–0.18 Use Grade 7 + thick wall
5-10 100 9 0.10-0.18 0.06-0.12 pH increase >10 Any Any >0.20 >0.12 Not recommended Engineering Beyond Tartrate Concentration Control

The titanium grade influences the complexation rate. Grade 7 (palladium stabilized) corrosion rates are 30-50% lower than those of Grade 2 in tartrate solutions, as the palladium stabilizes a thin passive coating even under complexing conditions. Grade 12 shows a moderate improvement. Wall thickness gives a corrosion allowance . 2 . 0 mm wall with 0 . 15 mm/year uniform corrosion lasts 13 years . Surface finish has little effect on uniform corrosion. By increasing the pH to 9 with sodium hydroxide the corrosion rate is reduced by 30–40 %. If the temperature is lowered from 100°C to 80°C, then the rate is cut in half.

Developing an informed specification

For a titanium sheath heater the dominating mechanism should be assumed to be uniform corrosion in 5% sodium tartrate at 103 °C. Use Grade 7 titanium to lower the corrosion rate to 0.10–0.18 mm per year. For a service life of 10 to 15 years use a wall thickness of 2.0 mm. Raise the pH to 9 ( if possible ) with sodium hydroxide or lower the temperature to 80C . During operation, the thickness of the heater is monitored by ultrasonic gauging once a year and the heater is replaced when the wall thickness reaches 0.5 mm. The engineer is aware that tartrate ions complicate titanium ions and inhibit repassivation, therefore they choose the right grade and wall thickness to ensure predictable uniform corrosion.