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?

Tartrate ion (C4H4O6 2- ) is a potent chelating agent for titanium in 5% sodium tartrate at 103°C (boiling). Tartrate creates soluble titanium-tartrate complexes: Ti⁴⁺ + 3C₄H₄O₆²⁻ → [Ti(C₄H₄O₆)₃]⁴⁻ Unlike chloride or fluoride, which cause pitting. This complexation eliminates Ti 4+ ions from the surface as they are generated, and also prevents the creation of a stable passive layer. Without the protective coating the titanium corrodes uniformly at 0.15-0.25 mm/year with no localised pitting. After the initial transient phase the corrosion rate is time independent.

The Mechanism of Uniform Corrosion of Tartrate

Tartrate ions possess several hydroxyl and carboxyl groups which form stable five-membered chelate rings with Ti(IV). The formation constant of the Ti-tartrate complex is exceptionally high (log K = 20). This ensures that any Ti 4+ liberated from the metal surface is promptly complexed and removed to solution. Only at very high local concentrations of Ti⁴⁺ is the solubility product of TiO₂ surpassed and so the passive film cannot form. The corrosion proceeds by direct dissolving of metal: Ti → Ti4+ + 4e- and fast complexation. The process is consistent since there is no passive film to be locally broken down.

Quantitative corrosion rate versus concentration of tartrate

The following has been established for the Grade 2 titanium under controlled testing for 500 hours in boiling sodium tartrate solutions (103°C). Corrosion rate 0.005-0.010 mm per year Surface Bright Mechanism Passive 0% tartrate (clean water) At 1% tartrate, the rate is 0.03–0.06 mm/year, the surface is light etched, and the mechanism is mixed. At 3% tartrate, the rate is 0.08–0.15 mm/year, the surface is uniformly etched and complexation is the dominant process. At 5% tartrate, the rate is 0.15–0.25 mm/yr, the surface is homogeneous and matte, and the process is complexation-dominant. The rate at 10% tartrate is 0.2–0.35 mm per year, the surface is uniformly matte, and the mechanism is severe complexation.

pH and Temperature Dependence of Rate of Complexation

The rate of complexation and the rate of corrosion depend on the pH and temperature. At 60°C, pH 7 with 5% tartrate, the corrosion rate is 0.04-0.08 mm/yr. The rate is 0.08-0.15 mm/year at 80°C and pH 7. At 100°C, pH 7, the rate is 0.15–0.25 mm/year. 0.10-0.18 mm/year at 100°C and pH 9 (higher pH decreases corrosion) At 100°C and pH 5, the rate is 0.20-0.35 mm/yr (lower pH accelerates corrosion). Activation energy is ~50kJ/mol.

Titanium Heaters - A Guide to Tartrate Concentration and pH Control

The table below gives recommendations of Grade 2 and Grade 7 titanium heaters operating in sodium tartrate solutions.

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

The rate of complexation depends on the quality of titanium. The corrosion rates of Grade 7 (palladium stabilised) are 30–50% lower than those of Grade 2 in tartrate solutions, since the palladium helps to maintain a thin passive film even under the presence of complexing conditions. Grade 12 is an intermediate level of improvement. Wall thickness gives a corrosion allowance; a 2.0 mm wall will last 13 years with a uniform corrosion of 0.15 mm per year. The surface polish has little effect on uniform corrosion. The corrosion rate is reduced by 30–40% on adding sodium hydroxide to increase the pH to 9. When the temperature is reduced from 100°C to 80°C the rate is reduced by 50%.

Providing an Informed Specification

For titanium sheath heater in 5% sodium tartrate at 103 °C. Uniform corrosion being the main process. Use grade 7 titanium to minimise the corrosion rate to 0.10 to 0.18 mm/year. Use a 2.0 mm wall thickness for a 10 to 15 year service life. If possible, either adjust the pH to 9 using sodium hydroxide or lower the temperature to 80°C. During operation, the heater wall thickness should be checked annually by ultrasonic gauges; when the wall thickness reaches 0.5 mm, the heater should be replaced. The engineer understands that the tartrate ions compound the titanium ions and inhibit repassivation resulting in predictable uniform corrosion. The engineer chooses the correct grade and wall thickness of the product.