For a Titanium Sheath Heater 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?

The tartrate ion (C4H4O6 2-) is a powerful chelating agent for titanium in 5% sodium tartrate at 103 °C. Tartrate creates soluble titanium tartrate complexes unlike chloride or fluoride (which lead to pitting) Ti4+ + 3C4H4O6 2- -> [Ti(C4H4O6)3] 4- In this complexation Ti4+ ions are scavenged from the surface as they are generated so that no stable passive film can be produced. In the absence of a protective coating, titanium corrodes uniformly at a steady rate of 0.15 to 0.25 mm per year without any localised pitting.

Corrosion Rate vs Tartrate Concentration (Quantitative)

103°C (boiling)

Sodium Tartrate Concentration (%) Corrosive Rate (mm/yr)Appearance of Surface Mechanism
0 (just water) 0.005-0.010Bright Passive 1 0.03-0.06Light etching Mixed light etching 3 0.08–0.15Etching uniform Complexing
5 0.15–0.25 Matte Complexation uniform dominating
10 0.20–0.35 Uniformity of matteness Strong complexation Influence 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 advice.

Assume that uniform corrosion is the major mechanism for the sodium tartrate solutions at high temperature. Use thicker walls (2.0-2.5 mm) to afford corrosion allowance. Palladium is not impacted by the tartrate complexing, hence there is no benefit to be had from Grade 7 titanium. Knowing the complexation mechanism of tartrate, the engineer can choose adequate wall thickness for consistent corrosion.