In a Titanium Electric Heater for a 20% Ammonium Carbonate Solution at 70°C, How Does the Carbonate Ion (CO₃²⁻) Change the Passivation Layer from TiO₂ to Titanium Carbonate (TiCO₃) and Reduce the Corrosion Rate to 0.005 mm/year?

In 20% ammonium carbonate at 70°C, the carbonate ion (CO3^2-) combines with the titanium surface to form a titanium carbonate (TiCO3) layer instead of the typical TiO2, In alkaline carbonate solutions this carbonate layer is very stable and protective. The corrosion rate of Grade 2 titanium is 0.003-0.008 mm/year, which is lower than in many neutral or acidic conditions. The layer is formed by the reaction TiO₂ + CO₃²⁻ → TiCO₃ + O²⁻ .

Mechanism of Carbonate Passivation

Carbonate ions are adsorbed on the titanium surface and displace oxygen in the passive film . The formed TiCO3 layer is dense, non-porous and insoluble in carbonate solutions. The layer thickness is 2-5 nm . The protective property is preserved around pH 8-11.

Quantitative corrosion rate versus carbonate concentration

(NH₄)₂CO₃ Concentration (%) Temperature (°C) Corrosion Rate (mm/year)Surface Layer 5 70 0.005-0.010 TiO 2 + TiCO 3 10 70 0.004-0.008 TiCO 3 20 70 0.003-0.006 TiCO 3 30 70 0.002-0.005 TiCO 3 Effect of pH on Stability of Carbonate Layer

pH Corrosion Rate (mm/year) Layer Stability <7 0.01-0.03 Unstable (TiCO₃ dissolved)
7-8 0.008-0.015 Moderate
8-11 0.003-0.008 Stable >11 0.005-0.010 Stable (but Ti forms titanates)
Making an Informed Specification

For a titanium heater in ammonium carbonate at 70°C, keep pH 8-11 for best carbonate passivation. The rate of corrosion is very low. The engineer knows carbonate passivation and uses titanium for long term service.