For a Titanium Heat Rod Operating in a 50% Potassium Carbonate Solution at 110°C, How Does the Dissolved Oxygen Level (0.5 ppm vs. 5 ppm) Change the Stability of the Titanate Passivation Layer?

A protective potassium titanate (K₂TiO₃) layer is formed on the titanium surface in 50% potassium carbonate (K₂CO₃) at 110°C. Titanate formation is critically dependent on dissolved oxygen (DO). At low DO (0.5 ppm) the titanate layer is thin (0.1–0.5 µm) and porous and provides poor protection. At high DO (5 ppm) the coating is thick (1-3 pm) and dense, decreasing corrosion rate 5-10 times. The titanate layer is formed by the reaction TiO 2 + K 2 O → K 2 TiO 3 where K 2 O is produced from the decomposition of K 2 CO 3 .

The Mechanism of the Effect of Dissolved Oxygen on the Formation of Titanate Layer

Oxygen is essential to keep the titanium surface in the passive condition in which TiO_2 can grow. The TiO2 reacts in presence of K2CO3 and forms K2TiO3. With a lack of oxygen, the TiO2 layer is thin and defective and the K2TiO3 layer formed is similarly defective. At low DO the layer contains cracks and pores that permit continued corrosion. At high DO, the layer increases uniformly and becomes protective. In the steady state the corrosion rate is determined by the passage of oxygen through the titanate layer.

Quantitative Titanate Layer Properties as a Function of Dissolved Oxygen

110°C in 50% K 2 CO 3 for 1,000 hours has proven the following in controlled testing for Grade 2 titanium. At 0 ppm DO (deaerated) the titanate layer thickness is 0.05–0.1 µm, the corrosion rate 0.15–0.30 mm/year and dark porous surface. At 0.5 ppm DO, the thickness is 0.1-0.3 µm, the corrosion rate is 0.08-0.15 mm/yr and the surface is gray with moderate porosity. Thickness is 0.5–1.0 µm. Corrosion rate is 0.03–0.08 mm per year. Surface is light gray with 2 ppm DO. At 5 ppm DO, the thickness is 1.0-2.0 µm, corrosion rate is 0.01-0.03 mm/year, and the surface is bright and protected. At 10 ppm DO the thickness is 2.0–3.0 µm, the corrosion rate is 0.005–0.015 mm per year, and the surface is bright and stable.

Influence of Temperature on Dissolved Oxygen Demand

With increasing temperature, the minimum DO required for stable passivation increases. The minimum DO at 80°C for stable passivation layer (corrosion rate <0.02 mm per year) is 1 ppm. The minimum is 2ppm at 100°C. At 110°C the minimum is 3ppm. The minimum is at 120 °C (4 ppm). 5 ppm, 130°C. The DO for a safety margin should be 2–3 times the minimum.

Guide to Managing Dissolved Oxygen Levels in Potassium Carbonate Heaters

Recommendations for dissolved oxygen levels in 50% K2CO3 at 110°C for Grade 2 and Grade 7 titanium heaters are given in the table below.

Measured DO (ppm) Stability of Titanate LayerExpected corrosion rate (mm per annum)Recommended Action:5 Excellent 0.005 to 0.015None 3-5Good 0.01–0.03 Monitor weekly 2-3 Marginal 0.03-0.08 Sparge air 1-2 Poor 0.08-0.15 Increase aeration
0.5 - 1 Unstable 0.15 - 0.30 Redesign; add oxidizer <0.5 Very unstable >0.30 Not acceptable
Engineering Beyond DO

Titanium grade impacts the necessary DO. Grade 7 (palladium stabilized) creates a stable titanate layer at lower DO levels (1-2 ppm at 110C) owing to palladium catalyzed oxygen reduction. The Grade 12 level is for intermediate enhancement. Wall thickness provides corrosion allowance; 0.03mm/year corrosion on a 2.0mm wall lasts for 60 years. The K2CO3 concentration influences the passivation and lower needed DO is observed at 40% K2CO3. DO need increases by 1-2 ppm due to the presence of chlorides in K 2 CO 3 solution (from impurities).

Making an Informed Speculation

For titanium heat rod in 50% K₂CO₃ at 110°C, ensure dissolved oxygen >5 ppm by air sparging or oxygen injection. Add DO probe with alarm set at 4 ppm and interlock at 3 ppm. For existing systems with DO < 3 ppm, install a sparger or increase the aeration rate . If aeration is not possible, dilute K2CO3 to 40% or lower the temperature to 100°C. The engineer controls dissolved oxygen above 5 ppm to promote a stable, protective potassium titanate layer and minimize corrosion.