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?

In 50 % potassium carbonate (K2CO3) at 110 °C a protective coating of potassium titanate (K2TiO3) is formed on the titanium surface. Dissolved oxygen (DO) is a key element of titanates production. At low DO (0.5 ppm) the titanate layer is thin (0.1-0.5µm) and porous and provides inadequate protection. At high DO (5 ppm), the layer is thicker (1–3 µm) and dense and reduces the corrosion rate by 5–10 times. The titanate layer is produced by the reaction TiO₂ + K₂O → K₂TiO₃, K₂O being obtained by breakdown of K₂CO₃.

Influence of Dissolved Oxygen on the Mechanism of Titanate Layer Formation

Oxygen is needed to keep the titanium surface in a passive state for TiO2 to develop. TiO 2 reacts in the presence of K 2 CO 3 to generate K 2 TiO 3 . Oxygen-deficient conditions result in a thin and defective layer of TiO$_2$, and hence a thin and defective layer of K$_2$TiO$_3$. At low DO the coating is cracked and porous and permits corrosion to continue. At high DO the layer grows consistently and is protective. In the steady state the corrosion rate is controlled by diffusion of oxygen through the titanate layer.

Quantitative Titanate Layer Characteristics as Function of Dissolved Oxygen

The following has been established for grade 2 titanium by controlled testing in 50% K2CO3 at 110°C. for 1,000 hours. At 0 ppm DO (deaerated) the titanate layer thickness is 0.05–0.1 µm, the corrosion rate is 0.15–0.30 mm/year and the surface is dark and porous. At 0.5 ppm DO the thickness is 0.1-0.3 microns, corrosion rate 0.08-0.15 mm/yr and the surface is grey and very porous. 2 ppm DO Thickness 0.5-1.0 µm Corrosion rate 0.03-0.08 mm/year Surface Light grey The thickness at 5 ppm DO is 1.0–2.0 µm, corrosion rate is 0.01–0.03 mm per year and the surface is bright and protected. The surface at 10 ppm DO is bright and stable, with a thickness of 2.0–3.0 μm and a corrosion rate of 0.005–0.015 mm per year.

Effect of Temperature on Dissolved Oxygen Demand

The minimum DO necessary for stable passivation increases with temperature. The minimum DO for a stable passivation layer (corrosion rate <0.02 mm per year) is 1 ppm at 80°C. Minimum is 2 ppm at 100°C. The minimum at 110°C is 3 ppm. The minimum is 4 ppm at 120 °C. 5 ppm minimum at 130 °C. The DO advised for a safety margin is 2-3 times the minimum.

Potassium Carbonate Heater Dissolved Oxygen Management Guide

The following table gives guidelines for dissolved oxygen in 50% K 2 CO 3 at 110°C for Grade 2 and Grade 7 titanium heaters.

Measured DO (ppm) Stability of Titanate LayerPredicted corrosion rate (mm/year)> Action recommended5 Excellent 0.005-0.015none 3 to 5Good 0.01–0.032-3 Marginal 0.03-0.08 Sparge air 1-2 Poor 0.08-0.15 Increase aeration Monitor weekly
0.5–1 Unstable 0.15–0.30 Redesign; add oxidiser <0.5 Very unstable >0.30 Not acceptable
Engineering Outside DO Control

Depending on the titanium grade, DO as required. Grade 7 (palladium stabilised) generates a stable titanate layer at low DO levels (1-2 ppm at 110oC) from palladium-catalyzed oxygen reduction. Grade 12 provides moderate progress. Wall thickness gives a corrosion allowance; a 2.0 mm wall corrodes at a rate of 0.03 mm per year and will survive 60 years. Passivation is influenced by the concentration of K₂CO₃; at 40% K₂CO₃, the needed DO is reduced. Chlorides in K2CO3 solution (from impurities) increase the DO requirement by 1-2 ppm.

Building a Well Informed Specification

For Titanium heat rod in 50% K2CO3 at 110C Keep dissolved oxygen above 5 ppm by air sparging or oxygen infusion. Install a DO probe with an alarm at 4ppm and interlock at 3ppm. For existing systems with DO < 3 ppm, add a sparger or increase the aeration rate. If aeration is not possible, dilute the K₂CO₃ to 40% or lower the temperature to 100°C. By regulating dissolved oxygen above 5 ppm, the engineer is able to produce a stable, protective potassium titanate layer and minimise corrosion.