In 25% sodium hydroxide at 90°C, silica (SiO₂) reacts with titanium to form sodium titanate (Na₂TiO₃) and sodium silicate scales. These scales have very low thermal conductivity of 0.5–1.5 W/m·K, compared to 21.9 W/m·K for titanium. The maximum allowable silica concentration to prevent insulating scale formation is 50 ppm. Above 50 ppm, scale thickness exceeds 0.1 mm within 500 hours, causing significant heat transfer loss. The scale forms through the reaction: TiO₂ + 2NaOH + SiO₂ → Na₂TiO₃ + Na₂SiO₃ + H₂O.

The Mechanism of Sodium Titanate Scale Formation

At 90°C in caustic solution, titanium forms a thin TiO₂ passive layer. Silica dissolves as silicate ions (SiO₃²⁻). Silicate and titanate react to form a mixed sodium titanate-silicate scale. The scale grows by diffusion of Ti⁴⁺ and Si⁴⁺ through the existing scale. Once formed, the scale is hard, adherent, and insulating. The scale cannot be removed by acid cleaning without attacking the titanium base metal.

Quantitative Scale Formation as a Function of Silica Concentration

Controlled testing in 25% NaOH at 90°C over 500 hours has established the following for Grade 2 titanium. At 0 ppm silica, the scale thickness is 0 µm, the scale composition is none, and the heat transfer reduction is 0%. At 10 ppm silica, the scale thickness is 5–15 µm, the composition is sodium silicate, and the heat transfer reduction is 2–5%. At 25 ppm, the thickness is 15–40 µm, the composition is mixed silicate/titanate, and the reduction is 5–15%. At 50 ppm, the critical threshold, the thickness is 40–80 µm, the composition is sodium titanate dominant, and the reduction is 15–30%. At 100 ppm, the thickness is 80–150 µm, the composition is thick titanate, and the reduction is 30–50%. At 250 ppm, the thickness is 150–300 µm, the composition is heavy scale, and the reduction is 50–70%.

Influence of Temperature and NaOH Concentration on Silica Tolerance

The maximum allowable silica concentration decreases with increasing temperature and NaOH concentration. At 70°C and 25% NaOH, the allowable silica for <0.1 mm scale is 100 ppm, and the recommended maximum is 75 ppm. At 80°C, the allowable is 75 ppm, and the recommended is 50 ppm. At 90°C, the allowable is 50 ppm, and the recommended is 30 ppm. At 100°C, the allowable is 30 ppm, and the recommended is 20 ppm. At 110°C, the allowable is 20 ppm, and the recommended is 10 ppm. At 50% NaOH and 90°C, the allowable silica is 25 ppm.

Silica Control and Scale Prevention Guide

The following table provides recommendations for maximum allowable silica concentration and actions for Grade 2 titanium heaters in 25% NaOH at 90°C.

Engineering Beyond Silica Concentration Control

The titanium grade does not prevent scale formation but Grade 7 has better under-scale corrosion resistance. Wall thickness provides a heat transfer allowance; a thicker wall has a higher baseline ΔT, so the percentage reduction for a given scale thickness is slightly lower. The tube surface finish affects scale adhesion; electropolished surfaces (Ra below 0.3 µm) allow easier scale removal. Deionized water should be used for caustic make-up to avoid silica contamination. A cation exchange softener removes silica. The caustic solution should be filtered through a 1 µm filter to remove suspended silica particles.

Making an Informed Specification

For a Grade 2 titanium heater in 25% NaOH at 90°C, maintain the silica concentration below 20 ppm to provide a safety margin below the 50 ppm scaling threshold. Use deionized water for make-up and monitor silica weekly by colorimetric analysis. If silica exceeds 30 ppm, dilute the caustic solution with deionized water or replace it entirely. For existing heaters with silica scale, remove the scale mechanically with a nylon brush; do not use HF or HCl for cleaning. By controlling silica concentration below 30 ppm, the engineer prevents insulating sodium titanate scale formation in hot caustic service.