When a Titanium Immersion Heater Is Installed in a 10% Sodium Metasilicate Solution at 80°C, Why Does the Silicate Ion Form a Protective, Non-Porous Film Only at pH >11, and What Happens at pH 9?

In solutions of sodium metasilicate (Na₂SiO₃) at 80°C, silicate ions polymerize on titanium surfaces. At pH values above 11, a continuous, non-porous, protective film of 0.5–2 µm thickness is formed from the silicate. At pH 9, the coating is porous, fractured and non-protective and allows penetration of chloride or other aggressive species and pitting. The discrepancy is due to the polymerization state of the silicate ions. At high pH, silicate occurs as monomeric SiO₄⁴⁻ ions, which create a dense glassy layer. At low pH, the silicate polymerizes to give colloidal chains which yield a porous fractured structure.

The Mechanism of pH-Dependent Silicate Film Formation

The polymerization of silicate ions SiO4 4- + H2O HSiO4 3- + OH- At high pH (>11) the equilibrium moves to monomeric ions that deposit as a thick amorphous silica layer. At pH 9-10 the ions polymerise to chain-like structures which build up a porous network. The film also uses sodium ions to create a sodium silicate glass. The ability of protection is defined by the density of the film and the absence of microcracks.

Quantitative Film Quality as a Function of pH.

The following has been established for Grade 2 titanium by controlled testing in 10% Na2SiO3 at 80°C for 500 hours: At pH 8 the thickness of the film is 0.1-0.3 µm, the porosity of the film is considerable, the corrosion rate is 0.15-0.30 mm per year and the pitting exists. At pH 9, thickness is 0.2–0.5 µm, porosity is high, corrosion rate is 0.08–0.20 mm per year, and pitting is occasional. At pH 10, the thickness is 0.5–1.0 µm, the porosity moderate, the corrosion rate 0.04–0.10 mm/year and pitting infrequent. At pH 11, the thickness is 1.0–1.5 µm, porosity is minimal, corrosion rate is 0.02–0.05 mm/year and pitting is absent. The thickness is 1.5-2.5 μm, the porosity is very low, the corrosion rate is 0.01-0.03 mm/year and no pitting occurs at pH 12. At pH 13, the thickness is 2.0–3.0 μm, the porosity is very low, the corrosion rate is 0.005–0.015 mm/year, and no pitting is observed .

Effect of Temperature on pH Requirement

The critical pH for the protective film is temperature dependent. The minimal pH is 10.5 at 60 o C while the suggested pH is 11.0. At 80°C, the minimum is 11.0 and the recommended 11.5. The minimum is 11.5 at 100°C and the suggested is 12.0. The minimum is 12.0 and the recommended is 12.5 at 120 deg C. greater temperatures favor silicate polymerization and so require greater pH for keeping silicates in monomeric form.

Sodium Metasilicate Service pH Control Guide

The table below is a guide for pH control to maintain a protective silicate film on Grade 2 titanium heaters in 10% sodium metasilicate at 80°C.

pH of Operation Film Quality Corrosion Rate (mm/year)Pitting Risk Recommended Action >11 Protective <0.05 None None 10–11 Marginal 0.05–0.10 Low Increase pH with NaOH 9–10 Poor, porous 0.10–0.20 Moderate Add NaOH to increase to >11 8–9 Non-protective 0.15–0.30 High Redesign solution <8 Unstable >0.30 Severe Not acceptable
Engineering Over pH Control

The required pH is dictated by the titanium grade. Grade 7 (palladium-stabilized) generates a protective silicate coating at pH 10.5 to 11.0, somewhat lower than Grade 2. Grade 2 is the same as Grade 12. Wall thickness offers a corrosion allowance if the film is not absolutely protective. The film formation is influenced by the silicate content. 5% of Na 2 SiO 3 requires a pH of 11.5. At 15% Na2SiO3 the pH required is 10.5. Chlorides (from impurities) raise the pH requirement by 0.5 to 1.0 pH unit. A pH meter calibrated for high pH should be used to monitor the solution.

Making a Spec With Knowledge

Add sodium hydroxide to maintain pH above 11.0 for a titanium immersion heater in 10% sodium metasilicate at 80°C. Install a pH controller with alarm at 10.8 and interlock at 10.5. Add NaOH to existing systems with pH below 11 to elevate the pH to 11.5. If pH cannot be increased, use Grade 7 titanium or lower the temperature to 60°C. Check for pitting on heater surface during operation. Immediately raise pH if pitting is seen. The engineer maintains the pH above 11 to develop a protective, non-porous silicate coating atop titanium.