In sodium metasilicate (Na₂SiO₃) solutions at 80°C, silicate ions polymerize on titanium surfaces. At pH >11, the silicate forms a continuous, non-porous, protective film with a thickness of 0.5–2 µm. At pH 9, the film is porous, cracked, and non-protective, allowing chloride or other aggressive species to penetrate and cause pitting. The difference arises from the polymerization state of silicate ions. At high pH, silicate exists as monomeric SiO₄⁴⁻ ions that form a dense, glassy film. At lower pH, silicate polymerizes into colloidal chains that produce a porous, cracked structure.

The Mechanism of pH-Dependent Silicate Film Formation

The polymerization of silicate ions follows: SiO₄⁴⁻ + H₂O → HSiO₄³⁻ + OH⁻. At high pH (>11), the equilibrium favors monomeric ions, which deposit as a dense amorphous silica film. At pH 9–10, the ions polymerize into chain-like structures that create a porous network. The film also incorporates sodium ions, forming a sodium silicate glass. The protective quality is determined by the density of the film and the absence of micro-cracks.

Quantitative Film Quality as a Function of pH

Controlled testing in 10% Na₂SiO₃ at 80°C over 500 hours has established the following for Grade 2 titanium. At pH 8, the film thickness is 0.1–0.3 µm, the film porosity is high, the corrosion rate is 0.15–0.30 mm per year, and pitting is present. At pH 9, the thickness is 0.2–0.5 µm, the porosity is high, the 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 is moderate, the corrosion rate is 0.04–0.10 mm per year, and pitting is rare. At pH 11, the thickness is 1.0–1.5 µm, the porosity is low, the corrosion rate is 0.02–0.05 mm per year, and no pitting occurs. At pH 12, the thickness is 1.5–2.5 µm, the porosity is very low, the corrosion rate is 0.01–0.03 mm per year, and no pitting occurs. 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 per year, and no pitting occurs.

Influence of Temperature on pH Requirement

The minimum pH required for a protective film increases with temperature. At 60°C, the minimum pH is 10.5, and the recommended pH is 11.0. At 80°C, the minimum is 11.0, and the recommended is 11.5. At 100°C, the minimum is 11.5, and the recommended is 12.0. At 120°C, the minimum is 12.0, and the recommended is 12.5. Higher temperatures promote silicate polymerization, requiring a higher pH to maintain the monomeric state.

pH Control Guide for Sodium Metasilicate Service

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

Engineering Beyond pH Control

The titanium grade affects the required pH. Grade 7 (palladium-stabilized) forms a protective silicate film at pH 10.5–11.0, slightly lower than Grade 2. Grade 12 offers similar behavior to Grade 2. Wall thickness provides a corrosion allowance if the film is not fully protective. The silicate concentration affects film formation; at 5% Na₂SiO₃, the required pH is 11.5. At 15% Na₂SiO₃, the required pH is 10.5. The presence of chlorides (from impurities) increases the required pH by 0.5–1.0 units. The solution should be monitored with a pH meter calibrated for high-pH solutions.

Making an Informed Specification

For a titanium immersion heater in 10% sodium metasilicate at 80°C, maintain the pH above 11.0 by adding sodium hydroxide. Install a pH controller with an alarm set at 10.8 and an interlock at 10.5. For existing systems with pH below 11, add NaOH to raise the pH to 11.5. If the pH cannot be raised, use Grade 7 titanium or reduce the temperature to 60°C. During operation, inspect the heater surface for pitting; if pitting is observed, increase the pH immediately. By controlling the pH above 11, the engineer ensures the formation of a protective, non-porous silicate film on titanium.