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?
Jul 16, 2026
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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 (thickness 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.
Quantitative Film Quality vs. pH
In 10% Na₂SiO₃ at 80°C for 500 hours:
| pH | Film Thickness (µm) | Film Porosity | Corrosion Rate (mm/year) | Pitting Present? |
|---|---|---|---|---|
| 8 | 0.1–0.3 | High | 0.15–0.30 | Yes |
| 9 | 0.2–0.5 | High | 0.08–0.20 | Occasional |
| 10 | 0.5–1.0 | Moderate | 0.04–0.10 | Rare |
| 11 | 1.0–1.5 | Low | 0.02–0.05 | No |
| 12 | 1.5–2.5 | Very low | 0.01–0.03 | No |
| 13 | 2.0–3.0 | Very low | 0.005–0.015 | No |
Effect of Temperature on pH Requirement
| Temperature (°C) | Minimum pH for Protective Film | Recommended pH |
|---|---|---|
| 60 | 10.5 | 11.0 |
| 80 | 11.0 | 11.5 |
| 100 | 11.5 | 12.0 |
pH Control Guide for Silicate Service
| Operating pH | Film Quality | Recommended Action |
|---|---|---|
| >11 | Protective | None |
| 10–11 | Marginal | Increase pH with NaOH |
| 9–10 | Poor, porous | Add NaOH to raise to >11 |
| <9 | Non-protective | Redesign solution |
Engineering Recommendation
For sodium metasilicate service at 80°C, maintain pH above 11 using sodium hydroxide addition. By controlling pH, the engineer ensures formation of a protective, non-porous silicate film on titanium.
