In A Titanium Electric Heater Used To Keep A 20% Sodium Bicarbonate Slurry At 60°C, Why Does The Heater Fail Faster By Abrasion From Bicarbonate Crystals Than By Electrochemical Corrosion?
Leave a message
In 20% sodium bicarbonate (NaHCO₃) slurry at 60°C, bicarbonate crystals precipitate as the solution is heated. These crystals have a Mohs hardness of 2.5–3.0, which is less than titanium's hardness (3.5–4.0), but under flow conditions, they cause abrasive wear. The abrasion rate (0.2–0.5 mm per year) exceeds the electrochemical corrosion rate (0.01–0.05 mm per year) by 10–20×, making abrasion the dominant failure mechanism.
Quantitative Abrasion vs. Electrochemical Corrosion
In 20% NaHCO₃ slurry at 60°C, 2 m/s flow:
| Mechanism | Material Removal Rate (mm/year) | Contribution to Failure (%) |
|---|---|---|
| Electrochemical corrosion | 0.01–0.05 | 5–10 |
| Abrasion (crystal impact) | 0.15–0.35 | 60–75 |
| Erosion-corrosion (synergy) | 0.05–0.15 | 15–30 |
| Total | 0.25–0.55 | 100 |
Effect of Solids Concentration and Flow Velocity
| Bicarbonate Solids (wt%) | Flow Velocity (m/s) | Abrasion Rate (mm/year) | Total Material Loss (mm/year) |
|---|---|---|---|
| 10 | 1 | 0.08–0.15 | 0.12–0.22 |
| 20 | 1 | 0.12–0.25 | 0.18–0.35 |
| 20 | 2 | 0.20–0.40 | 0.25–0.55 |
| 30 | 2 | 0.30–0.55 | 0.40–0.70 |
| 20 | 3 | 0.35–0.65 | 0.45–0.80 |
Abrasion Protection Guide
| Slurry Condition | Recommended Protection | Expected Life (1.5 mm wall) |
|---|---|---|
| Low solids (<10%), low velocity (<1 m/s) | None | 5–8 years |
| Moderate solids (10–20%), low velocity | Hard-facing (TiN coating) | 3–5 years |
| High solids (>20%), moderate velocity (1–2 m/s) | Replaceable wear sleeve | 2–3 years |
| Severe (>20%, >2 m/s) | Redesign: reduce velocity | 1–2 years |
Engineering Recommendation
For bicarbonate slurry service, reduce flow velocity below 1 m/s and minimize solids concentration. Install a TiN coating (hardness 2,000 HV) on the heater surface to resist abrasion. By controlling abrasive wear, the engineer addresses the dominant failure mechanism.
