For A Titanium Electric Heater Used To Melt A 50/50 Eutectic Salt (NaCl-KCl) At 700°C, Why Is A Thick (5 Μm) Anodized Oxide Layer Detrimental, While A Thin (0.2 Μm) Natural Oxide Provides Adequate Protection For Several Cycles?
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At 700°C in molten NaCl-KCl eutectic salt, titanium forms a natural oxide (0.1–0.3 µm) that grows slowly and remains adherent. A thick anodized oxide (5 µm) contains internal stresses and micro-cracks from the anodizing process. At 700°C, the thick oxide spalls due to thermal expansion mismatch, exposing fresh titanium to rapid oxidation. The thin natural oxide, grown in service, is more compliant and spall-resistant.
Quantitative Oxide Spalling vs. Initial Thickness
In molten NaCl-KCl at 700°C, thermal cycles (25°C → 700°C → 25°C):
| Initial Oxide Thickness (µm) | Oxide Source | Cycles to Spalling | Metal Loss per 100 Cycles (µm) |
|---|---|---|---|
| 0.1 (natural) | In-situ growth | >100 | 5–10 |
| 0.2 (natural) | In-situ growth | >100 | 5–10 |
| 1.0 (thermal, 400°C air) | Pre-oxidized | 20–40 | 20–50 |
| 3.0 (anodized) | Anodizing | 5–15 | 50–100 |
| 5.0 (anodized) | Anodizing | 3–8 | 100–200 |
| 10.0 (anodized) | Anodizing | 1–3 | 200–500 |
Effect of Salt Composition on Oxide Stability
| Salt Composition | Natural Oxide Life (cycles) | Anodized Oxide (5 µm) Life (cycles) |
|---|---|---|
| NaCl-KCl (50/50) | >100 | 3–8 |
| NaCl-KCl + 5% MgCl₂ | 50–100 | 2–5 |
| NaCl-only | 100–200 | 5–10 |
Oxide Preparation Guide for Molten Salt Service
| Desired Service Life (cycles) | Recommended Initial Oxide | Preparation Method |
|---|---|---|
| <10 | Any | Not recommended for thin wall |
| 10–30 | Natural or <1 µm thermal | Pickled only |
| 30–100 | Natural (in-situ grown) | Service growth only |
| >100 | Natural + controlled heat-up | Slow ramp to 700°C |
Engineering Recommendation
For molten chloride salt service at 700°C, avoid anodized or thick pre-formed oxides. Install the heater in the pickled condition and allow the natural oxide to grow during service with a slow initial heat-up (2°C/min). By using a thin natural oxide, the engineer prevents spalling and extends heater life.
