How Does The Inclusions Content (As Measured By ASTM E45, Type A Vs. Type C) In A Grade 2 Titanium Heater Tube Correlate With The Pitting Density In A 0.5% Bromine Solution At 50°C?
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In Grade 2 titanium, inclusions-primarily Ti₂Fe particles-act as pitting initiation sites. ASTM E45 classifies inclusions by type: Type A (sulfides, rare in titanium), Type B (aluminates), Type C (silicates), and Type D (globular oxides). In titanium, iron-rich inclusions (Type D) are most common and most detrimental. Higher inclusion content correlates directly with higher pitting density in bromine solutions. A severe inclusion rating (E45 rating >2) results in pitting density 5–10× higher than a clean material (E45 rating <1).
Quantitative Pitting Density vs. Inclusion Rating
In 0.5% Br₂ at 50°C for 500 hours:
| ASTM E45 Inclusion Rating | Ti₂Fe Particle Density (particles/mm²) | Pitting Density (pits/cm²) | Max Pit Depth (µm) |
|---|---|---|---|
| <0.5 (very clean) | <50 | 5–20 | 20–50 |
| 0.5–1.0 (clean) | 50–150 | 20–60 | 40–100 |
| 1.0–1.5 (moderate) | 150–300 | 50–150 | 80–180 |
| 1.5–2.0 (moderate-heavy) | 300–600 | 100–300 | 120–250 |
| >2.0 (heavy) | >600 | 200–500 | 200–400 |
Inclusion Type vs. Pitting Severity
| Inclusion Type (ASTM E45) | Typical in Titanium | Pitting Severity (relative) |
|---|---|---|
| Type A (sulfides) | Very rare | Low |
| Type B (aluminates) | Rare | Low |
| Type C (silicates) | Occasional | Moderate |
| Type D (globular oxides/Fe-rich) | Common | High |
Inclusion Control Guide
| Required Pitting Density (pits/cm²) | Maximum ASTM E45 Rating | Recommended Manufacturing |
|---|---|---|
| <20 | <0.5 | VAR melting (vacuum arc remelting) |
| 20–50 | <1.0 | Double VAR |
| 50–100 | <1.5 | Single VAR + clean feedstock |
| >100 | >2.0 | Not recommended for Br₂ service |
Engineering Recommendation
For bromine service at 50°C, specify titanium tube with ASTM E45 inclusion rating <1.0 (clean). Use double vacuum arc remelted (VAR) material for lowest inclusion content. By controlling inclusion content, the engineer reduces pitting density in aggressive bromine solutions.
