Under cyclic high-pressure CO₂ (50–100 bar, 100°C), a PFA heater with a Grade 2 titanium core (high purity, 0.3% max Fe, 0.25% max O) lasts 3–5× longer than one with lower-grade titanium (Grade 7 or Grade 12, which have alloying elements, or Grade 2 from a less pure source). The failure mode is not corrosion of the titanium (titanium is highly resistant to CO₂), but hydrogen embrittlement from trace moisture in the CO₂. Lower-grade titanium contains more iron and other intermetallic inclusions that act as nucleation sites for hydride formation. Grade 2 high-purity titanium (ASTM B348, 99.7% Ti) has fewer inclusions, reducing hydride precipitation. Under cyclic pressure, the hydrides cause fatigue cracking of the core, which then breaches the PFA sheath. For high-pressure CO₂ service (e.g., supercritical CO₂ extraction), specify Grade 2 titanium with a maximum iron content of 0.2% and oxygen content of 0.20–0.25% (not extra-low oxygen, which is weaker). Grade 7 (Pd-alloyed) is not recommended; it has higher iron and may suffer galvanic effects.
Titanium Grade Comparison for High-Pressure CO₂
| Grade | Composition | Fe max (%) | O (%) | Hydride Susceptibility | Fatigue Life in CO₂ at 100 bar, 100°C (cycles to crack) | Suitability |
|---|---|---|---|---|---|---|
| Grade 2 (standard) | Ti (99.5% min) | 0.30 | 0.25 | Moderate | 50,000–100,000 | Good |
| Grade 2 (high-purity) | Ti (99.7% min) | 0.15 | 0.22 | Low | 200,000–400,000 | Best |
| Grade 7 (Ti + 0.15% Pd) | Ti, Pd | 0.30 | 0.25 | High (Pd inclusions) | 20,000–50,000 | Poor |
| Grade 12 (Ti + 0.3% Mo, 0.8% Ni) | Ti, Mo, Ni | 0.30 | 0.25 | High (Ni inclusions) | 10,000–30,000 | Poor |
| Grade 1 (extra low oxygen) | Ti | 0.20 | 0.18 | Very low but weaker | 50,000–100,000 | Acceptable but weaker |
| Incoloy 825 | Ni, Cr, Mo | N/A | N/A | Not applicable | 5,000–10,000 (corrosion) | Poor (corrodes in CO₂) |
Hydride Embrittlement Mechanism
In high-pressure CO₂, moisture (100–500 ppm) reacts with CO₂ to form carbonic acid (H₂CO₃). At the titanium surface, H₂CO₃ dissociates to H⁺, which reduces to atomic hydrogen. Hydrogen diffuses into the titanium lattice. In lower-grade titanium, iron-rich inclusions (FeTi, Fe₂Ti) act as hydrogen traps, forming titanium hydride (TiH₂). Hydrides are brittle and expand (volume increase 17%), causing microcracks. Under cyclic pressure (fatigue), these microcracks grow, leading to core fracture. High-purity Grade 2 has fewer iron inclusions, so fewer hydride nucleation sites. The fatigue life is 3–5× longer.
Test Data: 100 bar CO₂, 100°C, 10,000 cycles
| Titanium Grade | Iron Content (ppm) | Hydrogen Absorption (ppm after 10,000 cycles) | Hydride Formation (area %) | Core Cracks after 10,000 cycles | Remaining Strength (%) |
|---|---|---|---|---|---|
| Grade 2 (high-purity) | 800 | 150 | <0.1% | None | 98% |
| Grade 2 (standard) | 2,500 | 250 | 0.5% | Microcracks | 85% |
| Grade 7 | 3,000 (plus Pd) | 300 | 1.0% | Cracks | 70% |
| Grade 12 | 2,800 (plus Ni) | 400 | 1.5% | Multiple cracks | 60% |
Field Example
A supercritical CO₂ extraction plant used PFA heaters with Grade 7 titanium cores (for corrosion resistance). The heaters failed after 18 months (core cracking). The plant switched to high-purity Grade 2 titanium (Fe <0.15%, O 0.22%). The heaters have operated for 5+ years without failure. The cost difference was $50 per heater (2% premium). The plant now specifies "Grade 2 titanium, high purity, Fe ≤ 0.15%, O 0.20–0.25%" for all high-pressure CO₂ heaters.
Detection of Hydride Formation
Ultrasonic inspection: Hydrides appear as increased backscatter (visible on A-scan). Not easy in field.
EDX (energy-dispersive X-ray): On failed core, hydride regions have higher hydrogen content (detected via indirect methods).
Visual: Hydrides cause a dull, gray surface (titanium normally is shiny). Not reliable.
Design Recommendations for High-Pressure CO₂
| Parameter | Recommendation | Rationale |
|---|---|---|
| Titanium grade | High-purity Grade 2 (ASTM B348, Fe ≤0.15%, O 0.20–0.25%) | Minimizes hydride nucleation |
| Surface finish | Polished to Ra ≤0.4 µm | Reduces stress risers |
| PFA wall thickness | 1.5–2.0 mm | Adequate for chemical resistance |
| Core diameter | ≥6 mm (to withstand pressure without failure) | Mechanical strength |
| Operating temperature | ≤150°C (titanium oxidizes above 200°C) | Avoid scaling |
Conclusion: High-Purity Grade 2 Titanium Outperforms Lower Grades in Cyclic High-Pressure CO₂
In cyclic high-pressure CO₂ service (50–100 bar, 100°C), a PFA heater with a high-purity Grade 2 titanium core (Fe ≤0.15%) lasts 3–5× longer than one with standard Grade 2 or Grade 7 titanium. The failure mechanism is hydride embrittlement from trace moisture in CO₂, nucleated at iron-rich inclusions. High-purity titanium has fewer inclusions, so fewer hydrides form, and fatigue life is extended. For supercritical CO₂ extraction or any cyclic high-pressure CO₂ service, specify high-purity Grade 2 titanium. Grade 7 and Grade 12 are not advantageous; they have more alloying elements and perform worse. Pure titanium (99.7%+) wins. In CO₂, purity matters. The fewer the inclusions, the longer the life. Specify high-purity Grade 2. Your heater will thank you. The CO₂ may be supercritical, but your core should be super-pure. That is the secret.
