What Is Tantalum? Acid Service Material
When to Specify Tantalum
Tantalum is a refractory metal with corrosion resistance comparable to glass in virtually all acid environments below 150°C. It resists hydrochloric acid, sulfuric acid, nitric acid, and most organic acids at concentrations and temperatures where zirconium, titanium, and nickel alloys fail. Tantalum is specified for the most extreme chemical process applications where equipment reliability justifies the very high material cost.
| Property | Value |
|---|---|
| Symbol | Ta |
| Atomic number | 73 |
| Density | 16.65 g/cm3 (twice the density of steel) |
| Melting point | 2996°C (5425°F) |
| Tensile strength | 345 MPa (50 ksi); annealed |
| Yield strength | 207 MPa (30 ksi); annealed |
| Elongation | 25% min |
| Thermal conductivity | 57.5 W/m-K |
| Max service temp | 150°C in acids; 300°C in air (oxidation onset) |
| Common grades | R05200 (unalloyed), R05400 (Ta-2.5W), R05252 (Ta-10W) |
Corrosion Resistance
Tantalum forms a stable, self-healing tantalum pentoxide (Ta2O5) film with properties similar to glass. Corrosion rates below 0.025 mm/year (<1 mpy) in:
| Environment | Concentration | Temperature | Tantalum Performance |
|---|---|---|---|
| HCl | All (to 37%) | To boiling | <0.025 mm/yr |
| H2SO4 | Up to 98% | To 200°C | <0.025 mm/yr |
| HNO3 | All | To boiling | <0.025 mm/yr |
| H3PO4 | All | To 150°C | <0.025 mm/yr |
| Acetic acid | All | To boiling | <0.025 mm/yr |
| HF | Any | Any | Rapid attack; avoid |
| Alkali (NaOH/KOH) | >10% | >50°C | Attacked; avoid |
| Fuming H2SO4 (oleum) | >98% | >150°C | Attacked |
Tantalum is vulnerable to hydrofluoric acid, fluoride-containing solutions, strong alkalis, and fuming sulfuric acid above 150°C. In HF service, Monel 400 is the standard alternative.
Tantalum vs Competing Materials
| Acid Environment | 316L | Alloy 20 | C-276 | Zirconium | Tantalum |
|---|---|---|---|---|---|
| HCl (20%, 80°C) | Fails | Fails | Good | Excellent | Excellent |
| H2SO4 (50%, boiling) | Fails | Good | Moderate | Excellent | Excellent |
| HNO3 (65%, boiling) | Good | Good | Moderate | Excellent | Excellent |
| Mixed HCl + H2SO4 | Fails | Fails | Moderate | Good | Excellent |
ASTM Specifications
| Product Form | ASTM Specification |
|---|---|
| Seamless pipe and tube | B521 |
| Plate, sheet, strip | B708 |
| Bar and rod | B365 |
| Forgings | B364 |
| Wire | B365 |
Applications
- Pharmaceutical manufacturing: reactor linings, heat exchangers, and piping for ultra-pure acid synthesis
- Chemical processing: HCl recovery, sulfuric acid concentration, bromine production
- Electronics: capacitor manufacturing, semiconductor processing equipment
- Bayonet heaters: immersion heaters in acid baths (utilizing tantalum thermal conductivity)
- Glass-lined vessel repair: tantalum plugs and patches replace damaged glass lining
Cost and Practical Considerations
Tantalum costs 30-100x more than stainless steel per kilogram. To manage costs, tantalum is most commonly used as:
- Thin linings (0.3-1.5 mm) bonded or loose-fitted inside carbon steel or stainless steel shells
- Bayonet heater tubes where tantalum provides both corrosion resistance and heat transfer
- Small-bore piping and instrument connections in critical acid circuits
Full solid tantalum construction is limited to small-diameter pipes, fittings, and instrument components.
Testing
Mill test certificates must report chemistry, mechanical properties, and grain size. NDT includes radiographic and ultrasonic examination. Hydrostatic testing follows ASTM B521 or the applicable equipment standard.
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