Ultramet diffuses highly corrosion-resistant tantalum into the surface of conventional stainless steels and nickel-based superalloys to dramatically improve resistance to acid corrosion, particularly nitric acid. Applications include valve components and piping used for the transfer of acids and other corrosive liquids.
Benefits of Tantalum Metal
- Provides extraordinary corrosion resistance because a tough impermeable oxide film forms on the surface when the metal is exposed to normal atmospheric conditions
- Exhibits a uniform grain structure
- Is impervious to chemical attack below 302°F (150°C)
- Is vulnerable only to hydrofluoric acid, acidic solutions containing the fluoride ion, and free sulfur trioxide
- Melts at 5425°F (2996°C)
- Exhibits high strength and ductility
- Demonstrates corrosion resistance comparable with that of glass
Advantages of Diffusion Coatings
- Thin (1–3 µm) surface layer that grades from pure tantalum at surface to a mixture of tantalum and substrate elements
- High bond strength, because tantalum is diffused into the substrate surface and chemically mixes with substrate elements
- Precise replication of intricate substrate features that requires no postprocess machining or polishing
- Little or no change to the treated substrate physical dimensions
- Original surface roughness maintained
- High corrosion resistance
- Application to various steels, chromium-coated materials, and nickel-based superalloys
- Ability to combine effectively with a conventional chromium coating to substantially increase the lifetime of the chromium coating
- Application to net-shape products as a final production step
The diffusion coating process itself requires no costly equipment, is readily scalable to large batches, is rapid (deposition times of <1 hour), and is economical because little tantalum is required. Ultramet developed this process under NASA funding to protect space shuttle propellant valves.
The tantalum diffusion coating significantly enhances the corrosion resistance and lifetime of chromium-coated components. Tantalum fills the microcracks that inevitably exist in chromium coatings and prevents corrosive liquids from reaching the component surface.