Tantalum is a reactive and refractory metal. The periodic table shows it between reactive metals (titanium, zirconium, and hafnium) and refractory metals (molybdenum and tungsten). Tantalum and its sister element niobium are similar to these two elements in many ways, but also have many differences.
Tantalum reacts with carbon, nitrogen, oxygen, and hydrogen, reactions that are common in reactive metals. Tantalum’s reaction with oxygen gives it excellent corrosion resistance.
This reaction occurs at room temperature, and an oxide layer quickly forms, protecting the tantalum from further attack. However, at temperatures above 200°C, the oxide layer becomes thicker and more pronounced. The layer appears tan, contrasting with the typical gray metal. As the temperature increases, oxygen will begin to interstitially migrate through the tantalum matrix, eventually leading to oxygen embrittlement. The concentration of oxygen and external pressure affect the rate of oxidation and oxygen absorption. It is difficult to generalize, but tantalum in an oxygen-rich environment should generally only be exposed to temperatures below 200°C.
Refractory Properties
Tantalum shares many properties with the refractory metals molybdenum and tungsten. It has an extremely high melting point of 2996°C, which is in the melting point range of other refractory metals. This high melting point makes it an excellent choice for high temperature applications. However, there is one caveat. Because it reacts with carbon, nitrogen, oxygen, and hydrogen, it cannot be used in environments where these elements or their compounds are present as gases or volatile substances. Therefore, its use at high temperatures is limited to vacuum or inert gas environments.
Density, Ductility, and Formability of Tantalum
Tantalum is one of the densest metals. With a density of 16.6 g/cm (0.600 lb/in), it weighs about twice as much as an equal volume of more common materials such as nickel, copper, and steel. Tantalum has a higher density than lead, but lower than tungsten and platinum.
Tantalum is easy to bend, stretch, and form. Its annealed hardness is at the lower end of the Rockwell B hardness scale, similar to pure copper. Its yield strength is similar to many of the more common non-ferrous elements.
Corrosion Resistance of Tantalum
Tantalum is a very corrosion resistant metal. The most commonly used alloy is Ta-2.5%W, which resists most common acids in a wide range of concentrations and process temperatures.
Tantalum is commonly used in high temperature, highly concentrated sulfuric and hydrochloric acid applications and in contact with bromine. Tantalum is often used when the process uses multiple media or when the media is changed frequently in batch processing. A serious limitation of tantalum is its lack of resistance to fluorides and fluoride ions and most strong bases. In many respects, the corrosion resistance of tantalum and Ta-2.5%W is similar to that of glass.
The addition of tungsten to tantalum greatly improves the material’s resistance to hydrogen absorption and corrosion.