Tantalum is an unusual metal for most people in the metallurgical world. Due to its obscurity, its nature is often unknown or misunderstood. A common question is "What kind of steel is that?" Or "Isn't tantalum one of the hardest metals?" This paper describes some properties of tantalum in order to eliminate some confusion. Table IV at the end of this article gives most of the general properties of tantalum in tabular form.
Tantalum is an active and refractory metal. The periodic table shows that it lies between the active metals (titanium, zirconium and hafnium) and the refractory metals (molybdenum and tungsten). Tantalum and its sister element niobium are similar to these two elements in many ways, but there are also many differences. Tantalum reacts with carbon, nitrogen, oxygen, and hydrogen. These reactions are common in active metals. The reaction of tantalum with oxygen gives it excellent corrosion resistance.
This reaction occurs at room temperature, and an oxide layer quickly forms, protecting the tantalum from further erosion. However, when the temperature exceeds 200°C, the oxide layer becomes thicker and more pronounced. The layer appears tan in contrast to the typical grey metal. As the temperature increases, oxygen will begin to migrate through the tantalum matrix, eventually leading to oxygen embrittlement. Oxygen concentration and external pressure affect the rate of oxidation and oxygen absorption. It is difficult to generalize, but tantalum in oxygen-rich environments can usually only be exposed to below 200°C.
Tantalum shares many properties with refractory metals molybdenum and tungsten. It has an extremely high melting point of 2996°C, which is the melting point range of other refractory metals. This high melting point makes it an excellent choice for high temperature applications. There is, however, 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.
