Tantalum is number 73 on the periodic table. It has a melting point of 2996°C and a density of 16.654 gm/cc. Tantalum is one of the refractory metals that offers a valuable combination of properties.

Tantalum and its alloys are midway between tungsten and molybdenum in density and melting points. Tantalum can be worked easily at room temperature.

Its thermal conductivity is one-fourth that of molybdenum and its coefficient of expansion is one-third greater. Its elevated temperature strength is low compared with tungsten and molybdenum.

Tantalum's corrosion resistance is unusually good in most commercial combinations of acids. Pure tantalum recrystallizes at approximately 2200°F (1204°C).

Tantalum has several unique properties that have made it essential to certain applications, making it well worth the high cost. It offers approximately the same corrosion resistance to most acids and caustics as glass.

In addition, it can be fabricated by bending, roll forming, and welding with relative ease by personnel experienced with the metal. Tantalum's ductility and density have made it popular with the military for armor penetration. Its density and nuclear stability make it a valuable material for containers of radioactive elements.

Tantalum, discovered by Ekeberg, a Swedish chemist in 1802, is a metal that is closely related to niobium. Tantalum works similarly to copper in forming operations. It can be cold-formed in both the grain direction and in the cross grain direction. It can be spun, drawn, and hydro-formed. Like copper, it work-hardens and when this occurs, it requires vacuum annealing before further working. Tantalum is readily weldable by EB and, if care is taken, by TIG in a dry-box. The welds are strong and can be stress-relieved by vacuum annealing. Welded tantalum can be further formed and even drawn.

Tantalum offers excellent "gettering" properties, making it popular in vacuum tubes to absorb products of out-gassing upon heat up of the tube components. It is also used to getter potential contaminants of niobium and its alloys as well as titanium during vacuum heat treating operations. Tantalum is used in vacuum furnaces where very high temperatures must be attained and where there can be no residual oxygen or hydrogen present during the cycle.

Tantalum also provides good thermal conductivity that, combined with its corrosion resistance, has made it the ideal choice for heat exchangers for acid processing equipment. It is superior to the nickel-based alloys in both these categories.

Tantalum also develops a stable oxide that is useful in electronics industry applications. It has gained acceptance as a suitable material for mass spectrometer filaments providing an alternative to rhenium, historically the only suitable material.

Tantalum is difficult to machine, however, and should be left to those experienced with it for machining operations. Mistakes in fabricating tantalum can prove very costly. Rembar only machines and forms refractory metals and has the necessary expertise to handle tantalum in the most cost-effective ways. Rembar stocks tantalum in sheet, rod, and wire forms. Powder is also available as a mill order.

Tantalum has gained wide acceptance for use in electronic components, chemical equipment, missile technology, and nuclear reactors. The electronics industry consumes the majority of tantalum produced (approximately 60%) for capacitors. Other industries concerned with corrosion, especially the chemical processing industry, are accounting for an increasingly large percentage of the market.

Tantalum can be used to fabricate valves for corrosive liquids and to manufacture heaters for acids and heat shields for rocket motors. It is also used as a component of ion implanters in the manufacture of semiconductors. Also, because tantalum does not have a low neutron absorption cross section, it is used for radiation shielding.

Tantalum mill products are used in the fabrication of corrosion resistant process equipment including reaction vessels, columns, bayonet heaters, shell and tube heat exchangers, U-tubes, thermowells, spargers, rupture diaphragms, and orifices.

Other properties of tantalum that are useful in its application are as follows:

• Corrosion Resistance

  Tantalum is almost completely immune to attack by acids and liquid metals. It equals glass in resistance to acids and it is impervious to liquid metals up to 1650°F Only a few reagents - hydrofluoric acid, fuming sulfuric acid, and strong alkalis - will begin to break through tantalum's corrosion barrier. This ability to resist practically everything has won tantalum favor among manufacturers of chemicals, chemical equipment, instruments, heating elements, and surgical implants.

• Good Thermal Conductivity

  Tantalum conducts heat better than the nickel alloys, ductile irons, and stainless and high temperature steels. Therefore, tantalum has become the efficient heat transfer surface, especially in acidic or corrosive environments.

  The corrosion-proof heating and cooling surfaces of tantalum remain smooth and clean under conditions that foul and scale so-called acid resistant materials. This is why a tantalum heat exchanger delivers a continuously high rate of heat transfer with practically no downtime.
  

• High Melting Point: 5425°F (2996°C)

  Of the refractory metals, tantalum is outranked only by tungsten at 6170°F and rhenium at 5732°F. The high temperature strength of tantalum, combined with its workability, has resulted in the fabrication of superior heat shields, heating elements, electrodes, and other high temperature parts.
  

• Superior Gettering Characteristics

  Tantalum absorbs surrounding gases and vapors extremely well at elevated temperatures. Electronic tube manufacturers have long-recognized the ability of tantalum to maintain high vacuum inside the tubes.
  

• Good "Valve Action"

  Tantalum forms highly stable anodic films. This property, combined with tantalum's acid resistance, has been exploited by manufacturers of rectifiers, capacitors, lightning arrestors, surge suppressors and optical lenses.
  

• Tough, Durable and Ductile

  Tantalum surpasses most of the other refractory metals in ductility, workability and weldability.  As an example, tantalum can be rolled, swaged, drawn, and formed cold. In drawing, stamping or spinning, it works like steel. When stamped, forged or annealed, tantalum will not spring back. It work-hardens slowly and will not age-harden.

Two alloys of tantalum are particularly well-suited for specific applications:

• 97.5% Tantalum; 2.5% Tungsten

  This alloy is particularly useful in applications where low temperature strength is important along with high corrosion resistance and good formability.  This alloy offers higher strength than pure tantalum while maintaining the fabricability-characteristics. It is available in basically the same sizes and shapes as pure tantalum, at a comparable cost.
  

• 90% Tantalum; 10% Tungsten

  This alloy should be considered when high temperatures of up to 4500°F and high strength in a corrosive environment are required.  The alloy has approximately twice the tensile strength of pure tantalum and yet retains tantalum's corrosion resistance and a good portion of tantalum's ductility. It is not available in as many forms, particularly thin-wall tubing, as pure tantalum or the alloy above. The cost is also somewhat higher than the above alloy.

Tantalum is clearly the refractory metal that is most resistant to corrosion. The only media that can affect it are fluorine, hydrofluoric acid, sulfur trioxide (including fuming sulfuric acid), concentrated strong alkalis, and certain molten salts.

The corrosion resistance of tantalum can be compared to that of glass. However, tantalum withstands higher temperatures and offers the intrinsic fabrication advantages of a metal.

Tantalum equipment is frequently used in conjunction with glass, glass-lined steel and other nonmetallic construction materials. Tantalum is also used extensively to repair damage and flaws in glasslined steel equipment.

Media	Concentration	Temp.	Ta
Acetic Acid	50%	Boiling	Nil
Bromine	Dry	200°F	Nil
Chlorine	Wet	220°F	Nil
Chromic Acid	50%	Boiling	Nil
Hydrochloric Acid	5%	200°F	Nil
	30%	200°F	Nil
Nitric Acid	65%	Boiling	<2 mpy
Sodium Hydroxide	10%	Room	*
Sulfuric Acid	40%	Boiling	Nil
	98%	Boiling	<2 mpy

* Material may become brittle due to hydrogen attack.

Tantalum is also resistant to attack by many liquid metals such as: Li <1000°C, Na, K + NaK <1000°C, ThMg <850°C, U <1400°C, Zn <450°C, Pb <850°C, Bi <500°C and Hg <600°C.

Tantalum has the ability form stable, passive oxides and therefore, it can provide unique solutions to many corrosion problems. However, tantalum cannot be used in air at temperatures exceeding 300'C. Refer to the table entitled Corrosion Resistance for additional information.

Tantalum is extremely workable. It can be cold-worked with standard equipment. Because of its bcc crystal structure, tantalum is a very ductile metal that can undergo cold reductions of more than 95% without failure.

It can be rolled, forged, blanked, formed and drawn. It is also machinable with high speed and carbide tools using a suitable coolant. Tantalum can also be resistance welded, electron beam or tungsten inert gas welded, brazed, and riveted. Tantalum does have a tendency to stick to tooling during metal forming operations. To avoid this, specific lubricant and die material combinations are required in high pressure forming operations.

Most procedures used in working and fabricating tantalum are conventional and can be mastered without very much difficulty. However, two important characteristics of tantalum must constantly be kept in mind:

• Annealed tantalum, like copper, lead, stainless steel, and some other metals is "sticky". Therefore, they have a strong tendency to seize, tear and gall.
• All forming, bending, stamping or deep drawing operations are normally performed cold. Heavy sections can be heated for forging to approximately 800°F.

Fabricating procedures should be planned so that a minimum of scrap results.

Most sheet metal work in tantalum is performed on metal with a thickness ranging from 0.004 to 0.060 inch. The instructions given here apply to metal in this thickness range. When using metal of greater thickness, it is suggested that you contact the factory for recommendations.

Blanking or punching presents no special difficulties. Steel dies are recommended for use. The clearance between the punch and die should approximate 6% of the thickness of the metal being worked. Close adherence to this clearance is important. The use of light oil is recommended to prevent scoring of the dies. A suitable lubricant is necessary.

Form stamping techniques are similar to those used with mild steel except that precautions should be taken to prevent seizing or tearing of the metal. Dies may be made of steel except where there is considerable slipping of the metal. In this case, aluminum/bronze or beryllium/copper should be used. Low melting alloys such as Kirksite may be used for experimental work or short runs. Rubber or pneumatic die cushions should be used where required. Annealed tantalum takes a permanent set in forming and does not spring back from the dies.

Deep drawing is an operation where the depth of the draw in the finished part is equal to, or greater than, the diameter of the blank. For deep drawing operations, only annealed tantalum sheet should be used.

Note that tantalum does not work-harden as rapidly as most metals, and that work-hardening begins to appear at the top, rather than at the deepest part of the draw. If the piece is to be drawn in one operation, a draw in which the depth is equal to the diameter of the blank can be made. If more than one drawing operation is to be performed, the first draw should have a depth of not more than 40% to 50% of the diameter.

Dies should be made of aluminum bronze, although the punch may be steel if not too much slippage is encountered. Sulphonated tallow, chlorinated oil, caster oil or Johnsons No. 150 Drawing Wax are suitable lubricants.

Spinning can be accomplished using conventional techniques. Steel roller wheels may be used as tools, although yellow brass may be used for short runs. Yellow soap or Johnsons No. 150 Drawing Wax may be used as a lubricant.

Annealing tantalum is accomplished by heating the metal in a high vacuum to temperatures above 2000°F.

Tantalum parts must not be cleaned by hydrogen firing. Cleaning and degreasing present no special problems and conventional methods and materials may be used. However, note that hot caustics must be avoided. Electronic tube parts that must be chemically cleaned require more careful treatment.

The first step for tantalum parts that have been blasted with steel grit is to immerse the parts in hot hydrochloric acid to remove particles of iron. The hydrochloric acid may be used as hot and as strong a solution as desired; it will not attack the tantalum. The parts should then be thoroughly rinsed with distilled water. Tap water often contains calcium salts that may be converted to insoluble sulfates in the subsequent cleaning process. If the tantalum parts have not been grit-blasted, the hydrochloric acid cleaning may begin with the second step below.

The second step is a chemical cleaning process. A hot chromic acid cleaning solution, commonly used for cleaning glass, may be applied. A saturated solution of potassium dichromate in hot concentrated sulfuric acid may be used for this purpose. However, chromium trioxide is preferred to potassium dichromate because its use eliminates the possibility of potassium residues in crevices or elsewhere on the tantalum parts. The cleaning solution should be applied at approximately 110'C and should maintain its red color at all times. When the liquid becomes muddy or turns green, it should be discarded.

After the chromic acid wash, a third step is applied to rinse the parts. The preferred rinse is hot distilled water. If running distilled water is not available, three lip washes will suffice, but it is important that all cleaning solution be removed. The parts should be dried in clean, warm air, free from dust. The parts should not be wiped with paper or cloth and they should not be handled with fingers.

Tantalum parts for electronic tubes are often blasted with steel grit to provide a greater radiation surface. The recommended procedure is a blast of a few seconds with No. 90 steel grit, at a pressure of 20 to 40 pounds. Sand, alumina, silicon carbide or other abrasives should not be used because they become embedded in the tantalum and cannot be removed with any chemical treatment that would not also damage the tantalum. This is followed by a thorough cleaning in hot hydrochloric acid as previously described.

The purpose of grit blasting is to increase the amount of surface per unit of area. Therefore, the blasting should be performed in a manner that will produce fine "whiskers" rather than mere indentations on the surface. Sharp particles of grit will do this, while dull ones merely indent the surface. To achieve best results, the blasting nozzle should be held at an angle nearly tangential to the work, rather than perpendicular to the work.

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