ABSTRACT
Tantalum is a metal little known to most of the metallurgical community. Its properties are often misunderstood and misrepresented. A general description of the properties of Tantalum (Ta) and its alloy Tantalum with 2.5%W addition (Ta-2.5%W), are discussed along with its corrosion, machining and welding properties. In order to give a more complete picture, several typical uses are described.
KEYWORDS
Tantalum, properties, uses, Ta-2.5%W, corrosion, reactive metals, refractory metals, sulfuric acid, hydrochloric acid
INTRODUCTION
Tantalum is an unusual metal to most people in the metallurgical community. Because of its obscurity, its properties are often unknown or misunderstood. Frequent questions are often heard, such as “What kind of steel is that?” or “Isn’t tantalum one of the hardest metals?” This paper describes some of the properties of tantalum with the intent of removing some of the confusion that exists. Table IV at the end of this article gives most of the general properties of tantalum in a tabular form.
CLASSIFICATION OF TANTALUM
Tantalum is a reactive metal and a refractory metal. The periodic table of the elements shows that it lies between the column of reactive metals (Ti, Zr, and Hf) and refractory metals, (Mo and W). Tantalum and its sister element niobium are similar to both groups in many ways and different in just as many ways. See Figure 1.
Figure 1. Relationship of tantalum to the reactive and refractory metals (Periodic Table Extract).
Tantalum will react with carbon, nitrogen, oxygen and hydrogen (1), see Figure 2. These reactions are common with the reactive metals. Tantalum’s reaction with oxygen gives it excellent corrosion resistant properties and will be discussed later.
Figure 2. Reaction of tantalum with oxygen (1)
This reaction takes place at room temperature, and an oxide layer forms quickly and protects the tantalum from further attack. However, at temperatures over about 200 C, this oxide layer becomes thicker, and more noticeable. The layer appears as a tan color in contrast to the typical gray metal. As temperature rises, oxygen will begin to migrate interstitially through the tantalum matrix eventually causing oxygen embrittlement. The concentration of the oxygen and the external pressure affect oxidation rate and oxygen absorption rate. Generalizations are difficult, but tantalum in oxygen-rich environments is often limited to exposure below about 200 C.
Similar reactions take place with nitrogen, carbon, and hydrogen. Hydrogen embrittlement is a significant problem for tantalum and will be discussed under corrosion resistance.
Refractory Characteristics
Tantalum has many properties in common with the refractory metals, molybdenum and tungsten. It has an extremely high melting point of 2996 C, which is the range of other refractory metals. This high melting point makes it an excellent selection for use in high temperature applications. However, one caution applies; due to its reaction with carbon, nitrogen, oxygen, and hydrogen, it may not be used in environments where these elements or their compounds are present as gases or volatile substances. Thus, it used at high temperature is limited to vacuum or inert gas environments.
Density, Ductility and Formability of Tantalum
Tantalum is among the densest metals. With a density of 16.6 g/cm³ (0.600 lb/in³ it weighs about twice as much as an equal volume of more common materials such as nickel, coper and steel. Ta has a higher density than lead and but less dense than tungsten & platinum; see Figure 3
Figure 3. Density of selected metals
Tantalum is easy to bend, stretch, and form. Its annealed hardness is on the low end of the Rockwell B scale and is similar to pure copper. Its yield strength is similar to many more commonly nonferrous elements, as shown in Figure 4.
The common alloy of tantalum, Ta-2.5W, has a higher yield strength and can provide more strength with thinner sections. This greater strength is provided while maintaining good ductility and formability.
Machining and welding Tantalum
Tantalum and tantalum alloys can be challenging to weld and machine. Their high melting point and reacting with gases present a challenge to a fabricator. However, experience at Apex Engineered Products has shown that while tantalum and Ta-2.5%W are different from other alloys, their fabrication is not necessarily difficult, if the right procedures are followed.
Table I
Machining tantalum successfully requires experience. High speeds and low coolant rates will generate heat and allow the tantalum to react with carbon, oxygen, and hydrogen to form extremely hard particles. These particles quickly dull the tool and cause more heat to be generated. Tantalum is also a soft gummy material that gels easily. To compensate for these properties, very slow turning speeds should be used and copious amounts of cooling fluids re needed. Water-soluble oils are adequate for most cooling; fluorocarbon lubricants can provide added benefits when needed. Table I (2) shows some recommendations.
Welding tantalum is usually done by the GTAW (TIG) process using a tungsten electrode and argon cover gases. Extra attention must be paid to keep air away from any heated surfaces. Welding should be done in high purity argon or helium filled glovebox. Alternately, argon or helium can be used with long trailing shields and backside shields (or backside purge). Precautions must be taken to prevent exposure of warm surfaces to air as weld embrittlement may occur. Both tantalum and its common alloy with 2.5% tungsten (Ta-2.5%W), can be welded in this manner. Due to the similarity of melting temperatures of tantalum and tungsten, the alloy will not segregate during solidification. Therefore, post weld heat treatments are not necessary.
In contrast to materials such as the B and C families of nickel alloys, Ta-2.5%W, maintains its wrought metal corrosion resistant properties in the weld. Due to segregation, the nickel alloys will always have corrosion properties in the weld that are significantly inferior to the published wrought alloy corrosion data.
The cleanliness of the surface of the tantalum can greatly affect weld quality. Hydrocarbons (oils) can react during welding to form brittle hydrides and carbides. The surface of tantalum should be degreased prior to welding.
Other methods such as electron beam welding (EBW) and plasma arc welding (PAW) can also be used. The same precautions to keep hot surfaces from exposure to air apply.
Corrosion resistance of Tantalum
Tantalum is an extremely corrosion resistant metal. Most commonly used as alloy Ta-2.5%W, it is resistant to most common acids in a variety of concentrations and process temperatures. Tale II (3) shows a summary of corrosion rates.
Tantalum is often used n high temperature, high concentration sulfuric and hydrochloric acid applications and in contact with bromine. Often tantalum is used when the process uses a variety of media or where the media are often changed as in batch processing. A serious limitation of tantalum is its lack of resistance to fluorides and fluoride ions as well as to most strong alkalis. In many ways the corrosion resistance of tantalum and of Ta-2.5%W is similar to that of glass.
Tantalum is often the material of choice when other materials fail. A good example is in sulfuric acid service. Figure 5 is an iso-corrosion chart for many materials in sulfuric acid (4, 5). The lines represent the temperatures and concentrations at which a 0.13mm per year or 5 mils per year (mpy) corrosion rate takes place.
Figure 5. Iso-Corrosion chart - sulfuric acid 0.13 mm/yr (0.005 in./yr) (4, 5)
From the chart it is clear that many metals can perform at lower temperatures and lower concentrations. Tantalum is clearly superior at high temperatures and high concentrations. This is often the case for other acid media and tantalum can be solution to serious corrosion difficulties in these media.
Figure 6 shows a bayonet heater manufactured by Apex Engineered Products for a sulfuric acid application. The entire unit is made of Ta-2.5%W, except for the tube sheet, which is loose-lined with Ta and Ta-2.5%W alloy 0.020”-0.024” (0.51 – .64mm) thick. Typical tube sizes for a bayonet heater are 1” (25.4 mm) diameter & wall thickness of 0.015”-0.025” (0.38-0.64 mm).
The most serious cause failure with tantalum in acid media is from hydrogen embrittlement. Ta & Ta-2.5%W reacts with hydrogen easily and forms a brittle solid solution matrix. Generally free hydrogen is not present in process media, but hydrogen can be liberated by galvanic reaction shown in Figure 7 at lower aqueous liquid temperature applications.
The combination of two dissimilar metals in an aqueous solution will generate hydrogen at one of the metals. Breaking the electrical connection between the tantalum from other metal can stop this galvanic cell and is the best solution to the problem. A non-conducting gasket placed between the metals will generally work. If this is not possible, Apex Engineered Products may be able to offer the use of an electroplated platinum spot that forces the hydrogen liberation to the platinum surface and minimizes the effect of the hydrogen on the tantalum (see Figures 8 and 9). The platinum spot addition is effective for an area ratio of tantalum to platinum of up to 10,000 to 1, but is typically applied in an area ratio of 1000 to 1 for fabricate equipment. The cost of platinum spotting is extremely small and is worth the effort to provide added hydrogen embrittlement resistance in extremely aggressive solutions.
Figure 10. Corrosion rate of tantalum and Ta-2.5%W in sulfuric acid, with and without platinum spots
A recent corrosion-testing program carried out by Apex Engineered Products has shown some very promising results when testing pure Ta and Ta-2.5%W, materials, with and without Pt spots, at high temperature in high concentrations of sulfuric acid. Table III shows the results of the test program. For a more robust solution Ultra 76® Tantalum alloy8 with a platinum series alloy additive provides excellent hydrogen embrittlement resistance as well as a stepwise improvement in corrosion resistance in HCl and H2SO4 environments compared to the corrosion rate of other Ta and Ta alloys illustrated in Figure 10 above.
The addition of tungsten to tantalum greatly improves the hydrogen absorption resistance as well as improving the corrosion resistance of the material, as shown in Figure 10. This has also been confirmed by an extensive study recently completed by Bayer AG. (Note: hydrogen absorption is typically not an embrittlement problem with tantalum or Ta-2.5%W below about 100 ppm.)
Typical Uses of Tantalum
Tantalum sees a wide variety of uses in today’s world. The largest single use is in tantalum capacitors. Anodized tantalum power capacitors are used in many common electronic devices today. Most other applications use the corrosion resistance of tantalum or its high temperature resistance.
The pharmaceutical industry finds tantalum very attractive because of its very low corrosion resistance. Corrosion by-products can contaminate process media and introduce contaminants into products. Figure 11 shows typical helical condensers designed and fabricated by Apex Engineered Products used in the pharmaceutical industry.
In the chemical process industry, very few materials can outperform tantalum in acid media. Tantalum is commonly used as tank and pipe linings for process vessels, as a heat exchanging material in shell and tube heat exchangers, condensers, and heaters. It is often used as autoclave liner in many high temperature, high-pressure applications. Tantalum can also be used to repair glass-lined vessels. Tantalum is often assumed to be used only in small equipment applications. However, tantalum equipment can be quite large. Figure 12 is a typical example of a larger application. This tantalum lined column (72 inches dia. X 117 feet long) replaced acid brick, which was very expensive to maintain and was causing significant downtime for the customer.
Tantalum is also used for heating elements and heat shields in high-temperature vacuum furnaces, as well as a crucible material for many high-temperature applications.
The thin film industry uses tantalum as a sputtering target source material for many electronic applications.
Tantalum also sees uses in the medical field for hemostat clips, stents, and pacemaker parts. Due to its expellant corrosion resistance, Tantalum has been found in the inert in the human body.
CONCLUSION
Industry has long found tantalum and its alloys to be a solution to some of its most difficult problems. Tantalum is known to have been in operation for decades in situations where any other material fails quicky. Often tantalum is chosen over less expensive materials because it offers long-term service and reduced downtime. Figure 13 is an excellent example of the short life cycle payoff with tantalum versus graphite. This case study by Zeneca Engineering shows the long-term savings by switching to tantalum (6).
Figure 13. Heat exchanger/condenser cost (6)
Tantalum is also versatile, easily fabricated material that can often outperform almost any alternative. While its properties are not widely known, and often misunderstood, and in some cases somewhat different from those of more common materials, it often supplies industry with practical solutions to difficult problems.
Some properties of tantalum are given in Table IV (2) in the References section below.
A - Atomic and Crystallographic Properties of Tantalum
B - Thermal Properties of Tantalum
C - Thermal Conductivity of Tantalum
E - Miscellaneous Properties of Tantalum
REFERENCES
1. G.L. Miller, “Tantalum and Niobium”, London: Butterworth Scientific Publications, (1959), p. 488
2. “Tantalum Metallurgical Products”, Newton, MA, H.C. Stack, Inc.
3. Corrosion Resistance of Tantalum and Niobium Metals”, Newton, MA, H.C. Starck, Inc.
4. M. Coscia and M.H.W. Renner, “Corrosion of Tantalum and Tantalum-2.5% Tungsten in Highly Concentrated Sulfuric Acids”, Materials Performance, 37(1) 52-57, (1998)
5. K. Moser, “The Manufacture and Fabrication of Tantalum”, Journal of Metals, April 1999, pp. 29-31, Figure 2
6. Zeneca Engineering, Manchester, U.K., Presented to The Institute of Materials, London, (October 1995)
7. Touloukian, Powell, Ho, and Klemens, Thermophysical Properties of Matter, Vol. 1, (Thermal Conductivity-Metallic Alloys), IFI/Plenum, New York-Washington, (1970), p. 362
8. Ultra 76® Tantalum is a Registered trademark of H.C. Starck Inc.
9. H.C. Starck Inc., Paul R. Aimone & Evan B. Hinshaw, “Tantalum Based Alloy That Is Reistant To Aqueous Corrosion”, US Patent 11,001,912 B2, U.S. Patent Office, May 11, 2021
10. K. Moser & E. Hinshaw, H.C. Starck Inc., “Tantalum: Properties and Typical Uses”, Technical Reprint, April 2000, pp. 183-188
Apex Tantalum Fabrication
Examples
Cleaning & Servicing Tantalum Heat Exchanger
Tantalum Heat Exchanger Fabrication
