The Corrosion Resistance of Tantalum
Tantalum is a refractory metal with a melting point of 3017°C. It is a tough, ductile metal, which can be formed into almost any shape. It is used in corrosion resistant applications in environments no other metal can withstand. The major limitation of tantalum is its reactivity with oxygen and nitrogen in the air at temperatures above 300°C.
Tantalum is the most corrosion resistant metal in common use today. The presence of a naturally occurring oxide film on the surface of tantalum is the reason for tantalum's extreme corrosion resistant properties in aggressive media. Its corrosion resistance in sulfuric acid and hydrochloric acid are second to none. It is inert to practically all organic and inorganic compounds. Tantalum's corrosion resistance is very similar to that of glass, as both are unsuitable for use in hydrofluoric acid and strong hot alkali applications.
Corrosion resistance of tantalum in sulfuric acid (H2SO4)
For this reason, tantalum is often used with glass-lined steel reactors such as patches, dip tubes, piping and overhead condensers. Tantalum is inert to sulfuric and hydrochloric acid in all concentrations below 150°C. The corrosion attack on tantalum is insignificant up to 205°C and tantalum has been in use up to 260°C.
Tantalum is not corroded by nitric acid in concentrations up to 98% and temperatures up to at least 100°C and has proven itself to be totally inert in many corrosion applications. Some heat exchanger installations have been in continuous use for over 40 years in multi-product research environments without so much as a gasket change due to corrosion.
Through the Tantaline® surface alloy, it is possible to apply corrosion resistant surface layers of pure tantalum on top of lower performance substrates like stainless steel.
Corrosion resistance of tantalum in hydrochloric acid (HCl)
Hydrogen Embrittlement of Tantalum
Despite the high corrosion resistance of tantalum, an often-reported problem is the risk of hydrogen embrittlement, which may lead to a fast and unpredictable failure of solid tantalum metal. Unlike traditional machined tantalum parts, Tantaline®'s tantalum surface alloy is not sensitive to the effects of hydrogen embrittlement.
In hydrogen embrittlement environments, the Tantaline® tantalum surface alloy offers:
- Longer lifetime than solid tantalum
- Increased operator safety
- No sudden failures of Tantaline® treated parts
- Tantaline® can be used safely in high pressure equipment
Because Tantaline®'s tantalum surface alloy is supported by a core that is not sensitive to hydrogen embrittlement, Tantaline® tantalum is safe in situations where solid tantalum components will fail due to hydrogen embrittlement.
The Corrosion Properties of Hastelloy®* (nickel alloys)
Nickel alloys (often referred to as Hastelloy®*) are a common material used when typical steel materials do not offer the corrosion performance needed. In general, Hastelloy®* offers a better corrosion performance than titanium. Only a few metals like titanium and zirconium are superior to Hastelloy®* for corrosion resistance in demanding media like sulfuric acid and hydrochloric acid.
When dealing with aqueous solutions to enhance the performance of nickel materials, the most important alloying elements are Fe, Cu, Si, Cr and Mo - with Cr and Mo playing a major role in nickels' corrosion resistance. By varying the concentrations of Cr and Mo in nickel alloys, the corrosive environments in which nickel alloys can be successfully applied are varied, but they are typically found in a range of acid, salt and alkali applications. The additions of chromium (15-30%) improves the corrosion resistance to oxidizing solutions, while the addition of molybdenum (up to 28%) significantly improves the resistance to non-oxidizing acids.
Focusing on some of the more corrosion resistant nickel alloys, C22, C276 and B2, they all have good corrosion resistance in a variety of media. In the case of hydrochloric acid (HCl), the corrosion resistance of these alloys depends greatly on the molybdenum content. The alloy with the highest concentration of molybdenum, B2, exhibits the best corrosion resistance.
Alloy concentrations details for C22, C276, and B2
Approximate Alloy Concentrations
In other solutions, such as nitric acid (HNO3), chromium is an essential alloying element responsible for providing the corrosion resistance in this environment. The weaknesses of nickel alloys evolve around the interaction with the media and its environment in the form of impurities. Under ideal testing conditions, B2 alloy, for example, works well in pure de-aerated H2SO4 and HCl but deteriorates rapidly when oxidizing impurities, such as oxygen and ferric ions, are present. Another important consideration is the presence of chlorides (Cl-). Chlorides generally accelerate the corrosion attack, but the degree of acceleration differs for various alloys.
The Corrosion Resistance of Titanium
Titanium is an established metal when dealing with corrosion applications. Titanium is available in a range of different alloys with the most corrosion resistant grades being titanium 7,11 (containing 0.15% palladium), and 12 (containing 0.3% Mo and 0.8% Ni).
In the chemical processing industry, titanium and its alloys offer good corrosion resistance in many process solutions and owe their corrosion resistance to a strong oxide film. The oxide film formed on titanium is more protective than on stainless steel, and it often performs well in media such as seawater, wet chlorine and organic chlorides. While titanium offers good corrosion resistance to these solutions, it certainly is not immune to them, especially at elevated temperatures, such as seawater at temperatures greater than 110°C.
Titanium solutions can be found in a variety of industries including chemical processing, pulp and paper, and marine applications. A major use for titanium is in seawater or brackish water applications. It is also used extensively in the production of chlorine.
The Corrosion Properties of Zirconium
Zirconium alloys exhibit excellent resistance to corrosive attack and work well in many organic and inorganic acids, salt solutions, strong alkalis, and some molten salts.
Zirconium owes its corrosion resistance to the natural formation of a dense, stable, self-healing oxide film on its surface. Unalloyed zirconium has excellent corrosion resistance to sulfuric acid up to 60% concentration at the boiling point and has excellent corrosion resistance in hydrochloric acid.
Zirconium is also highly resistant to most alkali solutions up to their boiling point. Zirconium’s corrosion resistance could be compared with titanium in many ways; however, zirconium is much more robust than titanium in withstanding organic acids, such as acetic, citric, and formic acids at various concentration and elevated temperatures.
For all of zirconium’s attributes it still can be corrosively attacked by fluoride ions, wet chlorine, aqua regia, concentrated sulfuric acid above 80% concentration, and ferric or cupric chlorides.
*Hastelloy® is a registered trademark of Haynes International