Tantalum corrosion behaviour

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Tantalum is one of the most versatile corrosion-resistant metals. Its corrosion behaviour can be compared with that of glass in most environments. This behaviour is attributed to the stable passive film of TajO, produced on the surface during exposure. 

Table 5.26 The corrosion behaviour of tantalum-molybdenum alloys in concentrated sulphuric and hydrochloric acids at 55°C solutions saturated with oxygen...

Levesque A, Bouteville A (2001) Evaluation of corrosion behaviour of tantalum coating obtained by low pressure chemical vapour deposition using electrochemical polarization. J Phys IVI1 Pr3-915-920... 

Zein El Abedin, S., Welz-Biermann, U., and Endres, F. (2005) A study on the electrodeposition of tantalum on NiTi alloy in an ionic liquid and corrosion behaviour of the coated alloy. Electrochem. Commim, 7(9), 941-946. 

Wang X, Zeng X, Wu G, Yao S, Lai Y, (2007), Effects of tantalum ion implantation on the corrosion behaviour of AZ31 magnesium alloy , Journal of Alloys and Compounds, 437, 87-92. 

On the other hand, metals such as Ta, Nb, Ti, Zr, Al, etc. (the valve metals ) do not exhibit transpassive behaviour, and in appropriate electrolyte solutions film growth at high fields rather than corrosion and/or oxygen evolution is the predominant reaction thus aluminium can be anodised to 500 V or more in an ammonium borate buffer titanium can be anodised to about 400 V in formic acid and tantalum can be anodised to high voltages in most acids, including hydrochloric acid. 

It is in its behaviour to caustic alkalis that zirconium shows itself to be superior to those other elements of Groups IV and V whose resistance to corrosion results primarily from an ability to form surface films. Thus, in contrast to tantalum, niobium and titanium, zirconium is virtually completely resistant to concentrated caustic solutions at high temperatures, and it is only slightly attacked in fused alkalis. Resistance to liquid sodium is good. Zirconium is thus an excellent material of construction for sections of chemical plant demanding alternate contact with hot strong acids and hot strong alkalis—a unique and valuable attribute. 

The anodic behaviour of platinum and certain of its alloys is of considerable technical importance, since they can be employed under a wide range of conditions without appreciable corrosion, and often in circumstances where no other metal can be used. Their use industrially has recently been extended by applying them as thin coatings to a substrate of a passive metal such as tantalum or, more commonly nowadays, titanium, to reduce the cost. Platinised titanium anodes are discussed in detail in Section 11.3. 

A hterature review was conducted, which provided some materials as candidates for the section I (Bunsen) of the process ceramics (SiC, Si3N4, AI2O3), glass, fluocarbons, Tantalum and Zirconium or Ni alloys. However, corrosion tests are necessary to assess the maximum temperature and acidity acceptable conditions, the long tenn behaviour and the corrosion mechanisms. 

In the methods used for the investigation of corrosion resistance, biocompatibility and bioadhesion the researchers try to simulate and imitate the natural in vivo condition of the implant. Only in the near past have efforts been made to standardize these tests. Because of a longtime decline of standardization the tests described up until now in literature differ and the results of such diversified tests are not comparable. Corrosion measurements, for example, are performed in different solutions with changing pH values and atmospheres (aerated or de-aerated). Only if different materials have been investigated in one test and under the same conditions does a comparison of their behaviour for this test seem possible. Nevertheless, regarding the differing test results, the most corrosion resistant materials seem to be the special metals (titanium, niobium, tantalum and their alloys), followed by wrought CoCr-based, cast CoCr-based alloys and stainless steel. 

Nickel is widely applied as structural material in molten salts owing to its high corrosion resistance, including a use in sodium-thermic production of tantalum powders. In this connection it is of interest to consider its behaviour in melts containing tantalum ions. For the experiments KCl-NaCl(l l mole), (Cs-K-Na)Cl(eutectic), KCI-KF(3 2 mole), (Li-Na-K)F(eutectic) melts containing 2.5% mole of potassium heptafluorotantalate K2TaF each were selected. 

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