Tantalum atmospheric corrosion

Corrosion resistance. They do not offer a good resistance to atmospheric corrosion as chromium nickel austenitic steels. Their resistance to corrosion is increased by addition of molybdenum and tantalum. 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

Derivation From tantalum potassium fluoride by heating in an electric furnace, by sodium reduction, or by fused salt electrolysis. The powdered metal is converted to a massive metal by sintering in a vacuum. Foot-long crystals can be grown by arc fusion. Corrosion resistance 99.5% pure tantalum is resistant to all concentrations of hot and cold sulfuric acid (except concentrated boding), hydrochloric acid, nitric and acetic acids, hot and cold dilute sodium hydroxide, all dilutions of hot and cold ammonium hydroxide, mine and seawaters, moist sul-furous atmospheres, aqueous solutions of chlorine. 

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. 

In one study, the corrosion resistance of tantalum in a vapor-liquid mixture from the system H3PO4-KCI-H2O containing 60-250 ppm fluoride has been investigated at 120°C and atmospheric pressure. Corrosion rates... 

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