Embrittlement tantalum/niobium

It is not subjected to hydrogen embrittlement as is tantalum, niobium and nickel alloys, and thus is able to sustain thermal and mechanical shock after exposure to gaseous hydrogen at high temperatures. 

Niobium like tantalum relies for its corrosion resistance on a highly adherent passive oxide film it is however not as resistant as tantalum in the more aggressive media. In no case reported in the literature is niobium inert to corrosives that attack tantalum. Niobium has not therefore been used extensively for corrosion resistant applications and little information is available on its performance in service conditions. It is more susceptible than tantalum to embrittlement by hydrogen and to corrosion by many aqueous corrodants. Although it is possible to prevent hydrogen embrittlement of niobium under some conditions by contacting it with platinum the method does not seem to be broadly effective. Niobium is attacked at room temperature by hydrofluoric acid and at 100°C by concentrated hydrochloric, sulphuric and phosphoric acids. It is embrittled by sodium hydroxide presumably as the result of hydrogen absorption and it is not suited for use with sodium sulphide. 

It is somewhat less corrosion resistant than tantalum, and like tantalum suffers from hydrogen embrittlement if it is made cathodic by a galvanic couple or an external e.m.f., or is exposed to hot hydrogen gas. The metal anodises in acid electrolytes to form an anodic oxide film which has a high dielectric constant, and a high anodic breakdown potential. This latter property coupled with good electrical conductivity has led to the use of niobium as a substrate for platinum-group metals in impressed-current cathodic-protection anodes. 

Tantalum and niobium are, like zirconium and titanium, reactive metals, that is, they rely on an oxide film for corrosion protection. The corrosion behavior of tantalum is similar to that of glass, that is, it can withstand most acids, but not hydrofluoric acid and caustic solutions. Tantalum is inert to nitric acid at all concentrations up to the boiling point and is resistant to hydrochloric acid at all concentrations up to 190 °C (Schussler and Pokross, 1987). There is only one commercially important tantalum alloy (Ta-2.5% W-0.15% Nb) which has corrosion resistance equivalent to that of tantalum (Hunkeler, 1997). When tantalum is coupled with other metals in any industrial application tantalum would generally become cathodic. When exposed to nascent hydrogen tantalum readily absorbs hydrogen and it is very sensitive to hydrogen embrittlement (HE) (Dillon, 1994). The corrosion behavior of niobium is similar to that... 

Tantalum is another reactive metal with excellent corrosion-resistance in most high-temperature mineral acids it cannot, however, tolerate hydrofluoric acid and is susceptible to hydrogen embrittlement. The corrosion behavior of niobium is similar to that of tantalum its general corrosion resistance is, however, lower. 

In dynamic tests platinum and gold were resistant to attack under all conditions, but niobium corroded at an appreciable rate, about 7 mpy at 15 fps. The corrosion rate of niobium depended on the flow rate of the solution, and at higher flow rates somewhat higher corrosion rates were observed. Static tests showed that tantalum and chromium are corroded only slightly under most conditions. If the solution contained dissolved hydrogen, tantalum was seriously embrittled highly oxygenated uranyl-sulfate solutions at temperatures above 250 C oxidized chromium to the soluble hexavalent state, and under these conditions the rate of attack was several mils per year. 

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