Tantalum-tungsten alloy

Tantalum-Tungsten Braun, Sedlatschek and Kieffer examined tantalum-tungsten alloys in 50% potassium hydroxide up to 80°C and in 20% hydrochloric acid at 20°C. In the alkaline solution the corrosion rate was a maximum when the tantalum was over 60 at.%. In hydrofluoric acid the alloy system exhibited the relatively low corrosion rates associated with tungsten until the tantalum concentration exceeded 80 at.%. 

Tantalum-hafnium and tantalum-zirconium alloys are less suitable for aggressive acid environments. Compared with tantalum-tungsten alloys, tantalum rhenium alloys are superior in corrosion resistance and to hydrogen embrittlement, but the major disadvantage is the high cost of rhenium. 

Corrosion rates for tantalum-tungsten alloys exposed to concentrated sulfuric acid at 181°C and 208°C as a function of tungsten content. (From M. Schussler and C. Pokross. 1985. Corrosion Data Survey of Tantalum, 2nd ed., Chicago Fansteel.)... 

The corrosion resistance of a series of tantalum-tungsten alloys was also studied in 50% KOH at 30°C and 80°C, 20% HF at 20°C, and a mixture with 1 part KOH and 3 parts K3Fe(CN)6. In the hydroxide, a maximum in corrosion rate was obtained at about 60at% tantalum. A maximum in electrical resistivity was foimd at the same composition. In 20% HF, the tantalum-timgsten alloy system exhibits the relatively low corrosion rates associated with tungsten as long as the tungsten content is at least 20%, below which corrosion rates increase markedly. In the hydroxide-ferrocyanide mixture, alloys exhibit little improvement over tantalum. 

Tantalum and 2kconium exhibit the highest corrosion resistance to HCl. However, the corrosion resistance of 2ironium is severely impaHed by the presence of ferric or cupric chlorides. Tantalum—molybdenum alloys containing more than 50% tantalum are reported to have exceUent corrosion resistance (see Molybdenumand molybdenum alloys) (69). Pure molybdenum and tungsten are corrosion resistant in hydrochloric acid at room temperature and also in 10% acid at 100°C but not in boiling 20% acid. 

Because several of the superalloys contain very little iron, they are closely related to some of the non-ferrous alloys. Some of the second- and third-row transition metals possess many of the desirable properties of superalloys. They maintain their strength at high temperatures, but they may be somewhat reactive with oxygen under these conditions. These metals are known as refractory metals, and they include niobium, molybdenum, tantalum, tungsten, and rhenium. 

Hydrogen chloride gas may be stored in steel cyhnders free of contaminants. Monel, pure nickel, or its aUoy, inconel, may also be used for storage and transportation up to 500°C. Hydrochloric acid may be stored in glass bottles or in containers made up of tantalum or tantalum-molybdenum alloys, or other alloys of zirconium, molybdenum, and tungsten. 

Tungsten alloys with tantalum in all proportions.1 Alloys of tungsten and tantalum which also contain cobalt, chromium or molybdenum have also been prepared.2... 

The corrosion resistance of various metals and alloys in high-temperature liquid lithium is shown in Figure 11. Unfortunately, lithium is much more corrosive than sodium. Consequently, it will be impossible to take full advantage of its many attractive heat-transfer properties until a satisfactory container material is found. The most corrosion-resistant pure metals in a static isothermal system are molybdenum, niobium, tantalum, tungsten, and iron. Of the commercially available structural materials, no alloys tested to date have had satisfacto corrosion resistance at a temperature above 1400 F. for extended time periods in systems where temperature differentials exist. Even though iron has good resistance in static isothermal lithium, iron and iron-base alloys suffer from mass trans-... 

For example, tantalum and molybdenum are alloyed with stainless steel to improve its corrosion resistance. Molybdenum alloys are used for extrusion dies and structural parts in space vehicles incandescent light filaments, x-ray tubes, and welding electrodes employ tungsten alloys. Tantalum is immune to chemical attack by virtually all environments at temperatures below 150°C and is frequently used in applications requiring such a corrosion-resistant material. 

Density is a particularly important characteristic of alloys used in rotating machinery, because centrifugal stresses increase with density. Densities of the various metals in Table 1 range from 6.1 to 19.3 g/cm. Those of iron, nickel, and cobalt-base superaHoys fall in the range 7-8.5 g/cm. Those alloys which contain the heavier elements, ie, molybdenum, tantalum, or tungsten, have correspondingly high densities. 

Nitrogen and carbon are the most potent solutes to obtain high strength in refractory metals (55). Particulady effective ate carbides and carbonitrides of hafnium in tungsten, niobium, and tantalum alloys, and carbides of titanium and zirconium in molybdenum alloys. 

Fused Salt Electrolysis. Only light RE metals (La to Nd) can be produced by molten salt electrolysis because these have a relatively low melting point compared to those of medium and heavy RE metals. Deposition of an alloy with another metal, Zn for example, is an alternative. The feed is a mixture of anhydrous RE chlorides and fluorides. The materials from which the electrolysis cell is constmcted are of great importance because of the high reactivity of the rare-earth metals. Molybdenum, tungsten, tantalum, or alternatively iron with ceramic or graphite linings are used as cmcible materials. Carbon is frequently used as an anode material. 

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