Refractory Metals Tantalum, Niobium, M o l y b d e n u m , Rhenium, a n d Tungsten
I
CLIFFORD A.
HAMPEL
Consulting Engineer, Skokie, 111.
High melting points, inertness, and good strength of these metals at high temperatures make them unusually interesting to the chemical industry
T U N G S T E N , RHENIUM. tantalum, molybdenum. and niobium are referred to as refractory metals because of their high melting points and their good retention of strength at elevated temperatures. They are not refractory in the chemical reactivity sense, because a t trmperatures much above a few hundred degrees Centigrade they become quite reactive with a large number of reagents, including air. However, each exhibits fine resistance to chemical attack by a wide variety of specific reagents in the temperature range encountered in chemical solution processes. This is one reason they are of unusual interest to the cheniical industry. These metals are all transition metals and are located in Groups V A (niobium and tantalum), VI A (molybdenum and tungsten), and VI1 A (rhenium) of the periodic table. The availability of thrse five metals varies considerably (9, 75). Xobium, molybdenum. and tungsten are in plentiful supply such that most conceivable applications could be met without difhcult). Production quantities or well over 10,000,000 pounds per year could be made available with 2 to 5 years lead time. Tantalum is much more limited in availability with a top availability of only tbvo or three times the current production of 500,000 pounds per year of total contained metal. Rhenium is quite scarce with only 10,000 to 30,000 pounds per year potentially available. As to the position of the United States o n the supply situation, we depend o n domestic sources for moll bdenum and rhenium, and upon foreign sources for most of our tungsten and niobium and all of our tantalum (75).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Raw Materials Molybdenum is obtained in Colorado from molybdenite ores which contain small concentrations of molybdenite or molybdenum disulfidc, MoS?. Molybdenite is also recovered as a by-product from copper operations in Arizona, Nevada, and Utah, and from tungsten operations in California. The United States has been the source of S5Y0 of the world output since 1925 ( 75). Separation of the highly disseminated mineral from gangue is accomplished by fine grinding and flotation to yield a concentrate containing 60 to 95% molybdenum disulfide. Some molybdenites contain rhenium in concentrations of 0.002 to 0.2%. The two rhenium sources in the United States are the flue dusts of the molybdenite roastinq operations of Miami Copper Co. of Miami. Arizona. and Kennecott Copper Corp. ( 7 5 ) .
Tungsten is obtained chiefly from ores containing kvolframite, ferrous-manganous tuqstate, Fe(Mn)W04, and scheelite, Ca\VO,. Most ores are low grade, containinq 0.4 to 2.5y0, and rarely more than 2y0 \ V 0 3 , which are concentrated to a minimum content of 60% TVO3. The operations used include steps such as crushing, grinding, milling, magnetic sepa-
These Metals Aren't Cheap
rations, flotation, tabling, and acid leaching. I n addition to concentrates produced in California, Nevada, Colorado, Idaho, and North Carolina, imports from Brazil, Peru, Bolivia, Australia, Korea, and Canada have provided the larger share of the domestic demands for tungsten ( 7 5 ) . Tantalum and niobium are derived from tantalite-columbite minerals found chiefly in pegmatites. The two elements are always found associated with each other, much the same as zirconium and hafnium. Niobium is about 11 times as prevalent as tantalum, however. The most important mineral source is a ferrous-manganous tantalite-columbite, (Fe,Mn)(Ta.Nb)?Os. If the TaaOj content exceeds the Nb20j content, the mineral is called tantalite, and if the reverse is true. it is called columbite. In pegmatic dikes or alluvial deposits the quantities of these minerals seldom exceed a few pounds per ton. Concentrates containing 60y0 or more combined pentoxides are obtamed by hand separation, washing, tabling, and electrostatic and electromagnrtic means. Mining of tantalum and niobium is predominantly a foreign industry. Belgian Congo, Brazil, Mozambique, Rhodesia, and .4ustralia are the principal sources of tantalum ore. Nigeria, Malaya, Belgian Congo, Xorway, and Brazil are the chief producers of niobium ore. Idaho is currently producing a small amount of niobium concentrate.
Metal Production
Price $,'cu.
Form
Sheet
Strip
Metal Tungsten Molybdenum Niobium Tantalum Rhenium
$/lb.
in.'
17 11 45 58 900
11.85 4.35 17.90 27 675h
n In most cases these metals are used to provide a certain volume rather than weight. b About 75% more than gold and almost four times as much as palladium.
The processes for extracting the pure metals from the concentrates vary with the metal, of course, but consist of three distinct stages: preparation of a pure compound by chemical treatments of the concentrates; reduction of this compound to metal powder; and consolidation of the powder into massive metal ingots. Tungsten. T h e two classes of tungsten concentrates require different chemi-
LESS COMMON E L E M E N T S cal treatments to produce pure tungstic oxide or tungstic acid which is subsequently reduced to metal powder. LYolframite concentrate, which contains ferrous and,'or manganous tungstate, F e W 0 4 or MnW04, is decomposed with fused alkali carbonate or aqueous alkali hydroxide to form soluble tungstates and a precipitate of iron and manganous hydroxides. If the solution of alkali hydroxide is used, this step can be conducted under pressure or in open tanks, the latter being the slower method. Purification of the soluble tungstate is accomplished by recrystallization. and the tungstate is converted to insoluble tungstic acid, HzW04. by neutralization with hydrochloric acid Other intermediate salts can be formed for purification purposes, among them ammonium paratungstate, (NH4)6W7024.6H20, and calcium tungstate, C a \ f 0 4 . Scheelite concentrates are treated with hydrochloric acid to form insoluble tungstic acid which is purified by dissolving it in sodium hydroxide. The sodium tungstate formed is converted to ammonium paratungstate which is crystallized from the filtered solution and can be further purified by recrystallization. While all tungsten destined to be used as metal is reduced with hydrogen (see illustration), some tungsten. which is to be converted subsequently to cast tungsten carbide. is reduced with carbon or hydrocarbons. The screened and blended tungsten poivder is compacted into chalklika, green ingots with steel dies in a press a t pressures of 15 to 30 tons per square inch. These compacts, typically 1 to 4 square inches in cross section and 24 to 30 inches long. are sintered under hydrogen b\r passing electric current through
them. Temperatures of about 3200' C. are reached in the sintering operation. Larger ingots (several hundred pounds) can be made by compacting poivder in a rubher bag under hydrostatic pressure, and sintering these large compacts by radiation heating to 1800 to 1950' C. under hydrogen atmosphere. Tungsten can be arc-melted by feeding a presintered consumable electrode into a vacuum furnace. Ingots of 50 pounds or more have been made. Because of the large grain size these must be extruded first, using glass as a lubricant. An 8 3 7 , reduction a t 1400" C. is achieved. The metal can be rolled to sheet a t 100" C. Sintered ingots, which are brittle a t room temperature, are reduced by hot swaging or rolling. Wire is draivn from bars that have been swaged to about 0.050 inch in diameter, and carbide dies followed by diamond dies are used as the size reduction proceeds. The temperature is maintained a t 400" to 800' C., the lower temperature for the smaller sizes of wire. Thin sheet is made by pack-rolling or with a pro-
tective metal envelope to reduce oxidation during hot rolling. The temperature to which tungsten must be heated during cold workingi.e,>below the recrystallization teniperature-varies with the metal thickness: the greater the thickness, the higher the temperature. Heating is done in a hydrogen or cracked ammonia atmosphere. and if the reduction steps are rapid, little oxidation of the metal occurs. Molybdenum, Molybdenite concentrates, containing 607, or more MoS2! are roasted in air at about 1000' C. to convert the sulfide to molybdenum trioxide, Moo,. which sublimes at temperatures above about 800' C. The M o o s vapor leaving the furnace is condensed and collected in bag filters. Purity of 99.977, is attained in the product. A technical grade product is made by roasting the concentrates a t 600' C. in a large excess of air under conditions that minimize volatilization of the oxide. Molybdenum oxide may be purified by treating it with ammonium hydroxide, followed by acidification and crystallization of ammonium paramolyb-
b Making tungsten. Tungstic acid and ammonium paratungstate are decomposed b y heating to yield the beautiful, yellow tungstic oxide. This oxide i s then placed in shallow heat resistant alloy boats, 12 to 18 inches long, which are inserted at regular intervals into horizontal tubes, 1 2 to 15 feet long, enclosed in a refractory-lined furnace. Here, the tubes are heated externally Pure hydrogen gas to about 850" C. flows through the tubes countercurrent to the boat movement and reduces the oxide to tungsten powder. One boat i s shoved into the tube, and another at the opposite end i s withdrawn. Particle size of the g r a y tungsten powder ranges from 0.5 to 10 microns, and metal characteristics are affected b y properties of the original oxide, rate of hydrogen flow, and time and temperature of the reducing operation
COURTESY W A H CHI*N(I CCRP
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date, (NH4)6M01024.4H20, which is decomposed to Moos by heating in air. The reduction of molybdenum trioxide to metal powder with hydrogen is conducted in a manner almost identical with that used on tungsten. Metal powder is consolidated by powder metallurgy techniques: compacting and sintering, similar to those described for tungsten. The final temperature is 2200' C. for electric current heating and 1630' C. for radiation heating. Primary processing of the sintered ingots is done by hot-rolling or swaging. The presence of oxygen in massive molybdenum makes the metal brittle, so small amounts of carbon are added before sintering to react with the oxygen in the metal powder and provide a n excess of u p to 0.03% carbon. Vacuum or inert atmosphere arc melting is used to produce ingots of molybdenum 1 ton or more in size. An arc is struck between a partially sintered bar which acts as a consumable electrode and a pool of liquid metal in a water-cooled copper mold in which the ingot forms. As practiced by Climax Molybdenum Co. ( 3 ) , molybdenum powder, mixed with carbon for deoxidation, is fed into a colleting device where it is compressed and partially sintered by passage of current through the compact. An alternating current is used for the sintering and melting, and the entire apparatus is maintained under a high vacuum. After the mold is filled, the ingot is removed for further processing. Ingots 1 2 to 26 inches in diameter and several feet long can be made, and the metal is of high purity. Because of the large crystal size of arc-melted ingots, the interstitial impurities, particularly oxygen, are concentrated a t relatively few grain boundaries. Cracking of the ingots results when forging or rolling is used to reduce them to bars and other forms. The initial breakdown is accomplished successfully by hot extrusion using glass as
COURTESY UNION C A R B I D E METALS CO.
Electrolytic cell wins pure tantalum metal from fluotantalate at its cathode. i s open to remove tantalum deposit on cathode
a lubricant. This cracking problem is not encountered when powder metallurgy ingots, with their small grain size, are reduced mechanically. Molybdenum must be hot-worked. Tantalum and Niobium. A major problem i n producing pure tantalum and niobium is that of separating these chemically very similar elements from each other as well as from other con-
~~
These Metals M a y Be Useful in Future Corrosion-Resistant Materials of Construction Metal
Heat
+ A4ir
Attacked by
Resistant to
Rhenium
Oxidizes above 350' C.
N?, H?, H:SOa, HC1
Strong oxidizing agents
Tantalum
Reactive at high temp.
Inert to most reactants below 150" C . ; liquid metals
F 2 ; H F ; SOS; conc. alkalis; Hz above 250' C . ; conc. HCl at >190° C.; conc. H2S04 and Hap04 at >200" C.
Niobium
Begins to react at 230' C.
Most acids i liquid metals
Most of the reagents to which tantalum is inert
Molybdenum
Oxidizes rapidly above 550' C.
Most mineral acids without oxidizers ; HF; HiPOi; HzSOr ; 12,H2; alkalis :liquid metals
"03;
Tungsten
Reaction begins at 400' C.
HF ; HCl ; H ~ S O;IHz ; liquid metals
"03;
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INDUSTRIAL AND ENGINEERING CHEMISTRY
aqua regia; fused alkalies; F2; Brg; Nz; S
aqua regia; s o h . of CuC12 and FeCl:; F?; Clz
Cell
stituents in the concentrate raw material. Until just a few years ago the classical Marignac process was used exclusively for these purposes. This technique ( 7 7) uses the difference in the solubilities of potassium fluotantalate, K2TaFi, and potassium niobium oxyfluoride, KZS b O F j , to effect the separation. The predominate process now in use is that of liquid-liquid extraction of tantalum and niobium separately from a HF-H?O solution with methyl isobutyl ketone (5). When a n acidic aqueous solution of the fluorides of tantalum and niobium, obtained by treating concentrates with hydrofluoric acid. is placed in contact with methyl isobutyl ketone, the tantalum fluoride is extracted by the ketone at lolv acidity, and the niobium fluoride at a high acidity. At the same time the impurities in the HF feed solution tend to remain in the aqueous phase. Clean extractions of very pure tantalum and very pure niobium can be obtained in a few extraction stages, and the ketone extracts can be stripped in turn of each component by use of distilled water. The pure aqueous phase of tantalum, most likely as H2TaFi, is treated with ammonium hydroxide to precipitate insoluble hydrated tantalum oxide,
LESS C O M M O N E L E M E N T S
COURTESY UNION CARBIDE METALS CO.
A A fiery arc, leaping between the watercooled tungsten electrode and the charge, melts niobium-base alloys in this nonconsumable electrode-inert atmosphere furnace
Ta20h.5Hz0; or potassium fluoride can be added to the solution to produce insoluble potassium fluotantalate: KZTaF,. Siobium is obtained as pure hydrated oxide, Nb;Os.jHzO, by the ammonium hydroxide treatment. Tantalum metal is made from the calcined, dry oxide by allowing it to react with tantalum carbide prepared by reducing Tap05 with carbon. T h e reaction, Ta?O:,
+ 5TaC
+
7Ta
+ 5CO
is conducted by heating pellets of the reactants in a vacuum induction furnace. Roundels of porous metal are obtained. Tantalum powder is made by converting the roundels to the hydride by exposure to hydrogen a t 500' C., then crushing the brittle hydride and dehydriding the powder by heating it to over 1000" C. in a vacuum in which the metal powder is cooled. Tantalum powder is also obtained by electrolysis of fused potassium fluotantalate a t about 900' C., and by reduction of the same compound with sodium in a steel bomb. The powder formed by these methods is washed with ivater and strong acids, such as aqua regia, recovered on a concentrating table, dried, screened, and blended. Xiobium metal is made by the niobium oxide-niobium carbide reaction conducted as is the one between T a z 0 6 and TaC. Both metals can be consolidated by powder metallurgy methods: compacting under pressure and sintering by passage of electric current through the compacts a t high temperature under a high vacuum. Ingots weighing no more than 10 to 15 pounds can be made by this technique.
Most of the tantalum and niobium now converted to massive metal is consolidated by vacuum consumable electrode arc melting whereby ingots weighing 100 pounds or more are produced. Partially sintered bars, pellets, or roundels are formed into electrodes and fed into the arc furnace where the mptal is melted and solidified in a water-cooled copper mold. A second arc melting of the first ingot is frequently used to obtain a purer metal. A similar arc melting consolidation can be done in a nonconsumable electrode furnace (illustrated) where an arc is struck between a tungsten electrode and the charge. Both processes result in considerable removal of harmful impurities from the initial metal. A more recent consolidation process is electron-beam melting whereby a consumable anode bar of metal is bombarded in a high vacuum chamber by high energy electrons from a cathode. T h e electron stream is focused on the end of the bar being melted and also on the upper end of the ingot being formed. Impurities escape from the pool of molten metal a t the latter point. A water-cooled copper mold is used to receive and solidify the metal. This technique yields the purest known forms of tantalum and niobium. The power consumption for both arc melting and electron-beam melting is only a fraction of that required for the sintering process. Tantalum and niobium are both worked cold and large reductions between vacuum anneals are possible. They are much simpler to work and reduce to mill forms than are tungsten and molybdenum which exhibit room temperature brittleness. Rhenium. The flue dusts from the roasting of molybdenites which contain rhenium are leached with water to dissolve the rhenium oxide, ReaO,. The acidic filtrate containing perrhenic acid, HRe04, can be treated \vith solid potassium chloride to precipitate the insoluble potassium perrhenate, KRe04, which is purified by repeated recrystallizations from water solutions. Alternatively, the acidic extract may be treated with ammonia to yield ammonium perrhenate, which is purified by repeated recrystallizations. Potassium perrhenate also can be converted to ammonium perrhenate. Rhenium metal powder is produced by the hydrogen reduction of K R e 0 4 a t 350' to 500' C., but hydrogen reduction of ammonium perrhenate is the preferred method. T h e finely ground. highly purified NHdReOl is placed in 8-inch long molybdenum boats to a depth of b / g inch. These boats are loaded into a long, horizontal, externally heated stainless steel pipe through which
purified hydrogen is passed. Animonium perrhenate decomposes a t about 380" C. and reduction occurs in about 1 hour a t 700' to 800" C. to form rhenium polvder. Powder metallurgy techniques are used to consolidate the metal. Powder compacts are pressed a t 25 to 30 tons per square inch, vacuum presintered a t 1200' C. for 2 hours, sintered under h>drogen a t 2850' C., and shaped by alternate cold working and annealing. The metal can also be arc-melted. Rhenium work hardens faster than any other metal, so the reduction in area betireen h!drogen anneals a t 1700' to 1800' C. is only 10 to 20%. The metal is available as rod, wire, strip, foil, and disks.
Chemical Technology Applications The chief applications of these refractory metals in chemical technology lie in the field of corrosion-resistant materials of construction. This has been and still is one of the major outlets for tantalum, but very little use has been made of the other metals in this group for this purpose for a variety of reasons. The corrosion properties of each of the metals varies with the specific reagent. its concentration. and the temperature of exposure. Rhenium. Because of its price and scarcity, it is doubtful that rhenium will be of interest as a material of construction for chemical equipment, and very few corrosion data about it are available. Rhenium is severely oxidized when heated to about 3.50' C. or above in air? and the Re20i which forms melts at 297' C. and boils at 363' C. However, it is not affected by nitrogen or hydrogen even a t elevated temperatures. Hydrochloric acid attacks rhenium a t a minute rate, if a t all, and the metal is also very resistant to sulfuric acid a t elevated temperatures. Strong oxidizing agents, like nitric acid. the halogens (except iodine), and hydrogen peroxide in ammoniacal solution. all readily attack rhenium. Alloys of rhenium ivith tungsten and lvith molybdenum have improved ductility. and such alloys might find use in corrosion applications. However, i t is likely that the uses of rhenium by the chemical industry will be limited to such items as thermocouples of tungstenrlienium Ivhich are capable of measuring and controlling temperatures up to 2500' C., and structural elements in electronic tubes. Tantalum is one of the most inert of all metals to reaction with chemicals a t temperatures below roughly 150' C. The only reagents which attack it rapidly are fluorine, hydrofluoric acid, and free sulfur trioxide. Alkalies react VOL. 53, NO. 2
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FEBRUARY 1961
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COURTESY WAH CHANO CORP.
Ingots are extracted from a vacuum electron beam furnace. In a recent consolidation process, electron beam melting, a consumable anode b a r of metal i s bombarded in a high vacuum chamber b y high energy electrons from a cathode. The electron stream i s focused on the end of the b a r being melted, and also on the upper end of the ingot being formed. Impurities escape from the pool of molten A water-cooled copper mold i s used to receive and metal a t the latter point. solidify the metal. This technique yields the purest known forms of tantalum and niobium
kvith it slondy, the rate increasing with alkaline concentration and temperature rise. At higher temperatures tantalum becomes increasingly reactive with a variety of reagents. including air. Tantalum reacts with molecular hl-drogen a t temperatures above about 250' C., forming an interstitial combination of maximum ratio, TaHo.,,. The absorption causes embrittlement which results from expansion of the crystal lattice. A similar result occurs a t lower
94
INDUSTRIAL AND ENGINEERING CHEMISTRY
temperatures when atomic hydrogen is discharged on a tantalum surface, as by cathodic electrolysis or chemical attack which releases hydrogen. Since tantalum becomes cathodic when in galvanic cell circuit with almost all metals except zinc. magnesium. and aluminum, it must be prevented from becoming cathodic in chemical equipment lest it fail structurally by the ensuing embrittlement. The hydrogen absorbed by tantalum can he removed by heating the metal to
800" C. or higher in a V ~ C U U I T I .but such treatment of a heat exchanger, for example, is not too feasible. Hydrochloric acid begins to corrode tantalum a t elevated temperatures. Tests a t 190' C. under pressure have shown a corrosion rate of less than 1 mil per year (0.001 inch per year) for concentrations up to 25% HCl, where some embrittlement resulting from hydrogen occurs (2). The rate of attack is 3.9 mils per year in 30y0 acid and 11.6 in 37% acid, with increasing embrittlement as the concentration increases. No attack occurs a t lower temperatures, regardless of concentration. Nitric acid a t 190' C. and concentrations up to 70% has a corrosion rate on tantalum of less than 1 mil per year ( 2 ) . Other tests a t temperatures of 200' C. or more with a variety of HNOI concentrations have shown nil attack (6, 7 ) . The presence of HCI or chlorides in the acid does not affect the resistance, and tantalum is inert to boiling aqua regia. In 9870 sulfuric acid a slow uniform attack begins a t about 175' C. At all concentrations below this temperature there is 110 attack. As the temperature of 98% H 2 S 0 4rises, the corrosion rate on tantalum rapidly increases; a t 200" C. it is 1.5 mils per year, a t 250" C. it is 29, and a t 300' C. it is 342 (72). As the concentration of the HzS04 is decreased, the rate of corrosion a t a given temperature tends to become less. For example, a test in a chemical company concentrator has shown a rate of 3.1 mils per year in 90% H2S04 a t 250' C. This is about '/IO that for 98% acid a t this temperature (7). The above exposures have resulted in no embrittlement of the tantalum. The presence of HCl or chlorides does not alter the resistance. O n the other hand, tests made a t the boiling points of 80, 85, and 95% H & 0 4 are reported to cause some brittleness of the tantalum samples (2). The respective corrosion rates and temperatures are 1.9 mils per year a t 202' C., 19.3 a t 225' C.? and 192 a t 290' C. At 190' C. the rate is less than 1 mil per year over the whole concentration range of 5 to 95% H2SO4 ( 2 ) . Mixed acid prepared from 88 parts of 957, H N O p and 12 parts of fuming HzSO4 (containing 20% SO,) has been reported to give a corrosion rate of 0.8 m i l p e r y e a r a t 1 2 l 0 t o 1 4 9 ' C . (70). The attack of fuming sulfuric acid (containing 15% SO$) on tantalum is rapid. Taylor has found that while the corrosion rate is 0.3 mil per year a t 23' C., it is 4 at 50' C., 9.2 a t 70" C., and 3900 a t 130" C. (7, 72). Phosphoric acid of 85% strength does not attack tantalum below about 180' C., but a t 225' C. the corrosion rate is 3.5 mils per year, and a t 250' C. is 20. Another study (2) has reported rates of less than 1 mil per year a t the boiling
LESS COMMON ELEMENTS point and a t 190' C. over the concentration range of 1 to 85% H3PO4. If the phosphoric acid contains hydrofluoric acid or more than a few parts per million of fluoride ion, as frequently is the case in commercial acid, attack on the tantalum is likely to occur. By contrast, chromium plating baths containing 40% CrOB and 0.5% fluoride ion have shown nil effect upon tantalum, possibly because of complex ion formation between chromium and fluoride ions (7). Alkalis in concentrated solutions attack tantalum even a t room temperature, but the metal is quite resistant to dilute solutions. In 10% S a O H a t room temperature a corrosion rate of 9.3 X niil per year has been found, and in 5 and 10% NaOH a t 100' C. the rate is 0.126 mil per year (73). IYhilp: fluorine attacks tantalum rapidly at room temperature, the metal is totally inert to dry or wet chlorine, bromine, and iodine below 150' C., and these elements dissolved in solutions of salts or acids likewise have no affect. Chlorine begins to react with tantalum a t about 250' C., but the presence of water vapor makes tantalum satisfactorily resistant a t temperatures as high as 375' C. (74). Bromine and iodine begin to react with tantalum a t about 300' C., forming TaBrb and TaIS, respectively. Sulfur and hydrogen sulfide react with tantalum a t red heat, forming tantalum sulfide, TaS2. Tellurium and selenium attack tantalum severely a t about 800' C. (8). When heated in air, oxygen, or nitrogen tantalum is unaffected below 250' C.? is tarnished in 24 hours at 300' C., and a t higher temperatures the reaction rate increases rapidly. No nitrides are formed in air oxidation. The reaction rate in oxygen is more rapid than in air, and a slower rate is found in nitrogen ( 7 ) . Tantalum reacts with carbon dioxide, carbon monoxide, and nitrogen monoxide a t temperatures above about 1100' C., but COz under 8 atm. pressure corrodes the metal a t 500' C. Carbon, boron, silicon, and phosphorus react directly with tantalum at elevated temperatures to form T a z C and TaC, TaB and TaB2, TaSi?, and T a P and TaP2, respectively. Tantalum exhibits remarkable resistance to liquid metals a t high temperatures in the absence of reactive gases such as (0,S, and COS). I t is suitable for use with sodium at 1200' C.; potassium, sodium-potassium alloys, lithium. and lead a t 1000" C.; bismuth at 900 O C. or higher ; mercury at 600 ' C. ; gallium at 450 ' C. ; and magnesium and uranium-magnesium and plutoniiimmagnesium alloys a t 1150' C. It has been satisfactory for several thousand
hours of service in molten metal fuel circulating loops containing magnesiumthorium alloy (63% hlg and 3 7 7 , Th) a t 800' C . , bismuth-uranium-manganese alloys (89.5, 10.0, 0.5. and 94.7, 5,0, 0.3%) a t 1160 and 1050' C., respectively ; and bismuth-uranium alloys (5 to 1Oyo V) a t 1100' C. (5). 'Tantalum fails in a few days when usrd as containers for an alloy of aluminum-thorium-uranium (76, 18>and 6%) a t 1000' C., an alloy of' uranium-iron (90 to 10%) at 900" C., and the eutectic alloy of uranium-chromium a t similar temperatures (5). Liquid aluminum reacts with tantalum rapidly to form the stable compound, Al:$Ta,aluminum tantalide. Zinc is reported to wet and attack tantaluni whose surface is abraded in zinc a t 400' C., but an industrial zinc producer has observed excellent corrosion resistance at 500' C. (5). The maintenance of the oxide film on the tantalum mas account for the latter result Niobium. LVhile somewhat similar to tantalum in corrosion resistance, niobium is attacked rapidly by some, and slowly by most of the reagents to which tantalum is inert. In no instance is it unaffected by media in which tantalum is corroded ( 7 ) . Further, niobium is more susceptible to hydrogen embrittlement than tanralum, so that even slow rates of attack by acids which cause hydrogen formation result in embrictlement of the metal. Like tantalum, i t must be prevented from becoming cathodic in a galvanic couple in electrolytes. Niobium has a high degree of resistance to most acids and acidic solutions at room temperature, one exception being 10% oxalic acid. This includes such reagents as sulfuric, hydrochloric, nitric, phosphoric, tartaric, lactic, acetic, and perchloric acids, aqua regia, 5% phenol, ammonium hydroxide, 302& hydrogen peroxide, and 10% ferric chloride. Like tantalum, i t is attacked rapidly by fluorine, hydrofluoric acid, free sulfur trioxide, fluoride ion in acidicsolutions. and concentrated alkalies.
However, as the temperature rises the corrosion resistance is much decreased to\vard almost all reagents, a notable exception being concentrated nitric acid jvhich has no effect on niobium a t 100' \Vhile tantalum is quite resistant to dilute alkalies, niobium is attacked and embrittled by even 1% S a O H or KOH at 100' C.: and is attacked slowly by 10% K a O H at room temperature (0.46 mils per year) (13). .4ir begins to oxidize niobium slowly at about 230' C., the rate accelerating as the temperature rises, and a t 390' C. a \c.hite oxide begins to appear on the surface. Heating in nitrogen results in nitride formation above 300' C. Niobium reacts rapidly with fluorine at room temperature ; with chlorine reaction begins at about 200' C. ; and with bromine and iodine at someivhat higher temperatures. The resistance of niobium to liquid metals at elevated temperatures closely approximates that of tantalum. This resistance, plus the density advantage of niobium and its lo\\, thermal neutron absorption cross section, makes its use very attractive in some types of nuclear reactors, for example, as a canning material for solid uranium. Molybdenum, Although molybdenum has very attractive corrosion resistance properties, the difficulty of obtaining strong, nonbrittle welds has restricted its fabrication into chemical equipment. However, by maintaining scrupulously clean welding conditions to prevent entry of oxygen and nitrogen into the welds, it is feasible to make \velds in arc-cast molybdenum that have moderate ductility at room temperature and as much ductility as the parent metal at slightly higher temperatures (1 00 0 to 200 ' C.) ( 3 ) . hlolybdenum has good resistance to mineral acids if no oxidizing agents are present, such as nitric acid, nitrates, ferric or cupric chloride. It has good resistance to hot and cold unaerated hydrofluoric acid, to hot and cold hydrochloric acid, and to hot and cold phosphoric acid in both dilute and con-
c.
Boiling and Melting Points of Refractory Metals Are Higher Than Those of Iron
c.
B.P., O C.
Den-., G./CC.
Tungsten Rhenium
3410 3130
5900 5900
19.3 21.0
Tantalum Molybdenum Niobium Iron
2996 2610 2468 1535
6100 5560 4927 3200
16.6 10.2 8.56 7.87
lI.P.,
a
Atomic Valencen
NO.
Atomic wt.
(e', 5, 4, 2 (7), 6,5349 3, 2 , 1, - 1 (51, 3 (61, 5, 4, 3, 2 (51, 3 (31, 2
74 75
183.92 186.31
73 42 41 26
180.95 95.95 92.91 55.85
Most reactive valences are in parentheses.
VOL. 53. NO. 2
FEBRUARY 1961
95
Present Production of These Metals in Unalloyed Form i s Small. (Tons of metal)
Prod.
U.8. in 1959 Consumptionh
2,500 0.5 259 30,000 576
4,200 0.5 259 20,000 576
Free World
Resources Tungsten
300,000
Rhenium
1,000 120,000 2,900,000 7,000,000
Tantalum Molybdenum Niobium a
prod.
700 0.5
122 1,200 66
.innual I'oten tial Prod.
12-25,000 5-15 500-750 12-25,000 12-25,000
( 9 , 75). Includes metals in alloyed form. As high purity metal.
centrated solutions. I n sulfuric acid it shows small corrosion rates in hot or cold solutions u p to about ?O% concentration, and is also fine in 75 and 95% H2S04 at 70' C. However, it is rapidly attacked in these last two solutions a t higher temperatures. Molybdenum corrodes slowly in concentrated nitric acid at room temperature, probably because of the formation of a passivating film, but is attacked rapidly by dilute nitric acid and hot concentrated nitric acid. It dissolves readily is aqua regia, and in mixtures of HF and "01, and H2S04 and " 0 3 a t room temperature. Organic acids, such as acetic, formic, lactic, tartaric, and oxalic, corrode molybdenum at a very slow rate (1-2 mils per year). It is not corroded by aqueous caustic alkalies, but dissolves in fused alkalies above about 600' C. in the absence of oxygen and at lower temperatures when oxygen is present. Fluorine attacks molybdenum at room temperature, chlorine at about 230' C., and bromine at about 450' C. The metal has excellent resistance to iodine up to a t least 800' C., and is used as internal components in the deBoer-van Arkel iodide process equipment. At temperatures above about 550' C. molybdenum oxidizes so rapidly in air or oxidizing atmospheres that its continued use in such exposures is impractical unless it is protected by a coating such as molybdenum disilicide. In nitrogen, nitriding begins a t 1500" C.; in carbon monoxide, carburizing starts a t 1400' C.; and in carbon dioxide, oxidation begins at 1200' C. Steam reacts with molybdenum a t about 700' C. Hydrogen does not affect the metal a t any temperature. Sulfur reacts with molybdenum at 440' C., and hydrogen sulfide at about 1200' C., although at lower temperatures a thin, adherent sulfide coating is formed. Molybdenum has outstanding resistance to liquid metals a t elevated temperatures, comparable to that of tantalum and niobium. I t can be considered suitable for long service with sodium up
96
1I et alc
to 1500' C., with bismuth to 1425' C., with lead to 1200' C., with lithium, potassium, and sodium-potassium alloys to 900' C., with magnesium to 700' C., with mercury to 600' C., and with gallium to 300' C. Tin severely attacks it at 1000' C., as do molten aluminum, cobalt, iron, and nickel ( 3 ) . Molten glass and nonferrous slags have negligible effect on molybdenum, and it is inert to refractory oxides, such as alumina, magnesia, and zirconia, up to a t least 1760' C. It is widely used as electrodes in glass furnaces. In all elevated temperature exposures an inert or nonoxidizing atmosphere must be present. Tungsten. LVhile it exhibits very interesting corrosion resistance properties, tungsten is brittle at room temperature, except for thin sections, and is most difficult to weld. It has not been used to more than a minor degree for these reasons in chemical equipment fabrication. Simple shapes, like cups, crucibles, boats, and nozzles, can be formed by spinning, drawing, forging, and slip casting. Tungsten is not attacked by hydrofluoric acid, and the corrosion rate is slow in hot, dilute, or concentrated hydrochloric and sulfuric acids. Nitric acid corrodes tungsten somewhat faster. Hot aqua regia and a mixture of H F and Hh-03 readily dissolve the metal. The attack by 10% phosphoric acid is slight. Solutions of cupric or ferric chloride corrode tungsten. In the absence of oxygen tungsten is not attacked by alkali solutions or ammonium hydroxide. Tungsten resists molten K O H or NaOH, but the presence of oxidants like K N 0 3 or KC103 cause a high rate of reaction. When used as an anode in dilute (20% or less) sulfuric acid bath plating operations, tungsten has a low corrosion rate, but when used as a n anode in alkaline solutions, the metal dissolves rapidly. Fluorine attacks tungsten a t room temperature, chlorine a t about 250" C., and bromine and iodine a t red heat.
INDUSlRIAL AND ENGINEERING CHEMISTRY
Water vapor oxidizes tungsten at above 700' C., although a recent study states that the reaction of 5y0 HgO in argon with tungsten does not become appreciable below 1600' K . ( d ) , Oxidation of the metal in air begins at about 400' C. and becomes rapid at above 600' C. In nitrogen, nitriding begins a t 2300' C. Hydrogen has no effect at any temperature. Gases like C O ? , CS,. S2CI2, CO. NO, and NO2 react with tungsten a t elevated temperatures. Hydrogen sulfide discolors it at 1200' C., and elements like carbon, boron, silicon, and sulfur combine directly with it at high temperatures to form binary compounds. For example, a mixture of tungsten and carbon begins 10 form tungsten carbide, LVC, a t 1200' C. Tungsten has good resistance to attack by liquid metals. I t is suitable for use with sodium and sodium-potassium alloys to 900' C., with mercury to 600' C., with gallium to 800' C., with bismuth to 980' C., and with magnesium to 600" C.
References (1) Albrecht, W. M., Klopp, LV. D., Koehl, B. G., Jaffee, R. I . , "Reaction of Pure Tantalum with Air, Nitrogen, and Oxygen," AIME Meeting, Chicago, Ill., Wov. 2, 1959. (2) Badger, F. S , personal communication, 1959; IND. ENG. CHEM.50, 1608-11 (1958); Union Carbide Metals Review 18-21 (Winter/l960). (3) Climax Molybdenum Co., "Molybdenum Metal," 1960. (4) Farber, M., J . Electrochem. $06. 106, 751-4 (1959). (5) Hampel, C. A , , "Rare Metals Handbook" (2nd ed.), Reinhold, New York, 1961. ._ __.
(6) Hampel, C. A , , IND.ENG.CHEM.48, 1979-81 (1956). ( 7 ) Hampel, C. .4., Corrosion 14, 557t-560t (1958). (8j. Hine., R. C., personal communication, 1960. (9) Jaffee, R. I., "A Brief Review of Refractory Metals," High Temperature Symposium, Alsilomar, Cal., Oct. 8, 1959. (10) Kaplan, N., Andru% R. J., IND. ENG.CHEM.40, 1946 (1948). (11) Placek, C., Taylor, D. F., IND.ENG. CHEM.48, 686-95 (1956). (12) Taylor, D. F., "Tantalum: Its Resistance to Corrosion," Electrochemical Society, Chicago Section, May 4, 1956. (13) Tingley, 1 . I., Dept. of Mines and Technical Survevs. Mines Branch, Ottawa, Canada, ' personal communication, 1959. (14) Tseitlin, K . L., Z h u r . Priklad. Khim. 29, 1281 (1956). (15) U. S. Bureau of jhlines, Bull. 585, Washington, 1960. RECEIVED for review Novembcr 3, 1960 ACCEPTEDNovember 16, 1960 Division of Industrial and Engineering Chemistry, 138th Meeting, ACS, New York, N. Y . , September 1960.
