Corrosion Resistance of Tantalum-Molvbdenum Alloys J
J
%''ALTER C. SCHURIB, SCHRADE F. RADTKE', AXD MICHAEL B. BEVER I)Zassuchusetts Institute of Technology, Cambridge, Muss.
A series of
alloys of tantalum and molybdenum from 0 to 100 atomic % tantalum in 10 atomic % steps was prepared by arc fusion of sintered compacts in an argon atmosphere. The resulting ingots were accurately cut, ground, and polished, and the surface of each sample was estimated by direct measurement of dimensions. The density, hardness, and other properties of these alloys were determined. In measurements of corrosion resistance carried out upon these alloys at 50' C. in solutions of concentrated hydrochloric, nitric, and sulfuric acids,
the tantalum-rich specimens were found to be unattaclced in the three acid media, whereas the molybdenum-rich alloys w-ere attacked at an increasing rate in the order sulfuric --+ hydrochloric + nitric acid. The type of curve of the variation in corrosion rate with composition was similar in the three media, although rates in individual acids varied. Corrosion in general w-as negligible above 50 atomic % tantalum. Where corrosion had occurred, the three acids produced films, although only in nitric acid was the film protective of the underlying metal.
V
diffusion pump. The heating was by means of a high frequency induction unit, the compacts serving as the heating element. The maximum temperature attained was in escess of 1600" C. maintained for 2 hours. Slow cooling was accomplished by gradually reducing the power over 2 hours, after which the furnace reached room temperature within 8 hours and could then be opened. The surfaces of the sintered compacts were clean and bright. The contact points between compacts and separators were carefully ground off and the compacts submitted to fusion. The arc fusion process of von Bolton ( I ) , as modified by Kroll (4),was found satisfactory for the production of sound alloys. Argon of 99.96% purity was employed as the inert atmosphere at about 0.1 at.mosphere pressure and the compact to be melted was held in a water-cooled copper cup which also served as anode. An arc was struck by means of a tantalum cathode held in a movable water-cooled electrode holder, which passed out of the Vgcor chamber by way of a Sylphon bellows with spring stiffener. An open circuit voltage of 30 to 35 volts was used, the closed circuit potential dropping to 20 to 25 volts, with a direct current of 200 to 400 amperes flowing between compact and tantalum electrode, as supplied by a heavy-duty welding generator. No metal was transferred to the compact from the movable electrode; rather a small gain in weight of the latter was observed after each run. The arc was moved over the surface of the compact until the upper part was completely melted; the compact was then turned over (by means of the movable electrode) and t.he lower part melted. With practice, smooth, round disks weighing up to 100 grams could be produced. After fusion the ingot was allowed to cool to room temperature in argon and then removed from the furnace. The crater temperature during the melting process wae found to be above the limit of the optical pyrometer employed-namelv, > 3200" C.-and as the temperature gradient between the molten surface and the bottom of the ingot was well over 3000" C. it was necessary to anneal the as-cast ingots to relieve quenching strains. Snnealing also assured homogeneity of composition of the metal grains. The annealing was carried out. in the same furnace used for sintering the compacts. The ingots were individually wrapped in tantalum foil and stacked vertically in the furnace, which after sealing was pumped down to less than 0.1-micron pressure and brought to a temperature in excess of 1600" C. by induction heating over a period of 2 hours. The ingots were held at this temperahre for 2 hours, then cooled at a rate of approximately 300' C. per hour until room temperature was reached. The ingots retained their bright metallic luster after this treatment. They were roughly circular in shape, about 1.5 inches in diameter, and 0.25 inch thick. The surfaces were smooth but not flat; they were therefore sectioned on a cut-off machine with Alundum wheels and with adjustabk pressure and feed devices. Water jets supplied cooling and lubrication for the cutting operation. Some of the alloys required as long as 1,5 hours for a single cut
i O S Bolton ( 1 ) and Buckle ( 2 )reported that tantalum and molybdenum form a continuous series of solid solutions. I t was considered to be of interest not only to investigate the physical properties of such a binary alloy system, such as density, hardness, thermoelectric power, electrical resistivity, linear coefficient of expansion, and magnetic susceptibility, as a function of the composition, but also to observe the corrosion characteristics of the alloys in concentrated acids. PREPARATION OF ALLOYS
The preparation of these alloys offered considerable difficulty. Nethods of attaining the necessary high temperatures were limited in view of the reactivity of tantalum and molybdenum with gases and with refractories under these conditions. The reactivity of tantalum toward oxygen, hydrogen, and nitrogen is familiar; and tantalum readily forms carbides and dissolves its own oxide as well as those of other metals present in common refractories. Tantalum also reduces oxides of other metals-for example, alumina. Although molybdenum is somewhat less sensitive to various gases than tantalum, only methods that would produce uncontaminated tantalum could be used so as not to introduce impurities into the alloy. For these reasons the alloys were prepared by the arc fusion of sintered compacts formed from mixtures of the powdered metals in proportions varying by steps of 10 atomic %. The tantalum powder employed contained 0.03% carbon and 0.01% iron; its particle size was -80 mesh. The molybdenum was reported to contain 0.05% oxygen; its particle size was -200 mesh. The sintering was carried out on compacts which had been pressed at 50 tons per square inch into the form of disks approximately 1.125 inches in diameter and 0.375 inch thick. The sintering was accomplished in a quartz tube furnace containing Alundum liners, on the inner side of which were several layers of sheet molybdenum, 0.015 inch thick; in turn, within these were several layers of 0.010 inch tantalum sheet. Thus the compacts mere completely isolated from any contact with the Alundum refractory. Individual compacts were piled vertically, separated by scrap tantalum separators, which were stamped with raised concentric circles to minimize contact between compact and separators. Ten ingots were sintered simultaneously in a vacuum of not over 0.1 micron of mercury, provided by a large-capacity mercury 1
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through the ingot. The cut pieces were ground to form rectangular bars on a' water-cooled emery belt; a metal jig was used to hold the bars in position. The surfaces were finish-ground by hand with No. 1 emery paper held on a plate glass surface until all six faces of the bar were plane. The finished bars were approximately 3/16 X inch in cross section and from 0.375 to 1.125 inches long. The dimensions of the bars were measured to 0.0001 inch. Within a longitudinal distance of 1 cm. the variation in the cross-sectional area of the bars averaged 0.05%. A q the percentage of,tantalum and the melting range of the alloys increased, proportionately more molybdenum was lost by vaporization and additional molybdenum was added to the original compacts to compensate for this loss. The compositions of the final sintered, arc-fused, and annealed ingots were determined by analysis and are shown in Table I. Metallographic examination of the entire range of alloys confirmed the conclusions of von Bolton (1)and Buckle ( 9 ) that the two metals form a continuous series of solid solutions, and further revealed that the ingots were dense and homogeneous with few i f any nonmetallic inclusions. Only a few ingots showed the presence of pores, which were small, spherical, few in number, and located generally near the surface of the ingot. The ingots after annealing were completely free from traces of segregation, and the grain size was found to be unaffected by the annealing process. After photomicrographs were taken of the specimens, their physical properties were measured.
CORROSION OF ALLOYS IN CONCENTRATED ACIDS
The apparatus employed for the study of the corrosion resistance of the tantalum-molybdenum alloys was designed to control as far as practicable the factors that affect the corrosion rate, such as temperature, nature and concentration of the corroding medium, nature of the gaseous atmosphere, rate of agitation of the corroding solution, and manner in which the specimen is supported in the corrosion medium. (One factor which was not controlled in the attempt to obtain specimens of uniform history was the grain size. The average diameter of the grains in the specimens of each composition was known, however.) A water thermostat of approximately 250-liter capacity was employed and was maintained at 55.0 * 0.08' C. by means of a system of immersion heaters, two constant and one intermittent. A float-type constant-level device provided for the continuous addition of water to the bath to replace that lost by evaporation without disturbing the temperature of the bath. As molybdenum is known to be moderately resistant to corrosion by acids and tantalum is very resistant (3, 5 ) , it was considered desirable to determine the corrosion rates of the alloys under conditions as conducive as possible to corrosion; consequently, all the runs were conducted with the medium saturated with oxygen at 1atmosphere. The gas was bubbled into jars containing the specimens a t a rate of 200 cc. per minute, as indicated by a flowmeter in the gas feed line
TABLEI.
COMPOSITION OF TANTALUM-MOLYBDENUM ALLOYS
Sample NO.
101 102 103 104 106 106 107 108 109 110 111
a27
Composition of Pressed Compacts At. % Mo At. % Ta
100 90 80 71 61 51 41 31 21 11 0
0 10 20 29 39 49 59 69 79 89
160
Tantalum in Ingots after Fusion and Annealing, At. % 0
10.10 20.08 30.02 39.96 49.47 61.21 71.47 82.77 91.42 100.
(Figure 2). Passage of the oxygen through the bubbler, and through the glass chimney provided to direct the bubbles vertically, caused a regular stirring of the corrosion liquid. Th? liquid streamed downward continuously over the metal specimens, which hung radially about the chimney. In this way the specimens were supplied with fresh, oxygen-saturated solution at all times. The corroding acid solutions were concentrated C.P. sulfuric (95.5%), hydrochloric (37%), and nitric (70%) acids. The corrosion jars, each holding 6.5 liters of solution,' were of such large capacity that the concentration of acid would not be reduced as much as 0.1% of its original value even if the samples had been completely consumed by the corrosion process. The jars were covered with glass plates, ground t o fit the jars, and were sealed by a silicone grease. Preliminary tests proved that none of this grease entered the corroding medium. Because of the flow of gas through the solutions a very small decrease in concentration of the hydrochloric and nitric acids was caused by entrainment, but the amount was too small to have any significant effect on the results. In no case was the concentration of the corroding medium found to vary more than 0.8% before and after the corrosion runs. Above each corrosion jar a water-cooled condenser was installed. The oxygen feed line passed down through the center of the condenser column, thus reducing the internal volume of the column and improving the contact of the effluent gas with the cooling surface of the condenser. The discharge water from the condensers led to a large jar in which the oxygen entering the system was saturated with the corroding medium at the same temperature as that of the gas leaving the system. A trap on the discharge side of the condenser prevented entrance of air into the system. The No. 774 Pyrex glass spiral hangers employed in mounting the specimens were made of 0.5-mm. rod. The spirals were approximately 12 mm. in diameter and permitted only point contact, or a t most Figure 1. Arc Fusion Furnace line contact, between the specimen and the support. The specimens 1. Cooling water inlet were placed in the hangers su that 2. Cooling water outlet the large plane surfaces were at an 3. Terminal lug 4. Rubber sleeve angle of 45" to the vertical. The 5. Sylphon bellows with spring stiffener specimens selected were dense bars 6. Brass furnace cover plate 7. Silicone rubber gasket free from pores or pits. Three speci8. Movable electrode holder-cathode (negative) mens of each composition were se9. Vycortube 10. Tantalum electrode lected, carefully washed to remove 11. Water-cooled crucible for supporting compacts oil or greasy film, and rinsed in tap during fusion. Serves as anode (positive) water and distilled water, then in 12. Brass plate 13 and 14. One tube serves as inlet for inert atmoabenzene, alcohol, and ether. The phere. The other tube leads to vacuum pump bars were then stored in a desicwhich serves to evacuate system
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U~f IN TABLE 11. CORROSION O F T . 4 S T , ~ L U ~ f - ~ ~ O L Y B D E N.&LLOYS COXCENTRATED SULFURIC IUD HYDROCHLORIC ACIDSAT 5b C.
Average Corrosion Rate, Mg./Sq. Dm /Dav -"
Sample NO. 101
102 103 104 105 106 107 108
109
110
111
Rejected.
,
At. % Ta
Concd. H~SOI
Concd. H Cl
0 10.10 20.08 30.02
0.80 0.88 0.78 0.96
1.80 1.67
39.96 49.97 61.21 7 1 A7 82.77 91 42
100
0.92 0 0 0 0 0 0
1.50 0.93 1.08 1.03 O. .' . l 0
0
0
Abnormal weight loss due t o chipping of specimen
Vol. 42, No. 5
per square decimeter per day. The average corrosion rates of the three specimens of each composition are shown in Table 11. The corrosion rates of the alloys in concentrated hydrochloric. acid are slightly greater than those observed with concentrated wlfuric acid, and the attack persists over a greater range in composition. The tantalum-rich alloys are again unattacked by the heid medium. CONCENTRATED NITRICACID. Specimens were removed fioir, corrosion jars containing 70% C.P. nitric acid, saturated with oxygen at I atmosphere, after 139, 240, and 330 hours Microscopic examination showed that where corrosion had occurred, the films were dense and hard and could not be wiped off. Corrosion again occurred only in the rase of molybdenum-rich alloys, from 70 atomic % molybdenum upnard: alloys containing 40 or more atomic % tantalum shon-ed n o corrosion (see Table 111). The
cator. Their weights were determined to 0.1 mg. and tlieii dimensions measured to 0.0001 inch with a micrometer caliper. The area of each surface was thus determined so as to evaluate the total surface of each specimen bar. Specimens were removed periodically during the corrosion run to determine the variation in rate of corrosion of each specimen with time. Each of three corrosion jars in the thermostat contained four hangers; each hanger held three specimens of a given alloy composition. The hangers were suspended from glass hooks placed radially around the bubbler chimney. After each corrosion run was completed, the specimens were carefully washed, dried, and weighed as before. Where the corrosion was pronounced, the bar was resurfaced, measured, and reweighed, so that it could be used for further study. Where films had formed on the metal surfaces, they were wiped off, wherever possible, or the film was cautiously brushed off in a manner to prevent accidental removal of uncorroded metal. The surfaces of each specimen were examined under a lowpower microscope before and after each run. After a given period during a run, the cover plates of the jars were removed and a sample of each composition wm removed from the hangers. In this way, simultaneous testing of the series of alloys could be accomplished under closely similar conditions. EXPERIMENTAL RESULTS
CONCENTRATED SULFURIC ACID. The corrosion run in 95.5% sulfuric acid continued for 504 hours in an oxygen-saturated solution at 55' C. Specimens 'Iyere removed after 210, 406, and 504 hours. Where corrosion had occurred microscopic examination indicated that corrosion had been uniform; there was no indication that the molybdenum had been preferentially removed, A slight, light-brown film could be wiped off readily. The corrosion, which occurred in the molybdenum-rich alloys, was of the order of 1 mg. per square decimeter per day, including the specimens of pure molybdenum. From the results shown in Table I1 it is evident that the corrosion ceases abruptly at the composition 50 atomic %each of tantalum and molybdenum. No corrosion of the alloys containing more than 50 atomic % tantalum was observed. The reported passivity of tantalum in concentrated sulfuric acid ( 5 )apparently persists until about 60 atomic yomolybdenum has been added, at which point very slow corrosion of the alloy becomes noticeable. The corrosion of n~olybdenum-richalloys was uniform and was not observed to vary with time. The loosely adherent film formed gave no protection to the metal surface and did not retard the m i rosion rate. CONCENTRATED HYDROCHLORIC ACID. Specimens were rcmoved from corrosion jars containing 37% C.P. hydrochloric acid, saturated with oxygen at 1 atmosphere, after 138, 239, and 250 hours. All specimens were found to be covered with a thin, transparent, brown to strawcolored film which was readily washed off the metal surfaces. Microscopic examination revealed that the corrosion was uniform with no pitting. The molybdenum-rich alloys were corroded itt a rate of approximately 2 nig.
Figure 2.
Corrosion Jar
Gas inlet tube Stopper Cooling water outlet 4. Water-cooled condenser 5. Gas outlet air trap 6. Cooling water inlet 7. Tygon sleeve gasket 8. Hook for hanging corrosion specimen hangers 9. Glass cover plate 10. Ground-glass surfaces o n jar and cover plate 11. Liquid level i n constant temperature bath and rorrosion jar 12. No. 774 Pyrex glass jar 13. Chimney to control internal liquid currents in corrosion jar 14. Fritted-glass hubbler 15. Enlarged view of corrosion specimen hanger showing method of placing specimens. Hanger is supported i n corroding medium by hanger hooks, 8 1. 2. 3.
the corrosion rate was observed to decline with time, so that at the end of 330 hours the rate had leveled off. The film formed on the alloy composed of 90 atomic 70 molybdenum and 10 atomic yo tantalum was bluish-white, hard, and tenacious. It did not flake off as in the case of pure molybdenum. Beneath the film microscopic examination showed that although the corrosion of the surface appeared uniform rather than preferential, there was indication of preferential attack of particular grains, the attack being greatest perpendicular to the longitudinal axis of the grains and least parallel to the longitudinal axis. The corrosion rate again was found to decrease with time, but not as rapidly as for pure molybdenum. In the case of the 80 and 70 atomic % molybdenum alloys, the transparent, thin, straw-colored films were difficult to remove, even with brushing. Beneath the films the metal surfaces were found to be uniformly attacked, with no preferential attack of any kind. The corrosion rates again declined with time. Alloys containing 40 or more atomic % ' tantalum showed no attark b y the nitric acid.
TABLE111. CORROSION OF TANTALUM-MOLYBDENUM ALLOYS IN CONCENTRATED NITRICACIDAT 55" C. Sample NO. 101
% Ta 0
102
10.10
103
20.08
104
30.02
105
39.96
106
49.47
107
61.21
108
71.47
109
82.77
110 111
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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91.42 100
Time Immersed,
Hours 139 231 331 139 231 331 139 231 331 139 23 1 331 139 231 33 1 139 231 331 139 231 331 139 231 331 139 23 1 331 139 231 331 139 23 1 331
292.06 160.55 129.70 94.61 86.91 68.88 10.46 6.26 4.27 0 0 0.579 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0
172.06 79.91 6.16 0.19 0 0 0
ACKNOWLEDGMENT
The gift of tantalum and molybdenum products by Frank H. Driggs and L. F. Yntema of Fansteel Metallurgical Corporation is gratefully acknowledged as of great assistance in this work. The authors are also indebted to several members of the Department of Metallurgy of this institute, in particular, to John Wulff, H. H. Uhlig, and A. R. Kaufman.
0
0
LITERATURE CITED
I
specimens of pure molybdenum showed a white to bluish-white film, which when removed appeared to be dense and hard. The attack of the metal appeared to be uniform over the entire surface of the specimen, with no indication of preferential attack of the clearly visible grain boundaries. This dense film is doubtless a barrier to continued attack of the metal surface by nitric acid, as
(1) Bolton, W. von, Z.EEektrochem., 11, 45 (1905). ,(2) Buckle, Metallforschung, 1, 53 (1946). (3) Fansteel Metallurgical Corp., North Chicago, Ill., Bull. 541
(1941).
(4) Kroll, Trans. Electrochem. SOC.,78,35 (1940). (5) Rohn, Z. Illetallkunde, 18,387 (1926). RECEIVBD November 21, 1949. Presented before the Division of Physical and Inorganio Chemistry a t the 117th Meeting of the AMERICAN CHm&frcaL SOCIETY,Detroit, Mioh.
Adjustment of Temperature Scale of Cox Charts C . E. REHBERG Eastern Regional Research Laboratory, Philadelphia 18, Pa. T h e value of C used in preparing the temperature scale of a Cox chart laid off according to the equation y = at/(t C) can be adjusted to any chosen value by adding or subtracting the appropriate number for each temperature on the calibrated scale. Thus any one type of ruled
Cox chart paper can easily be adapted to give straight lines for the temperature-vapor pressure relationship of any type of compound. This also constitutes a convenient graphical method for evaluating C in the Antoine equaB / ( t f C). tion, log' l = A
T
As generally used, the Cox chart presents a graphical representation of the Antoine (1)equation
+
HE Cox chart (3),as developed by Davis ( 4 ) and Calingaert and Davis (d), offers the most convenient way of recording and using data on the relation of vapor pressure to temperature. Since this relationship for a given compound is represented by a straight line on the Cox chart, reliable interpolations and extrapolations are readily made. Also, the reliability of the experimentally determined points can be estimated from the magnitude of the random deviations of the points from the straight line.
-
loglo P = A
- B / ( t + C)
(1)
where P = pressure in mm., t = 'C., and A , B, and C are constants. For the usual set of simplifying assumptions, C = 273, and the term (t C) becomes T in K. In practice, it is found
+
