Platinum-Vanadium and Nickel Baths for Inert Gas Fusion Oxygen Determination Dean H. Bollman A l b a n y M e t a l l u r g y R e s e a r c h C e n t e r , B u r e a u of M i n e s , U.S. D e p a r t m e n t of the I n t e r i o r , A l b a n y , Ore. 97327
Low and inconsistent results were encountered with an unalloyed platinum bath when analyzing thoriated and rirconiated tungsten for oxygen content by inert gas fusion. Data are given to show that vanadium additions to platinum baths made the recovery of oxygen easier from both of these tungsten-base alloys and also from titanium and FS-85 niobium alloys. Data are given to show that a nickel bath and wrap was superior to platinum for determining oxygen in zirconium and beryllium. In addition, a nickel bath and wrap works well for titanium and tungsten, but not for FS-85 niobium alloy. The importance of carbon solubility in the melt and the role of tin are discussed.
Several investigators, including Ashley and Longhurst ( I ) , Rottmann and Nickel ( 2 ) , and Lassner and Kraft ( 3 ) have shown that graphite precipitation in melts of nickel, iron, cobalt, and platinum is responsible for retention of both oxygen and nitrogen in the melt during the inert gas fusion and vacuum methods. Izmanova and Chistyakova ( 4 ) have used tin, copper, and even antimony to decrease the effect of graphite precipitation. Apparently, this is based on the assumption that if little or no carbon dissolves from the graphite crucible, the melt stays more fluid and no carbon can precipitate in the melt. However, there is nearly always some carbon present (and needs to be for reduction of oxides and formation of CO). It is not so much a question of how much carbon is present, but if it can be retained in solution. Table I shows carbon solubility in various metals. Fassel et al. (9) demonstrated convincingly that a platinum-20% tin bath is more effective than a platinum bath alone. However, tin is not invariably helpful; too much tin or too much added too quickly to a melt at operating temperature can cause graphite precipitation and ruin a bath because of the extreme insolubility of carbon in tin (Table
I). The amount of tin (0.16-0.17 gram) added intermittently as a tin capsule when calibrating with potassium acid phthalate standards, or when running samples, is permissible. All results in this paper are based upon calibration with potassium acid phthalate standards in tin capsules. R W. Ashley and T H Longhurst, At. Energy Can. Ltd.. AECL3743, Chalk River Ont.. Nov. 1970. J. Rottrnann and H Nickel. Fresenius' Z. Anal. Chem.. 247, 208 ( 1969). E. Lassner and G . Kraft J . Less-Common Metals. 22 (11, 83 ( 1970). T. A. Izrnanova and E. M . Chistyakova. S b Tr Tsent. Nauch lssled lnst. Chern Met.. 66, 59 (1969). R . P. Elliott. "Constitution of Binary Alloys, First Supplement." McGraw-Hill, New York. N . Y , 1965, pp 21 1 , 223. M . Hansen. "Constitution of Binary Alloys." McGraw-Hill. New York. N . Y 1958, pp 353, 360. 377, 3 7 8 , 3 8 0 F A. Shunk, "Constitution of Binary Alloys. Second Supplement. McGraw-Hill. New York. N.Y , 1969, pp 108, 166. J. R . Anderson and M . E. Bever, TP 2151. in "Metals Technology, Vol. 14,' AIME, New York. N.Y., April 1947. p 123. V A. Fassel. W. E Dallmann, and C. C Hill. Anal. Chem.. 38, 421 ( 19 6 6 ) .
T a b l e I. C a r b o n Solubilities in Selected M e t a l s Element
Nickel Cobalt Iron
Platinum Vanadium Copper Tin Antimony Manganese Beryllium
Carbon solubility, wt '/I
2.4 3.3
4.4-6.5 1.2 3.6-9.0 0.003 trace 0.094 6.86 < O . 3 solid solution
Temp. ' C
Source reference
1550 1550 1200-2000 1734 1630-2100 1700 boiling tin 1327 boiling 1450
Therefore, notwithstanding any other presentations in this paper, every inert gas fusion bath mentioned herein contains a variable amount of tin, usually 5-20'70. The benefits of tin may come from its ability to lower the melting point and produce a more fluid melt. Many reasons have been suggested for the beneficial effect of tin. An excellent discussion of these has been given by Fassel e t al., cited above (9). Iron, nickel, and cobalt have been used extensively as vacuum fusion baths for determining oxygen in metals, especially in Europe as shown by Friedrich and Lassner ( I O ) , who give a complete review; in the Soviet Union as exemplified by Klyachko e t al. ( 1 1 ) ; and in Japan by Kamada and Furuya (12), who used isotopic dilution to study absorption of carbon monoxide in the furnace. According to the latter two authors, it is desirable to use a combination bath, containing a metal which promotes reaction in the bath (Pt, Fe, etc.), and also a metal which prevents the absorption of carbon monoxide in the furnace (Sn). There are very few references to iron, nickel, and cobalt as baths for inert gas fusion determination of oxygen. In the extensive review by Dallmann for both carrier gas and vacuum fusion methods ( 1 3 ) , out of a summary of references, only one is to an unalloyed nickel bath employing inert (carrier) gas fusion. This sole reference is to one of Smiley's original works wherein he merely mentions that he tried nickel ( 1 4 ) . Platinum has generally ranked formost as bath material for inert gas fusion. For instance, Dallmann and Fassel (15) demonstrated the use of platinum baths with 5-20% tin for oxygen and nitrogen in 19 different base metals, but not including tungsten, manganese, or beryllium. Our attention was first called to the usefulness of nickel for oxygen by the paper of Ashley and Longhurst cited previously ( I ) . They decided for oxygen determinations of Zircalloy-2, that a 50% nickel-50% tin (10) K . Friedrich and E. Lassner. J. Less-Common Metals. 13. 156 (1967). (11) Yu. A. Klyachko. T A. Izrnanova, and E M . Chistyakova, Zavod. Lab., 29, 1425 (1963) (12) H . Kamada and K . Furuya. Bull. Chern. SOC.Jap.. 41, 1256 (1968). (13) W. E. Dallmann in "Encyclopedia of Industrial Cherntcal Analysis " Vol. 8 , Foster D. Snell and Leslie S. Ettre, Ed., John Wiley 8 Sons, Inc.. New York. N . Y . , 1969, pp 613-693. (14) W . G. Smiley, Arner. SOC. Test. Mater. Spec. Tech. Pub/., 222, 25 (1957). (15) W. E. Dallmann and V. A. Fassel, Anal. Chern.. 39, 133R (1967)
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 9, A U G U S T 1974
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Table 11. Oxygen in Titanium and Tungstena Inert gas fusion, bath
Initial charge, g
Pt
20-30 Pt
Pt-lO'jT,
v
Pt-20%
26.0 P t 2.7 V 0.10
Approx. sample wt g. 0.10 NBS 355 T i (Cert. value 3031, std dev 57) Oxygen, ppm, Av. 2560 3100 No. run 5 12 Std dev 150 190 NBS 356 T i Alloy (Cert. value 1332, std dev 77) Oxygen, ppm, Av. 1090 1330 No. run 4 6 Std dev 130 80 W-Tho? Oxygen, ppm, Av. Nil 2370 No. run Many runs 19 Std dev 150 W-Zr02 Oxygen, ppm, Av. Nil 810 No. run 7 Std dev 130
' Pt-V
Pi-257,
v
V
Pt-40';
2 1 . 3 Pt 7.0 V 0.09-0.14
1 6 . 0 Pt 11.0 v 0.11-0.17
3180
2610 3 270
2730 4 160
1
1060 2
1100 4 20
2320
2110
1
1
2180 4 120
1
1310
baths, 0.17 g Sn wrap only, 2170 ' C , 7 min.
bath was the most satisfactory. Previously, Kamin e t al. (16) had replaced 25-75% of the platinum in the baths with nickel for oxygen analyses in vanadium, niobium, and tantalum. Kallmann and Collier (17) used a nickel bath for oxygen in beryllium. Our interest in nickel was reinforced by a suggestion of Frank T. Coyle (18) that nickel has a rather universal application for inert gas fusion. A recent Bureau of Mines research project at the Albany Metallurgy Research Center involved the development of dispersion-strengthened tungsten in which both coarse and ultra-fine ZrO2 particles were formed in the tungsten matrix and excess zirconium was dissolved in the tungsten phase (19). When standard inert gas fusion techniques were used, difficulty was encountered when analyzing for oxygen content of samples from this study and similar WTho2 materials which were used as secondary standards. Quite by accident, while determining oxygen in vanadium, it was discovered that the presence of vanadium in the platinum bath significantly improved the oxygen determinations in the W-ZrO2 and W-Tho2 alloys. Subsequently, the platinum-vanadium bath composition was investigated extensively. Oxygen analyses in various metals were pursued using both nickel and platinum-vanadium bath materials.
Table 111. Oxygen in Titanium and Tungstena Method, IGF
The apparatus used for this study was a Leco 537 induction furnace with 589-606 loading device and a carbon dioxide analyzer similar to that of Lewis and Nardozzi (20). Graphite crucibles were 1-inch o.d., %-inch i.d., and 2 inches long. A pyrolitic boron nitride thimble ( 2 1 ) was used to hold the crucibles. Materials used included high-purity nickel foil, high-purity platinum foil, helium carrier gas, tin capsules, and vanadium from melted buttons. A careful temperature calibration of the apparatus was made, equating milliamperes plate current of the furnace with temperature in the graphite crucible. This was done using the melting points in graphite of high purity nickel, platinum, and hafnium as reference points. (16) G. J. K a m i n . J. W. O'Laughlin, a n d C V Banks. A n a / . Chem.. 35, 1653 (1963) (17) S. Kallmann and F . Collier.A n a / Chem.. 32. 1616 (1960) (18) F. T. Coyle, Kawecki Berylco Ind., l n c . . Boyertown. Pa.. private communication,1972 (19) R . Blickensdetfer, M . I . Copeland. and W . L O'Brien. U S . Bur. Mines, Rep. invest. 7521, Washlngton. D C . , June 1971 (20) L. L. Lewisand M J. N a t d o z z i , A n a / Chem , 38, 1214 (1966) (21) W . E. Dallrnann and V. A . Fassel, Ana/. Chem.. 38, 662 (1966).
ANALYTICAL C H E M I S T R Y , VOL.
46,
P t bath and wrap
Pt-lOCc V bath
Nickel bath and wrap
Initial charge, 4-24 Pt
g
Temp, "C Sample wrap,
2170 0.17 Sn 1 . 0 Pt
g
26.7 Pt 2.9 V 2170
T i Sponge 267 T i Sponge 268 T i Sponge 271 T i Sponge 266 NBS 355 T i (Cert. 3031) Std dev NBS 356 T i alloy (Cert. 1332) Std dev W-ThOs Std dev W-Zr02 Std dev
9 . 0 Ni
2030
0 . 1 7 Sn
Samples
EXPERIMENTAL
1192
v
22.7 Pt 5.5 V 0.10-0.15
0.17 Sn 0 . 7 5 Ni
02, Ppm
3350 3410 740 570 1160 1150 3.18% 3.36% 3080 3100 *2840 ( N = 26) ( N = 12) ( N = 3) 130 190 200
3240 710 1190 3.30% 3130 ( N = 19) 180
1350 ( N = 5) 80
1400
1330
970
(N= 6)
(N= 3) ( N
80
2370 ( N = 19) 150
810 ( N = 7)
130
250
=
5)
50
2360 ( N = 4) 40 790 ( N = 3) 40
a Various 0 ) levels, 3 methods, 7 min, 0.02- to 0.17-g samples. alone, no Sn wrap.
Sample
A graphite crucible was packed into the boron nitride thimble by surrounding it with carbon black. After insertion in the furnace, the general procedure was to blank the crucible down at 2450 "C with helium flowing, decrease the current to operating temperature, add the charge (bath), and degas at the same temperature. The apparatus was then calibrated with potassium acid phthalate standards at 100, 200, 300, and 400 micrograms of oxygen. Analyses were made of a number of metals with various baths under a variety of conditions. Conditions that were varied included temperature, fusion time, bath composition, sample wrap, and fluxes. Platinum-vanadium and nickel baths were investigated extensively. Data were developed for oxygen determinations in Zr, Ti, W, Be, and Nb alloy using the above baths. All solid samples were filed clean prior to oxygen determination. To find out why oxygen was retained in certain melts, several cooled melts were subjected to microprobe analysis. The buttons
NO. 9 . A U G U S T 1974
Table IV. Oxygen in Zirconium Powder. Zr-E - 15
'I
Zr-E-2i
Zr-E-20
Sample
~~
Initial charge, g Wrap, g
6 . 7 Pt 0 . 1 6 Sn 1 . 0 Pt
Recommended method, X No. run Oxygen, ppm, av Std dev
3 1640 40
7 . 5 Ni 0 . 1 6 Sn 0 . 7 5 Ni
6 . 7 Pt 0 . 1 6 Sn 1 . 0 Pt
7 . 5 Ni 0 . 1 6 Sn 0.75 Ni
6.7 Pt 0 . 1 6 Sn 1 . 0 Pt
X
X 4 1880 50
3 1540 180
1600 100
x
9 1630 210
3 650 a0
4
7 . 5 Ni 0 . 1 6 Sn 0 . 7 5 Ni
Inert gas fusion, 2170 'C, i min, 0.0i-to 0.12-g samples.
were broken away from the graphite crucibles, vertically sectioned, mounted and polished, and characterized using an electron microprobe with scanning beam micrograph capability to
identify phases. RESULTS AND DISCUSSION Oxygen in Titanium and Tungsten. Table I1 shows oxygen results in titanium and tungsten using platinumvanadium baths. Undoubtedly, one of the reasons that vanadium is helpful is its ability to dissolve carbon (Table I). Also, it forms a relatively low melting point carbide (V2C, mp ca. 2165 "C). Platinum does not form a carbide. For the bath to have a long life, platinum must be added to the crucible first, then the vanadium. This is a general rule: The bath metal with the lowest carbon solubility should be added first. If the metal with lesser carbon solubility is added second, graphite precipitation occurs. The vanadium has an amazing effect on the melt: It dissolves any slag on the melt and produces a very clear and highly fluid bath. At the same time, the platinum-vanadium alloy does not attack the crucible any faster than platinum alone. However, a platinum-vanadium bath is effective only within a fairly narrow range of vanadium content-about 10-20% vanadium. This is shown in Table 11. Further limited data indicate the minimum amount of vanadium required is about 8%. Although 20 weight 70is about the upper vanadium limit, this amounts to nearly 50 atomic 9% V. Waterstrat has studied the Pt-V alloy system ( 2 2 ) . After the vanadium effect was discovered, 10% each of iron, nickel, cobalt, and molybdenum were individually added to platinum baths, but none had the beneficial effect of vanadium. Table I1 shows that practically no response was obtained with an unalloyed platinum bath when analyzing thoriated and zirconiated tungsten. Microprobe studies revealed that the tungsten survived the conditions in the pure platinum baths. Pure tungsten without a platinum wrap acted similarly to the zirconiated and thoriated samples. Occasionally, the thoriated and zirconiated tungsten would not work in unalloyed platinum even with a platinum wrap. Microprobe studies also showed that little or no alloying of tungsten took place with platinum1070 cobalt and platinum-10% iron baths. Partial alloying took place with platinum-10% nickel and platinum-10% molybdenum baths. The buttons from the platinum-vanadium baths, where complete liberation of oxygen occurred, were the only ones containing a mixed carbide phase. There was always a tungsten-vanadium carbide phase present representing about 5% of the melt. Somehow the vanadium helped the tungsten to dissolve in the melt. Table I11 shows that oxygen in titanium and tungsten may be determined, using a platinum-10% vanadium bath, without any wrap or flux other than tin. This tin (22) R M Waterstrat Met Trans 4 , 455 (Feb 1973)
Table V. Oxygen in Berylliuma Sample
Initial charge, g Approx. sample wt, g Wrap, g Additional flux, g No. samples able to run per cruc. No. of runs Oxygen, ppm, av. Std dev Other value
6-252 Be
9.ONi
9.ONi
0.09-0.16 0 . 7 5 Ni 0 . 7 5 Ni
0.16-0.25 0 . 7 5 Ni 1 . 5 0 Ni
12
7 7 a7 7
11 217
49
Re-6
Re-7
9 . 0 Ni 0 . 0 3 - 0 . il 0 . 7 5 Ni
None 12 12 4870 200
200, USRM, Reno,
neutron activation Inert gas fusion, recommended procedure: 1900 "C, 14 min, 1: 1 blanks: samples.
wrap, however, is necessary in the case of titanium. This is also shown in Table 111. The platinum-10% vanadiumtin wrap procedure was effective a t all levels of oxygen content with titanium sponge and solid samples Pt:sample ratios should be maintained a t least 5:l for titanium and zirconiated and thoriated tungsten. Nickel Bath a n d Zirconium Powder Analysis. Oxygen in zirconium powders is shown in Table IV using both platinum and nickel. A nickel bath and wrap gave higher results than a platinum bath and wrap. The precise reason for this is unknown. However, zirconium forms a very high melting point carbide (ZrC, mp ca. 3450 "C). It is suspected that the zirconium powder particles act as nucleation sites for insoluble ZrC which disperses throughout the platinum melt and interferes with the complete extraction of oxygen. When observing the hot melts in the apparatus through a prism, no precipitation seems to take place with zirconium powders when using a nickel bath. This same pattern was followed on other zirconium powder samples in addition to those shown in Table IV. A nickel bath and wrap also works well for titanium and tungsten (Table 111). Adding vanadium to a nickel bath did not improve the bath as it did in the case of platinum: A nickel wrap was still necessar? . A nickel bath has one drawback. Nickel attacks the crucible faster than platinum. The crucibles easily burn through, lasting only 1 day (8 hours). However, melts generally are not functional anyway after cooling and reheating. because of carbon precipitation. Getters: Beryllium a n d Manganese. It is difficult to determine oxygen in beryllium. Kallmann and Collier ( I 7 ) have given a good discussion of the problems associated with analysis of beryllium for oxygen content, both vacuum fusion and inert gas fusion. In summary, beryllium has a high vaapor pressure and is a getter, it is difficult to alloy with many metals, and B e 0 is refractory and difficult to reduce. However, by using nickel, as was done by Kallmann, a satisfactory method for beryllium was
A N A L Y T I C A L C H E M I S T R Y . VOL. 46, NO. 9 , AUGUST 1974
1193
Table VI. Oxygen in FS-85Niobium Alloy Method
Inert gas fusion
Initial charge, g
9 . 0 Ni
6 . 0 Pt
Approx. sample wt, g Temp, "C
0.3-0.9 2170-2310 0.75 Ni 0.75-2.25 Ni 7-14
0.3-0.9 2170 1 . 0 Pt 1 . 0 Pt 7
Wrap, g
Additional flux, g Extraction time, min Recommended method, X No. of runs Oxygen, P P ~av , Std dev
12 45 10
evolved. There is an optimum temperature for beryllium analysis, and either a higher or lower temperature may cause problems. For our apparatus, the optimum temperature was about 1900 "C,but this temperature may be different for each apparatus. At this temperature, 14-minute extractions were required to achieve the highest recoveries of oxygen. Each beryllium sample was wrapped in 0.75 gram nickel, and a total flux (including the wrap) of 0.75-2.25 gram nickel, depending on sample size, was dropped with the sample. Also a blank of the same amount of nickel flux was run after every sample. Results on beryllium are shown in Table V. With the recommended procedure 12 beryllium samples (Be-6) and 13 blanks were run alternately in a single crucible before oxygen recovery decreased significantly. The longer extraction time without unduly high temperatures apparently assures reduction of oxides with very little gettering. Graphite precipitation may be a factor with beryllium also. That is why, if one has a functioning bath, everything should be done to maintain the carbon solubility. The temperature should remain as nearly the same as possible, especially if one is adding a sample of lesser carbon solubility. Adding carbon dissolvers as blanks, sample wraps, or fluxes should be helpful because the carbon solubility equilibrium is restored. From carbon solubility considerations, neither tin nor copper should be helpful for determining oxygen in beryllium. From alloying considerations, copper is better than tin; it alloys with beryllium. No alloying of beryllium and tin can be obtained below the melting point of beryllium, and an extended miscibility gap occurs with beryllium and tin in the molten state. When trying to determine oxygen in beryllium with a platinum bath, responses were either low or dropped off rapidly after the first sample. Manganese is even more volatile than beryllium, and samples of manganese had to be run a t even lower temperatures than beryllium (about 1610 "C), However, extraction times of only 7 minutes were necessary. Seventeen small manganese samples were run in one crucible with a nickel bath and a tin-nickel wrap before responses dropped off significantly. Tin is definitely helpful here and suppresses gettering by manganese. When Sn:Mn ratio in the nickel bath was maintained a t >15:1, no gettering problems were encountered. This is in line with the procedure for manganese of Shulte et al (23), who used vacuum fusion and a Ni-Sn bath. It is also consistent with the procedure of Goto et a2 ( 2 4 ) , who used a tin bath only with inert gas (argon) fusion for oxygen in manganese. FS-85 Niobium Alloy. The FS-85 alloy has a nominal (23) Y u A Shulte N A Gederevich and V S Shitikov Zavod Lab 3 4 , 1193 (1968) ( 2 4 ) H Goto S lkeda and A O n u r n a S o Rept Res lnst TohOku Univ Ser A 17, 237 (1965)
1194
Vacuum fusion
26.9 Pt 3.0 V 0.5-0.9 2170
14-40 Pt
none none
none none
0.6-1.8 1700-1960
7
5-10
X
X
X
8 64 14
6 63 10
40 61 8
composition of 2.8% tantalum, 10% tungsten, 1.0% zirconium, and the remainder niobium. This alloy proved to be the most difficult of all to analyze for oxygen and obtain consistent maximum values. A nickel bath with nickel sample wrap and flux did not suffice for this material. Satisfactory results were produced with a platinum bath and platinum wrap and flux, and with a platinum-10% vanadium bath with no wrap or flux whatsoever. However, the minimum platinum-tosample ratio in the bath, counting the vanadium as sample, must remain above 5:l. Therefore, only a small number of samples ( a t this low level of oxygen where large samples are required) may be run in one crucible. With a platinum bath only, the platinum-to-sample ratio must remain above 8:1. All results on FS-85 niobium are shown in Table VI. Results of many runs by vacuum fusion are also included to more sharply delineate the oxygen content of this material. Nickel Baths for Other Metals. Although beyond the scope of this work, determinations for oxygen in other metals indicate that with inert gas fusion, a nickel bath and wrap will work for solid samples of unalloyed niobium, chromium, copper, molybdenum, zirconium, hafnium, vanadium, and most steels.
CONCLUSIONS A nickel bath with nickel sample wrap is superior to a platinum bath for the inert gas fusion determination of oxygen in zirconium, beryllium, and manganese. It is equal to platinum for titanium and tungsten, including thoriated and zirconiated tungsten. Nickel does not suffice for FS-85 niobium alloy. A platinum-10% vanadium bath is superior to platinum alone for W-ThOZ and W-ZrOz. A platinum-10% vanadium bath allows the analysis for oxygen of titanium, WThOz, W-ZrOz, and FS-85 niobium without wrap or flux, other than tin in the case of titanium. An important factor for successful inert gas fusion determination of oxygen in metals is maintaining carbon solubility in the melt.
ACKNOWLEDGMENT The assistance given by Peter A. Romans, Albany Metallurgy Research Center, who performed the microprobe analyses is gratefully acknowledged. I am also grateful to Frank Coyle of Kawecki Berylco Inc. who first suggested to us the more universal use of nickel, and who supplied zirconium samples and information for their analyses. Received for review October 9, 1973. Accepted April 4, 1974. Reference to specific makes or models is made to facilitate understanding and does not necessarily imply endorsement by the Bureau of Mines.
A N A L Y T I C A L C H E M I S T R Y , V O L . 4 6 , NO. 9 . A U G U S T 1 9 7 4
