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The corrosion of molybdenum and tungsten in liquid sodium
Journal of the Less-Common Metals, 44 (1976) 169 - 176 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
THE CORROSION SODIUM
OF MOLYBDENUM
MARTEN G. BARKER
and CHRISTOPHER
Department
AND TUNGSTEN
169
IN LIQUID
W. MORRIS
of Chemistry, The University, Nottingham NG7 2RD (Gt. Britain)
(Received May 22, 1975)
Summary
The corrosion of molybdenum and tungsten has been examined in static and dynamic liquid sodium. Corrosion of molybdenum was not found to be oxygen dependent; no ternary oxide corrosion products were observed. Inclusion of labile carbon into the system containing molybdenum caused the formation of molybdenum carbide, MoaC, which was identified by its powder X-ray diffraction pattern. The corrosion of tungsten was found to be strongly influenced by the initial concentration of dissolved oxygen in the liquid metal; at low oxygen levels in the dynamic system the cubic phase, NasW04 (a, = 4.62 a), was identified by its X-ray diffraction pattern recorded through a matrix of sodium, whereas, at initially very high oxygen levels in static systems, the orthorhombic phase, Nas WOs, was identified. The solid-state interaction of sodium oxide with molybdenum and tungsten metals under vacuum give the ternary phases Na4Mo05 and Nas WOs, respectively, together with unreacted refractory metal and sodium vapour.
Introduction
The possible use of molybdenum-coated components for materials exposed to high-temperature, flowing, liquid sodium in fast-breeder reactors has lead to the need for more basic information on the behaviour of molybdenum exposed to high-temperature, liquid sodium. In Group 6A, the behaviour of chromium in liquid sodium has been studied [l] , but the behaviour of the remaining two elements is unknown: preliminary studies [2] on molybdenum indicated no corrosion products on the surface of the metal, and in the case of tungsten the phase or phases formed in corrosion remained unidentified. The object of the study was, principally, to determine the nature of any interaction of molybdenum and tungsten with both static and dynamic sodium, and to characterise any corrosion products that may have formed.
170 TABLE
1
The corrosion
of molybdenum
in static oxygen-doped
sodium
Sample
Sodium conditions
Corrosion temperature WI
MO 1
1000 ppm oxygen 3000 ppm oxygen. 3400 ppm oxygen Heavily doped with oxygen Heavily doped with oxygen Heavily doped with oxygen Heavily doped with oxygen
400
7
3.1479
550
24
3.1470
600
2
3.1467
600
16
3.1456
600
16
3.1460
600
16
3.1465
600
16
3.1467
MO 2 Mo3 Mo4 MO 5 MO6 Mo7
Time at corrosion temperature (days)
Metal lattice parameter (8)
Experimental (a) Static tests Using techniques similar to those described for niobium and tantalum [3], samples of molybdenum and tungsten metal in the form of sheet (2 X 1 X 0.1 cm) were subjected to a s~ium-oxygen environment. Tests were carried out using heavily-doped sodium in nickel pots at 400 - 650 “C for periods of 2 - 24 days. X-ray diffraction analysis of the plates was carried out using a Philips PW 1050/25 vertical goniometer with a modified chamber [3]. The lattice constant of the metal was determine by rne~u~g the position of a high-angle diffraction peak using a scan rate of l/8“ 2S per minute and computing the a, value from the known hkl value of the particular peak. Using the same techniques as above, molybdenum metal was placed in oxygen-containing sodium to which had been added (i) charcoal, (ii) a 316 stainless steel tab, and (iii) titanium carbide. (b) Dynamic tests Molybdenum and tungsten tabs (2 X 1 X 0.1 cm) were placed in the hot leg of a 321~stainless circulating sodium loop (1100 cm3 sodium capacity with nominal electromagnetic pumping) via an argon-filled, evacuable glove-box. Prior to commissioning, the loop had been run blank for several weeks with the hot leg operating between 400 and 500 “C and the cold leg between 200 and 300 “C. Before loading the samples into the loop, the glove-box atmosphere was gettered with a crucible of liquid sodium for a few hours and the whole of the loop cooled to ca. 120 “C. After sealing the
171 TABLE 2 The corrosion of molybdenum in static carbon-doped sodium Sample
Sodium conditions
Corrosion temperature (“C)
Time at corrosion temperature (days)
Metal lattice parameter (A)
Corrosion product
MO 8
Stainless steel tab added Stainless steel tab added Stainless steel tab added Stainless steel tab added Titanium carbide-doped Titanium carbide-doped Titanium carbide-doped Charcoal-doped Charcoal-doped Charcoal-doped Charcoal-doped
600
16
3.1492
-
600
16
3.1493
-
600
16
3.1452
-
600
16
3.1453
-
600
16
3.1494
-
16
3.1458
-
600
16
3.1475
-
600 600 600 600
16 16 16 16
3.1450 3.1467 3.1448 3.1444
MozC Mo2C Mo2C Mo2C
Mo9 MO 10 Moll MO 12 MO 13 MO 14 MO 15 MO 16 MO 17 MO 18
sample in position, the loop was operated at hot-leg temperatures between 440 and 650 “C and cold-leg temperatures between 120 and 260 “C for periods of, typically, five days. Using the reverse procedure, the sample was removed and subjected to X-ray diffraction analysis. (c) Solid state reactions Powdered molybdenum and tungsten metals and sodium oxide (prepared as previously described [4] ) were intimately mixed and heated under vacuum in nickel-foil crucibles. The resulting reaction mixture was examined by X-ray diffraction. Results (a) Static tests for molybdenum (i) Oxygen-doped sodium The conditions and results of these experiments are presented in Table 1 (samples Mol to Mo7). X-ray diffraction studies of the metal plates showed no evidence of either binary or ternary oxides of molybdenum. Other than molybdenum metal, the only phases observed were sodium and sodium oxide (Na20). The absence of corrosion products contrasts markedly with previous observations for other metal and metal-alloy systems studied using this technique, where large quantities of corrosion products are observed after relatively short periods of time at high oxygen activity [l, 3, 5, 61.
172
(ii) Carbon-doped sodium The conditions, and results of experiments of exposing molybdenum samples to oxygen-containing sodium doped with (i) charcoal, (ii) 316&ainless steel, (iii) titanium carbide, are presented in Table 2 (samples Mo8 to Mo18). These data indicated that no binary or ternary oxides had formed, which is in agreement with experiments Mol to Mo7. All diffraction lines could be assigned to molybdenum, sodium, and sodium oxide, except one of low intensity at d = 2.28 A, from the samples doped with charcoal. Initially, this phase (which persisted even after water washing) was not identified, but by comparison with the data obtained in the dynamic tests (next Section) the line was assigned to the phase MosC. (b) Dynamic tests for molybdenum The conditions and results of these tests are summarised in Table 3 (samples MO19 to Mo26). X-ray analysis of sample MO19 revealed a phase with d = 2.59, 2.28, 1.75,1.270 and 1.255 A, which persisted on water washing. The same phase was observed on all of the other samples in the dynamic tests except sample Mo25. The I = 100% diffraction line, d = 2.28 A, for this phase corresponded to the diffraction line observed for molybdenum samples exposed to charcoal-doped sodium (samples Mo15 to MO 18). The observed phase could be indexed in the hexagonal system with a, = 3.01 A, c, = 4.72 a and c/a = 1.57. By comparison with literature data [ 71 and standard diffraction traces, the hexagonal phase was identified as molybdenum carbide, MO&. It was concluded from this that the phase observed in the static tests was also molydenum carbide. (c) Static and dynamic tests for tungsten The details of these experiments are given in Table 4. The single dynamic test revealed only one phase other than the metal and sodium, and this* had the cubic NaCl-type structure with a, = 4.62 a. The diffraction peaks for this phase were found to correspond exactly with those of the compound NasW04 (a,, = 4.62 A) [8, 91, which can be readily prepared from the interaction of liquid sodium and NasW04 at 600 “C, followed by a low-temperature distillation of the excess sodium. From these data it was concluded that the corrosion product was NasWO*. For the static tests, other than tungsten and sodium, a phase or phases with identical peaks for each plate was observed. All the diffraction peaks observed could be assigned to the compound NaGWOs. For identification purposes, the phases Na4WQ and NaeWOs had been prepared by solid-state reaction of sodium oxide (NasO) and Na2W04. The X-ray diffraction data for these phases agreed with that given by Reau et al. [lo]. However, there was no evidence for any of these less alkali-rich phases on the plate surface. (d) Solid-state reactions The results and conditions of the experiments are given in Table 5. The ternary oxide phases were identified by comparison with standard diffraction
173 TABLE
3
The corrosion
of molybdenum
by sodium
in a dynamic
system
Sample
Hot leg temperature (“C)
Cold leg temperature (“C)
Time (days)
MO 19
470
260
5
3.1467
MO 20
440
140
5
3.1455
MO 21a and 21b
450 170
150 Frozen
4 7
(a) 3.1463 (b) 3.1466
350 575 650 530 150 440
120 120 150 150 120 120
(a) 3.1501 (b) 3.1492 3.1448 3.1455
640
130
3.1419 3.1459
MO and MO MO MO
22a t 22b 23 24 25
MO 26 MO Standard
TABLE
3.1452
Corrosion products as observed by X-ray diffraction Mo2C a, = 3.01 A c, = 4.72 A c/a = 1.57 v. small quantity of MozC by comparing metal and carbide intensities d = 2.22 8, and d = 2.28 A, respectively. v. small quantity of MozC for both (a) and (b) (by comparing metal and carbide intensities at d = 2.22 A and d = 2.28 A, respectively). (a) MozC (b) Mo2C Mo2C Mo2C MO& not positively identified from X-ray data Mo2C No carbide observed in material as examined by X-ray diffraction
4
The corrosion Plate
(50)
Lattice parameter (A)
of tungsten
Temp (“C)
Wl w2 w3 w4
550 550 550 Dynamic with hot leg at 470 “C; cold leg at 260 “C W Standard *48 wt. ppm according
Conditions
Time (days)
Initial oxygen level (wt.ppm)
Corrosion product(s)
Metal lattice parameter (A)
Static Static Static
5 5 5 5
10,000 8000 3000 *
Nae Woe Nau Woe NaGWOe NaaW04
3.1662 3.1662 3.1664 3.1678
to Noden’s
3.1672 data [ 191.
data presented by R6au et al. The interaction of the two metals with sodium oxide in the solid state at such a low temperature is typical of several M + Na20 systems (M=Ti, V, Nb, Ta [ll],Zr, Fe and Cr [H] ).
174 TABLE 5 Solid state reactions Reactants -
Reaction temperature (“C)
4Naz0 f MO 300 2NazO + W 360
Equilibration temperature CC)
Reaction time (h)
Products
450 600
18 18
NaqMoOs + Na NaGWOe + Na
Discussion
A consideration of the chemical potential of carbon dissolved in liquid sodium at different concentrations and temperatures [ 131 shows that at 500 “C carburization of molybdenum metal is thermodynamically favourable for sodium-carbon solutions at carbon concentrations greater than 10e3 ppm. Molybdenum carbide (MO&) [ 141 and chromium carbide (Cr7C3) [15 ] have reasonably similar free energies of formation at 500 “C. This would imply that molybdenum metal could act as a very effective carbon sink in systems with approximately the same carbon potentials as chromium carbide, i.e., stainless steels, until equilibrium is achieved. Thus, from the data presented in Table 3 it can be concluded that, in the temperature range covered, the magnitude of the carbon potential in the 321~stainless steel loop was less than the magnitude of the carbon potential of the molybdenum-molybdenum carbide couple. The appearance/non-appearance of molybdenum carbide in the dynamic system would appear to be a function of the hot-leg temperature, although this is somewhat confused by the fact that MO19 appeared to have formed far more carbide than any of the other samples (as detected by the X-ray diffraction ~~hnique). Thus, it is probable that the later results were affected by the decarburization of the 321~stainless steel loop, which would lower the drlving force of reactions in experiments undertaken later in the lifetime of the loop. The appearance of molybdenum carbide in the carbon-doped static system is in agreement with the thermod~~i~ prediction, as is the nonappearance of carbide in the titanium c~bide-doped system. The absence of carbide formation in the stainless steel-doped static tests is probably a function of the quantity of available carbon in the small stainless steel tabs. In no experiment, static or dynamic, was there any evidence for the formation of either a binary or a ternary oxide corrosion product on the moly~enum surface, although it is known that some ternary sodium-molybdenum-oxides are stable towards liquid sodium under the conditions of this investigation [ 161. Weight-loss experiments on the static oxygen-doped experiments indicated no measurable weight change, which is in agreement with the observation that no significant amount of corrosion took place.
175
Tungsten Preliminary investigations on the corrosion of tungsten metal in oxygendoped liquid sodium by Wood [2] did not allow the identification of any of the observed corrosion products, but these products did appear to relate to the reactions of tungsten powder and sodium tungstate in oxygen-rich sodium. On comparing the diffraction data from Wood’s study with data from the present study it appears that the product of these reactions was Nas WOs. The experimental results presented in Table 4 would suggest that the low oxygen-activities encountered in the dynamic sodium system (i.e.,
176
Conclusions The observed behaviour of molybdenum metal is seen to be very different from that of the other Group 6A metals, where ternary oxide formation is observed under very similar conditions. The formation of molybdenum carbide, but not tungsten carbide, in the dynamic system, would appear to arise as a consequence of the lack of ternary oxide formation on the molybdenum metal. This would appear to be due to kinetic effects rather than to thermodynamic ones, as it has been shown that some sodiummolybdenum-oxygen ternary oxides are stable towards liquid sodium [16]. The kinetic aspect in liquid sodium is further emphasised by the observation that both molybdenum and tungsten metals react with sodium oxide at a temperature less than 400 “C in the solid state to give ternary oxides, but on moving to the liquid sodium system only tungsten shows ternary oxide formation, even in sodium saturated with oxygen at 600 “C. It can be concluded that the corrosion of molybdenum, unlike vanadium, niobium, tantalum, chromium and tungsten, should not be oxygen dependent, but at carbon levels greater than 10e3 ppm at 500 “C, the formation of molybdenum carbide (MO&) will take place, which may have a significant effect on any molybdenum coatings, especially if the carbide is formed by a wedging mechanism [ 181. Acknowledgments The authors wish to thank the Central Electricity Generating Board, Berkeley Nuclear Laboratories, for their financial support for this work. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
M. G. Barker and D. J. Wood, J. Less-Common Met., 35 (1974) 315. D. J. Wood, Ph. D. Thesis, Univ. Nottingham, 1971. C. C. Addison, M. G. Barker and D. J. Wood, J. Chem. Sot., Dalton Trans., (1972) 13. M. G. Barker and D. J. Wood, J. Chem. Sot., Dalton Trans., (1972) 9. M. G. Barker and D. J. Wood, J. Chem. Sot., Dalton Trans., (1972) 2451. M. G. Barker and D. J. Wood, J. Less-Common Met., 34 (1972) 215. J. D. H. Donnay, Crystal Data, American Crystallographic Association, 1963. H. Kessler, A. Hatterer and C. Ringenbach, C.R. Acad. Sci., Ser. C, 277 (1973) 763. A. J. Hooper, Ph. D. Thesis, Univ. Nottingham, 1971. J.-M. Rbau, C. Fouassier and P. Hagenmuller, Bull. Sot. Chim. Fr., (1967) 3873. M. G. Barker, to be published. M. G. Barker and D. J. Wood, Reactivity of Solids, Chapman and Hall, London, 1972, 623. M. R. Hobdell, CEGB Rep. RD/B/N1931,1972. A. Solbakken and P. H. Emmett, J. Am. Chem. Sot., 91(1969) 31. C. E. Wicks and F. E. Block, U.S. Bur. Mines Bull., (1963) 605. M. G. Barker and C. W. Morris, to be published. F. A. Shunk, Constitution of Binary Alloys, First Supplement, McGraw-Hill, New York, 1969. R. L. Klueh, in J. E. Draley and J. R. Weeks (eds.), Corrosion of Liquid Metals, Plenum Press, New York, 1970, p. 137. J. D. Noden, J. Br. Nucl. Energy Sot., 12 (1973) 57.
THE CORROSION SODIUM
OF MOLYBDENUM
MARTEN G. BARKER
and CHRISTOPHER
Department
AND TUNGSTEN
169
IN LIQUID
W. MORRIS
of Chemistry, The University, Nottingham NG7 2RD (Gt. Britain)
(Received May 22, 1975)
Summary
The corrosion of molybdenum and tungsten has been examined in static and dynamic liquid sodium. Corrosion of molybdenum was not found to be oxygen dependent; no ternary oxide corrosion products were observed. Inclusion of labile carbon into the system containing molybdenum caused the formation of molybdenum carbide, MoaC, which was identified by its powder X-ray diffraction pattern. The corrosion of tungsten was found to be strongly influenced by the initial concentration of dissolved oxygen in the liquid metal; at low oxygen levels in the dynamic system the cubic phase, NasW04 (a, = 4.62 a), was identified by its X-ray diffraction pattern recorded through a matrix of sodium, whereas, at initially very high oxygen levels in static systems, the orthorhombic phase, Nas WOs, was identified. The solid-state interaction of sodium oxide with molybdenum and tungsten metals under vacuum give the ternary phases Na4Mo05 and Nas WOs, respectively, together with unreacted refractory metal and sodium vapour.
Introduction
The possible use of molybdenum-coated components for materials exposed to high-temperature, flowing, liquid sodium in fast-breeder reactors has lead to the need for more basic information on the behaviour of molybdenum exposed to high-temperature, liquid sodium. In Group 6A, the behaviour of chromium in liquid sodium has been studied [l] , but the behaviour of the remaining two elements is unknown: preliminary studies [2] on molybdenum indicated no corrosion products on the surface of the metal, and in the case of tungsten the phase or phases formed in corrosion remained unidentified. The object of the study was, principally, to determine the nature of any interaction of molybdenum and tungsten with both static and dynamic sodium, and to characterise any corrosion products that may have formed.
170 TABLE
1
The corrosion
of molybdenum
in static oxygen-doped
sodium
Sample
Sodium conditions
Corrosion temperature WI
MO 1
1000 ppm oxygen 3000 ppm oxygen. 3400 ppm oxygen Heavily doped with oxygen Heavily doped with oxygen Heavily doped with oxygen Heavily doped with oxygen
400
7
3.1479
550
24
3.1470
600
2
3.1467
600
16
3.1456
600
16
3.1460
600
16
3.1465
600
16
3.1467
MO 2 Mo3 Mo4 MO 5 MO6 Mo7
Time at corrosion temperature (days)
Metal lattice parameter (8)
Experimental (a) Static tests Using techniques similar to those described for niobium and tantalum [3], samples of molybdenum and tungsten metal in the form of sheet (2 X 1 X 0.1 cm) were subjected to a s~ium-oxygen environment. Tests were carried out using heavily-doped sodium in nickel pots at 400 - 650 “C for periods of 2 - 24 days. X-ray diffraction analysis of the plates was carried out using a Philips PW 1050/25 vertical goniometer with a modified chamber [3]. The lattice constant of the metal was determine by rne~u~g the position of a high-angle diffraction peak using a scan rate of l/8“ 2S per minute and computing the a, value from the known hkl value of the particular peak. Using the same techniques as above, molybdenum metal was placed in oxygen-containing sodium to which had been added (i) charcoal, (ii) a 316 stainless steel tab, and (iii) titanium carbide. (b) Dynamic tests Molybdenum and tungsten tabs (2 X 1 X 0.1 cm) were placed in the hot leg of a 321~stainless circulating sodium loop (1100 cm3 sodium capacity with nominal electromagnetic pumping) via an argon-filled, evacuable glove-box. Prior to commissioning, the loop had been run blank for several weeks with the hot leg operating between 400 and 500 “C and the cold leg between 200 and 300 “C. Before loading the samples into the loop, the glove-box atmosphere was gettered with a crucible of liquid sodium for a few hours and the whole of the loop cooled to ca. 120 “C. After sealing the
171 TABLE 2 The corrosion of molybdenum in static carbon-doped sodium Sample
Sodium conditions
Corrosion temperature (“C)
Time at corrosion temperature (days)
Metal lattice parameter (A)
Corrosion product
MO 8
Stainless steel tab added Stainless steel tab added Stainless steel tab added Stainless steel tab added Titanium carbide-doped Titanium carbide-doped Titanium carbide-doped Charcoal-doped Charcoal-doped Charcoal-doped Charcoal-doped
600
16
3.1492
-
600
16
3.1493
-
600
16
3.1452
-
600
16
3.1453
-
600
16
3.1494
-
16
3.1458
-
600
16
3.1475
-
600 600 600 600
16 16 16 16
3.1450 3.1467 3.1448 3.1444
MozC Mo2C Mo2C Mo2C
Mo9 MO 10 Moll MO 12 MO 13 MO 14 MO 15 MO 16 MO 17 MO 18
sample in position, the loop was operated at hot-leg temperatures between 440 and 650 “C and cold-leg temperatures between 120 and 260 “C for periods of, typically, five days. Using the reverse procedure, the sample was removed and subjected to X-ray diffraction analysis. (c) Solid state reactions Powdered molybdenum and tungsten metals and sodium oxide (prepared as previously described [4] ) were intimately mixed and heated under vacuum in nickel-foil crucibles. The resulting reaction mixture was examined by X-ray diffraction. Results (a) Static tests for molybdenum (i) Oxygen-doped sodium The conditions and results of these experiments are presented in Table 1 (samples Mol to Mo7). X-ray diffraction studies of the metal plates showed no evidence of either binary or ternary oxides of molybdenum. Other than molybdenum metal, the only phases observed were sodium and sodium oxide (Na20). The absence of corrosion products contrasts markedly with previous observations for other metal and metal-alloy systems studied using this technique, where large quantities of corrosion products are observed after relatively short periods of time at high oxygen activity [l, 3, 5, 61.
172
(ii) Carbon-doped sodium The conditions, and results of experiments of exposing molybdenum samples to oxygen-containing sodium doped with (i) charcoal, (ii) 316&ainless steel, (iii) titanium carbide, are presented in Table 2 (samples Mo8 to Mo18). These data indicated that no binary or ternary oxides had formed, which is in agreement with experiments Mol to Mo7. All diffraction lines could be assigned to molybdenum, sodium, and sodium oxide, except one of low intensity at d = 2.28 A, from the samples doped with charcoal. Initially, this phase (which persisted even after water washing) was not identified, but by comparison with the data obtained in the dynamic tests (next Section) the line was assigned to the phase MosC. (b) Dynamic tests for molybdenum The conditions and results of these tests are summarised in Table 3 (samples MO19 to Mo26). X-ray analysis of sample MO19 revealed a phase with d = 2.59, 2.28, 1.75,1.270 and 1.255 A, which persisted on water washing. The same phase was observed on all of the other samples in the dynamic tests except sample Mo25. The I = 100% diffraction line, d = 2.28 A, for this phase corresponded to the diffraction line observed for molybdenum samples exposed to charcoal-doped sodium (samples Mo15 to MO 18). The observed phase could be indexed in the hexagonal system with a, = 3.01 A, c, = 4.72 a and c/a = 1.57. By comparison with literature data [ 71 and standard diffraction traces, the hexagonal phase was identified as molybdenum carbide, MO&. It was concluded from this that the phase observed in the static tests was also molydenum carbide. (c) Static and dynamic tests for tungsten The details of these experiments are given in Table 4. The single dynamic test revealed only one phase other than the metal and sodium, and this* had the cubic NaCl-type structure with a, = 4.62 a. The diffraction peaks for this phase were found to correspond exactly with those of the compound NasW04 (a,, = 4.62 A) [8, 91, which can be readily prepared from the interaction of liquid sodium and NasW04 at 600 “C, followed by a low-temperature distillation of the excess sodium. From these data it was concluded that the corrosion product was NasWO*. For the static tests, other than tungsten and sodium, a phase or phases with identical peaks for each plate was observed. All the diffraction peaks observed could be assigned to the compound NaGWOs. For identification purposes, the phases Na4WQ and NaeWOs had been prepared by solid-state reaction of sodium oxide (NasO) and Na2W04. The X-ray diffraction data for these phases agreed with that given by Reau et al. [lo]. However, there was no evidence for any of these less alkali-rich phases on the plate surface. (d) Solid-state reactions The results and conditions of the experiments are given in Table 5. The ternary oxide phases were identified by comparison with standard diffraction
173 TABLE
3
The corrosion
of molybdenum
by sodium
in a dynamic
system
Sample
Hot leg temperature (“C)
Cold leg temperature (“C)
Time (days)
MO 19
470
260
5
3.1467
MO 20
440
140
5
3.1455
MO 21a and 21b
450 170
150 Frozen
4 7
(a) 3.1463 (b) 3.1466
350 575 650 530 150 440
120 120 150 150 120 120
(a) 3.1501 (b) 3.1492 3.1448 3.1455
640
130
3.1419 3.1459
MO and MO MO MO
22a t 22b 23 24 25
MO 26 MO Standard
TABLE
3.1452
Corrosion products as observed by X-ray diffraction Mo2C a, = 3.01 A c, = 4.72 A c/a = 1.57 v. small quantity of MozC by comparing metal and carbide intensities d = 2.22 8, and d = 2.28 A, respectively. v. small quantity of MozC for both (a) and (b) (by comparing metal and carbide intensities at d = 2.22 A and d = 2.28 A, respectively). (a) MozC (b) Mo2C Mo2C Mo2C MO& not positively identified from X-ray data Mo2C No carbide observed in material as examined by X-ray diffraction
4
The corrosion Plate
(50)
Lattice parameter (A)
of tungsten
Temp (“C)
Wl w2 w3 w4
550 550 550 Dynamic with hot leg at 470 “C; cold leg at 260 “C W Standard *48 wt. ppm according
Conditions
Time (days)
Initial oxygen level (wt.ppm)
Corrosion product(s)
Metal lattice parameter (A)
Static Static Static
5 5 5 5
10,000 8000 3000 *
Nae Woe Nau Woe NaGWOe NaaW04
3.1662 3.1662 3.1664 3.1678
to Noden’s
3.1672 data [ 191.
data presented by R6au et al. The interaction of the two metals with sodium oxide in the solid state at such a low temperature is typical of several M + Na20 systems (M=Ti, V, Nb, Ta [ll],Zr, Fe and Cr [H] ).
174 TABLE 5 Solid state reactions Reactants -
Reaction temperature (“C)
4Naz0 f MO 300 2NazO + W 360
Equilibration temperature CC)
Reaction time (h)
Products
450 600
18 18
NaqMoOs + Na NaGWOe + Na
Discussion
A consideration of the chemical potential of carbon dissolved in liquid sodium at different concentrations and temperatures [ 131 shows that at 500 “C carburization of molybdenum metal is thermodynamically favourable for sodium-carbon solutions at carbon concentrations greater than 10e3 ppm. Molybdenum carbide (MO&) [ 141 and chromium carbide (Cr7C3) [15 ] have reasonably similar free energies of formation at 500 “C. This would imply that molybdenum metal could act as a very effective carbon sink in systems with approximately the same carbon potentials as chromium carbide, i.e., stainless steels, until equilibrium is achieved. Thus, from the data presented in Table 3 it can be concluded that, in the temperature range covered, the magnitude of the carbon potential in the 321~stainless steel loop was less than the magnitude of the carbon potential of the molybdenum-molybdenum carbide couple. The appearance/non-appearance of molybdenum carbide in the dynamic system would appear to be a function of the hot-leg temperature, although this is somewhat confused by the fact that MO19 appeared to have formed far more carbide than any of the other samples (as detected by the X-ray diffraction ~~hnique). Thus, it is probable that the later results were affected by the decarburization of the 321~stainless steel loop, which would lower the drlving force of reactions in experiments undertaken later in the lifetime of the loop. The appearance of molybdenum carbide in the carbon-doped static system is in agreement with the thermod~~i~ prediction, as is the nonappearance of carbide in the titanium c~bide-doped system. The absence of carbide formation in the stainless steel-doped static tests is probably a function of the quantity of available carbon in the small stainless steel tabs. In no experiment, static or dynamic, was there any evidence for the formation of either a binary or a ternary oxide corrosion product on the moly~enum surface, although it is known that some ternary sodium-molybdenum-oxides are stable towards liquid sodium under the conditions of this investigation [ 161. Weight-loss experiments on the static oxygen-doped experiments indicated no measurable weight change, which is in agreement with the observation that no significant amount of corrosion took place.
175
Tungsten Preliminary investigations on the corrosion of tungsten metal in oxygendoped liquid sodium by Wood [2] did not allow the identification of any of the observed corrosion products, but these products did appear to relate to the reactions of tungsten powder and sodium tungstate in oxygen-rich sodium. On comparing the diffraction data from Wood’s study with data from the present study it appears that the product of these reactions was Nas WOs. The experimental results presented in Table 4 would suggest that the low oxygen-activities encountered in the dynamic sodium system (i.e.,
176
Conclusions The observed behaviour of molybdenum metal is seen to be very different from that of the other Group 6A metals, where ternary oxide formation is observed under very similar conditions. The formation of molybdenum carbide, but not tungsten carbide, in the dynamic system, would appear to arise as a consequence of the lack of ternary oxide formation on the molybdenum metal. This would appear to be due to kinetic effects rather than to thermodynamic ones, as it has been shown that some sodiummolybdenum-oxygen ternary oxides are stable towards liquid sodium [16]. The kinetic aspect in liquid sodium is further emphasised by the observation that both molybdenum and tungsten metals react with sodium oxide at a temperature less than 400 “C in the solid state to give ternary oxides, but on moving to the liquid sodium system only tungsten shows ternary oxide formation, even in sodium saturated with oxygen at 600 “C. It can be concluded that the corrosion of molybdenum, unlike vanadium, niobium, tantalum, chromium and tungsten, should not be oxygen dependent, but at carbon levels greater than 10e3 ppm at 500 “C, the formation of molybdenum carbide (MO&) will take place, which may have a significant effect on any molybdenum coatings, especially if the carbide is formed by a wedging mechanism [ 181. Acknowledgments The authors wish to thank the Central Electricity Generating Board, Berkeley Nuclear Laboratories, for their financial support for this work. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
M. G. Barker and D. J. Wood, J. Less-Common Met., 35 (1974) 315. D. J. Wood, Ph. D. Thesis, Univ. Nottingham, 1971. C. C. Addison, M. G. Barker and D. J. Wood, J. Chem. Sot., Dalton Trans., (1972) 13. M. G. Barker and D. J. Wood, J. Chem. Sot., Dalton Trans., (1972) 9. M. G. Barker and D. J. Wood, J. Chem. Sot., Dalton Trans., (1972) 2451. M. G. Barker and D. J. Wood, J. Less-Common Met., 34 (1972) 215. J. D. H. Donnay, Crystal Data, American Crystallographic Association, 1963. H. Kessler, A. Hatterer and C. Ringenbach, C.R. Acad. Sci., Ser. C, 277 (1973) 763. A. J. Hooper, Ph. D. Thesis, Univ. Nottingham, 1971. J.-M. Rbau, C. Fouassier and P. Hagenmuller, Bull. Sot. Chim. Fr., (1967) 3873. M. G. Barker, to be published. M. G. Barker and D. J. Wood, Reactivity of Solids, Chapman and Hall, London, 1972, 623. M. R. Hobdell, CEGB Rep. RD/B/N1931,1972. A. Solbakken and P. H. Emmett, J. Am. Chem. Sot., 91(1969) 31. C. E. Wicks and F. E. Block, U.S. Bur. Mines Bull., (1963) 605. M. G. Barker and C. W. Morris, to be published. F. A. Shunk, Constitution of Binary Alloys, First Supplement, McGraw-Hill, New York, 1969. R. L. Klueh, in J. E. Draley and J. R. Weeks (eds.), Corrosion of Liquid Metals, Plenum Press, New York, 1970, p. 137. J. D. Noden, J. Br. Nucl. Energy Sot., 12 (1973) 57.