ORNL-TM-4188 0L o EFFECT OF FeF, ADDITION ON 'MASS TRANSFER IN A HASTELLOY N—LiF-BeF, -UF, THERMAL CONVECTION LOOP SYSTEM J. W. Koger THIS DOCUMENT CONFIRMED As NCLASSIFIED DI BYVISION OF CLASSIF ICATION DATE ZLle /75 This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights, O Ty N o » » ) 0 ORNL-TM-4188 Contract No. W-7405-eng-26 METALS AND CERAMICS DIVISION EFFECT OF FeF, ADDITION ON MASS TRANSFER IN A HASTELLOY N — LiF-BeF,-UF, THERMAL CONVECTION LOOP SYSTEM J. W. Koger NOTICE This report was prepared as an account of work sponsored by the United States Government, Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of theit contractors, subcontractors, or their employees, “makes any warranty, express or implied, ot assumes any legal liability or responsibility for the accuracy, com- pleteness or uscfulness of any information, apparatus, | . product or process disclosed, or represents that its use would not infringe privately owned rights. DECEMBER 1972 OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee 37830 operated by UNION CARBIDE CORPORATION for the U.S. ATOMIC ENERGY COMMISSION DISTRIBUTION OF THIS DOCUMENT IS UNLIMITE L] =~ [ CONTENTS ADSIACT . ... oo it e e et i 1 Introduction . . .. ..o i e e e e et i e 1 Background . ... ... e e e e et 1 Experimental System ...........iiiiiiiiiii i e i e e 8 Results and Discussion .. ... ... ..ottt iiit it ieaiiet s insanaeeanaeaesnaaanns 11 003 173 113 Lo 3 T3P 19 1ii £ i ” EFFECT OF FeF, ADDITION ON MASS TRANSFER IN A HASTELLOY N—LiF-BeF, -UF, THERMAL CONVECTION LOOP SYSTEM J. W. Koger ABSTRACT The compatibility of Hastelloy N with high-purity LiF-BeF;-UF4 (65.5-34.0-0.5 mole %) in a low-flow temperature-gradient system (maximum temperature 704°C, minimum temperature 538°C) was shown to be quite good. (The maximum corrosion rate was 0.04 mil/year over 29,500 hr of operation.) Subsequent experimental additions of FeF, increased the mass transfer of the system; specifically, the maximum weight loss rate before FeF additions was 1 X 10™ mg cm™ hr™!, while after addition the rate was 6 X 10”2 mgcem™ hr™? Cracks which transformed into voids were found in the specimens after exposure to the salt containing FeF,. INTRODUCTION The Molten Salt Reactor Program has been concerned with the development of nuclear reactors which use fluid fuels that are solutions of fissile and fertile materials in suitable carrier salts. A major goal has been to achieve a thermal breeder molten salt reactor (MSBR). One concept considered was a two-fluid MSBR. The fuel would be 233UF, or 235UF, dissolved in a salt consisting of LiF and BeF, (66-34 mole %). The blanket would be ThF, dissolved in a carrier of similar composition. Hastelloy N, a nickel-based alloy used in the Molten Salt Reactor Experiment (MSRE) was favored as the material out of which the reactor would be constructed. The design of the two-fluid MSBR showed the fuel salt entering the core at 538°C and leaving at 704°C.1 As part of our materials program for molten salt reactor development, we studied the compatibility of Hastelloy N with fuel salt. One such experiment was a thermal convection loop (NCL-16), which was operated at a maximum temperature of 704°C and a minimum of 538°C. During the operation of NCL-16, the MSRE was shut down and selected portions were examined. The Hastelloy N removed from the MSRE apfieared sound, but all metal surfaces that had been exposed to fuel salt showed shallow intergranular cracking when strained at 25°C.2 We subsequently used loop NCL-16 to investigate the possibility that the attack in the MSRE was related to the localization of normal corrosion processes to grain boundaries. In our study of cracking, we twice added 500 ppm FeF, to the loop and exa.mmed the corrosion spec1mens for signs of cracking. BACKGROUND In the beginning of the Molten Salt Reactor Program, several fluorides were considered as diluents for the UF, fuel.3:4 After much investigation and consideratidn of nuclear properties and chemical stability,$-6 BeF, and " LiF were selected as the diluent. 1. MSR Program Semiannu. Progr. Rep. Feb. 28, 1966, ORNL-3936, p. 172. 2. H. E. McCoy and B. McNabb, Intergranular Cracking of INOR-8 in the MSRE, ORNL-4829 (November 1972). 3. W.R. Grimes, MSR Program Semiannu. Progr. Rep. July 31, 1964, ORNL-3708, pp. 214-26. 4. W. R. Grimes, “Molten Salt Reactor Chemistry,” Nucl. Appl. Technol. 8, 137 (1970). 5.- Alvin Glassner, The Thermochemical Properties of the Oxzdes Fluorides, and Chlorides to 2500°K, ANL-5750, Argonne National Laboratory. 6. L. Brewer, L. A. Bromley, P. W. Gfl!es, and N. L. Lofgren, MDDC-1553 (1945); L. Brewer in The Chemistry and Metallurgy of Miscellaneous Materials; Thermodynamics, L. L. Quill, ed., McGraw-Hill, New York, 1950, pp. 75-192. ORNL-DWG 71-5270R2 I I 'I I I I I I 900 - . 500_ - I T T . T = /848 ' : X (EUTECTIC) = 0,3280+ 0.0004 % Tmax = 459.1 £0.2 800 . as0 |- LIQUID = -4 X (EUTECTIC= 0.531+0.002 700 400 LipBeF, + — 5 LiIF + ' . LIQUID f; LIQUID . & 600 }—- 350 ' . — 555 g . 030 035 040 045 0.50 0.55 < , W & / - m st~ 0 el : ‘ A = 458.9+0.2°C ! o BeF, (B-QUARTZ TYPE) i i + LIQUID | ! 400 |- I : - ' ) 363.5 £0.5°C LiF + Lt - . : . ' LiBeF3 + Befp LizBeF, LipBefy + BeF, {8-QUARTZ TYPE) _ 300 — ] E" ) 280°C . (a-QUARTZ TYPE)\_ 5| LizBeFg + uP| LiBeFy + BeF, (B-QUARTZ TYPE) / LiBeF3 @ gz7ec 200 l | | i - g | 1 1 ’ 0 oA 0.2 03 . 04 05 0.6 0.7 0.8 0.9 1.0 XB.F’ {mole fraction) Fig. 1. The system LiF-BeF,. The phase behavior of systems based upon LiF and BeF, as the major constituents has, accordingly, been examined in detail.”? Fortunately for the molten fluoride reactor concept, the phase diagram of LiF-BeF, -UF, is such as to make it useful as a fuel. ] : : The binary system LiF-BeF, has melting points below 500°C over the concentration range from 33 to 80 mole % BeF,.7:8 The phase diagram, presented in Fig. 1, is characterized by a single eutectic (52 mole % BeF,, melting at 360°C) between BeF, and 2LiF+BeF,. The compound 2LiF-BeF; melts incongruently to LiF and liquid at 458°C. LiF-BeF, is formed by the reaction of solid BeF, and solid 2LiF-BeF, below 280°C. The phase diagram of the BeF,-UF, system (Fig. 2) shows a single eutectic containing very little UF,. That of the LiF-UF, system (Fig. 3) shows three compounds, none of which melts congruently and one of which shows a low-temperature limit of stability. The eutectic mixture of 4LiF-UF, and 7LiF-6UF, “occurs at 27 mole % UF, and melts at 490°C. The ternary system® LiF-BeF,-UF,, of primary importance in reactor fuels, is shown as Fig. 4. The system shows two eutectics. These are at 1 mole % UF, and 52 mole % BeF, and at 8 mole % UF, and 26 mole % BeF,; they melt at 350 and 435 C respectlvely : Moreover, the system shows a very wide range of compositions melting below 525 C. The corrosion resistance of metals to fluoride fuels has been found to vary directly with the “noblhty” of the metal — that is, inversely with the magnitude of the free energy of formatlon of fluorides involving the metal. Accordingly, corrosion of multiéomponent alloys tends to be manifested by the selective oxidation and removal of the least noble component. In the case of Hastelloy N, corrosion is selective with fespect to chromium. The selective removal of chromium by fluoride mixtures depends on various chemical 7. R.E. Thoma, ed., Phase Diagram of Nuclear Reactor Materials, 0RNL—2548 (Nov. 6, 1959) , 8. L. V. Jones, D. E. Etter, C. R. Hudgens, A. A. Huffman, T. B. Rhinehammer, N. E. Rogers, P. A. Tucker, and L. J. Wlttenberg, *“Phase Equilibria in the Ternary Fused-Salt System LiF-BeF,-UFq,” J. Amer. Ceram. Soc. 45, 79 (1962). A} .x o 1100 1000 900 @ o o TEMPERATURE (°C) ~ Q o . 600 500 400 LiF ORNL~LR-DWG 28598A 1100 1000 1 /. i/. o9 900 o 800 P o UF, + LIQUID TEMPERATURE (°C) 700 £ ’ / J. 600 [ — ‘ é«o«-mumm-—uu—. —e—¢—so g ——— e 500 —QHIGHBQF2+L|QU D QHIGHBeFZ +UFR; i | | 10 20 30 40 50 60 70 80 90 UF, UF4 (mole %) Fig; 2. The system BeF,-UF,. ORNL-LR-DWG 47457A " / 4LiF -UF, \ LiF-UF, LiF - UF, 0 20 30 40 50 60 70 80 90 UR, UF4 (mole %) Fig. 3. The system LiF-UF,. ALL TEMPERATURES ARE IN °C £ = EUTECTIC P = PERITECTIC [UR] = PRIMARY PHASE FIELD LiF - UF, Fig. 4. The system LiF-BeF,-UF,. ORNL-DWG 66—-7634 o a reactions, as follows: 1. Due to impurities in the melt, for example, FeF, + Cr=CrF, + Fe (1) 2HF +Cr=CrF, +H, . - (2) 2. Dissolution of oxide films from the metal surface, for example, 2Fe’* (from film) + 3Cr = 2Fe + 3Cr**. L 3) 3. Due to constituents in the fuel, particularly, Cr+ 2UF4 = 2UF3 + CIF2 . . _ (4) If pure salt containing UF, (and no corrosion products) is added to a Hastelloy N loop operating polythermally, all points of the loop initially experience a loss of chromium in accordance with the Cr-UF, reaction, Eq. (4), and by reaction with impurities' in the salt (such'as HF, NiF,, or FeF,). Impurity reactions go rapidly to completion at all temperature points and are important only in terms of short-range corrosion effects. The UF, reaction, however, whose equilibrium is temperature-dependent, provides a mechanism by which the alloy at high temperature is continuously depleted and the alloy at low temperature is continuously 'enriched in chromium. All parts of the loop are attacked as the corrosion-product (CrF,) concentration of the salt is increased by the impurity and UF, reactions. Eventually the lowest temperature point of the loop achieves equilibrium with respect to the UF, reaction. Howe}fer, in regions at higher temperature, because of the temperature dependence for this reaction, a driving force still exists for chromium to react with UF,. Thus, the corrosion-product concentration will continue to increase, and the temperature points at equilibrium will begin to move away from the coldest temperature point. At this stage, chromium is returned to the walls at the coldest point in"the system. The rise in corrosion-product concentration in the circulating salt continues until the amount of chromium returning to the walls exactly balances the amount of chromium entering the system in the hot-leg regions. Under these conditions, the two positions of the loop at equilibrium with the salt are termed the “balance - points,” and they do not shift measurably with time. Thus, a quasi-steady-state situation is eventually achieved in which there is a fixed chromium surface concentration at each point-in the loop and chromium is transported at very low rates. This idea is supported by the fact that concentrations of CrF,, UF,, and ‘UF, achieve steady-state concentrations in the salt even though attack slowly increases with time. A schematic of this mass transfer process is shown in Fig. 5. Subsurface voids are often formed in alloys exposed to molten salts The formation of these voids is initiated by the ox1dat10n and removal of chromium from exposed surfaces. As the surface is depleted in chromlum chromium from the interior diffuses down the concentration gradlent to the surface. Since diffusion occurs by a vacancy process and in this particular situation, is essentially nondirectional, it is possible to build up an excess number of vacancies in the metal. These precipitate in areas of disregistry, principally at grain boundaries and impurities, to form voids. These voids tend to agglomerate and grow in size with increasing time and/or temperature. Studies have demonstrated that such subsurface voids are not interconnected with each other or with the surface. Voids of this same type have also been developed in Inconel by high-temperature oxidation tests and high-temperature vacuum tests in which chromium is " ORNL-DWG 67-6800R HOT SECTION DIFFUSION TO SURFACE SOLUTE ESCAPE THROUGH ™ NEAR-SURFACE LIQUID LAYER — ~ T DIFFUSION INTO BULK LIQUID l _ .T__‘_.____,,.__‘__u__v TRANSPORT TO COLD PORTION OF SYSTEM \\\\\\\\\\\\\\\\\\\\\\\\\‘l N COLD SECTION —— —_— S - Al {x SUPERSATURATION — —-i— * NUCLEATION . o GROWTH TO STABLE CRYSTAL SIZE -_-__;OR L SUPERSATURATION AND DIFFUSION "THROUGH LIQUID - NUCLEATION AND GROWTH ON METALLIC WALL — —— - — OR DIFFUSION INTO WALL Fig. 5. Temperature-gradient mass transfer. selectively removed.? Voids similar to these have also been developed in copper-brass diffusion couples and by the dezincification of brass.1® All of these phenomena arise from the so-called Kirkendall effect, whereby solute atoms of a given type diffuse out at a faster rate than other atoms comprising the crystal lattice can diffuse in to fill the vacancies which result from outward diffusion. The removal of the least noble constituent is often preferential along grain boundaries. In time, given a continuing electrochermcal process, this will lead to crevices in the grain boundaries. Diffusional processes within a crevice may lead to its broadening and ultimately to the formation of pits. However, if the root of the crack is anodically polarized relative to the walls, knife-line attack will continue. Such a condition may arise if the walls of the crevice become covered with a very noble material (mckel or molybdenum) This covering by a noble constituent can occur either by the noble material remaining on the wall when the least noble constituent is removed or by dlssolutxon of all the alloy constituents with subsequent prempltatlon of ‘the more noble constituents. 9. A. DeS. Brasunas, “Sub-Surface Porosuy Developed in Sound Metals during High-Temperature Corrosion,” Metals Progr. 62(6), 88 (1952). 10. R. W. Balluffi and B. H. Alexander “Development of Porosnty by Unequal Diffusion in Substltutlonal Solutnons SEP 83, Sylvania Electric Products (February 1952). . - Table 1. Thermal convection loops that have operated with LiF-BeF,-UF, salts » Maximum Hours Salt Alloy temperature Remarks CO) operated % LiF-BeF,;-UF, Inconel 6002 _ 677 1000 General subsurface voids to (53-46-1 mole %) | 2% mils Inconel 600 677 8664 Heavy intergranular voids to 7 mils Inconel 600 - 677 8760 Moderate to heavy intergranular voids to 7 mils Inconel 600 732 8760 Heavy intergranular voids to - 15 mils Inconel 600 732 8760 Heavy intergranular voids to - 15 mils Hastelloy N? : 677 © 1000 No attack Hastelloy N - 677 8760 No attack Hastelloy N . 732 8760 Light surface pitting LiF-BeF, Inconel 600 677 1000 Few voids <1 mil (71-29 mole %) * Inconel 600 677 1000 Few voids <1 mil Inconel 600 - 677 8760 Light to moderate intergranular voids to 5 mils Inconel 600 732 8760 Moderate to heavy intergranular . ' ' voids to 6% mils Hastelloy N 677 1000 Light surface roughening Hastelloy N - 677 3114 Light surface roughening : Hastelloy N 732 8760 Heavy surface roughening LiF-BeF5-UFq4 -Inconel 600 677 1000 General intergranular attack (62-37-1 mole %) <1 mil- Inconel 600 732 1000 Intergranular voids to 3 mils Inconel 600 - 732 1000 Intergranular voids < 2 mils Inconel 600 132 1000 Intergranular voids to 4 mils Inconel 600 677 8760 Heavy intergranular and general ' voids to 5 mils ) Inconel 600 732 8760 Heavy intergranular voids to 14 mils * Hastelloy N 677 1000 No attack Hastelloy N 677 8760 Light surface roughening Hastelloy N 732 8760 Light surface roughening LiF-BeF,UF,4 Inconel 600 677 1000 Intergranular voids <1 mil (60-36-4 mole %) Hastelloy N 617 1000 Light surface roughening Hastelloy N 677 8760 Moderate surface roughening LiF-BeF,-UF,4 Hastelloy N 677 1000 No attack (70-10-20 mole %) Hastelloy N 732 1000 Moderate surface roughening 415% Cr—7% Fe—bal Ni. , 57% C1—5% Fe—16% Mo—bal Ni.. . Table 1 lists the results of previous Hastelloy N and Inconel 600 thermal convection loop tests using salts made up of LiF, BeF,, and UF,.11~18 There were no corrosion specimens in the loops, so no weight change data are available. Yet, it is interesting to compare the behavior of the various salts, the various alloys, and different times and temperatures. In all cases the Hastelloy N showed better corrosion resistance, and, in general, the higher peak temperature and longer times resulted in greater corrosion. The 11. MSR Quart. Progr. Rep. Sept. 1, 1957 0RNL~2378 p- 3. s 12. 13. 14. 15. 16. 17. E/ . [ 3] Ibid., Oct. 31, 1957, ORNL-2431, pp. 23-29. Ibid., Jan. 31, 1958, ORNL-2474, pp. 51 -54. Ibid., Qct. 31, 1958, ORNL-2626, pp. 53—-53. Ibid., Jan. 31, 1959, ORNL-2684, pp. 75-76. Ibid., Apr. 30, 1959, ORNL-2723, pp. 51-54. Ibid., July 31, 1959, ORNL-2799, pp. 47-55. Ibid., Jan. 31 and Apr. 30, 1960, ORNL-2973, pp. 33-36. salts containing no more than 1 mole % UF, at 677°C only produced light surface roughening on the Hfiste]loy N. A little more attack was produced at 732°C and by the salts with the larger amounts of UF,. Based on these results for our specific salt and temperature conditions, Hastelloy N should be quite resistant to attack. ' EXPERIMENTAL SYSTEM The thermal convection loop is an excellent corrosion test system.that is intermediate in complexity and cost between isothermal capsules and pumped loops. The loop is particularly suited for small-scale tests that involve flow and terriperature gradient mass transfer. The flow in the systerfi results from the difference in density of the liquid in the hot and the ‘cold leg. A schematic of a thermal convection loop is shown in Fig. 6, and an actual photograph of loop NCL-16 is seen in Fig. 7. Thermal convection loop NCL-16 contained 14 specimens, 7 in each leg. Twelve specimens were titanium-modified Hastelloy N, and two specimens were standard Hastelloy N. Their compositions are given in Table 2. The loop itself was constructed of standard Hastelloy N. The test specimens were 1.9 X 0.95 X 0.076 cm and weighed approximately 1 g, with a surface area of 3.5 cm?. They were measured to within 0.0025 cm to obtain surface area and were triply weighed to within 0.01 mg. The specimens were attached by wires to the speéiméh fixture, which consisted of 0.32-cm-diam rod welded to 0.63-cm-OD Hastelloy N tubing. Salt for analysis was dip-sampled from the harp portion of the loop into a hydrogen-fired copper container attached to 0.63- mg -2 o1 cm “ hr. 704°C 676°C 560°C 10,686 hr +0.6 mg/em? 500X 29,509 hr 14 29,509 hr 29,509 hr 448 hr-with 500 ppm FeF, 1097 hr with 500 ppm FeF, 2 —2.5 mg/cm 500X 1] 15 " ORNL DWG. 73-4386 29,509 hr 29,609 hr 29,509 hr 29,509 hr 1097 hr with 500 ppm FeF» 1097 hr with 500 ppm FeF5 1097 hr with 500 ppm FeF, 1097 hr with 500 ppm FeF, 839 hr with 500 ppm FeF5 1800 hr with 500 ppm FeF 5 5150 hr with 500 ppm FeF, 5800 hr with 500 ppm FeF, s additional additional additional additional —~19.0 mg/cm?2 —30.2 mg/cm2 500X 500X ~15.5 mg/cm? 200X [ —25.0 mg/em? 500X Fig. 12. Optical micrographs of specimens exposed to LiF-BeF3-UF, (65.5-34.0-0.5 mole %) at various temperatures g and times. 91 RN T Fig. 13. Optical micrographs of specimens from various portions of the loop at the end of the test. Approximatety 36,400 hr exposure. o> .y 17 - Table 3. Activity coefficients estimated for various fluorides - in molten LiF-BeF; solutions at 1000°K . AF°f P LT g Approximate a Species ~ {kcal per fiuorme) Equlllbnum stgdled " mole fraction Y UF, -95.3 UF4 + 2H,0 = UO; + 4HF 0.02 0.55 UF,3 -1014 UF4 + '4H, = UF; + HF 0.0001 50 - FeFy ~665 + TFeFy+Hy=Fe+2HF 0.005 .16 CtFy, -74 . CrFy3+Hy;=Cr+2HF - 0.001 0.5 @Based on the crystalline solid as reference state. ‘/3 -in. Hastelloy N specimen rod from the hot leg was inadvertently broken dunng one of the specimen removal penods The rod had been exposed to the unaltered fuel salt for 10,301 hr, to the salt with 500 ppm FeF, for 1100 hr, and to the salt with an additional 500 ppm FeF, for 3254 hr. The total exposure time was 14,655 hr. The rod was sectioned and examined at different positions which represented different exposure temperatures (Fig. 14). Voids were quite evident at the hottest positions, with the amount and depth of the voids decreasing with decreasing temperature. ~ Observations of the specimen rods correlate quite well with the speclmen behavior. The balance point (no gains or losses) as determined from the specimen welght change is about 621°C, so all but one of the rod specimens should have been attacked, and this is what we found. It is interesting to compare values for actual mass transfer in a loop system such as NCL-16 with calculated values based on quite basic chemical equilibrium experiments. Data obtained from studies of hydrogen reduction equilibria by Long!® and by Blood2? can be used to compute reasonable (and consistent) estlmates of the activity coefficients of the several species in fluoride salt melts. T hese activity coefficients based on the crystalline solid as reference state are shown in Table 3. From these values and the values of AF °f for the several compounds (where AF° =RT 1n K), we may assess the extent of the reaction wn+m#&m+wn,,'g*;“7{.g'_' L g @) where the UF4, CrF,, and UF; are in solutlon in the fuel salt- and the chromitm is at the relatlvely low activity it has in unaltered Hastelloy N. For this reaction, auF,’ aCng (N7)UF3 (N‘r)cu-*, K, 24x10fl_ (5) aum dce. (NY)UF4 acy If, mmally, the salt were comp!etely pure and the metal contained no oxide (so that all CiF, was generated by th1s reaction), then 19. MSR Program Semiannu. Progr. Rep. Jan. 31, 1964, ORNL-3626, pp. 119-29. 20. C. M. Blood, Solubility and Stability of Structural Metal Difluorides in Molten Fluoride Mixtures, ORNL-CF- 61-54 (Sept. 21, 1961); C. M. Blood et al., “Activities of Some Transition Metal Fluorides in Molten Fluoride Mixtures,” in Proceedings of the International Conference on Coordination Chemistry, 7th, Stockholm and Uppsata, June 25-29, 1962, Butterworths, London, 1963. : 18 73-4385 ORNL DWG. | = u __O0.0_ 687°C 1 - Wl €00'0] X00S | S3HINI L00°0 u S000) Ui £00°0) ] Fig. 14. Hastelloy N specimen rod -exposed to LiF-BeF,-UF4 (65.5-34.0-0.5 mole %) for 14,655 hr (4354 hr with 1000 ppm FeF, in the salt). s 19 For reaction of the fuel from NCL-16 (Nyf, = ayfr, = 0.005) with Hastelloy N (ac, = 0.083), the equilibrium indicated in Eq. (6) is satisfied at Neep, =07X 107 57 and Nyp, =14X 107, Accordingly, less than 3% of the UF4 would be reduced to UF3, and the chromium fluoride concentration - of the melt would be 70 ppm (as Cr). In our system we produced 400 ppm Cr as CrF, in 29,500 hr; thus ‘other sources such as FeF, were available for the oxidation of chromium. For the reaction FeF, + Cr==Fe + C1F, , ’ (1) according to ref. 20, _ NCngNFe =——2 " -6000, e, N " NciNrer, 0 where the chromium is in solid solution in the Hastelloy N (¢ = 0.083) and the iron is crystalline iron at unit activity. Thus, the reaction should proceed until the ratio ' =500, ' (8) NFng which means that large amounts of CrF; can be produced by very little FeF,. Depletion of chromium at the surface of the alloy will lower this ratio. If we attribute 70 ppm Cr as CrF, to oxidation by UF4 and 330 ppm Cr as C1F, to oxidation by FeF,, which decreased by about 100 ppm in 29,500 hr, the calculated equilibrium constant for the FeF, reaction in the first 29,500 hr is 40. During the periods of FeF, additions the calculated equilibrium constant ranged from 12.5 to 36. Thus the depleted alloy caused the equilibrium constant to be lowered an order of magnitude. CONCLUSIONS 1. The compatibility of Hastelloy N with LiF-BeF,-UF, (65.5-34.0-0.5 mole %) which contains few impurities in a low-flow temperature gradient system (maximum temperature 704°C, minimum temperature 538°C) is quite good. The maximum corrosion rate was 0.04 mil/year. | _ ' 2. Additions of impurities such as FeF; increase the mass transfer of the system; specifically, the maximum weight loss rate before impurity additions was 1 X 10™* mgem™ hr ™!, while after addition the rate was 6 X 10”2 mgem ™ hr7!, | 3. Cracks which transformed into voids were found in the specimens exposed to the impurity-laden salt. However, these cracks were not equivalent in appearance to those noted in Hastelloy N in the MSRE; thus attack by FeF, was not the primary cause of the crack formation. ) ¥ (3) Central Research Library INTERNAL DISTRIBUTION (79 copies) ORNL -- Y-12 Technical Library - Document Reference Section (10) Laboratory Records Department Laboratory Records, ORNL RC 'ORNL Patent Office G. M. 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