ORNL-TM-4172 CORROSION OF TYPE 304L STAINLESS STEEL AND HASTELLOY N BY MIXTURES OF BORON TRIFLUORIDE, AIR, AND ARGON J. W. Koger . UMENT CONFIRMED AS THIS DOCUNCLASS‘F!ED DIVISION OF CLASSlF{lCATlON N BY T, .Cu ¢ N av [(ev DATE__ (o] 1 2/ND3 14 % OAK RIDGE NATIONAL LABORATORY OPERATED BY UNION CARBIDE CORPORATION e« FOR THE U.S. ATOMIC ENERGY COMMISSION 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. ORNL-TM-4172 Contract No. W-7405-eng-26 METALS AND CERAMICS DIVISION CORROSION OF TYPE 304L STAINLESS STEEL AND HASTELLOY N BY MIXTURES OF BORON TRIFLUORIDE, AIR, AND ARGON 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 Aiomic 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, com- pleteness or usefulness 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 1S UNL\M\T)E 17 Abstract . . . Introduction . Background Data iii CONTENTS Results of the Current Experiments . Conclusions . Acknowledgments CORROSION OF TYPE 304L STAINLESS STEEL AND HASTELLOY N BY MIXTURES OF BORON TRIFLUORIDE, AIR, AND ARGON J. W. Koger ABSTRACT Corrosion of type 304L stainless steel and Hastelloy N was studied in gaseous mixtures of BF3, air, and argon at 600, 300, and 200°C. 1In some tests the alloy specimens and gases were in contact with molten fluoride mixtures. At 600°C under air, specimens of both alloys were completely destroyed in both fuel salt and fluoroborate salt mixtures. No significant weight changes were measured for Hastelloy N immersed in salt exposed to argon or BF3, but type 304L stainless steel lost weight under the same conditions. This weight loss was greater under BF3; than under argon and greater in the fluoroborate mixture than in the fuel salt. With no salt immersion, the various gases produced very small changes in the alloys at 200°C. Also with no salt, only small changes were noted at 300 and 600°C for Hastelloy N in any gas and for the stainless steel in air. At 600°C, large quantities of chromium and iron oxides were pro- duced on the stainless steel by the air-BFj; mixture and large amounts of unidentified corrosion products by BF3, argon-BFs, and argon-air. Drying the argon decreased the amount of cor- rosion. Hastelloy N was more resistant than type 304L stain- less steel to corrosion by the gases tested in these experiments. INTRODUCTION Over the past few years, increased interest has been shown in the use of a sodium fluoroborate mixture, NaBF,—8 mole % NaF, as a coolant in a Molten-Salt Breeder Reactor. The physical properties of the salt mixture have been determined' and many experimem:sz"12 have been undertaken to s, Cantor, J. W. Cooke, A. S. Dworkin, G. O. Robbins, R. E. Thoma and G. M. Watson, Physical Properties of Molten Salt Reactor Fuel, ORNL-TM-2316 (August 1968). 27, W. Koger and A. P. Litman, Compatibility of Hastelloy N and Croloy 9M with NaBF,-NaF-KBF, (90-4-6 mole %) Fluoroborate Salt, ORNL-TM-2490 (April 1969). ascertain the corrosion of various alloys in this salt. The use of this mixture in a high-temperature environment is somewhat complicated by the fact that the NaBF, dissociates by the reaction NaBF, = NaF + BF3(g) , and the effect of the BF; gas must be considered. The equilibrium pressure above a melt of NaBF,—8 mole % NaF as a function of temperature is given by log P = 9,024 — 5920/T(°K) torr The maximum design temperature for the coolant salt mixture is 621°C, and this corresponds to a partial pressure of BF3; of 252 torr. Because of the appreciable vapor pressure of BF; at operating temperatures, cover gases 3J. W. Koger and A. P. Litman, Catastrophic Corrosion of Type 304 Stainless Steel in a System Circulating Fused Sodium Fluoroborate, ORNL-TM-2741 (January 1970). *J. W. Koger and A. P. Litman, Compatibility of Fused Sodium Fluoro- borates and BF3 Gas with Hastelloy N Alloys, ORNL-TM-2978 (June 1970). °J. W. Koger and A. P. Litman, MSR Program Semiannu. Progr. Rep. Feb. 29, 1968, ORNL-4254, pp. 221-25. ®J. W. Koger and A. P. Litman, MSR Program Semiannu. Progr. Rep. Aug. 31, 1968, ORNL-4344, pp. 264—66 and 285-89. ’J. W. Koger and A. P, Litman, MSR Program Semianrmu. Progr. Rep. Feb. 28, 1969, ORNL-4369, pp. 246—53. 83. W. Koger and A. P. Litman, MSR Program Semiannu. Progr. Rep. Aug. 31, 1969, ORNL-4449, pp. 200208, ®J. W. Koger, MSR Program Semiannu. Progr. Rep. Feb. 28, 1970, ORNL-4548, pp. 242—52 and 265-72. 19J. W. Koger, MSR Program Semiannu. Progr. Rep. Aug. 31, 1970, ORNL-4622, pp. 168—78. '13. W. Koger, MSR Program Semiannu. Progr. Rep. Feb. 28, 1971, ORNL-4676, pp. 192—215. '23. W. Koger, MSR Program Semiannu. Progr. Rep. Aug. 31, 1971, ORNL-4728, pp. 138-53. containing equivalent concentrations of BF3; must be maintained in the free volume of the pump bowl of dynamic systems or used for gas sparge operations. BACKGROUND DATA An early report on corrosion experiments with gaseous boron tri- fluoride®® noted that the BF; reacted rapidly with traces of moisture to give hydroxyfluoboric acid (HBF3OH) and HF. It also showed that BF3; and glass reacted at an appreciable rate just above 200°C. Under the condi- tions of those experiments, there was no appreciable attack by BF3; on any metal or alloy examined at temperatures up to 200°C. A systematic study of the compatibility of Hastelloy N with BFj3; had not been undertaken before this study, but several observations indicated possible compatibility problems. These observations were often made in the course of experiments where compatibility was not the primary objective, and the results are difficult to interpret. However, these miscellaneous obeervations do contribute to our understanding of metal-BFj;-moisture reactions. 1% BF; contacted chromium metal (a consti- In one experimental program in almost all alloys considered for use in a molten salt system) 60 hr at 650°C. The chromium sample gained weight (about 4%), and the weight gain showed a linear dependence on the square root of the reaction time. The surface of the chromium sample contained substantial quantities of Cr;03 and minor quantities of CrF,;~CrF;. Thus, commercially available BF; did promote the oxidation of pure chromium. 12 In all Hastelloy N thermal convection loop tests>" that have involved the fluoroborate mixture, specimens have been placed in the vapor }3F. Hudswell, J. S. Nairn, and K. L. Wilkinson, "Corrosion Experiments with Gaseous Boron Trifluoride" J. Appl. Chem. 1: 333-36 (1951). '%J. H. Shaffer and H. F. McDuffie, MSR Program Semiannu. Progr. Rep. Aug. 31, 1967, ORNL-4191, pp. 163—64. space above the hot and cold leg surge tanks to provide some data on the compatibility of BF3 with Hastelloy N. The vapor space contained about 220 torr BF; and 5 psig He. Since the mass transfer rates in these systems were quite dependent on salt purity, which varied from loop to loop, the weight changes of the specimens exposed to the vapor (Table 1) are scattered. However, in none of the cases were the losses excessive. Based on uniform removal of all alloy constituents, the maximum corrosion rate was 0.04 mil/year. In another experiment Hastelloy N specimens were exposed to static BF3; vapor for 6800 hr at 605°C in capsules that contained the fluoroborate mixture.® The BFj; pressure was varied in each capsule. Table 2 shows the measured weight changes. The weight changes were quite small, and no composition changes were noted in any of the specimens. Another experience with BF3; involved a pump loop (PKP-1), which was used to test a molten-salt pump with the fluoroborate mixture.?®21% An Inconel 600 bubbler tube, which had been used for BF3; addition and salt level indication, had been in service 11,567 hr and was suspected to have become plugged just before the loop shutdown. During most of the test program the total gas flow rate was 370 cm®/min. The gas was He—13.5 vol % BF3. The outside of the tube was exposed to the fluoroborate mixture at 550°C. After removal from the loop, examination disclosed a plug at the bottom of the inside of the tube. Above the plug, in order, were a thin black film, a gray thin deposit with a sprinkling of green particles, a bright green deposit, and finally green material covered with magnetic powder. The green deposit was Na3CrFg, and the magnetic mixture was equivalent to NizFe. Figure 1 shows a cross section of the tubing wall near the liquid-gas interface. We saw a zone of metallic crystals on the inner surface to a thickness of approximately 20 mils. 1In view of the apparent loss of wall thickness in this area, these crystals probably represent vestiges of the original metal surface. Figure 2 shows a higher '5A. N. Smith, MSR Program Semiannu. Progr. Rep. Feb. 28, 1971, ORNL-4676, pp. 55-57. 183, W. Koger, ibid., pp. 211-15. Table 1. Weight Losses of Hastelloy N Specimens Exposed to BF3; in a Thermal Convection Loops Temperature Time Weight Loss Rate of Weight Loss (°C) (hr) (mg/cm?) (mg cm™? hr™ 1) 530 4,150 0 0 x 107° 530 21,983 0.3 1.4 530 23,427 0.3 1.3 530 32,090 0.15 0.47 593 19,433 0.4 2,1 607 4,150 0.4 9.7 607 21,933 2.2 10 607 23,427 0.3 1.3 607 32,090 2.0 6.2 687 19,433 0.3 1.5 Table 2. Weight Losses of Hastelloy N Specimens Exposed to Various Pressures of BF3; at 605°C for 6800 hr.?2 BF3 Pressure Weight Loss (psia) (mg/cm?) 4,2 0.03 64.7 0.3 114.7 0.4 414.7 0 #J. W. Koger and A. P. Litman, Compatibility of Fused Sodium Fluoroborates and BF3 Gas with Hastelloy N Alloys, ORNL-TM-2978 (June 1970). i [ { Outside | Inside Fig. 1. As-Polished Cross Section from Near Liquid-Gas Interface of Inconel 600 Bubbler Tube from PKP-1 Pump Loop, 100%. magnification of the inside surface, both as polished and etched, and tends to corroborate that the crystals are a result of material leaving the sur- face rather than material deposition. The material on the inside of the tube above the liquid level appeared to be the result of attack by water- derived impurities in the BFj. On removing the pump rotary element from the loop and dismantling the various pump components, Smith!? found that the inner heat baffle plates had been severely attacked, as evidenced by holes in some places and by severe pitting in other placeé. The attack was not homogeneous on any one plate or constant from one plate to another. The top surface of the top 17A. N. Smith, MSR Program Semiannu. Progr. Rep. Aug. 31, 1971, ORNL-4728, pp. 31—34. » n ¥ -Exposed to He—13.5 vol % BF3; for 11,500 hr. (a) As Polished, 500%. Y-103137 £ Fig. 2. Inside Surface of Inconel 600 Bubbler Tube from PKP-1 Pump (b) Etched with Aqua Regia, 500x, plate was severely pitted, but the other surfaces that combined to form the chamber above the top plate (shaft, inner surface of impeller housing support cylinder, and lower surface of the cooling o0il chamber) were relatively free of attack. All baffle plates except the one below the top plate had holes corroded completely through. The inner heat baffles fitted very tightly against the inner surface of the upper impeller housing sup- port cylinder, so that probably most of the gas flow was past the inner annulus between the baffle plate and the pump shaft. The attack was attri- buted to the intimate contact of purge gas (or more correctly, the moisture in the purge gas) with puddles of salt that lay on the baffle surfaces. We believe that the temperature in this region was 280°C or less, so the water—-salt reaction products appear to be highly corrosive even at rela- tively low temperatures. Because of the findings in this pump bowl and other questions concern- ing the role of BF3; in corrosion of alloys, we felt that the compatibility of BF3 and various gas mixtures with candidate alloys for molten salt use should be studied systematically. As a first test, we exposed type 304L stainless steel and Hastelloy N to argon, air, BF3;, and mixtures of these for 100 hr at temperatures from 200 to 600°C. RESULTS OF THE CURRENT EXPERIMENTS The reaction chamber was a 30-in. length of 2-in. IPS nickel pipe, mounted horizontally in a 3-in. tube furnace. The reacting gases were admitted through a penetration in the end plate that was welded to one end of the pipe. Sheathed thermocouples also penetrated the end plate and extended into the central region on the heated zone. Several thermo- couples were attached along the outside of the nickel pipe. The other end of the reaction chamber, which extended some 10 in. out of the tube furnace, was closed by Teflon in a threaded pipe cap. The gas manifold system provided for the introduction of helium, BF3, argon, air, or mix- tures of these gases at known flow rates into the reaction chamber. The system was sealed from the atmosphere by bubbling the gas effluent through a fluorocarbon oil. Metal samples were carried in nickel boats inserted through the threaded access port. 1In several runs, metal specimens were placed with a fuel salt, LiF—20 mole 7 BeF,—11l.7 mole % ThF4—0.3 mole % UFy, or a coolant salt, NaBF4—8 mole % NaF, in the nickel boats. The tests lasted 100 hr, and the gas flow rate was 100 cm®/min. The metal specimens were approximately 0.75 x 0.375 X 0.030 in. and weighed about 1 g. Speci- mens were weighed before and after each test, and reaction products were analyzed whenever possible. The nominal amounts of the major alloying constituents of Hastelloy N and type 304L stainless steel are given in Table 3. Table 3. Nominal Compositions of Alloys Content, wt % Alloy Ni Cr Fe Mo Hastelloy N 70 7 5 17 Type 304L stainless steel 10 18 69 0 Table 4 gives the results for the metal-salt combinations exposed to various gases at 600°C for 100 hr. As was expected, air in combination with the fluoride salts produced highly corrosive conditions, which destroyed not only the specimens but the nickel boats. None of the weight changes measured for the Hastelloy N immersed in salt and exposed to any of the gas mixtures other than air were significant. For the stainless steel specimens immersed in salt, BF3; gas caused more attack than argon. A larger moisture content in the BF; probably caused the greater attack. The fluoroborate mixture was more aggressive toward the stainless steel than the fuel salt. Weight losses rather than weight gains occurred because the corrosion products were dissolved from the specimens by the salt. Table 5 gives the results for alloys exposed to the various gas mixtures without salt at temperatures from 200 to 600°C for 100 hr. 1In most cases, weight gains were found, since the corrosion products remained on the specimens and were not carried away. 10 Table 4. Weight Changes of Hastelloy N and Type 304L Stainless Steel Exposed to Various Gases while Immersed in Fluoride Salts at 600°C for 100 hr. Gas Flow 100 cm®/min Weight Change,a mg/cm? Alloy Salt Argonb BF3C Ar-BF ;3 Hastelloy N Fue1d ~0.03 +0.03 0 Coolant® +0.06 0 —0.06 Type 304L stainless steel Fueld 0.3 —1.4 —1.6 Coolant® —1.2 —5.2 —2.4 a . . . s . Specimens exposed to air containing 7.5 ppm moisture were completely destroyed. b24 ppm moisture. 50 ppm moisture. d1iF20 mole % BeFs~11.7 mole % ThF4—0.3 mole % UFy. ®NaBF,—8 mole % NaF. For Hastelloy N, the only significant changes measured at 600°C were in the mixtures containing air. This effect can be attributed to too small an amount of chromium in the Hastelloy to provide good resistance to air oxidation. At 200 and 300°C, all changes were rather small. For the stainless steel at 600°C, the air-BF; mixture produced a large amount of chromium and iron oxides, which is shown in Fig. 3(b). The stainless steel specimen from which the oxides in Fig. 3(b) had been removed is shown in Fig. 3(c), and a relatively unaffected stainless steel specimen that had been exposed to air at 200°C is shown in Fig 3(a). By itself, BF3 at 600°C also produced a large amount of corrosion product on the stainless steel and was more aggressive than argon. Significant weight gains of the stainless steel specimens were produced at 300°C and 600°C by all gas mixtures except air. At 200°C, most changes were rather small. Reaction of the stainless steel with argon, air, and an argon-BFj; mixture resulted in a red corrosion product on the surface, whereas reaction with BFj3 by itself resulted in the formation of white material on the surface. These red and white corrosion products could not be 11 Table 5. Weight Changes of Hastelloy N and Type 304L Stainless Steel Exposed to Various Gases at 200 to 600°C for 100 hr. Gas Flow 100 cm3/min Weight Change, mg/cm® os elsome T ‘ Hastelloy N Stainless Steel Argon 24 600 +0.06 +0.1 3002 0 +0.3° 200 +0.03 +0.05 Argon® 2 600 +0.03 +0.05 300% 0 +0.17 200 —0.03 +0.03 BF 4 <50 600 0 +5.2 3002 +0.2 +3.3¢ 200 —0.03 0 Air 7.5 600 +0.2 +0.03 3002 +0.03 —0.03° 200 +0.03 0 Argon-BF 3 24 600 —0.03 +0.22 3002 +0.03 +5.85% 200 0 0 Argon®-BF 4 2¢ 600 +0.03 +0.05 3002 +0.3 +2.0 200 0 0 Air-BF; 600 +0.2 +119.28 300° 0 +25.4 200 0 —1.2 Argon-air 24 600 —0.1 +0.8 3002 0 +10.0" 200 —0.03 +0.2 aNear gas entrance. bAdherent red layer on surface. “Dried by passing through heated titanium sponge. dLarge amount of nonadherent white material on surface; 0.0290 g removed. e In the argon. fLarge amount of nonadherent red material on surface; 0.0745 g removed. gLarge amount of oxide on surface, identified by x-ray diffraction as 25 mole % Cr,0; and 75 mole Z Fe»03. hLarge amount of material flaked off. T L i ' ' i , Fig. 3. (a) Type 304L Stainless Steel Specimen Exposed to Air at ? 200°C for 100 hr. (b) Chromium and iron oxides removed from a similar specimen after exposure to mixed air and BF3; for 100 hr at 600°C. ;; (c) Specimen from which oxide in (b) had been removed. 13 identified by x-ray analysis but contained iron, chromium, oxygen, and, where BF; was involved, fluorine. Thus, the corrosion products were probably complex mixtures of metal oxides and perhaps fluorides. The use of drier argon (2 ppm moisture as opposed to 24 ppm) lowered the weight gain of the stainless steel specimens by about one-half. The effect of moisture was more dramatic when the gas was an argon-BF:; mixture (Table 5). Even though air had almost no effect on the stainless steel, the air-BF3 mixture had the worst effect of any of the gases, much worse than just BF3. The air in combination with argon also produced more corrosion products than either of the two gases by themselves. In combination with salt, BF3 had the worst effect. These results underline the problems that can result in a system that allows air and/or moisture to come in contact with molten fluoride salts or with the BFj3; vapor. Except in air at 600°C, Hastelloy N was much more resistant to corrosion than the type 304L stainless steel. CONCLUSIONS 1. At 600°C, air completely destroyed Hastelloy N and type 304L stainless steel specimens that were immersed in either a fuel salt or the fluoroborate coolant mixture. 2. No significant weight changes were measured for Hastelloy N immersed in salt exposed to argon or BF3 at 600°C, but type 304L stainless steel lost weight. 3. For the stainless steel immersed in salt, the BFj3; gas was more corrosive than the argon. 4, Stainless steel weight'losses were larger in the fluoroborate mixture than in the fuel salt. 5. At 200°C, the various gases produced very small changes in the alloys. 6. At 300 and 600°C, only small changes were noted in the Hastelloy N specimens. 7. Small changes were noted in the stainless steel exposed to air at 300°C, but large amounts of corrosion products were produced by BF3, argon-BF3, air-BF3;, and argon-air. 14 8. At 600°C, the air-BF; mixture produced large quantities of chromium oxide and iron oxide on the stainless steel; also large amounts of corrosion products were produced by BF3, argon-BF;, and argon-air. 9. Drying the argon decreased the amount of corrosion. 10. Hastelloy N is more resistant to corrosion by the gases tested in this experiment than type 304L stainless steel. ACKNOWLEDGMENTS It is a pleasure to acknowledge E. J. Lawrence, who operated the experiment and was responsible for the weight change measurements of all the corrosion specimens. I am indebted to H. E. McCoy, J. H. DeVan, and T. S. Lundy for constructive review of the manuscript. Special thanks are extended to C. E. Zachary for metallographic work, R. M. Steele for the x-ray diffraction, and the Metals and Ceramics Reports Office for invaluable assistance. 15 INTERNAL DISTRIBUTION (77 copies) (3) Central Research Library ORNL — Y-12 Technical Library (5) Document Reference Section (10) Laboratory Records Department Laboratory Records, ORNL RC ORNL Patent Office . Adamson, Jr. Baes Bamberger . Beall Bohlmann WM E L. Compere Cook Culler Cunningham Dale DeVan DiStefano . Engel . Ferguson Frye, Jr. Gilpatrick Grimes . Grindell Harms . Haubenreich Hill Huntley Inouye - - . (3) mERWErnr Uy RnEENnOOQQ ?uwzomwommwwmzmr*m EXTERNAL DISTRIBUTION (24 copies) - - . - - - - I.."'f--af-«U‘PUPUCDg}C-cM'EKP*UPE"UUEOD:FUEW'J:WHD>MC—¢*U ORNL-TM-4172 R. W. J. G HEEWE O Al Kasten Koger Lawrence . Lotts Lundy Lyon . MacPherson MacPherson Martin McClung . McCoy . McHargue McLain McNabb E. S. McNeese Meyer Patriarca - - - "GEZE‘.L—‘OSC§ Perry Rosenthal Savage Scott Shaffer Slaughter Smith Strehlow Thoma Trauger . Weir White . Wilson BABCOCK & WILCOX COMPANY, P.O. Box 1260, Lynchburg, VA 24505 B. Mong BLACK AND VEATCH, P.0. Box 8405, Kansas City, MO 64114 C. B. Deering 16 BRYON JACKSON PUMP, P.0. Box 2017, Los Angeles, CA 90054 G. C. Clasby CABOT CORPORATION, STELLITE DIVISION, 1020 Park Ave.,, Kokomo, IN 46901 T, K. Roche CONTINENTAL OIL COMPANY, Ponca City, OK 74601 J. A. Acciarri EBASCO SERVICES, INC., 2 Rector Street, New York, NY 10006 D. R. deBoisblanc T. A. Flynn THE INTERNATIONAL NICKEL COMPANY, Huntington, WV 25720 J. M. Martin UNION CARBIDE CORPORATION, CARBON PRODUCTS DIVISION, 12900 Snow Road, Parma, OH 44130 R. M. Bushong USAEC, DIVISION OF REACTOR DEVELOPMENT AND TECHNOLOGY, Washington, DC 20545 David Elias J. E. Fox Norton Haberman C. E. Johnson T. C. Reuther S. Rosen Milton Shaw J. M. Simmons USAEC, DIVISION OF REGULATIONS, Washington, DC 20545 A, Giambusso USAEC, RDT SITE REPRESENTATIVES, Oak Ridge National Laboratory, P.0. Box X, Oak Ridge, TN 37830 D. F. Cope Kermit Laughon C. L. Matthews USAEC, OAK RIDGE OPERATIONS, P.O. Box E, Oak Ridge, TN 37830 Research and Technical Support Division USAEC, TECHNICAL INFORMATION CENTER, P.0. Box 62, Oak Ridge, TN 37830 (2)