ORNL-TM-2490 Contract No. W-7405-eng-26 4 ‘T }':} METALS AND CERAMICS DIVISION COMPATIBILITY OF HASTELLOY N AND CROLOY 9M WITH 'NaBF,-NaF-KBF, (90-4-6 mole %) FLUOROBORATE SALT J. W. Koger and A, P. Litman I.EGAL NOTICE “ , . ’ | This report was prepared as an account, of Government -ponsored work. Neither the United L ’ . i i Btates, nor the Commission, nor any person acting on behalf of the Commission: f - A, Makes any warranty or representation, expressed or implied, with respect to the accu- rlcy. completeness, or usefulness of the information contained in this report, or that the use S , - pf any information, apparatus, method, or process disclosed in this report may not infringe - | privately owned rights; or - " . . B, Assumes any liabilitiea with respect to the usze of, or fior damages resulting from the’ ~ use of any information, apparatus, method, or process disclosed in this report. . As used in the Rbove, ‘“‘person acting on behalf of the Commiasion’ includes any em- i ployee or contractor of the Commission, or employee of such contractor, to the extent that ‘ such employee or contractor of the Commission, or employee of such contractor prepares, i disseminates, or provides access to, any information pursuant to his emiployment or contract 3 with the Commisaion, or his employment with such contractor. s o - CAPRIL 1969 OAK RIDGE NATIONAL LABORATORY - Oak Ridge, Tennessee . .. operated by - ' UNION CARBIDE CORPORATION (- - | ~ for the U. S. ATOMIC ENERGY COMMISSION o 9w GISTRIBUTION OF THIS DOCUMENT 15 URTH Ty ( i .‘J i *) S s <) ) v Abstract { e e - . & @ . - Introduction . « « + « & o o o « & Experimental Procedure . . . . . - Materials Sectibfi and Fabrication Salt Preparation . . . Operations . . . .'; c e e e e e Resulfs’ .. .'. e e e e e NCL-10 (Hgstelloy N) . NCL-12 (Croloy 9M) . . Dis cus S ion . - » - - ¥ - * - - - Thermodynamics of System Corrosion . Kinetics of System Corrosion . . . . ~ Salt Purification . . Recommendations . . . « « « + « o . . Acknowledgments . . . . . . . - ) COHCIuSionS . - - s e . s e . e . . . llllll i O O 3 v W K Wowowwwwn e BT IR WH OO ” * e ~ 'COMPATIBILITY OF HASTELLOY N AND CROLOY 9M WITH - NaBF;-NaF-KBF; (90-4-6 mole %) FLUOROBORATE SALT J. W. Koger and A. P. Litman . .ABSTRACT The compatibility of a relatively impuré (> 3000 _ppm impurities) fluoroborate selt, NaBF,-NaF-KBF, (90-4-6 mole %) s with Hastelloy N and Croloy 9M was evaluated in natural cir- culation loops operating at a maximum temperature of 605°C - with a temperature difference of 145°C. The Croloy 9M loop - '(NCL’—J.E.’) was completely plugged -after 1440 hr of operation .. ~and the Hastelloy N lo (NCL-10) was three-quarters plugged after 8760 hr (one"ygar' ‘of operation. All major alloying elements of the container materials mass transferred during operation as the result of nonselective attack by virtue of . the initial oxygen and water contamination of the salt. Satu- ration.concentrations of 700 ppm Fe and 470 ppm Cr were deter- “mined for the fluoroborate salt at 460°C.. The mechanism of - corrosion of the system is as follows. Initially, metal fluo- ride compounds that are soluble in the salt are formed in the hot leg. The reverse reaction occurs in the cold leg and causes the metal to deposit and to diffuse into the cold leg. This contimes until (1) an equilibrium concentration of one or more metasl fluorides is reached in the salt at the cold- leg temperature and these compounds start depositing (e.g., NCL-10) or, (2) the equilibrium constant of the reaction deposited (g.g., NCL-lZ) - changes so much with temperature that the pure metal is , i J?NTRODUCTION Nuclear reactors that use molten fluorides as fuels are under : * development for 'thr_e' ‘dual purpose of power prodfié_fiioh and thorium con- ” 've'zjs;idnl.- 1 -:H_ea.t: genérafed ;ifi,-the ‘core region of such ,a,i_ziioitéh—éa.lt | “breeder reactor is tra.nsferi;é&--”f'r_omthe fuel-containing fluoride . selt to a 'secondary,co'olé.nfbfféiré\iit -of fluoride sa.ltmthcut fuel, tl;;at IMSR Program Semiann. Progr. Rept. Feb. 29, 1968, .ORNL-'-4254. then dissipates the heat to steam. 2 In the Molten-Salt Reactor Experiment (MSRE), the nickel-base alloy Hastelloy N has proved to be an effective container material for the fluorlde salt that contains the fuel and for the LiF-BeF, diluent. Many potential fissile® (contalnlng UF, ), fertile® (containing ThF4), or combination! (containing both UF; and ThF,;) salts proposed for the MSER have been or are being tested for their compatibility with Hastelloy N and other container materials. The choice of a salt for the secondary coolant, however, remains_open. Test programs naveronly considered coolant salts like NaF-ZrF, and, recently, LiF-BeF.. While these salts have demonstrated excellent compatibility with Hastelloy N, there is need for cheaper fluoride mixtures that melt at 1owor tempera- tures. On the baéis of low cost (approximately 50é/lb) and a relatively low melting point (about 400°C, necessary for transferring heat to super- critical steam), fluoroborate salts -especially'NaBF4 — with small additions of NaF and/or KBF; have recently become of interest as secon- dary coolants. Little is known, however, about the corrosion behavior of these salts. _ The space diagram of the ternary NaF-NaEF,-KBF, systém (Fig. 1) shows that the salt mixture under consideration lies very near the NaBF, corner and has a melting point close to 390°C. Note that oxygen impu- rities that form BF30H-compounds significantly lower the melting point of the mixture below that shown by the diagram. The vapor pressure of fluoroborate salts, especially NaBF,, is higher than that of other fluoride salts,’ because in the temperature range of interest the sodium fluoroborate maintains such an equilibrium with its components that a significant amount of boron trifluoride gas is present in the system: NaBF, =NaF + BF;(g) . (1) °P. R. Kasten, E. S. Bettis, and R. C. Rdbertson, Design Studies of 1000-Mr(e) Molten Salt Breeder Reactors, ORNL-3996 (August 1966). 3H. S. Booth and D. R. Martin, Boron Trifluoride and Its Derivatives, Wiley, New York, 1949. : _ Y - 7 ) ,NaBF4 - NoF - KBF4 (90-4-6 mole %) 386’ / 398¢° NoBF, / : " B4ge ORNL-DWG €8-5560R 4 . KBFy 408° ) 384£'°8 ) A (576°) 4000 S 450° — - 4 , 328° 700 180" e 00e 0\ 355 o _ — = 4400 . g0/ — o 650, .. B50° - ' s50° L ol 900° - . , f 650° 9502 - L1000 © NoF A o ' . 780° 800° NaF &= A \/ e L LN L \ KF Fig. 1.. Space Diagramuof the NaF-NaBF,-KBF,-KF System. Little is known about the corrosion behav1or of the BF3 gas. A few corro- ~sion exper:unents have 'been run wz.th very dry BF; gas »" and llttle appre- c:La.ble attack was found up to 200°C on metals such as brass y sta:.nless steel nz.ckel, Monel, mlld steel, and many others. o - ThlS report describes the first comprehens1ve study of the compu __patlbllity of a relatively 1mpure fluoroborate sa.lt with Hastelloy N : a.nd Croloy OM alloys. ‘The e:@erlments were de51gned to yield :Lnfor- mation on temperature grad:.ent mass transfer, the major form of corro- _ sion in fluorlde salt systems Two loops were operated'W1th NaBF4-NaF-KBF4 (90-4-6 mole ‘75) salt at a maxmm temperature of 605° C ‘W‘l‘bh 8 tempera.ture ,",dlfference of 145° C to obta.ln the data presented. These temperatures . - ;reactor. - ' EXPERIMENTAL PROCEDURE - match those proposed for fluoroborate sa.lts in 8 molten saJ.t breeder . The natural circulation loops for this progrem were of the type . 'isho&nfin Fig. 2. | Flowresul'bsfi'omthe difference in'density:"of-" the “F. Hudswell, J. 8. Nairn, and X. L. Wilkinson, J. Appl Chem | 1, 333 (1961) ™~ =+ o w o - o = Q. Natural Circulation Loops in Test Stands. 2. Fig. ” - galt in the hot and cold pertions of the loop. We estimated the velocity of salt flow in the test:loops to be 7 ft/min. | Materials Section and Fabi'icatioh - " The Croloy 9M materle.l was from Bebcock & W:Llcox Company heat 18760 a.nd was vapor blasted before fabrication to remove oxides. The loop, . '_NCL-12, was fabricated from-O. 750-in. -OD tubing with'a wall thickness of 0.109 in. The_-material "was_TiG welded and inspected to meet ‘existing “ internal standards. The welded ereas were torch annealed before and: ‘after welding. .. - | S The Hastelloy N materla.l was from Union Carbide Corporation, Materials Systems Division heat 5096. The loop, NCL-10, was fa.brica.ted from 0. 672-in. -0D tubing with a wall thickness of O. 062 in. , TIG welded and inspected to meet the seme stendards as those required for the Croloy 9M material. The conmos:.tions of both alloys are given in Table 1. Table 1. Chemical Composition of Alloy Test Materials _ Composition (wt %) Alloy — , , : Ni Cr M .Fe C M S P si Hastelloy N 70.8 7.47 15. 59 2,. 01 0.07 0.5 0.005 0.6 (NCL-10 : Croloy 9M - ,8 87 0 98 89.00 0.09 0.48 O 012 0.010 0.47 - (NeL-12) - | | A | ) Sa.lt Prepa.ratlon - The sa.lt was prepa.red 'by the Fluor:.de Process:mg Group of the B . Reactor Chemistry Divlslon at ORNL. ~ This was their first experience _ _w:.th a fluoroborate salt, and they prepared it by techniques established __'for other _fluor:.de__s_a_lts._ = The___zja._w materials —~ relatively mpure NaBF, 9 SMSR Program Semiann. Progr. Rept. Jan. 31, 1967 ORNL-3626, p. 146. ‘NaF, and KBF, (90-4-6 mole %) (Table 2) — were loaded into a container lined with nickel, which was then evacuated and purged several times with helium.' Then, the materials were melted and heated to 400° C under " a helium atmosphere. Next, the liquid was sparged with hélium."sinée this caused a large increase in pressure, the reactor vessel was vented. Large quantities of BF3, which had caused the increase in pressure, were " then released. Later steps included sparges with a mixture of hydrogen fluoride and hydrogen for several days at 550°C to remove oxides and a sparge with hydrogen to remove structural metal impurities. The salt was then transferred to a fill tank made of Hastelloy N in preparation for filling the loop. _ The chemical analysis (Table 3) of the prepared selt disclosed two important compositional changes germane to this test series:. (1) a significant loss of BF; during preparation, and (2) a high content of oxygen and water. Since little was known asbout the corrosivé behavior of this salt or the effect of impurities and because information on its compatibility with the container alloys was needed quickly, we decided to use it as prepared. Table 2. Composition of Salt Mixture Before Purification Mole % | Wt % Quantities ILoaded in Container Compound KBF,, | - 5.88 6.85 NaBF, 90.20 | 91.61 NaF | 3.92 | . 1.53 ‘Chemical Analysis Element K | | o | 2.10 Na | 20.00 - B | 9.60 F | 68.30 ot n - Teble 3. Chemical .Ana.lysis“of ‘Salt Before Fill Element ' B Content (wt %) kK R 2.20 Na o o 25.80 B o | | 9.65 Fo L 60440 NG < 5% cr S - A Fe I | e s e 0 - ' 3000% Bo . 900, 2100% a'Pa.rts- p_ei-_'. million. ' OPERATIONS The hot portion of each loop was heated by sets of clamshell heaters, with the input power controlled by silicon controlled rectifiers .(SCR units) and the temperature éont_rolléd by a Leeds and Ndrthrup Speedomax H Series 60 type C.A.T. (current proportioning) controller. The loop tem- peratfires were mea'sured by Chromel-P vs Alumel thermocouples that were spot welded to the. outside of the tubing, covered by a la.yer of qua.rtz tape, and then covered W1th stalnless steel shim stock. - Before each loop was filled with salt, it was degreased with a.cetone ‘and heated to l?O‘?,C ~under v_s_a._c_:gum }:o_ remove any moisture in the system. We checked for leaks with a helium mass spectrometer leak detector while the interior of the loop was evacuated to < 5 X 1077 torr. All lines from the fill tank to the loop that were exposed to‘the fluoroborate _ ¥ salt were of the same material as the loop and were cleaned and tested “ in the same manner as the loop. All temporary line connections were made with stainless steel compression fittings. | | The loops were filled by heating the loop, the salt pot, and all connecting lines to a minimum of 530°C and applying helium pressure to the salt pot to force the salt into the loop. Air was continuously blown on the freeze valves leading to the dump and flush tanks to pro- vide a positive salt seal. Tubular electric heaters controlled by vari- able autotransformers furnished the heat to the cold-leg portions. Once the loop was filled the heaters were turned off and the insulation was removed to obtain the proper temperature difference by exposing portlons of the cold leg to ambient air. The first charge of salt was dumped after 24 hr in the loops at the maximum operating temperature with some circulation and little tempera- / ture gradient. This flush removed surface oxides and other impurities that could have been left in the loops. The loops were then‘refilled : | ~ with qéw salt and put into operation. A helium cover gas under slight | positive pressure (about 5 psig) was maintained over the salt in the loops during operation. Each loop was operated at a maximum temperature of 605°C and with a temperature difference of 145°C. A temperature pro- file around the loops is shown around the schematic of the loop in Fig. 3. During circulation each loop contained about 920 g of salt that con- - tacted 1200 cm® of surface and traveled 254 cm around the loop. Temperature excursions indicated a flow stoppage in NCL-12 (Croloy 9M) after 1440 hr. Attempts to drain the loop were unsuccess- ful, and the loop was allowed to cool with the test salt in place. In - NCL-10 (Hastelloy N),Va significant increase of temperature in the hot leg accompanied by a simultaneous temperature decrease in the cold leg . occurred after 8335 hr of operation. A perturbation of this type indi- t cates a disruption in salt flow and can indicate plugging. This tem- - perature cycle ceased after 1 hr, and no further incidents occurred Q;J " L4} o ORNL-DWG 68-11780 SURGE TANK sosec L=l | T — TS OsS | g | v | . | sa0%c VT L o \ o CLAMSHELL ! 0 _}7 HEATERS l ! ' 1 | B . - e | el | —NsuLaTioN | 32.51in. { I i,/ ANt _ i L P! s | | | ' | | | 1 : LUl 540°C \' = kN DUMP TANK - ‘Fig. 3. Schematic of MSRP Natural Circulation Loop. during the life of the lbop-,:The_looP’was shut down after 8760 hr (1 year), and the salt was dréified‘intoda @ump'tank-in.a_fiormal manner. 'RESUI.TS : Analyses of loop components and salt were made by standard chemical analy31s, metallographlc examlnatlon, and electron mlcroprdbe analy31s. ‘The results are dlscussed below 'NCL-lO'(Hastelloy N) Visual. -A partlal plug 4n the lcwer part of the cold leg (Flg. 4) _closed approximately 75¢ of the cross-sectional area of the plpe._ Analysis 10 TUBE WALL NO; CrFs PLUG Fig. 4. Plug Formed in NCL-10 (Hastelloy N) Containing NaBF,-NeF-KBF, (90-4~6 mole %) After 8760 hr at a Maximum Temperature of 605°C and Tem- perature Difference of 145°C. 8X. Reduced 40%. showed this emerald-green plug to be single crystals of NasCrFg (ref. 6). Smaller amounts of two complex iron fluorides, NajFeFg and NaFeF,, were also identified in the cold leg. | ' Chemical. — The analysis of the salt ceke (Fig. 5) collected from - the dump tank of NCL-10 is given in Table 4. The concentrations of nickel, molybdenuin, iron, and chromium in this salt were higher than those of the salt before test. The average chromium concentration was 470 ppm. | | Due to cooling, nickel was segregated in the top and bottom portions of the cake, reaching 11.15 and 4.47 wt % in these portions, respectively. ’ ‘The analysis indicated that the water content of the salt — 400 to 900 ppm - 6MSR Progrem Semisnn. Progr. Rept., Aug. 31, 1967, ORNL-4191, p. 228. O L2 o Bottom layer 4.47° 11 BPhoto 74747 Fig. 5. Drain Salt Cake From NCL-10 (Hastelloy N). Table 4. Impurity Analysis: NaBF,-NaF-KBF, (90-4-6 mole %) Anelysis (ppm) Ni Cr .M - PFe 0 H,O Before test 87 83 7 After test 8 Top slag 11.15 & * 1000 1.35 Center layer 90 ~ ~ 210 160 ® 1500 7300 146 4200 270 1500 1400 3000 4850 1200 1750 3540 3120 3550 3660 400 900 1800 11300 2800 2,6 Seight percent. 12 before test — increased to 1300 to 2800 ppm. The oxygen analyses showed mich scatter, but we believe that the oxygen content also increased. . Presumsbly these increases are due not to air inleakage but to other factors discussed later in this report. | The crossover line to the cold leg, the cold leg, and the crossover line to the hot leg all showed slight increases in'wal} thickness due to deposition of eomplex surface layers (Fig. 6). Chemical analysis of the layers disclosed that they were primarily'metailic nickel (60 to 90 wt %) and molybdémum. A small quantity of iron was present 1n.prox1m1ty to the ‘base metal, but chromium was conspicuously absent. o Metallurgical. — Micrometer measurements of the hot leg of the loop disclosed 1 to 2 mils of metal loss and slight surface roughening. Metallographic examination of an area from the hottest section _(605°C), however, showed a smooth surface with an 6ccasienal penetra- tion along a grain boundary (Fig. 7). Microprobe traces’ of this hottest section for all the alloying elements showed no compositional gradients. The chromium trace for this section is givenrin Fig. 8. These results indicate a general dissolutive attack in the hot-leg section. An area at the entry of the cold leg (520°C) showed a duplex surface structure, and the areas numbered in Fig. 9 were analyzed with the micro- probe for the various elements (Fig. 10). The outermost layer, aresa 1, is quite high in nickel ~ up to 87 wt %. We believe this is most likely a region where nickel deposited during the test. Area 2 is a region con- taining essentially only'molybdenum and nickel. Close to the original metal surface there is an increase in nickel, a smaller increase in molybdenum, and little change in chromium and jron. Note that with an increase in nickel and molybdenum metal the concentration of chromium and iron would show a decrease even though there were no chenge in the actual amounts. ' Figure 11, a photomicrograph of the coldest section NCL-lO (460°C) shows a spongy deposit on the surface and a loosely adherent corr031on product beyond this region. At higher magnifications, it was seen that the tightly attached spongy deposit varied in thickness from O to 6 um 7Da.ta not corrected for absoxptlon, secondary fluorescence or atomic number effects. iy o N “‘; ‘! ‘ 1 éll | ;;32.5in. - 0007 inches ' . NeLo | SALT: NaBF,NaF-KBF,; (90-4-6 mole %) - TEMPERATURE :605°C, AT= 145°C, TIME =8760hr . ‘ L) ) o " ORNL-DWG 67-9342R LOCATION OF Naz Crfg PLUG\_ Fig. 6. 'Surfa,ce' Layers in NCL-10 (Hastelloy N) After 8760 hr Operation. Y-81402 __ I-. e [ 0,007 INCHES & 500x | Jer I o 0i00175 INCHES "2000x ! Fig. 7. Hastelloy N Hot-Leg Section NCL-10 (605°C) Shown at Different Magnifications After 8760 hr in Fluoroborate Salt. Etchant: Glyceria Regisa. L t: I . " + ORNL-DWG 68- 9658 L 10 L ‘ o . & L Oy Oy - O A 0 i_D . ;—..‘ o W ~ \J O O U L) o gV &08 ‘ = o s 2 o | & x =~ 9 6 Q- ke B é L 1 = o z TRACES OF Ni,Fe, AND Mo N o SHOWED NO GRADIENT L s ~ :—-———\A————- | - Bp o — — _ — o {‘O : 10 ; | - : 20 - 30 S ‘ » 40 Fig.‘S; Penetration Curve of Chrom Fluoroborate Salt. | R DISTANCE () ium ithbfi7Beg (605°C) of NCL-10 (Hastelloy N) After 8760 hr in' 2 16 Y-83857 N [ 0.007 INCHES I 500X 1w > 0.00175 INCHES I (b) Fig. 9. Entry to Cold Leg (520°C) of NCL-10 (Hastelloy N) Operated for 8760 hr in Fluoroborate Salt. Etchant: Glyceria Regia. (a) 500x. (b) Numbered areass were analyzed by microprobe. 2000X. ) 1) 0 @ ORNL-DWG 68-9659 / ) TS | \= I _ | / ™~ Ni _ - . I | | s | So--—o-—-.-—-o..___‘,__.__. & o N/ —— N - e H L o . : e LAYER ————=1 oriGINAL - b sureace g ESTIMATED COMPOSITION (wt %) 20 } 0. Fig. 10. Penetration Curve of Constituents of Hastelloy N at Cold-Leg 0 e ® e =3 ::....‘-—.L—'“‘ = _ 0 e © o s e e g U s ’."-. ¢ . ' Fe 20 3 DISTANCE (1) 0 | 40 Entry (520°C) of NCL-10 Operated for 8760 hr in Fluorcborate Salt. LT 18 Y-81411 1-5 I | 0.007 INCHES & 500X o 1% Y-824u0 o F2060ox 1 0.00175 INCHES 5 Fig. 11. Coldest Section (460°C) of NCL-10 (Hastelloy N) Shown at Different Magnifications After 8760 hr in Fluoroborate Salt. Etchant: Glyceria Regia. ’ an 3 " w) 19 and that it resembled the area seen in the entry to the cold leg, <hough ~ much thinner. The spongy layer is high in nickel, with some molybdenum, and sppears, like the;area 1l of Fig. 9, to have resulted from nickel deposition (Fig. 12). There was only 63% metal in this spongy layer; the balance was fluoride compounds. The original surface is high in nickel and molybdenum and low in chromium and iron and is an area where nickel and molfibdenum have diffused into the matrix. No analysis of the loosely adherent material Was possible The last section of the 1oop examined'was an area at the entrance _ _to the hot 1eg Although the salt was being heated.here, the temperature was st111 low enough (< 530° C) to make this an area where materlal'was ' belng depos1ted. Again, a duplex layer'was seen (Fig. 13) end found to be predominantly nickel, 657tor90% metal and the balance fluoride com- . pounds (Fig. 14). X-ray diffraction analysis of the fluoride compounds disclosed an unidentifiable crystal structure, probably a mixture of several fluoride compounds. The surface layer had somewhat more nickel and molybdenum‘than originally,'and, thus, smaller concentrations of chromium and iron. + NCL-12 (Croloy 9M) Visual — When this”loop'Was sectioned for examination, a dark gray'plug that completely fllled the cross-sectional area fOr a vertical _: distance of 1 in. was found 1n the cold leg (Fig. 15) Also, small, ~ green crystals were seen in the drain line, and a metallic layer was 'found against the 1n31de of the tubing in the cold section and in the The th1n metallic 1ayer-about 2. 5 mils thick was dep081ted on the in31de of the tubing in the entire cold-leg section (over one-half the loop) In some places, the materlal had become detached from the tubing f:;and'was found in the frozen salt (Flg 16) The metallic layer repre- | sented sbout 4% of the total mass of the mater:l.al (mostly salt) removed o Vfby'mechanical mesns) from the loop. Chemical analysis revealed the layer -tobe90wt%Feanlewt%Crmetal. ORNL—DWG 68—9660 _ | 80 r63 7% METAL | BALANCE FLUORIDE SALTS | | 0 | I /\ ESTIMATED ORIGINAL SURFACE 40\! COMPOSITION (wt %) SPONGY et e e——— LAYER 20 20 R - 20 DISTANCE () Fig. 12. Penetration Curve of Constituents of Hastelloy N in Coldest Section (460°C) of NCL-10 Operated for 8760 hr in Fluoroborate Salt. 0c [- 0.007 INCHES 5 500X I [on t2000x 1 0.00175 INCHES I Fig. 13. Entry to Hot Leg (530°C) of NCL-10 (Hastelloy N) Shown at Different Magnifications After 8760 hr in Fluoroborate Salt. Etchant: Glyceria Regia. ' ' ' ' o 22 ORNL—~DWG €8-9€61 80 e . ‘—.—Ii—.—fl——‘—.-;d'—'_.fi_._ ¢ o “Ni | | i | 65-90% METAL | 60 |— BALANCE + FLUORIDE SALT | ESTIMATED = ‘ ! ORIGINAL 5 -~ LAYER , SURFACE = I 5 | - { o 40 1 o | a ] s I O o i | | | 20 : | . P __,..—-:—1.-—0—o—o—-c—dr—ojo—._._., V4 ' .‘3—0'—.--0—0—0—-.—-0-—0—-1.—.—0—0 “s... 0 * s__-—l-‘ 0 10 : 20 30 40 " DISTANCE (p) - Fig. 14. Penetration Curve of Constituents of Hastelloy N in Hot~- Leg Entry (530° c) of NCL-10 Opera.ted for 8760 hr in Fluoroborate Salt. P £ Fig. 15. (croloy oM) Qperated in Fluorcoborate Salt for 1440 hr at a Maximum Tem-‘ FLUOROBORATEVSALT ’ Iron Dendrite Plug in Coldest Section (460° C) of NCL-12 perature of 605°C and a Terperature leference of 145°C. % DEPOSITED METAL LAYER Fig. 16. Cross Section of Tubing of NCL-12 (Croloy 9M) Showing Deposited Metal Layer. Loop operated for 1440 hr in fluoroborate salt at g maximm temperature of 605°C and e temperature difference of 145°C. Reduced 22%. (a) Cross section of NCL-12 tubing. (b) Enlarged view of metal and salt interface. 30X, = S , - ' 24 The dark-gray'plug located in the coldest part of the 1oop'was com~ - posed of dendritic crystals. we found sxmllar crystals adhering to spec1- mens in the hot leg (Fig. 17), but we assume that these crystals, growing and circulating in the salt stream, attached themselves_tb-fhe-specimens while the loop was cooling. - Chemical. — Chemical analysis showed that the dark-gray plug and the material on the specimens were essentially pure iron with less than 1% of other elements (shown below). Elements ~ Content (wt %) Fe ) 99.00 B 0.03 Cr < 0.05 Mn < 0.01 Mg 0.05 Pb < 0.02 Si 0.02 Cu © 0.05 Mo 0.02 The results of chemical analysis of the green crystals in the drain leg are given below. Elements wt % Na 7 7 1015 B | 24 F o 45.9 K | ~ 0.058 Fe - | 18 Cr | 12 Mn , | 1.5 The stoichiometry of this green depbsit, based on this analysis, color, and other factors, is roughly 2NaF-FeF,-CrF;-BFs, which corresponds to chromium and iron fluorides mixed with the salt. 73674 TR R ' Fig. 17. Pure Iron Crystals from NCL-12, Which Operated for 1440 hr in Fluoroborate Salt at a Maximum Temperature of 605°C and a Temperature Difference of 145°C. 10X. Reduced 12%. (a) Crystals adhering to speci- men, and () crystals removed from specimen for photographing. ~ - 26 Table 5 gives the composite of the salt before and after operation. The significant changes in the salt chemistry due to test are the incresses in the chromjum and iron content from 54 to 255 and 28 to 700 ppnb respec- tively; ' | Table 5. Cdmposition of Salt Before and After Operation in NCL-12 Composition (wt %) - Composition (ppm) K N B F ©Ni Cr Fe M S 0 _ , - Before Test ' Theoretical 2.10 20.00 9.60 68.3 (calculated) | | ‘Before Fill 2.20 25.80 9.65 60.4 <5 54 28 3000 During Fill 1.98 18.83 9.38 66.2 87 83 146 1400 After Test Hot leg 1.55 19.72 9.29 67.1 265 700 < 20 7 3000 Cold leg 1.89 20.71 9.27 67.3 255 700 < 20 < 2 3200 Metallurgical. — Metallographic examination of the hot-leg loop tubing (Fig. 18) disclosed a fairly smooth surface, and micrometer mea- surements showed an average 2.5 mil loss from a nominal;pipe diameter. Electron microprobe analyses were made on tubing fromrthe hot- and - cold-leg sections of NCL-12: the hot-leg analysis (Fig. 19); consistent with the metallographic results, showed no iron or chromium concentration gradients; the cold leg analysis (Fig._20) showed an increase of about 4 wt % in iron concentration and a decrease of about 4 wt % in chromium concentration (i.e., an iron-rich surface layer)- iDISCUSSION i The tests described are the first study of the compatibility of a fluoroborate salt with container materials of interest to molten=salt reactors. Unfortunately, little was known at the time of the test about the purification'of the salt or the characteristics of mass transfer in A [ I I~ 100X e Fig. 18. Hot Leg (605°C) of NCL-12 (Croloy 9M), Which Operated for 1440 hr in Fluoroborate Salt. a.nd ethyl alcohol. -Etchant: Picric acid, hydrochloric acid, DISTANCE (p2) o4 'ORNL-DWG 68-9663 25 90 foo—o - oo l oo o o 2| '&(fl —~ o0 o o o "0.© = & ° ° ° — 0|0 n2 % g6 ' h.lg S s 8¢ | | & el 1l 8 e.e o Y | E 3 et eyt — et _ L, : - : 8 8 ] Cr s l Lo Lo 4 T s l 0p - 0 b— — _ L i 0 100 = 200 300 - 400 Fig. 19. Penetration ‘Curve for Iron and Chromium in the Hot Leg (Croloy,?M) » Which Operated for 1440 hr in Fluoroborate (605°C) of NCL-12 Salt. 28 ORNL-DWG 68-9662 94 o S o | o . 90 \ 5 : ' o - o < O~ 0 0 o o 0 & &cc - Y g0 o " Y0o0o ¢ o 3 Fe = o o c o ~ 86 C':D 12 & 1t 8 7 . Q ® = / 19 ° © { ® 4 0 0 100 200 - .300 | 400 | | | DEflANCE(yJ Fig. 20. Penetration Curve for Iron and Chromium in the Cold Leg. (460°C) of NCL-12 (Croloy 9M), Which Operated for 1440 hr in Fluoroborate Salt. systems containing such salts. The use of loops of an old design pre- vented our having any removeble specimens, but permanent hot-leg specimens were used in NCL-12. Our expectation, based on previous experience w:ith fluoride salts, was that temperature gradienf mass transfer of the least noble constit- uent (i.e., chromium and iron) would be the limiting factor. But salt analyses after the test indicated that all the major alloying elements of the container materials had mass transferred. The nonselective attack was also: confirmed by metallographic examination and microprobe analysis of the loop specimens and piping. In view of the mofrement of nickel and molybdenum, it is obvious that highly oxidizing conditions, due to water and oxygen in the salt, were present during the operation of the loops. In analyzing NCL-10 and -12, we reviewed earlier studies® (1late 1950's) to single out the cases of mass transfer of nickel and molybdenum. 83. H. DeVan, unpublished data, 1957-1959. i -y i 29 ‘We. found several instances where this had occurred in salts containing KF. One such 1oop, constructed of Hastelloy N, operated for a year with ‘a NaF-LiF-KF salt (11. 5=46. 5-42 mole %) at & maximum temperature of 690°C w:Lth a temperature dlfference of 90°C. No anslysis for oxygen or water ‘was made before test. Examination after test disclosed green crystals embedded in the salt. Sa]:.t_analyses showed significent increases in nickel, iron, molybdenum, and chromium content. X-ray analysis showed that the green crystals were mixtures of sodium and potassium chromium fluoride complex compounds and that most of the KF present in the salt 'was actually KF-2H>0. , Another loop, constructed. of Inconel s (N1—18% Cr—lO% Fe) operated about one-half Yyear with the seme sa:l.t and at the same temperatures as above. After test, metallographlc examination showed heavy attack in -the hot-leg portlon of the 1oop After test only the chromium and iron contents of the salt were determined; the chromium content had increased significantly from 60 to 900 ppm. X-ray analysis of the salt disclosed 7 -that the salt was about 15% KF 2H20 The relatlvely well~ deflned x-ray -pattern of this hydrate suggested that it was not formed when the cold melt was exposed to air but was carr::.ed as a part of the salt mxture & The compound KFe 2H20 is known to be thermally stable. The literature states that KF easily forms a series of crystal hydrates, whereas LiF and NaF crystallize anhyd.rously.g It is also noted that it is easy to form KBF30H and‘NaBF3OH and that they are quite stable. Thus we have seen. that dur:mg the molten-sa.lt reactor corros:l.on durlng ‘operation. f-program, salts conta:ming KE‘ have on occaslon ‘been very sggressive toward | metals. ' We believe that the reason for this is the combined water asso- clated with that alkali metal fluoride selt. We agaln stress that we ' fou.nd no leakage of air 1nto any of the loops. This suggests that hydrated KF_ is not removed by the pur:l.flcatlon process. Paradoxically, . it appears that combined water 1n the fluoroborate salt m:.xture can be S released and w:.ll react to produce HF by | Hgo +, BF4 —-HF + BF30H .]_ (@) | 9I. G. Ryss, The Chem:Lstry of Fluor1ne and Its Inorgan:r.c Compounds, pp. 521-28 and 815-16, AEC-tr-3927, Pt. 2, (February 1960). 30 The generation of HF in the system leads to the following reactions with ~the elements of the container material: 2HF + Ni Examination of the data for the other salt shown in Table 6 ~ (NaF-ZrF,-UF;) leads to the conclusion that chromium metal would not plate out in that system and that any product of mass transfer would be _a fluoride compound. At 600°C this salt in equilibrium with pure chromium will support a higher concentration of chromium fluorides than it will 4G, M. Adamson, R. S. Crouse, and W. D. Manly, Interim Report on Corrosion by Alkali-Metal Fluorides: Work to May 1, 1953, ORNL-2337 (March 20, 1959). 15G. M. Adamson, R. S. Crouse, and W. D. Manly, Interim Report on Corrosion by Zirconium-Base Fluorides, ORNL-2338 (Jan. 3, 1961). . i 33 ~in contact with Inconel at 800°C. For gross deposition of compounds to -occur, the concentration of the compound at the temperature of interest (cold zone) must exceed the saturation concentration of the metal fluoride corrosion product present in the system. We believe that the mechanism in NCL-10 (Hastelloy N) is similar to the one discussed above for the NaF;Zer system and that in time the con- centratlon of chromium.-or more accurately the concentration of mixed metal chromium fluoride -1n the fluoroborate salt at 605°C exceeded the saturation concentration at 460°C and allowed deposition of large quan- tities of complex compounds. | | Both NCL-10 and NCL-12 operated with the same salt at the same tem- perature, yet they'were plugged by different mechanisms at different rates. A chromium-rich plug was found in the Hastelloy N loop (containing | 7% Cr—5% Fe) and an iron plng'was found in the Croloy 9M loop (containing 9% Cr-bal Fe). Thus, it appears that when these fluorcborate salts ~are contained in alloys with between 7 and 9 wt % Cr, the iron content of the alloy controls the comp031t10n of the temperature-gradaent mass~transfer dep081t. | Kinetics of System Corrosion With the fOreg01ng data and evaluatlon, it is now p0381b1e to deter- mine when plugging started ;n_NCL-lO and =12 and the dlsp031t10n_of each element at various times. . R We calculated the interim concentrations for chromium and iron in _ NCL-12 from knowledge of . the amount of these elements after test, the "welght of the iron plug, and an assumed reaction-rate constant and mode of chemical attack (from later experlments) 16 The results of the cal- culatlons, presented in Table 7, show that the saturation value of iron " in the cold section, 700 ppm or 1120 mg, was exceeded shortly after :. 130 hr; at that tlme pure 1ron started dep081t1ng and eventually caused e complete plug. o | R Table 8 shows the same klnd of calculatlons for the operatlon of NCL-lO. In thls_case, it,appears that the saturation value of chromlum 167, W. Koger and A. P. Litman, MSR Program Semiann. Progr. Rept., __Féb 29, 1968, ORNL 4254 PP 218—225. Table 7. Calculated Concentration of Alloying Elements from NCL-12 Present in the Salt at Various Times ‘.Time - Maximum Aversge Area Total Deposited in Concentra.t ion in Salt m b Deposited (hr) Attack Attack Attacked™ Material Ribbon Form Fe Cr As Plug " (mg/em?) (mg/em?) = (cm?) Iost (mg) (mg) (mg) (mg) (ppm) (mg) (ppm) 1.4 2 1 650 650 250 360 225 40 25 57.6 4 2 650 1300 500 720 450 80 50 130 6 3 650 1950 750 1080 675 120 75 1440 20 10 650 - 6500 2450 1120 700° 400 250° 2530 B0aloulated from M = 24/% o ovkt where K = rate constant, 10~? cm?®/sec at 607°C, MW = weight loss, ¢ = concentration of chromium snd iron, p = density of Croloy 9M, and t = time. bIncludes ‘surge tank and lines and one-half of total area. By chemical analysis. e ®a !7 | . - ‘. , 1 Table 8. Ca.lc_:ulated Concentratlon of Alloying Elements from NCL-lO Present in the Salt at Va.rlous Times Ares . Concentration in Salt Ma.:dnmm Average : '. Time ' 'b . Total Deposited on (hr) Attack® Attack. Attacked Material - Cold ; Fe Cr = Ni (mg/mnz) (mg/cm?) (cm ) 1 Dost (mg) ‘Surface — —— g ‘ . Lot i mg) (ppm) (mg) - (ppm) (mg) (ppm) 4000 :33':@1 L1900 550 ‘,..':-10,450 2100 350 383 420 459 6350 6950 1150 1260 8760 .56 28 . 550 15,400 3300 509 555 427 470 9300 10,000° 1560 1700° aée.'ltmiated fr‘oifi Mz = Z/J- o o‘/f{_ where K= ra.te consta,nt, 10-12 2/:sec: at 607°C, W = Welght loss, | e = concentration of chromium, iron, nickel, and mol:fbdenum, P o= density of Hastelloy N, a.nd | -t = time. 1"’In(:].v.n:'ie:se surge tank and lines and one-half total a.rea.. | By chemica.l ana.lysis. B ge 36 in’ the cold leg, 470 ppm, was reached after about 4000 hr of operation; at that time, the chromium, as Na3CrFg, started deposmtlng in large amounts. 1’ It is interesting to note that in both cases the amounts of the alloylng elements in the salt and in the plug were in dbout the same ratio as they are in the base metal. Capsule tests and other loop tests | have shown that when conStituents of an alloy react with the liquid medium they will be found in the liquid and/or deposited in the ratio at which they existed in the alloy.l® This behavior was found in both loops of this experiment. ' Nickel and molybdenum mass transfer products, not normally found in relatively pure fluoride salf systems, also fbllcw this pattern. As has been stressed here, the presence of gross quantities of nlckel and molyb- denum indicates strongly oxidizing conditions. Salt Purification The salt used in these experiments contained many impurities that the old techniques, successful with other fluoride salts, did not remove. Since these experiments, improvements have been made in the fluoroborate salt preparation. Only very pure salt (99.9%) is now used as starting material. In fact, the salt has so few impurities that no steps are 17 Several suggestions, besides the removal of the water or oxygen from the system, are offered to improve the service of the container materials using this fluoroborate salt. The most obvious change is to lower the hot-leg temperature to 540°C thus lowering the reaction-rate constant an order of magnitude. It would then take three times as long to duplicate the previous plugging. Another possibility is to raise the temperature in the cold-leg section. This would raise the saturation value of the elements in the salt, and depositing would not occur until later. But this probably would not improve the life by the same factor as above. Probably the most important improvement is removing corrosion Products from the system on a continuous or batch-wise basis since deterioration (thlnnlng) of the container wall by dissolutive corrosion is not a problen in these systems. 187, R. Weeks and D. H. Gurinsky, "Solid Metal-Liquid Metal Reactions in Bismuth and Sodium,"” pp. 106161 in Liquid Metals and Solidification, American Society for Metals, Metals Park, Novelty, Chio, 1958. R 37 taken during processing,to remove the structural metals. Purification procedures for removing water and oxygen are now under study. A new procedure is alse used in the.melting and preparation of the fluorcborate salt to prevent'loss‘of BF3 vapor and the ensuing change of composition. In brief, the loaded salt is evacuated to about 380 torr and heated to 150°C in a vessel lined with nickel and is held for about 15 hr under these conditions. -If the rise in vapor pressure is not excessive, (no volatile impurities) the salt is heated to 500°C, while still under vacuum, and agitated with helium for a few hours. The salt is then ready for transfer to the fill vessel. CONCLUSIONS 1. Natural clrculatlon loops, fabricated from. Hastelloy N and Croloy 9M, that c1rculated impure (> 3000 ppm.1mpur1t1es) NaBF, -NaF-KBF, (90-4-6 mole %) at a max1mum;temperature of 605°C with a temperature difference of 145°C evidenced serious temperature-gradient mass transfer. The mass transfer involved migration of all major constituents of the con- stituents of the container materials and resulted in restricting flow in the Hastelloy N-leop by'dePOSition of Na3CrFe¢ crystals andfcomplete plug- ging of the Croloy 9M loop by iron dendrites. - 2. The nonselective corrosion observed was due to the presence of water, chemically'bound'to the fluordborate salts, that reacted durlng heating to form HF. | 3. The driving fbrce fbr mass transfer'was the temperature depen-_ ‘dence of the equllibrrwm constant between the conta1ner material con- ?stltuents and the,most stable,fluorlde compounds that cen,form in the ‘system. 4. The saturatlon concentratlons for iron and chromlum in the test salt at. 460 C'were found to be 700 and 470 ppm, respectlvely - 'EECOMNDATIONS | 1. Other, morerhighlfi'purified'fluoroborates should be extensively tested for corrosion before these coolants can be qualified for molten- salt reactor service. 38 2. Based on knowledge to data, iron-base and iron-containing alloys should.be avoided in molten-selt reactor coolant circuits that use - fluordborate salts. ACKNOWLEDGMENTS _ It is a pleasure to acknowledge that E. J. Lawrence supervised con- struction, operation, and disassembly of the test loops during'the course of this program. We are also 1ndébted.to H. E. McCoy, Jr. and J. H. DeVan for constructive review of the manuscrlpt Special thanks are extended to the Metallography Group, especially H. R. Gaddis, H. V. Mateer, and R. S. Crouse, and to the Analytical Chemistry Division, Graphic Arts Department, and the Metals and Ceramics Division Reports Office for invaluable assistance. - 32. 1-3, 6-15. 16. 17. 18. 19. 20. 2L. 22. 23. 24, 25. 26. 27. 28. 29. 30. 31 33. ’ 3. i 35. 36. 37. 38. 39. 40. 41. 42. 43. 45, 46, 4T, 48. 49, . 50. 51. - 52, 53, 54. 55. 56. v -39 ORNL-TM-2490 INTERNAL DISTRIBUTION Central Research Library 58. E. L. Compere ORNL — Y-12 Technical Library 59. K. V. Cook Document Reference Section 60. W. H. Cook Laboratory Records o 6l. L. T. Corbin Laeboratory Records, ORNL RC 62. B. Cox , ORNI: Patent Office 63. R. S. Crouse R. K. Adams 64. J. L. Crowley - G. M. Adamson 65. F. L. Culler R. G. Affel 66. D. R. Cuneo J. L. ‘Anderson 67. J. E. Cunningham R. F. Apple 68. J. M. Dale C. F. Baes - 69. D. G. Davis J. M. Baker 70. R. J. DeBakker S. J. Ball 71. J. H. DeVan C. E. Bamberger 72. S. J. Ditto C. J. Barton 73. A. S. Dworkin H. F. Bauman 74. TI. T. Dudley S. E. Beall 75. D. A. Dyslin R. L. Beatty 76. W. P. Eatherly M. J. Bell 77. J. R. Engel M. Bender 78. E. P. Epler - C. E. Bettis ~ 79. D. E. Ferguson E. S. Bettis 80. L. M. Ferris D. S. Billington 8l. B. Fleischer R. E. Blanco 82. A. P. Fraas F. F. Blankenship 83. H. A. Friedman J. O. Blomeke 84. J. H Frye, Jr. R. Blumberg _ 85.. W. K. Furlong E. G. Bohlmann 86. C. H. Gabbard C. J. Borkowski 87. R. R. Gaddis G. E. Boyd 88. R. B. Gallaher J. Braunstein - 89. R. E. Gehlbach M. A. Bredig 90. J. H. Gibbons R. B. Briggs .. - ' 91. L. O. Gilpatrick H. R. Bronstein 92. P. A. Gnadt G. D. Brunton 93. R. J. Gray - D.. A. Csnonico 9. W. R. Grimes - S. Cantor 95. A. G. Grindell W. L. Carter 96. R. W. Gunkel - G. I. Cathers- 97. R. H. Guymon 0. B. Cavin el - og., J. ‘P. Hammond - J. M. Chandler - 99, B. A. Hannaford F. H. Clerk 100. < P. H. Harley - - W. R. Cobb 101. . D. G. Harman: H. D. Cochran 102. W. O. Harms - C. W. Collins 103. C. S. Harrill 104. 105. 106. 107. 108. 109-111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131-140. 141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151. 152-161. 162. 163. 164. 165. 166. 167. 168. 169. 170. 171. 172, 173. 174. P. R. P, D. J. .M. H. D. P. R. A. T. W. H. W. P. R. M. M. C. T. H. J. D. S. dJd. R. A. w3 EPHAPPELEOSO N. E. G. N. R. R. W. K. P. W. 40 Haubenreich Helms Herndon Hess Hightower Hill . Hoffman Holmes Holz Horton Houtzeel L. R. Hudson Hantley Inouye H. R. J. T. J. R. W. T. J. V. S. W. B. I. S. - W. E. A. E. J. J. o HOHONEHN YR 2Py Jordan Kasten Kedl Kelley Kelley Kennedy Kerlin- Kerr Keyes Kiplinger Kirslis Koger Korsmeyer Krakoviak Kress Krewson Lamb Lane Larson Lawrence Lawrence Lin Lindauer Litman Llewellyn . Long, dJr. Lotts Iundin Lyon Macklin MacPherson MscPherson Mailen Manning | Martin- . Martin Mateer 175. 176. 177. 178. 179. 180. 181. 182. 183. 184. 185. 186. 187. 188. 189. 190. 191. 192. 193. 194. 195. 196. 197. 198. 199, 200. 201. 202, 203. 204. 205. 206. 207. 208. 209. 210. 211. 212. 213. 214. 215. 216. 217. 218. 219. 220. 221. 222, 223. 22/, 225. QEEEmEPa T. H. PRUrQaYEp HPEHEE RO P J. H. C. J. W. G. A. F. G. 0. P. I. R. Dunlap H. Mauney McClain W. McClung . E. McCoy . L. McElroy . K. J MeGlothlan McHargue E. McNeese R. McWherter Metz Meyer . P. Milford Moore M. Moulton W. Mieller A. Nelms H. Nichol P. Nichols L. Nicholson D. Nogueirs C. Oakes Patriarca M. Perry W. Pickel B. Piper E. Prince L. Ragan I.. Redford Richardson Robbins C. Robertson Robinson Romberger Ross Ssvage Schaffer Schilling Scott L. Scott E. Seagren E. Sessions H. Shaffer H. Sides M. Slaughter N. Smith J. Smith P. Smith L. Smith G. Smith Spiewak C. Steffy P AQEEQ = oy 226. 227. 228. . 229, 230. 231. 232. 233. 234. 235. 236. 237. 238. 239. 240. 256. 257. 258, 259, 260. 261. 262. 263. 264 . 265. 266-267. 268. 269. 270. 271. 272. 273. _' 2740 275, 276. 277. _278. - 279. 280. 281. 282. 283, 284—298 G. dJd. D. C. H. A. F. c. W. W. T. A. D. C. G. D. H.‘ M. J. W. S. E. D. R. A. R. - C. H. A. A. R. H. 41 Stoddart - - 241. C. F. Weaver Stone 242, B. H. Webster Strehlow 243, A. M. Weinberg Sundberg 244, J. R. Weir Talleackson 245, W. J. Werner Taylor , 246. K. W. West Terry . 247. M. E. Whatley E. - F . M. . B. E. M. ® S. L. G. G. F. B. M. Thoma, . . 248, J. C. White Thomason . 249. R. P. Wichner Toth ~ 250. L. V. Wilson Trauger ' 251. Gale Young Unger ' -~ 252. H. C. Young Watson : ' : 253. J. P. Young Watson 254. E. L. Youngblood Watts - 255. F. C. 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