3] " v} «) » .\ ORNL~TM~2741 Contract No. W-7405-eng-26 METAIS AND CERAMICS DIVISION CATASTROPHIC CORROSICN OF TYPE 304 STAINLESS STEEL IN A SYSTEM CIRCULATING FUSED SODIUM FLUOROBORATE J. W. Koger and A. P. Litman LEGAL NOTICE- This report was prepared as an account of Government sponsored work. Nelther the Unlted Hiates, nor the Commission, notT &ny person acting on behalf of the Commission: A. Makes any warranty or representation, expressed or implied, with respect to the scon- T8EY, ¥ or usefi of the infx th tainad in this report, or that the use 1 of mny information, spparatus, method, or process disciosed in this report may not infringe : . privately owned rights; or ' . . 1 B, Assumes any labilities with respect to the use of, or for damages resulting from the i uso of any information, apparatus, method, or process disclosed tn this report. - As used in the above, “person acting oa bebalf of the Commission® includes sny em- ployes or contractor of the C: or employes of such , 0 the axtent that such empioyee or contractor of the Commission, or employes of such contractor prepares, disseminates, or provides nccess to, any information parsnant to his smployment or contract with the Commission, or his employment with such eontractor. ) - JANUARY 1570 QAK RIDGE NATIONAL IABORATORY Oak Ridge, Tennessee .. operated by UNION CARBIDE CORPORATION for the _ U.S. ATOMIC ENERGY COMMISSION NLIMITED (ENT IS U EISTBIBUTION oF THIS pOCUMENT | > » x] v "l ¥\ Abstract . - ® - - - . ® . o » o . - . - - . Introduction . . « « v v o 4 v 0 v o0 0. . Results - . L] » ' . * . . - 7 . . ’ . . - . . Visual and Metallographic . . . . . . Chemj.call . - * ® - .‘ . . . V . L . - * » ~ Physical Property Changes . . . . . . Discussion . « « ¢« o ¢ ¢ ¢ ¢ o o ¢ v o o Impurity Effects . . « ¢« ¢« ¢ ¢« ¢« &« o & Effect of Imposed Electromotive Force Stainless Steel Corrosibfi c s s s & e Dissimilar-Metal Corrosion . c e e e Corrosion Mechanism and Mode . . . . . SUMMATY « o o « o o o o o o o o s » o C onc luS i—on S - - . - - - - . * - * - * - . Recommendations . . . . . e e e e e e Page o W W EKEEKE 14 16 16 16 » ® CATASTROPHIC CORROSION OF TYPE 304 STAINLESS STEEL IN A SYSTEM CIRCUILATING FUSED SODIUM FLUOROBORATE J. W. Koger and A. P. Litman ABSTRACT A type 304 stainless steel liquid level probe contacted fluoroborate salt (NaBF,~8 mole % NaF) in an Inconel 600 pump loop at constant temperatures in the range 540 to 690°C for 192 hr. The probe exhibited heavy attack, evidenced by severe leaching of Cr, Fe, Mn, and Si from the alloy. Equivalent wniform attack was about 4 mils/day. Corrosion of the stain- less steel, which 1s inferior to nickel-base alloys in fused fluorides, became catastrophic in this system duvue to dissimilar- metal effects. INTRODUCTION The PKP-1 loop, constructed of Inconel 600, is a forced-circulation loop and part of the Fuel Pump High Temperature Endurance Test Facility. The locop is used for performance testing centrifugal pumps of the type developed for the Molten-Salt Reactor Experiment (MSRE). The loop was altered to accept NaBF;—8 mole % NaF as the circulating medium as part of the program to qualify the salt for use as a coolant in molten-salt reactors. The experimental program currently involves the measurement of the cavitation pressure of the pump as a function of temperature of the molten salt. For these experiments the loop is operated under nearly isothermal conditions in the range 540 to 690°C. A liquid-level probe (Fig. 1) was installed in the pump bowl of PKP-1 on June 20, 1968, to indicate changes in the liquid level that occurred independently of changes in salt density. The probe was ini- tially thought to be constructed with an Inconel 600 outer sheath, but later examination showed it was type 304 stainless steel. The instru- ment used a 1000-Hz electrical signal across a conductance probe immersed in the salt and had an output signal that was a linear function B b i E: 2 i L g i Gmn»m“fim Lfl%a, o S . 3 ; L il s e sl : : ; i S G i i Photo 75764 Level Probe iqu Exposed to NaBF,—8 mole % NaF in id- L 1 Loop for 192 hr at 540 to C. PKP- 690° of immersion depth.l The salt temperatures seen by the probe are given in Table 1. Signals from the probe stopped shortly after installation. The probe was removed on June 28, 1968, after 192 hr in the salt, and extensive corrosion had obviously occurred. This paper describes our metallurgical analysis of the probe, discusses the corrosion phenomena that occurred, and details the significance of the incident for the Molten-Salt Reactor Program. | | Table 1. Salt Temperatures in MSRP-PKP-1 Pump Loop Duration Temperature (hr) (°c) 21 540 R 552 26 649 53 690 RESULTS Visual and Metallographic The portion of the probe that was immersed in salt was 0.25 in. in diameter X 0.030 in. wall thickness. As shown in Fig. 2, heavy attack occurred over the lower 2 in. of this section. The level of the salt on the probe during operation is not known and appears to have varied. For analysis, the tubing was cut into eight 3/4-in.-long samples, num~- bered such that sample 1 was furthest from the exposed end. Figure 3 shows two magnified views of the end of the probe (sample 8). Severe distortion and very large pits are seen. In some places the wall was completely penetrated. We calculated that the cor- rosion in this region was equivalent to a uniform attack of 3.75 mils/day (1.4 in./year). Damage decreased with increasing distance from the probe 'private commmnication, A. N. Smith, ORNL, to J. W. Koger, 1968. Lol L . INCHES Fig. 2. ILower End of Liquid-Level Probe that Contacted NaBF;—8 mole % NaF in PKP-1 Loop for 192 hr at 540 to 690°C. (a) Entire active length. (b) Bottom 2 in. e g i A " » tip. Pitting was less severe on sample 7 (about 1 in. from the tip), but the photomicrographs (Fig. 4) show that this area was also heavily attacked, The attack in this region extended for about 2ijils through the tubing (about 3 mils/day). Figure 5, a photomicrograph of the upper end of sample 7,.sh0ws-a preferential attack in the grain boundaries, with the voids linking to form holes. Here the attack only extends for about 10 mils through the tubing, thus, demonstrating the varying level of the salt, as indicated by corrosion, on the probe. Chemical The salt was chemically analyzed just before the probe was placed in the system and just after it was removed. The results are given in Table 2. The Li, Be, U, and Th are frbm an earlier fuel salt used in this lobp. As expected, no significant changes in the amounts of the‘ corrosion products ifi the salt resulted from corrosion of the probe because (1) the surface area of the probe was small compafed to that of the loop, and (2) the volume of salt was large ccmpared to the quantity of corrosion products removed. The reported changes in the iron and oxygen concentrations are attributed to sampling and analytical procedures. o Each probe sample was analyzed, and the results are given in Table 3. We removed the attacked area from the base metal on sample 8 and determined the composition of each region. We found that the base material of the probe was type 304 stainless steel and not Inconel 600 as originally thought by project personnel. The analysis of the attacked area disclosed that mainly Cr,'Fe, Mn; and Si had been leached from the base metél by the salt. This removal resultéd in an apparent enrichment in Ni, Mo, and Cu, although in the areas of complete dissolu- tion, of course, all the alloying elemehfs were removed. Also, about 20% unidentified material was associated with the attacked area. « " ~t crostructure of Lower End of S ainple 7. -Mi (b) 500x i (a) 100x of Sample 7. crostructure of Upper End . m ig. 5 ig. F (b) 500x% e e " 1 Table 2. Analysis of Fluoroborate Salt in PKP-1% , ) ' - Content, ppm - ' " Content, wt % Element .. Before - After ‘Element - Before After . Insertion Removal Insertion = Removal cr 7 93 . Na 21,0 21.5 Fe 272 229 B 9.18 9.31 Ni 27 25 F . 66.7 65.9 0 1149 " L 0.8 0.193 | D Be 0.18 0.19 U 0.229 0.241 T 0.170 0.172 a‘W’a.‘l:t-':r'con1:en1:---n0'l_: analyzed. Table 3. Chemical Composition of Sections of Type 304 Stainless Steel LiQuidgLevel Probe in PKP-1 Loop Sample Portion - | - Content, % Analyzed @ Cr - Fe Ni Mn Mo 51 Cu _ - Before Test Nominal 18-20 Major 8-10 1.5 0.6 composition o : type 304 stainless steel After Test , 1 Base metal 18 69 9.4 1.4 0.4 0.52 0.12 -2 and lsyer, 17 68 8.1 1.3 0.14 -0.44 0.10 3 if any 17 68 11.0 1.4 0.14 . 0.58 0.12 4 17 - 68 1.7 1.4 0.13 0.64 0.12 5 16 - 67 11,2 1.3 0O.11 © 0.70 0.10 -6 --16 - 67 9.0 1.2 0.12 0.46 0.10 -7 | 16 67 10.2 1.3 0.12 0.52 0.11 g Base metal 18 69 10.0 1.7 0.15 0.60 0.10 8 - Attacked 1 15 . 60,0 0.1 1.00 0.27 0.50 10 Physical Property Changes Initial examination disclosed that some parts of the probe were highly ferromagnétic. Each of the eight-Samples was tésted with a Radio Ffequency Laboratory gaussmeter No. 1890 to determine its magnetic field strength (Table 4). This is a rapid nondestructive test suitable for many engineering systems. This device is quite useful on iron- or nickel-base alloys that are selectively attacked enough to change their magnetic properties. The magnetic field stfength of the samples increased as the end of the probe was approached. Table 4. Magnetic Field Strength and Seebeck Effect Seebeck Effect Semple ( gauss ) ( instrument units ) 1 0.20 | 5.0 2 0.20 7.5 3 0.20 10.0 4 0.30 | 9.5 5 0.50 11.5 6 0.75 13.5 7 2.00 14.0 8 Too fragile to be Too fragile measured but highly | ferromagnetic Stendards Monel | 0.20 330 Nickel 0.50 27 Type 304 stain- - 0.06 8.5 less steel ' _ Inconel ~ 0.04 - =13.0 Hastelloy N 0.06 ' _ -11.5 ¥ » » 11 Another testing device utilized in this study was a metal compari- son meter.? This instrument,nondestructively identifies metals by - measuring the Seebeck effect® of the unknown metal and comparing the value to that Obtained'from‘a'piece of known metal. The standards built into the device are nickel, type 304 stainless steel, Monel, Inconel 600, and Hastelloy N. Table 4 gives the relative Seebeck effect readings for the various samples and the standards. No attempt was made to determine absolute values; The examination results were that the Seebeck effect increased as the end of the probe was approached in agreement with the gaussmeter tests. ' ‘The metallurglcal evaluation showed that ‘the attack by the fluoro- borate salt drastically changed the composition and properties of the type 304 stainless steel_probe during service. The highly attacked region had a final composition near that of 78 Permalloy (Ni—22% Fe), a highly ferromegnetic material. The changes of the magnetic field strength and the Seebeckueffect'as a function of attack were‘also indic- ative of comp051tion changes and are often more sens1t1ve to small com- p051t10n changes than chemlcal analys1s. The gradatlonrln these . properties along the probe, as opposed to'an abrapt change, indicates that the salt level varied during exposure. . | DISCUSSION To account for the heavy attack Observed we cons1dered several ipotentlal corrosion mechanlsms._ 2Prlvate communlcatlon, J. Summers, Y-12, to J. W._KOger, 1968. - 3The -Seebeck . effect is the phenomenon of a current passing between two. metal junctions held at: dlfferent temperatures, when no other source ~~ of emf is present. The actual measuring unit consists of two copper probes, one of which is heated, and a millivoltmeter. The probes are placed on the metal to be tested, and the induced emf is read. 12 Impurity Effects Salt analyses were obtained to decide if the salt itself was exces- sively corrosive. Past work” has shown that fluorcborates that contain at least 2000 ppm water and oxygen grefhighly corrosive to iron- and nickel-base alloys. These impurities react with the salt to form HF, which attacks almost all the constituents of the container materiéls.‘ The most common evidence of this is detection of an increase in the con- centration of the more noble elements, such as nickel, in the salt. This fias not found for the system here, so we conclude, even,éonSidering the surface area and volume mismatch of probe to salt and container system, that the impurity effects on the probe corrosion were small. Effect of Imposed Electromotive Force . We also believe that the passage of current through the probe and the associated emf had no effect on the corrosion rate, since other probes containing essentially the same elements in different combina- tions have shown no deleterious effects under an imposed emf in prior exposures to fused fluorides.’ Stainless Steel Corrosion In the last two decades a continuing corrosion program at ORNL has been seeking to determine the compatibility of various fused fluoride mixtures with nickel- and iron-base a.llo;grs.l*_’10 Most of these tests 4J. W. Koger and A. P. Litman, Compatibility of Hastellqy N and Croloy 9M with NaBF;-NaF-KBF, (90~4—6 mole %) Fluoroborate Salt, ORNI~TM-2490 (April 1968). 53, W. Koger, unpublished data from MSRP Natural Circulation Loop Corrosion Program, 1967 through 1969, 5G. M. Adamson, R. S. Crouse, and W. D. Manly, Interim Rlort on Corrosion by Alkali-Metal Fluorides: Work to May 1, 1953, ORNL~-2337 (March 20, 1959). Declassified May 9, 1959. ' 7G. M. Adamson, R. S. Crouse, and W. D. Manly, Interim Report on Corrosion by Zirconium-Base Fluorides, ORNL-2338 (Jan. 3, 1961). n »y 13 ‘have used natural-circulation loops as the testing device. No tests have been conducted to determine the compatibility of stainless steel with fluoroborate salts,'although both stainless steels and fluoro- borate salt have been separately tested with other materials. Table 5 summarizes recent data®:10 obtained from natural-circulation loop tests and compares the compatibilities of some fluoride salts with type 304 stainless steel and'with Hastelloy N. In 5000-hr tests Hastelloy N loses about seven times as much weight in NaBF¢—8 mole % NaF, 4 mg/cm , &8s it does in 1ith1umrbery111um type fuel salt, O. 6 mg/cm . By analogy and with knowledge of the modes and mechanlsms of fluoro-_' borate salt corr031on, we assume that type 304 stalnless steel exposed to the same fluoroborate salt for 5000 hr at 605 C in an all-stalnless- steel system.would lose about seven times as much'welght as in a 87, H. DeVan and R. B.'Evans_rrx, Corrosion Behavior of Reactor Materials in Fluoride Salt Mixtures, ORNL-TM-328 (September 19, 1962). 9J. W. Koger and A. P. Litman, MSR Program Semiann. Progr. Rept. Feb. 29, 1968, ORNL-4254, pp. 218-225. _ 105, W. Koger and A. P. thman, MSR Program Semiann. Progr. Rept. Table 5. Weight Lbss-of;Ailoys Expesed-to Various Salts at Different Temperatures for 5000 hr ' ) A Ma.ximum ' i Weight Uniform Metal ~Salt .. Temperature (°c) Loss ' R ~ (°c) (mg/cm.) (mils/year) Hastelloy N = IiF-BeF,-ThF, 675 55 04 0.03 R ~ (73-2-25 mole %) - - - | Hastelloy N = LiF-BeF,-UF, 705 170 0.6 0.05 S ‘(65 5-34.0-0. 5 mole %) | e - ' ‘Hastelloy N - NaBFz,-Na.F 605 15 4.0 0.3 ' Type 304 stain- L1F—BeF2-ZrF4—UF4-ThF4 675 . 100 25.0 1.8 ~less steel (70-23-5-1-1 mole %) - | Type 304 stain- NaBF,-NaF ~ . 538-690 - 175.0 12.6 less steel (928 mole %) : | T T SEstimated from comparlsen with the behavior of Hastelloy N in NaBF;~8 mole % NaF of the same impurity level and assuming an all-stainless- steel system. 14 lithium-beryllium type fuel salt; that is, 175 vs 25 mg/cm?. Corrosion ~of this magnitude would be severe, equivalent to about 13 mils/year : (0.035 mil/day) attack, more than can be tolerated in a;molteu-salt reactor. However, this rate is only 1% of the maximum corrosibn_rate, 1.4 in./year (1370 mils/year) experienced by the probe. Dissimilar-Metal Corrosion It should be noted thatrthe above comparison of corrosion rates of stainless steel and Hastelloy N is really valid only in systems where all the container material exposed to the salt is of the same composi- tion. In the PKP-1 system described in this work, the iron-base type 304 stainless steel was surrounded by the more noble and more cor- rosion resistant nickel-base Inconel 600 which can be assumed to set the oxidizing potential of the system. Thus, it is not surprising that the less noble and more active stainless steel underwent much more corro- sion (1370 vs 13 mils/year) than it would had the entire System been stainless steel. This well-knoun effect is termed dissimilar-metal cor- rosion and has been noted in both salt and liquid m.t-z'ta.l-syste_ms,-lo»'11 Corrosion Mechanism and Mode Our interpretation of the catsstrOPhic corrosion that occurred selectively on austenitic stainless steel in this dissimilar-metal test system is consistent with related thermodynamic and electrochemical | phenomena., Examination of Table 6 shows that the elements removed from the stainless steel are those whose fluorides are more stable than-NiFg, This is in agreement with Bakish and Kern'? who found almost all the chromium and most of the iron removed from Inconel 600 exposed to 20% K,TaF,; in equimolar KCl-NaCl at 800°C. Measuring galvanic cells with a molten KC1l-NaCl-KF electrolyte, they found the nickel electrode 115, H. DeVan, A. P. Litman, J. R. DiStefano, and C. E. Sessions, Lithium and Potassium Corrosion Studies with Refractory Metals, ' ORNL~TM-1673 (December 1966). - 12R. Bakish and F. Kern, "Selective Corrosion of’ Inconel, " Corrosion 16, 553t (1960). | R - ¥ . N uf 15 Table 6. Relative Stability of Fluorides® | e Free Energy of Compound Formation at 1000°K - | (kcal/gram-atom F) siFy o -4 Cem 75 © FeFs | —68 MR, -6l MoFy o 58 CuF; | - =49 ®Based on A. Glassner, The Thermochemical Properties of the Oxides, Fluorides and Chlorides _bCompoundslof the metals known- to be removed from the stainless steel (Table 3). | more noble than iron by 0.3 v and chromium by 0.7 v. Recent ORNL mea- surementsl3 of electrode potéhtials'in molten fluorides agree qualitaQ tively with Bakish and Kern's data and our corrosion results. Thus, it is clear that the corrosive action of a fluoride salt on an alloy with or without dissimilar-metal mass transfer is fundamentally an electrochemlcal process whereln some or all of the alloy constltuents are oxidlzed to thelr 1onic state with the formatlon of fluorlde com- pounds. The rate may ‘be controlled by elther solld-state ‘diffusion or boundary-layer dlffus1on, w1th the attack concentrated at reglons of highest energy,_such as graln boundaries,'subgraln boundarles, certain : crystallographlc planes, and dislocatlons. 134, w. Jenklns, G. anmntov, and D. L. Manning, "Electrode Potentials of Several Redox Couples in- Mblten Fluorldes,“ Journal of the Electro— T'chemlcal 8001ety, in press. R 16 Summary The severe corrosion of type 304-staifi1ess steel exposed to a fluoroborate salt in an Inconel 600 system was interpreted. Drawing analogies from similar systems we conclude that the mainICause_of the catastrophic corrosion was dissimilar-metal corrosion. The stainless steel was the least noble part of the test system and thus suffered the brunt of the attack. However,'we believe that the corrosion of stain- 1ess_steel in en 8ll-stainless-steel system by fused fiuoroborate salt of the impurity level that existed in this case would still be exces- sive. We showed that from cemparison of free energies and electrode potentials one can predict the relative stability of the constituents of an alloy in qued.fluorides;' CONCLUSIONS 1. Type 304 stainless steel in an Inconel 600 system in the tem- perature range 540 to 690° C was severely corroded by NaBF4—8 mole % NaF. 2. The fact that the type 304 stainless steel.was the least noble part of an Inconel 600 system increased the amount of corrosion. 3. 'The mode of ettack invelves leaching large quantities of Cr, Fe, Mg, and Si from the stainless steel leaving a highly ferromagnetic nickel alloy. _ 4. The order of element removal in type 304 stainless steel by the fused sodium fluoroborate salt is in agreement with electrode poten- tial measurements and free energy data .in other halide salt systems and also is in agreement with other corrosion studies. B 5. Type 304 stainless steel in an all-stainless-steel system - exposed to fluordborate salt of the impurlty level used in these experiments would corrode too much to be useful in engineering systems. - ' . RECOMMENDATTONS 1. 'This experience emphasizes the necessity for more careful con- trol over materials being placed in & system containing a relatively uncharacterized fused fluoride salt. y " ) 17 2. In our judgment, low corrosion in nickel- and iron-containing alloys is favored by decreasing chromium and iron éoncentrations; that is, in order of decrefisifig corrosion resisfance, we find modified Hastelloy N (containing no iron), Hastelloy N, Inconel 600, and stain- less steel. While the general use of Hastelloy N alloys in the Inconel PKP-1l loop service is recommended, dissimilar-metal corrosion effects prohibit the use of any alloy less noble than Inconel 600. &4 n “ 67. 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