WMASTER OAK RIDGE NATIONAL LABORATORY 7y o operated by UNION CARBIDE CORPORATION /™ NUCLEAR DIVISION - for the ™ U.S. ATOMIC ENERGY COMMISSION « ORNL- TM- 2021, Vol. 1 o0 o > T - i = O > a Q w > il 8 a‘* ' : EFFECT OF ALLOYING ADDITIONS ON CORROSION BEHAVIOR OF NICKEL - MOLYBDENUM ALLOYS |IN FUSED FLUORIDE MIXTURES Thesis) Jackson Harvey DeVan ’ A This report is a portion of a thesis submitted to the Graduate Council of the University of Tennessee in partial fulfillment of the requirements for the degree of Master of Science. ERTRRUTION OF THIS DOCUMEN 15 UNLIMITED LEGAL NOTICE This report was prepored as an account of Government sponsorad work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Mokes any warranty or representotion, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privatsly owned rights; or 8. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process diiclesed in this report. As used in the above, '‘person acting on behalf of the Commission’’ includes any employse or contractor of the Commission, or employse of such contractor, to the extent thot such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employment or contract with the Commission, or his employment with such contractor. LEGAL NOTICE This report was prepared as an account of Government aponsored work. Neither the ¥United States, nor the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or rejresentation, expressed or implied, with respect to the accu- racy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed In this report may not infringe privately owned rights; or ORNL - TM.._ 2 02 :|_ B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. VOl . I Ag used in the above, “‘person acting on behalf of the Commission”’ includes any em- ployee or contractor of the Comrnission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employment or contract with the Commisgion, or his employment with such contractor. Contract No. W-7405-eng-26 METALS AND CERAMICS DIVISION EFFECT OF ALIOYING ADDITIONS ON CORROSION BEHAVIOR OF NICKEL- MOLYBDENUM ALLOYS IN FUSED FLUORIDE MIXTURES Jackson Harvey DeVan MAY 1969 This report is a portion of a thesis submitted to the Graduate Cogncil of the University of Tennessee in partial fulfillment of the requirements for the degree of Master of Scilence. OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee operated by UNION CARBIDE CORPORATION for the U.S. ATOMIC ENERGY COMMISSION WEIRIFULUON OF THIS DOCUMENE 15 UNHWM iii CONTENTS Abstract . . . v v v v i e e e e e e e Introduction . « .+« v ¢ ¢« ¢« ¢« + ¢ ¢ o o Review of Related Work . « o« + o« o s o o Corrosion by Fluoride Mixtures . . . Corrosion Reactions . . « « . . Reduction of UF, by Chromium . . Corrosion of Nickel-Molybdenum Alloys. Materials and Procedures . . « o o o o @ Tegt Materials . . . « . « « « . . . Test Equipment . . . . . . . . . . Salt Preparation . . . . . . « . . . Operating Procedures « « « « ¢ « « o Test Examination . .« ¢« ¢ « o o & & Results and DisCussion . « v o « s o o & Chromium « ¢« « &« &+ ¢ o o o o o o o o Corrosion~Product Concentrations . Metallographic Results . . . . . Aluminum . v v ¢ 4 o« ¢ o o o« o o o Corrosgsion-Product Concentrations . Metallographic Results . « . . . Titanium . o + & ¢ ¢ ¢ o o o + o o . Corrosion-Product Concentrations Metallographic Results . . . . . Vanadium . . « ¢ ¢ ¢ v ¢ ¢ v 0 o 4 Corrosion-Product Concentrations Metallographic Results . . . . . Iron . o v ¢ ¢ 0 i e v e e e e e e Corrosion-Product Concentrations -Metallographic Results . . . . . J 5 00 e oo DWW W W W W wWww NN DN NN H P e DO O OO0 0 W ®eEeuw KX P ;e oW iv Page Nio-biu-zn . . . . . * ® - & . . » e . ” - * - - - L] - . . . » . - 32 Corrosion-Product Concentrations . + v o « o v o o o + o« + o 32 Metallographic Results . v v ¢ o o & o « o o o« o o+ o o« o o o 33 TUNESEEIL o v v v v o o v o o o o o o 4 s o e e e e e e e e e . 34 Corrosion-Product Concentrations . . + « o « o o o o o « o« o« 34 Metallographic Results . . « « « « ¢« v o o« o « o . . . ... 36 Relative Thermodynamic Stabilities of Alloying Constituents . . 36 General Discussion of Alloying Effects . « v v v v ¢ « v & » o . 38 Summary and ConcluSions .+ + « « « o o « & « o o o o o o o 0w oo o 40 ACKNOwledgZmENtS « v 4 s v 0 e e e e e e e e e e e e e e e e e e e . 4R EFFECT OF ALLOYING ADDITIONS ON CORROSION BEHAVIOR OF NICKEL- MOLYBDENUM ALLOYS IN FUSED FLUORIDE MIXTURES ABSTRACT Fused fluoride mixtures containing UF, have been developed as fuel solutions for high-temperature nuclear reactors. To develop container materials for such mixtures, we investigated the corrosion properties of nickel-molybdenum alloys with vari- ous solid-solution strengthening additions. These evaluations utilized thermal convection loops which circulated salt mix- tures between a hot-zone temperature of 815°C and a cold-zone temperature of 650°C. The alloys selected for study contained 17 to 20% Mo and various percentages of Cr, Al, Ti, V, Fe, Nb, and W. Loops of individual alloys were exposed to the salt mixture NaF-LiF-KF-UF, (11.2-45,3-41.0-2.5 mole %) for periods of 500 and 1000 hr. Measurements of the concentrations of corrosion products in after-test salt samples indicated the corrosion susceptibility of alloying additions to increase in this order: Fe, Nb, V, Cr, W, Ti, and Al. However, metallographic examinations of loop surfaces showed relatively light attack for all alloys except those containing combined additions of aluminum and titanium or aluminum and chromium. A nickel-base alloy containing 17% Mo, 7% Cr, and 5% Fe, designated Hastelloy N, was found to afford the best combina- tion of strength and corrosion resistance among the alloy compositions tested. INTRODUCTION Molten fluorides of uranium, thorium, or plutonium, in combination with other fluoride compounds, have wide applicability as fuels for the production of nuclear power.! Because of their high boiling points, these mixtures can be contained at low pressures even at extremely high operating temperatures. Their chemical and physical properties impart addi- tional advantages such as excellent stability under irradiation and large IR. C. Briant and A. M. Weinberg, "Molten Fluorides as Power Reactor Fuels," Nucl. Sci. Eng. 2, 797-803 (1957). solubility ranges for both uranium and thorium. These factors have prompted design studies of molten fluoride fuel systems in conjunction with thorium-uranium thermal breeders, uranium-plutonium converters, and uranium burners. The development of reactors which incorporate a circulating fluoride salt is predicated on the availability of a construction material which will contain the salt over long time periods and also afford useful structural properties. The container material must also be resistant to air oxidation, be easily formed and welded into relatively complicated shapes, and be metallurgically stable over a wide temperature range. In order to provide a material for initial reactor studies, several commercially available high-temperature alloy systems were evaluated with respect to the above requirements. As a result of these studies, Inconel, a nickel-base alloy containing 15 wt % Cr and 7 wt % Fe, was found to afford the best combination of desired properties and was utilized for the construction of the Aircraft Reactor Experiment.2 Extensive corro- sion tests,3’4 as well as posttest examinations of the ARE,” confirmed the suitability of Inconel as a container material for relatively short- term fluoride salt exposures. Corrosion rates encountered with this alloy at temperatures above 700°C, however, were excessive for long-term use with most fluoride fuel systems. Utilizing experience gained in corrosion testing of commercial alloys, we initiated an alloy development program to provide an advanced container material for fluoride fuel reactor systems. The reference alloy system was composed of nickel with a primary strengthening addition of 15-20% Mo. This composition afforded exceptional resistance to fliuo- ride attack but lacked sufficient mechanical strength and oxidation W. D. Manly et al., Aircraft Reactor Experiment — Metallurgical Aspects, ORNL-2349“(1§575. °G. M. Adamson, R. S. Crouse, and W. D. Manly, Interim Report on Corrosion by Alkali-Metal Fluorides: Work to May 1, 1953, ORNL-2337 (1959). . “G. M. Adamson, R, S. Crouse, and W. D. Manly, Interim Report on - Corrosion by Zirconium-Base Fluorides, ORNL-2338 (1961). W. B. Cottrell et al., Disassembly and Postoperative Examination of the Aircraft Reactor Experiment, ORNL-1868 (1958). resistance at desired operating temperatures. To augment these latter properties, additions of various solid-solution alloying agents were evaluated, among them Cr, Al, Ti, Nb, Fe, V, and W. An optimum alloy composition was selected on the basis of parallel investigations of the mechanical and corrosion properties which were imparted by each of these additions. The composition best suited to reactor use was determined to be within the range 15-17 wt % Mo, 6-8 wt % Cr, 4—6 wt % Fe, 0.04-0.08 wt % C, balance Ni. Subsequent studies of the alloy, desig- nated Hastelloy N, have shown it to be extremely well suited for appli- cations demanding long-term compatibility with fluoride salts in the temperature range 650-800°C. The present research was concerned with the corrosion effects resulting from additions of alloying elements to the nickel-molybdenum system and an analysis of the thermodynamics of the corrosion process as indicated by these alloying effects. REVIEW OF RELATED WORK Corrosion by Fluoride Mixtures Corrosion Reactions The corrosion of nickel-base alloys, containing iron and chromium, by fluoride fuel mixtures has been found to result from a combination of the following types of oxidation reactionss © 7 1. Reactions’ involving impurities in the salt 2HF+QI_=CI‘F2 + Hp , (l) NiF, + Cr = CrFp + Ni , (2) FeF, + Cr = CrFp + Fe . (3) *W. D. Manly et al., "Metallurgical Problems in Molten Fluoride Systems," Progr. Nucl. Energy, Ser. IV 2, 164 (1960). 7"Solid-solution alloying elements are underlined. 2. Reactions involving impurities in or on the metal, for example 2Ni0 + ZrF, = ZrO, + 2NiFs (4) followed by reaction (2). 3. Reactions involving components in the salt 2UF, + Cr = CrF, + 2UF; , (5) 1l i 3UF, + Cr = CrF3 + 3UF; . (6) The extent of the first four of these reactions, which proceed strongly to the right and to completion rapidly, can be reduced by maintaining low impurity concentrations iIn the salt and on the surface of the metal. Reactions (5) and (6), on the other hand, are indigenous to fluoride systems which derive their usefulness through the containment of UF,. While the role of chromium has been investigated extensively in connec- 8 considerably less information is available tion with these reactions, regarding the thermodynamics of these reactions for the other alloying elements which were of interest in the present study. Reduction of UF, by Chromium The reaction of UF,; with chromium is found to be strongly influenced by the reaction medium employed.8 In melts composed principally of NaF-Zr¥F, or NaF-BeFp, the reaction produces only divalent chromium, that is, 2UF, + Cr = 2UF3 + CrTy . (7) However, in the case of NaF-KF-LiF-UF, mixtures used for this investiga- tion, the reaction between chromium and UF, produces both divalent and 87. D. Redman, ANP Quart. Progr. Rept. Dec. 31, 1957, ORKL~2440, pp. 78-82. trivalent chromium. At equilibrium, approximately 80% of the total chromium ions in the mixtures are observed to be trivalent, giving the net reaction 2.8UF, + Cr = 0.2CrFp + 0.8CrF3 + 2.8UF3 . (8) The equilibrium constant for this reaction is given by (a )O.Z(Q )O.B(G )2.8 K = CrFs CrF, UF4 . (9) CHPLICN where & represents thermodynamic activity. Because of the relatively small concentrations of CrF, and UFs which are attained in the salt solu- tions at equilibrium, the activities of each of these components can be closely approximated by their mole fractions, in accordance with Henry's law. Thus, for a salt system of fixed UF, concentration, assuming the reference states for salt components to be the infinitely dilute solution, (NCng)O.Z(NCrF3)O.8(NUF3)2.8 K = . (10) (g, )25 (o) For a system initially containing no UFi, CrF,, or CrF;, it follows that NCrF2 = 1/4NCrF3 = 1/14NUF3. In such systems where the change in UF, concentration is small, Eq. (9) reduces to _ 5/19 Ny, * Norp,) % (11) where 4/19 o 14/19 iy - 5@;{) (TU% / O~ The constant KN has been experimentally determined for the mixture LiF-KF-NaF-UF; (11.2—41.0-45.3-2.5 mole %) by equilibrating it with pure chromium (agr = 1) at 600 and 800°C (ref. 8). Under these conditions, the constant is equivalent in value to the mole fraction of chromium ions in the salt at equilibrium. The measured values of KN are listed in Table 1. Table 1. Eguilibrium Concentrationsa of Chromium Fluorides for the Reaction of Pure Chromium with UF, Concentration of Chromium Equilibration Tons in NaF-LiF-KF-UF, Temperature (11.2-41.0-45.3-2.5 mole %) (°C) ppm mole fraction (KN) 600 1100 1.0 x 10~° 800 2600 2.4 % 1072 8J. D. Redman, ANP Quart. Progr. Rept. Dec. 31, 1957, ORNL-244C, pp. 7882, Because the chromium-UF, reaction is temperature dependent, chemi- cal equilibrium beiween these two components is precluded in systems of uniform alloy composition where the circulating salt continually experi- ences a temperature change. In such systems chromium tends to be con- tinually removed from hotter portions and deposited in cooler portions. A theory relating the rate of this movement to the operating parameters of the container system has been formulated by Evans.’ Corrosion of Nickel-Molybdenum Alloys Because of the oxidation of chromium by fuel-bearing fluoride salts, alloys employing large percentages of this element were not satisfactory ’R. B. Evans, ANP Quart. Progr. Rept. Dec. 31, 1957, ORNL-2440, pp. 104—113. as container materials except at temperatures where diffusion rates in the alloys were relatively low. Accordingly, evaluations were made of several commercial alloys in which chromium was not employed as a major alloying addition. Based on these tests, alloys of nickel and molybdenum appeared to offer the most promising container system for achieving rela- tively high reactor operating temperatures. Unfortunately, commercially available nickel-molybdenum alloys which exhibited excellent corrosion properties were not well suited to contemplated reactor systems because of three adverse characteristics: (l) poor fabricability; (2) a tendency to age-harden and embrittle at service temperatures between 650 and 815°C (ref. 10); and (3) poor resistance to oxidation by air at elevated temperatures. The scale formed on exposure of these alloys to high- temperature air was of the type NiMoO,, which upon thermal cycling between 760 and 350°C underwent a phase transformation and spalled as a consequence of a resultant volume change.ll By means of an alloy development program, however, it was considered plausible to eliminate the undesirable features of the commercial materials while retaining their inherent corrosion resistance. The initial objec- tive of this program was to provide an alloy which did not embrittle under the thermal treatments imposed by reactor operation. By experimenting with various compositions of binary nickel-molybdenum alloys, 1t was determined that lowering the molybdenum concentration to a level of 15-17% served to avoid detrimental age-hardening effects in the alloy system.12 Although such an alloy system was satisfactory from the standpoint of corrosion resistance, it was necessary to augment the oxidation and strength characteristics of the system through additional solid-solution alloying agents. The corrosion effects which resulted from these addi- tions were the subjects of the present study. 1R, E. Clausing, P. Patriarca, and W. D. Manly, Aging Characteristics of Hastelloy B, ORNL-2314 (1957). iy, Inouye, private communication. 127, W. Stoffel and E. E. Stansbury, "A Metallographic and X-ray Study of Ni Alloys of 20-30 Per Cent Mo," Report No. 1 under Subcontract No. 582 under Contract No. W-7405-eng-26, Knoxville, Tenn., Dept. of Chem., Eng. of the Univ. of Tenn. (1955). MATERIALS AND PROCEDURES Test Materials The nickel-molybdenum alloy compositions selected for study were supplied by the Metals and Ceramics Division at ORNL and, under subcon- tract agreements, by Battelle Memorial Institute and Superior Tube Company. Alloys furnished by ORNL were induction-melted under vacuum, while those furnished by subcontractors were induction-melted in air using a protective slag. Each alloy heat, which ranged in weight from 12 to 100 1b, was either forged or extruded into a 3-in.-diam tube blank and was subsequently drawn into 1/2-in.-0D seamless tubing by the Superior Tube Company. The cold-drawn tubing was annealed at 1120°C. Table 2 lists the experimental alloy compositions used for this corrosion study. Test Eguipment The method selected to evaluate the corrosion properties of these alloys was based on the following considerations: 1. The method necessarily had to be tailored to the use of rela- tively small quantities of material, since it was practical to produce only small heats of the many alloys desired for study. 2. Tubing was considered to be a highly desirable form in which to test the material, since production of the material in this form was carried out as an adjunct to evaluating the fabricability of each alloy. 3. Previous demonstrations of the effects of temperature gradient in the salt and the salt flow rate on the corrosion behavior of container materials in fluorides made it mandatory that corrosion tests be con- ducted under dynamic conditions, that is, conditions which provided for the continuous flow of salt through a temperature gradient. The thermal convection loop, which had been used extensively for Inconel corrosion studies and had been developed into an extremely straightforward and reliable test device, was judged to be the best form of experimental device for meeting these requirements. This device con- sists of a closed loop of tubing, bent to resemble the outline of a harp, Table 2. Compositions of Experimental Alloys Used for Corrcsion Studies Composition, wt % Composition, at. % Heat i Number Wi Mo Cr Fe Ti Al Wb W v Ni Mo Cr Fe Ti Al Nb W v Series T OR 30-1 80.12 16.93 2.83 85.50 11.10 3.41 -2 78.55 16.65 4.62 83.60 10.80 5.55 -4 73.65 16.37 9.21 78.30 10.60 11.04 -6 78.50 15.11 .40 g82.70 9.72 7.60 37A-1 77.0 20.39 2.62 83.30 13.50 3.20 43A-3 73.30 20.34 6.34 78.90 13.40 7.71 Series IT OR 30-7 82.10 15.93 1.88 85.60 10.15 4.26 -8 80.30 17.80 1.89 85.90 11.60 2.47 _ -9 81.10 16.8 .09 88.10 11.20 0.72 -10 81.10 16.60 2.23 86.40 10.80 2.73 -11 79.80 16.53 3.68 85.10 10.80 4,12 -12 80.00 16.80 3.22 86.70 11.10 2.20 -19 79.00 16.90 4 .10 g87.10 11.40 1.44 -20 79.20 16.60 4,18 84.10 10.80 5.11 =21 78.90 16.40 471 85.50 10.90 3.62 S T23012 82.00 17.42 0.53 87.40 11.40 1.22 OR 1491 86.58 11.23 2.19 88.16 6.99 4. 85 Series ITL OR 30-13 79.93 17.56 1.56 0.95 84.48 11.35 2.02 2.18 =14 79.53 16.50 1.52 2.45 82.11 10.42 1.92 5.51 =16 77.74 16.00 3.65 1.49 1,12 81.01 10.20 4.30 1.90 2.54 -22 77.65 15,90 5.69 1.16 0.60 80.27 10,05 6.64 2.6l 0.39 -33 74.07 15.15 5.C1L 5.07 0,70 77.26 9.66 5,90 5.56 1.59 B2897 76.13 20,50 1.25 1.32 83.61 13.77 1.68 0.92 B2898 76.30 20.50 2. 44 1.31 82.60 13.20 3.24 0.90 B3276 69,12 21,10 7.58 2.16 75,17 14,00 9,32 1.48 B3277 66.95 21.60 7.82 1.31 2.32 7L.72 14.20 9.4%7 3.06 1.57 S T23011 71.50 15.06 3.84 0,83 4.17 4.90 79.04 10.20 4.79 1.27 2.91 1.72 S T23013 74.42 15.20 0.58 4.57 5.23 g83.14 10.36 1.40 3.23 1.85 S T23014 80.86 16.70 2,19 0.57 85.22 10.76 2.71 1.30 ®0R denotes heats furnished by the Metals and Ceramics Division; 8 T by Superior Tube Company; and B by Battelle Memorial Institute. 10 two legs of which are heated and two of which are exposed to the cooling effects of ambient air. Flow results from the difference in density of the salt in the hot and cold portions of the loop. The configuration and dimensions of the loop design are presented in Fig. 1. All loops were fabricated of seamless tubing having an out- side diameter of 0.500 in, and a wall thickness of 0.035 in. The tubing was assembled by the Heliarc welding technique using an inert gas backing. During operation the loops were heated by a series of clamshell resistance heating elements located as shown in Fig. 1. To fill the loop required that we apply heat to both the cold- and hot-leg sections. Auxiliary heating for this purpose was provided by passage of electric current directly through the tube wall. When the loop was filled, as determined by an electrical shorting probe near the top of the loop, the auxiliary heat source was turned off and the heating elements were turned on. Insulation was then removed from the cold leg to whatever amount was required to establish a predetermined temperature gradient. Loop temperatures were measured with Chromel-P—Alumel thermocouples located as shown in Fig. 1. The thermocouple junctions, in the form of small beads, were welded to the outside tube wall with a condenser dis- charge welder and covered by a layer of quartz tape, which in turn was covered with stainless steel shim stock. All tests were operated so as to achieve a maximum mixed-mean salt temperature of 215°C and a minimum salt temperature of 65C°C. The maximum salt-metal interface temperature, which was attained near the top of the vertical heated section, exceeded the maximum bulk salt temperature by approximately 95°C (ref. 13). The salt-flow rate under these temperature conditions was established from heat balance calculations to be in the range of 5 to 7 ft/min, 13Measurements of the maximum inside wall temperature could not practically be made in each loop test; however, values of this tempera- ture were recorded by means of heat balance calculations and imbedded thermocouples using a specially instrumented test loop which exactly simu- lated the geometry and temperature profile used in the corrosion experiment. 11 T ORNL-LR~DWG 27228 R I ! | 4 T.C. NO. 4 T.C.NO.1-3 3-in.R —twi 6-in. CLAM SHELL HEATERS — CONTROL & NO. 2 2 -__"fij’ “ ¢ (o (] | | i P | ; IR c | s |1 | Pl ! | } METALLOGRAPHIC SAMPLES | |l : + THERMOCOUPLE LOCATIONS || i ! | 17 .E! P 7 N | | ! I 4 ‘ - | I v i | t | ] N L ' L] ! ‘. 7 , 3-in. R\ 750 T.C. NO. 6 — 6-in. CLAM SHELL 7 e~ 1T HEATERS 37 / e~ - . / .—"x__ w:"“-___- / T~ T B\ Fig. 1. Schematic Diagram of Thermal Convection Loop Used for Evaluations of Experimental Nickel-Molybdenum Alloys. The locations of thermccouples and test samples are shown. 12 Salt Preparation The fuel mixture used in these studies was of the composition shown in Table 3. We selected the LiF-KF-NaF-UF, composition (Salt 107) on the basis that the oxidation of container constituents by a given con- centration of UF; tended to be greater for this mixture than for other mixtures of practical importance. Thus, achievement of satisfactory compatibility with this mixture in effect provided a container material of ultimate versatility with respect to all fuel mixtures. Table 3. Composition of Fluoride Mixture Used to Evaluate Experimental Nickel-Molybdenum Alloys Salt Number: 107 Liquidus Temperature: 490°C Component Mole % Weight % NaF 11.2 9.79 LiF 45,3 24 .4 KF 41.C 49, 4 Uy, 2.5 16.3 The fluoride mixtures were prepared from reagent-grade materials and were purified by the Fluoride Processing Group of the Reactor Chemistry Division. In general, the procedure for purification was as follows: (1) the dry ingredients, except for UF,, were mixed, evacuated several times for moisture removal, and then melted under an atmosphere of helium; (2) the molten mixture was held at 815°C and treated with hydrogen for 4 hr to purge hydrofluoric acid from the mixture; (3) the mixture was cooled to 205°C under a helium atmosphere and UF, was admitted. Upon the addition of the UF,, the mixture was remelted, heated to 815°C, and then treated again with hydrogen to purge the excess hydrofluoric acid. All mixtures were prepared in 300-1b gquantities and apportioned into 50-1b containers, after which samples were submitted for analysis of Ni, Fe, and Cr. It was required that each of these elements be present 13 in amounts less than 500 ppm as determined from individual batch analyses, A second before-test analysis of each salt mixture was obtained from a sample of the salt taken as it was being admitted to the test loop. Operating Procedures Fach loop was thoroughly degreased with acetone and checked for leaks using a helium mass spectrometer. After thermocouples and heaters were assembled and insulation was applied, the loop was placed in a test stand, as shown in Fig. 2. The salt charging pot was connected to the loop with nickel or Inconel tubing, and both the loop and the charging pot were heated to €50°C under a'dynamic vacuum of less than 50 p Hg. Helium pressure was then applied to the charging pot in order to force the salt mixture from the pot to the loop. After filling, scalt was allowed to stand in the loop at 650°C for approximately 2 hr, so that oxides and other impuri- ties would be dissolved from the container surface into the salt mixture. This mixture was then removed, and a fresh salt mixture was admitted from the fluoride charging pot. A helium cover gas under slight positive pressure (approx 5 psig) was maintained over the salt mixture during all periods of testing. At the end of test, power to the loop was cut off and insulation was stripped from the loop so as to freeze the salt mixture as rapidly as possible. Test Examination Each loop was sectioned with a tubing cutter into approximately 6-in. lengths. Five Z2-in. sections were then removed from the loop posi- tions indicated in Fig. 1 for metallographic examination. Two of the remaining 6-in. sections, one from the hottest section of the loop (specimen H) and one from the coldest section (specimen C), were obtained for salt chemistry studies, and the remaining loop segments were held in storage until all examinations of the loop were completed. Salt removal was accomplished by heating each section in a small tube furnace at 600°C in helium., The salt was collected in a graphite - » " Fig. 2. Stands. ( 15 crucible located below the furnace windings. The five 2-in, sections of tubing were examined metallographically, and the salt samples were sub- mitted individually for petrographic and chemical analyses. .If layers of corrosion products were discovered on the tube wall, samples of the tubing and salt contained in that section were also submitted for x-ray diffraction examination. RESULTS AND DISCUSSION Alloying effects were evaluated in terms of both the corrosion products entering the salt mixtures and the metallographic appearance of the alloy after test. Results have been grouped in this section according to the alloying element studied. Chromium Corrosion-Product Concentrations Effects of chromium additions were examined in six ternary alloys with chromium contents of 3.2 to 11.0 at. %. One loop of each of these compositions was operated with Salt 107 for 500 hr under the test condi- tions described above, The compositicns investigated and the attendant concentrations of chromium ions in salt samples taken at the conclusion of these tests are shown in Table 4. The extent of reaction between chromium and fluoride constituents, as indicaked by the chromium ion con- centrations, increased with the amount of chromium in the alloy. This increase is illustrated graphically in Fig. 3, where the data are com- 114 15 It may be noted pared with data for Incone and for pure chromium. that the chromium concentrations of the salts were less than those for Inconel loops operated under identical temperature conditions. A hori- zontal line, which represents the chromium ion concentration at 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 (1959), 17, D. Redman, ANP Quart, Progr. Rept. Dec. 31, 1957, ORNL-2440, pp. 78-L82. 16 Table 4. Corrosion-Product Concentrations of Salts Tested with Nickel-Molybdenum-Chromium Alloys Alloy Composition Tegt Chromium Concentration in Heat (at. %) : Salt Samples® (mole %) Number Duration (hr) Cr Other Components Sample H Sample C Other OR 37A-1 3.20 13.5 Mo, bal Ni 5C0 0.01%4 0.0180 ©.0213 OR 30-1 3.41 11.1 Mo, bal Ni 500 0.0222 0.0365 0.0291 OR 30-2 5.55 10.8 Mo, bal Ni 5C0 0.0375 0.0352 0.0376 OR 30-2 5.55 10.8 Mo, bal Ni 1000 0.0509 0.0509 0.0543 OR 30-6 7.60 9,72 Mo, bal Ni 500 0. 0606 0.0606 0.0566 OR 43A-3 7.71 13.4 Mo, bal Ni 500 0.0453 0.0476 0.0425 OR 30-4 11.04 10.6 Mo, bal Ni 5C0 0.0819 0.0814 0.0699 ®Sample designations "H" and "C" are discussed on page 13; "other" designates salt samples obtained from metailographic specimens. equilibrium with pure chromium at 600°C, is seen in Fig. 3 to be above the measured chromium ion concentrations for all alloys tested. Thus, the observed concentrations were less than those required for the forma- tion of pure chromium crystals in the coldest portion of the loops (approx 650°C). As discussed previously, the concentration of chromium ions under the conditions of these tests should be governed by the relation [Eq. (11)] N . = _ 5/19 Cr ions (NCI'F2 * NCI"F3) - K2CYC£ 3 where NCr sons is the mole fraction of chromium ions in Salt 107 at equilibrium with an alloy of given chromium activity, o, . If we assume Cr op? the resultant chromium ion concentrations for these alloys should lie within a region which is that NCr giveg an approximate measure of & bounded above by the function determined at a temperature equivalent to the maximum loop temperature and below by the function determined at the minimum loop temperature. Bounds using experimental values of K 17 ORNL—LR—DWG 46946 3000 _— /// - " — 200 2000 ms:fififii—f”"' rATY 1600 = CONCENE e —{ 150 = \CTED E PRED | a e | — ! § 1200 =r,-,——=""—— PURE Cr-800°C e — = T ] —_—t 100 < L 0 x 1om3:f”" 0 o = o Y ol ¢ 800 2 o — £ 8 = 600 T3 3” 2 — T — o z 3 —{ 40 400 2 i RANGE OF CHEMISTRY SAMPLES H AND C - 500 hr TESTS — , e AVERAGE CONCENTRATIONS iN ] e - METALLOGRAPHIC SAMPLES —500 hr TESTS - | ' E RANGE OF CHEMISTRY SAMPLES H AND C —1000 hr TESTS ¢ AVERAGE CONCENTRATIONS IN —{ 20 200 |- METALLOGRAPHIC SAMPLES — 1000 hr TESTS _| | L 003 0.04 0.06 0.08 0.0 0.2 0.4 0.6 0.8 10 ATOM FRACTION OF CHROMIUM IN ALLOY Fig. 3., Concentration of Chromium Ions in Salt 107 as a Function of Chromium Content in Nickel-Molybdenum-Chromium Alloys. measured at 800 and 600°C (Table 1), which reasonably approximate the maximum and minimum loop temperatures, are plotted in Fig. 3 and are seen to include only two of the observed loop concentrations, both corre-~ sponding to chromium compositions greater than 7 at. %. However, except for the alloy with least chromium content, all concentrations lie rel- atively close to the lower bound and roughly approximate the slope of this bound. The deviation of measured corrosion-product concentrations in these tests from the predicted concentration ranges may have resulted primarily from inaccuracy in the assumption that N . equals & o However, assuming C C aCr to be accurately known, the corrosidE:product concentrations in these tests would nevertheless have been lower than those predicted, since cor- rosive attack would necessarily reduce the chromium content and hence the 13 chromium activity at the surface of the alloys relative to the original chromium activity. We also examined the possibility that the test times were not sufficiently long for the chromium concentration of the salt to have completely attained the maximum or steady-state level. To evaluate this latter point, sufficient material of one of the alloy compositions, heat OR 30-2, was obtained to permit the operation of a 1000-hr test. Results of this test, which are shown in Table 4 and Fig. 3, indicated that only a small increase in chromium concentration occurred between the 500- and 1000-hr intervals. It was concluded, therefore, that the 500-hr test provided a reasonably close estimate of the limiting corrosion- product concentrations associated with the temperature conditions of these experiments. Additional data on the effects of chromium in nickel-molybdenum alloys were afforded by tests of five alloys which contained chromium in combina- tion with one or more other alloying elements, as shown in Table 5. Not- withstanding the additional alloying agents, the concentrations of chromium-containing corrosion products following 500-hr tests of these alloys in Salt 107, as shown in Table 5, were comparable to the concen- trations associated with the simple ternary chromium-containing alloys. The corrosion-product concentrations for the alloys with multiple additions are plotted in Fig. 4, and are seen to show the same general variation with chromium content as the ternary alloys (see Fig. 3). Also shown in Table 5 and Fig. 4 are the results of 1000-hr tests of four of the alloys containing multiple additions. After all but one of these tests, the chromium ion concentrations in the salts were slightly higher than for 500-hr tests of the same alloys; in one test, the chromium ion concentration was unaccountably lower. The chromium ion concentra- tions of these multicomponent alloys again fell near those predicted on the basis of equilibrium data for pure chromium at 600°C and were con- siderably less than the concentration needed to deposit pure chromium at 600°C. The chromium activities in all of the alloys tested would appear on the basis of corresponding corrosion-product concentrations to be lower than the activity of chromium in Inconel. However, it is lmportant to note in Figs. 3 or 4 that any alloy in which the chromium activity Table 5. Containing Chromium in Combination with Other Alloying Elements Corrosion-Product Concentrations of Salts Tested with Nickel-Molybdenum Alloys Chromium Concentration in 11 iti . , Heat Alloy Composition, at. % Test Salt Samples,® mole 9% - Duration Number (hr) Chromium Other Components Sample H Sample C Other OR 30-16 4.30 2.5 Al, 1.90 Ti, 10.2 Mo, 500 0.0227 0.0236 0. 004 bal Ni S T23011 4."79 1.27 Al, 2.91 Nb, 1.72 W, 500 0. 0490 0.0490 0. 0426 10.2 Mo, bal Ni OR 30-33 5.90 1.59 Al, 5.56 Fe, 9.66 Mo, 1000 0.0689 0.0731 0.0620 bal Ni OR 30-22 6. 64 0.43 Fe, 0.39 Nb, 2.61 Al, 500 0.0583 0.0555 0. 0490 10.05 Mo, bal Ni OR 30-22 6. 64 0.43 Pe, 0.39 Nb, 2.61 Al, 1000 0.0416 0. 0402 0.0370 10.05 Mo, bal Ni B3276 9.32 1.48 Nb, 14.0 Mo, bal Ni 500 0.0615 0.06el15 0.0578 B3277 9.47 1.57 Nb, 3.06 Al, 14.2 Mo, 500 0.0698 G. 0700 0. 0624 bal Ni B3277 9.47 1.57 Nb, 3.06 Al, 0.073L1 0. 0657 0.0740 bal Ni 14.2 Mo, 1000 aSam_ple designations "H" and "C" are obtained from metallographic specimens. discussed on page 13; "other" designates samples 6T 20 UNCLASSIFIED ORNL—LR—DWG 46945 3000 } | } o 1 | : — ! | ; /// f / | | } . - — 200 2000 N — e T e . OoC L - 80 b o I Lo u..,!__“_.__.___— ONS S WSO SO —_ 800/ — e — 4,,, L_ CoNCEN/ — 150 o : PRE & 1200 —t PURE Cr — 600°C i = r— g — - . —_— PRt B e B e S e (010 5 4000 [= A e S T 3 - T — — 80 ™ = | S 800 : E~‘ PR INCONEL _156655 | g — . S | ] g NTRM\ON ; 52 = — CONCE = 460 o 2 L 3 : CcTed ’ ° S 800 ; | pREOVV_Z. - — f £ = l ' \ o —' * . ~ | | — o I e | (:5 T i E, I RANGE OF CHEMISTRY SAMPLES | a0 400 4 H AND C - 500 hr TESTS ; AVERAGE CONCENTRATIONS IN - ) , | : ® METALLOGRAPHIC SAMPLES -500 hr TESTS | - - i lqvl RANGE OF CHEMISTRY SAMPLES | : | H AND € —1000 hr TESTS | r ‘ g AVERAGE CONCENTRATIONS IN ; : METALLOGRAPHIC SAMPLES —1000 hr TESTS L | : ‘ — 20 200 + . : i i ! [ | | | ; ; i i | 0.03 0.4 0.06 008 040 0.2 0.4 0.6 0.8 1.0 ATOM FRACTION OF CHROMIUM IN ALLOY Fig. 4. Concentration of Chromium Ions in Salt 107 as a Function of Chromium Contents in Nickel-Molybdenum Alloys Containing Multiple Alloy Additions. exceeded a value of 0,035 would support the formation of pure chromium at 600°C in any case where the concentration of Cr¥F,; and CrF:; approached equilibrium with the alloy at temperatures of 800°C or above. Thus, unless the activity coefficients for chromium in these alloys are much smaller than unity, the possibility for the deposition of pure chromium in cold-leg regions cannot be completely excluded for any of the alloys tested, In the case of ZrF,- or ReF,-base salt mixtures, the temperature dependence of the Cr-UF, reaction is much less than for Salt 107. It 16 has been shown-" that in these mixtures, alloys having a chromium 16y, R. Grimes, ANP Quart. Progr. Rept. March 10, 1956, ORNL-2106, pp. 9699, 21 activity equivalent to or less than that for Inconel cannot support chromium ion concentrations at 800°C which are sufficiently high to main- tain pure chromium at 600°C., Consequently, mass transfer of chromium by deposition of pure chromium crystals in the latter salt mixtures should not be possible for any of the alloys listed in Tables 4 and 5. Metallographic Results Metallographic examinations of the ternary alloys shown in Table 4 showed little evidence of corrosive attack other than shallow surface roughening. Void formation, which characteristically had been found in 7 was not detected nickel-~chromium alloys under similar test conditions,? in any of these alloys. The relative depths of attack which occurred for the alloy containing 5.55 at. % Cr (heat OR 30-2) after 500- and 1000-hr tests, respectively, are compared in Fig. 5. Although the depth of attack was similar at both time intervals, the intensity of surface pitting was somewhat greater after the 1000-hr exposure. The intensity of surface roughening was also found to increase with increasing chro- mium concentration. This is seen in Fig. & which compares photomicro- graphs of two alloys containing different chromium contents. Cold-leg sections of these loops showed no evidence of corrosion and contained no visible deposits of corrosion products. The depths of attack observed metallographically for alloys con- taining chromium in combination with other additions are shown graphi- cally in Fig., 7. Attack in these alloys was manifested as surface roughening after 500 hr and as a combination of surface pitting and shallow void formation after 1000 hr. Depths of corrosion were in gen- eral higher than for the ternary chromium-containing alloys, particularly in alloys which contained aluminum additions. 171, s. Richardson, D. C. Vreeland, and W. D. Manly, Corrosion by Molten Fluorides, ORNL-1491 (Sept. 1952). T-11327 " F— T T [ o 0.014 INCHES @ 250x HO B (a) ifi!% , s ‘ SE N e | o IN‘_‘I" - | " . 0.014 INCHES o 2s0x 1@ T 28 "r*j;j(b_ , Jwfiffj' Flg. 5. Appearance of Hot Leg Surface of Ternary Nickel—Molybdenum _ _A110y‘Contain1n 5.55 at. % Cr Following Exposure to Salt 107 ‘Heat OR 30-2. f;) After 500 hr -and (b) after 1000 hr. - - i i ] ] 23 o J [ oy :0.014 INCHES I 250% [ ke N - & 0.014 INCHES Fig. 6. Hot-Leg Sections of ‘Nickel-Moiybdenum—-clhrdzi_i_iti_m Thermal Convection Loops Following 500-hr Exposure to Salt 107. (a) 3 2 Cr=13.5 Mo—bal Ni " (at. %), (v) 11.0 cr-10.6 Mo-bal Ni (at. %) 24 ORNL- LR—DWG 46944 S 500 hr - SALT 107 § kol < 4 §§§“ > NN 1000 hr - SALT 107 % Q 3 D\ @ N \\\ o @ / & \ % = N Q // § / I 4 - || | - | & ) AV U sss ° o Lzza 2 Sk\ Mo | 10.36|12.89 | 11.35] 10.76 [ 1400 | 10.2 | 10.2 |13.77 | 14.20 | 10.42 | 9.66 | 10.05 Cr 9.32 | 4.30 | 4.79 9.47 590 |6.64 ® 5 Fe 5.56 | 0.43 =2 Q S Al [ 140|561 |2.48 | 1.30 2.54 | 1.27 3.06 | 5.51 | 1.59 | 2.61 2 & i 202|271 1.90 1.68 1.92 - " Loop tests of 500~ and 1000-hr duratlons were carrled out w1th ter-' ',nary alloys containlng 2.20 and 3. 62 at % Nb respectlvely..-As shown _1n Tdble 10, the nldblum.concentratlons in ‘salts exposed. to these alloys '_1ncreased from a value near O. 015 mole % in the 500-hr tests to values _of_from'0.022 to 0.025 mole %_;n the 1000-hr tests. - Salts tested with Ten 33 Table 10. Corrosion-Product Concentrations of Salts Tested with Nickel-Molybdenum-Niobium Alloys Alloy Composition Niobium Concentration in Heat (at. %) Test Sal 3 . Duration alt Samples® (mole %) Number (hr) Nb Mo Ni Sample H cample C Other OR 30-12 2.20 11.1 bal 500 0.00363 b 0.0135 OR 30-12 2.20 11.1 bal 1000 0.0225 0.0207 C.0233 OR 30-21 3.62 10.9 bal 500 0.0155 0, 0174 0.0L53 OR 3C-21 3.62 1C.9 bal 1000 G.C0244 0.0228 0.0259 a, . . 1 . Sample designations "H' and "C" are discussed on page 13; "other” designates samples obtained from metallographic specimens. bNot determined. alloys which contained niobium combined with other additions showed very much lower niobium concentrations than did the simple ternary alloys. As seen in Table 11, concentrations in the multiple-addition tests were less than 0.002 mole % for niobium contents as high as 3.62 at. %. Thus, it appears that corrosion products formed by reactions with other com- ponents in these alloys, namely titanium, aluminum, and chromium, were effective in inhibiting reaction of the salt with niobium,. Metallographic Results Neither ternary alloy containing niobium showed significant attack in metallographic examinations of 500-hr tests; however, the presence of very small subsurface voids to depths of approximately 0.001 in. was detected in examinations of the 1000-hr tests, as illustrated in Fig. 11. Corrosion results determined for alloys containing niobium together with other additions are summarized in Fig. 7. Except where aluminum and chromium were both present, attack in these alloys was less than 0.002 in. in depth. 34 Table 11, Corrosion-Product Concentrations of Salts Tested with Nickel-Molybdenum Alloys Containing Niobium in Combination with Other Alloying Elements Alloy Composition Niobium Concentration in Test Heat (at. %) Duration Salt Samples? (mole %) Number (hr) Nb Other Components oample H Sample C Other OR 30-22 0.39 10.0 Mo, 6.6 Cr, 500 0.00129 0.00181 0. 00207 2.6 Al, bal Ni OR 30-22 0.39 10.0 Mo, 6.6 Cr, 1000 0.00052 0.00104 0.00052 2.6 A1, bal Ni B2898 0.90 13.2 Mo, 3.24 Ti, 500 <0. 0005 <0. 0005 b bal Ni B2897 C.92 13.8 Mo, 1.7 Ti, 1000 0. 00104 0. 00052 C.001z¢2 bal Ni B3276 1.48 14.0 Mo, 9.3 Cr, 500 0. 00078 0.00052 0.00078 bal Ni B3277 1.27 14.2 Mo, 9.5 Cr, 500 <0. 0005 <0.0005 <0, 0005 3.1 A1, bal Ni B3277 1.27 14.2 Mo, 2.5 Cr, 100C 0.00259 C.00181 0. 00104 3.1 Al, bal Ni S T23013 3.23 1C.4 Mo, 1.4 Al, 500 <0. 0005 <0. 0005 <0.0005 1.8 W, bal Ni a Sample designations "H" and "C" are discussed on page 13; "other" designates samples obtained from metallographic specimens. bNot determined. Tungsten Corrosion-Product Concentrations Single alloying additions of tungsten to the nickel-molybdenum sys- tem were evaluated at levels of 0,72 and 1l.44 at. %, respectively. Loop tests of both alloys were conducted for 500 hr, at which time tungsten concentrations in the salt mixtures had reached levels of 0.029 to 0.032 mole %. In 500-hr tests of two alloys which contained tungsten in addition to chromium, niobium, or aluminum, the concentration of tungsten detected in salt samples was below 0,010 mole %. Compositions of these alloys and their attendant salt corrosion-product concentrations are shown in Table 12, s o 35 q‘ I T i~ = [on = im O, 014 INCHES % 250% i o Fig. 11. Appearance'of;Voi&s in Nickel-Molybdenum Alloy Containing 3.62 at. % Nb after 1000-hr Exposure to Salt 107. Heat OR 30-21. Table 12. Corrosion;Préduct'Concentrations of Salts Tested with Nickel-Molybdenum Alloys Containing Tungsten Alloy Comp051t10n Tungsten Concentration in designates samples obtalned from metallographlc specimens. - bNot determined Test a Heat (at. %) Duration Salt Samples (mole %) Number — (hr) W Other Components _ Sample H Sample C Other OR 30-9 0.72 11.2 Mo, bal Ni 500 0.0318 . .0.0287 0.0325 OR 30-19 1.44 1L.4 Mo, bal Nt 500 0.0384 0.0311 0.0310 523011 1.72 10.2 Mo, 1.3 Al, 500 . 0.00500 0.00983 b - 2,9 Nb, 4.8 Cr, | | ; o : ~ bal Ni ,_w_""_'w : | - . S T23013 1.85 10.4 Mo, 1.4 41, 500 0.00223 0.00105 b Samples designatlons "H" and "C" are discussed on page 13, "other" 36 Metallographic Results Metallographlc exemanation of loops constructed of nlckel-molybdenum- tungsten alloys revealed only”sllght,surface pitting, as illustrated in Fig. 12. The addition of aluminum?together with tungsten led to heavy surface pitting;”while the,addition_of;elpminum, chromium, and niobium produced subsurface voids to a depth'of 0.0025 in., as shown in Fig. 7. T-11961 s e N | o 0.014 INCHES 1@ 250x% Fig. 12. Hot-Leg Surface'OffNickel-Mblybdefium.Alloy'Contdining_ 1.44 at. % W after 500-hr Exposure to Salt 107. Heat OR 30-19. -Relative'Thermodynamic SteoilitieeeofAlloying Constituents Slnce the salt mlxtures supplied for th1s 1nvest1gatlon were very 7un1form 1n compositlon, one can presume that the same relative act1v1t1es "of'UFl and UF3-eXJsted*at the start of each test. It follows, therefore, that the relative extent of reaction which occurred between the salt mlxtures and an alloylng element xM + = : M+ yUF, = MF + yUF; X 37 for which o O”MXF “ur, - xy K, = = (12) M UF'3 should be directly related to the activity of the fluoride compounds, MxFy’ involving that element. An indication of the relative activities of these compounds is provided by a consideration of their standard free energies of formation, as derived from the reacticn I = M + £ Fa(g) MF (13) where _ % RT M T Xy In Table 13 are listed the standard free energies of formation, per gram- atom of fluorine, of fluoride compounds at 800 and 600°C associated with each of the alloying elements investigated. Values are given for the most stable compounds (that is, those with most negative free energies) reported18 for these elements, and are listed in order of decreasing stabilities. The resultant order suggesfs that corrosion-product concentrations associated with each element at a given activity should have increased in the following order: W, Nb, Fe, Cr, V, Ti, and Al. In Fig. 13, the general ranges of corrosion-product concentrations actually observed for these components, when present as single alloying additions, are plotted as a function of alloy content. It is seen that, 18y, Glassner, The Thermodynamic Properties of the Oxides, Fluorides, and Chlorides to 2500°K, ANL-5750. 38 Table 13. Relative Thermodynamic Stabilities of Fluoride Compounds Formed by Elements Employed as Alloying Additions (Data Compiled by Glassner)® Most Suable AG8OOOC AGSOO o Element Fluoride N\ Compound (-~E§E£h-——;\ (___EQEEE____‘ p g atom of F/ g atom of F/ Al AlF4 —&7 —92 Ti TiF3 ~85 —90 Vv VF> —80 84 Cr CrFs —72 =77 Fe Fels —66 —66 Nb NbF 5 —58 —60 W WF 5 —46 ~48 a A. Glassner, The Thermodynamic Properties of the Oxides, Fluorides, and Chlorides to 2500°K, ANL-5750. with the exception of niobium and tungsten, the concentrations per atomic per cent of addition do increase in the exact order predicted. Only tungsten noticeably deviates from this predicted behavior; the causes for this deviation have not been established, although the number of tests completed on alloys with tungsten additions were quite limited. General Discussion of Alloying Effects The corrosion-product concentrations associated with either iron, niobium, or tungsten alloying additions were much lower when these ele- ments existed in multicomponent alloys than in simple ternary alloys. The reason for this behavior is undoubtedly associated with the presence of the more reactive alloying additions in the multicomponent alloys. Consider, for example, a system containing comparable additions of chromium and iron, for which the corrosion reactions are Cr + 2UF, = CrFp + 2UF;3: AGI (]_4) FeF, + 2UF3: AG.. - (15) Fo + Uk IT 500 400 300 200 60 50 40 CONCENTRATION DETECTED IN SALT {mole % x 103) 30 20 10 39 ORNL ~ LR— DWG 46943 Al/ / —— \—" S 7 / y , 2 /7 7/ Nb/ 0 T / / / /1N / 3/}/ Fig. 13. 2 3 4 S 6 7 8 9 | 2 ALLOY CONTENT (at. %) Comparison of Corrosion-Product Concentrations Formed in salt 107 by Various Alloying Additions as a Function of Alloy Content. 40 Since AGI is considerably more negative than AG the equilibrium UF; ) concentration produced by the first reaction isliigher than that which would be produced by the second reaction. According to the law of mass action, therefore, the FeF, concentration at eguilibrium should be reduced in the presence of chromium compared to a system containing iron only. While this means of corrosion inhibition was effective for those alloying components less reactive than chromium, its effect was not apparent among elements of relatively high reactivities. Thus, chromium corrosion-product concentrations were the same in the case of alloys con- taining both aluminum and chromium as in the case of alloys containing chromium alone. Similarly, the presence of aluminum did not noticeably reduce the titanium-corrosion products associated with titanium-containing alloys. For most of the multicomponent alloys, the amount of chromium and titanium, on an atomic percent basis, exceeded that of aluminum; conse- quently their activities could have approached the activity of the alumi- num component. Nevertheless, the high aluminum concentrations of fluo- ride mixtures after these tests suggest a level of UF:z higher than would have been produced by either chromium or titanium alone, so that some inhibitive action would be predicted. The fact that none occurred may suggest that the measured aluminum concentrations were somewhat higher than actually existed,!”® SUMMARY AND CONCLUSIONS The corrosion properties of solid-solution alloying elements in the Ni—17% Mo system were investigated in molten mixtures of NaF-LiF-KF-UF, (11.2—41.0-45.3-2.5 mole %). The corrosion susceptibility of alloying additions was found to increase in the following order: Fe, Nb, V, Cr, W, Ti, and Al, With the exception of tungsten, the susceptibility of 19Considerable difficulty was in fact encountered in analyses of this element by wet chemical methods. Analyses of a limited number of samples were consequently made using semiquantitative spectirographic techniques. Concentrations obtained by the latter technique were lower by a factor of approximately 2/3 than the values that were reported in Tables € and 7. 41 these elements to corrosion increased in approximately the same order as the stabilities of fluoride compounds of the elements. The corrosion-product concentrations produced by either iron, nio- bium or tungsten alloying additions in the above salt mixture were much lower when these elements existed in combination with chromium, titanium, or aluminum than when they existed as single alloying additions. In con=- trast, corrosion-product concentrations associated with chromium, tita- nium, or aluminum were unchanged by the presence of other alloying constituents. The metallographic examinations of all alloys investigated under this program showed considerably less corrosion than Inconel under equiv- alent conditions. Surface roughening or shallow pitting was manifested in hot-leg sections of all the loops tested. ©Shallow subsurface voids were also seen in aluminum- and niobium-containing systems. Alloys con- taining more than one alloying addition invariably were attacked to depths greater than alloys containing each of the additions individually. The greatest depths of attack, which ranged from 0.003 to 00,0045 in. in 1000-hr exposures, were incurred in alloys which contained combined addi- -tions of aluminum and chromium or aluminum and titanium. Alloys without either aluminum or titanium in no case exhibited attack to depths of more than 0,002 in, and generally showed attack in the range from 0.001 to 0,002 in. In the case of the majority of alloys tested, the depth of attack increased at a greater rate between O and 500 hr than between 500 and 1000 hr. This finding is in asgreement with the concentrations of corro- slon products, which in general increased only slightly between the peri- ods of 500 and 1000 hr. Both results suggest that nearly steady-state conditions were established within the first 500 hr of test operation. In view of the generally favorable results of these tests, one has considerable freedom in the choice of potential alloying components for nickel-molybdenum alloys for molten salt service., Only titanium and aluminum appear to afford potential corrosion problems, particularly if used as combined additions or in combination with chromium. Thus, the choice of an optimum alloy composition for any given application can be judged mainly on the basis of strength and fabrication requirement. 42 ACKNOWLEDGMENTS The many loop experiments reported here were carried out under the skilled hand of M. A. Redden. The author is also indebted to W. 0. Harms for his editorial assistance and guidance as thesis advisor. 1-3, 4=5, 6—15. lé. 17. 18. 19. 20, 21. 22. 23. 24, 25, 26, 7. 28. 29. 30. 31. 32. 33, 34, 35, 36. 37, 38. 39, 40, 41, 42, 43, 45, 46, 47. 48, 49, 50. 51. 52. 53, 5. 55. 56. 57, 43 INTERNAL DISTRIBUTION Central Research Library ORNL Y-12 Technical Library Document Reference Section Laboratory Records Laboratory Records, ORNL-RC ORNL, Patent Office HEFNPLORERDREIRNOANIOERDHEASSINEAANHAD TR X Adams Adamson Affel Anderson Apple Baes Baker Ball Bamberger . Barton Bauman . Beall . Beatty Bell ender . Bettis Bettis Billington . Blanco . Blankenship . Blomeke Blumberg G. Bohlmann J. Borkowski E. Boyd Braunstein A. Bredig B. Briggs R. Bronstein D. Brunton A. Canonico Cantor L. Carter I. Cathers B. Cavin Cepolina M. Chandler H. Clark R. Cobb D. Cochran EEEE e E R OsEHmnEOOE NS GQEY 58. 59. €0, el. 62. 63. ST 65, 66, 67, 68. 69. 70. 71-80. 81. 82. 83. 84, g5, 86. 87. 88. 89. 20. 91. 92. 93. %. 95. 926. 97. 98. 99, 100. 101. 102, 103, 104, 105. 106. 107. 108, 109, 110. 111. 112, oW aoasgsunduodgaxdyrEiigdn Qg mryudHGEOH PN OO oG E REO QR m< = 0 » SD_tIJZmOQCE}b"UD:‘ZQDU.O';E:EUW’;E:':N';C:brdigtljw:'d?drd?f—:}mgififi.fii_fi.z.fljw.b‘}_' ORNL-TM-2021 Vol. I Collins Compere Cook Cook Corbin Crowley Culler Cuneo Cunningham Dale Davis DeBakker DeVan Ditto Dworkin Dudley Dyslin Eatherly Engel Epler Evans I11 Ferguson Ferris Fraas . Friedman frye, Jr. Furlong Gabbard Gallaher Gehlbach Gibbons Gilpatrick Grimes Grindell Gunkel Guymon . Hammond Hannaford Harley Harman Harms Harrill Haubenreich Helms Herndon 113, 114, 115-117. 118. 119. 120, 121. 122, 123, 124, 125, 126. 127, 128. 129, 130. 131. 132, 133. 134, 135, 136, 137. 138. 139, 140. 141. 142. 143, 144, 145. 146. 147. 148, 149. 150, 151. 152, 153, 154, 155, 156, 157, 158, 159, 160, 161. 162. 163. 164, 165, 166. 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