A series of austenitic Fe-Cr-Mn steels was exposed to circulating lithium at temperatures up to 500/sup 0/C. Two groups of the alloys, which contained 12 to 30 wt % Mn and 2 to 20 wt % Cr, were sequentially exposed for periods greater than 3000 h in a type 316 stainless steel thermal convection loop. Mass transfer of manganese caused very large weight losses from the steels containing 30 wt % Mn. However, the actual magnitude of corrosion losses for alloys containing 12 to 20 wt % Mn was difficult to establish due to competing surface reactions involving chromium

DeVan, J. H.

Tortorelli, P. F.

UNT Digital Library

lipCORROSION OF Fe-Cr-Hn ALLOYS IN THERMALLY CONVECTIVE LITHIUM*P, F. Tortorelli and J. H. DeVanOak Ridge National LaboratoryOak Ridge, Tennessee 37331 MASTERAbstractA series of austenitic Fe-Cr-Mn steels was exposed to circulatinglithium at temperatures up to 500°C. Two groups of the alloys, which con-tained 12 to 30 wt % Mn and 2 to 20 wt % Cr, were sequentially exposed forperiods greater than 3000 h in a type 316 stainless steel thermal convectionloop. Mass transfer of manganese caused very large weight losses from thesteels containing 30 wt % Mn. However, the actual magnitude of corrosionlosses for alloys containing 12 to 20 wt % Mn was difficult to establish dueto competing surface reactions involving chromium.*Research sponsored by the Office of Fusion Energy, U.S. Department ofEnergy under contract DE-AC05-84OR21400 with the Martin Marietta EnergySystems, Inc .By acceptance of this article, t h . rnWV OCnAOl IEpublisher or recipient acknowledges KAJN£ ~ O O U 4 * 1 ~ ' " J . Othe U.S. Government's right toretain a nonexclusive, royalty-free D E 8 f llicense in and to any copyrightcovering the article.DISCLAIMERThic report wat prepared at aa aoooaat of work sponsored by aa agency of the United StatasGovernment. Neither tlie Uafted States. Government aor aay agency thereof, acr any of theiremploveei, maket any warranty, express or implied, or anaaiet any legal liabMHy or loaaoaai-bility for the aocaracy, completeness, or isefaiatai of aay iaformatioa, apaaraftw, prodact, «rprooeif disclosed, or represoats that its me would aot iafnafe privately owaed lajjhtt. Rafcr*eoce herein to any specific commercial prodact, procon, or twvioe by trade aaaw, t tataMik,maaufactarer, or otherwise doe* not aeooMarily ooaatitaUi or iaeply Hs imeadatioa, or favoriac by the United States G o w m e a t or aay agaacy theraoT. H e viewtand opiaioas of authors expressed herein do aot aecisssarily Mate or nflect that* of theUnited States Oownuneat or any anacy thereof.U3TNMJTNM QF TWS DKUMBIT I tCORROSION OF Fe-Cr-Mn ALLOYS IN THERMALLY CONVECTIVE LITHIUM*P. F. Tortorelli and J. H. DeVanOak Ridge National LaboratoryOak Ridge, Tennessee 37831IntroductionAustenitic Fe-Cr-Mn alloys can serve as an alternative to the tradi-tional Fe-Cr-Ni stainless steels. Such alloys are of particular interest tofusion technology because of their substantially lower residual radio-activity compared to that for the standard nickel-containing austeniticstainless steels [1]. However, for fusidn applications,.the Fe—Cr-!in steelsmust first exhibit resistance to both radiation damage and corrosion. Ofparticular interest is the corrosion of Fe-Cr-Mn alloys by molten lithium,which is a possible coolant and/or tritium-breeding fluid for D-T fusionreactors. To date, studies of the corrosion of these alloys by moltenlithium were conducted under static conditions [2—6]. In this paper, quali-tative results on the corrosion of a number of Fe-Cr-Mn alloys exposed tothermally convective lithium are presented and compared to the earlier workin a static lithium environment.Experimental proceduresFe-Cr-Mn steel specimens were exposed to lithium in a type 316stainless steel thermal convection loop that allowed removal and insertionof specimens without discontinuing the lithium flow [7] and had previouslyoperated for many thousands of hours with specimens of other alloys. Thelithium density gradient caused by the imposition of a temperature dif-ference of 150°C across the loop resulted in an average flow rate of| ^Research sponsored by the Office of Fusion Energy, U.S. Department ofI Energy under contract DE-ACO5-84OR214OO with the Martin Marietta Energy| Systems, Inc.approximately 25 mm/s. Such a slow flow rate would be characteristic oflithium being used as a semistagnant tritium breeding fluid. Typicalimpurity concentrations of the purified lithium were 30 to 100 wt ppm ofnitrogen and 30 to 130 we ppm of oxygen.In order to characterize the corrosion of a variety of Fe-Cr-Mn alloys,eight specimens were grouped near the top of the loop's hot leg, where theexposure temperatures of these clustered specimens ranged from 485 to 500°C.The rema. • j.ng coupon positions around the loop were occupied by type 316stainless steel specimens. Two loop experiments were conducted in this wayfor total exposure times of 3340 and 3096 h, respectively. The firstexperiment contained Fe-Cr-Mn specimens provided by H. R. Brager of theHanford Engineering Development Laboratory (HEDL); in the second series,alloys from HEDL and Oak Ridge National Laboratory (the latter developed byP. J. Maziasz and R. L. Klueh) were exposed. The compositions of thesealloys are listed in Table 1.ResultsThe weight changes measured for the Fe-Cr-Mn alloys exposed in the twothermal convection loop experiments are shown in Figs. 1 and 2. The steelscontaining 30 wt % Mn suffered very high weight losses that increased withexposure time. In contrast, the 12 to 20 wt Z Mn alloys exhibited a mixtureof weight gains and losses that were not always raonotonic with tine.However, in every case, each 12 to 20 wt % Mn specimen gained weight overapproximately the last 2000 h of exposure.The large number of Fe-Cr-Mn alloys exposed in these experimentsprecludes inclusion of the microstructural and compositional data for eachspecimen (but opher data can be found elsewhere [8]). Therefore, resultsshowing the general representative behavior of the alloys are presented.For the 30 wt % Mn alloys, exposure to lithium resulted in the developmentof rough, porous surfaces on which sone. "nodules" were also observed (seeFig. 3) Energy dispersive x-ray (EDX) analyses indicated that these nodu-les were principally chromium and that the underlying corroded surfaces-were severely depleted in manganese (to below 3 wt % ) . In contrast, thecorroded surfaces of the 12 to 20 wt % Mn alloys tended to rzve many more ofthese chromium-rich nodules. On a few of these specimens, an underlying,porous surface could be observed in gaps between nodule aggregates (seeFig. 4a), while, as shown in Fig. 4b, the nodules completely covered thesurfaces of other coupons. Examination of polished cross sections showedthat porosity developed on every specimen and that the depth of the porouslayer varied among the specimens without any direct relationship to thedensity of the overlying nodules. Figure 5 shows extremes ia the developmentof these porous layers; it includes microprobe elemental x-ray maps indi-cating manganese depletion throughout the porous corrosion layers andchromium-rich phases both on the surface and within these altered zones.These observations were confirmed by wavelength dispersive x-ray profiles ofthe cross sections of selected specimens obtained with the electronmicroprobe; typical results are shown in Fig. 6 and indicated manganesedepletion and surface enrichment in chromium. Compositional data obtainedfrom EDX analyses of the normal and cross sectional surfaces in the scanningelectron microscope were consistent with the electron raicroprobe results.The location of type 316 stainless steel specimens in the cold legduring r.he loop experiments allowed the cold zone deposition reactions toalso be characterized by weight changes and raicrostructural analyses. Thecoldodt loop coupon (350°C) gained considerable weight, particularly duringthe experiment in which the 30 wt % Mn steels were exposed in the hot zone.Numerous large deposits formed at the coldest loop coupon positions (seeFig. 7). EDX analyses showed that such deposits were principally composedof nickel and manganese.DiscussionThe necessity of exposing a range of specimen compositions in a loop ofyet another composition precludes an unambiguous interpretation of thermalgradient mass transfer effects because of the introduction of concentration(dissimilar metal) gradients. This point was recognized when the experi-ments were initially conceived, but as discussed below, some importantinformation about the corrosion of Fe-Cr-Mn alloys by thermally convectivelithium was nevertheless gained from these experiments.The observation of mixed weight gains and losses for the steels con-taining 12 to 20% Mn indicated a competition between dissolution and weightgain reactions. In agreement with results from experiments in staticlithium [2], the present EDX analyses, microprobe imaging, and depth profiledata (see Figs. 5 and 6) indicate that the reactions include concentrationgradient chromium transport among the specimens and loop wall in the hotzone, a small amount of nickel redistribution to the Fe-Cr-Mn specimens, andpreferential dissolution of manganese. Such competing reactions can lead toa net weight gain if the addition of reacted material due to dissimilarmetal mass transfer is greater than that lost by dissolution of manganeseand other elements.Each of the specimens clustered in the 485 to 500°C zone of the lithiumloop, including those with significant net weight gains, wsrs found to havecorrosion layers characteristic of the type caused by preferential dissolu-tion of an alloy. These layers, containing dark-etching phases or pores areshown in Fig. 5 and bear a strong res emblaze to layers on Fe-Ni-Cr steelsexposed to lithium [9]. In the present case, the porous layers appear to beassociated with the selective leaching of manganese while, for Fe-Cr-Nisystems [9], they are related to preferential depletion of nickel. In thecase of the 30 wt % Mn alloys (X75, R77, and R80), this preferential disso-lution obviously dominated the corrosion processes and resulted in the largeweight losses shown in Fig. 1. For the 12 to 20 wt % Mn specimens, theporous layer was generally deeper and more uniform for the alloys showingnet weight losses. Conversely, the Fe-Cr-Mn steel with the largest weightgain after 3096 h (Age 18-18+) showed a very thin dissolution zone, and theone with the next greatest increase in weight (R87) showed a somewhatthicker, but very irregular corrosion layer. Given the trend of the weightchange versus exposure time curves (see Figs. 2 and 3) and the observationof many chromium-rich reaction products on "op of the dissolution zones (seeFigs. 4—6), it appears that, in many cases, manganese depletion makes theprincipal contribution to the weight change of the Fe-Cr-(12—20 wt %) Mnalloys during the early stages of exposure. Subsequently, chromiumtransport and reaction become dominant and lead to weight gains. Thepresence of materials of different compositions in the hot zone probablyexacerbated the latter reactions.The exact nature of the reaction(s) involving chromium is not known.The starting concentrations of chromium, nitrogen, and carbon (or the con-centration ratios of Cr/N and Cr/C) did not correlate with the weight change:.ata or microstructural results. Nevertheless, the chromium reactions maybe driven by the relatively high nitrogen concentrations of some of theFe-Cr-Mn alloys (see Table 1). Reactions between Cr and N and between Li,Cr, and N are possible [5,10,11] and could lead to increased dissolutionand/or reaction product formation at specimen surfaces. The significantintergranular penetration observed for some of these alloys in staticlithium [4—6] was related to the presence of such reaction products at grainboundaries. In contrast, there was no evidence for intergranular attack ofthe alloys exposed in the present experiments even though several of thecompositions were common to both studies. The reason for such a discrepancymay possibly be attributed to greater dissolution and preferential leachingunder the nonisothermal loop conditions. Alternatively, differences inlocal solution chemistry and/or interstitial transfer gradients betweenisothermal and thermally convective conditions may explain the absence ofgrain boundary attack in the present study.The observation of deposits on type 316 stainless steel specimens inthe cold leg (see Fig. 7) and the large measured weight gains for thecoldest coupons indicated substantial mass transport occurred duringexposure to lithium. Much of the deposited material was manganese, whichcame from the hot leg specimens. The source of the nickel in these depositswas the nickel-containing surfaces in the hotter area of the loop.A quantitative comparison between the corrosion of Fe-Cr-Mn alloys bythermally convective lithium and that of similarly exposed Fe-Cr-Mi steelscannot be made because of the corap.lications arising from hot leg con-centration gradients in the present experiments and the relative limitedamount of data for the former alloys. However, it is apparent that theFe-Cr-Mn steels suffer from the same effects of nonisothermal lithium expo-sure as do nickel-containing austenitic alloys (preferential dissolution,porosity formation, and extensive mass transport of soluble species to coldzone). Indeed, the measured weight losses of the steels containing30 wt % Mn after 3340 h at 490 to 500°C (217—391 g/m2) were much greaterthan that measured for a Fe-Cr-30 wt % Ni alloy exposed for 2832 h at 500°C(28 g/m2) [12], despite the competing weight gain reaction with chromium.Such Fe-Cr-Mn alloys are clearly not suitable for use in lithium systems atelevated temperature. Furthermore, the 12 to 20 wt % Mn alloys experiencedsignificant degradation despite the low net weight changes and are notexpected to be particularly corrosion resistant in thermaily convectivelitnium.ConclusionsFe-Cr-Mn alloys containing 30 wt % Mn suffered large weight losses dueto mass transfer of manganese. Such steels are clearly not suitable for usein elevated temperature lithium systems.In Fe-Cr-(12—20 wt %) Mn alloys, competing mechanisms of weight lossresulting from manganese transport and weight gain due to chroniua reactionswere observed. For these alloys, ii appeared that manganese depletion domi-nated initially, but that it was then offser by chromium reactions that vrereexacerbat&d by dissimilar metal gradients in the hot leg. Despite theirlower long-term radioactivity, the corrosion of such Fa-Cr-Mn alloys bylithium appears problematical.References[1] F.W. Wiffen and R.T. Santoro, in: Proc. Topical Conf. on FerriticAlloys for Use in Nuclear Energy Technologies, Warrendale, Pa., 1984(AIME, 1984) pp. 195-200.[2] V. Coen et al., J. Nucl. Mater. 86S86 (1979) 217—275.[3] E. Rued! et al., Material Behavior and Physical Chemistry in LiquidMetal Systems (Plenum, New York, 1982) pp. 97—104.[4] E. Ruedl et al., J. Nucl. Mater. 110 (1982) 28-36.[5] E. Ruedl and T. Sasaki, J. Nuc.l. Mater. 116 (1983) 112-122.[6] E. Ruedl et al., J. Nucl. Mater. 122&123 (1984) 1247-1251.[7] P.F. Tortorelli and J.H. DeVan, in: Proc. 3rd Intern. Conf. OR LiquidMetal Engineering and Technology, Oxford, 1984 (BNES, London, 1984)pp. 81—88.[8] P.F. Tortorelli and J.H. DeVan, in: ADIP Semiannual Progress Reportsfor Periods Ending Sept. 30, 1984; Mar. 31, 1985; Sept. 30, 1985; andMar. 31, 1986, DOE/ER-0045/13,14,15, and 16 (U.S. Department of Energy,Office of Fusion Energy, 1984—1986).[9] P.F. Tortorclli and J.H. DeVan, Microstructural Sci. 12 (1985) 215—26.[10] W.F. Calaway, in: Proc. 2nd Intern. Conf. on Liquid Metal Technologyin Energy Production, Richland, Wa., 1980 (U.S. Department of Energy,1980) pp. 18-8-18-26.[11] M.G. Barker et al., J. Nucl. Mater. 114 (1983) 143-149.[12] P.F. Tortorelli and J.H. DeVan, J. Nucl. Mater. 103&104 (1981)633-638.Table 1. Compositions of alloys exposed to thermallyconvective lithium between 500 and 480°CaAlloyX75R77R8013-13+Age J -18+AMCRR88R87N1T-32PCMA-2PCMA-4PCMA-6PCMA-7PCMA-9CN13316 SSMn3030301818C1715151217191419181.72Cr221018181015151815101615201717CompositionNi0.50.50.50.50.50.70.50.51.50.010.010.010.010.0114.010C0.10.60.500.10.10.20.30.10.10.060.090.180.380.260.050.05(wt0000000000000.o.% ) b '?J.15.15.10.4.4.06.30 •.1.4.001.002.003.005.0060000000000000,0,0.0.Si.4.4.4.6.6.6.4.4.6.04.02.02.02.03.6.4FebaibalbalbalbalbalbalbalbalbalbalbalbalbalbalbalaPGMA alloys developed at Oak Ridge National Laboratory;other alloys supplied by Hanford Engineering DevelopmentLaboratory.^Analyses provided with alloy material.cElectron beam techniques revealed Mn concentration of12 wt 7.. This correction probably applies to the 18-13+ alloyalso.Figure captionsFig. 1. Weight loss versus time for Fe-Cr-30 wt % 1-In alloys exposed tothermally convective lithium. Part of first loop experiment.Fig. 2. Weight change versus tine for Fe-Cr-(12—20 wt %) Sin andFe-Cr-(12—14 wt %) Ni alloys exposed to thermally convective lithium in(a) first and (b) second loop experiments. ("P" denotes "PCMA".)Fig. 3. Surface of alloy X75 (30 wt % Mn) after exposure to thermallyconvective lithium at 495°C for 3340 h.Fig. 4. Typical surfaces of 12 to 20 wt % Mn alloys, exposed tothermally convective lithium for 3096 h. (a) NIT-32, 485°C. (b) R87,500°C.Fig. 5. Backscattered electron images (BSI) and elemental x-ray mapsof Fe-Cr-Mn alloys exposed to thermally convective lithium for 3096 h.(a) NIT-32, 485°C. (b) Age 13-18+, 490°C.Fig. 6. Wavelength dispersive x-ray depth profile for PCMA-4 exposedto thermally convective lithium for 3096 h at 485°C.Fig. 7. Cold leg deposits of Ni-Mn formed on 350°C coupon after 3340 hof exposure of Fe-Cr-(12—30 wt % Mn) alloys in the hot leg of a lithiumthermal convection loop.ORNL-DWG 8 6 - 8 6 4 5THERMALLY CONVECTIVE Li400E\COCOCOo_ lH-XCDi—i3002001000 -500 1000 1500 2000 2500EXPOSURE TIME (h)3000 3500FIG. 1WEIGHT CHANGE (g/m2 ) WEIGHT CHANGE ig /m2 )o h-. »s - . - . • • . • • " . . • . ' . ' • :Fig. 3Fig. 4ORNL-DWG 85-17934Fe-19Mn-10Cr, thermally convective Li, 3096 h, 485°C80 -60 -40 -UJ(JOo20 -0 -6 8 10 12DEPTH (urn)14 18FIG. 6Fig. 7

Corrosion of Fe-Cr-Mn alloys in thermally convective lithium

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Corrosion of Fe-Cr-Mn alloys in thermally convective lithium

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