Tortorelli 1981 Journal of Nuclear Materials

7/27/2019 Tortorelli 1981 Journal of Nuclear Materials

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J ournal of Nuclear Materials 103 & 104 (1981) 675-680North-Holland ublishing ompany

615

CORROSION OF TYPE 316 STAINL ESS STEE L IN MOLTE N LiF-LiCl-LIBr]

P. F. Tortorelli, J . H. DeVan, and J . R. Kaiser

Metals and Ceramics Division,Oak Ridge National Laboratory

Oak Ri dge, Tennessee 37830

The properties of LIF-LICl-LIBr salt make it attractive as a solvent for extractingtri tium from a fusion reactor li thium blanket. Consequently, the corrosion of type316 stainless steel by flowing (about 15 mm/s> LiF -LiCl-L iBr at a maximum temperatureof 535C was studied to determine whether compatibility with the structural materialwould be limiting in such a system. The corrosion rate was found to be low(


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lmm-diam ir idium wires welded to 3-mm-OD(l/8-in.) nickel ri sers. The electrode areaexposed to the melt was 10 to 20 mm2. Thevoltammograms were recorded versus the ir idi umquasi-reference electrode (I r QFS), which was

poised at the equil ibri um potential (E,q) of themelt. I n this manner, relati ve changes i n theoxidati on potential of the salt and in the con-centration of certain impuri ties and corrosionproducts can be qualitatively monitored. Theequil ibrium potential of the salt is a measureof the tendency of the melt to react with oxi-dizable or reducible materials that it contacts.For melts that are not poised by a redox buffer,relati ve changes in the equil ibrium potentialare conveniently observed by determining volt-ammetricall y the potential difference (00between the melt equil ibrium potential and thecathodic limit of the melt, which forLIF-L iCl-L iBr is the reduction of Li+ Wedefine the cathodic li mit for these measurementsas the potential at 16-mA cell current (seeFig. 2). Other reference markers may also beused as long as the same conditions aremaintained. A decrease in the absolute value of.!E, resulting in a cathodic shift in Eeq, for

, I NSUl . ATI ON I l l

c3

c4

C6

c7

CS

Figure 1 : Type 316 stainless steel thermal-convection loop. The designations on each sideindicate the sample numbers.

example, indicates that the melt has become lessoxidizing; an increase in the magnitude of il Eindicates the opposite.

4'

0 e~ /I

0 -0 .5 - f . O - i . 5 -2 .0VOLTAGE ( V)

Figure 2 : Typical voltammetry scan for pureLiF-LfCl-LiBr salt.

3. RESULTS

Coupons of type 316 stainless steel were exposedto flowing LiF -LiCl -LiBr for a total of 7855 h.After 2675 h of exposure, 0.8 g of li thi um wasadded to the loop salt (7.3 kg). The resultingweight change behavior as a function of time forthe specimen at the maximum temperature (535C)position is shown in Fig. 3. Note that after2175 h of coupon exposure with a constant corro-sion rate (-3785 h of salt circulation), thespecimen weight loss increased dramatically whenli thium was added to the salt. The corrosionrate then quickl y decreased to a value smallerthan that measured before the li thium addition.This type of behavior was not li mited to the535C specimen. As shown in Table 1, all theloop coupons exhibited similar trends in weightloss rate with exposure time. The corr osionrates in Table 1 are those measured during the2175-h period before the li thium addition(peri od l), duri ng the 430 h immediatelyfollowing the addition (peri od 2), and duri ngthe remainder of the loop test (period 3). Theyare taken from the slopes of the least-squaresfit ted li nes in the given time intervals. Thedata show that the li thium addition caused anincreased specimen weight loss at all loop posi-tions for a short period of time. Ihe data inTable 1 also indicate that the lower corrosionrates during periods 1 and 3 were not very sen-sitive to specimen position or to temperaturewithin the loop. I ndeed, every loop coupon suf-fered weight losses ranging between 2.0 and 0.6pm/year. During period 2, the rate of weight


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678 PF Tortorelli et al. 1 Type 316 stainless steel in molten LiF-LiCI-LiBr

10pm

Figure 6 : Type 316 stainless steel exposed to,LiF-LiCl-LiBr or 7855 h (with lithium addedduring exposure) in cold leg. (a) Exposed at5oo"c. (b) 465C.

Electron microprobe analysis was used to definethe concentration rofiles of Fe, Ni, Cr, and MOin the near-surface rea of selected specimens.These results, shown in Fig. 7, indicated thatnickel was somewhat depleted from the surfacesof the coupons exposed at 535 and 510C sincethe nickel lines drop off below the respectivecoupons' surfaces. Chromium may be slightlydepleted also. The specimen exposed at 465Cdid not appear to have been depleted of theseelements [see Fig. 7(c)]. Analysis of the saltafter 675 h of loop operation revealed no sig-nificant increase in the concentration f

chromium, nickel, or iron. This would thereforeindicate that the nickel was depleted from thesteel after the lithium was added.

Periodic voltammetry scans of the salt weretaken throughout the time the loop wasoperating. The behavior of AE during this timeis shown in Fig. 8. In general, AE was approxi-mately constant until the lithium was added tothe salt. At that point it shifted very closeto zero potential and then gradually increasedas loop operation continued. The data points inFig. 8 that represent measurements taken withthe specimens out of the loop are denoted byopen circles. Comparison with the data points

immediately following the open circles showsthat in all cases (except just after the lithiumwas added) AE became greater when the specimenswere reinserted and then rapidly returned toapproximately its former value. These rela-tively small changes indicate the transienteffect of impurities introduced during specimeninsertion on the oxidation potential of thesalt.

Cr

,o\

l b)SURFACE

-Fe

Cr

Figure 7 : Microprobe traces of near-surfaceareas of type 316 stainless steel coupon crosssections after 7855 h exposure to LiF-LiCl-LiBr(with lithium added during exposure).(a) Exposed at 535C. (b) 51O'C. (c) 465C.

-4.6 c1

-4.6

-4.4

-1.2

; -1.0wa

-0.8

-0.6

Li

m ok r;:LiF-LiCI-LiBr I

. SPECIMENS IN LOOP

NSOUTOFLOOP

71

SALT CIRCULATION TIME (h)

Figure 8 : Limit potential versus salt cir-culation time for the type 316 stainless steelthermal-convection oop operated between 535 and465C.


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P.F. Tortor ell i et al . / Type 316 stainl ess steel i n molt en Li F-LiCl -Li Br 679

4. DISCUSSION

Two general aspects of halide salts becomeimportant in defining their corrosivi ty.Halides of the type used in this investigation

are relatively stable in the presence of themetallic elements composing austenitic stainlesss tee16 ; that is, reactions between these saltsand containment materials are li mited thermo-dynamically. On the other hand, materialsexposed to molten halides general ly do not formpassivating films because the corrosion productstend to have relati vely high solubili ties.Molten fluoride salts, which have been exten-sively studied in previous TCL investigations,afford relevant examples of the aforementionedbehavior (see, for example, refs. [2,4,5]).Typically, flowing nonisothermal fluoride saltwill oxidize and remove the least noble elementfrom the metal contai ner and then may sub-sequently deposit it in another porti on of thesystem. Results from a nickel-base molybdenum-chromium-iron alloy (Hastelloy N) in acirculating, nonisothermal salt composed ofLIF-BeF2-ThF4-UF4 have shown that fluoride saltswil l oxidize chromium by reaction first withimpurities in the salt such as HF , Ni F2, and FeF2and later by possible reaction with constituentsof the salt [4]. Impurity reactions expected are

2HF + M


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680 PI; Tortorelli et ul 1 Tl,pc 316 stainless steel in molten LiF-LiCI-LiBr

as Eq. (1). However, we expect that the corro-sion will not be sufficiently evere at suchtemperatures to limit the use of the saltbecause the corrosion in the lithium systemitself will be the limiting factor.

5. SUMMARY

The results from this study indicate that type316 stainless steel has a low corrosion rate(

Tortorelli 1981 Journal of Nuclear Materials

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