Analysis of Corrosion of Hastelloy-N, Alloy X750, SS316 and SS304 in Molten Salt High-Temperature Environment

energies

Article Analysis of Corrosion of Hastelloy-N, Alloy X750, SS316 and SS304 in Molten Salt High-Temperature Environment

Ketan Kumar Sandhi and Jerzy Szpunar *

Department of Mechanical Engineering, College of Engineering, 57 Campus Drive, University of Saskatchewan, Saskatoon, SK S7N 5A9, Canada; [email protected] * Correspondence: [email protected]

Abstract: Nickel superalloy Hastelloy-N, alloy X-750, stainless steel 316 (SS316), and stainless steel 304 (SS304) are among the alloys used in the construction of molten salt reactor (MSR). These alloys were analyzed for their corrosion resistance behavior in molten fluoride salt, a coolant used in MSR reactors with 46.5% LiF+ 11.5% NaF+ 42% KF. The corrosion tests were run at 700 ◦C for 100 h under the Ar cover gas. After corrosion, significant weight loss was observed in the alloy X750. Weight loss registered in SS316 and SS304 was also high. However, Hastelloy-N gained weight after exposure to molten salt corrosion. This could be attributed to electrochemical plating of corrosion products from other alloys on Hastelloy-N surface. SEM–energy-dispersive X-ray spectroscopy (EDXS) scans of cross-section of alloys revealed maximum corrosion damage to the depth of 250 µm in X750, in contrast to only 20 µm on Hastelloy-N. XPS wide survey scans revealed the presence of Fe, Cr, and Ni elements on the surface of all corroded alloys. In addition, Cr clusters were formed at the triple junctions of grains, as confirmed by SEM–EBSD (Electron Back Scattered Diffraction) analysis. The order of corrosion resistance in FLiNaK environment was X750 < SS316 < SS304 < Hastelloy-N.

Keywords: MSR; corrosion resistance; Hastelloy-N; alloy X-750; stainless steel 316; stainless steel 304; high temperature corrosion Citation: Sandhi, K.K.; Szpunar, J. Analysis of Corrosion of Hastelloy-N, Alloy X750, SS316 and SS304 in Molten Salt High-Temperature 1. Introduction Environment. Energies 2021, 14, 543. https://doi.org/10.3390/en14030543 Molten salt reactors (MSRs) are among six of the Gen IV nuclear reactor concepts. These reactors use molten salt mixed with nuclear fuel as a heat transfer media. Since Academic Editor: Luca Pasquini molten salt remains in liquid state during the operation of MSR, the reactor operates at near Received: 4 January 2021 atmospheric pressure, which reduces the chances of leakages and explosions. Moreover, ◦ Accepted: 20 January 2021 MSR operates at relatively higher temperatures (700–850 C), which results in higher Published: 21 January 2021 thermal efficiency [1]. These molten salt reactors have negative coefficient of reactivity, which makes MSR passively safe reactors [2,3]. Some MSR design concepts have been Publisher’s Note: MDPI stays neutral already proposed—Seaborg Technologies Wasteburner (SWaB) reactor burns the spent fuel with regard to jurisdictional claims in from light water reactors (LWRs), and this allows for the minimization of nuclear waste [4]. published maps and institutional affil- Another design, advanced high temperature reactor (AHTR), has high thermal efficiency, iations. large power output (3400 MW [th]), and uses thorium as fuel [5]. The chloride and fluoride salt mixtures have been proposed for use in the MSR [6]. The high temperature stability, high heat capacity, low vapor pressure, high boiling point, and stability in nuclear environment are characteristics that make these salts an excellent Copyright: © 2021 by the authors. coolant for MSR application [7]. Although MSR reactors have many advantages, the salts Licensee MDPI, Basel, Switzerland. are extremely corrosive to materials used in construction of the reactor [8]. The fluoride This article is an open access article salts with eutectic mixture of LiF–NaF–KF (FLiNaK) with mol % of 46.5-11.5-42 are one of distributed under the terms and the candidates to be used in MSR. conditions of the Creative Commons Many studies have been performed to develop corrosion resistance material for use Attribution (CC BY) license (https:// in MSR [9–11]. All alloys when exposed to this highly corrosive salt (FLiNaK) at elevated creativecommons.org/licenses/by/ temperatures (700–850 ◦C) tend to corrode. Some of these alloys are usually protected from 4.0/).

Energies 2021, 14, 543. https://doi.org/10.3390/en14030543 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 10

Energies 2021, 14, 543 2 of 10

temperatures (700–850 °C) tend to corrode. Some of these alloys are usually protected oxidationfrom oxidation in air in by air their by their protective protective oxide oxide layer. layer. However, However, in the in MSR,the MSR these, these protective protec- layerstive layers tend tend to dissolve, to dissolve, and and corrosion corrosion occurs occurs by by removing removing the the oxide oxide forming forming elements.elements. AlloysAlloys containingcontaining Al,Al, Cr,Cr, Si,Si, andand MoMo formform fluoridesfluorides whenwhen exposedexposed toto fluoridefluoride salts.salts. TheThe necessarynecessary conditioncondition forfor thethe formationformation ofof fluoridesfluorides (e.g., (e.g., CrF CrF2,2, FeF FeF22,, NiFNiF22)) isis thatthat thethe GibbsGibbs freefree energyenergy ofof formationformation ofof fluoridefluoride mustmust bebe negativenegative [[9].9]. FigureFigure1 1 shows shows the the Gibbs Gibbs free free energyenergy ofof formationformation ofof fluoridefluoride forfor differentdifferent metals.metals. TheThe candidatecandidate metalsmetals fromfrom thisthis graphgraph forfor useuse inin moltenmolten saltsalt environmentenvironment areare Ni,Ni, Mo,Mo, and W. TheThe raterate ofof corrosion,corrosion, however,however, alsoalso dependsdepends onon otherother factors such as galvanic coupling of different metals in the alloy, temperature,temperature, andand concentrationconcentration ofof elements.elements. ConsideringConsidering this,this, anan alloyalloy withwith 70%70% Ni,Ni, 16%16% Mo,Mo, 7%7% Cr,Cr, andand 5%5% FeFe waswas developeddeveloped inin OakOak RidgeRidge NationalNational LaboratoryLaboratory [[12].12].

◦ FigureFigure 1.1. GibbsGibbs freefree energyenergy ofof formationformation (kJ/mol)(kJ/mol) of fluoride fluoride for different metals at 850 °CC[ [10]10].. A corrosion study was also performed by Ignatiev et al. on nickel-based alloys A corrosion study was also performed by Ignatiev et al. on nickel-based alloys Kh80MTYu, KhN80M-VI, and MONIKR, exposed to a fluoride salt environment (<700 Kh80MTYu, KhN80M-VI, and MONIKR, exposed to a fluoride salt environment (<700 °C) ◦C) for 1200 h in a naturally circulated molten salt loop consisting of hot and cold regions for 1200 h in a naturally circulated molten salt loop consisting of hot and cold regions to to simulate the actual service condition in MSR [13]. It was found that the Cr depletion simulate the actual service condition in MSR [13]. It was found that the Cr depletion oc- occurred in the FLiNaK environment; however, this process can be slowed down by curred in the FLiNaK environment; however, this process can be slowed down by remov- removing impurities from the molten salt. Moreover, the accumulation of depleted Cr was ing impurities from the molten salt. Moreover, the accumulation of depleted Cr was ob- observed on the cold region of the salt loop. This shows the importance of temperature in served on the cold region of the salt loop. This shows the importance of temperature in controlling the mechanism of corrosion in FLiNaK environment. Sellers et al. [14] studied corrosioncontrolling behavior the mechanism of multiple of corrosion alloys in ninein FLiNaK sealed environment. stainless steel Sellers 316 crucibles. et al. [14] However, studied onecorrosion crucible behavior leaked andof multiple reacted withalloys furnace in nine refractory sealed stainless material, steel producing 316 crucible harshs. However, corrosive vaporsone crucible that corroded leaked and all the reacted nine crucibles,with furnace leading refractory to catastrophic material damage, producing of all harsh specimens. corro- Thissive vapors study demonstrated that corroded theall the highly nine corrosivecrucibles,nature leading of to experimental catastrophic damage setup required of all spec- for suchimen experiments. This study [14 demonstrated]. Slama et al. studied the highly MoNiCr, corrosive an alloy nature developed of experimental by COMTES setup FHT required for ◦ such experiment [14]. Slama et al. studied MoNiCr, an alloy developed by Inc., at 700 C in LiF–BeF2 environment for a duration of up to 3 months. The tests revealed theCOMTES Cr and FHT Mo elementsInc., at 700 were °C depletedin LiF–BeF from2 environment the alloy and for were a duration accumulated of up in to the 3 months. molten salt.The Crtests depletion revealed was the seen Cr and primarily Mo elements on grain were boundaries depleted [15 from]. Another the allo studyy and with were Ni–Mo accu- supermulated alloy in GH3535the molten concluded salt. Cr depletion that the corrosion was seen occurs primarily by dissolution on grain boundaries of Cr and Mo[15]. from An- theother grain study boundaries with Ni– [Mo16]. super alloy GH3535 concluded that the corrosion occurs by dis- solutionIn the of presentCr and Mo study, from Hastelloy-N, the grain boundaries X750, SS316, [16]. and SS304 were tested in a FLiNaK environmentIn the present at 700 ◦study,C for 100Hastelloy h. This-N, study X750, aimed SS316 at, understandingand SS304 were the tested corrosion in a FLiNaK mecha- nismenvironment of these at alloys 700 °C and for their 100 behaviorh. This study when aimed they areat understanding used in the MSR the at corrosion the same mech- time. Foranism the of first these time, alloys EBSD and and their XPS behavior examination when was they performed are used onin the corroded MSR at alloy the samplessame time. to understandFor the first thetime nature, EBSD of and corrosion XPS examination products and was a roleperformed of the grain on corroded boundaries alloy and samples triple junctions in chromium depletion in an MSR environment.

Energies 2021, 14, x FOR PEER REVIEW 3 of 10

Energies 2021, 14, 543 to understand the nature of corrosion products and a role of the grain boundaries3 of and 10 triple junctions in chromium depletion in an MSR environment.

2. Materials and Methods 2. MaterialsHastelloy and-N Methods and X-750 were procured from Haynes International in the form of small sheetsHastelloy-N of size 2″ and× 2″ X-750 × (0.063 were–0.125 procured″). Stainless fromHaynes steel 316 International and 304 of size in the 5″ form × 5″ of× 1.5 small mm sheetswas obtained of size 2” from× 2” YIEH× (0.063–0.125”). United Steel Stainless Corporation. steel 316The and nominal 304 of chemical size 5” × composition5” × 1.5 mm of wasthese obtained alloys is from given YIEH in Table United 1. Steel Corporation. The nominal chemical composition of these alloys is given in Table1. Table 1. Nominal chemical composition (% by weight) of alloys used in this study. Table 1. Nominal chemical composition (% by weight) of alloys used in this study. C Si Mn Ni Cr Mo Cu Fe Nb + Ta X750 C0.08 Si 0.5 Mn 1 NiBalance Cr17 Mo- Cu0.5 Fe5 Nb +1.2 Ta HatelloyX750-N 0.080.06 0.5 1 1 0.8Balance Balance 177 -16 0.5 54 1.2- Hatelloy-NSS316 0.060.024 1 0.60 0.8 0.98Balance 10.13 716.77 162.02 0.10 Bal 4ance - - SS316SS304 0.024 0.030.60 0.75 0.98 2 10.1310 16.7718 2.02- 0.10- BalanceBalance - - SS304 0.03 0.75 2 10 18 - - Balance -

Small coupons of the size 15 × 10 × 1.5 mm were cut from original sheets. All samples wereSmall abraded coupons progressively of the size 15using× 10 SiC× 1.5grinding mm were paper cut of from grit originalsize 320, sheets. 500, and All 800 samples to ob- weretain abradedthe same progressivelysurface finish usingon all SiCsamples. grinding The samples paper of were grit sizethen 320,washed 500, in and 1M 800 NaOH to obtainsolution, the samecleaned surface using finish acetone on all, and samples. dried in The warm samples air. wereThe samples then washed were instored 1M NaOH in vac- solution,uum desiccator cleaned usingto avoid acetone, oxidation/corrosion and dried in warm before air. The testing. samples The were weight stored of the in vacuum samples desiccatorwas recorded to avoid before oxidation/corrosion corrosion test. before testing. The weight of the samples was recordedEutectic before salt corrosion mixture test. with 46.5LiF–11.5NaF–42KF mol % was prepared. Pure nickel crucibleEutectic obtained salt mixture from Delta with-Scientific 46.5LiF–11.5NaF–42KF Laboratory was mol filled % waswith prepared. this salt mixture. Pure nickel All 4 cruciblealloy samples obtained were from submerged Delta-Scientific in the salt. Laboratory The crucible was filledwas then with placed this salt in MTI mixture. Corp AllGSL 41500× alloy samplestubular furnace were submerged (see Figure in2 for the detailed salt. The set crucible-up). Argon was gas then was placed filled in in MTI the tubular Corp × GSLfurnace 1500 andtubular corros furnaceion test (seewas Figureset to 2run for at detailed 700 °C set-up).for 100 h. Argon After gas this was test filled, the samples in the tubular furnace and corrosion test was set to run at 700 ◦C for 100 h. After this test, the were left in furnace to cool down. Crucible was removed from the furnace and samples samples were left in furnace to cool down. Crucible was removed from the furnace and were taken out. For cleaning salts from sample surfaces, 1M Al(NO3)3 solution was pre- samples were taken out. For cleaning salts from sample surfaces, 1M Al(NO ) solution pared and samples were ultrasonically cleaned in this solution. Samples3 3 were then was prepared and samples were ultrasonically cleaned in this solution. Samples were cleaned using deionized water and dried in warm air. The weight of all samples was rec- then cleaned using deionized water and dried in warm air. The weight of all samples was orded after corrosion tests. recorded after corrosion tests.

(a) (b)

Figure 2. (a) Schematics of staticFigure immersion 2. (a) Schematics set-up used of static for molten immersion salt corrosion set-up used tests. for (b molten) MTI Corpsalt corrosion GSL 1500 tests× tubular. (b) MTI furnace used for corrosion test.Corp GSL 1500× tubular furnace used for corrosion test.

TheThe crystallographic crystallographic characterization characterization of corrodedof corroded samples samples was was performed performed using using X-ray X- diffractometerray diffractometer (Rigaku (Rigaku Ultima Ultima IV X-ray IV X-ray Diffraction Diffraction (XRD)) (XRD with)) with Cu Cu Kα Kαradiation radiation at at 44 44 mAmA and and 40 40 kV.kV. MeasurementsMeasurements were taken in in the the 2θ 2θ rangerange of of 10° 10 to◦ to 80° 80 with◦ with 0.02° 0.02 step◦ step size. size.The Thechemical chemical composition composition of surface of surface of corroded of corroded samples samples was wasdetermined determined using using an X an-ray X-ray photoelectron spectroscopy (XPS) Kratos AXIS Supra system equipped with 500 mm Rowland circle mono-chromated aluminum Kα 1486.6 eV source and combined with hemi-spherical analyzer (HSA) and spherical mirror analyzer. Multiple spots of size of

300 × 700 µm were analyzed at different positions on the surface of samples. All survey Energies 2021, 14, 543 4 of 10

scans were collected in the binding energy range −5 to 1200 eV with 1 eV step size and 15 keV accelerating voltage and 15 mA emission current. For analyses of surface morphology, a Hitachi SU6600 Scanning Electron Microscope (SEM) was used. The distribution of elements on corroded surface were analyzed using Oxford Instruments X-Max 80 mm2 Large Area SDD Silicon Drift Detector Energy-Dispersive X-ray Spectroscopy (EDXS). To analyze the depth of corrosion and cross-sectional analyses, we hot mounted the samples using Poyfast-Struers and polished. The EDXS maps and line scans were collected from the cross-section of the corroded samples.

3. Results and Discussion 3.1. Weight Loss Measurements The weight of the corroded samples changed because of removal of the certain elements from the alloy. The weight loss per unit surface area (w) of the sample was calculated. This weight change per unit surface area (w) was compared for tested alloys. The maximum w = 9.98 mg/cm2 was registered for alloy X750, which showed the maximum depletion of Cr from the surface. SSS316 and SS304 also lost weight after corrosion, and the weight losses per unit surface area (w) were 1.99 mg/cm2 and 2.13 mg/cm2, respectively. The Hastelloy-N, on the other hand, interestingly gained weight, and the weight loss per unit surface area (w) was −3.10 mg/cm2. This gain in weight could have been because of deposition of corrosion products from the other samples on the surface of Hastelloy-N. From the weight loss/gain trend, it seems the corrosion resistance of alloys tested was X750 < SS304 < SS316 < Hastelloy-N.

3.2. Analysis of Surface Morphology after Molten Salt Corrosion Tests Figure3 shows the scanning electron microscope (SEM) secondary electron images (SEI) of alloys X750 (a), Hastelloy-N (b), SS304 (c), and SS304 (d) after corrosion test of 100 h at 700 ◦C in molten salt environment. The surface morphology of all samples showed interesting features; the sample of X750 alloy was heavily corroded and formation of chromium oxide can be seen in form of layered structure in the inset image in the figure at higher resolution. This was later confirmed to be chromium and oxide by SEM EDXS. The morphology of alloy Hastelloy-N showed granular surface and grains with micro-holes, which were observed in the high-resolution image. Both stainless steel samples were also corroded, and the remnants of chromium oxide layer was seen on the surface of sample SS304 (Figure3c). At high-resolution (inset image), grains of nickel were observed. The surface of SS316 showed (Figure3d) two distinct surfaces, the granular surface in the middle and the smooth outer surface. From these images, all the samples showed corrosion on the surfaces; cross-sectional images, discussed later, will show the depth of corrosion. For analyses of distribution of selected elements (Ni, Fe, Cr, O) on the surface of corroded alloys, we collected SEM-EDXS maps at multiple regions on the samples. Figure 4 shows the elemental maps of top surfaces. The alloy X750 showed the strong presence of Cr and O on the surface. The Cr was distributed evenly on the surface showing uniform corrosion in this alloy. The layered-like structure observed earlier was confirmed to be chromium oxide. This Cr present on the surface came from the base alloy and diffused to the surface for the duration of the corrosion test. On the surface of Hastelloy-N, similar uniform corrosion was seen with comparatively less Cr. However, both stainless steel samples showed non-uniform corrosion. In SS316, the middle region of the scanned map showed a Cr oxide patch and Fe in the outer region of patches. Similarly, SS304 had remnant protective chromium oxide layer. Fe and Ni were seen around these remnants. From these top surface EDXS scans, it can be concluded that the uniform corrosion occurred in Ni base alloys X750 and Hastelloy-N, and non-uniform corrosion took place in steel samples. Energies 2021, 14, x FOR PEER REVIEW 5 of 10 Energies 2021, 14, 543 5 of 10

Figure 3. ScanningScanning electron electron microscope microscope images images of of (a (a) )X750, X750, (b (b) )Hastelloy Hastelloy-N,-N, (c ()c stainless) stainless steel steel 304 304 Energies 2021, 14, x FOR PEER REVIEW(SS304)(SS304), , and and ( (dd)) stainless stainless steel steel 316 316 (SS316) (SS316) after after exposure exposure to to molten molten LiF LiF–NaF–KF–NaF–KF (FLiNaK) (FLiNaK) salt6 salt ofat at10 700 °C◦C for for 100 100 h. h.

For analyses of distribution of selected elements (Ni, Fe, Cr, O) on the surface of corroded alloys, we collected SEM-EDXS maps at multiple regions on the samples. Figure 4 shows the elemental maps of top surfaces. The alloy X750 showed the strong presence of Cr and O on the surface. The Cr was distributed evenly on the surface showing uniform corrosion in this alloy. The layered-like structure observed earlier was confirmed to be chromium oxide. This Cr present on the surface came from the base alloy and diffused to the surface for the duration of the corrosion test. On the surface of Hastelloy-N, similar uniform corrosion was seen with comparatively less Cr. However, both stainless steel samples showed non-uniform corrosion. In SS316, the middle region of the scanned map showed a Cr oxide patch and Fe in the outer region of patches. Similarly, SS304 had remnant protective chromium oxide layer. Fe and Ni were seen around these remnants. From these top surface EDXS scans, it can be concluded that the uniform corrosion occurred in Ni base alloys X750 and Hastelloy-N, and non-uniform corrosion took place in steel samples.

Figure 4. SEMSEM–energy–energy dispersive X-rayX-ray spectroscopyspectroscopy (EDXS)(EDXS) elemental elemental maps maps of of Ni, Ni, Cr, Cr, O, O, and and Fe Fe on onthe the surface surface of alloyof alloy X750, X750, Hastelloy-N, Hastelloy- SS316,N, SS316 and, and SS304 SS304 after after 100 100 h exposure h exposure to molten to molten FLiNaK FLiNaK salt saltat 700 at ◦700C. °C.

For better understanding of corrosion, the polished cross-sections of the samples were scanned using SEM–EDXS, and the elemental maps of different elements were collected for all alloys tested. Figure 5 shows these collected elemental maps for X750 alloy’s cross-section—these scans show the presence of possibly TiAl gamma prime (γ′) particles in the base alloy. The region beneath the outer surface showed mostly Ni and very little Cr, which indicates that Cr had been depleted from this region. In Hastelloy-N (Figure 5), the cross-section showed similar depletion of Cr. Moreover, characteristic Si–Mo clusters were observed in the base of Hastelloy-N. In addition, a very thin layer of nickel was detected on the outer surface and a thick layer of Fe and Cr was also found close to surface. This may be attributed to plating of corrosion products on the surface from other alloys tested. A similar Cr- and Fe-depleted band was also observed in the SS304 sample. A layer of oxide was present close to the surface of this alloy. In the SS316 alloy, depletion of Fe, Cr, and Mn was also noticed. All these elemental maps of cross-sections showed the extent of corrosion in the cross section of alloy. It is possible to conclude from these maps that corrosion resistance was the lowest in alloy X750 and that Hastelloy-N had the highest corrosion resistance.

Energies 2021, 14, 543 6 of 10

For better understanding of corrosion, the polished cross-sections of the samples were scanned using SEM–EDXS, and the elemental maps of different elements were collected for all alloys tested. Figure5 shows these collected elemental maps for X750 alloy’s cross- section—these scans show the presence of possibly TiAl gamma prime (γ0) particles in the base alloy. The region beneath the outer surface showed mostly Ni and very little Cr, which indicates that Cr had been depleted from this region. In Hastelloy-N (Figure5), the cross-section showed similar depletion of Cr. Moreover, characteristic Si–Mo clusters were observed in the base of Hastelloy-N. In addition, a very thin layer of nickel was detected on the outer surface and a thick layer of Fe and Cr was also found close to surface. This may be attributed to plating of corrosion products on the surface from other alloys tested. A similar Cr- and Fe-depleted band was also observed in the SS304 sample. A layer of oxide was present close to the surface of this alloy. In the SS316 alloy, depletion of Fe, Cr, and Mn was also noticed. All these elemental maps of cross-sections showed the extent Energies 2021, 14, x FOR PEER REVIEW of corrosion in the cross section of alloy. It is possible to conclude from these7 of maps 10 that corrosion resistance was the lowest in alloy X750 and that Hastelloy-N had the highest corrosion resistance.

FigureFigure 5. Cross-sections 5. Cross- SEM–EDXSsections SEM elemental–EDXS maps elemental of alloy maps X750, Hastelloy-N,of alloy X750, SS304, Hastelloy and SS316-N, afterSS304 100, and h corrosion SS316 at 700 ◦Cafter in molten 100 h FLiNaK corrosion environment. at 700 °C in molten FLiNaK environment.

To measure the exact depth of corrosion in all alloys the line scans were collected from To measure the exact depth of corrosion in all alloys the line scans were collected the cross-sections of the corroded samples at multiple regions. The depth of corrosion from the cross-sectionswas measured of the ascorroded change in samples composition at multiple of Cr and regions. Fe compared The todepth the base of alloy.corro- Figure sion was measured6 depicts as change the SEM–EDXS in composition line scans of of Cr selected and Fe elements compared (Ni, Fe, to Cr) the collected base alloy. from cross- Figure 6 depicts thesection SEM of–EDXS alloys afterline scans exposure of selected to molten elements FLiNaK salt(Ni, for Fe, 100 Cr) h atcollected 700 ◦C. from From these cross-section of alloyselements, after the exposure corrosion to depthmolten can FLiNaK be measured salt fo asr 100 the h changes at 700 °C. in theFrom Cr, these Fe, and Ni µ elements, the corrosionintensity depth along can the linebe measured scan. The maximum as the changes depth of in corrosion the Cr, Fe of 247, andm Ni was inten- registered for alloy X750 and minimum depth of corrosion of 19 µm was recorded for Hastelloy-N. sity along the lineHowever, scan. The for steelmaximum samples depth SS304 andof corrosion SS316, similar of 247 depths µm of was corrosion registered of 30 µ mfor and 40 alloy X750 and minimumµm, respectively, depth were of corrosion recorded. Therefore, of 19 µm from was these recorded EDXS line for scans, Hastelloy the increasing-N. However, for steeltrend samples of corrosion SS304 was and Hastelloy-N SS316, similar < SS304 depth < SS316s of

Figure 6. SEM–EDXS line scans in cross-sections of alloy X750, Hastelloy-N, SS304, and SS316 after corrosion in molten FLiNaK salt at 700 °C for 100 h.

3.3. XRD and XPS Analyses of Alloys after Corrosion Characterization of the corrosion products on the surface of all alloys was performed using XRD and XPS. Figure 7a shows the XRD spectra of alloys after corrosion in FLiNaK environment. For Hastelloy-N, only Ni and Cr peaks were observed. However, for X750,

Energies 2021, 14, x FOR PEER REVIEW 7 of 10

Figure 5. Cross-sections SEM–EDXS elemental maps of alloy X750, Hastelloy-N, SS304, and SS316 after 100 h corrosion at 700 °C in molten FLiNaK environment.

To measure the exact depth of corrosion in all alloys the line scans were collected from the cross-sections of the corroded samples at multiple regions. The depth of corrosion was measured as change in composition of Cr and Fe compared to the base alloy. Figure 6 depicts the SEM–EDXS line scans of selected elements (Ni, Fe, Cr) collected from cross-section of alloys after exposure to molten FLiNaK salt for 100 h at 700 °C. From these elements, the corrosion depth can be measured as the changes in the Cr, Fe, and Ni inten- sity along the line scan. The maximum depth of corrosion of 247 µm was registered for alloy X750 and minimum depth of corrosion of 19 µm was recorded for Hastelloy-N. However, for steel samples SS304 and SS316, similar depths of corrosion of 30 µm and 40 Energies 2021, 14, 543 µm, respectively, were recorded. Therefore, from these EDXS line scans, the increasing7 of 10 trend of corrosion was Hastelloy-N < SS304 < SS316 < X750.

FigureFigure 6. 6. SEMSEM–EDXS–EDXS line line scans scans in in cross cross-sections-sections of of alloy alloy X750, X750, Hastelloy Hastelloy-N,-N, SS304, SS304, and and SS316 SS316 after after corrosioncorrosion in in molten molten FLiNaK FLiNaK salt salt at at 700 700 °C◦C for for 100 100 h h..

3.3.3.3. XRD XRD and and XPS XPS Analyses Analyses of of Alloys Alloys after after Corrosion Corrosion CharacterizationCharacterization of of the the corrosion corrosion products products on on the the surface surface of of all all alloys alloys was was perform performeded usingusing XRD XRD and and XPS. XPS. Figure Figure 7a7a shows the XRD spectra of alloys after corrosion in FLiNaK environment.environment. For For Hastelloy Hastelloy-N,-N, only only Ni Ni and and Cr Cr peaks peaks were were observed. observed. However, However, for for X750 X750,, additional NiO and Fe2O3 peaks were identified. Both steel samples SS304 and SS316 showed iron oxide and nickel peaks. In XPS survey scans, shown in Figure7b, Ni 2p was identified at binding energy 856.5 eV, Fe 2p at 712.2 eV, and Cr 2p at 577.2 eV, and other peaks of F 1s at 685 eV (metallic fluoride), K 2p at 293 eV, and Cu 2p at 932 eV (doublet separated by 19.75 eV) were also noted. The sources of this Cu were alloys X750 and SS304 with 0.5 wt % and 0.1 wt % of Cu, respectively. The atomic concentration of different elements can be calculated from the XPS survey scans using the following Equation (1):

Cx = (Ix/Sx)/(ΣIi/Si) (1)

where Cx is the atomic concentration of the x element, Ix the peak area of this element x, and S is the relative sensitivity factor (RSF) of this element. CasaXPS software was used for the calculation of atomic concentration, and the results were plotted as bar charts, as shown in Figure7c. The atomic concentration of Cr on surface of alloy X750 was 23.6%, while on Hastelloy-N, only 2.9% was detected. Furthermore, SS304 and SS316 showed 7.0% and 4.1% of Cr, respectively. The atomic concentration of nickel, on the other hand, was minimal in SS304, with only 5.1%, and maximum in Hastelloy-N at 12.2%. Moreover, nickel atomic concentrations for X750 and SS316 were 7.0% and 6.0%, respectively. From these XRD and XPS results, we can conclude that Hastelloy-N was the least corroded and X750 was the most corroded alloy. This was mainly supported by the presence of Cr diffused to the surface from the base alloy. However, the atomic concentration on the surface may not always represent the actual corrosion, since the corrosion products from one alloy can become deposited on another alloy. Moreover, because of plating of the corrosion products from one alloy to another, the weight loss or gain will also not represent the true corrosion comparison between the alloys. However, the EDXS cross-section line scans presented in Energies 2021, 14, x FOR PEER REVIEW 8 of 10

additional NiO and Fe2O3 peaks were identified. Both steel samples SS304 and SS316 showed iron oxide and nickel peaks. In XPS survey scans, shown in Figure 7b, Ni 2p was identified at binding energy 856.5eV, Fe 2p at 712.2 eV, and Cr 2p at 577.2 eV, and other peaks of F 1s at 685 eV (metallic fluoride), K 2p at 293 eV, and Cu 2p at 932 eV (doublet separated by 19.75 eV) were also noted. The sources of this Cu were alloys X750 and SS304 with 0.5 wt % and 0.1 wt % of Cu, respectively. The atomic concentration of different elements can be calculated from the XPS survey scans using the following Equation (1):

Cx = (Ix/Sx)/(ΣIi/Si) (1)

where Cx is the atomic concentration of the x element, Ix the peak area of this element x, and S is the relative sensitivity factor (RSF) of this element. CasaXPS software was used for the calculation of atomic concentration, and the results were plotted as bar charts, as shown in Figure 7c. The atomic concentration of Cr on surface of alloy X750 was 23.6%, while on Hastelloy-N, only 2.9% was detected. Furthermore, SS304 and SS316 showed 7.0% and 4.1% of Cr, respectively. The atomic concentration of nickel, on the other hand, was minimal in SS304, with only 5.1%, and maximum in Hastelloy-N at 12.2%. Moreover, nickel atomic concentrations for X750 and SS316 were 7.0% and 6.0%, respectively. From these XRD and XPS results, we can conclude that Hastelloy-N was the least corroded and X750 was the most corroded alloy. This was mainly supported by the presence of Cr diffused to the surface from the base alloy. However, the atomic concentration on the surface may not always represent the actual corrosion, since the corrosion products from one alloy Energies 2021, 14, 543 can become deposited on another alloy. Moreover, because of plating of the corrosion8 of 10 products from one alloy to another, the weight loss or gain will also not represent the true corrosion comparison between the alloys. However, the EDXS cross-section line scans pre- sentedFigure 6in are Figure the best 6 are way the tobest compare way to thecompare corrosion the corrosion damage in damage the investigated in the investigated alloys in alloysmolten in salt molten environment. salt environment.

Figure 7. a b Figure 7. (a() )XRD XRD spectra spectra of of samples samples after after corrosion corrosion in FLiNaK in FLiNaK environment environment,, (b) XPS ( ) XPS wide wide survey sur- scansvey scans from fromthe surface the surface of alloys of alloysafter corrosion after corrosion test, and test, (c)and bar chart (c) bar comparing chart comparing the atomic the con- atomic centrationsconcentrations calculated calculated from from XPS XPS wide wide survey survey scans scans for Ni for 2p, Ni 2p,Fe 2p Fe, 2p,and and Cr 2p. Cr 2p. 3.4. Microstructural Analysis of Corroded Alloys Using EBSD

For better understanding the corrosion in molten ﬂuoride salts at elevated temperature, we collected EDXS and EBSD scans from the cross-sections of the alloys. Figure8a shows the Cr elemental map for alloy X750 and corresponding EBSD scan. In the EBSD graph, red arrows are pointing at the corrosion initiation sites in the vicinity of outer surface. Similarly, Figure8b depicts the Mo elemental map with the red arrows pointing at the corrosion initiation sites. The white arrow is pointing at the Mo–Si compound in the Hastelloy-N. Figure8c,d both show similar corrosion sites. Interestingly, these corrosion initiation sites were located at the triple junction of the grains. This may be attributed to high elastic energy accumulated at these junctions, which provide sites for Cr transport to the surface and corrosion initiation. The white arrow in Figure8d points at the grain boundary where the corrosion was not started yet. It is clear from these results that the corrosion started ﬁrst at the triple junction, then at the grain boundaries, followed by general corrosion. Energies 2021, 14, x FOR PEER REVIEW 9 of 10

3.4. Microstructural Analysis of Corroded Alloys Using EBSD For better understanding the corrosion in molten fluoride salts at elevated temperature, we collected EDXS and EBSD scans from the cross-sections of the alloys. Figure 8a shows the Cr elemental map for alloy X750 and corresponding EBSD scan. In the EBSD graph, red arrows are pointing at the corrosion initiation sites in the vicinity of outer surface. Similarly, Figure 8b depicts the Mo elemental map with the red arrows pointing at the corrosion initiation sites. The white arrow is pointing at the Mo–Si compound in the Hastelloy-N. Figure 8c,d both show similar corrosion sites. Interestingly, these corrosion initiation sites were located at the triple junction of the grains. This may be attributed to high elastic energy accumulated at these junctions, which provide sites for Cr transport to the surface and corrosion initiation. The white arrow in Figure 8d points at the grain Energies 2021, 14, 543 boundary where the corrosion was not started yet. It is clear from these results that9 of the 10 corrosion started first at the triple junction, then at the grain boundaries, followed by general corrosion.

Figure 8.8. EDXS elementalelemental mapmap alongalong withwith EBSD EBSD maps maps showing showing different different grains grains in in random random colors colors for (fora) X750,(a) X750 (b), Hastelloy-N,(b) Hastelloy (-cN) SS316,, (c) SS316 and, (andd) SS304 (d) SS304 after after corrosion corrosion in molten in molten FLiNaK FLiNaK salt at salt 700 at◦ C700 for 100°C for h. 100 h.

4. Conclusions The corrosioncorrosion teststests of of 100 100 h h were were conducted conducted in in molten molten FLiNaK FLiNaK environment environment at 700at 700◦C for°C for alloy alloy X750, X750, Hastelloy-N, Hastelloy- stainlessN, stainless steel steel 316, 316 and, and stainless stainless 304 in304 argon in argon cover cover gas. Ele-gas. mentalElemental analysis analysis of theof the distribution distribution of corrosionof corrosion products products after after corrosion corrosion and and analysis analysis of EDXS,of EDXS, XRD, XRD and, and XPS XPS results results demonstrated demonstrated that that Hastelloy-N Hastelloy-N performed performed better better thanthan anyany other alloyalloy duringduring corrosioncorrosion tests,tests, withwith thethe corrosioncorrosion depthdepth inin thisthis alloyalloy beingbeing aboutabout 1010 timestimes lowerlower thanthan forfor alloyalloy X750. The corrosion process changed the elemental composition of thethe surfacesurface layer of all investigatedinvestigated alloys,alloys, and poor corrosion performance was corre- latedlated withwith acceleratedaccelerated diffusiondiffusion ofof CrCr toto thethe surfacesurface ofof thethe alloy.alloy. TheThe amountamount ofof CrCr onon thethe surfacesurface ofof X750X750 waswas aboutabout 1010 timestimes higherhigher thanthan onon thethe surfacesurface ofof Hastelloy-N.Hastelloy-N. AnalysisAnalysis ofof corrosioncorrosion nucleationnucleation sitessites allowedallowed usus toto identifyidentify mostmost frequentfrequent corrosioncorrosion sitessites atat thethe tripletriple junctionjunction ofof grains,grains, followedfollowed byby graingrain boundariesboundaries andand generalgeneral corrosion.corrosion. FromFrom thethe weightweight lossloss measurements,measurements, the corrosioncorrosion resistanceresistance ofof alloysalloys testedtested cancan bebe classiﬁedclassified asas X750X750 << SS304 << SS316 < Hastelloy Hastelloy-N.-N.

AuthorAuthor Contributions:Contributions: Conceptualization, K.S K.K.S.;.; methodology, methodology, K.S. K.K.S.;; writing writing—original—original draft prepa- draft preparation,ration, K.S.; writing K.K.S.; writing—review—review and editing, and editing, K.S. and K.K.S. J.S.; supervision, and J.S.; supervision, J.S.; project J.S.; administration, project administra- J.S.; tion,funding J.S.; acquisition, funding acquisition, J.S. All authors J.S. All have authors read have and read agreed and to agreed the published to the published version version of the manu- of the manuscript.script. Funding: This researchresearch receivedreceived nono externalexternal funding.funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors would like to acknowledge the Saskatchewan Structural Sciences Centre (SSSC) and Haynes International for providing the equipment and materials used in this research. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. LeBlanc, D. Molten salt reactors: A new beginning for an old idea. Nucl. Eng. Des. 2010, 240, 1644–1656. [CrossRef] 2. Haubenreich, P. Engel Experience with the Molten-Salt Reactor Experiment. Nucl. Appl. Technol. 1970, 8, 118–136. [CrossRef] 3. He, X.; Song, J.; Tan, J.; Zhang, B.; Xia, H.; He, Z.; Zhou, X.; Zhao, M.; Liu, X.; Xu, L.; et al. SiC coating: An alternative for the protection of nuclear graphite from liquid ﬂuoride salt. J. Nucl. Mater. 2014, 448, 1–3. [CrossRef] Energies 2021, 14, 543 10 of 10

4. Seaborg Technologies Seaborg Wasteburner—Molten Salt Reactor; Seaborg: Copenhagen, Denmark, 2015; White Pap. 2015; SEAB-WP- 2015-001. 5. Generation IV International Forum (GIF) Annual Report; Atomic Energy Commision: Paris, France, 2014. 6. Serrano-ópez, R.; Fradera, J.; Cuesta-López, S. Molten salts database for energy applications. Chem. Eng. Process. Process Intensif. 2013, 73, 87–102. [CrossRef] 7. Sohal, M.S.; Ebner, M.A.; Sabharwall, P.; Sharpe, P. Engineering Database of Liquid Salt Thermophysical and Thermochemical Properties; INL/EXT-10-18297; Idaho Natl. Lab.: Idaho Falls, ID, USA, 2010. [CrossRef] 8. Guo, S.; Zhang, J.; Wu, W.; Zhou, W. Corrosion in the molten fluoride and chloride salts and materials development for nuclear applications. Prog. Mater. Sci. 2018, 97, 448–487. [CrossRef] 9. Ignatiev, V.; Surenkov, A. Material Performance in Molten Salts. In Comprehensive Nuclear Materials; Elsevier: Moscow, Russia, 2012; Volume 5, pp. 221–250. ISBN 9780080560335. 10. Koger, J.W. Fluoride Salt Corrosion and Mass Transfer in High Temperature Dynamic Systems. Corrosion 1973, 29, 115–122. [CrossRef] 11. Ouyang, F.Y.; Chang, C.H.; Kai, J.J. Long-term corrosion behaviors of Hastelloy-N and Hastelloy-B3 in moisture-containing molten FLiNaK salt environments. J. Nucl. Mater. 2014, 446, 81–89. [CrossRef] 12. DeVan, J.H.; Evans, R.B., III. Corrosion Behavior of Reactor Materials in Fluoride Salt Mixtures; U.S. Atomic Energy Commision: Oak Ridge, TN, USA, 1962. 13. Ignat’ev, V.V.; Surenkov, A.I.; Gnidoi, I.P.; Fedulov, V.I.; Uglov, V.S.; Panov, A.V.; Sagaradze, V.V.; Subbotin, V.G.; Toropov, A.D.; Afonichkin, V.K.; et al. Investigation of the corrosion resistance of nickel-based alloys in fluoride melts. At. Energy 2006, 101, 730–738. [CrossRef] 14. Sellers, R.S.; Anderson, M.H.; Sridharan, K.; Allen, T.R. Failure analysis of 316L stainless steel crucible by molten fluoride salt interaction with clay bonded silicon carbide. Eng. Fail. Anal. 2014, 42, 38–44. [CrossRef] 15. Salma, P.; Marecek, M. Corrosion testing of nickel alloy for molten salt reactors. J. Achiev. Mater. Manuf. Eng. 2015, 70, 78–85. 16. Liu, T.; Dong, J.; Xie, G.; Wang, Y.; Li, H.; Li, Z.; Zhou, X.; Lou, L. Corrosion behavior of GH3535 superalloy in FLiNaK molten salt. Jinshu Xuebao/Acta Metall. Sin. 2015, 51, 1059–1066. [CrossRef]