Comparison of Crevice Corrosion of Fe-Based Amorphous

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Comparison of Crevice Corrosion of Fe-Based AmorphousMetal and Crystalline Ni-Cr-Mo Alloy

X. SHAN, H. HA, and J.H. PAYER

The crevice corrosion behaviors of an Fe-based bulk metallic glass alloy (SAM1651) and aNi-Cr-Mo crystalline alloy (C-22) were studied in 4M NaCl solution at 100 C with cyclicpotentiodynamic polarization and constant-potential tests. The corrosion damage morpholo-

gies, corrosion products, and the compositions of corroded surfaces of these two alloys werestudied with optical three-dimensional reconstruction, scanning electron microscopy (SEM),energy dispersive spectroscopy (EDS), and Auger electron spectroscopy (AES). It was foundthat the Fe-based bulk metallic glass (amorphous alloy) SAM1651 had a more positivebreakdown potential and repassivation potential than crystalline alloy C-22 in cyclic poten-tiodynamic polarization tests and required a more positive oxidizing potential to initiate crevicecorrosion in constant-potential tests. Once crevice corrosion initiated, the corrosion propaga-tion of C-22 was more localized near the crevice border compared to SAM1651, and SAM1651repassivated more readily than C-22. The EDS results indicated that the corrosion products ofboth alloys contained a high amount of O and were enriched in Mo and Cr. The AES resultsindicated that a Cr-rich oxide passive film was formed on the surfaces of both alloys, and bothalloys corroded congruently in the crevice corrosion damage areas.

DOI: 10.1007/s11661-008-9697-9 The Minerals, Metals & Materials Society and ASM International 2008

I. INTRODUCTION

IN recent years, amorphous alloys that have largesupercooled liquid regions before crystallization and highresistance against crystallization have been developed,and this has enabled the production of bulk amorphousalloys in the thickness range of 1 to 100 mm by usingvarious casting processes.[14] These bulk amorphous

alloys exhibit a number of useful properties, such as highmechanical strength, high hardness, and, for somecompositions, high corrosion resistance.[2,5] In addition,Fe- and Co-based bulk amorphous alloys also show goodsoft magnetic properties which cannot be obtained fromcrystalline-type magnetic alloys.[2,5] The high boroncontent of the Fe-Cr-Mo-C-Bbased amorphous alloysalso makes them an effective neutron absorber andsuitable for nuclear criticality control applications. Thesebulk amorphous alloys extend their possible applicationsand have attracted much attention as new materials inscientific and engineering fields. Recent reviews coverseveral classes of corrosion-resistant amorphousalloys.[6,7] Several Fe-based bulk amorphous alloys with

highcorrosion resistance have been reported.[711] Panget al.[9,10] reported that Fe-Cr-Mo-C-Bbased amor-phous alloys had a corrosion rate of 10-3 to 10-2 mm/yearin 1, 6, and 12 M HCl solutions at room temperatureand did not suffer pitting corrosion even when the alloyswere polarized anodically up to 1.0 V (Ag/AgCl) in 12 MHCl solution. Farmer et al. reported that one of theFe-Cr-Mo-C-Bbased amorphous alloy SAM1651

(Fe48Cr15Mo14B6C15Y2), also known as SAM7,showed much more resistance to corrosion in aggressiveenvironments, such as 5 M CaCl2 at 105 C, thancrystalline type 316L stainless steel and Ni-Cr-Mo-basedalloy C-22.[12,13] Farmer et al. also reported that thecorrosion resistance of Fe-based amorphous alloySAM295 (Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4) was comparable to that of Ni-Cr-Mobasedcrystalline alloy C-22 in natural seawater at 30 C and90 C.[14] The bulk metallic glass exhibited thermalstability, and the corrosion resistance was maintainedafter prior exposure to temperatures up to the glasstransition temperature (approximately 570 C).[14]

The Fe-based amorphous alloys can be applied asthermal-spray coating. The results of standard salt-fogtests of ASTM B 117 showed that the amorphous alloycoatings were resistant to rusting in salt fog.[15] Unlikestainless steel and nickel-based corrosion-resistant alloy,which lose high corrosion resistance when used as athermal-spray coating, the corrosion resistance of theamorphous alloy thermal-spray coating is almostunchanged compared to the bulk alloy.[13,14] Due to thelow cost of the alloy compared to nickel-based alloys andthe high corrosion resistance, the Fe-based bulk metallicglass alloy is of interest for commercial applications.

X. SHAN, Senior Research Associate, H. HA, Graduate Student,and J.H. PAYER, Professor, are with the Materials Science andEngineering Department, Case Western Reserve University, Cleveland,OH 44106. Contact e-mail: [email protected]

This article is based on a presentation given in the symposiumentitled Iron-Based Amorphous Metals: An Important Family ofHigh-Performance Corrosion-Resistant Materials, which occurredduring the MSandT meeting, September 1620, 2007, in Detroit,Michigan, under the auspices of The American Ceramics Society(ACerS), The Association for Iron and Steel Technology (AIST), ASMInternational, and TMS.

Article published online November 18, 2008

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Alloy C-22 (UNS N06022), a nickel-chromium-molybdenum (Ni-Cr-Mo) crystalline alloy, has highuniform corrosion resistance in oxidizing environmentsand superior corrosion resistance in reducing corrosionmedia. The crystalline alloy C-22 also exhibits excellentresistance to pitting and crevice corrosion in low pH, highchloride oxidizing environments. For both the Fe-basedamorphous alloys and Ni-Cr-Mo crystalline C-22 alloy,the high corrosion resistance is due in large part to theformation of a Cr-rich oxide passive film on the surface.

For alloys that depend on the durability of a passivefilm for corrosion resistance, localized corrosion is animportant degradation mode to be evaluated for corro-sion performance over long exposure periods. Thelocalized corrosion of C-22 has been examined by anumber of researchers.[1625] The objective of this workis to study and compare the localized corrosion behaviorof an Fe-based bulk metallic glass SAM1651 andNi-Cr-Mobased crystalline alloy C-22.

II. MATERIALS AND METHODS

Two materials are compared in this study: Fe-basedbulk metallic glass SAM1651 and Ni-Cr-Mo-basedcrystalline alloy C-22. The compositions of these twoalloys are shown in TableI. The SAM1651 rods with adiameter of 4.2 mm were obtained from Oak RidgeNational Lab (ORNL). They were made by drop castinto a copper mold. The alloy C-22 was obtained fromHaynes International Inc., Kokomo, IN.

For C-22, two types of specimen were used. One was amultiple crevice assembly (MCA) specimen[26] madefrom 2-mm-thick plates and shown in Figure1.A polytetrafluoroethylene (PTFE) tape covered multi-contacts Al2O3ceramic crevice former was used in MCA

crevice corrosion tests. The assembly was tightened witha grade 2 titanium bolt and nut with an applied torqueof 7.91 Nm (70 in-lbs). For comparison with theamorphous alloy, a C-22 cylindrical specimen with a6-mm diameter and a 3- to 4-mm length was also used inthe cyclic potentiodynamic polarization test and theconstant-potential crevice corrosion test. The length ofthe SAM1651 alloy specimen was 4 to 7 mm. For theSAM1651 and C-22 cylindrical specimens, a modifiedcrevice assembly holder was made and is shown inFigure2. The holder was made of grade 2 titanium, anda titanium bolt was used to apply the pressure on thecrevice former and create a crevice gap. The creviceformer was made from 3.2-mm-diameter high purity

Al2O3 ceramic rod and had a length of 2.5 mm. Thecontact surface was ground with 600-grit SiC paper to

obtain a flat surface. In the tests with the modifiedcrevice assembly holder, the surface of the creviceformer that contacted the specimen was covered withPTFE tape. Two crevice formers were used in themodified crevice assembly, and the applied torque was1.0 Nm (9 in-lb). For both the MCA specimen andcylindrical specimen, the test surfaces were ground with600-grit SiC paper and ultrasonically cleaned withmethanol before the crevice corrosion tests.

The test solution was 4M NaCl prepared using

American Chemical Society certified reagent-grade NaCl,and the test temperature was 100 C. The pH value ofthe solution, as measured with pH paper, was four.The standard three-electrode method was used to run thecrevice corrosion test. The counter electrode was rare-earth coated titanium wire and was separated from the

Table I. Nominal Composition of SAM1651 and Mill Composition of C-22

Alloys B C Cr Mo W Y Co Fe Ni

SAM1651 (at. pct) 6 15 15 14 0 2 0 48 0SAM1651 (wt pct) 1.2 3.4 14.9 25.7 0 3.4 0 51.3 0C-22 (at. pct) 0 0 24.7 8.4 0.9 0 2.2 4.2 59.6C-22 (wt pct) 0 0 21 13.1 2.8 0 2.1 3.8 57.2

Fig. 1MCA: (a) dimensions of MCA specimen and (b) specimenassembly.

Fig. 2Modified crevice corrosion test assembly for cylindrical spec-imen; electrical contact lead to sample is not shown.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 40A, JUNE 20091325


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main compartment of the test cell with a frittedglass tube.The reference electrode was a saturated calomel electrode(SCE) which was kept at room temperature and linked tothe test cell througha salt bridge. The test cell was open toair through a water-cooled condenser to prevent thechange of solution concentration due to the evaporation.A Gamry PC4/750 potentiostat (Warminster, PA) wasused for the cyclic potentiodynamic polarization test. Inthe cyclic potentiodynamic polarization test, the solutionwas deaerated with argon, and the open circuit potential

(OCP) of the specimen was monitored. When the changeof the OCP was less than 1 mV/hour, the cyclic potentio-dynamic polarization test was performed. The scanstarted from 50 mV below the OCP and swept towardto a more positive potential. The scan was reversed whenthe current density equaled or exceeded 5 mA/cm2 (basedon the total surface area, which includes the surface areaexposed to solution and the surface area under the crevicecontacts). The scan rate was 0.167 mV/s. The breakdownpotential was determined by the extrapolation of (a)passive current and (b) ascending current, while therepassivation potential was determined by the intersec-tion of reversescanwiththe passive current in the forward

scan. In the constant-potential crevice corrosion tests, thetest solution was in a naturally vented condition. For eachtest condition, the test was repeated two to four times forrepeatability and reliability.

The corrosion-morphology examination and energydispersive spectroscopy (EDS) were performed ona FEI*/PHILIPS** XL-30 SEM. Auger electron

spectroscopy was performed on a Perkin-ElmerPHI680. The corrosion damage depth profiles andthree-dimensional reconstruction were performed withan InfiniteFocus Microscope made by Alicona ImagingGmbH in Grambach/Graz, Austria.

III. RESULTS

A. Cyclic Potentiodynamic Polarization

Figure3 shows the cyclic potentiodynamic polariza-tion results of SAM1651 and C-22 in deaerated 4MNaCl at 100 C assembled with PTFE-tape coveredceramic crevice formers. Both results were obtained onthe creviced cylindrical specimens with an assemblyshown in Figure2. The measured corrosion potentialof SAM1651 was -0.640 to -0.580 V-SCE, and thiswas slightly higher than the corrosion potential ofC-22 tested under the same condition, which had anaverage value of -0.650 V-SCE. The cyclic potentio-dynamic polarization curves of SAM1651 showed a

shoulder feature when the specimen changed frompassive to transpassive in the forward scan. Thisshoulder feature has also been observed on specimenstested without crevice formers by other research-ers.[8,12] The breakdown potential of SAM1651 was+0.340 to +0.410 V-SCE, while the breakdownpotential of C-22 was -0.110 to -0.060 V-SCE. Thebreakdown potential of SAM1651 was approximately400 to 520 mV more positive than that of C-22.During the reverse scan, both alloys exhibited a

hysteresis loop. The repassivation potential ofSAM1651 was +0.190 to +0.280 V-SCE, while therepassivation potential of C-22 was -0.190 to -0.130V-SCE. The repassivation potential of SAM1651 wasnearly 400 mV more positive than that of C-22.Observing a range of values for breakdown andrepassivation potentials for specimens tested withcrevice formers is not unusual and has been reportedby other researchers.[27] This spread in values is largelydue to variation in the locations and extent of crevicecorrosion damage on the creviced specimens. Post-examination showed that crevice corrosion formedunder the crevice contacts for both alloys, and this

confirmed that the hysteresis loop formed during thereverse scan, which is an indication of the formationof localized corrosion. The cyclic potentiodynamicpolarization results indicate that SAM1651 has con-siderably higher localized corrosion resistance thanC-22 at the more oxidizing potential range.

B. Constant-Potential Crevice Corrosion Test

Two potentials were used in the constant-potentialtests: -0.150 V-SCE and +0.150 V-SCE. These poten-tials were selected based upon the cyclic potentiody-namic polarization curve for MCA C-22.[19,24] The-0.150 V-SCE potential was 30 to 50 mV above the

repassivation potential of MCA C-22 specimen in 4MNaCl 100 C solution. The +0.150 V-SCE potential wasselected to be even more aggressive. The latter was 330to 350 mV above the repassivation potential andapproximately 90 mV below the breakdown potentialof MCA C-22 specimen in 4M NaCl 100 C solution.Matching potentials were used for the SAM1651 testsfor direct comparison. Figure4 shows the current vstime relationship in the -0.150 V-SCE constantpotential crevice corrosion test of SAM1651 and C-22in 4M NaCl 100 C solution with PTFE-tape coveredceramic crevice formers. Both alloys were tested withcylindrical specimens and the same size crevice formers.During the 28-day tests, the corrosion current ofSAM1651 remained at less than 0.1 lA, and no indica-tion of initiation of crevice corrosion was observed.Post-test examination of the SAM1651 specimen alsoconfirmed that no corrosion initiated on either the alloysurface exposed to solution directly or on the alloysurface under the crevice formers. The SAM1651specimen surface remained shiny metallic as before thetest.

For C-22, crevice corrosion initiated, and four crevicecorrosion development stages can be identified from thecurrent vs time curve: initiation, propagation, stifling,

*FEI is a trademark of FEI Company, Hillsboro, OR.

**PHILIPS is a trademark of Philips Electronic Instruments Corp.,Mahwah, NJ.



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and arrest. After the anodic potential of-0.150 V-SCEwas applied, crevice corrosion of the C-22 specimeninitiated in less than 5 minutes as indicated by the sharpincrease of the corrosion current, and the minimumcurrent before the initiation of crevice corrosion was0.75 to 1.1 lA. After the initiation of the crevicecorrosion, the overall corrosion current increased withincreasing test time, and the maximum corrosion currentof 30 lA was reached after 500 hours of exposure.During the propagation period, the corrosion current ofC-22 showed multiple onset/arrest events as indicated bycurrent rise/drop serrations along the curve. After580 hours of testing, the corrosion current of C-22dropped to 0.07 lA and showed an overall tendency todecrease with increasing test time. This indicates that theC-22 specimen became repassivated. The repassivation

behavior of C-22 afterextended corrosion has also beenreported elsewhere.[24,28]

With increasing the test potential to +0.150 V-SCE,crevice corrosion initiated on the SAM1651 specimen.As with C-22, four stages of damage evolution areindicated in the corrosion process of SAM1651: initia-tion, propagation, stifling, and arrest. Figure5 showsthe current vs time curve for one of the SAM1651specimens tested under +0.150 V-SCE in 4M NaCl100 C solution for 28 days. During the test, themaximum corrosion current was approximately 15 lA.For another SAM1651 specimen tested under the samecondition, the maximum corrosion current was 0.5 lAduring the propagation period of crevice corrosion.During the 28 days of testing, the total amount ofcharge to the SAM1651 specimens was in the range of

Fig. 3Cyclic potentiodynamic polarization curve of (a) cylindrical SAM1651 specimen and (b) cylindrical C-22 specimen in 4M NaCl 100 Csolution, deaerated.

Fig. 4Constant-potential crevice tests with (a) cylindrical SAM1651 and (b) cylindrical C-22 in 4M NaCl 100C solution (E= -0.150V-SCE). Note the current for cylindrical SAM1651 remained in the passive range throughout the test, i.e., I


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0.4 to 1.77 coulombs, compared to 30 coulombs for theC-22 specimen tested under -0.150 V-SCE. The

SAM1651 tested under the more severe oxidizingcondition exhibited a corrosion rate significantly lowerthan that of C-22. After testing for 600 hours, corrosioncurrent of SAM1651 dropped to below 0.1 lA as theSAM1651 repassivated. The constant-potential testresults indicate that SAM1651 has higher localizedcorrosion resistance than C-22 at more oxidizing con-ditions. For SAM1651, a higher oxidizing potential wasrequired to initiate crevice corrosion, and when localizedcorrosion initiated, SAM1651 repassivated more readilythan C-22.

C. Corrosion Morphology

Figures6(a) and (b) show the optical corrosionmorphologies of cylindrical SAM1651 and MCA C-22after similar amounts of corrosion. TableII quantita-tively summarizes the corrosion damages of theSAM1651 and C-22 specimens. In Figure6(a),the corrosion products of SAM1651 remained on thesurface, while most of the corrosion products onthe C-22 specimen in Figure6(b) had been removedbefore the photograph was taken. For SAM1651 inFigure6(a), the specimen was tested at +0.150V-SCE in4M NaCl 100 C solution for 211 hours, and the totalamount of charge to the specimen was 0.77 coulombs.Because there were two crevice formers with the

specimen and no corrosion on the surface exposed tothe test solution directly, the average amount of chargeto each corroded crevice contact was 0.39 coulombs.For the C-22 specimen in Figure6(b), the test wasperformed on MCA specimen at -0.150 V-SCE in 4MNaCl 100 C solution for 90.5 hours, and the amount ofcharge to the specimen during the test was 1.0 coulomb.Because only one contact from the 12 crevice contacts ofthe crevice former was corroded during the test period,the amount of charge to the corroded contact of theMCA C-22 specimen was essentially equal to the charge

to the whole specimen. The crevice corrosion ofSAM1651 initiated at locations under the crevice formerand approximately 0.2 mm from the edge of the crevice

Fig. 5Constant-potential crevice test with cylindrical SAM1651 in 4M NaCl 100 C solution (E =+0.150 V-SCE): (a) entire time (0 to 672 h)and (b) later stage after repassivation (480 to 672 h).

(a)

(b)

(c)

un-corroded area

corroded area

corroded area

un-corroded island

corroded area

un-corroded island

un-corroded area

corroded area

crevice border

Fig. 6Corrosion morphology of (a) cylindrical SAM1651,E = +0.150 V-SCE, Qtotal = 0.77 coulomb; (b) MCA C-22, E =-0.150 V-SCE, Qtotal = 1.0 coulomb; and (c) cylindrical C-22,E = -0.150 V-SCE, Qtotal = 30 coulombs after crevice corrosiontests in 4M NaCl, 100 C.



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contact. For C-22, the crevice corrosion also initiatedunder the crevice former but nearer the edge of thecrevice contact.

For each alloy, the crevice corrosion propagatedboth along the crevice and into the alloy. However,the corrosion penetration rate of C-22 into the alloywas greater than that of SAM1651, and the corrosiondamage distribution of C-22 was more localized. Atthe end of the crevice corrosion test that was stoppedafter 1.0 coulomb, the center region of a C-22 MCAcrevice contact was not corroded, as shown inFigure6(b). The corroded area on the C-22 specimenhad a width of approximately 0.64 mm, a surface areaof 4.8 mm2, and the maximum corrosion penetrationinto the alloy was 25lm located at approximately0.1 mm from the edge of the crevice contact. For theSAM1651 cylindrical specimen after 0.77 coulombs of

corrosion, except the outer 0.2-mm border region, allthe other surfaces under the crevice contacts werecorroded, including the center region. The corrodedarea of SAM1651 had a radius of 1.3 to 1.4 mm andsurface area of approximately 10 mm2 (sum of twocrevice contacts), and the corrosion -penetration depthinto the alloy was less than 5 lm. For a similaramount of corrosion, the crevice corrosion ofSAM1651 propagated further inward along the creviceand caused a larger corroded area, while the maxi-mum corrosion penetration into the SAM1651 alloy

was more than five times less than that of C-22.Compared to SAM1651, the crevice corrosion propa-gation of C-22 was more localized and nearer the

crevice edge.The more localized crevice corrosion of C-22 was alsoobserved on the cylindrical C-22 specimen tested withthe same crevice former as for SAM1651. Figure6(c)shows the corrosion morphology of a cylindrical C-22specimen; the current vs time curve of this specimen isshown in Figure4(b). The total amount of charge was30 coulombs, or 15 coulombs on average from the twocrevice contacts. As shown in Figure 6(c), in the centerregion of the C-22 crevice contact, there was anuncorroded island, even though the amount ofcorrosion with the cylindrical C-22 was nearly 40 timeshigher than that of the SAM1651 specimen shown inFigure6(a).

Figure7 shows higher magnification of the surfacemorphology of the corroded area of cylindricalSAM1651 and MCA C-22 shown in Figures 6(a) and (b).For SAM1651, the corrosion morphology consisted ofmany larger pits with a diameter of several microns andsmaller pits with a submicron diameter, as shown inFigures8(a) and (b). Spherical particles were observedon the surface of SAM1651, as shown in Figure 8(b).Auger electron spectroscopy analysis showed that theseparticles contained high concentrations of Y and O inthe ratio of Y2O3. It is likely that these oxide particles

Table II. Corrosion Damage of SAM1651 and C-22 in 4M NaCl, 100 C Solution

SpecimenDuration

(h)Qtotal

(Coulombs)

Crevice Contacts,Number/Total Area

(mm2)Corroded

Area (mm2)

MaximumPenetration,

Location (lm)

SAM1651, cylindrical 211 0.77 2/15.1 ~10 200 lm)C-22, MCA 90.5 1.0 1/6.0 4.8 25 (~100 lm)C-22, cylindrical 672 30.0 2/15.1 9.4 485 (0 to 300 lm)

Notes:1. SAM1651 was tested under +0.150 V-SCE; C-22 was tested under -0.150 V-SCE.

2. There is an uncorroded ring under the crevice contacts of SAM1651, and the width of the ring is approximately 0.2 mm.3. No uncorroded island on SAM1651 specimen; uncorroded islands were found on both C-22 specimens.4. The location of the maximum penetration is the distance to the crevice-contact border.

Fig. 7Corrosion morphologies of (a) cylindrical SAM1651, E = +0.150 V-SCE, Qtotal = 0.77 coulomb; and (b) MCA C-22, E = -0.150V-SCE, Qtotal = 1.0 coulomb after crevice corrosion tests in 4M NaCl, 100 C.



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were present in the melt prior to solidification of thealloy. During the corrosion process, the substratearound the particles was corroded, and the larger pitmorphology formed. The surfaces within the larger pitswere composed with the submicron pits as on theremainder of the surface. The submicron pits were thetypical corrosion morphology of the amorphous alloy.

For C-22, a characteristic of the corroded surface isthat the grain boundaries were discernible, as shown inFigure7(b), and this was categorized as type-I corrosionmorphology of C-22 by Rebak.[27] The depth of thegrain-boundary penetration of C-22 was slight. Anothercharacteristic of the corroded surface of C-22 is that thecorroded surface was composed with many shallow pitswith the diameter of several microns or less, and thesurface of the pits was smooth. This type of C-22corrosion morphology was categorized as type-II cor-rosion morphology by Rebak.[27] Both types of corro-sion morphologies of C-22 are related to its crystallinestructure.[27]

D. Corrosion Products

After crevice corrosion tests where crevice corrosionoccurred, black corrosion products were found underthe crevice contacts for both SAM1651 and C-22specimens. Figures9 and 10 show the morphologiesand compositions of the corrosion products of cylindri-cal SAM1651 and MCA C-22, respectively. The smoothsurfaces of the corrosion products shown were in

contact with the PTFE-tape covered crevice former.The corrosion products of these two alloys were similarboth in morphology and composition and both showedsimilar crack patterns through the corrosion products.For both alloys, the corrosion products contained a highamount of O and were enriched in Mo and Cr. Thecorrosion products of C-22 were also enriched in W. ForSAM1651, the corrosion products were depleted in Fe,while for C-22, the corrosion products were depleted inNi. During crevice corrosion, the environment inside thecrevice becomes highly acidic from hydrolysis of metal

Fig. 8Higher-magnification corrosion morphology of cylindrical SAM1651 after crevice test: (a) corroded surface and (b) corroded surface andinclusion.

Fig. 9SEM and EDS analysis of cylindrical SAM1651 corrosion products under crevice former.



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cations.[29,30] The pH of the bulk solution was four,while the local environment of the corroding creviceregion can be more severe. The pH in thecrevice can bemuch lower and even lower than zero.[18,31] Based uponE-pH (Pourbaix) diagrams, W, Mo, and Cr can formstable oxides in highly acidic environments, while Feand Ni form soluble corrosion products in highly acidicenvironments. The former elements are primarilyretained within the corroded crevice, while the latterelements diffuse/migrate out of the crevice and into thebulk test solution. More detailed analysis ofthe corro-sion products of C-22 has been presented.[25]

E. Element Distribution of the Corroded Surface

Figure11 shows the AES composition depth profileon the corroded surface of cylindrical SAM1651 and

MCA C-22 under the crevice contacts. The analyseswere in areas of crevice corrosion, and the looselyadhered, bulk corrosion products had been removedfrom the surface to expose the repassivated metal. Forboth alloys, high amounts of oxygen were found in theouter layers with nickel depletion from C-22 and irondepletion from SAM1651 surfaces, respectively. Theanalysis indicates a Cr-rich oxide layer (passive film) wasformed on the surfaces. However, the compositions ofthe passive films of these two alloys are different. Byconsidering the three main elements, Ni (Fe forSAM1651), Cr, and Mo, the normalized concentrations(Cr pct + Mo pct + Ni(Fe) pct = 100) of these threeelements are shown in Figure12. For SA1651, on thetop surface, the normalized Mo concentration is 32 at.pct, and the Mo/Cr ratio is 3.2. The normalized Moconcentration forms a small depletion region at the

Fig. 10SEM and EDS analysis of MCA C-22 corrosion products under crevice former.

Fig. 11AES composition depth profile of corroded (a) cylindrical SAM1651 and (b) MCA C-22 surfaces.



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depth of 1.6 nm with the normalized concentration of

21.6 at. pct. The normalized Mo concentration decreasesto the bulk concentration, which is 20 at. pct, after8 nm. The normalized Cr concentration on the topsurface of SAM1651 is 10 at. pct, and it showsenrichment at 1.6 nm with a value of 26 pct. After6.4 nm, the normalized Cr concentration of SAM1651decreases to the bulk concentration, which is 17.5 at.pct. No significant Cr depletion region was found belowthe top surface. The normalized Fe concentration on thetop surface of SAM1651 is 58.2 at. pct, and it shows aslight depletion at the depth of 1.6 nm with a value of52.2 at. pct. At 8 nm, the normalized Fe concentrationof SAM1651 increases to the bulk concentration, whichis 62.5 at. pct. No significant Fe-concentration enriched

region was found on SAM1651 under the top surface.For C-22, the normalized Mo concentration on the

top surface is 26.6 at. pct, and the Mo/Cr ratio is 1.0,and these values are lower than that of SAM1651,especially the Mo/Cr ratio which is more than threetimes less than that of SAM1651. The normalized Moconcentration decreases to the lowest value of 9.6 pctafter 3.6 nm, and at this depth, the Mo/Cr ratio is 0.35.After 14 nm, the normalized Mo concentration in-creases to near the bulk concentration, which has anormalized concentration of 16 at. pct. The depth atwhich Mo concentration reaches the bulk concentrationin alloy C-22 is 50 pct deeper than that of SAM1651.

The normalized Cr concentration on the top surface ofC-22 is 27.5 at. pct, and it increases to 42.3 at. pct at thedepth of 1.2 nm and then starts to decrease. The Crforms a slightly depleted region at the depth of 7 nm,and then the normalized Cr concentration starts toincrease and reaches the bulk concentration at the depthof approximately 19 nm with a value of 26 at. pct. Thenormalized Ni concentration on the top surface of C-22is 46 at. pct, and it decreases to 43.3 at. pct after 1.2 nm.Then the concentration of Ni starts to increase andforms an enriched region at a depth of 7 nm. After

7 nm, the Ni concentration starts to decrease and

reaches the bulk concentration at approximately 22 nm.From the elemental composition profiles, the passive-film thickness on corroded SAM1651 surface and thecorroded C-22 surface are estimated to be approxi-mately 3 to 5 nm. The high corrosion resistance of thesetwo alloys is due a chromium-rich oxide passive filmformed on the surface. The high amount of Mo in thepassive film of SAM1651 is one important reason for theimproved localized corrosion resistance of SAM1651. Itis also shown in Figure 11 that the composition of thebulk composition is observed in less than 20 nm beneaththe oxide layer for both alloys. This indicates that bothalloys were corroded congruently under the crevice,i.e.,all of the elements in the alloy reacted at the same

atomic percentages as the bulk alloy. Some elementswere incorporated in solid products that remainedwithin the crevice, while others were transported outof the crevice. More detailed analysis on the corrodedsurface and corrosion products of C-22 after crevicecorrosion was published elsewhere.[25] The detailedanalyses on the composition profiles of the SAM1651surfaces after grinding and after different amounts ofcorrosion will be published in the future.

IV. CONCLUSIONS

The crevice corrosion behavior of Fe-based bulk

metallic glass SAM1651 and crystalline Ni-Cr-Mo alloyC-22 are compared in high temperature (100 C),concentrated brine (4M NaCl) by cyclic potentiody-namic polarization, and constant-potential crevice cor-rosion tests as follows.

1. SAM1651 has a more positive breakdown potentialand repassivation potential than C-22.

2. A more positive oxidizing potential was requiredto initiate crevice corrosion on SAM1651 than onC-22.

Fig. 12Normalized AES composition depth profile of corroded ( a) cylindrical SAM1651 and (b) MCA C-22 surfaces.



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3. Once the crevice corrosion is initiated, SAM1651repassivated more readily than C-22.

4. The crevice corrosion propagation of C-22 is morelocalized compared to SAM1651, i.e., the corrosionwas nearer the crevice border and deeper.

5. The corrosion products of two alloys have similarmorphology. Both have a high amount of O andare enriched in Mo and Cr, and C-22 corrosionproducts are also enriched in W. For SAM1651, thecorrosion products are depleted in Fe, while for

C-22 the corrosion products are depleted in Ni.6. A chromium-rich oxide, passive film is observed on

the surfaces of both alloys in corroded regions thathad repassivated.

7. In regions where crevice corrosion was observed,both alloys are corroded congruently, i.e., the bulkalloy composition is reached after 15 to 20 nm intothe alloy.

ACKNOWLEDGMENTS

The authors thank the Science and TechnologyProgram of the Office of the Chief Scientist (OCS),Office of Civilian Radioactive Waste Management(OCRWM), and the United States Department ofEnergy (DOE) for support. The work was performedunder the Corrosion and Materials Performance Coop-erative, DOE Cooperative Agreement No. DE-FC28-04RW12252. The views and opinions of the authorsexpressed herein do not necessarily state or reflect thoseof the United States Department of Energy. Theauthors also acknowledge the support of the DefenseScience Office (DSO) of the Defense AdvancedResearch Projects Agency (DARPA). Approved for

Public Release, Distribution Unlimited.

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Comparison of Crevice Corrosion of Fe-Based Amorphous