Corrosion Behavior of Alloy 600 Coupled with Electrodeposited Magnetitein Simulated Secondary Water of PWRs

Soon-Hyeok Jeon, Geun-Dong Song and Do Haeng Hur+

Nuclear Materials Safety Research Division, Korea Atomic Energy Research Institute,989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea

The corrosion behavior of Alloy 600 coupled with magnetite was investigated in simulated secondary water of pressurized water reactorsusing a potentiodynamic polarization test and zero-resistance ammeter. Passive film formed on the surface of Alloy 600 was also analyzed usingX-ray photoelectron spectroscopy. Alloy 600 was the anodic element of the galvanic pair since its corrosion potential was less noble than that ofthe magnetite. Galvanic coupling increased the corrosion current density of Alloy 600 due to the shifting of the potential of Alloy 600 to thepositive value. The passive film of coupled Alloy 600 was more slowly stabilized and was thinner and less protective than that of non-coupledAlloy 600. [doi:10.2320/matertrans.M2015322]

(Received August 11, 2015; Accepted October 5, 2015; Published November 13, 2015)

Keywords: Alloy 600, magnetite, galvanic corrosion, passive film, electrodeposition

1. Introduction

Magnetite is released from the surface of carbon steel incontact with cooling water of a secondary circuit systemin the pressurized water reactors (PWRs). The magnetiteparticles are transported into a steam generator, accumulatedon the top of tube sheet and deposited on the surface of steamgenerator tubes and their support structures. The magnetiteadhering to the steam generator tubes reduces heat transferfrom the primary to secondary side and causes anacceleration of the corrosion of the steam generator tubes.1­3)

There are many investigations on the effect of magnetiteon the galvanic corrosion of carbon steel.4­8) Fushimiet al.4­6) claimed that magnetite accelerated the corrosion ofcarbon steel with a galvanic coupling. The acceleration wasdue to the generation of hydrogen on the magnetite as well asthe reduction of the magnetite itself in sulfate solution.7) Jeonet al.8) reported that the corrosion rate of carbon steel coupledto magnetite increased by about 3.5 times than that of non-coupled carbon steel when they are contacted to theequivalent area ratio under alkalized reducing conditionsof pH 9.5 which is a typical secondary water chemistry ofPWRs.

Magnetite having a porous structure is deposited on the Ni-based alloys tube in operating steam generators as well as thecarbon steel piping.9,10) Therefore, the surface of the Alloy600 tube is in electrically contact with the porous magnetitedeposits. However, there are no researches regarding thegalvanic corrosion behavior between Alloy 600 and magnet-ite in the simulated secondary water of PWRs. Suchresearches are very important because magnetite particlesare deposited on the surface of steam generator Alloy 600tubes in the operation conditions.

In order to elucidate the corrosion behavior of Alloy 600coupled with magnetite, it is necessary to use the dense andadhesive magnetite specimens. Electrodeposition is onemethod to produce the magnetite layer on an Alloy 600substrate. Magnetite can be formed by a chemical reaction

after the oxidation of Fe(II)11­13) or reduction of Fe(III)14,15)

on the electrode surface. Recently, Kothari et al.16) haveshown that magnetite is easily produced by electrodepositionon the stainless steel substrate in Fe(III)-triethanolamine(TEA) solution at 333­363K.

In this study, the magnetite was electrodeposited on thesurface of Alloy 600 substrate by electrodeposition in theFe(III)-TEA solution for simulating magnetite deposited onthe outer diameter surface of Alloy 600 steam generatortubing. Using the magnetite specimens produced by electro-deposition, the galvanic corrosion behavior and passivationcharacteristics of Alloy 600 coupled with magnetite wasinvestigated under simulated wet layup conditions.

2. Experimental

2.1 Material preparationAlloy 600 specimens for magnetite electrodeposition were

machined into 10 © 5 © 1mm. Alloy 600 substrates weremechanically ground up to #1000 grit silicon carbide paperand then ultrasonically cleaned in acetone and deionizedwater. The chemical compositions of Alloy 600 werepresented in Table 1.

2.2 Electrodeposition of the magnetiteMagnetite films were electrodeposited in an alkaline

solution of Fe2(SO4)3 complexed with TEA. The concen-trations in the deposition bath were 0.09M Fe2(SO4)3, 0.1MTEA, and 2M NaOH. The solution was prepared by addingFe2(SO4)3 dissolved in water to a solution of NaOH and TEAunder stirring. The resulting gray-green solution was heatedto 353K and stirred at 200 rpm. The electrodeposition of themagnetite on the Alloy 600 substrate was conducted using a

Table 1 Chemical compositions of the Alloy 600 (mass%).

C Cr Fe Si Mn Ti Al Ni

0.02 15.7 10.0 0.1 0.3 0.1 0.06 Bal.

+Corresponding author, E-mail: dhhur@kaeri.re.kr

Materials Transactions, Vol. 56, No. 12 (2015) pp. 2078 to 2083©2015 The Japan Institute of Metals and Materials EXPRESS REGULAR ARTICLE

PAR273 potentiostat (EG&G) with Power suite software anda three-electrode cell. A saturated calomel electrode (SCE)was used as a reference electrode and a platinum wire wasused as a counter electrode. Magnetite was electrodepositedin the deposition solution at an applied potential of ¹1.05VSCE for 1800 s at 353K.

After electrodeposition, the magnetite specimens werecarefully rinsed with deionized water and dried in an oven for5min at 333K. The morphology of electrodeposited magnet-ite layer was analyzed using scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS) using aQUANTA 3D FEG FIB-SEM. The phase composition ofmagnetite layer was determined by X-ray diffraction (XRD)with a Rigaku D/Max-2500 diffractometer (Cu-K¡ radia-tion). The electrodeposited magnetite on Alloy 600 substratewas also milled by a focused ion beam (FIB) toward avertical direction of the magnetite using a QUANTA 3D FEGFIB-SEM.

2.3 Corrosion testsTwo different electrochemical techniques were used to

study galvanic corrosion in simulated secondary water ofPWRs: potentiodynamic polarization test and ZRA measure-ments. The potentiodynamic polarization tests were carriedout using a PAR273 potentiostat (EG&G) with Power suitesoftware. ZRA test was performed using Gamry Potentiostat/galvanostat/ZRA (Reference 600) embedded with Gamryframework system. The potentiodynamic polarization testwas conducted in deaerated aqueous solution at 298K.The pH of the test solution was adjusted to be 9.5 withethanolamine (ETA), which is used as a major organicreagent to control the pH of secondary water in PWRs. Thistest environment was designed to simulate wet layupconditions in a steam generator. The detailed parameters forwet layup are given elsewhere.17) The solution was deaeratedby blowing a high purity nitrogen gas at a rate of600 cm3·min¹1 during the tests. A saturated calomel electrode(SCE) and a platinum wire were used as the reference andcounter electrode, respectively. The exposed surface ofmagnetite and Alloy 600 specimens was 1.3 cm2. After theopen circuit potential (OCP) was stabilized, polarization scanwas started from the OCP to the cathodic or anodic directionwith a scan rate of 1 © 10¹3 V·s¹1. The corrosion currentdensity (icorr) of Alloy 600 and magnetite was calculated bythe Tafel extrapolation of the cathodic and anodic curvebetween 50 and 100mV away from the corrosion potential(Ecorr). The galvanic potential (Ecouple) and the galvaniccurrent density (icouple) of the pair were calculated from thepotentiodynamic polarization curves by the mixed potentialtheory.

The galvanic corrosion behavior between Alloy 600 andmagnetite was also measured in the same test solution usingthe Reference 600 instrument potentiostat in ZRA mode. Forgalvanic coupling measurements, the exposed surface ofmagnetite and Alloy 600 specimens was 1.3 cm2. The arearatio between magnetite and Alloy 600 specimens are theequivalent area ratio (1 : 1). Alloy 600 specimen wasconnected to the working electrode and the magnetitespecimen was connected to another working electrode. Afterthe open circuit potential (OCP) was stabilized, the speci-

mens were electrically connected though the ZRA. Thevariation of galvanic current density of non-coupled andcoupled Alloy 600 with the magnetite was measured during1800 s. All electrochemical tests were conducted at least threetimes per specimen for reproducibility and reliability.

2.4 XPS surface analysisTo elucidate the passivation behavior of non-coupled and

coupled Alloy 600 with magnetite, magnetite was electro-deposited on the half surface of Alloy 600 substrate formaking the coupled Alloy 600 with magnetite contacted tothe equivalent area ratio (1 : 1). The films of the non-coupledand coupled Alloy 600 with the magnetite were formed bythe immersion test in the alkaline solution at 298K for 15days. The depth profile and chemical species of the filmsformed on Alloy 600 specimens were analyzed using XPS.The XPS analysis was carried out using a ThermoFisherScientific (Theta Probe AR-XPS) X-ray photoelectronspectrometer with an Al K¡ X-ray source (1486.6 eV)operated at 15 kV and 150W under a base pressure of2.7 © 10¹7 Pa. The binding energies of all peaks werecorrected by the reference C1s peak at 284.5 eV.

3. Results and Discussion

3.1 Electrodeposition of the magnetiteFigure 1 shows the linear sweep voltammogram of Alloy

600 substrate in the Fe(III)-TEA bath at 353K. The potentialwas swept from the open circuit potential to ¹1.30VSCE ata scan rate of 50mV·s¹1. The electrochemical reduction ofFe(III)-TEA was started at potentials more negative than¹0.95VSCE. The first reduction wave of this linear sweep wasmeasured approximately between ¹0.95 and ¹1.20VSCE.The second reduction wave was also observed in the potentialrange from ¹1.20 to ¹1.30VSCE.

The electrodeposition of magnetite film in a Fe(III)-TEAsolution can be simplified by two steps. The first step is thatthe Fe(III)-TEA solution is electrochemically reduced to Fe2+

and TEA. The second step is that the electrochemicallyproduced Fe2+ reacts chemically with Fe(III)-TEA solutionto produce magnetite film. The proposed mechanism isexpressed in the following reactions (1) and (2).16,18­20)

-1.30 -1.25 -1.20 -1.15 -1.10 -1.05 -1.00 -0.95 -0.90 -0.85 -0.80-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

Cur

rent

Den

sity

, I /

A c

m-2

Potential, E / V vs. SCE

OCP

Fig. 1 Linear sweep voltammogram of Alloy 600 substrate in Fe(III)-TEAsolution at 353K.

Corrosion Behavior of Alloy 600 Coupled with Electrodeposited Magnetite in Simulated Secondary Water of PWRs 2079

FeðTEAÞ3þ þ e� ! Fe2þ þ TEA ð1ÞFe2þ þ 2FeðTEAÞ3þ þ 8OH�

! Fe3O4 þ 2TEAþ 4H2O ð2ÞTherefore, the first reduction wave observed between ¹0.95and ¹1.20VSCE can be attributed to a one electron reactioncorresponding to the above reaction (1).16,18) On the contrary,the second reduction wave between ¹1.20 and ¹1.30VSCE

seems to be due to a two-electron process corresponding tothe reduction of Fe(II) to Fe.18­20)

In this work, the magnetite was electrodeposited on Alloy600 substrate at the potential of ¹1.05VSCE (first reductionwave region) for 1800 s. Figure 2 presents a SEM-EDS andXRD analysis of the magnetite electrodeposited on Alloy600 substrate. The magnetite has highly faceted and densemorphologies. This morphology is homogeneous on thewhole surface of the deposited magnetite specimen(Fig. 2(a)). The XRD pattern of the magnetite electro-deposited on Alloy 600 substrate shows that the magnetiteis all crystalline, and only has peaks corresponding tomagnetite (Fig. 2(b)).

Figure 3 shows the SEM image of cross sectional electro-deposited magnetite films on Alloy 600 substrate. Thethickness of the magnetite films was ranged from 1.3 to2.2 µm, with an average of 1.8 µm. Adherent and densemagnetite deposited on Alloy 600 substrate is shown. Nocrack or hole can be observed at the interface betweenmagnetite film and Alloy 600 substrate, which confirms thatmagnetite film tightly bonded to Alloy 600 substrate.

3.2 Corrosion behavior of Alloy 600 coupled with themagnetite

Figure 4 shows the potentiodynamic polarization curves ofAlloy 600 and the magnetite in the alkaline solution at 298K.The icorr of Alloy 600 and the magnetite at its OCP wascalculated from the potentiodynamic polarization curves bymeans of the Tafel extrapolation method. The Ecouple andicouple of the pair were also calculated by means of the mixedpotential theory. These electrochemical corrosion parametersare summarized in Table 2. When Alloy 600 and themagnetite are electrically contacted, Alloy 600 is the anodicelement of the pair because the Ecorr of the magnetite is higher

than that of Alloy 600. As shown in Table 2, the icorr ofAlloy 600 was determined to be 1.05 µA/cm2 by the Tafelextrapolation method and the icouple of the pair was measuredto be 1.67 µA/cm2 by the mixed potential theory. Therefore,

(a) (b)

28 32 36 40 44 48 52 56 600

500

1000

1500

2000

2500

Inte

nsity

(CPS

)

2θ (degree)

(220)

(311)

(222)

(400)(422)

(511)

3 μm

Fig. 2 SEM image and XRD analysis of magnetite electrodeposited on the Alloy 600 substrate at the applied potential of ¹1.05VSCE inFe(III)-TEA solution at 353K: (a) SEM image of magnetite surface and (b) XRD analysis.

2 μm

Magnetite

Alloy 600 substrate

Pt coating

Fig. 3 SEM image of cross section of magnetite electrodeposited on Alloy600 substrate at the applied potential of ¹1.05VSCE in Fe(III)-TEAsolution at 353K.

10-9 10-8 10-7 10-6 10-5 10-4-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

Pote

ntia

l, E

/ V v

s. S

CE

Current Density, I / Acm-2

Alloy 600 Magnetite

Fig. 4 Potentiodynamic polarization curves of Alloy 600 and magnetite inthe alkaline solution at 298K.

S.-H. Jeon, G.-D. Song and D. H. Hur2080

the corrosion rate of galvanic coupled Alloy 600 will increaseby about 1.6 times than that of non-coupled Alloy 600 whenthey are galvanically contacted to the equivalent area ratio(1 : 1). Consequently, it is expected that the galvaniccoupling with the magnetite accelerates the corrosion rateof Alloy 600 based on the mixed potential theory.

Figure 5 shows the galvanic current density variation ofnon-coupled and coupled Alloy 600 with magnetite in thealkaline solution at 298K, which was measured using ZRA.In the Alloy 600/magnetite couple, the galvanic currentdensity of Alloy 600 specimen was the positive valuescorresponding to the anodic reaction. This indicates thatAlloy 600 specimen is the anode of the couple, like the result

predicted in Fig. 4. The corrosion current density of coupledAlloy 600 was also higher than that of non-coupled Alloy600. The above results obtained from ZRA test show asimilar tendency as the result calculated from the mixedpotential theory: galvanic coupling between Alloy 600 andmagnetite increased the corrosion current density of Alloy600. In addition, the corrosion current density of the twoAlloy 600 specimens decreased rapidly with time. Thisdecrease in corrosion current density with time can beattributed to the formation of passive film on the surface ofAlloy 600. Burstein et al.21) also showed that the galvaniccurrent density tends to diminish as the metal passivates byoxide film growth during the initial stage and to stabilizefrom the final stage. In addition, it should be noted that thecurrent density of coupled Alloy 600 was higher and wasstabilized more slowly than that of non-coupled Alloy 600.This result indicates that the galvanic coupling between Alloy600 and the magnetite has a negative effect on the passivationbehavior of Alloy 600.

Although ZRA measurement showed the lower corrosioncurrent densities as compared with the result predicted fromthe mixed potential theory due to the stabilization of passivefilms, these results show a similar tendency as the resultpredicted that the galvanic coupling between Alloy 600 andthe magnetite increased the corrosion current density of Alloy600.

3.3 The film on Alloy 600 coupled with the magnetiteFigure 6 shows the XPS depth profiles of passive surface

film on non-coupled and coupled Alloy 600 with magnetiteafter the immersion test in the alkaline solution at 298K for15 days. Taking the half-height of the oxygen in Fig. 6 as anestimate of the film/substrate steel interface,22) the passivatedfilms formed on the non-coupled and coupled Alloy 600 withthe magnetite require approximately 24 and 17 s of etching,respectively. This indicates that galvanic coupling betweenAlloy 600 and the magnetite formed the thinner and lessprotective passive film of Alloy 600. This result can besupported by the fact that a higher corrosion current densityof the coupled Alloy 600 was observed in Fig. 5.

Figure 7 shows that the deconvolution of Fe, Cr, Ni, and Orelated chemical species by the XPS in the outer surface film

Table 2 Electrochemical corrosion parameters of Alloy 600 coupled withthe magnetite obtained from polarization curves in the alkaline solution at298K.

Alloy 600 Magnetite

Ecorr (VSCE) ¹0.474 ¹0.406

Ecouple (VSCE) ¹0.440

icorr (µA/cm2) 1.05 1.17

icouple (µA/cm2) 1.67

0 200 400 600 800 1000 1200 1400 1600 18000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Time, t / s

Cur

rent

Den

sity

, I/ μ

A c

m-2

Non-coupled Alloy 600 Coupled Alloy 600

Fig. 5 Variation of galvanic current density of non-coupled and coupledAlloy 600 with magnetite in the alkaline solution at 298K using ZRA test.

(a) (b)

0

10

20

30

40

50

60

70

80

90

100

Ato

mic

%

Etch Time, t / s

Fe Cr Ni O

0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 7000

10

20

30

40

50

60

70

80

90

100

Ato

mic

%

Etch Time, t / s

Fe Cr Ni O

Alloy 600Alloy 600 Alloy 600/MagnetiteAlloy 600/Magnetite

Fig. 6 XPS depth profile of the passive film formed on the non-coupled and coupled Alloy 600 with magnetite after the immersion test inthe alkaline solution at 298K for 15 days.

Corrosion Behavior of Alloy 600 Coupled with Electrodeposited Magnetite in Simulated Secondary Water of PWRs 2081

formed on the non-coupled and coupled Alloy 600 withmagnetite. The binding energies (Eb) obtained from thedeconvoluted XPS profiles of the primary compounds in thefilms are listed in Table 3. The signals corresponding to theNi 2p spectra are the strongest in both the non-coupled andcoupled Alloy 600 with magnetite. The Ni 2p peak can beseparated into three constituent peaks, representing themetallic state (Ni (M)), nickel oxide (NiO), and nickelhydroxide (Ni(OH)2). The Cr 2p peak can be separated intothree constituent peaks, representing the metallic state (Cr(M)), chromium(III) oxide (Cr2O3), and chromium hydroxide(Cr(OH)3). The Fe 2p spectra can be separated into fourconstituent peaks representing the metallic state (Fe (M)),iron(II, III) oxide (Fe3O4), iron(II) oxide (FeO) and, iron(III)hydroxide (FeOOH). The results revealed that iron(III) is theprimary iron oxidized species in the passive film. Galvaniccoupling increased the ratio of metallic state of passive filmformed on the Alloy 600. Especially, galvanic couplingbetween Alloy 600 and the magnetite significantly increasedthe ratio of Ni (M) of passive film. These results mean thatthe surface of coupled Alloy 600 is more corroded than thatof non-coupled Alloy 600 due to the retardation of passivefilm stabilization. Based on the XPS results, galvaniccoupling between Alloy 600 and the magnetite has a negativeeffect on the passivation behavior of Alloy 600.

In this study, the galvanic effect on the corrosion rate ofAlloy 600 was measured when the magnetite on the outsidesof steam generator tubes was deposited. If Alloy 600 and themagnetite are galvanically contacted to the equivalent arearatio (1 : 1) at 298K, the corrosion rate of coupled Alloy 600will increase by about 1.6 times than that of non-coupledAlloy 600.

However, in actual steam generators, the degree of galvaniceffect is expected to be more severe and the corrosion rate ofAlloy 600 coupled with the magnetite will increase. Theseresults can be explained by the following two reasons. First,operating temperature of steam generator in PWRs is the hightemperature ranges up to 553K. In addition, a local temper-ature increase is expected within the magnetite deposited onthe surface of Alloy 600 tubing, since heat transfer though thetubing is hindered by the deposits. The porosity of passivefilm increases with temperature and intrinsic modification ofthe chemical composition and/or physical structure of thepassive film takes places. These changes lead to an increase ofdefects in the passive film and generate higher galvaniccorrosion rate.32) Second, the ratio of the cathodic to anodicarea on the outer surface of steam generator tubes coveredwith the porous magnetite is larger than the equivalent area(1 : 1). Increasing the ratio of cathode to anode area increasesthe corrosion rate of anode due to the area ratio effect.

0

2000

4000

6000

8000

10000

12000

14000

Cou

nts

(cps

)

Binding energy, eV

0

5000

10000

15000

20000

25000

30000

35000

Cou

nts

(cps

)

Binding energy, eV

Ni 2p

Ni (M)

Ni(OH)2

NiO

Cr 2p

Cr (M)

Cr(OH)3

0

1000

2000

3000

4000

5000

6000

Cou

nts

(cps

)

Binding energy, eV

Cr2O3

Fe 2p

FeOOH

Fe3O4

FeO

Fe (M)

(b)

582 580 578 576 574 572 860 858 856 854 852 850 716 714 712 710 708 706

Cr (M)

0

2000

4000

6000

8000

10000

12000

14000

Cou

nts

(cps

)

Binding energy, eV

Cr 2p

Cr(OH)3

Cr2O3

582 580 578 576 574 572 716 714 712 710 708 7060

1000

2000

3000

4000

5000

6000

Cou

nts

(cps

)

Binding energy, eV

Fe 2p

Fe (M)

FeOOH

Fe3O4

FeO

0

5000

10000

15000

20000

25000

30000

35000

Cou

nts

(cps

)

Binding energy, eV

Ni 2p

Ni(OH)2

NiO

(a)

860 858 856 854 852 850

Ni (M)

Fig. 7 XPS component analyses of the outer passive film formed on the non-coupled and coupled Alloy 600 with magnetite afterimmersion test in the alkaline solution at 298K for 15 days: (a) non-coupled Alloy 600 and (b) coupled Alloy 600.

Table 3 The binding energies of some chemical species for the XPS analysis.

SpeciesBinding

energies (eV)Species

Bindingenergies (eV)

SpeciesBinding

energies (eV)

Cr(M)2p3/2 574.023­25) Ni(M)2p3/2 852.723,26,27) Fe(M)2p3/2 707.026)

Cr2O3 2p3/2 576.024) NiO2p3/2 854.028,29) Fe3O4 2p3/2 708.231)

Cr(OH)3 2p3/2 577.324) Ni(OH)2 2p3/2 855.830) FeO2p3/2 709.431)

FeOOH2p3/2 711.831)

S.-H. Jeon, G.-D. Song and D. H. Hur2082

In our research group, electrochemical experiments areongoing to validate the galvanic effect on the corrosion rateof steam generator tubing coupled with magnetite in the hightemperature up to 553K.

4. Conclusions

(1) Galvanic effect on the corrosion of Alloy 600 coupledwith magnetite was predicted by mixed potential theoryand verified by ZRA measurements. Alloy 600 was theanode of the couple because the corrosion potential ofthe magnetite was higher than that of Alloy 600.Galvanic coupling increased the corrosion currentdensity of Alloy 600 due to the shifting of the potentialof Alloy 600 to the positive value.

(2) Galvanic coupling has the negative effect on thepassivation behavior of Alloy 600. The passive filmof coupled Alloy 600 was more slowly stabilized andwas thinner and less protective than that of non-coupledAlloy 600. As a result, the ratio of metallic states suchas Ni (M), Cr (M), and Fe (M) of outer surface film oncoupled Alloy 600 was higher than that of non-coupledAlloy 600.

(3) The pure and dense magnetite was adherently electro-deposited on the Alloy 600 substrate in Fe(III)-TEAsolution at 353K. This electrodeposition method isuseful for simulating the magnetite deposited on thesurface of Alloy 600 in nuclear steam generator tubingof a nuclear power plant.

Acknowledgements

This work was supported by the National ResearchFoundation of Korea (NRF) grant funded by the Koreagovernment (MSIP). This work was also partially supportedby the Nuclear Power Core Technology DevelopmentProgram of the Korea Institute of Energy TechnologyEvaluation and Planning (KETEP) granted financial resourcefrom the Ministry of Trade, Industry & Energy, Republic ofKorea (2013T100100029).

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Corrosion Behavior of Alloy 600 Coupled with Electrodeposited Magnetite in Simulated Secondary Water of PWRs 2083