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DOI: 10.1007/s10967-007-0715-y Journal of Radioanalytical and Nuclear Chemistry, Vol. 273, No.1 (2007) 8590

02365731/USD 20.00 Akadmiai Kiad, Budapest

2007 Akadmiai Kiad, Budapest Springer, Dordrecht

Corrosion study of heat exchanger tubes in pressurized water cooled nuclearreactors by conversion electron Mssbauer spectroscopy

Z. Homonnay,1 P. . Szilgyi,1 E. Kuzmann,2 K. Varga,3 Z. Nmeth,3 A. Szab,3 K. Rad,3

J. Schunk,4 P. Tilky,4 G. Patek41Department of Nuclear Chemistry, Etvs Lornd University, Budapest, Hungary

2 MTA-ELTE Research Group for Nuclear Methods in Structural Chemistry, Hungarian Academy of Sciences,

Etvs Lornd University, Budapest, Hungary3Department of Radiochemistry, University of Veszprm, Veszprm, Hungary

4Paks Nuclear Power Plant, Paks, Hungary

(Received June 30, 2006)

57Fe-conversion electron Mssbauer spectroscopy (CEMS) a sensitive tool to analyze the phase composition of corrosion products on the surfaceof stainless steel was applied to study real specimens from the Paks Nuclear Power Plant, Hungary. The primary circuit side of the heatexchanger tubes was studied on selected samples cut out from the steam generators during regular maintenance. Mostly Cr- and Ni-substitutedmagnetite, amorphous Fe-oxides/oxyhydroxides as well as the signal of bulk austenitic steel of the tubes were detected. The level of Cr- and Ni-

substitution in the magnetite phase could be estimated from the Mssbauer spectra. It is suggested that CrNi substitution occurs simultaneously sothat the inverse spinel structure of magnetite is preserved up to a certain limit which appears to be roughly at[Fe3+]tet[Fe

2+1/4Ni

2+3/4Fe

3+1/4Cr

3+3/4]octO4. Further decrease of the iron content of this phase results in the formation of nickel chromite of regular

spinel structure, with very low Fe content. This transformation may be responsible for the hybrid structure of the protective oxide layer, beingsubstantially accelerated by previously performed, factory developed and proposed AP-CITROX decontamination cycles.

Introduction

Nuclear energy production tends to return into thefocus of interest because of the constantly increasingenergy need of the world and the green house effect

problems of the strongest competitor oil or gas based power plants. In addition to the construction of newnuclear power plants, lifetime extension of the existingones is the most cost effective investment in the energy

business. However, feasibility and safety issues becomevery important at this point, and corrosion of theconstruction materials should be carefully investigated

before decision on a potential life-cycle prolongation ofa reactor.

Corrosion products in the primary circuit of watercooled nuclear reactors have been investigated usingCEMS and other methods by several groups.14 CEMS(57Fe-conversion electron Mssbauer spectroscopy) is asensitive tool to analyze the phase composition of

corrosion products on the surface of stainless steel,5giving information mostly from the outhermost 300 nmlayer due to the penetration range of conversionelectrons.

It is well known that the main corrosion product onstainless steel in wet environment under chemicallyreductive conditions is magnetite (Fe3O4).

5,6 Magnetitehas an inverse spinel structure: it has Fe3+ ions on thetetrahedral (A) sites and Fe3+ and Fe2+ ions in equal

population on the octahedral (B) sites of the ideal unitcell. Iron ions may be easily substituted by other metalions (Cr3+, Ni2+, Co2+) if the respective elements are

present in the steel. Since octahedral sites arecrystallographically equal in the unit cell, the Mssbauer

spectrum of magnetite contains only two sextets. Onerepresents the tetrahedral Fe3+ with isomer shift typicalto high spin Fe3+ with an internal magnetic hyperfinefield B = 49.1 T, while the other sextet has an isomershift between those characteristic of Fe3+ and Fe2+ withhyperfine field B = 45.3 T).7 The averaged valence stateat site B is due to fast electron hopping between Fe2+

and Fe3+ at the octahedral sites.ROSENBERG and FRANKE8 have systematically

studied the effect of substitutional disorder on the ironvalence states of Fe3xMexO4-like systems (Me=Ni, Co,Cr) using Mssbauer spectroscopy. They haveconcluded that substitution of Ni2+ or Co2+ into theFe(B) sites lead to the appearence of sextets withhyperfine fields 47.0, 49.6 and 51.7 T besides theaveraged Fe(B) line, so the mixed valency state willacquire more Fe3+ character. For Cr3+ substitution theFe(B) site became more Fe2+ in character since thespectrum contained three more Lorentzian sextets,

besides the Fe(B) magnetite one, with hyperfine fields of38.2, 41.7 and 44.0 T. This shows that analysis of theMssbauer spectrum of magnetite formed on the surfaceof stainless steel can help estimate the substitution levelin the spinel structure.

Recently, we have reported corrosion study ofstainless steel tubes from the steam generators of thePaks Nuclear Power Plant using various analyticalmethods, and concluded that the protective oxide layeron the primary curcuit side of the tubes has a hybridstructure with substantial mobilty.6,911 We report here,a more detailed CEMS analysis of selected samples in

order to find out if the substitution level of magnetite onthe surface and the mobility of the protective oxide layer


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Z. HOMONNAY et al.: CORROSION STUDY OF HEAT EXCHANGER TUBES

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may be correlated. Possible relation with previousAP-CITROX6,12 decontamination cycles is also

proposed.

Experimental

The experiments have been performed on numerous(>25) austenitic stainless steel tube specimens (type:08X18H10T (GOST 5632-61) outer diameter: 16 mm,average wall thickness: 1.6 mm) originating fromdifferent steam generators of the Paks Nuclear PowerPlant. In addition to CEMS, the samples have beenstudied by several methods including voltammetry,XPS, SEM-EDX and XRD and such results have been

published elsewhere.6,912

The 1.52 cm long tube pieces were cut into twohalves along their axes and then flattened gently so thatthey could be mounted to the CEMS detector. Visible

damage was not observed on the surface, or if was, itwas painted with an organic paint which completelyabsorbed conversion electrons and thus prevented falsedetection of deeper oxide layers or bulk steel.

The CEMS spectra were recorded at roomtemperature with a conventional Mssbauerspectrometer (Wissel) in constant acceleration mode.The conversion electrons were detected with a constant-flow type proportional counter specially designed forCEMS technique (Ranger). The counter gas was amixture of 96% He and 4% methane. A 57Co(Rh) sourceof 500 MBq provided the -rays. Calibration was done

by measuring an -Fe foil in transmission mode, whichis the reference of the isomer shifts published in thiswork. Computer analyisis of the spectra was carried out

by using Msswinn 3.0. Lorentzian line shape andcommon linewidth (within one spectrum) were assumedin all cases.

Results and discussion

The CEMS results on the specimens cut out from thesteam generators resulted in various types of spectrawhich may be classified in two characteristic groups. Inthe first group of samples, the Mssbauer spectra werecomposed mostly of a doublet and a singlet in additionto a very minor sextet if any. Four of such spectra areshown in Fig. 1 as examples. The ramarkable feature ofthese spectra is the marked presence of the singlet,which is assigned to the bulk steel, and the absence ofmagnetite, a typical corrosion product of stainless steelin reductive primary coolant. The small sextetsometimes observed may be assigned to magnetite onlytentatively because it cannot be resolved to two sub-sextets due to low intensity.

The high intensity of the singlet is difficult tounderstand because the thickness of the oxide layer on

the surface was found to be sometimes more than

12 m, and thus conversion electrons from bulk steelshould not be able to reach the detector if the corrodedsurface has a layered structure. Since possible mirco-cracks on the surface due to shaping of the specimenmay account only for a minor part of this signal; it was

concluded earlier6,911

that the oxide layer has a hybridstructure: a relatively thick layer composed of a more orless random mixture of grains of iron depleted magnetite(spinel ferrite or chromite), amorphous Fe(OH)3 andsteel.

The doublet is assigned to amorphous Fe(OH)3(and/or FeO(OH)), and is present in various amounts inevery sample.

The second typical group of samples showedMssbauer spectra with large fraction of magnetite (Fig.2). However, regular magnetite has a somewhat differentMssbauer spectrum as shown in Fig. 3. The sextet withsmaller hyperfine field and averaged Fe2+/Fe3+ character

has an intensity twice as high as that of the other because there are two octahedral sites for eachtetrahedral site in the unit cell of spinel (Fig. 4).(Differences in Mssbauer-Lamb factors which causessome deviation from the 2:1 ratio are normallyneglected.)

In our measurements, although the Mssbauerparameters of the sextets show no noticeable differencefrom those of pure magnetite, the relative intensities arevery different: the intensity of the sextet assigned to siteB is anomalously low (Table 1).

This shows that the population of Fe at site B

decreased, which must be due to substitution of Fe bythe most abundant alloying elements of the steel: Cr and Ni. Chromium is expected to show up in the form ofCr3+ ions in a corrosion process, and since the crystalfield stabilization energy for Cr3+ in octahedral field is224.5 kJ/mol in contrast to the tetrahedral field where itis only 66.9 kJ/mol,13 chromium will be substitutedsolely for Fe3+ at site B. The analogous figures for Ni2+,expected corrosion product of nickel, are 122.1 kJ/moland 35.9 kJ/mol, respectively, thus nickel will alsosubstitute at site B, however, for Fe2+.

As mentioned earlier, ROSENBERG and FRANKE8

found new sextets in the Mssbauer spectrum ofmagnetite when either Cr or Ni were single substitutingelements. Both were found to substitute at site B. That isreasonable since by single substitution, the electronhopping between Fe2+ and Fe3+ becomes unbalanced.We have found no new sextets, only the ratio of sextet Aand sextet B changed. Taking into account the site

preference of Ni2+ and Cr3+, one can logically concludethat this system (i.e., Cr-Ni steel) offers a uniqueopportunity for simultaneous substitution of Cr3+ and

Ni2+ at site B in the inverse spinel structure of magnetiteformed during the corrosion of stainless steel, and thiscan explain the Mssbauer spectra.


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Fig. 1. Examples of CEMS spectra indicating hardly any (or no) magnetite on the corroded surface of stainless steel tubesfrom the steam generators of Paks NPP

Fig. 2. Examples of CEMS spectra indicating large amount of magnetite on the corroded surface of stainless steel tubesfrom the steam generators of Paks NPP


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Fig. 3. Simulated Mssbauer spectrum (CEMS) of pure magnetite at room temperature, showing two sextets assigned to terahedral(site A, dashed line) and octahedral (site B, solid line) lattice positions

Table 1. Mssbauer parameters of the sextets of Ni- and Cr-substituted magnetite in selected samples; : isomer shift relativeto -Fe,B: magnetic field (induction),I: relative spectral area in the Mssbauer spectrum

Sextet A Sextet B IB/IASpectrum

, mm/s B, T I, % , mm/s B, T I, %A 0.38 49.9 59.9 0.69 46.4 25.4 0.424B 0.36 49.6 59.7 0.69 46.1 31.7 0.532C 0.34 50.0 43.2 0.63 46.2 21.1 0.488D 0.35 50.4 30.6 0.71 46.3 25.6 0.833

Fig. 4. The unit cell of the (inverse) spinel structure for magnetite.

Tetrahedral sites (A) are highlighted by indicating Fe3+-O bonds.Octahedral sites (B) contain Fe3+ and Fe2+ with equal populations

Now the question arises how far this simultaneoussubstitution may proceed. Recording more than 25spectra on various samples, it is remarkable that in themagnetite rich ones, the relative fraction of sextet B isnever observed to diminish below a certain limit,namely, about 4050% of sextet A (Table 1). This mayindicate a substitution limit at a composition of roughly[Fe3+]tet[Fe

2+1/4Ni

2+3/4Fe

3+1/4Cr

3+3/4]octO4. In the other

group of samples the intensity of magnetite (?) seems todrop rather suddenly, and the two sextet description

appears to be invalid. To explain these findings, thefollowing picture can be proposed.Since we have several examples where the

Mssbauer spectra do not show any magnetically split inFe-containing phase while the thickness of the protectiveoxide layer is found to be more than 1 m, the spinel

phase covering the surface must be composed of mainlynickel chromite, NiCr2O4 (not detectable by Mssbauerspectroscopy). Nickel chromite has the normal spinelstructure,14,15 thus Ni2+ is at site A and Cr3+ is at site B.This structural difference explains why a continuoustransition from the inverse spinel Cr/Ni substitutedmagnetite to normal spinel Ni-chromite is not possible.A smooth internal transformation would involve


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exchange of A site Fe3+ ions with B site Ni2+ ions bydiffusion jumps:

Fe3+A(Fe2+, Ni2+)B(Cr

3+, Fe3+)BO4

Ni2+A(Fe2+, Fe3+)B(Cr

3+, Fe3+)BO4,

followed by further chromium substitution:

Ni2+A(Fe2+, Fe3+)B(Cr

3+, Fe3+)BO4

3

3

Fe

Cr

Ni2+A(Fe2+, Cr3+)B(Cr

3+, Fe3+)BO4,

which (the first step) is against the site preference ofNi2+. Therefore, one has to assume that thetransformation should occur by dissolution of theinverse spinel type substituted magnetite followed bynucleation and growth of the normal spinel nickelchromite. This can result in a sudden change in thecomposition of the spinel. Note that since chromium has

a very strong preference for the octahedral sites (B),Fe3+ cannot substitute for it easily in nickel chromite.Fe2+ has also preference for the octahedral sites (Fe3+

has no preference at all due to its highly symmetric 3d5

valence shell configuration), thus noticeable substitutionfor Ni2+ at the tetrahedral site A is not expected either.The obvious result is low solubility of iron in nickelchromite. This explains why we found sextets of verysmall intensity in many cases, and now these sextetsmust be assigned to Fe impurity in nickel chromiterather than to magnetite with very low iron content. Theformation of Ni-ferrite and Fe-chromite may also be

considered; both phases are normal spinels.The high Ni- and Cr-content of the corrosion layeron the steel was verified by EDX measurements.9

Fig. 5. Correlation between the Cr-content (first column), Ni-content(second column) measured by EDX, and the magnetite content (thirdcolumn) measured by CEMS in the protective surface oxide layer ofcorroded stainless steel tubes. (Note that percentage values ontainedfrom EDX and CEMS have quite different meanings, but it does not

effect the observed tendencies)

As can be seen in Fig. 5 by the example of threecharacteristic samples, when the Cr- and Ni-concentration increases in the surface oxide layer, theamount of magnetite detectable by CEMS decreasesdown to even zero. Therefore, the absence of the signal

of magnetite in the CEMS spectra can serve as anindication of a quite thick oxide layer composed ofnormal spinels of high Ni- and Cr-concentration.

Conclusions

On the basis of our results we conclude that the protecting oxide layer formed on the surface of thestainless steel tubes in the steam generators of thenuclear reactors has various compositions depending onthe operation time and decontamination history. We findthat Fe depletion from the surface layer (Fe is the most

soluble metal components of steel in the primarycoolant) causes transformation of the originally inversespinel magnetite with low Ni- or Cr-substitution levelmostly into the normal spinel Ni-chromite by adissolution-nucleation process. During thistransformation, part of iron may precipitate in the formof amorphous Fe(OH)3 and/or of FeO(OH).

This mechanism can explain the hybrid structure ofthe protective oxide layer which is undesirable for goodcorrosion resistance.

As discussed in our previus papers,911 the surfacedecontamination by properly performed non-

regenerative version of the AP-CITROX procedure(developed by Russian steam generator manufacturer)can initiate (or/and accelerate) the above describedundesired transformation process, and the resultantoxide layer may become mobile which can beresponsible for delivering large amount of corrosion

products to the reactor core.

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P. BARADLAI, G. HIRSCHBERG, J. SCHUNK, P. TILKY, J. Radional.Nucl. Chem., 246 (2000) 131.

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6. K. VARGA, The role of interfacial phenomena in thecontamination and decontamination of nuclear reactors, in:Radiotracer Studies of Interfaces, Vol. 3, G. HORNYI (Ed.),Interface Science and Technology, Elsevier B.V., Amsterdam,2004, p. 313.

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