AC response of RRDE during the passivation of iron

Vol.3I, pp. 627~35, 1990 Printedin GreatBritain

0010---938X/90$3.130+0.00 © 1990PergamonPresspie

Corrosion Science,

AC RESPONSE OF RRDE DURING TIlE PASSIVATION OF IRON

N. BENZEKRI, R. CARRANZA, M. KEDDAM and !!. TAKENOUTI LPI5 du CNRS, Physique des Liquidcs et Eleetrochimie I..abofaloire de I'Universil(~ Pierre el Marie Curie Tour 22, 4, place Jussieu 75252 Paris Cedex 05 - FRANCE

ABSTRACT Electrochemical impedance spectroscopy (EIS) is often applied to the study of passivation kinetics, but by Ihis nteans, the formaliou of the passive film itself and parlicularly its rclaxalion phem)menon remain hypothetical. The use of the RRDE (Rotatillg Ring Disc Electrode) technique is in this context particularly advantageous sittce it allows us to materialize the foln|atiot| of a fihn or of an adsorbed inlerinediate species through the charge stored at the electrode surface. The most marked feature, we observed, is the emission efficiency of Fe Ill. This emission is negligibly small at its high and low frequency limits, whereas it is significant at the intermediate frequency range, that is I to 0.011tz, according to the polarization conditions. Our efforts were focussed at describing this specific experimental observation on the basis of a passivation model. KEYWORDS Electrochemical impedance, Reaction migration, Surface charge.

model,

Collection efficiency,

Field

assisted

INTRODUCI'ION The passivation of iron in a sulfuric acid snediusn has been extensively investigated for a long tithe. It is largely recognized that the metal passivation takes place by tile formation of a surface fihn or of an adsorbed layer which decreases drastically the rate of active dissolution. The active-passive transition in Ihe immediale vicinity of tile Flade potential is characterized by a negative slope regioll of the polarization curve associated with impedance diagl~ms with a low frequency loop bending towards the negative real part region. Iiowever investigation of this crucial domain for the understanding of passive film buildup is difficult on the basis of data restricted to Electrochemical hnpedance Spectroscopy (EIS) since the faradaic processes consist of simultaneous dissolution and film forming reaction. Particularly the relation between relaxation phenomena and the h)rmation t)f 2-D or 3-D surface layers rctnains largely hypothetical. In order to oblain new information we have recently introduced the application of the Rotating Ring Disc Electrode (RRDE) under a stnall amplitude AC perturbation of the disc polarization (I-3). This technique was first proposed by Albery and his coworkers (4-5) in the early 70's. They showed theoretically that 627

628

N. BENZEKRIet al.

this method offers evidence of the h)rmation of sub-monolayer in the course of a reaction process. More recently, Tsuru et al. (6) investigating iron dissolution in the transient regime designed a channel-flow device with a Pt electrode located down-stream acting as the collecting electrode of the species formed at the upstream iron anode. In fact we have shown (7) that the contribution of hydrodynamic coupling between up- and down-slream electrodes can only be taken into account accurately by performing experiments in the frequency domain. In our early papers concerning the passivation of iron by the RRDE teclmique, only the formation of ferrous ion was measured by the ring. ilowever, it is largely known that the iron dissolves in ferric species in the passive range. The dissoluliou valencies may change from two to three in the passivation range (8), Ihal is to say, at potentials more anodic thaq the Flade potential, and where Ihe polarization curve shows clearly a negative slol)e. For this reason, new experiments tnc.'tsnring both ferrous and ferric ion emissions from the disc were performed. In Ihe passivity range, furlhermore, the passive layer grows beyond the monomolecular layer leading thus to 3-D fil=n. The passive current is then controlled by the field assisted migration process (9-11). EXPERIMENTALS The principle of the RRDE technique under AC pcrlu,bation of the disc electrode and the experimental set-up arc given elsewhere in II,is proceedings (12). The geolnelry of Ihe RRI)E used in Ihis study was 2.8, 3.0 and 4.2111111 respectively to the disc and the inner an(I outer ring diameters. Its sleady-slale collection efficiency NS is equal Io 0.456. The disc electrode was Fe (Johnsnn-Mallhcy) and the ring was Pt (Lyon-Alemand). The electrolyte used was I M - I I 2 S O 4 (Merck, pro-analyst quality). The disc was polished with emery paper up to # 1200 grade, and Pt ring was slightly polished together with Fc disc with alumina before each experiment. The ring electrode was polarized at 0.80V v s SSE (saturated sulfate reference electrode) to oxidize the ferrous ions leaving the disc to the ferric state, or at - 0 . 4 0 V v s SSE to detect the entissio,i of ferric ions f()rmed. The rotation speed of RRDE was set generally at 900 rpm. 1'lie RRDE experiments were first pcrh)rntcd in 10mM-Fc2(SO4)3 added to the tnolar sulfttric acid solution. The disc was I't on which the reduction of ferric into ferrous ions was carried out. At the ring, the oxidation of the latter was taking place. Sincc this redox reaction involves no adsorption process, N((o) was identified to Nt(<0) (of Eq I of the reference 12). One of these results is shown in Fig. I. On this figure, it can bc seen Ihat AC response at the ring becomes van!shingly small for frequencies higher than 1011z. Because of this high frequency cut-off fcaturc of the RRDE, no reliable results about Nd(00 ) can be obtained for frequencics higher than this value.

AC Response of RRDE during the passivation of iron 10 .2

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I

40.0

630

N. BENZEKRIet al.

RESULTS Figure 2 depicts thc polarization curve obtained in tile passivation range, that is, at the foot o f Ihc bell-shaped polarization curve. The passive current close to 7 p A . c m - 2 observed cxperimenlally shows that no significant electrolyte leakage is taking place Iletwccn line disc elcclrode and its cylimler wall. Tiffs is an iml)orlaut imlicati(m to inst, rc r e p r o d u c i l l l c results. the p o l a r i z a l i o n c o u d i t i o u s at which EIS

"lhc poinls m.'Irkcd on the curve indicate and R R D E measurcpnculs were pcrror,ncd.

f

As e x a m p l e s , Fig.3 shows the rcsults obtained at three polarization potentials marked A to C iu Fig.2. The point A is typical for tile passivation state and C may be considered to bc close to tile characteristics of tile passive slate. "i lie up-most row of Fig.2 concerns tile electrode iml)cdances : they show tile well-known feature for the p a s s i v a t i o n process. The c h a r a c t e r i s t i c frequency, thai is, the f r e q u e n c y corresponding Io line nnaximum ill the absolute value of imaginary part, decreases w h c , tile passivatiou progresses. The second row illustrates the results relative to Nd il, the emission efficiency of ferrous ions at tile disc. These results were ahcady given in ot, r previous work (2-3) except for tile most anodic polcmial. In fact, only the potcntiostat regulation can be used iu tills potential range to fulfill tile stability c o n d i t i o n s of line system. On the otlner hand, when tlnc clcctrodc is polarized at the potential close to tile fully passive range, the impedaucc increases considerably. Consequently, tile current change ill tile disc b e c o m e s very small making the R R D E measurements particularly difficult. It call bc seen, however, on these diagrams that Nd II exhibits the typical features derived for a charge transfer in c o m p e t i t i o n witln a faradaic charge storage in a passivatiug layer (12). Its characteristic frequency is equal to thai observed in the impcdancc diagram. The third row of Fig.3 are relative to Nd I11 : the einissiou efficiency of ferric lolls at the disc. The diagram depicts a quite parlicular feature : a sickle shape. That is to say, there is (almost) no emissioq of Fe III neither at high frequency nor at h)w frequency limits. In fact Ndlll((}) equal to zero does not aecessarily means thai there is no ferric dissolution, since ill this case N(U) no longer coincides wilh NS the steady state emission efficiency (I-3,12), but that the steady state current decrease shown ill Fig.2 is entirely due to line decrease ill tile flux of Fe II in agreement with the shape of Nd II. I1 call be observed further on these diagrams that the frequency at which the real part of Nd III is maxinuun corresponds to tile maxi,num of tile real part in tile impedance diagrams. Tiffs establishes that tile passivation mechauisnl as secn from Ihe RRDE data involves not one faradaic frequency domain as fcalurcd by thc impcdaucc bt, I two domains Ihe higher onc being hardly separated from the d o u b l e layer c a p a c i t a n c e - c h a r g e transfer rcsistancc contribution. The lowest row of tile figure 3 displays tile differential charge stored at tile electrode surface. To calculate this term, both Nd II and Nd III were taken into consideration. In our previous work, only bldll was considered to calculate it, but as far as tile low frequency limit of this value is concerned, no error was involved since Nd III tends towards zero when the frequency decreases. Another peculiar feature that we can pointed out from these diagrams is that Nd I!1 diagram is cntirely located in tile negative imaginary range, that is to say tile emission of ferric ions is c o n t r o l l c d through a surfacc c h a r g e which b e h a v e s like a dissolution intcrmcdiate contrasting with Nd II as staled above.

AC Response of RRDE during the passivation of iron

631

DISCUSSION The most particular fcaturc was obscrvcd in the results concerning Nd I11. From thcsc diagrams t w o conclusions can be drawn. Firstly, thc emission o f Fe i l l involves tire formation o f a charge of Ihc dissolulion inlcrmediate typc and nol Ihal of thc passivc film. Secondly, Nd Ill in Ihe Nyqulst plane has a sickle-shape. These experimental observations introduce a gcncral form fi)r the reaction modcl :

Fc

¢-~

Q

Film

; surface species

,L Fc It

Ill Fe !11

; solution species

Whatever thc dclailcd model, it should iuvolvc a chargc Q , which plays a double rolc : as a source o f Fc I!1 ions rclcascd to Ihc solulion on ovic hand and as an intcrmedialc of the formation of the passlvc film on the oilier hand. The build-up of Q should bc Ihc fastest process, Ihcll inducing Ihc dissolulion hllo Fc III. The transformation o f Q inlo tire passive fihn malcrial has to bc the slowcsl one. By Ihis way, at the mcdiunl frcqucncy only tire d i s s o l n l i o n o f Fe I I I through Q is significant. When Ihc frcqucncy decreases, the film formation becomes prevailing, and Q dccrcascs to zero making the dissolulion of Fc ill vanishingly small. This situation is illustrated in Fig.4. This is a necessary condition to explain the sickle-shape of Nd Ill diagram observed cxpcrimcnlally. It may be noteworthy that only Ihc overall charge is cxpcrimcnlally available, and the partial charge as concerned in Fig.4 is merely calculalcd on Ihc basis of thc reaction model.

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PART

Fig.4 : Schematic rcprcscmation of & Q / 6 E .

632

N. BENZEKRIet al.

2-D model

Tile simplest model Fc

which fulfills these general k2 ¢=~ k-2

kl ,[

Fe li

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k3 ,[,

Fell

requirement

is given below :

Fe II1

~" k5

[111

Felll

Thc chemical dissolution of passive fihn (ks) is added to describe the steady passive current. Let 01, 02 and 0 3 bc tile fractional surface coveragcs by Fe, Fe II and Fe Ill r e s p e c t i v e l y , [~ the surface conccntralion at coverage unily, and reaction rates follow Ihc Tafcl law, k i = ki, 0 exp(bi.E), then the following relations can be derived : I = F I 2(kl + k2) 01 - (2k-2 - k3 - k4) 02 - k . 4 0 3 } d02 ~ = k201 - (k-2 + k3 + k4) 02 - k - 4 03

(i) (2)

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(3)

01 + 0 2 + 0 3

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~ 1

~,li = kl 01

(5)

• I11 = k3 02 + k5 03

(6)

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;

dQ Ill d0~ dE = F p dE

(7)

(8)

Thcsc cqualions were solvcd according to Ihc usual manncr in EIS technique. Thc results shown in Fig.5 arc calculated by diglt,H simulation setting the suitable values [or Ihc kinclic parameters. A comparison of diagrams given ill this figure to those in Ihc c o h m m A of Fig.3 shows that lhc agreement between cxperimenls and model is reasonably good. llowcver, when thc polarization potential becomes more anodic, a sig,lificant discrcpancy was observed. We devised therefore a reaction model taking into account the progressive covcragc of the electrode by a 3-D layer according to a 3-D nuclcalion and growth kinetics.

AC Response of RRDE during the passivation of iron

633

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: Calculaled rcsulls, see line columu A of Fig.3. 3.10 "8 exp (5.E) ; k 2 = 4.10 .8 exp (20.E) ; k. 2 = 4.10 .8 cxp (-30.E) ; 3.1U "8 exp (10.E) ; k4 = 10 .7 exp (IO.E) : k.4 = l0 "12 exp (-U.I.E) ; l0 1 0 ; (k i is expressed in Inol/cm2.s) ; [~ = l0 -8 m o l / c n l 2 ;

Cd = 1 0 0 p F / c m 2.

3-D model

Tlne behaviour o f tile 3-D structure as given abovc : Fe

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was

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writlen

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lllll

Felil

The formation of ferrous ion ( k l ) is considered [o obey Ihe Tafel law, whereas Qm is tile bulk cllarge associated to mobile ferric species wlfich migrate through tile fihn according to Ihc Inigh field assislcd migration (9-11) : .E - EF.

IIIF = io exp C - ' T - ~ ) whilst as ;

tile s u r f a c e

coverage

(9) ct by tile 3-D p a s s i v e

l a y e r is a s s u m e d

to d e p e n d

on E

634

N. BENZEKRIet

al.

1

Ct = i + e x p { -b (E + 0 . 1 ) }

(io)

1 = ot iii F + (I - or) I l ; ! l = 2F kl,0 exp (bl E)

(11)

The balance of Qm taking into account the various contributions :

dQm

(12)

dt = I i l F - I S - I F and the net growth rate of the passive film: dl kg ~'~ = I F - IC In addition, we considered

where

dot

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I +jo) z

that dot/dE varies with a localized lime constant "L

(14)

:

electric field strength in the fihn. thickness of passive fihn. dissolution current from Qm. film fornlation current from Qm. rate of the chemical dissolution of the fihn. constant li,lki,le ! to Qm through the Faraday law. fixed charge f a r a d a i c a l l y s l o r e d in the fihn lattice.

l:

Is = kqQm : IF = k/Qm : Ic: kg:

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l

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= 3.0 106V/cm ; E = 5.4 105V/cm ; EF = 0.2V ; kq -- .I s "1 ,

k 1 = . 0 6 s -1 ; k g = 2.104 A.s/cm 3 ; b = 3 7 V ' l ; t

= 1 0 0 s , Cd = 5 0 p F / c m 2.

1.S

AC Responseof RRDE during the passivationof iron

635

Evaluation of the charge stored in terms of differential capacitance on the basis of tile thickness given by the high field migration leads to 6.7mF.cm "2 (I0) a value substantially smaller than that provided by the RRDE: AQ/:/AE = 27mF.cm -2. A similar discrepancy observed with the capacitance deduced from very low frequency impedance data (10) was previously ascribed to the amount of charge involved in the dissolution process. Thi.n interpretation no longer holds for RRDE which essentially discriminates the dissolution component. The only acceptable explanation seems to be the storage of cations in the outer part of the passive film often described as the hydrated or colloidal layer. 6 Q m / A E is the transient charge storage as illustrated in Fig.4 which is the necessary condition to simulate the diagram for Nd III. The agreement between experiments and model is here again satisfactory, however, it was imposible to explain, on the basis of this model, the electrode behavior at less anodic potentials. CONCLUSION Passivation kinetics of iron in the vicinity of the Flade potential has been investigated by combining measurements of complex impedance and complex emission efficiencies of Fe It and Fe II!. A satisfactory interpretation must involve the relaxation of a transitory charge, small with respect to that involved in the passive film, acting as an intermediate of both film growth and Fe III dissolution. Ilowever in spite of the rather narrow potential range explored, it was necessary to switch from a 2-D to a 3-D layer model as the system progressively enters the passive state. This may result from some weakness of the model but certainly reflects the steep transition undergone by the film in the Flade region. This study illustrates the outstanding contribution of a.c. measurements with RRDE when devising a model of a film forming process. ACKNOWI.FI'K]EMENT R.C. is grateful to C.N.E.A. of Argentina and to C.R.O.U.S. of France for their financial support during his post-doctoral terms. REFERENCES

(i) (2) (3)

(4)

(5) (6) (7)

(8) (9) (10) (11) (12)

N. Bcnzckri, M. Kcddam and H. Takcnouti, in "Surfacc Inhibition and

Passivation", cd. E. McCafferty and R.J. Brodd, The Electrochenlical Society, Princeton (NJ), 86-7, 524 (1986). N. Benzekri, M. Keddam and It. Takenouti, Electrochim. Acta, in press. N. Benzekri, R. Carranza, M. Keddam and H. Takenouti, 174th Meeting of the Electrochem. Soc., Chicago (OH), October 1988, Extended Abstracts 88-2, n ° 186 (1988), Symposium Volume, Transient Techniques in Corrosion Science and Engineering, p.183 (in press). W.J. Albery and M.L. Hitchman, "Ring-Disc Electrodes", Oxford University Press, 1971. W.J. Albery, A.H. Davis and A.J. Mason, Faraday Disc. of the Chemical Soc., n ° 56, 317 (1974). T. Tsuru, N. Nishimura and S. Haruyama, Materials Sei. Forum, 8, 429 (1986). N. Benzekri, Thesis "Contribution au dEveloppement de 1'Electrode disqueanneau en courant alternatif. Application aux m6canismes de dissolution et passivation anodique", Paris, July 1988. K.E. tleusler, Ber. Bunsenges. Phys. Chem., 72, 1197 (1968). K.J. Vetter, Z. Elektrochem., 58, 230 (1954). M. Keddam, J.F. Lizee, C. Pallotta and H. Takenouti, J. Electrochem. Sot., 131, 2016 (1984). M. Keddam and C. Pallotta, J. Electrochem. Soc., 132, 781 (1985). C. Gabrielli, M. Keddam and H. Takenouti, this proceedings.