The influence of different cations on the eletrochemical and ellipsometric behaviour of iron in alkaline media

E/ecrrochunrca Am, Vol 36, No 7, pp 1143-l 150, 1991

0013-4686/915300+000 0 1991 Pergamon Press plc

Prnued rn Great Bntm

THE INFLUENCE OF DIFFERENT CATIONS ON THE ELECTROCHEMICAL AND ELLIPSOMETRIC BEHAVIOUR OF IRON IN ALKALINE MEDIA S JUANTO,R S SCHREBLER,* J 0 ZERBINO,J R VILCHE and A J ARVIA Instltuto de Investlgaclones Rslcoquhmcas Te6ncas y Aphcadas (INIFTA), Facultad de Clenclas Exactas, Unlversldad Naclonal de La Plata, Sucursal 4-C C 16, (1900) La Plata, Argentina (Recerved 24 May 1990, m revzsedform 20 September 1990) Abstract-The mfluence of Lr+, Na+, K+, Cs+, NH:, Ca2+ and Ba2+ tons on the electmchemrcal and elhpsometnc behavtour of Fe m alkahne soluhons has been studied at 25°C On the basis of a duplex layer structure for the anodtc layer, the results show only a small influence of the nature of the catrons m those process concemmg the inner bamer layer spectfically Othetwtse, there 1s a remarkable mfluence of the solutton consntuents on the growth and reachons at the outer porous portion of the anodtc layer In thrs case, the mfluence of cahons can be described m terms of a spectfic adsorpnon at the outer plane of the mner bamer layer, a catton we effect and a coprectpttatton effect The relative contnbutron of each effect depends on both the charge and the SEX of cations m solutton Key words passmty m dtfferent media

of iron, cation effects, duplex layer structure, alkahne solutions, elhpsometry of iron

INTRODUCTION The electrochemtstry of iron m alkaline soluttons has been studted for many years m relatton to metal corrosion and passtvation and to the iron electrode m alkahne battenes, particularly m NaOH[l-31 and KOH aqueous soluttons[4,5] and to a lesser extent m Ca(OH),[6] and LiOH-contaimg soluttons[7,24] The use of saturated Ca(OH), simulates the iron pieces wnhm pores m concrete[6] Most of these studies have focused on deterrmmng the structure and the composition of the surface oxide layers [4,5,8] More recently these aspects of iron electrochermstry have been mvesttgated through Raman spectroscopy[9, lo], photocurrent spectroscopy[l 11, X-ray photoelectron spectroscopy[l], elhpsometry [6, 12-151, and Mossbauer spectroscopy[l6] The anodtzation of iron m alkahne solution mvolves at least two Qstmgmshable oxtdatton levels At low potenttals iron 1s firstly oxidized to hydrous Fe(OH),[8, 171, and subsequently, at lugher potentials to non oxtde-oxyhydroxlde species[4, 17-201 In strongly alkaline solutions the mam reactton product m the imttal oxtdatlon stage is HFeO; [19], which m a secondary reaction yields msoluble Fe(OH), The exact composttton of the anodic layer or whether the passlvatmg film on iron m alkaline soluttons conststs of a btlayer or a single layer structure is still not completely understood Results from zn sztu elhpsometry[ 12, 13, 15,2 11, Raman spcctroscopy[9, lo], and XPS of passive layers on iron with an electrode transfer m a close system[l], favour

*Permanent address Instrtuto de Quimlca, Facultad de Ctencras B&as y Matematicas, Umverstdad Cat&a de Valparaiso, Chile

a duplex passive layer structure[l3, 15,221 The latter can be described as a thm inner iron oxide layer (layer I) and an outer non-protecttve porous oxyhydroxtde layer (layer II) which probably results from iron hydroxide residues that accumulate durmg the sequence of oxtdatton and reduction cycles (ORC) under penodtc potential scans Layer II apparently behaves as a matnx where the Fe(II)/Fe(III) redox process takes place along the ORC[ 1,l S] A stmtlar structure has recently been concluded from experiments mvolvmg a prolonged anodtzation at hrgh posittve potenttals[l5,21] In contrast, the passive layer has been also described as a single phase layed 1,231 m which the electrical and optrcal behavtour become influenced by potential dependent dtstnbuttons of Fe(I1) and Fe(II1) m the oxide film By growmg the surface film under potenttostattc condttions, the mcorporatlon of foreign cations mto the oxide structure was deduced from the analysts of current transients made at different temperatures[25] The adsorptron of Ca’+, Ba2+ and Mg2+ on iron was also detected by means of differential capacity measurements[26] Otherwse, the presence of Ca2+ ions m the oxide fllm was concluded from elhpsometnc measurements[6] Nevertheless, m tms respect, further work 1s required m order to establish quantttattvely the mfluence of the solution composttton on the behavtour of anodtcally formed oxide layers on iron Tlus work mvestigates the influence of the cations present m alkaline solutions on the electrochemical and elhpsometnc response of anodically formed oxide layers on polycrystalhne iron For this purpose LtOH, NaOH, and CsOH, and Ca(OH),, Ba(OH)2, and NH40H solutions at pH 12 6 and 25°C were used It appears that the composition of the electrolyte modifies considerably the characteristics and

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2. EXPERIMENTAL The expenmental arrangement was essentially the same as that already described m previous pubhcatlons[lS, 211 The working electrode (specimen) consIsted of a polycrystalhne non (“Specpure”, Johnson Matthey Chemicals Ltd) &SC (0 85 cm* apparent area) mounted m a PTFE holder Each specimen was gradually polished starting wth 400 and 600 grade emery papers and fimshmg with 0 3 and 0 1 pm gnt alumma-acetone suspensions Afterwards it was cleaned with acetone and repeatedly rmsed Hrlth tnply dlstllled water Finally, It was cathodlcally polanzed for 5 mm m the potential regon of net hydrogen evolution The resulting electroreduced surface speamen yielded a reproducible electrochemical response Electrode potentials were measured against a hydrogen electrode m the same solution The reference electrode was connected through a Luggm-Haber capillary tip Potentials m the text are referred to the nhe scale A large area Pt plate was employed as counterelectrode The followmg electrolyte solutions were used 0 04M LlOH, 004M NaOH, and 0 04M CsOH (group A), and NH,OH, Ba(OH),, and saturated Ca(OH), (group B) at pH 12 6 They were prepared from tnply distilled water previously boiled to remove CO,, and analytical grade reagents The electrolysis cell was mounted m a Rudolph Research type 437-02/200 B manual elhpsometer (maximum resolution 0 01’) provided with a 150 W tungsten lamp wth filter (546 1 mn) and a RCA IP 21 photomultlpher The incidence light beam angle was fixed at 69” and that of the compensator at 135” Electrochemical runs were performed with either single (STPS) or repetitive (RTPS) tnangular potenteal sweeps between fixed cathodic (E,,) and anodlc (E,,) swltchmg potentials at different potential sweeps (u) m the range 0 0004V/s I u < 2OV/s Elhpsometnc data at fixed wavelength of a freshly polished iron electrode which was electroreduced at E = - 1 3 V were mltlally measured The correspondmg polanzer (P,) and analyser (A,) readings yield the refractive mdex (n,) and the absorption coefficient (k,) The complex refractive mdex of the substrate, ii, = n, - rk,, derived from P,, and A0 was m good agreement with the value for the bare iron surface reported m the hterature[27,28] The elhpsometnc readings (P and A) of the specimen covered with the anodlc layer were taken at either E,, or EsB after applying one of the three following procedures Procedure I involved a RTPS treatment at different v m order to accumulate a certain amount of the anodlc layer at the electrode surface Procedure II consisted of a potential sweep at 20 V/s followed by a potential holding at Es,+to produce the growth and stablhzatlon of the anodlc layer Procedure III comprised a potential cycling for a certain time, and subsequently switching off the current either at EsE or Es B Then, the elhpsometnc

readings were made at the correspondmg stationary open clrcult potential, te at the potential where

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Fig 1 Voltammograms run m 0 04M NaOH at u =004V/s between Es,= -13VandE,,=OV Thefirst cycle and the voltammograms after 30, 90 and 120mm cycling are shown

the RTPS perturbation was again started In any case, the elhpsometnc readings were presented as conventional 6P us SA plots, where 6P = P - POand dA=A-A, 3. RESULTS The RTPS voltammograms m 0 04 M NaOH (reference

of polycrystallme iron system) run between

1 6

01 -I-

Fig 2 Voltammograms run m 0 04 M LIOH (a) and 0 04 M CsOH (b) at v=OO4V/s between E,,= -1 3V and Es, = OV The lst, 5th, lOth, lSth, 30th, 80th and 110th cycles are shown

Electrochemical and elhpsometnc hehavlour of iron E,,=-126VandES,~=O04Vatv=004V/s,that IS, under the condltlons of procedure I, show two mam ano&c current contnbutlons (II and III) durmg the first postttve potential going sweep, and two mam cathodic wntnbutions (IV and V) preceding the HER durmg the reverse scan The latter process can be clearly observed at potenttals lower than - 1 2 V (Fig 1) It should be noted, however, that this type of voltammogram, as shown m prevtous pubhcatlons[4,5, 15,201, exhibits addtttonal contnbuttons which can be enhanced under certam expenmental condltlons These contnbutlons are not dtstmgushable m Fig 1, but some of them can be seen m Ftg 2 They are located at potenttal ranges which are mdtcated by I, III’ and Iv’ m Fig 1 The nomenclature and the assignments of the &fferent current contnbutlons follow that indicated m prevtous pubhcations referred to iron voltammetry m strongly alkaline media[4,5, 15,201 The preceding descnption of the voltammogram changes gradually along the potential cycling unttl a stable voltammogram 1s finally attamed Peaks III-III’ and IV’-IV always behave as those related to conlugated redox reactions The voltammograms run under comparable conditions m 0 04M LlOH (Fig 2a) and m 0 04 M CsOH (Fig 2b), exhibit features which are quahtatlvely similar to those described m Fig 1 This descnpbon applies to the electrochenucal response of iron m the group A solutions Neverthe-

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(aI

NaOH

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(bl

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Fig 4 Voltammograms run m NH,OH (a), saturated Ca(OH), (b), and x M Ba(OHh (c) solutions, pH 12 6, at v=OO4V/sbetweenE,,=-13VandE%,=OV Thefirst cycle and the voltammograms after merent potential cychng

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Fig 3 6P us 6A plots Data resulting at E,, (curve R) and E,,a (0) (a) 0 04 M NaOH, (b) 0 04 M LlOH, (c) 0 04 M CsOH Freshly electroreduced iron electrode held 5 mm at E,,(x)

times

are shown

less, it should be noted that the accumulation of charge m NaOH and CsOH soluttons becomes greater than m LtOH solutton Prevtous works have shown that the voltammograms of iron m NaOH and KOH solutions at the same pH exhlht under comparable expenmental wndlhons prac~ally no dtfference[4,5] The elhpsometnc plots (Fig 3) resaultmg from procedure 1 m the group A solutions exhtht two envelopes movmg clockwise with the increase of the voltammetnc charge, and as a charactensttc feature the lmtlal change of the elhpsometnc parameters exhlbtts a negative slope The outer loop measured at Es,acorresponds to the oxidized state of the anodlc layer, whereas the inner loop resultmg at Es,c, belongs to the reduced state Furthermore, the elhpsometnc loops are greater m 0 04 M CsOH than m either 0 04 M NaOH or 0 04 M LlOH The voltammograms of iron m group B soluttons under potential cychng show an accumulation of charge much smaller than m group A solutions (Fig 4) Correspondmgly, the elhpsometnc plots (Fig 5) exhibit an mltlal poslhve slope for both loops Likewise, at a constant potential cychng ttme, the difference between the elhpsometnc readings at E,,, and Es, m group B solutions becomes considerably smaller than that found for group A solutions A very dlstmcttve feature of these solutions 1s the crossing of the elhpsometnc loops at a certain time of the ORC perturbation routme

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Fig 5 6P USSA plots Data resultmg at E,, (curve R) and E,, (0) (a) NH&H, (b) WOW,, (c) WOW,

When the results obtained for solutions of groups A and B are compared one concludes that under slmdar expenmental condlhons the increase of the anodlc voltammetnc charge, either the absolute (Q) or the relative (Q/Q,,) value, with cycling time (Fig 6), where Q, is the anodlc voltammetnc charge obtained from the first cycle, changes with the electrolyte composltlon m such a way that the slope (q) of those plots decreases m the order q[KOH] = q[NaOH] 2 q [CsOH] > q[LiOH] g q[NH,OH] > q[Ba(OH),] z q[Ca(OH),] Furthermore, the values of P and A measured at Es,c (Figs 3 and 5) and the gradual voltammetnc changes along the ORC (Rgs 1, 2 and 4) indicate that it 1s qmte likely that a i!5i!O7 E 15-

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E \, 0

WI K+ ,.I

cm*

20

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60 cychng

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tme / mm

Fig 6 Dependences of Q (a) and Q/Q0 (b) on the cycling time Data from voltammograms run under the condltlons Indicated m Figs 2 and 4

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Fig 7 Influence of the solution composltlon at pH 12 6 on the 6P us 6A plot at E,, = - 1 3 V (open symbols cathodic, C) and E,, = 0 V (solid symbols anodlc, A) for the Ist, the 2nd, and the 3rd potential scan at u = 0 2 V/s (a) LIOH, (b) NaOH, (c) CsOH, (d) NH,OH, (e) Ca(OH),, (f) Ba(OH),

part of the anodlc product either m the electrooxldlzed or the electroreduced form remains on the iron surface even at high negative potentials This different elhpsometnc behavlour of the correspondmg Iron oxide layers can be more clearly seen by employmg those potential routines which include either a few potential cycles at lugh tr (Fig 7) or a fast potential step followed by a prolonged potential holding at a certain preset value (Fig 8) Elhpsometnc data at Es,a = 0 01 V and Es,c = - 1 26 V, during a few potential cycles at u = 0 2 V/s (procedure I), obtained m the different solutions (Rg 7), show that for group A solutions the elhpsometric parameters shift m the same directions For a constant cycling time the magnitude of the change observed m the optical parameters becomes for 0 04 M CsOH (Fig 7c) greater than for 0 04 M LlOH and 0 04 M NaOH (Fig 7a and b) Otherwise, dlfferent elhpsometnc behavlours were found m group B solutions Thus, for NH*OH solution, the elhpsometnc readings at either E,,a or E,,c result practically comcldent m the first three potential cycles (Fig 7d), but for saturated Ca(OH), solution the value of 6P Increases m Jumping the potential from E,, to Es,a The same effect can be observed as both Es,c and Es,+ Increase along the ORC (Fig 7e) The 6P us 6A plots correspondmg to the iron oxide layer formed m Ba(OH), solutions are intermediate between those found for group A solution and m saturated Ca(OH), In these cases the elhpsometrlc parameters change also m a different way for both groups of

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Electrochenucal and elhpsometnc behavlour of iron

CsOH

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Independently of the ORC Nevertheless, as E,, IS postttvely shifted, the elhpsometrrc plots for group A soluttons show tmmdately the appearance of two loops correspondmg to the oxtdrzed and the reduced states of the surface layer, respectrvely Tins fact whtch resembles that depicted m Fig. 3, suggests that the charactenstrcs of the reduced state of the passive layer appears to be independent of E,,, although the barner layer thickness increases according to E,, It can also be compared to that depicted m Fig 5 for group B solutrons where the same conclusion appears to be vahd Under these crrcumstances, the large shift of the outer loop resulting for Es, set at a value more posttrve than the potentral range of peak III, can be associated wtth the oxidation reactton at peak III, re rt is mainly related to those processes occurrmg in the outer part of the passive layer Accordmgly, the large an&c and cathodic voltammetrrc charges obtained by changing E,, from -0 54 to 0.01 V result m the gradual accumulatron of spectes at the outer part of the layer durmg cychng wtthm a more restncted potential range (Figs 9 and 10) Strmlar results obtained for iron electrodes m NaOH soluttons have been reported elsewhere[ 151

1

&P/degree

Fig 8 Influence of the solution composltlon at pH 12 6 on the SP us S A plot STPS at D = 2OV/s between EW = - 1 3 V (open symbols cathtic, C) and different E,,, values (solid symbols antic, A) mcludmg an m&m&ate 60 mm holding at E,, E, = -0 44O(lA), 0 010 (2A), 0 150 (3A), and 0 225 V (4A) solutions

when the potential

4. DISCUSSION The mfhtence of the electrolyte compostnon on the prepasstvatron and passtvatton of iron m alkaline soluttons 1s considerably dtfferent dependmg whether one deals wtth the early stages of the ano&c layer

routine mvolves a poten-

tial scan at v = 2OV/s from E,, = - 1 36V up to dtfferent Es, values, followed by a potenttal holding durmg 60 mm at Es,* (procedure II) (Fig 8) Correspondmgly, for monovalent catton contatmng solutions the elhpsometrtc plot moves wtth a negative slope as the holdmg ume (t’) at E,, 1s increased, whereas for saturated Ca(OH), it moves m the opposite &r&ton From results shown m Fig 8, it can be concluded that at constant Es,a the tmttal change of the opttcal parameters detected after the fast potentral sweep, becomes vtrtually independent of the solutron composttton Thrs fact suggests that the imttal amount of am&c product (Inner layer) IS determmed by the preset E,,a values, whereas the mfluence of the solutton compostnon affects prmcipally the charactensttcs of those surface products which are accumulated (outer porous layer) durmg a prolonged anodtsatton For group A soluttons the elhpsometnc data obtamed throughout procedure III by setting E,,c = -1 36Vand E,,a= -0 53 V, that ts, m the potential range precedmg peak III (Figs 9 and lo), become nearly similar for 0 04 M LIOH and 0.04 M CsOH even after 250 cycles The difference between the successive opttcal readings at E,$ and Es,a starting from the first cycle onwards are practically constant and independent of the number of cycles These results indicate that the opttcal parameters of the iron oxide layer produced at potentials lower than the potential range of peak III, become nearly the same,

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E/V Fig 9 (a) Voltammograms run m 0 04 M LlOH at v=OO4V/s, E,,= -1 3V and E,,= -053V After 250 potential cycles the anodlc potential hmlt was changed to E,, = 0 V (b) bP vs SA plots correspondmg to Qfferent cycling times as mdlcated for procedure III with the perturbing potential routme mdxated m (a)

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Fig 10 (a) Voltammograms run m 0 04 M CsOH at 0=004V/s, E,,=-13V and ES,=-053V After 250 potential cycles the anodlc potential hmlt was changed to E,,a= 0 V (b) SP US A4 plots correspondmg to different cycling times as described for procedure III with the perturbmg potential routme mdxated m (a)

growth (inner layer formation) or mth the thlckenmg of the anodlc layer (outer layer formatlon)[lS, 291 The mltlal changes resulting m the elhpsometnc plots shown m Fig 8, are virtually independent of the electrolyte composltlon This fact suggests that the charactenstlcs of the mltlally formed inner barner layer, 1s mainly determined by the Es, value On the other hand, the different changes observed m the elhpsometnc plots for the electrolyte solutions of groups A and B, become clearer after a prolonged anodlzatlon at Es,a (Fig 8), and they are

comparable to those displayed m Figs l-5 which involve a prolonged potential cycling As recently dlscussed[36], these changes of the elhpsometrlc plots can be associated with the amount of iron hydroxide precipitated m the different electrolyte solutions Thus, the amount of u-on hydroxide preclpstated m Ca(OH), solution 1s apparently greater than that formed m NaOH solution at the same pH, but the outer part of the passive layer formed on Fe m NaOH solution appears to be more compact than that produced m Ca(OH), solution Hence, the changes m the slope of the elhpsometnc plots suggest that the thickening of the outer porous layer 1s considerably influenced by the electrolyte composition The growth of the outer porous layer can be produced either by potential holdmg (Fig 8), or by potential cycling (Fig 7) In both cases the transport of iron ions through the barrier layer 1s reqmred, although the overall process 1s undergone under

different condlhons, particularly concermng how far the system 1s from equlhbnum The effect of E,,a on the Fe(II)/Fe(III) redox couple at the outer oxide layer (Figs 9 and 10) 1s associated with the complex peak III The results m Fig 6b show that the accumulation of charge for those solutions containing Ca2+ and Ba*+ ions IS considerably smaller than that observed for those solutions containing single charged cations, re both Ca*+ and Ba*+ ions exhlblt an mhlbltlon effect on iron corroslon[30,31] Previous dlscusslon of electrochemlcal and elhpsometnc data of Fe m saturated Ca(OH),[l5], allowed us to conclude that the refractive index of the anodlc layer smoothly increases along the potential cycling This fact was interpreted m terms of a progressive decrease of the average amount of water remaining m the anodlc layer, the entire effect being dependent on the cation present m the solution Therefore, It seems reasonable that the cation miluence on the voltammetnc accumulation of charge could be attnbuted at least to two contnbutlons, one derived from the ion size m solution and another one related to the specific ionic charge The correspondmg parameters are given m Table 1, where rp denotes Pauhng lomc radu and d, stands for the average distance between ions and the nearest water molecule[32-341 The anodlc layer structure produced on Fe m the solutions used m this work can be related to the film model already described for Fe m 0 04 M NaOH[ 151 It consists of an inner part made of a Fe,O,-type film as supported by optlcal data[l3,35-371 and an outer part mainly formed by the relatively thicker FeOOH/Fe(OH), hydrous layer Hence, as the structure of the passive layer 1sequivalent to two electrode capacitors m senes one should expect that m the potential range where the inner passlvatmg layer 1s formed, the adsorption of blvalent cations at the outer plane of the inner layer should become stronger than that of monovalent cations Accordmgly, a decrease of the strength of the electnc field asslstmg the displacement of iron ions from the substrate through the inner layer to the outer layer region should be expected This explanation 1s consistent with the scarce influence of cations at the mner layer level and their possible insertion mto the passlvatmg layer structure, mainly at the inner/ outer layer boundary region, as it has been demonstrated specifically for the case of Ca*+ containing solutlons[6, 36,381

Table 1 Ionic sizes of different cations Involved m the present work taken from Refs[32-341 Catlon

r,lnm

d, inm

Ll+ Na+ K+ cs+ NH: Ca2+ Ba2+ Fe*+ Fe’+

0 068 0 095 0 133 0 169 0 148 0 099 0 135 0 074 0 065

0 207 0 237 0 273 0 308 0 305 0 242 0 260 0212 0204

d, = 0 1376 + 1 0167 r,,

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Electrochemical and elhpsometnc behavtour of Iron

As it has been proposed elsewhere[9,13, 151 the outer porous anodlc layer can be considered as a sort of matnx contamng the oxldzed (Fe3+) and/or the reduced (Fe*+) cations, the prevtuhng species being determmed by the applied potential con&tions Furthermore, the charge accumulated at the outer layer increases ~th the cation present m group A solutions m the followmg way Ll+ < Cs+ < Na+ Accordmgly, the hfferences between the elhpsometnc data for the oxldlzed and the reduced states at the outer layer m those solutions contalmng single charged metal cations are relatively large (Fig 3) This fact indicates that the potential cychng effectively promotes changes m the optical response of the ano&c oxide layer Othemse, these changes become considerably smaller for the anodlc oxide layers produced m group B solutions conttimng double charged metal cations (Fig 5) The aforementioned explanation 1s consistent with the spectroscopic charactemtlon data of passive films of iron wluch have shown the gradual conversion from Fe(H) to Fe(II1) valency state[39], the presence of water m the surface layer at the early stage of iron passlvation[40], and the amorphousness of the hydrous polymenc-hke passive layer[41]

5. CONCLUSIONS There 1s only a slight dependence of the barner layer growth on the electrolyte composltlon at least for the solutions employed m the present work In contrast there 1s a conslderable influence of the solution constituents on the growth and reactions takmg place at the outer porous layer The influence of the cations on the entire process can be described m terms of the followmg contnbutlons (1) specific adsorption at the outer plane of the inner barner layer, this effect largely predominates for blvalent cations, (u) cation size either solvated (Ll+) or nonsolvated (Cs’) which decrease the growth rate of the outer layer as compared to Na+ or K+, either by changmg the actlvlty of water or by mterfermg the kinetics of the Fe-Fe’+ + 2e- reaction, (m) copreapltation effect, particularly for Ca’+ and Ba2+ contaming solutions, a fact wluch 1s supported by the relatively lower optical indexes (lower density) obtamed for the outer layer m these solutions as compared to monovalent cation containing solutions (see Figs 9 and 10 and Ref [36]) The behavlour of Fe m NH,OH solution follows the typical response m group B solutions while cycling, but the anodlc growing at a constant potential tends to become similar to that observed m group A solutions On the other hand, one should expect that besides the adsorption effect Just described for blvalent cations the reactions at the outer layer growth should also change Hrlth either the cation charge or the value of d,, that is, the hydration sheath structure At first right no simple correlatron emerges from those data shown m Fig 6 If one takes exclusively the results obtained wth monovalent cations It appears that Ll+ as a hydrated ion (d, = 0 207 nm) and Cs+ and NH: as non-hydrated ions (rp = 0 169 nm and rP = 0 148 nm, respectively) are hmdenng the outer

layer growth, whereas those ions such as Na+ and K+ mvolvmg d, values not too far from those of Fe2+, are apparently offermg the rmmmum hmdrance effect to the outer layer growth. TIM fact can be explamed as an addttlonal interference of the catlons through thar s@.~fic size, either with or Hrlthout a strongly bound solvatlon sheath, mto the formation of the Fe(OH),/Fe(OOH) constituents at the outer layer regon Tlus mterference m the outer layer growth rate can be due ather to a modification of the local activity of water reqmred for the reaction or to the proper ion field effect on the kmetlcs of the process As extreme examples, the former case can be assigned to the presence of hydrated Li+ ions, whereas the latter can be related to solution constituents such as Cs+ and NH,+ These effects should also be present m the case of blvalent catlons, but probably largely masked by the specdic adsorption effect already described m precedmg paragraphs In addition, the possible contnbution of preapitatlon reactions should not be discarded to explain the results obtained m the Ca2+ and Ba2+ contammg solutions (&[Ca(OH),] = 4 68 x lO-‘j and & [Ba(OH), 8H,O] = 2 55 x lo-‘)[42] Finally, the different elhpsometnc behavlour of the anodlc layers accumulated m Ba(OH), and NH,OH solutions through prolonged anodzatlon (Fig 8) are consistent Hrlth the fact that soluble Fe2+ species are produced durmg the electroreduction of the passive layer Acknowledgements-Tius research work was financmlly supported by the ConseJo Naclonal de Investlgaclones Clentificas y T&xxas and the Cormsl6n de Investigaclones Cientificas de la Provmcta de Buenos Anes

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