study of

Electrochimica Acta51(2005)

745–753

Study of lepidocrocite?-FeOOH electrochemical reduction in

neutral and slightly alkaline solutions at25?C

H.Antony a,L.Legrand a,?,L.Mar′e chal b,S.Perrin b,Ph.Dillmann c,A.Chauss′e a

a Laboratoire Analyse et Environnement,UMR8587CNRS-Universit′e d’Evry-CEA,Universit′e d’Evry Val d’Essonne,

rue du P`e re Jarland,91025Evry,France

b Laboratoire d’Etude de la Corrosion Aqueuse DEN/DPC/SCCME,

CEA Saclay,91191Gif Sur Yvette,France

c LRC CEA DSM01.27,Laboratoire Pierre S¨u e,CEA/CNRS,DSM/DRECAM/SCM,CEA Saclay,91191Gif Sur Yvette,France

Received22February2005;received in revised form13May2005;accepted26May2005

Available online18July2005

Abstract

The electrochemical reduction of lepidocrocite?-FeOOH was investigated at25?C in neutral or slightly alkaline solutions containing chloride,sulphate or bicarbonate anions by means of thin lepidocrocite?lm electrodeposited on inert gold substrate or graphite/lepidocrocite powder composite electrode.Electrochemical measurements were coupled to in situ electrochemical quartz crystal microbalance(EQCM) and ex situ SEM and FTIR analysis.The reduction of lepidocrocite occurs in all the electrolytes considered here.The initial reduction product is adsorbed ferrous ion,Fe ads2+.The desorption of Fe ads2+is promoted as pH increases,leading to an increase of reduction depth.This promotion is related to the formation of secondary Fe II-containing species,as revealed by SEM and FTIR.The comparison of?-FeOOH reduction potentials and iron corrosion potentials let us state that the galvanic coupling is possible.

?2005Elsevier Ltd.All rights reserved.

Keywords:Lepidocrocite;Electrochemical reduction;Indoor atmospheric corrosion;EQCM

1.Introduction

French nuclear waste could be packaged in low alloy steel containers for interim storage which could last more than 100years.In the storage conditions,the condensation on the container walls can conduct to atmospheric corrosion caused by a succession of wet and dry periods.These periods,called wet–dry cycles,are due to variations of relative humidity and temperature.

During the atmospheric corrosion of iron,different phases (iron oxides and iron oxy-hydroxides)constitute the rust layer [1,2].An example of a rust layer,which has aged in environ-mental and indoor condition during150years,is shown in Fig.1[3].The four main phases constituting the rust layer are ?Corresponding author.Tel.:+33169477705;fax:+33169477655.

E-mail address:ludovic.legrand@chimie.univ-evry.fr(L.Legrand).goethite(?-FeOOH),lepidocrocite(?-FeOOH),magnetite (Fe3O4)and maghemite(Fe2O3).The ratio between the two oxy-hydroxide phases(?/?)is called protective ability index because goethite is supposed to be a protective constituent whereas lepidocrocite is supposed to participate to the corro-sion processes[4,5].

In order to predict the damage due to atmospheric cor-rosion and so the evolution of the rust layer,a mechanistic modelling has been established for each stage of the wet–dry cycle[6].

During the?rst stage(wetting stage),the anodic dissolution of iron is mainly balanced by the reduction of lepidocrocite:?-FeOOH+e?+H+→?-Fe·OH·OH(1)

This reduction is considered to be the rate-limiting step of the corrosion.The amount of oxidized metal depends on the quantity of reduced lepidocrocite.

0013-4686/$–see front matter?2005Elsevier Ltd.All rights reserved. doi:10.1016/j.electacta.2005.05.049

746H.Antony et al./Electrochimica Acta51(2005)

745–753

Fig.1.SEM micrograph of a150years old rust layer. During the second stage of the cycle(the wet period),the reduction of dissolved oxygen on the lepidocrocite,which has been previously reduced,controls the mechanism:

O2+4e?+2H2O→4OH?(2) The amount of oxidized metal mainly depends on the oxy-gen diffusion in the electrolyte and in the rust layer.

At the end of the cycle,the blocking of the anodic sites is considered to describe the extinction of electrochemical corrosion during the drying.

This short description of the modelling show the rele-vant role of lepidocrocite in the corrosion processes.As a consequence,in the present study,we report results on the electrochemical reduction of lepidocrocite.A thin?lm of lepidocrocite electrodeposited on Au substrate[7]was used in order to get information on the reduction mechanism.Pow-der/graphite composite electrode was also used since it might better mimic the behaviour of a corrosion layer,due to the low pore volume of electrolyte.Moreover,as we plan to further investigate the electrochemical behaviour of some archaeo-logical artefact rust samples by using rust/graphite composite electrode,it was then of importance to have information about the electrochemical response of such electrode in the case of FeOOH compounds.

2.Experimental

2.1.Chemicals

Na2SO4·10H2O(ACS Reagent,Aldrich),NaHCO3 (ACS Reagent,Aldrich),NaCl(RP Normapur,Prolabo), FeCl2·4H2O(99%,Aldrich),HCl(1M,Titrinorm,Pro-labo),NaOH(30%solution,RP Normapur,Prolabo),1-methyl-imidazole(99%,Aldrich,pH-buffering agent with p K a～7.2),1,4-piperazinediethanesulfonic acid,disodium salt(PIPES)(Aldrich,pH-buffering agent with p K a～6.8), N-tris[hydroxymethyl]methl-3-aminopropane-sulfonic acid (TAPS)(Aldrich,pH-buffering agent with p K a～8.4)and graphite(Fluka)were used without further puri?cation.The solutions were prepared with18M cm nano-pure water and deaerated with Argon(U,Air

Liquide).Fig.2.XRD pattern of lepidocrocite powder.Cu K?;step time=38s;step size=0.05;2θ;(#)peak because of apparatus.

2.2.Synthesis of lepidocrocite powder

Lepidocrocite(?-FeOOH)powder was synthesised at

25?C following a procedure drawn from Schwertmann and

Cornell[8].One hundred millilitres of18M cm nano-pure

water were introduced into a200mL glass beaker equipped

with a stirrer,a combined pH electrode and a burette con-

taining1M NaOH solution.Then,4g of FeCl2·4H2O were added and the mixture was left in contact with air under stir-

ring(500rpm).NaOH(about30mL)was continuously added

during the synthesis in order to maintain the pH within the

6.7–6.8range.After about3h,the completion of the oxida-

tion reaction was obtained,as revealed by the orange colour

of the suspension.Filtration was done and the solid was dried

at ambient temperature.Pure lepidocrocite was obtained as

revealed by XRD analysis(Fig.2).The iron content of lepi-

docrocite powder,as determined by complexometric titration

of Fe III by EDTA[9],is58wt%.The comparison of the latter

value with the theoretical one may indicate that our lepi-

docrocite powder is partially hydrated.

2.3.Electrochemical synthesis of lepidocrocite thin?lm

Lepidocrocite thin?lms were synthesised at the surface

of gold substrate by potentiostatic oxidation(E=0V)of

soluble Fe(II)in0.4M NaCl/0.02M Met-Imidazole/0.01M

FeCl2solution at pH7.5and T～22?C under argon bub-

bling.The main features of electrolysis and electrodeposits

are only summarised here;complementary information can

be found in our previous paper[7].The current density was

in the0.05–0.15mA cm?2range;it decreased with oxida-

tion time duration,consistently with the progressive surface

coverage by ferric oxy-hydroxide particles.The coulombic

charge values were between several tenths to several hundreds

mC cm?2.A mass gain was observed along the oxidation

experiment.A m–Q linear relationship was obtained and

the formation of partially hydrated lepidocrocite,FeOOH,

n H2O with n～1,was assumed from the value of m–Q

H.Antony et al./Electrochimica Acta51(2005)745–753747

plot slope.FTIR,XRD and SEM measurements showed the formation of pure lepidocrocite thin?lms with a tendency to(001)preferentially oriented growth.The mass of lepi-docrocite deposit and coulombic charge involved during the synthesis will thereafter be expressed as m o and Q o,respec-tively.

2.4.Electrodes

Polished gold disc(Goodfellow,99.95%,laminated, 0.785cm2)or5MHz Au/Ti polished quartz crystal(Max-tek Inc.,1.37cm2)were used as a working electrode for the electrochemical studies of lepidocrocite thin?lms.The working electrode used for the electrochemical studies of lepidocrocite powder consisted of a Pt grid with a geometric area of1cm2on which the cathodic material,a mixture of lepidocrocite(20wt%)and graphite(80wt%),was pressed (mass:15–20mg,pressure:5tonnes cm?2for1min).Au wire and homemade AgCl/Ag(0.1M NaCl)electrode were employed as the counter and reference electrodes,respec-tively.All potentials are referred to SHE.

2.5.Apparatus

Electrochemical measurements were conducted with an EGG PAR273A potentiostat/galvanostat.Electrochemical quartz crystal microbalance(EQCM)measurements were performed with a Maxtek Inc.RQCM plating monitor cou-pled to an AUTOLAB PGSTAT30potentiostat/galvanostat (Eco Chemie).IR spectra were recorded on a Bruker IFS28 FTIR spectrometer.A re?ection–absorption tool(Grazeby Specac)with variable incidence angle allowed us to anal-yse the thin?lm directly on the disc surface without scrap-ing procedure.The FTIR spectrum of lepidocrocite powder was taken using transmission mode and KBr pellet.The electrical conductivities of lepidocrocite/graphite composite electrodes were measured in air at22±0.5?C by complex impedance measurements(frequencies from106to1Hz) using a frequency response analyser(Solartron1255)cou-pled to a potentiostat-galvanostat(EGG PAR273A).The lepidocrocite/graphite mixtures were pressed into pellets by applying a pressure of5–6tonnes cm?2for1min,using a hydraulic press.The pellets were1.25cm2in surface area and their thickness ranged from165to215?m.For the con-ductivity measurement,the pellet was placed between two blocking stainless steel electrodes.

3.Electrochemical reduction of lepidocrocite thin

?lms

3.1.E–Q and m–Q transients

The reduction behaviour of lepidocrocite thin?lm was studied at T=25?C in neutral or slightly alkaline pH-buffered solutions containing chloride,sulphate or bicarbonate

anions,Fig.3.(a)E–Q and(b) m–Q transients recorded for the reduction of ?-FeOOH thin?lm in0.1M NaCl/0.05M PIPES solution at pH7.5and T=25?C,I c=12.7?A cm?2;(c) m–Q transient recorded for the deposi-tion of?-FeOOH thin?lm(see text for conditions);(d)mirror curve of(c) representing the theoretical m–Q transient that should be obtained for the reduction of?-FeOOH into soluble Fe II species.

by applying a cathodic current density,I c(between5and 35?A cm?2).The E–t and m–t transients recorded with the electrochemical quartz crystal microbalance were con-verted to E–Q and m–Q transients(Fig.3,curves a and b).The m–Q transient obtained for the deposition of lep-idocrocite(Fig.3,curve c)thin?lm and the corresponding mirror curve(Fig.3,curve d)are also reported.With a depo-sition coulombic charge Q o=100mC cm?2,the deposited mass of lepidocrocite,m o,is115?g cm?2and the slope is110g F?1.The latter value is higher than the theoreti-cal value that should be expected for the formation of?-FeOOH from soluble ferrous species,89g F?1,indicating that the lepidocrocite thin?lm may be partially hydrated or that the EQCM response may be in?uenced by the deposit morphology and by the presence of electrolyte in the inter-particle spaces.The potentiometric curve displays a potential decrease followed by a plateau around?0.2V;both pro-cesses are related to the reduction of?-FeOOH thin?lm since they are not observed on the blank curve of Au elec-trode.The end of this reduction process is revealed by the potential drop.At the beginning of reduction, m gradu-ally decreases;then,a quasi-linear variation is reached from which a negative slope value of69±2g F?1can be deter-mined.Finally, m becomes constant as the potential drop occurs.The slope value is lower than the one expected from the one-electron dissolutive reduction of solid?-FeOOH into soluble Fe II species.From Fig.3,several values can be computed such as the potential value recorded on the potential plateau at Q=Q?/2,E?/2,the coulombic charge involved during lepidocrocite reduction(Q?),the masses of reduced and dissolved lepidocrocite(m r,d),of reduced and not dissolved lepidocrocite(m r,nd)and of not reduced and not dissolved lepidocrocite(m nr,nd).These values are expressed as a percentage by dividing by Q o or m o.A part of the

748H.Antony et al./Electrochimica Acta51(2005)745–753 lepidocrocite thin?lm,40%,is not affected by the reduction

process since the reduction ef?ciency,%Q?,is only60%.

Taking into account the intercept values of curves(b)and(d)

with the vertical line at Q=Q?,the%m r,d,%m r,nd and%

m nr,nd values can be determined,33,27and40%,respectively.

3.2.In?uence of the quantity of deposited lepidocrocite

Reduction experiments were performed in0.1M

NaCl/0.05M PIPES solution at pH7.5and T=25?C,with

various quantities of deposited lepidocrocite(Q o from25to

250mC cm?2,i.e.m o from～26to～290?g cm?2).Fig.4

reports the evolution of E?/2(curve a),%Q?(curve b),%

m r,d(curve c)and%m r,nd(curve d),as a function of Q o.The

in?uence of Q o on E?/2values is almost negligible,only a very

low diminution is observed.At low Q o,%Q?values should

approach100%.As Q o increases,%Q?decreases;beyond

～100mC cm?2,rather stable%Q r values are obtained,about

55–60%.A similar tendency is observed for%m r,d values.

Regarding%m r,nd values,only a low diminution from about

30to25%is recorded in the Q o range investigated here.

3.3.In?uence of pH

The in?uence of pH on the lepidocrocite reduction was

investigated in0.1M NaCl solution at T=25?C.In the

5.8–8.2pH domain,0.05M PIPES(p K a=

6.8)was used

as

Fig.4.Evolution of E?/2(a),%Q?(b),%m r,d(c)and%m r,nd(d),as a function of the quantity of deposited lepidocrocite Q o;0.1M NaCl/0.05M PIPES solution at pH7.5and T=25?C,I c=12.7?A cm?2

.Fig.5.Evolution of E?/2(a),%Q?(b),%m r,d(c)and%m r,nd(d),as a function of pH;0.1M NaCl/0.05M PIPES or TAPS solution at T=25?C,

I c=12.7?A cm?2,Q o=100mC cm?2.

a pH-buffering agent;in the8.2–9.2pH domain,PIPES was

replaced by TAPS(p K a=8.4).In curve(a)of Fig.5,we report

E?/2values as a function of pH.A linear relationship is dis-

played and a slope of?0.183±0.007V can be determined.

At pHs around6,the reduction ef?ciency%Q?(Fig.5,curve

b)is low,～20%.Then,it increases with pH and reaches

almost100%at pHs8.5–9,%m r,d(Fig.5,curve c)and%

m r,nd(Fig.5,curve d)values increase with pH(～10%at pHs

around6and～50%at pHs8.5–9).

3.4.In?uence of cathodic current density

The in?uence of cathodic current density,I c,for values

between5and35?A cm?2,on the lepidocrocite reduction

was investigated in0.1M NaCl/0.05M PIPES solution at

pH7.5and T=25?C.E?/2(Fig.6,curve a)becomes more

and more cathodic as I c increases.Below～10?A cm?2,

%Q?(Fig.6,curve b)increases as I c decreases.Beyond ～10?A cm?2,%Q?is stable(～60%);%m r,d(Fig.6,curve c)and%m r,nd(Fig.6,curve d)values are slightly affected by

cathodic current density,only a low diminution is observed

for the values below～10?A cm?2.

3.5.In?uence of anion

The reduction behaviour of lepidocrocite thin?lm was

investigated at pHs7.5and9(T=25?C),in solution

H.Antony et al./Electrochimica Acta51(2005)745–753

749

Fig.6.Evolution of E?/2(a),%Q?(b),%m r,d(c)and%m r,nd(d),as a function of the reduction current density;0.1M NaCl/0.05M PIPES solution at pH 7.5and T=25?C,Q o=100mC cm?2.

containing0.1M NaCl or0.05M Na2SO4or0.1M NaHCO3, with a cathodic current density of12.7?A cm?2.The corre-sponding values of E?/2,%Q?,%m r,d and%m r,nd are given in Table1.Results indicate that at pH7.5,the nature of anion has a negligible in?uence on the electrochemical reduction process.At pH9,similar data are obtained in chloride and sul-phate solutions whereas the reduction depth is slightly lower in bicarbonate solution.

4.Electrochemical reduction of lepidocrocite powder

Curve a of Fig.7reports the typical reduction E–Q tran-sient of a lepidocrocite/graphite composite electrode in0.1M Table1

In?uence of the nature of anion on the reduction of?-FeOOH thin?lm

0.1M NaCl0.05M Na2SO40.1M NaHCO3 pH7.5

E?/2(V)?0.240±0.015?0.240±0.015?0.240±0.015 %Q?605663

%m r,d333136

%m r,nd272527

pH9

E?/2(V)?0.517±0.010?0.518±0.010?0.443±0.010 %Q?969884

%m r,d404240

%m r,nd5556

44Fig.7.E–Q transients recorded for the reduction of(a)?-FeOOH/graphite composite electrode and(b)blank graphite electrode in0.1M NaCl/0.05M PIPES solution at pH7.5and T=25?C,I c=25?A mg?1,?-FeOOH mass fraction=20%.

NaCl/0.05M PIPES solution at pH7.5and T=25?C.The E–t transients were transformed to E–Q(1)t or E–Q(2)t transients. Q(1)t and Q(2)t,the quantities of transferred electrons nor-malised with respect to1g of lepidocrocite or with respect to

1mol Fe,respectively,are obtained from either

Q(1)t=

i c t

mF

(in mole of electrons per gram of lepidocrocite)(3) or

Q(2)t=

i c tM

0.58mF

(in mole of electrons per mole of lepidocrocite)(4) where i c is the cathodic current(A),t the time(s),M the molar mass of iron(55.85g mol?1),m the mass of ferric product in the composite electrode(g),F the faraday con-stant(96,485C mol?1)and0.58is the mass fraction of iron in the lepidocrocite powder.%Q?is then obtained from

%Q?=100Q(2)?(5) The rapid drop of potential observed in the curve of blank graphite electrode(Fig.7,curve b)indicates that graphite does not give any reduction response.The applied cathodic current was adjusted with respect to the mass of lepidocrocite powder mixed with graphite in the electrode,25?A mg?1. E?/2and%Q?values are determined from the curve,as indicated.The values are lower than those recorded for the reduction of lepidocrocite thin?lm.

The dependence of E?/2and%Q?on lepidocrocite mass

fraction in composite electrode was investigated(Fig.8).The

continuous diminution of E?/2(Fig.8,curve a)can be par-

tially related to the decreasing conductivity(κ, ?1cm?1)of

composite electrode as the weight percentage of lepidocrocite

750H.Antony et al./Electrochimica Acta 51(2005)

745–753

Fig.8.Evolution of E ?/2(a)and %Q ?(b)as a function of ?-FeOOH mass fraction in composite electrode;0.1M NaCl/0.05M PIPES solution at pH 7.5and T =25?C,I c =25?A mg ?1.

increases (the values of log κare ?2.25,?2.50,?6.70and ?7.10at lepidocrocite wt%of 80,90,95and 100,respec-tively).In the 15–90wt%range,%Q ?does not signi?cantly vary (Fig.8,curve b),the values remain between 34and 39.Beyond ～90wt%,lower %Q ?values are recorded and a more pronounced decrease of E ?/2is observed.Then,the mass fraction of ～90%corresponds to the critical graphite ?lling content or percolation threshold [10],beyond which the electrochemical response should be affected.Below a mass fraction of 15%,larger %Q ?values are recorded.In Fig.9,we report the evolution of E ?/2and %Q ?as a function of cathodic current density.Whereas the decrease of E ?/2with I c could be expected,the increase of %Q ?is more surprising and may be explained by local pH changes in pore solution within the composite electrode.Beyond ～120?A g ?1,a pseudo-stabilisation seems to be reached.The in?uence of anion was also evaluated for lepi-docrocite/graphite composite electrode in solutions at pH 7.5.Results are reported in Table 2

.

Fig.9.Evolution of E ?/2(a)and %Q ?(b)as a function of the reduction current density;0.1M NaCl/0.05M PIPES solution at pH 7.5and T =25?C,20wt%?-FeOOH.

Table 2

In?uence of the nature of anion on the reduction of ?-FeOOH/graphite com-posite electrode

pH 7.50.1M NaCl

0.05M Na 2SO 40.1M NaHCO 3E ?/2/V ?0.543±0.030

?0.591±0.003

?0.600±0.020

%Q ?

34±2

46±1

23±1

5.Ex situ analysis of electrodes

The reduction process was studied through SEM and FTIR.Fig.10reports the SEM images of a thin ?lm (a)before and (b)after its reduction in Na 2SO 40.05M/0.05M PIPES solution at pH 7.5and T =25?C.The lepidocrocite ?lm is homogeneous and it is constituted by low size particles (less than 200nm length).After the reduction process,some parts of the ?lm exhibit a new morphology characterized by agglomerated platy particles.A signi?cant contraction of the ?lm results from the drying of the reduced ?lm under argon ?ow,indicating that it was hydrated in a rather large extent.At more alkaline pHs,%Q ?approaches 1and almost the whole reduced surface displayed the same morphology as in Fig.10b.The data of FTIR analysis are given in Fig.10c for a reduction experiment performed in Na 2SO 40.05M/0.05M TAPS solution at pH 9.The spectrum recorded before reduc-tion is consistent with that of lepidocrocite,as indicated

by

Fig.10.SEM images of a thin ?lm (a)before and (b)after its reduction in Na 2SO 40.05M/0.05M PIPES solution at pH 7.5.(c)FTIR spectra of a thin ?lm before and after reduction in Na 2SO 40.05M/0.05M TAPS solution at pH 9.T =25?C.

H.Antony et al./Electrochimica Acta51(2005)745–753751

the observation of characteristic modes at740,1023and 1145cm?1;the shape of the spectrum should indicate that lepidocrocite particles have a preferential orientation of c-axis perpendicular to the surface[7].After the reduction process,a large attenuation of the lepidocrocite modes is observed,concomitantly with the appearance of new bands, which may be attributed to sulphate(ν4mode at605cm?1 andν3modes at1040,1080and1110cm?1),to carbonate(ν3 modes at1375and1485cm?1)and to Fe O lattice modes (460and505cm?1).A modi?cation of the OH region(not shown)is also noted.The detection of carbonate mainly arises from the contamination of solutions or?lm surface by atmo-spheric carbon dioxide.

6.Discussion

Results obtained from either thin?lm or powder indicate that lepidocrocite can be electrochemically reduced in the condition of atmospheric corrosion of iron.The more neg-ative potential values obtained with lepidocrocite/graphite composite electrode mainly result from the large ohmic drop induced by the geometrical and morphological features of this kind of electrode(thickness and porosity)[11].

The comparison of lepidocrocite reduction potential val-ues(Table1)and iron corrosion potential values(Table3) indicates that the iron/?-FeOOH galvanic coupling is possi-ble.Lepidocrocite can act as a depolarizer towards metallic iron during the wetting stage of atmospheric corrosion.

The present investigation on composite electrode allowed us to emphasis the in?uence of several parameters and will be helpful for our further studies on other synthetic ferric phases and rust samples collected from corroded artefacts.

The slope of the E?/2–pH straight line(Fig.5,curve a)can give information on the reduction mechanism,since it re?ects the number of protons exchanged,with respect to one elec-tron.The value,?0.183V,is not consistent with the bulk reac-tion proposed by Stratmann et al.[12],in which lepidocrocite crystals is assumed to be reduced via a solid state reaction into Fe(OH)(OH)with the transfer of one proton(with respect to one electron).In fact,this value is very close to?0.177V, indicating that the number of protons to number of electrons ratio is3.Then,the only possible reaction is the reduction of lepidocrocite into Fe2+with the transfer of three protons with respect to one electron(electrochemical step).Moreover,the experimental potential values(Table3)are in agreement with the theoretical values for the reduction with a dissolution mechanism(Table4).The fact that the reduction coulom-bic charge Q?strongly depends on pH may be explained by the occurrence of subsequent adsorption/desorption equilibrium and chemical reaction.The following reduc-tion mechanism is proposed and is more consistent with the second reaction already proposed by Stratmann and Hoffmann[13]:

?-FeOOH+3H++e?→Fe ads2++2H2O(6) Fe ads2+?Fe aq2+(7) Fe aq2+→secondary Fe II-containing species(8) Even though Stratmann and Hoffmann[13]have found by M¨o ssbauer spectrometry the secondary Fe II to be Fe(OH)(OH),the present study has clearly revealed that the nature of these secondary Fe II-containing species depend mainly on the composition and pH of solution and may be, on the reduction rate(current density)and hydrodynamic conditions.At pHs near6,Fe aq2+do not signi?cantly react with solution species and the reduction process takes place until the lepidocrocite surface becomes saturated by Fe ads2+ species in equilibrium with a given concentration of Fe aq2+ in the vicinity of electrode surface.

At pHs near6,%Q?is close to22%and both%m r,d and %m r,nd are close to11%(see Fig.5).Following the reduc-tion process,the mass of remaining lepidocrocite is about 90?g cm?2(i.e.78%of m o),corresponding to a volume of 2.2×10?5cm3(the density of lepidocrocite is4.09g cm?3 [8]).The parameters of lepidocrocite orthorhombic cell are a=0.388nm,b=1.254nm and c=0.307nm[8].By assum-ing that the surface of lepidocrocite exposed to electrolyte is mostly constituted by(a–c)planes[7,8],the surface occu-pancy of one Fe atom can be estimated,about0.1nm2. Taking into account that11%of Fe ads2+are present at lep-idocrocite surface and assuming that all surface sites are occupied,a value of lepidocrocite speci?c surface area can be computed,76m2g?1.The latter value is consistent with the values reported in the literature for lepidocrocite pow-der samples(70–80m2g?1from Ref.[8],62m2g?1from Ref.[9])and con?rms consequently our assumption of iron adsorption/desorption for the reduction mechanism.

The consumption of Fe aq2+in Eq.(8)provokes a dis-placement of equilibrium in the desorption sense,leading to a refreshment of lepidocrocite surface,which allows the reduction process go further.Reaction(8)is promoted by pH increase from both thermodynamic and kinetic view.As

Table3

E?/2and iron corrosion potential values recorded in the various electrolyte solutions investigated here

pH6/7.5/9

0.1M NaCl0.05M Na2SO40.1M NaHCO3

E?/2(V,thin?lm)+0.01/?0.240/?0.517nd/?0.240/?0.518nd/?0.240/?0.443 E cor(V)?0.506/?0.571/?0.599nd/?0.579/?0.600nd/?0.576/?0.629 nd:not determined.

752H.Antony et al./Electrochimica Acta51(2005)745–753

Table4

Theoretical E values for the lepidocrocite reduction with a dissolution or a bulk reaction

Reactions E?(V)E–pH slope value(V)Theoretical reversible E values(V)pH6/7.5/9?-FeOOH(s)+3H++e?=Fe2++2H2O0.814a?0.177*+0.107/?0.159/?0.424

?-FeOOH(s)+H++e?=Fe(OH)2(s)0.060b?0.059?0.294/?0.383/?0.471

*E values calculated by assuming[Fe2+]=10?6M.

a From Refs.[24,25].

b From Refs.[24,26].

atmospheric corrosion is a phenomenon which occurs with thin electrolyte thicknesses and porous diffusion,pHs which should be observed in such electrolyte could be close to the acidiest ones studied in this paper due to the presence of Fe II species in the solution.One of the most reasonable assump-tions in this case is that the reduction of lepidocrocite is occurring at the rust layer surface since Fe aq2+do not sig-ni?cantly react with solution species at pHs near6as it has been shown.In the pH range investigated here,the species that can be expected are magnetite[14],green rusts incorporating either carbonate or sulphate or chloride ions[15–17],ferrous hydroxi-anions such as Fe2(OH)3Cl[18]or Fe2(OH)2CO3 [15],siderite[19],amorphous phases and may be soluble complexes[20].Some of these compounds were already identi?ed in the corrosion products of iron archaeological artefacts(magnetite,siderite and Fe2(OH)3Cl on objects cor-roded in soil[21–23])but only magnetite and amorphous phases(for recent samples,i.e.several10years)were evi-denced in addition to goethite and lepidocrocite in the speci?c case of atmospheric corrosion of iron.The formation of one or another species is chemically driven by the reduction con-ditions.The present ex situ analysis of reduced electrodes by means of FTIR and SEM put into evidence the con-sumption of lepidocrocite and formation of new solid phases, therefore,validating Eq.(8).However,the nature of reduced products could not be obviously determined and then,a more accurate identi?cation of the secondary Fe II-containing species is necessary;this study may be performed by cou-pling electrochemical reduction and in situ microRaman analysis.

Moreover,the nature of the secondary Fe II-containing species must be discussed in term of experimental condi-tions and especially the composition of the electrolyte that is formed on the rust layer during the wet stage in atmo-spheric environment.Stratmann and Hoffmann[13]seems to have clearly identi?ed by M¨o ssbauer spectroscopy the for-mation of Fe(OH)(OH)via a solid state reduction.Actually, the author also notes that M¨o ssbauer spectroscopy does not allow to detect phase such as precipitated Fe(OH)(OH)which should be in the form of freshly gel-like,such kind of phase presents a too low probability for the recoil-free absorption of a?-quantum.The observation of Fe(OH)(OH)-like spec-tra during in situ reduction of?-FeOOH means that this reaction is ocurring in a solid state.By taking into account that all the experiments made by this author have been con-ducted under very thin electrolyte?lms without controlling of the pHs,the in?uence of such conditions should be inves-tigated since the results presented in our paper have rather revealed a reduction of lepidocrocite via a clear dissolution pathway.

7.Conclusion

The electrochemical reduction of lepidocrocite thin?lm has been studied in function of several parameters:quantity of deposited lepidocrocite,pH,anion added in the electrolyte and cathodic current density.Our results enable to conclude that the mechanism of reduction takes place with a dissolu-tion process.The galvanic coupling between lepidocrocite reduction and iron oxidation is possible.This result con?rms the key role of lepidocrocite in the atmospheric corrosion process.

The reduction of lepidocrocite has also been performed with powder compacted electrode.This kind of electrode might give a better mimic of a corrosion layer.In order to complete this work,the electrochemical reduction of the other compounds of the rust layer has to be studied.In par-allel,a wide characterization study of various archaeological artefacts,which have been subjects of atmospheric corro-sion conditions,will give and/or con?rm the presence and the distribution of the most important rust layer compounds such as goethite,lepidocrocite,maghemite and magnetite. Moreover,reduction experiment will also been performed with powder coming from these old archaeological arte-facts for knowing the electrochemical behaviour of the rust layer.

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