Corrosion Science\ Vol[ 39\ No[ 00\ pp[ 0792Ð0708\ 0887Þ 0887 Elsevier Science Ltd[ All rights reserved[\ Pergamon Printed in Great Britain[

9909Ð827X:87:,Ðsee front matter

PII] S9909Ð827X"87#99966Ð7

LANTHANIDE COMPOUNDS AS ENVIRONMENTALLY!FRIENDLY CORROSION INHIBITORS OF ALUMINIUM ALLOYS]

A REVIEW

M[ BETHENCOURT\a F[ J[ BOTANA\a J[ J[ CALVINO\ M[ MARCOSb

and M[ A[ RODRIłGUEZ!CHACOłNa

aDepartamento de Ciencia de los Materiales e Ingenier(�a Metalu�rgica y Qu(�mica Inorga�nica\ Facultad de Cienciasdel Mar\ Universidad de Ca�diz\ Pol(�gono R(�o San Pedro s:n[ Apdo[ 39\ Puerto Real\ 00409 Ca�diz\ Spain

bDepartamento de Ingenier(�a Meca�nica y Disen½o Industrial\ Escuela Superior de Ingenier(�a\ Universidad de Ca�diz\c:Sacramento\ 71\ 00992 Ca�diz\ Spain

Abstract*Currently\ chromates are among the most common substances used as inhibitors or incorporated inanticorrosive pretreatments of aluminium alloys[ However\ these compounds are highly toxic and their useproduces serious environmental hazards[ Consequently\ an intense research e}ort is being undertaken to replacechromates by more ecological compounds[ In recent years\ several authors have begun studies of the behaviourof lanthanide compounds in the corrosion protection of metallic alloys[

After a brief overview of the use of chromates and their ecological alternatives in corrosion protectionsystems\ the paper reviews progress from the literature published to date concerning the application of lanthanidecompounds as corrosion inhibitors of aluminium alloys in aqueous solution[ Aspects of the mechanism involvedin the cathodic inhibition process by lanthanides are also dealt with[ In addition\ these compounds have beenused to prepare chromate!free conversion coatings by full!immersion and electrochemical activation methods[Treatments proposed for these conversion coatings are discussed in detail[

From the comprehensive review presented in this article\ it is concluded that lanthanide compounds ful_l thebasic requirements for consideration as components of more environmentally!friendly formulations] low toxicityand acceptable protective capacity[ However\ methods proposed up to now present various limitations that restricttheir application in practice[ Further research e}orts are necessary to develop feasible treatments for industry[Þ 0887 Elsevier Science Ltd[ All rights reserved

Keywords] aluminium alloys corrosion\ green inhibitors\ chromate!free conversion coatings\ lanthanide com!pounds

CORROSION INHIBITION BY CHROMATES

Cr"VI# compounds\ mainly chromates\ are widely applied as corrosion inhibitors in aqueousmedia[ A wide range of metals and alloys\ such as iron\ steels\ aluminium alloys\ zinc\copper\ lead and others\0Ð5 can be protected using chromates[ Their high e.ciency:costratio has made them standard corrosion inhibitors[6 Nevertheless\ due to their oxidisingnature\ chromate concentration in the medium must be periodically checked\ in order toavoid undesirable corrosion results\ since concentrations lower than a critical value canpromote a pitting process[7Ð00 Even if the chromate content is at an optimum\ the presence

Manuscript received 01 June 0886^ in amended form 17 April 0887

0792

M[ Bethencourt et al[0793

of reducing agents in solution can cause their concentration to fall below the critical range[01

Moreover\ an upper concentration limit must not be exceeded either\ in order to maintaintheir inhibiting properties\ for example\ when chromates are used as pigment of primers[02

In the anticorrosive pretreatments of metallic alloys\ they are in common use across arange of di}erent industries\ from aerospace to the automotive and naval sectors[ In thisapplication\ chromium is incorporated into protective coatings applied to metallic surfaces[These coatings provide e}ective corrosion protection and\ in addition\ they facilitate theapplication of further _nishing treatments\ in particular\ painting[ In the speci_c case ofsurface _nishing procedures for aluminium alloys\ chromates have been applied in threedi}erent ways] incorporated into conversion coatings\ as an additive in anodising bathsand as pigment in painting primers[03Ð06

By their general working mechanism\ chromates can be considered as anodic oxidisinginhibitors[07\08 There are several models related to the growth of protective chromium!rich_lms[19\10 However\ in spite of their extensive use\ the full details of precisely how they workare still not known[ At the present time\ it is accepted that better knowledge of the inhibitionmechanism of chromates is required\ and attempts to provide this are continuously beingpublished[11

In spite of their high e.ciency and established industrial application\ process plantsusing chromates will have to be shut down progressively over the next few years[ Recently!enacted environmental laws in many countries have imposed severe restrictions on chromateuse due to its high toxicity01\12 and consequent environmental hazards[13

Ecolo`ical alternatives to chromatesRegarding the protection of aluminium alloys\ in recent years more ecological alter!

natives have been investigated to replace chromates in their di}erent _elds of application[Thus\ e}orts towards this have been focused on the search for new corrosion inhibitors andnew formulations of both anodising baths and conversion coatings[14 In agreement withCohen\14 the development of viable alternatives to chromates is still in its infancy[ Many ofthe new systems are only in the experimental phase and a wide variety of approaches isbeing investigated[

A variety of passivating inhibitors have been tested[ Ions such as MoO1−3 \ MnO1−

3 \SiO1−

3 and WO1−3 have shown e.ciencies similar to that of chromate in protecting alu!

minium alloys[15Ð18 Other anions\ like benzoate or nitrite\ have been shown to give a highdegree of protection to stainless steels exposed to solutions containing chlorides[15\29\20

In relation to replacements for chromic acid anodization on aluminium\ di}erentattempts have been reported[21Ð23 The most promising candidate in this _eld seems to bethin!_lm H1SO3 anodization[22 The advantage claimed for this treatment is that it avoidsthe fatigue performance limitations resulting from conventional H1SO3 anodising[

Regarding chromate!free conversion coatings\ various di}erent processes have beenproposed based on phosphate and sulphate solutions\ also incorporating nitrite or boraterespectively[24Ð26 Also\ in relation to chromate!free primers for aluminium alloys\ passivatinginhibitors such as vanadate\ molybdate or phosphate have been successfully tested[27Ð39

A di}erent approach\ studied for the replacement of chromates as corrosion inhibitors\involves using cathodic or mixed inhibitors[ The behaviour of various salts containingzinc\ calcium\ barium or manganese has also been studied[30Ð32 In this respect\ lanthanidecompounds are among the substances that have been investigated as an alternative to

Lanthanide compounds as environmentally!friendly corrosion inhibitors of aluminium alloys] a review 0794

chromates\ both as cathodic inhibitors and in the development of conversion coatings[ Inthe following section\ a review of the information available on this topic is presented[

LANTHANIDE SALTS AS CORROSION INHIBITORS

It is well known that lanthanide ions form insoluble hydroxides33 which enable them tobe used as cathodic inhibitors[ Lanthanides have a low toxicity and their ingestion orinhalation has not been considered harmful to health34 whilst the toxic e}ects of their oxidesare similar to those produced by sodium chloride[35 Furthermore\ lanthanides can beconsidered as economically competitive products36 because\ as elements\ some of them arerelatively abundant in nature^37 cerium\ for instance\ is as plentiful as copper[38 Productionof lanthanides has shown a continuous increase in recent years[49 Taking all these facts intoaccount\ it is reasonable to consider the development of corrosion protection methods usingthis family of compounds[

At _rst\ the lanthanide elements\ principally cerium\ were used in protection againstcorrosion at high temperatures[ Using several incorporation methods\ cerium and\ to alesser extent\ other Rare Earth "RE# elements have been used to improve the adherence ofoxide _lms developed during the oxidation processes of di}erent metallic alloys[40Ð47 In thisparticular application\ special attention has been paid to ionic implantation as the lan!thanide incorporation method[48Ð54

Surface _lms generated on RE!doped alloys have improved behaviour against bothhigh temperature oxidation and aqueous corrosion[ Thus\ Lu and Ives54 showed thatimplantation of cerium on AISI 205 stainless steel decreases its corrosion rate in NaClsolution by two orders of magnitude[ These authors suggest that the growth of a CeO1

surface _lm blocks both anodic and cathodic active surface areas\ thus leading to the above!mentioned decrease in corrosion rate[

The earliest applications of lanthanides as corrosion inhibitors were stimulated by theneed to replace chromates imposed by new international environmental standards[ Thereare several papers in the literature dealing with the use of lanthanides as corrosion inhibitorsfor several metals and alloys such as zinc\55Ð58\ bronze\69\60\ nickel\61 mild steels62Ð64 orstainless steels[65Ð78 Since chromates are in most extensive use in the pre!treatment ofaluminium alloys\ most of the research to date has been focused on the application of REsalts to the anticorrosive protection of these alloys[

LANTHANIDES AS CORROSION INHIBITORS OF ALUMINIUM ALLOYS

The pioneering studies concerning the application of lanthanides for the corrosionprotection of aluminium alloys are those by Hinton\ Arnott and Ryan[89Ð84 These authorsstudy the e}ect of increasing the concentration of CeCl2\ on both uniform and pittingcorrosion of AA6964 "AlÐZn# alloy[ The degree of protection provided by the addition ofcerium\ in concentrations ranging from 9 to 0999 ppm\ to NaCl solution was evaluated byweight loss and linear polarisation measurements[ Corrosion rate values calculated fromdata included in89 allow study of the evolution of this parameter as a function of inhibitor

M[ Bethencourt et al[0795

Fig[ 0[ Evolution of corrosion rate of alloy AA6964 in NaCl solutions as a function of CeCl2 concentration[Data adapted from reference 89[

concentration\ Fig[ 0[ This _gure shows that the uniform corrosion rate decreases steeplyup to 099 ppm of CeCl2[ Higher concentrations of CeCl2 only produce a stabilisation in therate of corrosion[ The optimum inhibitor concentration was found to correspond to aconcentration of 099 ppm of CeCl2 in the solution[ Moreover\ good agreement between theresults obtained from both experimental techniques can be observed in this _gure[

Regarding pitting corrosion\ some conclusions can be obtained from the polarisationcurves included in Ref[ 89[ Pitting generation resistance\ R\ can be evaluated as the di}erencebetween the pitting nucleation potential\ Epit\ and the corrosion potential\ Ecorr[ The linearpolarisation curve\ carried out in bare solution\ shows a nearly horizontal anodic branchin which pitting nucleation potential is very close to Ecorr\ indicating an almost zero R value[Figure 1 plots the evolution of R parameter as a function of CeCl2 concentration[ Accordingto this _gure\ resistance to pitting corrosion increases slowly from 9 to 099 ppm and showsthe steepest increase in the range between 099 and 0999 ppm\ whilst a steady value of R isobtained for higher concentrations up to 09999 ppm of CeCl2[

Similarly\ the inhibition behaviour of other lanthanide salts such as YCl2\ LaCl2\ PrCl2and NdCl2\ has been investigated by the same authors[56\57\80Ð84 A trend in the evolution ofcorrosion rates with lanthanide salt concentrations similar to that depicted in Fig[ 0 isobserved[ The protective power of RE trichloride has also been compared with other saltssuch as FeCl1\ CoCl1 and NiCl1[ Figure 2 plots the values for degree of protection providedby the above mentioned salts[ In these experiments\ the corrosion rates of aluminium alloyAA6964\ in NaCl 2[4) plus 0999 ppm of inhibitor\ have been measured[ According to dataincluded in Fig[ 2\ the best degree of inhibition is achieved with Ce2¦ cations[ It is signi_cantthat the degree of inhibition provided by the optimum concentration of CeCl2 is similar tothat obtained with chromate\ showing that lanthanide salts can be considered both ase.cient and as ecological alternatives to chromate[

The alloys studied by Hinton et al[ are mainly used in the aerospace industry[ Never!

Lanthanide compounds as environmentally!friendly corrosion inhibitors of aluminium alloys] a review 0796

Fig[ 1[ Evolution of pitting resistance as a function of CeCl2 concentration for alloy AA6964 in NaCl solutions[Data computed from reference 89[

Fig[ 2[ Degree of protection against uniform corrosion of alloy AA6964 in NaCl solutions provided by additionsof 0999 ppm of di}erent metallic chlorides[ Data adapted from Refs[ 89Ð84[

theless\ other studies following Hinton have been carried out on alloy AlÐMg 4972\developed for speci_c applications in marine environments[ The protection of this alloyagainst uniform and pitting corrosion by lanthanide salts in seawater has been studied inRefs[ 85Ð099 In\86 the e}ect of CeCl2 concentrations from 9 to 0999 ppm in 2[4) NaClaerated aqueous solutions is considered[ Figure 3\ taken from this reference\ shows that theevolution of the corrosion rates with increasing inhibitor concentration is similar to thatobserved for the AA6964 alloy in Fig[ 0[ For the AlÐMg alloy\ a steep decrease of corrosionrate due to the addition of inhibitor is evident up to 099 ppm concentration whilst an almost

M[ Bethencourt et al[0797

Fig[ 3[ Evolution of corrosion rate of alloy AA4972 in NaCl solutions as a function of LnCl2 concentration[Data adapted from reference 86[

stable value for the degree of protection is observed for higher inhibitor concentration[ Itis also notable that there are no substantial di}erences between the di}erent lanthanidesalts tested in this study[

The highest degree of inhibition against uniform corrosion is observed for a 499 ppmconcentration of cerium chloride in the basic NaCl solution[ For this inhibitor concen!tration\ the uniform corrosion rate is reduced by more than 89)\ a value similar to boththat obtained with sodium chromate and the rate measured by Hinton et al[ for AA6964alloy[

In Refs[ 86 and 099 the increase of pitting nucleation resistance due to the presence ofcerium chloride in NaCl solutions is analysed from cyclic polarisation curves\ Fig[ 4[ Inthis _gure\ Epit corresponding to AA4972 alloy in NaCl solution can be measured\ incontrast to AA6964[ From this value\ the pitting nucleation resistance\ R9"E9

corr−E9pit#\ in

this solution can be determined[ Figure 4 also includes the cyclic polarisation curve recordedwith the addition of CeCl2[ The value of Epit remains close to E9

pit\ while the value cor!responding to Ecorr is lower than that obtained in the bare solution[ These modi_cations_nally lead to an increase of the pitting nucleation resistance\ R[ From calculated values ofR and R9\ the pitting nucleation protection percentage\ Ppit\ can be evaluated for di}erentconcentrations of Ce\ La and Sm trichlorides "Figure 5#[ An increase of the pitting

Lanthanide compounds as environmentally!friendly corrosion inhibitors of aluminium alloys] a review 0798

Fig[ 4[ Cyclic Polarisation diagrams for AA4972 in bare NaCl solution and with 149 ppm addition of CeCl2[The {{9|| superscript values in the _gure are those corresponding to the absence of inhibitor[

Fig[ 5[ Estimated values of the degree of protection against pit nucleation as a function of Ln2¦ concentration\for alloy AA4972 in NaCl solutions[

M[ Bethencourt et al[0709

Fig[ 6[ Evolution of the degree of protection against pit growth as a function of inhibitor concentration\ foralloy AA4972 in NaCl solutions[

nucleation resistance is observed in the presence of LnCl2[ It is signi_cant that\ with a499 ppm concentration of CeCl2\ the degree of protection is greater than 199)\ cor!responding to a R value greater than the maximum obtained by Hinton and co!workersfor AA6964[

In Refs[ 88 and 099\ the protection against pitting growth\ Pg\ is also evaluated in termsof the percentage of area lost in the anodic hysteresis loop of the cyclic polarisation diagram\plotted in EÐi style[ Fig[ 6 shows that LnCl2 addition leads to an increase of the pittinggrowth resistance[ From data included in Figs[ 5 and 6\ it can be concluded that the additionof 499 ppm of CeCl2 provides the best improvement in the behaviour of the alloy AA4972against both pitting nucleation and pitting growth processes[

Other studies of corrosion inhibition by CeCl2 for di}erent aluminium alloys can befound in the literature[ For instance\ Liu studies the protection against corrosion of AlÐZnÐMg alloys in solutions containing NaCl as a function of CeCl2 concentration and pH[090

Similar studies have been done using Ce"NO2#2 and Ce1"SO3#2\ although the inhibitor e}ectof cerium is masked in these cases by the presence of the corresponding oxidising anionssuch as nitrates and sulphates[091 In Ref[ 092\ a pitting nucleation delay of two days isreported for AA5950 alloy and AA5950ÐSiC composites in the presence of Ce in 2[4)NaCl aqueous solutions[

In summary\ lanthanide salts\ particularly cerium trichlorides\ have been shown to bepromising cathodic inhibitors against uniform and localised corrosion of a large variety ofaluminium alloys[ It may reasonably be expected that future developments in this _eld willprovide e.cient alternative inhibitors to the classic\ but highly toxic\ chromates[

Lanthanide compounds as environmentally!friendly corrosion inhibitors of aluminium alloys] a review 0700

Fig[ 7[ Linear Polarisation diagrams for alloy AA4972 in bare NaCl solution "solid line# and with 149 ppmaddition "dotted lines# of CeCl2[

THE MECHANISM OF INHIBITION

Based on linear polarisation curves similar to those shown in Fig[ 7 for AA4972 alloy\88

lanthanide chlorides have been classi_ed as cathodic inhibitors[ This assumption was madein the earliest research by Hinton with AA6964\ taking into account the relative positionof cathodic branches in the polarisation diagrams from tests with and without inhibitors[

Recently\ Isaacs has con_rmed the cathodic nature of lanthanide inhibitors[093 In Ref[093 a simple method is proposed to discover the inhibition mechanism of cerium salts onAA1913 aluminiumÐcopper alloy in NaCl!containing solutions[ In the tests proposed byIsaacs\ an aluminiumÐcopper galvanic couple is formed[ This couple would mimic theelectrochemical behaviour of the cathodic copper!containing intermetallic precipitateswithin the alloy[ The results obtained from these tests showed that the corrosion inhibitionprocess was related to the development of a cerium!rich _lm over the cathodic coppersurface[ Results reported in Ref[ 094 reveal further details regarding the origin of theprotective e}ect observed in AA4972 samples treated with cerium!containing solutions[According to Ref[ 094 samples treated for 37 hours in a 499 ppm CeCl2 solution develop asurface coating in which the major component is Al\ whereas cerium accumulates indispersed islands ðFig[ 8"a#Ł[ An enlargement of the island marked with an arrow in Fig[8"a# can be observed in Fig[ 8"b#[ The EDS spectrum included in Fig[ 8"c#\ shows that thisisland contains mainly cerium[ The presence of cracks\ observed in Fig[ 8"b#\ allows us toanalyse the actual nature of the metallic substrate underlying these cerium islands[ TheEDS spectrum recorded on this area ðFig[ 8"d#Ł indicates that the cerium islands haveformed over the Al5"MnFe# precipitates[ If we recall that these precipitates are the mostcathodic ones in the alloy\ a feasible mechanism for the inhibition process can be envisaged[Thus\ during the _rst stages of the corrosion process\ OH− groups would be generated overthe cathodic intermetallic precipitates[ This proposal has been supported by the results of

M[ Bethencourt et al[0701

Fig[ 8[ "a# SEM image recorded on a sample of alloy AA4972 after a 37 hours of inmersion in 499 ppm CeCl2solution^ "b# high magni_cation view of one of the islands observed in "a#^ "c# EDS spectrum recorded in "0#^ "d#

EDS spectrum recorded in "1#[ Data taken from reference[094

Davenport\ Isaacs and co!workers\ who detected local increases of pH at the cathodic siteson alloy AA1913 due to the oxygen reduction[095Ð009

The hydroxyl groups formed over the cathodic sites would react with the cerium ionspresent in the solution[ This reaction gives rise to the formation of cerium islands observedin Fig[ 8"a\b#[ The blockage of the cathodic sites by these islands decreases the availablecathodic current and\ therefore\ reduces the overall corrosion rate[

LANTHANIDE!CONTAINING CONVERSION COATINGS ON ALUMINIUMALLOYS

Based on the previously mentioned _lm forming properties of lanthanide salts and theprotective behaviour of such _lms\ di}erent treatments have been proposed in order todevelop alternative conversion coatings to those based on chromates[ In general terms\ two

Lanthanide compounds as environmentally!friendly corrosion inhibitors of aluminium alloys] a review 0702

di}erent methods have been used to grow lanthanide _lms on aluminium alloys] fullimmersion and electrochemical activation[

As in the case of their use as inhibitor\ the earliest attempts to obtain lanthanide!containing conversion coatings by full immersion were made by Hinton et al[81 In thisresearch\ coatings were formed on AA6964 using 0999 ppm CeCl2 solutions and a minimumcorrosion rate value was observed after 89 hours of immersion[89 Scanning ElectronMicroscopy "S[E[M[# studies\ included in Refs[ 89 and 82 con_rm this time period as theoptimum for the formation of a cerium!containing _lm that covers the whole exposedsurface[ Nevertheless\ according to the authors\ after an immersion period of about 19hours\ a signi_cant protection level is reached[ Unfortunately\ such long time periods makethese immersion treatments commercially unattractive[

The in~uence of lanthanide salts in the protection against corrosion of aluminiumÐlithium alloys AA1989 and AA7989 has been studied in aerated 9[4M Na1"SO3# and2[4) NaCl solutions and deaerated 2[4) NaCl solutions[000 In this research\ an e}ectiveprotection against pitting corrosion provided by pretreatments of full immersion in cerium!containing solutions is concluded[ However\ it is also observed that the protective e}ect ofthis coating is limited to a very short period of time[

In a similar fashion\ Mansfeld et al[ have proposed a full immersion method to developcerium protective _lms on alloys AA1980 and AA5950\ and on metal matrix compositessuch as AA5950ÐSiC and AA5950ÐGraphite[001 Previously\ the behaviour of cerium saltsas inhibitors of these materials had been studied in 9[4N NaCl aqueous solutions at roomtemperature\ using Electrochemical Impedance Spectroscopy "EIS#[001Ð005

The treatment tested by Mansfeld consists of exposing the material to a solutioncontaining 0999 ppm of CeCl2 open to the air and at room temperature for a period ofseven days[ The _lms obtained were tested by EIS techniques after immersion in NaClcontaining solutions after di}erent time intervals[

These cerium conversion coatings not only show a good performance against corrosionbut they also improve the adherence of epoxy layers deposited onto the metallic surfaces[The degradation behaviour of such epoxy!coated samples was studied by EIS after drillinga 9[5mm hole into the coated material[ After 4 days of exposure to the NaCl solution\ thesurface of the coated AA5950 alloy showed _liform corrosion starting from the edge of thehole[ However\ Al:Gr and Al:SiC metal matrix composites did not show any kind ofcorrosion phenomena even after three months of exposure to the aggressive medium[ Byincreasing the test aggressiveness\ only a soft delamination was observed on the Al:SiCafter 72 days\ whilst the other composites remained unaltered[

A new and rapid _lm deposition method\ named {{Cerating||\ has been developed andpatented by Hinton and Wilson[006 In this process\ the metallic alloy is immersed in anaqueous cerium salt solution containing both oxidising agents and organic additives[ TheCerating method has been successfully applied not only to aluminium alloys but also to awide range of other metallic alloys\ such as zinc\ galvanised steels\ stainless steels\ cadmiumand magnesium[ However\ according to the authors\ the corrosion performance of thesecoatings in the salt spray test was not satisfactory[

Recently\ further improvements of this method have led to a new patented treatmentnamed {{Cerate Coating||[007 Basically\ the Cerate coating is obtained by full immersion ofthe metallic alloy for 09 minutes in an aqueous solution of cerium chloride with a 9[2) by!volume hydrogen peroxide content\ at 205K and pH0[8[008 A prior surface cleaning anddeoxidising treatment is applied[ A silicate solution is used as a post!treatment stage in

M[ Bethencourt et al[0703

order to seal the coatings obtained[ This process has been successfully applied to AA1913aluminiumÐcopper alloy[ The linear polarisation data for bare and {{Cerate Coated||AA1913 alloy in sodium chloride!containing solutions show a decrease in the corrosionrate of about 84)[ The corrosion potential is decreased by almost 199mV\ whilst thepitting nucleation potential remains at a stable value[ Because of this\ a high pittingnucleation resistance can be achieved with this coating[ The sealed Cerate coating survived225 hours in the salt spray test before corrosion was apparent[ The aircraft industry requireschromate conversion coatings to protect aluminium substrates for at least 057 hours in thistest[ The level of protection provided by the coating depends heavily on the pre!treatmentprior to coating and the sealing stage after coating[

SolÐgel routes have also been researched in the deposition of CeO1 _lms019 and com!mercial organometallic precursors are available[ However\ high temperature conditions arerequired to generate an oxide from the initially!deposited gel _lm[ These preparationconditions limit the use of solÐgel procedures to generate _lms on aluminium alloys[

In relation to alloys used in marine applications\ in Ref[ 010 AA4972 samples have beentreated by full immersion in CeCl2 solutions[ The electrochemical characterisation datareported in this study indicate a signi_cant improvement in the behaviour of the alloyagainst both uniform and pitting corrosion[ EIS data suggest that this improvement isrelated mainly to the resistance of the coating formed by this treatment[

A second group of treatments\ based on electrolytic activation methods have also beenproposed in the literature[ Thus\ Hinton and co!workers propose a method based ongalvanostatic treatment with cathodic currents from 9 to 9[1mAcm−1 in solutions con!taining CeCl2[81 The best results are obtained for 9[0mAcm−1 currents and an exposuretime of 29 minutes in a 0999 ppm CeCl2 solution[ Samples coated by this method show areduced corrosion rate in NaCl solution of one order of magnitude[ However\ the improve!ment in pitting corrosion resistance is not as high as that observed when applying the fullimmersion method or when the lanthanide salts act directly as inhibitors in the aggressivemedium[ Although the electrolytically!deposited coating reduces the required treatmenttime by comparison with the full immersion procedure\ the degree of corrosion protectionis also reduced[ The presence of small holes in the coating due to hydrogen evolution hasbeen suggested as the cause of the loss of protective properties of the _lm[

A better performance was observed by these authors for coatings developed in Ce"NO2#2solutions applying a cathodic potential of 89V for 049 seconds on AA6964 aluminiumalloy[81 In these samples\ the corrosion rate is reduced by one order of magnitude andpitting nucleation resistance is higher than 299mV[ Based on Auger spectroscopy data\these authors suggest that the improvement of the protective properties of the _lm seemsto be related to an enrichment in Ce¦2[ Given that high DC potentials in conjunction withhighly volatile organic solvents are required with this technique\ large scale depositionscould present practical di.culties[

Mansfeld has also proposed a two step\ electrolytically activated pre!treatment basedon cerium salts for the AA5950 alloy\ self!named {{stainless aluminium||[011 At _rst\ thealloy is immersed for two hours in dilute hot Ce"NO2#2 and CeCl2 solutions[ After this pre!treatment\ the alloy is anodically polarised to 9[4V vs[ SCE in 9[0M Na1"MO3#[ A highlystable oxide layer is obtained on the metal surface which inhibits the oxygen reductionreaction[ This process has been also successfully tested for aerospace aluminium alloys suchas AA6964 and AA1913[012

The inhibitor e}ect derived from the joint action of cerium and molybdate had pre!

Lanthanide compounds as environmentally!friendly corrosion inhibitors of aluminium alloys] a review 0704

viously been studied by Baldwin and co!workers for several alloys employed in the aerospaceindustry[013 One of the most e}ective systems found by Baldwin consists of the applicationof organic epoxy _lms pigmented with Ce1"MO3#2[

Regarding the ceriumÐmolybdate treatment\ Kendig and Thomas014 have investigatedthe contribution of each of the two components\ Ce and Mo\ to the overall protective e}ect[In order to study the role of Ce¦2 ions\ AA5950 samples were treated in boiling Ce2¦

solutions\ as described by Mansfeld[011 Afterwards\ an anodic polarisation at 9[4V vs[ SCEin bu}ered borate "pH8[5# was applied for two hours[ To investigate the role of Mo\ thecerium pre!treatment was replaced by four hours of immersion in a boiling 9[929M NaNO2

solution[ The polarisation step was carried out\ in this case\ in the sodium molybdatesolution[ Finally\ a treatment that eliminates the contribution of the two heavy metals fromthe {{stainless aluminium|| processes was considered by combining the two mentionedtreatments\ i[e[\ boiling in NaNO2 solution and anodic polarisation in borate bu}er[ Thesamples pre!treated by the three methods were tested by EIS in 9[4M NaCl aeratedaqueous solutions[ EIS spectra included in Refs[ 011 and 014 suggest that Ce and Mo actsynergistically in the protection against the corrosion process[

To summarize\ in general\ the di}erent treatments commented on above enhance thebehaviour against corrosion of several aluminium alloys[ However\ data reported to datesuggest that conversion coatings based on lanthanide compounds present various handicapsthat limit their application on an industrial scale[

CONCLUSIONS

Environmental regulations in industrialized countries are increasing the pressure toeliminate\ in the short term\ a number of compounds widely used in industrial coatings toprevent corrosion[ Hexavalent chromium!based processes are one of the most a}ected bythese regulations[ A number of {{green|| alternatives to chromates are currently emerging\oriented primarily towards minimizing environmental impact and\ secondarily\ towardsproviding e}ective corrosion inhibition[

From the comprehensive review of the literature presented in this article\ lanthanide!based compounds seem to ful_l the basic requirements to be considered as a component ofmore environmentally!friendly formulations] low toxicity and acceptable inhibition power[

In relation to the corrosion protection of aluminium alloys by lanthanide compounds\most of the recently published studies have focused on the study of their behaviour asinhibitors in aqueous solutions[ Several attempts have been made to incorporate lanthanideinto non!chromate conversion coatings[ There is a lack of research into lanthanides asalternatives to chromic anodizing[ Regarding the metallic materials studied\ lanthanideshave been tested as corrosion inhibitors of aerospace aluminium alloys and\ to a lesserextent\ of those alloys commonly used in marine environments[

In respect of the _rst approach cited above\ various authors have shown that lanthanidesalts are e}ective inhibitors against uniform and pitting corrosion in aqueous environmentsfrom tests with several aluminium alloys[ These compounds show an e.ciency similar tothat obtained using classical Cr"VI# based inhibitors[ Cerium salts have proven to be themost e.cient lanthanide corrosion inhibitors[ The optimum inhibitor concentration variesconsiderably according to the speci_c alloy to be protected[

Dealing with the inhibition mechanism\ results obtained in the study of di}erent systemssuggest that lanthanide salts behave as cathodic inhibitors[ They act by blocking the

M[ Bethencourt et al[0705

cathodic sites in the metallic alloys[ During the inhibition process\ protective surface _lmsincorporating the lanthanide element\ are formed[ This property has been used by severalauthors to design various methods of producing lanthanide!based conversion coatings[

The applications of rare earths in the anticorrosive pretreatments of aluminium alloyshave been carried out either by immersion or by electrolytic activation methods[ Reasonableprotection levels have been demonstrated for lanthanide!containing conversion coatingsprepared by some of these treatments[ However\ the methods proposed up to now havedi}erent limitations that restrict their application in practice[ Further research e}orts arenecessary to develop industrially feasible treatments[

Taking all the above observations into account\ there is reasonable justi_cation forbasing future research and development of new high!performance and environmentally!friendly anti!corrosion coatings for aluminium alloys on lanthanide compounds[

Acknowled`ements*This work has received _nancial support from the Spanish Government {{Comision Inter!ministerial de Ciencia y Tecnolog(�a "CICYT#||\ Projects MAR84!1900\ and MAT86!0964!C92!90[

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