Evaluation of Self-healing Ability in Protective Coatings Modified Withn

Electrochimica Acta 60 (2012) 31 40

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Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com

Evalua cocombin d cnanoco s

M.F. Mon kaa

G.C. Kord c, Ma ICEMS, Institu ortugb Sol-Gel Labor s, Greec CICECO-Depa -193

a r t i c l e i n f o

Article history:Received 2 September 2011Received in revised form 26 October 2011Accepted 26 October 2011Available online 23 November 2011

Keywords:Self healingCoatingsNanocontaineCorrosion

a b s t r a c t

Nowadays, there is a strong demand on the search of thinner, but more effective organic coatings forcorrosion protection of metallic substrates, like galvanised steel, used in the automotive industry. In orderto guarantee effective corrosion protection of these coatings, and because chromate-based pigmentscannot be used, one of the most attractive strategies consists on the modication of the organic matrix

1. Introdu

Organic method forof technicalprovide corcorrosion atorganic coaseveral metever, despittechnologiemetals in agthe decreasindustry, to

CorresponE-mail add

0013-4686/$ doi:10.1016/j.rs

with nano-additives lled with corrosion inhibitors, which can be released to the active sites. In thiswork, two different nano-additives are explored as potential self-healing materials for the developmentof active protective coatings. These additives are layered double hydroxides and cerium molybdate hollownanospheres loaded with mercaptobenzothiazole, as a corrosion inhibitor. These additives were addedto epoxy primers, individually, or combining the two nanoadditives in the same layer.

The electrochemical behaviour and the potential of self-healing ability were studied by electrochemicalimpedance spectroscopy, scanning vibrating electrode technique and scanning ion-selective electrodetechnique. The results reveal that both types of nanocontainers can provide effective corrosion inhibitionon articial induced defects, at different stages of the degradation process. Moreover, the results alsoshow that there is a synergistic effect concerning corrosion inhibition and self-healing potential when amixture of the two nanocontainers is used. The mechanism of self healing is presented and discussed interms of effect of organic inhibitor and role of the nanocontainers, including effect of cerium ions releasedfrom cerium molibdate nanoparticles.

2011 Elsevier Ltd. All rights reserved.

ction

coatings are the most common and cost effective corrosion protection of metallic parts in a wide range

applications. One of the main roles of the coating is torosion protection by creating a barrier that delays thetack of aggressive species. Throughout the last decadestings have been widely applied for the protection ofals with signicant progress in the recent years. How-e signicant improvements in anti-corrosion coatingss, problems continue in the long-term protection ofgressive environments, with very high costs. Moreover,e of the weight of components in the transportation

reduce fuel consumption and CO2 emission, are also

ding author. Tel.: +351 218419769; fax: +351 218419771.ress: [email protected] (M.F. Montemor).

imposing urgent challenges like the use of lightweight alloys andthinner coating layers produced by eco-friendly routes. The appli-cation of thinner coatings demands new strategies to extend theirlifetime and to improve their effectiveness, as for example thedevelopment of smart coatings. Nowadays coatings showing self-healing ability are emerging as promising technical solutions forimproved durability of painted materials. According to the liter-ature, [1] in a smart healable material a key property is altered,in a controlled fashion, in response to the introduction of a pre-determined external stimulus. This denition can also be applied tomany materials and several properties, including coatings used forcorrosion protection of metallic parts. Some examples of successfulsmart functionalities in organic coatings have been proposed in theworks by White et al. [24] that aim at developing self-healing abil-ity based on encapsulated polymerising agents, for the self repairof damaged coating matrix. New synthesis routes and strategieshave been reported, highlighting the potential of smart coat-ings for corrosion protection. Strengths and weaknesses of these

see front matter 2011 Elsevier Ltd. All rights reserved.electacta.2011.10.078tion of self-healing ability in protectiveations of layered double hydroxides anntainers lled with corrosion inhibitor

temora,, D.V. Snihirovaa, M.G. Tarybaa, S.V. Lamaasb, J. Tedimc, A. Kuznetsovac, M.L. Zheludkevichto Superior Tcnico, Technical University of Lisbon, Av Rovisco Pais, 1049-001 Lisboa, Patory, Institute of Materials Science, NCSR DEMOKRITOS,153 10 Agia Paraskevi Attikirtment of Ceramics and Glass Engineering, University of Aveiro, Campus Santiago, 3810/ locate /e lec tac ta

atings modied witherium molibdate

, I.A. Kartsonakisb, A.C. Balaskasb,.G.S. Ferreirac

alceAveiro, Portugal

32 M.F. Montemor et al. / Electrochimica Acta 60 (2012) 31 40

systems have been discussed as well as scale up and related costs.A detailed review of this topic and related strategies can be foundelsewhere [4,5].

The development of smart coatings, modied with nanoaddi-tives, speciastarted in tmicron parreport an imself-healingwith the uscorrosion ihalloysites hydroxyapain nanoconing approacfar and reqsion inhibitto provide ing life. Thiprimer coattainers, inclused in thicerium mowith mercamodied coment of prouse of variointend to hifor effectiveimpedance technique (trode techn

The resuscreening orelevance ounderstandFrom the imof the differerties of thetechniques self-healingThe results of additivesare combin

2. Experim

2.1. Prepara

Galvaniscoating (wiadditives). Tthickness ispolymer mindustry.

The coatrosion inhibhydroxides2-mercaptotainers androsion perfcommerciacalcined atstructure. Tquent step aqueous so

XRD diffractograms acquired after the calcinationrehydrationtreatment showed peaks ascribed to the crystalline structure ofLDHs. The loading content of MBT in the LDH (determined by HPLC)was found to be 1013 wt.%.

prepe strae reps of d poerisal metollowtes iontaaturvacu0 nm

o set coateMo ond eig

odit in ted indingrefered.

thic usined wcomp

was

ectro

the ancentaine, con. Themtoz do

pote conratednter de. T2. Th

evol weree cors, a

NaComp

activle to

to alfect wa lozlled bstrammET pT qully designed for the mitigation of the corrosion activityhe early 2000s with the addition of inhibitors or subticles to the coatings [69]. Generally, these worksprovement of the corrosion resistance and interesting

effects. The self healing ability was much improvede of pH sensitive reservoirs, acting as containers for

nhibitors, as for example inhibitor lled particles or[10,11], polyelectrolyte layers [12,13] or pH sensitivetite particles [14]. The use of inhibitors encapsulatedtainers is still in its infancy but it is certainly a promis-h. Another aspect of this technology, not explored souiring deeper studies, is the use of mixtures of corro-or containers, sensitive to different stimulus and ablecorrosion protection at different stages of the coat-s aspect is explored in this work, in which thin epoxyings modied with different inhibitor loaded nanocon-uding their mixtures were studied. The nanocontainerss work are layered double hydroxides (LDH) [15] andlybdate (CeMo) hollow nanospheres [16], both loadedptobenzothiazole (MBT) as a corrosion inhibitor. Theatings were applied on galvanised steel. The assess-spective self-healing ability demands for the combinedus electrochemical techniques. In this paper we alsoghlight the potential of three methods that are essential

assessment of the self-healing ability: electrochemicalspectroscopy (EIS) and the scanning vibrating electrodeSVET) combined with the scanning ion-selective elec-ique (SIET).lts show how EIS can be powerful as a tool for thef promising smart protective systems and highlight thef the parameters extracted from the EIS analysis to theing of the corrosion behaviour of the smart coatings.pedance results it was possible to assess the impact

ent additives (including mixtures) in the barrier prop- coating. On the other hand, localised electrochemicalgive detailed knowledge on the corrosion activity and

extent in the presence of defects exposing the metal.highlight the different kinetics of inhibition of each type

and the synergistic effect observed when both additivesed within the same coating.

ental

tion of the coated materials

ed steel samples were coated with a model epoxy basedthout anti-corrosive pigments or any other functionalhe lms were applied with a roll-coater and the dry lm

approximately 2 m in order to simulate the organicatrix of anti-corrosion systems used in the automotive

ings were modied with nanocontainers lled with cor-itor. The following additives were used: layered double

and cerium molybdate hollow nanospheres lled withbenzothiazole (MBT). The preparation of LDH nanocon-

their physicalchemical properties, including anticor-ormance, are described in detail elsewhere [15]. First,l synthetic hydrotalcite (Mg2Al6(CO3)(OH)164H2O) was

650 C (2 h), to remove carbonate anions from thehe incorporation of MBT was achieved in a subse-

consisting of hydration of the calcined sample, using anlution containing 0.1 M MBT in anionic form (pH 10).

Theand thcles arconsistchargepolymsolgedate htempladate cusing sunder 230 2

Twprimerwith Cthe secequal wwas mamounof storthe loaposes, prepar

Theminedequippthese 15 keV

2.2. El

All impedthe copurpossolutioFAS2 F100 kHcircuitnal). Aa satuas couelectro3.4 cm

Thecessesthat thcoating0.05 Mcase, crosionpossibcientThe decreate controiron su

A cothe ASand SIE[18].aration and characterisation of the CeMo nanoadditivestegies used to encapsulate the inhibitor in these parti-orted in literature [16]. Summarily, the synthetic routea two-step process. First of all, templates of anioniclystyrene (PS) spheres were prepared using emulsiontion. Second, the PS containers were coated via thehod to form a cerium molybdate layer. Cerium molyb-

spheres were obtained after the burn off of the PSn air at 550 C. Furthermore, the obtained cerium molyb-iners were loaded with the corrosion inhibitor MBT,ated solution of MBT in acetone into a sealed containerum conditions. The containers are hollow with a size of. The CeMo containers were loaded with MBT [16,17].

s of modied coatings were prepared: in one set, theing was loaded with LDH lled with MBT (LDH/MBT) orlled with the same inhibitor (CeMo/MBT) in 4 wt.%. Forset of samples, both type of containers were mixed, inht amounts, to modify the primer layer, i.e. the coatinged with a mixture of LDH and CeMo containers (sameotal) loaded with the same inhibitor, MBT. The amounthibitor is not the same for the modied coatings because

capacity of each NC is different. For comparative pur-ence coatings, i.e. coatings without additives were also

kness and the morphology of the coatings were deter-g SEM. A JEOL JSM7001F scanning electron microscopeith Oxford light elements EDS detector was used inlementary experiments. An electron beam energy of

applied for SEM analysis.

chemical studies

coated specimens were studied by electrochemical spectroscopy (EIS) in order to evaluate the effect ofers on the barrier properties of the coating. For thisated samples were immersed in 0.5 M NaCl aqueouse EIS measurements were performed using a Gamrystat with a PCI4 Controller in a frequency range fromwn to 10 mHz. All the spectra were recorded at openntial, applying 10 mV sinusoidal perturbations (rms sig-ventional three-electrode cell was used, consisting of

calomel reference electrode, a coiled platinum wireelectrode and the coated metallic coupon as workinghe area of the working electrode was approximatelye electrochemical cell was placed in a Faraday cage.ution of the corrosion activity and the self-healing pro-

studied by the SVET and SIET techniques. To guaranteerosion onset starts exactly at the same time for all thedefect was created on the coating prior immersion inl (pH 6.5). A more diluted solution was used in thisared to the EIS measurements, in order to delay cor-ity of the blank samples. With this concentration it isfollow the non-inhibited system for a period of time suf-low effective comparisons with the inhibited samples.as created by using a punch machine, which is able to

enge like defect of approximately 0.2 mm side. In thisconditions the defect reaches the Zn layer, but not thete.ercial system from Applicable Electronics, controlled byrogram (Sciencewares), was used to perform the SVETasi simultaneous measurements as previously reported

M.F. Montemor et al. / Electrochimica Acta 60 (2012) 31 40 33

Insulated PtIr probes (Microprobe, Inc.) with platinum blackdeposited on a spherical tip of 15 m diameter was used as vibrat-ing electrode for the SVET system. The probe was placed 100 3 mabove the surface, vibrating in the planes perpendicular (Z) andparallel (X)was 18 mand 325 Hzexperiment

The locpH-selectivnonadecylpmicroelectrable pH bu55.2 1.0of H+ was on prepara[14,19].

The loca26 26 gridconditions. was 0.6 and40 min. 81immersion.

Such a mmaps can bevolution oof data tre[20,21] andcathodic (Iciz current dtime of the

Icathodic =

Ianodic =

xm

where the scanned areunits of A/cathodic anbe expresse

3. Results

3.1. Barrier

Fig. 1(a)ed with LDrelevant deformation othickness ocross sectioexample, foin Fig. 1(b).modied waddition of and CeMo not induce (Fig. 1(b)) foers can be oit is possiblewithout for

Fig. 2 shin NaCl 0.5

EM imT and

ditiveMo/DH + CeMo)/MBT. The EIS response for the reference sampler the LDH/MBT modied coating is very similar at the earlyof immersion. In both cases there is a marked capacitivese, extending in the frequency range from 100 kHz to 10 Hz,esistive response in the low frequency end. The response cangned to the coating capacitance and the resistive response toistance of the electrolyte within the coating pores, respec-For the systems modied with CeMo/MBT the EIS response

a shorter capacitive range and the lowest impedance valuese entire frequency domain. Moreover, a new time constant

to interfacial processes develop in the medium/low fre- range.

system consisting of a mixture of LDH/MBT and CeMo/MBTd impedance values, which are below those of the systemed with LDH/MBT and above those of the coating modiedeMo/MBT.the coatings displayed a drop of the impedance values, forhan one order of magnitude, during the rst 24 h of immer-his drop is consequence of electrolyte uptake that formstive pathways through the coating. Simultaneously, the highncy time constant (related to the coating barrier proper-hifts to higher frequencies, and for the coating modiedeMo, it becomes out of the measured frequency range. Thision reveals that the barrier properties are poorest for the

modied with CeMo/MBT containers. After 24 h of immer-d up to one week of immersion, all the other 3 coatingsted identical impedance values over the entire frequencyn, despite the differences in the relaxation times and timents, which demanded different equivalent circuits to modeltem as proposed in Fig. 3. to the samples surface. The amplitude of vibration, vibration frequencies of the probe were 124 Hz (Z)

(X). Only the vertical component was used for treatingal data and calculating total current.alised pH measurements were carried out usinge glass-capillary microelectrodes lled with 4-yridine-based liquid membrane. The pH-selectiveodes were calibrated using commercially avail-ffers and demonstrated linear Nernstian response

mV/pH in a pH range from 2 to 10. The local activitydetected 50 5 m above the surface. More detailstion of the microelectrodes can be found elsewhere

l pH and ionic current density (iz) were mapped on a in a 0.05 M NaCl solution under open circuit potentialThe time of acquisition for each SVET and SIET data point

2.3 s respectively, resulting in a total scan time of about3 scans for each sample were recorded every day of

assive amount of local ionic current distribution data-e reduced to one very informative plot demonstratingf total cathodic and anodic currents in time. This methodatment was proposed by McMurray and co-workers

used by other groups [22,23]. Total anodic (Ianodic) andathodic) ionic current density were calculated integratingensity distribution across the area of each scan at themeasurement, Eqs. (1) and (2):

xmax

xmin

ymaxymin

[iz(x; y) < 0dx]dy (1)

xmax

in

ymaxymin

[iz(x; y) > 0]dxdy, (2)

xmax, xmin, ymax and ymin are the coordinates of thea of each sample. Thus, integrating current density withcm2 and spatial dimensions of scanned area of m, totald anodic currents arising through the scanned area willd in A.

and discussion

properties of the smart coatings

shows the SEM image obtained on the coating modi-H/MBT. The image reveals a uniform surface without

fects such as holes or cracks. There is no evidence off LDH agglomerates on the surface of the coating. Thef the coatings was determined by observation of thens and it was found to be between 2 and 2.2 m. Anr the coating modied with CeMo/MBT, is presented

The thickness of the blank coating and of the coatingith LDH/MBT was also in the same range. Therefore, theLDH, the addition of CeMo or the addition of mixed LDHdid not change the thickness of the coating and doesthe formation of large agglomerates. Near the interfacer the CeMo modied coating, some CeMo nanocontain-bserved. The SEM images indicate that for thin coatings

to achieve very effective dispersion of nanocontainers,mation of undesirable defects in the coating.ows the EIS Bode plots evolution during immersion

M for the samples coated with the blank coating

Fig. 1. SLDH/MB

(no adwith Ctives (Land fostages responand a rbe assithe restively. revealsover threlatedquency

Therevealemodiwith C

All more tsion. Tconducfrequeties) swith Cevolutcoatingsion anpresendomaiconstathe sysages obtained in two samples tested: (a) coating modied with (b) coating modied with CeMo/MBT (cross section).

es), coating modied with LDH/MBT, coating modiedMBT and coating containing a mixture of the two addi-


mersion in 3% NaCl for: (a) 1 h and (b) 24 h.

The impcuits depictthese systethem, capa(CPE) whictherefore dexplanationcircuit propin cascade, capacitancesurface polbeen used timens [24].obtained froCPE1/R1 accthe relaxatiand the CPE

Fig. 4 deing (R1) forbecause it iideally shotainers to follows thestages of imwere measmodied worders of mcoating mo

Fig. 3. E

sion all the values dropped below 1 105 cm2 and con-o dewestodinsta

incrrom8

atingPE wth th

evot featies sion,ith

ispe in th

negating

the or thr defFig. 2. EIS Bode plots obtained for the different coatings after im

edance spectra were tted using the equivalent cir-ed in Fig. 3(a) and (b). Due to the dynamic nature ofms, different equivalent circuits were used. For all ofcitances were replaced by a constant phase elementh accounts for non-homogeneity of the systems andeviations from the ideal capacitive behaviour. Detaileds of the CPE have been previously reported [24]. Theosed in Fig. 3(a) is composed of two time constantsaccounting for the electrolyte resistance (Rs), coating

and pore resistance (CPE1/R1) and double layer andarization effects (CPE2/R2). This equivalent circuit haso describe the behaviour of galvanised steel coated spec-

The one in Fig. 3(b) was used to simulate the resultsm 24 h onwards. It makes use of three time constants:ounts for the coating properties, CPE2/R2 is assigned toon processes observed in the medium frequency range3/R3 describes the low frequency behaviour.picts the evolution of the pore resistance of the coat-

the various coated samples. This parameter is relevants related to the barrier properties of the coatings anduld not be affected by the addition of the nanocon-a great extent. The evolution of this resistance (R1)

same trend for all the coated samples. At the earlymersion, the highest values, around 1 108 cm2,

immertinue tThe loing mthe co(CPE1)tude, f1 10the coof the Cone wi

Therelevanproperimmerbility wgood ddefectsa morethe coibility,inhibitneitheured for the reference coating and for the coatingith the LDH/MBT. The lowest values, which are twoagnitude lower (1 106 cm2), were observed for thedied with CeMo containers. Within the rst 24 h of

quivalent circuits used for the tting of the EIS measurements.Fig. 4. Evolutitime constantcrease attaining values generally below 1 104 cm2. values were systematically observed for the coat-ed with CeMo/MBT containers. The admittance ofnt phase element for the high frequency processeased with time, by more than one order of magni-

values around 1 109 1/cm2 sn to values above1/cm2 sn, or even higher (above 1 107 1/cm2 sn for

modied with CeMo containers). The exponent valueas typically above 0.9 for all the systems, except for thee mixture (exponent between 0.8 and 0.85).lution of the coating pore resistance highlights sometures: the LDH containers do not damage the barrierof the coating, neither at early nor at later stages of

revealing that this system has a very good compati-the epoxy matrix. Moreover, this trend also suggestsrsibility of the LDH/MBT containers and the absence ofe coating. On the other hand, the addition of CeMo has

ative impact, affecting the earlier barrier properties of. This may indicate poorer dispersibility and compat-presence of defects or some spontaneous leaching ofat can interact negatively with the matrix. However,ects nor large agglomerates could be detected by theon of the coating resistance estimated by tting the highest frequency.


Fig. 5. (a) Curr NaCof the surface.

SEM/EDS anpreferentiapolymeric mture of bothbecause thepartially repclose to the

3.2. Corrosi

EIS is a vthe coatingan average rlocalised eving. Howevthe great adtion of the cdefects of tsmall defecSIET. Currenon the expoquantify thcathodic cuionic curren

For the diately afteimmersion,was above SVET resultFig. 5. The a(Fig. 5(a)), rface was m(Fig. 5(b)), aaround 5.3,

ocal ed con th

Zn(aq

nH

= 7.9ent density maps obtained on the reference sample after 18 h of immersion in 0.05 M

alysis, suggesting that most probably the presence ofl pathways was created by poorer compatibility with theatrix. Nevertheless, the coating prepared with a mix-

inhibitors evidenced slightly better properties probably total amount of CeMo containers was reduced as it waslaced with LDH, which showed barrier properties very

reference system.

As lhydratbased

Zn

Zn2+(aq) +

pK1hydon inhibition ability of the smart coatings

ery powerful technique to investigate the evolution of properties and corrosion inhibition ability, but it givesesponse of the surface, making it difcult to understandents that occur in small defects formed in the coat-er, spatially resolved electrochemical techniques havevantage of giving local information, regarding evolu-orrosion processes (progress and mitigation) on activehe coating. The ability to inhibit corrosion activity ints created on the coating was investigated by SVET andt density maps and pH maps were taken periodicallysed samples up to 34 days of immersion. In order toe overall electrochemical activity, the total anodic andrrent densities were calculated integrating the maps oft density.blank coating, corrosion activity was detected imme-r immersion in the aggressive solution. After 1 h of

the anodic current density measured over the defect70 A/cm2 and increased continuously with time. Thes obtained after 18 h of immersion are depicted innodic current density attained values near 200 A/cm2

evealing strong corrosion activity. The pH near the sur-apped at the same time over the same area by SIET,nd the results show that the pH on the anodic area is

thus revealing dissolution of the exposed Zn.

where hydr

stant of thethat the locmeasured ppH increase

During tof the coatwere carriestrongly da

Fig. 6 shoand cathodgrated cathequal, with

Icathodic = IaHoweve

localised cuanodic activare partiallytion, with rprobe measthe ionic udetect all tthe OH ionow to the sin this casel; (b) pH map obtained after 18 h of immersion; (c) optical micrograph

acidication occurs only due to the hydrolysis of formedations Zn(aq)2+, their concentration can be estimatede Eq. (4).

)2+ + 2e (3)

2O [Zn(OH)n](2n) + nH+

6 K1st = 1.10 106, (4)olysis constant pK1hyd is calculated from stability con-

Zn(OH)+ complex K1st. A simple calculation [25] showsal activity of generated Zn(aq)2+ corresponding to theH 5.3 is around 2.5 103 M. In the cathodic areas thed to values above 6.8.he rst 18 h of immersion there was a clear degradationing (Fig. 5(c)) around the original defect. Experimentsd only for 24 h as after this period the coating wasmaged.ws the time evolution of the total currents (both anodic

ic) for the system modied with LDH/MBT. Ideally, inte-odic and anodic currents at any given time should be

overall current being zero:

nodic; Ioverall = Icathodic + Ianodic = 0 (5)r, in practice SVET is not always able to measure allrrents. In the case of a coated sample, with a defect,ity is localised in the defect while the cathodic reactions

taking place under the coating. This causes alkalinisa-elease of OH, under the coating. The SVET vibratingures the potential differences arising in solution due toxes produced on the sample surface. Thus, it may not

he activity taking place under the coating, as much ofs generated underneath the coating may not be able toolution. This means that even though the Eq. (5) is valid

, total cathodic current available for SVET detection will


Fig. 6. Evolution of the total anodic and cathodic currents obtained integratingcurrent density distribution for the coating modied with LDH/MBT.

be smaller than total anodic current. On the other hand, there isa time frame necessary to complete the measurement, and duringthis time, the anodic and cathodic currents may suffer some slightchanges, leading to deviations from the ideal behaviour.

The rst but weak signs of corrosion activity for the coatingmodied with LDH/MBT were detected after 45 h of immersionwith a small increase of the corrosion activity over the defect anda small acidication of the anodic areas. From this time, and upto 1 day of immersion, there was very little activity, revealing astrong inhibition effect when compared to the blank coating. After18 h of immof Zn (Fig. in the corrothat attainearound 5.4 activity, thelower than onwards, thtime after w

The coatvery distincat the earl

Fig. 8. Evolution of the total anodic and cathodic current obtained integrating cur-rent density distribution for the coating modied with CeMo/MBT.

reached value 5.04 corresponding to high dissolution rate of Znwith local Zn2+ concentration reaching 1 102 M (SIET map is notshown). The overall anodic current density values reached a max-imum after 35 h of immersion (Fig. 8). However after this peakactivity, the current densities decreased attaining very low values.The current density map (Fig. 9(a)) obtained after approximately20 h of immersion clearly illustrates this trend: both anodic andcathodic activities are low and do not exceed 5 A/cm2. The pHin the defe

rate ed f

ion aterioting

ion aoatinant cion a. Thisve coBT oersion the pH was 5.5, indicating weaker dissolution7). After 1819 h of immersion there was an increasesion activity characterised by values of current densityd 100 A/cm2 after 2 days of immersion, and pH values(SVET-SIET maps are not shown). Despite this increased

values of the current density were about two timesthose obtained for the reference coating. From this timee current intensity increased until 3 days of immersion,hich the experiment was concluded.ing modied with the CeMo/LDH containers shows at behaviour: evident corrosion activity was observedy stages of immersion. The pH in the anodic defect

lution remaincorrosing dethe coacorrosied cimportcorrosperiodeffectiLDH/MFig. 7. Coating modied with LDH/MBT nanocontainers. (a) Current density map obtainect increased up to 5.68, corresponding to lower disso-of Zn2+; approximately 4.4 104 M. This trend wasor more than 3 days of immersion. An increase of thectivity was observed after 4 days of immersion and coat-ration was detected after one week. Comparatively to

modied with LDH/MBT these results indicate that: (1)ctivity develops much earlier in the CeMo/MBT mod-g, probably due to the poorest barrier properties; (2)orrosion inhibition was observed after a maximum ofctivity; (3) corrosion inhibition extended over a longer

evolution suggests that CeMo/MBT containers are alsorrosion inhibitors, acting delayed comparatively to thenes.d after 18 h of immersion in 0.05 M NaCl; (b) pH map.


Fig. 9. Coating modied with CeMo/MBT nanocontainers. (a) Current density map obtain

When thtrochemica(Fig. 10). Tligible (Figsevident bettime the loThus, at thifor the LDHsion times 18 h of immditions for (2425 h onup to 3 daysobserved foanodic currwas kept fo

The overmodied wresults (Figdepicted, bement was sThe evolutimixed systecess, whichdifferent m

Fig. 10. Evolucurrent densit

the ers a

his iDH/Mmixeassined inhe Cel houMO/Mina

sion. crealays BT awer imbincoatiion. Tot ino on tre [2xposd at e two types of containers are mixed, the localised elec-l measurements also reveal a kind of mixed behaviourhus, corrosion at early stages is delayed, being neg-. 10 and 11(a)). The activity over the defect becameween 18 and 24 h, reaching a maximum value. At thiscal pH at the anodic sites was around 5.2 (Fig. 11(b)).s stage the behaviour is very similar to that observed/MBT modied coatings inhibition at earlier immer-and increase of corrosion activity after approximatelyersion. Increased activity in the defect creates the con-activating CeMo/MBT containers and for longer timeswards), the anodic currents decrease, being negligible

of immersion. This behaviour is now similar to the oner the CeMo/MBT coatings that showed a decrease of theent and a stabilization of the pH around 5.66. This trendr several days of immersion (Figs. 10 and 11(c) and (d)).all anodic current was depicted for the three systemsith nanocontainers for better comparison of the SVET. 12). The current values for the blank sample are notcause they were much higher and because the experi-

topped after 1 day due to severe damage of the coating.on of the anodic currents (Fig. 12a) show that for them there are two steps for the corrosion inhibition pro-

suggests different kinetics of inhibition and eventually

delayscontaincess. Twith Lin the after pobservFig. 8 tseverathe Cepredomimmerused totwo deLDH/Mthe slothis cosmart inhibitrelies nbut alsliteratuafter ereleaseechanisms triggering the corrosion inhibitor. LDH/MBT

tion of the total anodic and cathodic current obtained integratingy distribution for the coating modied with LDH/MBT and CeMo/MBT.

work [26] thexchange eby an exchaof aggressivtial, providobserve anchloride ioMBT inhibiis an excessmay decreaNeverthelein the blanprovided bysevere detein the LDHuntil relevaping effect, blank systeover the atted after 20 h of immersion in 0.05 M NaCl; (b) pH map.

corrosion activity at early stages, meaning that thesere extremely effective at the onset of the corrosion pro-

s conrmed by the behaviour of the coating modiedBT only as well. However, contrasting with this one,

d system the overall anodic current started to decreaseg a maximum, attaining values very similar to those

the CeMo/MBT modied coatings. In fact, as shown inMo/MBT additives start to inhibit the defect only afterrs of immersion and after some corrosion activity. Thus,BT show slower kinetics of inhibition, which start to

te in the mixed coating after approximately one day of The complementary behaviour of both additives can bete a synergistic effect, in which the combination of thecorrosion onset due to: (1) early inhibitive ability of thend decreases of the longer term corrosion activity due tonhibition ability of CeMo/MBT. This trend suggests thatation can be an interesting approach to develop newngs with self-healing potential and different kinetics ofhe reasons for the effective self-healing effect observed

the inhibitor activity only (which is MBT in both cases)he active role of LDH and CeMo containers. According to6], the release of MBT from LDHs proceeds very rapidlyure to NaCl. Thus, a high concentration of inhibitor isearly stages in the active sites. It was shown in a previous

at after a period of chloride exposure the chemical ion-

quilibrium is attained. The leaching of inhibitor occursnge mechanism (release of an inhibitor and entrapmente chlorides) and the release of an inhibitor is sequen-

ing protection on demand. Therefore it is expected to important delay of the corrosion activity because thens are trapped by the LDH and because the releasedts corrosion onset. For longer immersion times, there

of chloride ions and the amount of released inhibitorse, leading to an increase of the anodic current densities.ss, the values were less than half of the values observedk coating, indicating that corrosion inhibition was still

the released inhibitor. In the case of the blank coatingrioration was observed within the rst day, whereas/MBT system the coating lasted for more than 3 daysnt deterioration occurs. In addition to the chloride trap-the inhibition of corrosion activity, comparatively to them, involves the formation of a protective inhibitor layeracked surface. The effectiveness of MBT and thiazoles as


Fig. 11. Coatinafter 3 h, (b) af

corrosion inand aluminthe formati

The inhitainers behliterature, Cing aluminicerium molsensitive tosolve in mog modied with a mixture of LDH/MBT and CeMO/MBT nanocontainers. Current densitter 18 h, (c) after 40 h and (d) optical image after 7 days.

hibitors has been demonstrated for zinc, steel [2729]ium [30,31] and the mechanism is suggested to involveon of inhibitive adsorption layers.bitive effect of CeMo/MBT may also rely on the con-aviour and on inhibitor activity. According to previouseMo empty containers have a positive effect in protect-um substrates [16]. Literature [32] also points out thatybdates can work as ion exchangers and that they are

pH being stable in alkaline environment and able to dis-re acidic conditions. If the inhibitor could be released

due to instated in the acorrosion aied as littlproperties oself healingdissolutionlution in acrelease of tused as inhy maps obtained after different immersion times in 0.05 M NaCl: (a)

ability of CeMo, in the more acidic environment cre-nodic areas, this would explain the drop observed after

ctivity peaks. However the exact mechanism is not clar-e is known in literature about the exact ion-exchangef these containers. Another possible explanation for the

effect involves both cerium and molybdate ions due to of the nanocontainers. These particles may suffer disso-idic media (formed over the anodic areas) and thereforehese ions is likely to occur. Indeed these species can beibitive pigments as reported in literature [33]. Therefore


Fig. 12. Evoluing current denand the mixtu

cerium andplaces, playin literaturean effectivecontainers t

In the ption of the cachieved byrosion inhibeffect on thewith differefor organic

4. Conclus

Modicadouble hydmercaptobeproperties same epoxysome losses

The abiliers lled win defects fscanning viThese additimmersion

Cerium mercaptobeafter a periois effective

attributed either to the organic inhibitor or release of activespecies, such as cerium ions from the nanocontainers, themselves.

The addition of a mixture of nanocontainers, lled with 2-mercaptobenzothiazole, results in a synergistic inhibition effect

mbihis isers.ntai

for p

wled

autht, funontrs-Schir te

s ackueseject

.G.T. RH/BRef. S

nces

. Murp Blaisz

Rule,.

Blaiszu. RewaagterialsVoevohnol. .S. Fetion of the total anodic (a) and cathodic (b) current obtained integrat-sity distribution for the coatings modied with LDH/MBT, CeMo/MBT

re (LDH + CeMo)/MBT.

that cotion. Tcontainnanocoability

Ackno

Theprojecvided cThomaand theSalak iPortugthe proand Mand SFgrant (

Refere

[1] E.B[2] B.J.[3] J.D.

205[4] B.J.

Ann[5] S. Z

Ma[6] N.

Tec[7] M.G/or molybdenium species can be formed on the healeding an inhibitive role as effective inhibitors reported

[3436]. The results obtained in this work evidence corrosion inhibition ability of these CeMo/MBT nanohat extended over longer periods.resent work it is demonstrated that effective inhibi-orrosion processes in articially induced defects can be

the addition of LDH and CeMo loaded with MBT as cor-itor. The results highlight, by the rst time, a synergistic

self-healing ability potential of mixtures of containers,nt kinetics of inhibitor release, acting as smart additivescoatings.

ions

tion of epoxy based coatings with layeredroxides anion-exchange containers lled with 2-nzothiazole has no negative impact on the barrierof thin epoxy coatings. However, modication of the

matrix with cerium molybdate nanoparticles induces in terms of barrier properties.ty of layered double hydroxides ion exchange contain-ith mercaptobenzothiazole to inhibit corrosion activityormed in the epoxy coating was demonstrated by thebrating probe and ion-selective electrode techniques.ives delay corrosion onset, being very effective at earlierstages.molybdate hollow nanospheres lled with 2-nzothiazole show very good inhibition potentiald of relevant corrosion activity. The corrosion activity

ly inhibited for longer immersion times and can be

49 (2004[8] S.V. Lama

Ferreira, [9] K. Arama

[10] D.G. ShchMhwald

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Such behaviour clearly demonstrates the potential ofners mixtures to design smart coatings with self-healingrolonged coating lifetime.

gments

ors acknowledge the MUST (NMP3-LA-2008-214261)ded by the FP7 programme and all the partners that pro-ibutions to this work: Production of model coatings: Mrmidt-Hansberg and Valerie Gandubert from Chemetall

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Evaluation of self-healing ability in protective coatings modified with combinations of layered double hydroxides and ceri...1 Introduction2 Experimental2.1 Preparation of the coated materials2.2 Electrochemical studies

3 Results and discussion3.1 Barrier properties of the smart coatings3.2 Corrosion inhibition ability of the smart coatings

4 ConclusionsAcknowledgmentsReferences

Evaluation of Self-healing Ability in Protective Coatings Modified Withn

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