1-s2.0-s0300944004002279-main

Progress in Organic Coatings 52 (2005) 339350

Comparative electrochemical studiesinh

b, A.Av. Rovniversid

ber 200

Abstract

The antic and zimpedance s (SVETthat zinc wa lvanisepigments, th bes, althe various p urringsamples. Fo enciesformed by m ticorro 2004 Elsevier B.V. All rights reserved.

Keywords: Anticorrosive pigments; Corrosion; EIS; SVET; Zinc; Phosphate; Chromate

1. Introdu

Decadesof a few pigwide rangechoice for mtal and heaAmong theline up as sis probablyin many pa

Althougchromate pchromate iosame waychromate ioreduced tooxidation o

CorresponE-mail ad

0300-9440/$doi:10.1016/jction

of industrial practice have lead to the selectionments with excellent anticorrosive properties in aof situations. Chromates have long been the firstany applications, but they represent environmen-

lth threats that are leading to their replacement.new non-toxic anticorrosive pigments that today

ubstitutes for traditional pigments, zinc phosphatethe most widely accepted, and it is incorporated

int formulations.h not fully understood, the inhibitive action ofigments is now well accepted to be based on thens that leach out to solution, where they act in the

as other chromate soluble inhibitors. In solution,ns are present in the hexavalent form and becomethe trivalent form to counterbalance the anodicf the metal substrate. It is the trivalent chromium

ding author.dress: [email protected] (A.M. Simoes).

that is usually found in the passive layer. On iron, films ofoxide spinels with Cr3+ and Fe3+ have been reported [13].The Cr/Fe atomic ratio seems to depend on the pH, oxygenand chromate content of the solution. Other authors describethe film composition as a mixture of oxides and hydroxidesof iron and chromium [46]. Hydroxylated layers may bepresent [2] and adsorbed Cr(VI) has also been observed [7].On aluminium alloys, the action of chromate has also beenstudied and an inhibiting effect on the oxygen reduction re-action at the copper precipitates was observed [8]. Studies ofchromate passive layers on zinc are scarce but a recent studyreports a passive film of Cr(III) with the absence of zinc [9].Zinc yellow, being a mixed salt of zinc chromate, potassiumchromate and zinc hydroxide, has, in addition to CrO42,two more ionic species of inhibitive nature, Zn2+ and OH.It has been referred that for the same concentration of chro-mate, zinc yellow has a high inhibitive power compared toother simple chromate salts [10]. OH leads to a rise in pH,decreasing the corrosivity, particularly at the anodic sites,whereas Zn2+ precipitates as Zn(OH)2 at the cathodic sites.The protective layers formed on the metal prevent either the

see front matter 2004 Elsevier B.V. All rights reserved..porgcoat.2004.09.009zinc phosphate as corrosionA.C. Bastosa, M.G.S. Ferreiraa,

a Chemical Engineering Department, Instituto Superior Tecnico,b Departamento de Engenharia de Ceramica e do Vidro, U

Received 28 May 2004; received in revised form 9 Septem

orrosive performance of two inhibitive pigments, zinc chromatepectroscopy (EIS) and the scanning vibrating electrode techniques protected from corrosion in both extracts. In tests using hot dip gae SVET detected the anodic and cathodic distribution along the scririmers. On the contrary, EIS was able to distinguish processes occ

r primers with anticorrosive pigment, a time constant at high frequetal ions from the substrate and inhibitive ions leached from the anof zinc chromate andibitors for zincM. Simoesa,

isco Pais, 1049-001 Lisboa, Portugalade de Aveiro, Aveiro, Portugal

4; accepted 9 September 2004

inc phosphate, was compared using electrochemical) in pigment extracts in 0.1 M NaCl. It was observedd steel painted with an epoxy primer incorporating the

though no significant differences were observed amongon the metal surface exposed by the scribe in differentwas attributed to a layer of protective nature, probablysive pigments.

340 A.C. Bastos et al. / Progress in Organic Coatings 52 (2005) 339350

release of metal ions from the surface towards the solutionor the oxygen or proton reduction, by, for instance, impedingthe flow of electrons needed for such reactions. A hydropho-bic charactlayers, as wchromate isanions such

Phosphaand also inpigment wamodes of ation of soathe binder bphates repocathodic reiron and alders the accneutralizintrolling thepaints withtrasting wipoor, possithe low soltion seemsphosphate

In this wclassical banew alterneither incorous extractand three vphate primamount ofon the primtenders. Cousing electscanning viestablishedeither coateentiate themakes it supotential ditributions.ues are con

troduced tosome work

2. Experim

2.1. Bare m

Zinc 99(produced1 cm 1 cmthread copp

mounted (Fig. 1a), whereas for the SVET measurements, zincsamples were epoxy mounted, leaving exposed a rectangle of1 mm2 (Fig. 1b). The specimens were polished with SiC grit

rs of grades 220, 500, 800 and 1000, washed in deion-water (Millipore), degreased with ethanol and dried withressed air.

Coated samples

ree primers were used: a two-component epoxy primerzinc phosphate (commercial product), an epoxy primeraddition of chromate (modification of the commercialer by replacing the phosphate pigment by zinc yellow)he corresponding epoxy clear coat. The primers wereed toratestests wng. Dral thic

20e sam

a cloe. Forcut, s

dges ag. 1c.el, acrder wcm2, a

Expos

gmenby ma

l for 24ame a

c yellphateolution

Electr

ET mcs Inc

1cterizat

NaClate ext

hate ext

etrohmison GLductivelolecularer has also been proposed for the Cr(III) oxideell as a lower zeta potential of oxide layers whenadsorbed, hindering the adsorption of aggressiveas chloride [11].

te has been used for decades as soluble inhibitorconversion coatings. The use of zinc phosphate ass proposed for the first time in 1965 [12]. Severalction have been proposed, including the forma-

ps of zinc [13], in addition to an improvement ofarrier properties [14]. Studies with soluble phos-rt phosphate layers at both the anodic [15] andgions [16]. Tertiary phosphates precipitate withso with zinc, forming a protective layer that hin-ess of oxygen or the oxidation of the base metal. A

g ability, capable of inhibiting corrosion by con-pH, was also suggested [10]. Real exposure ofzinc phosphate has led to very good results, con-

th accelerated tests where the results were quitebly due to the slow kinetics of the reactions or toubility of the pigment [17]. The pH of the solu-to play a major role, since the solubility of zinc

is higher in acidic media [18].ork, two anticorrosive pigments were studied: thesic zinc potassium chromate (zinc yellow) and theative, zinc phosphate. The pigments were testedporated in an epoxy primer or by preparing aque-s. A two-component epoxy primer was selectedariations were studied: a commercial zinc phos-er, a modification of the former by replacing thephosphate by chromate, and a clear coat, baseder formulation but without any pigments or ex-mparative electrochemical tests were performed

rochemical impedance spectroscopy (EIS) and thebrating electrode technique (SVET). EIS is a well-technique in corrosion research; it can work withd or uncoated samples and its capability to differ-

processes occurring in the electrochemical systemitable for this kind of study. The SVET measuresfferences in solution, created by uneven ionic dis-By a prior calibration, the measured potential val-verted to currents. The SVET technique was in-the field of corrosion in the early 1970s [19] andhas been published since then [2025].

ental

etal samples

.95% pure, in the shape of 1-mm thick foilby Goodfellow Ltd., UK) was used. For EIS,

samples were electrically connected via a multi-er wire with silver conductive adhesive and epoxy

papeizedcomp

2.2.

Thwithwithprimand tapplisubstThecoatidigitandin thallowscribwere

the ein Fiscalpcylin3.80

2.3.

Pi[26],NaCthe sas zinphosthe s

2.4.

SVtroni

TableChara

0.1 MChromPhosp

a Mb Crc Ind Mhot-dip galvanised steel (HDG) by dipping theinto the paint for 1 s and leaving them to dry.ere performed 1 month after application of the

y film thickness measured using an Elcometer 355kness gauge was 40m for EIS measurementsm for SVET measurements. The smaller thicknessples for the SVET study was chosen in order toser approach of the tip to the metal surface at thethe SVET measurements, squares of 2 cm 2 cmcribed and glued to an epoxy sample holder andnd sides isolated with epoxy adhesive, as shownFor EIS, a scribe of 1 cm long was made using aoss the coating and down to the substrate; a plasticas then glued to the surface, exposing an area ofs shown in Fig. 1d.

ure media

t extracts were made based on existing literaturegnetic stirring 1 g of pigment in 500 mL of 0.1 Mh, followed by filtering twice. The pigments were

s used in the paints, i.e., zinc chromate knownow (K2CrO43ZnCrO4Zn(OH)22H2O) and zinc(Zn3(PO4)22H2O). Table 1 shows some details ofs used.

ochemical techniques

easurements were made using Applicable Elec-., equipment. Tape glued around the sample hold-

ion of the exposure media (T= 20.0 C)pHa Conductivityb

(mS cm1)Concentration

6.20 9.44 ract 6.55 10.90 |Cr|total = 1.1 102 Mcract 6.43 9.46 |PO43|= 4.8 105 Md632 pH meter, electrode 6.0220.100.P 31 conductimeter.y coupled plasma.absorption spectrometry.

A.C. Bastos et al. / Progress in Organic Coatings 52 (2005) 339350 341

Fig. 1. Sampl(c) coated sam

ers workedplatinum tidistance ofplitude. Eaimage wasequipmentparallel andnormal toactive metasion of metregions is drents corresas a produc

EIS metation. Theworking elrated Calomwere madesolution wato 5 mHz ftential signthe open cisoftware anarea of thees used in the experiments. (a) Mounted bare zinc for EIS measurements in extracts,ple glued with epoxy for SVET measurements (d) cell for EIS measurements on s

as solution reservoir. The microelectrode had ap of 40m and was made to vibrate at an average

200m above the surface, with 40m of am-ch scan comprised 20 20 points and an opticalacquired before each scan. Although the SVETused can detect ionic currents in two directions,normal to the surface, only results from the flux

the surface will be presented. In general, for anl immersed in an electrolytic solution, the diffu-al cations from the metal oxidation in the anodicetected as a positive current, whereas negative cur-pond to OH emerging from the cathodic regionst from oxygen reduction.

asurements were made using Gamry instrumen-three-electrode cell consisted of the sample as

ectrode, a platinum counter electrode and a Satu-el Electrode (SCE) as reference. Measurements

inside a Faraday cage, at room temperature and thes quiescent and exposed to air. Usually, a 50 kHzrequency range was swept with a sinusoidal po-al with an amplitude 10 mV rms, superimposed torcuit potential. Fitting was made using ZView 2.70d all the spectra were corrected to the geometricalworking electrode.

Measuremade withagainst the

3. Results

3.1. Bare m

SVET csodium chthe salt solwith the dcathode (Fhalf of thetime, the anafter appro(Fig. 2c an

When ththe currentsured on twas only wthe patternscopic and(b) mounting for SVET measurements of bare zinc in extracts,cribed coated samples.

ments of the open circuit potential (OCP) werean AUTOLAB PGstat 20 apparatus and are plottedpotential of the SCE reference.

etal electrodes

urrent mapping made on pure zinc immersed inloride solution shows significant differences. Inution without pigment, activity occurred alwaysevelopment of one single anode and one singleig. 2a). Each of these electrodes occupied nearlysurface in the first minutes of immersion. Withodic area became more localized (Fig. 2b) until,

ximately 1 day, a pit was observed on the surfaced d).e NaCl solution contained the pigment extractsinhibition was obvious. The current values mea-

he surface were significantly reduced and thereeak indication of cathodic or anodic activity, withof the surface corresponding to scattered micro-weak activity, corresponding to currents below


Fig. 2. SVETchromate extrAlso shown a

6A cm2with the chrequired abefore theactivity wasion. For lligible withthe inspectand l).

The opewas practicof zinc (Fislightly higmore anodimately 3 hthe protectled to a sigmaps of the ionic currents measured 200m above pure zinc exposed to: 0.1 M NaCl (ac); zinc phosphate extract in 0.1 M NaCl (eg); zincact in 0.1 M NaCl (ik). Times of immersion are (top to bottom): 5 min, 1 h and 1 day. Scanned area: 1.25 mm 1 mm; Scale units: A cm2.re pictures of the surface after 1 day of immersion in each solution (d, h, l).

with the phosphate (Fig. 2eg) and 2A cm2romate (Fig. 2ik). Inhibition by the phosphatelonger time before its full action was felt, i.e.,phosphate film was formed, since some anodics measured in the first few minutes of immer-

onger exposure times the activity remained neg-both extracts (Fig. 2g and k), as confirmed by

ion of the surface at the end of the test (Fig. 2h

n circuit potential of pure zinc in 0.1 M NaClally constant and near the equilibrium potential

g. 3). With the phosphate, the potential was onlyher, although at the beginning it started from a

ic value and then rose during a period of approx-, which probably corresponds to the growth ofive film. In contrast, the chromate pigment hasnificant anodic polarization, reaching potentials

Fig. 3. Open circuit potential of pure zinc in 0.1 M NaCl, either uninhibitedor containing phosphate extract or chromate extract.


Fig. 4. Impedchromate extr

of 0.945chromiummajor comobserved ible curve fbetween thrent densitystate is achagrees withorder of relmate > pho

The impsolutions isspectrum hat high freThe low-frminutes ofwhich meaof an air-fothe chloridance spectra obtained for pure zinc at different times of exposure to 0.1 M NaCl,act (c); (a1), (b1) and (c1) are Bode plots, whereas (a2), (b2) and (c2) are the corre

V. This value is within the region of stability ofoxides/hydroxides, which are known to be theponents of the passivating film. The fluctuationsn the phosphate curve, in opposition to the sta-or the chromate, indicate differences in reactivitye two protective films, and agree with the cur-measurements with the SVET. After a stationary

ieved, the order of the corrosion potential valuesthe SVET observations and follows the expected

ative protective efficiency of the pigments (chro-sphate > reference system).edance response of pure zinc exposed to the threepresented in Fig. 4. In the absence of inhibitor, theas two relaxation constants, one well developedquencies and a small one at lower frequencies.equency process was clearly resolved in the firstimmersion but disappeared after 3 h of exposure,ns that it may be associated with the presencermed film of oxides that becomes dissolved in

e solution. Meanwhile, the high frequency pro-

cess becamto a rise ofgresses (poobtained fospectrum mwith a poroder oxygenthe spectraangle plot,the low freconstant. Trevealing thchange wit1 k cm2the extractsplots, inclusults, also ssolutions; isemi-circlecorrosion pwithout inhibitor (a), or containing phosphate extract (b) orsponding Nyquist plots.

e shifted to lower frequencies, which correspondsthe double layer capacitance as corrosion pro-

ssibly due to increasing active area). The spectrar pure zinc in NaCl solution is identical to theodelled by Titz et al. [27] for a metal coveredus oxide layer, in which corrosion proceeds un-diffusion control. In the presence of the extracts

are simpler, with only one maximum in the phasealthough the slope in the impedance modulus atquencies suggests the presence of a second timehe capacitance is smaller than described above,e inhibiting effect of the pigments, and does not

h time. The low frequency resistance, which wasin the NaCl solution, increased in the presence of, confirming a slower corrosion rate. The Nyquistded in Fig. 4 for a better visualization of the re-how the difference in behaviour among the threen the absence of inhibitor the spectrum has twos, the first of which is well-defined and reveals therocess, whereas in the presence of the extracts the


Fig. 5. Gene ut inhirameters for 1 = 1.31R2 = 631 cm ) = 3.31Y0(Q2) = 6.51 3.

semi-circlevalues in th

For theshown in Fstant phaseZ= 1/Y0(jof Y0 can bthe spectrawas made uing a highand a low-these electring, changehigh-frequefer resistanthe corrosiwith a capattributed tfilm of oxicess ceasedspectrum isituations oscribed byclearly resoence of a sbelow 10ral equivalent circuit and fittings for pure zinc in 0.1 M NaCl witho(a) Rs = 34.4 cm2; Y0(Q1) = 1.62 105 F cm2 sn1; n(Q1) = 0.872; R2; 2 = 3.9 104. Fitting parameters for (b) Rs = 26.2 cm2; Y0(Q1 106 F cm2 sn1; n(Q2) = 0.500; R2 = 1.14 106 cm2; 2 = 1.0 10is always incomplete and reaches much highere scale.equivalent circuit model fitting of the spectra, asig. 5, the capacitance was replaced by a con-element (CPE) with an impedance given by

)n. Provided the exponent n is close to 1, the valuese taken as a measure of the capacitance. Althoughin Fig. 5 are apparently quite different, the fittingsing the same equivalent circuit, generically hav-

frequency time constant, described by R1 and Q1frequency time constant (R2, Q2). The values ofical components, as well as their physical mean-however between the two systems. In NaCl, thency time constant corresponds to the charge trans-ce and double layer capacitance associated withon process, whereas the low-frequency process,acitance of approximately 600F cm2, can beo the mass transport across the porous air-formeddes. As described above, this low-frequency pro-

to be detected a few hours after immersion. Then Fig. 5(b), which is representative of both thef chromate and of phosphate extracts, can be de-the same circuit, although only R1 and Q1 arelved; a closer observation shows in fact the pres-econdary slope in the impedance modulus plot,

1 Hz.

The timto the threeresistance,104 cm2mate. In thfirst hours,the air-formwas highera monolayesion; the slthe behavioon the surf

The valu1 mF, a vafer across tues of Y0 wchromate elayer capactime. The hcorrespondphysical tetance of a cto the doubuncoveredSavinell [2of coveragebitor (a) and with zinc chromate extract (b). Fitting pa- 103 cm2; Y0(Q2) = 6.06 104 F cm2 sn1; n(Q2) = 1; 106 F cm2 sn1; n(Q1) = 0.903; R1 = 1.74 105 cm2;e evolution of the fitted parameters correspondingsystems is presented in Fig. 6. The charge transferR1, was in the range of 103 cm2 in 0.1 M NaCl,in the phosphate extract and 105 cm2 in chro-

e NaCl solution, the resistance decreased after thepossibly due to the activation of the zinc surface ased film became dissolved. With the pigments R1

from the first measurements, meaning that at leastr of film was formed in the first instants of immer-ight growth of R1 with time reveals no change ofur in the film, but rather the build-up of the film

ace.

e of Y0 increased for zinc in 0.1 M NaCl to nearlylue that suggests some influence of mass trans-he gel-like layer of corrosion products. The val-ere almost constant both in phosphate and in the

xtracts, with values below that of a typical doubleitance, and with a slight tendency to decrease withigh values of the exponent n reveal that the CPEs to a nearly capacitive response, as expected. Inrms, Y0 can be either associated with the capaci-ompact dielectric oxide film or it may correspondle layer capacitance over the fraction of area leftby the passivating film, as discussed by Zeller and8]. According to this last interpretation, a fraction

of 99% can be estimated from the capacitance


Fig. 6. Fit pa of expand to chroma

values for aEstimationresistance,phate and 9

3.2. XPS in

XPS anphate and cphosphoroua mixture ogle Zn 2p3/or Zn(OH)probably duchromiumexposed to140.2 eV.

3.3. Scribe

EIS meaGalvanizedand with asion, the re(Fig. 7), anbare metalmain differexplained bsample areties of thequencies, wimpedancehaving a many significtribute to thmeasured.

After 24the sampleof this syste

enciescircuicm2)

and Qdl co

le layeibingcases

.1 Hzk cmss haitancealueser, inaviou

ng capr uptakbe expr chromis filmthe prg the2080le layetive arameters for the impedance spectra obtained for pure zinc at different timeste extract in 0.1 M NaCl.

ny of the two pigments, after 25 h of immersion.of the inhibition efficiency by the charge transferR1, gives an inhibition of 90% with the phos-9% for the chromate.

spection

alysis of pure zinc after immersion in the phos-hromate extracts revealed well-defined peaks ofs and chromium, respectively. Cr was present asf Cr(III) and Cr(VI). Zinc was detected by a sin-2 peak, at 1022.7 0.3 eV, corresponding to ZnO2. The O 1s peak was measured at 532 eV and ise to OH, which means that both the zinc and the

are present mainly as hydroxides. In the samplesthe phosphate extract, a P 2p3/2 peak appeared at

d samples

surements were performed on samples of Hot DipSteel (HDG), with the various paint formulations

scribe reaching the zinc layer. After 1 h of immer-sponse was strikingly similar for all the coatings

frequalent(1 kRHFand Qdoubdescrsome

low 0510procecapachas vFurtha behcoatiwateever

eitheof thwithdurinRct =doubthe acd also identical to the spectra obtained for the

exposed to plain NaCl solution (Fig. 4a). Theence is related to the higher impedance values,y the smaller ratio of the area exposed to the totala, roughly estimated as 1:501:100. The proper-primer film, typically observed at the high fre-ere not detected, probably because of the highof the film; being in parallel with the scribe anduch higher impedance, the coating does not allowant flow of current, and therefore it does not con-e impedance spectrum in the range of frequencies

h of free corrosion, the impedance was lowest inwithout pigment, which reveals the higher activitym, and another time constant appeared at the high

ratio of Y0ing that thpigment. Twas minimthe epoxypthe same rasamples at

The SVtribution abin 0.1 M Ncurrent denences amon

example, FHDG samposure to 0.1 M NaCl only, to phosphate extract in 0.1 M NaCl

. Fitting of the spectra was made using the equiv-t in Fig. 8, where the high frequency impedanceis due to the electrolytic solution at the scribe.

HF describe the high frequency process, and Rctrrespond to the charge transfer resistance and ther capacitance, respectively. Finally, a componentthe mass transfer impedance was considered inin order to account for a small tail appearing be-

. The high frequency process had a resistance of2, as shown in Fig. 9, whereas the Y0 for this

s a value of roughly 108 to 105 F cm2. Thisis too high for a polymer film, which generally

in the range of 1010 to 108 F cm2 [27,29].the presence of chromate Y0 decreases with time,r that is the opposite to the known evolution ofacitance values (which tend to increase due toe). This high frequency time constant can how-

lained by the formation of the passivating film, ofium hydroxide or zinc phosphate. The formationstarts in the first minutes of exposure, possibly

ecipitation of a monolayer, and grows afterwards,first 2448 h. The low frequency process givesk cm2 and is maximum for the chromate. Ther CPE grows with time, revealing the growth of

rea underneath the coating, i.e., delamination. The

with and without pigment is roughly 0.1, mean-e loss of adhesion was weaker in samples withhe delamination rate, estimated from the CPE,um in the epoxychromate coating, followed byhosphate and finally by the clear coat. This waste that was determined by visual inspection of thethe end of the test.ET was also used to study the ionic current dis-ove the scribed samples, for the three coatingsaCl. The evolution of the current maps and thesity values have not show any significant differ-g the specimens with the various coatings. As anig. 10 shows SVET measurements of the scribedle, coated with the clear epoxy paint. In the first


Fig. 7. BodeNaCl. (a1) an

23 days othe cathodthere wereage. For loalong the sdisappearefrom the sc

4. Discuss

The inhupon a numand Nyquist impedance plots of scribed samples of HDG, coated with epoxy primerd (b1): Bode plots; (a2) and (b2): corresponding Nyquist plots.

f immersion there was a clear separation betweene and the anode along the scribe (Fig. 10a) andno signs of delamination in the micro-video im-

nger exposure times, however, the anodic activitycribe increased and the signs of cathodic activityd from the map, as delamination progressed awayribe (Fig. 10b).

ion

ibiting power of anti-corrosive pigments dependsber of parameters, namely the pigment solubility,

the pH, thethe solutioin the extrathe chromaproved to bto zinc in 0of the formchromate, thibition byby the SVEalso the mothe highestopment ofwith pigment, after 1 h (a) and 1 day (b) of exposure in 0.1 M

properties of the substrate and the composition ofn. The levels of inhibitor concentration achievedcts were much lower for the phosphate than forte, although in both cases the inhibiting effect hase sufficient to provide high inhibition efficiency

.1 M NaCl. In both cases, inhibition was the resultation of a layer of inhibitor on the surface. In thehe film was formed instantly after immersion. In-the phosphate was a slower process, as confirmedT and the OCP measurements. The chromate wasre efficient inhibitor. On the bare metal, it led tocharge transfer resistance and also to the devel-

a protective layer characterized by a capacitance


Fig. 8. Equiv s of Hphosphate pr .05 1Y0(Qdl) = 6.76 .2 10Y0(QHF) = 1.2 = 6.792 = 2.2 10

decrease ocompared w

When ththe solutiona slow proficient concThe mechadescribed bof film formoxidized to

3Zn + 2C

Chromate iaged areasare left expavailable foFor the conthe tests, winhibit the

An interof a time ccal spectrafrequencyproperties oother hand,rosion prodalent circuit and fitting of impedance spectra for the scribed sampleimer (b). Fitting parameters for (a) Rscribe = 775 cm2; Y0(QHF) = 3 107 F cm2 sn1; n(Qdl) = 0.630; Rct = 7.33 104 cm2; 2 = 5

0 105 F cm2 sn1; n(QHF) = 0.551; RHF = 5.47 103 cm2; Y0(Qdl)4.f approximately two orders of magnitude, whenith the reference solution.ese pigments are used in a coating, leaching tois the first step towards inhibition. Since this is

cess, inhibition can only be detected after a suf-entration of inhibitor is achieved near the metal.

nisms of passivation by zinc chromate are usuallyy an oxidizing action of CrO42. In the processation, Cr(VI) is reduced to Cr(III), while Zn is

Zn(OH)2, according to the reaction scheme

rO42 + 5H2O 3Zn(OH)2 +Cr2O3 + 4OH

s usually considered to act especially at the dam-of the coating, where the electropositive metalsosed to the aggressive medium, and are thereforer providing free electrons for chromate reduction.ditions of our experiments and for the duration ofe found none of the pigments had the power to

corrosion occurring at the defect.esting feature was related with the developmentonstant at the high frequencies. Unlike the typi-observed with defective painted samples, the highprocess could not be attributed to the dielectricf the coating, due to its high capacitance. On theit cannot be simply due to the precipitation of cor-ucts, or it would have to be even more relevant in

the clear colayer. The Soccurred atsivating laywhere it inare usuallystrong thermto Cr(III). Itions aboveitation of ththe coatingsome pigmdissolved pcoating, chwith an effipH [26]. Ooxide layera mixture ozinc and chductors, foto the thick

C = 0AL

where isuum, A thedielectric pDG in 0.1 M NaCl with chromate primer (a) and with08 F cm2 sn1; n(QHF) = 0.844; RHF = 9.04 103 cm2;4

. Fitting parameters for (b) Rscribe = 990 cm2;106 F cm2 sn1; n(Qdl) = 0.768; Rct = 2.73 105 cm2;at. This process has then to be due to a passivatingVET, however, has shown that the anodic activitythe scribe irrespective of the pigment. This pas-er, therefore, can only be underneath the coating,hibits the cathodic reaction. Chromate pigmentsclassified as oxidizing inhibitors, because of theodynamic tendency of Cr(VI) to become reduced

t is claimed to inhibit steel corrosion at concentra-104 M [30]. According to our results, the precip-is layer seems to occur preferentially underneath

, as water permeates across the coating and leachesent, and then at the scribe, after diffusion of theigment. At the pH values expected underneath theromate acts as mixed anodiccathodic inhibitor,ciency that is only slightly smaller than at neutraln the other hand, at this pH the thickening of Zns is easier. Therefore, the film probably consists off chromium and zinc oxides and hydroxides. Bothromium oxides are know to behave as semicon-

r which the capacitance is inversely proportionalness:

the relative permitivity, 0 the permitivity of vac-area and L is the film thickness. Provided the

roperties of the layer remain constant, the drop


in capacitaimportant tlayer of Cr(across the tamount ofthe result o

The lowtrum, whicgives the athe same amate > pho

4.1. SVET

SVET mone large cThe fact ththe scribe scorrosion creactions tBierwagenthe cathodireveals thascribe, i.e.,Fig. 9. Fit parameters of the impedance spectra of scribed painted HDG at d

nce means that the thickening of the film is morehan its growth along the surface, i.e., as if a mono-OH)3 was formed as soon as the water permeatedhin polymer coating, through leaching of a smallchromate. After that, the capacitance decreased asf film thickening.

frequency semicircle in the impedance spec-h is actually related to the corrosion process,ctual rate of delamination, and the ranking is

s observed with the pigments extracts, i.e., chro-sphate > clear coat.

apping at the scribes showed one large anode andathode at the scribe in the first days of immersion.at both the anode and the cathode are located athows that the scribe itself constitutes a completeell, without the need for further electrochemical

o balance the electrons. As already observed byand co-workers [23], for longer immersion timesc currents cease to be detected by the SVET. Thist the cathodic reaction takes place away from theunderneath the coating, and is thus a distinct sign

of the delamactions havthe cathodiof the cathdue to the r

O2 + 2H2Oand occursincrease ofbonds and/accumulatiof this excward flow oby the inwand Wangsolution mthe front oto the delamfound an ein a thin are[32]. Thisther acrossity, howevefrom the bifferent times of exposure to 0.1 M NaCl.

ination process. Since the cathodic and anodic re-e to balance each other, then a large ratio betweenc and the anodic areas will lead to a small densityodic current. The cathodic current is essentiallyeduction reaction of oxygen:

+ 4e 4OH

at the metalcoating interface, thus leading to anpH (responsible for the rupture of coating metal

or to saponification of the coating) but also to anon of anions in a confined region. Compensationess of anions can be achieved either by the out-f the hydroxyl anions themselves or, more easily

ard flow of cations. As pointed out by Leidheiser[31], the high concentration of Na+ in the outerakes them the most likely cations to migrate tof the delamination. Migration of sodium cations

ination front was confirmed by Stratmann, whoxcess of sodium in the delaminated area but alsoa considered to be beyond the delamination front

migration of cations can occur in two ways: ei-the coating or from the scribe. This last possibil-r, would mean that other cations would migrate

ulk into the scribe, generating a significant ionic


Fig. 10. SVE with cle0.1 M NaCl. A

flow aboveaway fromSVET, in tcontrast, fofor the flowgradients adetect.

5. Conclu

Protectiphosphate wagreement

Evaluatizinc in pigmcorrosion rmate > phoimpedanceequivalentfirst stage,coating starelaxation cings, was apigment unlaminationcapacitancecoat.

appinhownwhereT maps of ionic currents measured 200m above a scribe in HDG coatedlso presented are micro-video images of the corresponding sample area.

the scribe, equivalent to that of Zn2+ ions flowingthe surface. Such a flow should be detected by thehe same way as anodic currents are detected. In

Mhas sway,r a flow of ions across the coating, the cross areais much larger, leading to very small potential

bove the coating, which are necessarily difficult to

sions

on of zinc by pigments of zinc chromate and zincas monitored by EIS, SVET and OCP, with good

between the three techniques.on of the electrochemical parameters for the bareent extracts allowed the ranking of the decreasing

esistance in the extracts in 0.1 M NaCl as: chro-sphate > plain salt solution. Interpretation of thespectra of the scribed coatings has shown that thecircuit is identical to that of the bare metal in awhereas the processes occurring underneath thert to appear in a second stage. A high frequencyonstant observed in the scribed, pigmented coat-ssigned to the formation of a protective layer ofderneath the coating. The decreasing rate of de-

, taken as the rate of growth of the double layer, was the same, i.e., chromate > phosphate > clear

inhibits the

Reference

[1] J.E.O. M[2] E. McC

559.[3] W. Meis

(1983) 4[4] Z. Szkla

(1974) 1[5] A.I. Onu[6] S. Virtan[7] H.S. Isa

Electroc[8] W.J. Cla

trochem.[9] C. Gabri

Electroc[10] M. Svob[11] M. Kend

Hexavaleof CorroAlloys f

[12] J. Barrac[13] J.E.O. M[14] M. Beir

Org. Coar epoxy paint, after (a) 2 days and (b) 7 days of exposure to

g of the local ionic fluxes above the metal surfacethat zinc corrosion in NaCl occurs in a localizedas the presence of any of the two pigment extracts

establishment of this local activity.

s

ayne, P. Ridgway, Br. Corros. J. 3 (1974) 177.afferty, M.K. Bernett, J.S. Murday, Corros. Sci. 28 (1988)

el, E. Mohs, H.-J. Guttman, P. Gutlich, Corros. Sci. 2365.rska-Smialowska, R.W. Staehle, J. Electrochem. Soc. 121146.chukwu, J.A. Lori, Corros. Sci. 24 (1984) 833.en, M. Buchler, Corros. Sci. 45 (2003) 1405.acs, S. Virtanen, M.P. Ryan, P. Schmuki, L.J. Oblonsky,him. Acta 47 (2002) 3127.rk, J.D. Ramsey, R.L. McCreery, G.S. Frankel, J. Elec-Soc. 149 (2002) B179.

elli, M. Keddam, F. Minouflet-Maurent, K. Ogle, H. Perrot,him. Acta 48 (2003) 935, 1483.oda, J. Mleziva, Prog. Org. Coat. 12 (1984) 251.ig, R. Buchheit, Corrosion Inhibition of Al and Al Alloys bynt Cr Compounds a mechanistic overview, in: Proceedingssion/2000Surface Conversions of Aluminum and Ferrousor Corrosion Resistance, NACE, 2000.lough, J.B. Harrison, J. Oil Col. Chem. Ass. 48 (1965) 341.ayne, Br. Corros. J. 5 (1970) 106.

o, A. Collazo, M. Izquierdo, X.R. Novoa, C. Perez, Prog.at. 46 (2003) 97.


[15] K. Aramaki, Corros. Sci. 43 (2001) 591.[16] I.M. Zin, S.B. Lyon, V.I. Pokhmurskii, Corros. Sci. 45 (2003) 777.[17] M.J. Austin, Inorganic anticorrosive pigments, in: ASTM Manual

17Paint and Coating Testing Manual, ASTM, Philadelphia, 1995,Chapter 27.

[18] J.A. Burkill, J.E.O. Mayne, J. Oil Col. Chem. Ass. 71 (1988) 273.[19] H.S. Isaacs, G. Kissel, J. Electrochem. Soc. 119 (1972) 1628.[20] H.S. Isaacs, Y. Ishikawa, Applications of the vibrating probe to lo-

calized current measurements, in: R. Baboian (Ed.), ElectrochemicalTechniques for Corrosion Engineering, NACE, Houston, 1986.

[21] K. Ogle, V. Baudu, L. Garrigues, X. Philippe, J. Electrochem. Soc.147 (2000) 3654.

[22] F. Zou, C. Barreau, R. Hellouin, D. Quantin, D. Thierry, Mat. Sci.Forum 289292 (1998) 83.

[23] J. He, V.J. Gelling, D.E. Tallman, G.P. Bierwagen, J. Electrochem.Soc. 147 (2000) 3661.

[24] S.M. Powell, H.N. McMurray, D.A. Worsley, Corrosion 55 (1999)1040.

[25] I.M. Zin, R.L. Howard, S.J. Badger, J.D. Scantlebury, S.B. Lyon,Prog. Org. Coat. 33 (1998) 203.

[26] A. Amirudin, C. Barreau, R. Hellouin, D. Thierry, Prog. Org. Coat.25 (1995) 339.

[27] J. Titz, G.H. Wagner, H. Spahn, M. Herbert, K. Juttner, W.J. Lorenz,Corrosion 46 (1990) 221.

[28] R.L. Zeller III, R.F. Savinell, Corr. Sci. 26 (1986) 389.[29] A.S. Castela, A.M. Simoes, Corr. Sci. 45 (2003) 1631.[30] J.G.N. Thomas, The mechanism of corrosion prevention by in-

hibitors, in: L.L. Shreir, R.A. Jarman, G.T. Burstein (Eds.), Cor-rosion, vol. 2, Butterworth-Heinemann, Oxford, 1994, Chapter17.3.

[31] H. Leidheiser Jr., W. Wang, Rate controlling steps in the ca-thodic delamination of 1040 m thick polybutadiene and epoxy-polyamide coatings from metallic substrates, in: H. Leidheiser Jr.(Ed.), Corrosion Control by Organic Coatings, NACE, Houston,1981.

[32] W. Furbeth, M. Stratmann, Corros. Sci. 43 (2001) 207.

Comparative electrochemical studies of zinc chromate and zinc phosphate as corrosion inhibitors for zincIntroductionExperimentalBare metal samplesCoated samplesExposure mediaElectrochemical techniques

ResultsBare metal electrodesXPS inspectionScribed samples

DiscussionSVET

ConclusionsReferences