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Progress in Organic Coatings 59 (2007) 214–223

Surface evaluation and electrochemical behaviour of dopedsilane pre-treatments on galvanised steel substrates

W. Trabelsi a, P. Cecılio b, M.G.S. Ferreira b,c,∗, K. Yasakau c,M.L. Zheludkevich c, M.F. Montemor b

a ENIT, Campus Universitaire, BP 37, Le Belvedere Unite de Recherche de Corrosion, Tunes, Tunisiab Instituto Superior Tecnico, ICEMS, DEQ, Av. Rovisco Pais 1049-001 Lisboa, Portugal

c University of Aveiro, Department of Ceramic and Glass Engineering, Campus de Santiago 3810-193 Aveiro, Portugal

bstract

The present work aims at evaluating the surface morphology and the corrosion resistance of galvanised steel substrates pre-treated with bis-triethoxysilylpropyl] tetrasulfide silane (BTESPT) solutions doped with cerium nitrate or zirconium nitrate. The silane pre-treatment leads to theormation of a silane coating in the metallic surface. The surface morphology of this coating was studied by atomic force microscopy (AFM) andcanning electron microscopy (SEM).

The corrosion behaviour of the pre-treated substrates during immersion in 0.05 M NaCl was assessed by the scanning vibrating electrode

echnique (SVET) and by electrochemical impedance spectroscopy (EIS).

The results show that the presence of dopant improves the protective properties of the silane pre-treatment. Moreover, it was found that pre-reatments based on silanes doped with cerium nitrate are more resistant against corrosion development due to the corrosion inhibition propertiesf the cerium ions.

2006 Elsevier B.V. All rights reserved.

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eywords: Silanes; Pre-treatment; AFM; SEM; EIS; Cerium; Zirconium; Dopa

. Introduction

Multifunctional pre-treatments are emerging as a technicalolution for the pre-treatment of metallic substrates such as steel,alvanised steel, aluminium alloys and copper [1–20]. Somef these multifunctional pre-treatments are based on functionalilanes and present interesting advantages such as competitiverice and compatibility with the actual environmental concerns.

Pre-treatments based on silane formulations lead to the for-ation of a silane coating on the metallic surface that can provide

ifferent surface functionalities, creating an interface compati-le with a wide range of organic paints. The silane moleculesontain hydrolysable siloxane groups, which establish covalentonds with the native oxide/hydroxide film, initiating the forma-

ion of a multifunctional self-assembled coating on the metallicubstrate. The final result of the silane pre-treatment is a denseilicon and oxygen rich coating, which constitutes a protective

∗ Corresponding author at: University of Aveiro, Department of Ceramic andlass Engineering, Campus de Santiago, 3810-193 Aveiro, Portugal.el.: +351 243 370354; fax: +351 234 425300.

E-mail address: mgferreira@cv.ua.pt (M.G.S. Ferreira).

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300-9440/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2006.09.013

hysical barrier. However, the silane coating may present smallores and pinholes or micro cracks that facilitate electrolyte dif-usion and the accumulation of aggressive species at the interfaceoating/substrate. A coating providing good barrier propertiess characterised by high resistance and low capacitance valuesnd can work as an effective anti-corrosion pre-treatment.

The enhancement of the barrier properties of the silane coat-ngs is of paramount importance for the improved protectionnd longer lifetime of the silane pre-treatments. This work aimst studying new pre-treatments based on doped silane coat-ngs. These pre-treatments must present longer durability andmproved resistance against electrolyte uptake, comparatively tohe non-doped coatings. The final goal is to enhance the barrierroperties of the silane coatings and, simultaneously, to makehe coatings “smarter” in the presence of corrosion activity.

In a previous work [21] the anti-corrosion behaviour of theis-[triethoxysilylpropyl] tetrasulfide silanes (BTESPT) dopedith cerium nitrate or with zirconium nitrate was assessed

y electrochemical impedance spectroscopy (EIS). The resultshowed that the presence of dopant enhances the coating resis-ance by about 3 orders of magnitude comparatively to theon-doped coatings. The results also showed that the substrates

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re-treated with the cerium-doped coating present lower cor-osion rate. This behaviour is consequence of the corrosionnhibition properties of the cerium ions as reported in literature22–27].

The addition of cerium nitrate to hybrid sol–gel coatings wastudied in recent works [28–30] and the results show that ceriumnhances the corrosion protection of aluminium and zinc sub-trates.

Zirconium ions have also been used for modification ofol–gel and silane coatings. Modification of sol–gel coatingsith zirconium nanoparticles was successfully tested for cor-

osion protection of aluminium alloys [31]. Furthermore, theybrid sol–gel coatings with incorporated zirconium oxideanoparticles doped with cerium provide long-term corro-ion protection. The nanostructured zirconia particles can pro-ide prolonged release of the cerium ions, playing the rolef nanoreservoirs for controllable storage and release of thenhibitor [32].

The previous research results [21,31–33] showed that ceriumnd zirconium are good candidates for incorporation in theilane coatings aiming at improved barrier properties. Thus, theresent work investigates the role of the dopant on the protectiveerformance of the silane coating. The morphological character-sation of the coating was performed by atomic force microscopyAFM) and scanning electron microscopy (SEM). The abilityf the doped coating to mitigate corrosion was evaluated bylectrochemical impedance spectroscopy and by the scanningibrating electrode technique (SVET). The results showed thathe Ce-doped coatings present higher coating resistance andonger lifetime. The electrochemical results also showed thathe corrosion rate of substrates pre-treated with the Ce-dopedoatings decreases by more than 2 orders of magnitude whenompared to the Zr-doped coatings.

. Experimental procedure

.1. Pre-treatments

The bis-[triethoxysilylpropyl] tetrasulfide silane (Sigma–ldrich product) solutions were prepared by dissolving 4% (v/v)f silane in methanol (90.5%, v/v) and 5.5% (v/v) of aque-us solutions of the inorganic salt. Solutions of 1 × 10−3 Me(NO3)3 and 1 × 10−3 M Zr(NO3)3 were prepared for incor-oration in the silane solution. The choice of the concentrationf the cerium nitrate solution was based in previous researchork [33] in which the optimum concentration of dopant was

ssessed by EIS measurements. In that work cerium concentra-ions ranging from 1 × 10−1 to 1 × 10−4 M were studied and itas found that very low concentrations did not induce corrosionrotection, whereas high concentrations seem to have negativempact on the barrier properties of the coating. The optimumoncentration was around 1 × 10−3 M Ce(NO3)3.

The modified silane solutions were stirred during 1 h and

ept for 3 days before use. The metallic substrate consisted ofalvanised steel coupons having a zinc coating weight of approx-mately 140 g/m2 and a thickness of approximately 10 �m,ithout post annealing. The galvanised steel coupons were pre-

taio

ic Coatings 59 (2007) 214–223 215

ared according the procedure described elsewhere [21,33]. Thealvanised steel coupons were immersed in the doped silaneolution for 10 s as described elsewhere [21,33].

.2. Microscopic techniques

The changes observed in the surface of the silane coatinguring ageing of the silane coating during ageing in 0.005 MaCl solutions was evaluated using a SEM apparatus (Hitachi-4100 system with electron beam energy of 25 keV) fitted withDS.

The morphology of the silane coatings was assessed by thetomic force microscopy using a Nanoscope Digital Instrumentsith a NanoScope III controller using a silicon tip covered withtIr5 in taping mode to prevent scratching of the pre-treatedurface.

Measurements were performed periodically during immer-ion in 0.005 M NaCl solution.

.3. Electrochemical techniques

The scanning vibrating electrode technique measurementsere performed using an Applicable Electronics apparatus. The

canned area was 1 mm × 1 mm.Electrochemical impedance spectroscopy was performed

sing a three electrode arrangement, a frequency response anal-ser and an electrochemical interface from Solartron. The fre-uency range was swept between 50 kHz and 0.05 mHz. Forresentation purposes the values of the phase angle were cor-ected to positive values.

The electrochemical impedance experiments were conductedt the corrosion potential during immersion in 0.05 M NaCl. Thisoncentration is higher than that used for the microscopic stud-es in order to accelerate the corrosion processes. To evaluatehe corrosion inhibition performance of the dopant, the metallicubstrates were immersed during 1 day in the aggressive solu-ion and after this period an artificial defect was created on theurface of the coatings. The defect was made using a needle.he defect was approximately circular and its diameter around00 �m.

. Results and discussion

.1. AFM results

The morphology of the surface of the doped silane coatingsas assessed by atomic force microscopy. Figs. 1–3 depict theorphology and the evolution of the surface roughness of the

oped silane coatings during ageing in 0.005 M NaCl. Fig. 1ahows the AFM scans obtained on the Ce-doped coatings beforemmersion. The silane coating is very uniform, showing a nanos-ructured surface. The surface roughness is lower than 10 nm andhe dimensions of the grains existing on the surface are smaller

han 100 nm (Fig. 1c (I)). The larger scan (Fig. 1a (II)) evidencesvery uniform surface film. The behaviour of the Zr-doped coat-

ngs is slightly different. Although the dimensions of the grainsbserved in the 1 �m scan are identical to those observed for

216 W. Trabelsi et al. / Progress in Organic Coatings 59 (2007) 214–223

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ig. 1. (a) AFM scans for the Ce-doped silane coatings before immersion: (I)mmersion: (I) 1 �m scan; (II) 20 �m scan. (c) Surface roughness measured on

erium, the surface roughness observed in the 20 �m (Fig. 1cII)) scan increases by about three times.

After 1 day of immersion the morphology of the surfacehanges and the roughness increases (Fig. 2). The Ce-dopedoating still shows a nanostructured surface film (Fig. 2a).owever, there is formation of agglomerates, which lead to an

ncrease of the surface roughness. The height of these agglom-rates is around 10 nm and their width is around 200 nm (Fig. 2c

I)). The remaining surface film still presents reduced rough-ess. This trend is also observed in the 20 �m scan (Fig. 2c (II)).he 1 �m scan obtained on the surface of the Zr-doped coat-

ng also presents reduced roughness, however slightly higher

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scan; (II) 20 �m scan. (b) AFM scans for the Zr-doped silane coatings beforeFM scans depicted in (a and b): (I) 1 �m scan; (II) 20 �m scan.

han the Ce-doped coating (Fig. 2c (I)). The largest scan20 �m scan) reveals a depletion that may account for coatingissolution.

As the immersion time elapses, the morphology of the Ce-oped silane coating remains practically unchanged. After 5ays of immersion, for example, the AFM scans depicted inig. 3a (I) are very similar to those obtained after 1 day of

mmersion (Fig. 2a (I)) and the surface roughness is still lower

han 10 nm. However, in the 20 �m scan it is possible to observehe formation of some deposits that may be associated with therowth of the first zinc corrosion products. Concerning the Zr-oped coating (Fig. 3b), the surface structure becomes more

W. Trabelsi et al. / Progress in Organic Coatings 59 (2007) 214–223 217

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ig. 2. (a) AFM scans for the Ce-doped silane coatings after 1 day of immersioilane coatings after 1 day of immersion in 0.005 M NaCl: (I) 1 �m scan; (II) 20I) 1 �m scan; (II) 20 �m scan.

eterogeneous and there is formation of defects that could bebserved in both scans. The “valley-like” area in the middle ofhe 1 �m scan suggests dissolution of the silane coating (Fig. 3bI)). The 20 �m scan (Fig. 3b (II)) shows that the Zr-containingoating is now very thin because it is possible to follow theriginal topography of the galvanised layer. The nucleation ofinc corrosion products can be observed on the surface. Fig. 4

resents the AFM scans obtained on the non-doped BTESPTilane coating. The images show that the structure of the coatings much more heterogeneous than that of the doped coatings.

oreover, the non-doped coating presents more defects (darker

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.005 M NaCl: (I) 1 �m scan; (II) 20 �m scan. (b) AFM scans for the Zr-dopedscan. (c) Surface roughness measured on the AFM scans depicted in (a and b):

reas in Fig. 4I) and seems to be thinner than the doped coatingsince the morphology of the native zinc surface can be observedn Fig. 4II.

The AFM results showed that the addition of the dopant leadso the formation of very uniform silane coatings. Among theoped coatings, the Ce-containing coating presents the mostomogeneous structure and lowest surface roughness. The Ce-

ontaining coatings also seem to be slightly thicker than the non-oped coatings. Previous works [29] also reported an increasef the thickness of the sol–gel coatings with increasing concen-ration of cerium ions.

218 W. Trabelsi et al. / Progress in Organic Coatings 59 (2007) 214–223

Fig. 3. (a) AFM scans for the Ce-doped silane coatings after 5 days of immersion in 0.005 M NaCl: (I) 1 �m scan; (II) 20 �m scan. (b) AFM scans for the Zr-dopedsilane coatings after 5 days of immersion in 0.005 M NaCl: (I) 1 �m scan; (II) 20 �m scan. (c) Surface roughness measured on the AFM scans depicted in (a and b):(I) 1 �m scan; (II) 20 �m scan.

Fig. 4. AFM scans for the non-doped silane coatings before immersion: (I) 2 �m scan; (II) 20 �m scan.

W. Trabelsi et al. / Progress in Organic Coatings 59 (2007) 214–223 219

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ig. 5. SEM picture obtained on non-doped silane coating (a), Zr-doped coatingb) and Ce-doped coating (c). Images were obtained before immersion.

.2. SEM results

The surface morphology of the different surface coatingsas investigated by FEG/SEM. The SEM images depicted inig. 5 show the morphology of the surface before immersion.

he micrographs reveal the presence of some important fea-

ures on the different coatings. The Zr-doped coating (Fig. 5b)eveals the presence of few cracks on the coating, which mayontribute for the earlier deterioration of the coating observed in

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ig. 6. SEM picture obtained on non-doped silane coating (a), Zr-doped coatingb) and Ce-doped coatings (c), after 1 week of immersion in 0.005 M NaCl.

he AFM scans after immersion. Nevertheless, these cracks arerobably limited to the outer layers of the coating, because goodarrier properties for the coating were observed in the electro-hemical measurements. After 1 week of immersion (Fig. 6) theurface of the non-doped coating is completely covered of zincxide/hydroxides. The doped coatings also revealed the presencef zinc corrosion products. However, the density of these corro-ion products is higher on the Zr-doped coating. Furthermore,areful examination of the surface shows that the structure ofhe substrate is more deteriorated in the Zr-doped coating, beingdentical to that observed for the non-doped coating. In the casef the Ce-doped coating, the surface presents non-attacked areasnd the corrosion products are likely to grow through defects ofhe surface film.

The microscopic results account for the longer durability ofhe Ce-doped silane coatings. The addition of small amounts of

his dopant to the silane solution improves the protective prop-rties of the coatings, making it more resistant to deteriorationnder immersion in NaCl solutions. The SEM and the AFMesults confirm that the Ce-doped coatings are less prone to cor-

2 rganic Coatings 59 (2007) 214–223

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osion attack. These results are in very good agreement withhe results of electrochemical impedance spectroscopy measure-

ents published elsewhere [21]. Such measurements showedhat the capacitance (Qc) of the doped coatings was about 3rders of magnitude lower than that of the non-doped coatingsor the same electrolyte concentration (0.005 M NaCl). An iden-ical factor was found for the ratio between the coating resistanceRc) of the doped and the non-doped coatings. Furthermore, therevious [21,33] EIS results also revealed that the corrosion rateas lower for the substrates pre-treated with the Ce-doped silane

olutions.

.3. SVET results

The electrochemical behaviour of the pre-treated substratesuring immersion in the aggressive solution (0.005 M NaCl) wastudied by SVET. In order to accelerate the corrosion processnd to evaluate the role of the dopant on the corrosion rate ofhe substrate, an artificial defect was made on the pre-treatedamples, either doped or non-doped.

Fig. 7a–c depicts the SVET images obtained on the non-oped coating and on the doped coatings after approximately 1ay of immersion. The results showed that the anodic activitys reduced, especially for the Ce-doped coating, evidencing theood barrier properties of these silane coatings.

After 1 day of immersion, an artificial defect was created onhe surface of the coatings. Fig. 8a–c shows the electrochemicalehaviour of the surface some hours after the formation of theefect. All the substrates showed localised anodic activity inhe defect. For the Ce-doped coating important cathodic activityould also be observed nearby the anodic spot [33].

After 2 days of immersion the SVET maps (Fig. 9) showedhat the non-doped coatings and the Zr-doped coatings stillevealed anodic activity on the defect, whereas no activityould be observed for the Ce-doped coating. The SVET resultshowed that the nature of the dopant affects the electrochemicalehaviour of the substrate after corrosion onset. In the case ofhe Ce-doped coating the corrosion process is inhibited.

.4. EIS results

The EIS results obtained for the substrates pre-treated witherium nitrate and zirconium nitrate prior and after defectormation during immersion in 0.05 M NaCl are depicted inigs. 10 and 11, respectively. The total impedance values mea-ured in the low frequency range can be used to assess theorrosion resistance of the system. Increased impedance val-es account for higher corrosion resistance that results formmproved barrier properties and/or lower corrosion rate of theubstrate. The EIS spectra showed that the low frequencympedance values are dependent on the nature of the dopant.uring the first day of immersion the substrates pre-treatedith the Ce-doped solutions showed impedance values higher

han 107 � cm2, whereas the Zr-doped coatings show slightlyower impedance values. For example, after 1 day of immer-ion the impedance values for the Zr-doped coating were around× 106 � cm2. At this time, a defect was created on the surface

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ilane solutions (a) [2], Ce-doped solutions (b) and Zr-doped solutions (c).esults were obtained after 1 day of immersion in 0.005 M NaCl and priorefect formation.

f the coatings. Then, spectra were taken after different periodsf time: 3 h after defect formation (def 3 h), 12 h after defectormation (def 12 h) and 1 day after defect formation (def 1d).

After creation of the artificial defect, both coatings showed anmportant decrease of the impedance values. The impedance ofhe Ce-doped coating decreased by about five times, whereas thempedance of the Zr-doped coating decreased by about 2 ordersf magnitude. However, the impedance of the Ce-doped coatingas kept approximately constant whereas that of the Zr-doped

oating continuously decreased with time.The EIS spectra were fitted using the equivalent circuit

escribed elsewhere [21,33]. This equivalent circuit includeswo time constants: one in the high frequency range that sim-lates the properties of the coating, i.e., coating resistance

W. Trabelsi et al. / Progress in Organic Coatings 59 (2007) 214–223 221

Fig. 8. SVET results obtained on the substrates pre-treated with the undopedsilane solutions (a), Ce-doped solutions (b) and Zr-doped solutions (c). Resultsw

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seffFtvalues of α for the high frequency time constant were above 0.9,slightly decreasing with time, especially after defect formation.

ere obtained 6–8 h after defect formation.

nd coating capacitance and another one, at low frequencies,ccounting for the charge transfer reactions. The equivalent cir-uit used constant phase elements to simulate the capacitiveesponse of the EIS spectra. The CPE behaviour is generallyttributed to distributed surface reactivity, surface inhomogene-ty, roughness or fractal geometry, electrode porosity and tourrent and potential distributions associated with electrodeeometry. The CPE is associated with an exponent α and it haseen considered to represent a circuit parameter with limiting

ehaviour as an ideal capacitor if α = 1 and a resistor for α = 034].

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ig. 9. SVET results obtained on the substrates pre-treated with the undopedilane solutions (a) [33], Ce-doped solutions (b) [33] and Zr-doped solutions (c).esults were obtained 2 days after defect formation.

The results of the numerical simulation of the impedancepectra are depicted in Figs. 12 and 13. Fig. 12 depicts thevolution of the coating resistance (Rc) obtained from the highrequency relaxation processes and the evolution of charge trans-er resistance (Rct) obtained from the low frequency response.ig. 13 depicts the evolution of the constant phase element for

he coating (Qc) and for the double layer processes (Qdl). The

or the low frequency time constant the α values were in theange 0.9–0.8.

222 W. Trabelsi et al. / Progress in Organic Coatings 59 (2007) 214–223

Fig. 10. Impedance plots obtained for the Ce-doped coatings during immersion in NaCl. A defect was formed after 1 day of immersion. Spectra were taken 3 h afterdefect formation (def 3 h); 12 h after defect formation (def 12 h); 1 day after defect formation (def 1d).

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ig. 11. Impedance plots obtained for the Zr-doped coatings during immersionefect formation (def 3 h); 12 h after defect formation (def 12 h); 1 day after de

During the first day of immersion (i.e., before defect forma-ion) there was a slow decrease of the coating resistance (Rc) as

xpected due to progressive electrolyte uptake. With time it wasossible to see that the coating resistance (Rc) is slightly higheror the Ce-doped coating. The coating CPE (Qc) showed a smallncrease with time, being higher for the Zr-doped coatings. Dur-

ig. 12. Evolution of the coating resistance and charge transfer resistance forhe doped coatings during immersion in NaCl. A defect was formed after 24 hf immersion.

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Cl. A defect was formed after 1 day of immersion. Spectra were taken 3 h afterormation (def 1d).

ng this period the values of α were around 0.95, thus revealinghat the Qc can be used as an indicator of the coating capaci-

ance. The evolution of Rc reveals that the coatings become moreonductive with time due the development of conductive path-ays. After 24 h of immersion an artificial defect was formedn the surface and the EIS spectra show important changes.

ig. 13. Evolution of the coating capacitance and double layer capacitance forhe doped coatings during immersion in NaCl. A defect was formed after 24 hf immersion.

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he resistance of the doped coatings (Rc) showed an importantecrease due to the localised destruction of the coating. Simul-aneously, another relaxation process became evident in the EISpectra, in the low frequency range. This is associated with theevelopment of the corrosion processes and its resistance wasssigned to the charge transfer resistance (Rct). This resistanceas determined by numerical fitting and it was possible to see

hat the charge transfer resistance (Rct) of the Zr-doped coat-ngs decreased by more than 2 orders of magnitude, contrastingith the charge transfer resistance (Rct) of the Ce-doped coat-

ngs that remained above 1 × 106 � cm2, being approximatelyonstant during time. The CPE associated with the corrosionrocess (Qdl) increased with time, being higher for the Zr-dopedoating. The Qdl values for the Zr coating increased from 1 to0 �F/cm2 (with α values ranging between 0.9 and 0.8) The EISesults showed that the Ce-doped silane coatings present reducedorrosion activity when compared to the Zr-doped coatings. Athe end of the electrochemical tests, the corrosion rate (which isnversely proportional to Rct) of the substrates pre-treated withhe Ce-doped coatings was approximately 2 orders of magnitudeower than the corrosion rate of the substrates pre-treated withhe Zr-doped coating. This behaviour results from the fact thaterium possesses self-healing properties, being able to inhibitorrosion activity. Such behaviour could not be observed forhe Zr-doped coatings. The results showed that the addition ofmall amounts of dopant to the silane solution affects both theoating resistance and CPE (Rc and Qc) and the corrosion pro-ess (Rct and Qdl). The addition of cerium led to higher coatingesistances suggesting that these coatings present lower poros-ty and/or conductivity and higher thickness. Furthermore, thenhibiting effect of cerium led to an important decrease of theorrosion rate after corrosion onset as stated by the EIS andVET results.

The results obtained in this work combine the use of micro-copic and electrochemical techniques with the aim at assessinghe protective barrier properties and the anti-corrosion perfor-

ance of the doped coatings. The results obtained in this workvidence the good anti-corrosion performance of doped silaneoatings and are an important contribution for the developmentf more effective multifunctional pre-treatments for galvanisedteel substrates.

. Conclusions

Cerium nitrate and zirconium nitrate can be used as effectiveopants for silane-based pre-treatments for galvanised steel. Theresence of dopant leads to the formation of silane coatingsresenting a very uniform and nanostructured surface.

The microscopic studies performed on the galvanised steelubstrate pre-treated with the doped silane solutions show thathe addition of dopant improves the barrier properties of theilane coating, increasing the coating lifetime and delaying the

orrosion processes.

The electrochemical results reveal that after corrosion onset,he presence of cerium reduces the corrosion rate of the sub-trate. Thus, the cerium-doped coatings are able to mitigate the

[

[

ic Coatings 59 (2007) 214–223 223

orrosion phenomenon. Such effect could not be observed forhe non-doped coating or for the Zr-doped coating.

cknowledgements

NATO Collaborative Linkage Grants PST. CLG.979427.artners of this project are strongly acknowledged. ECSC P482-F7 contract 7210-PR/382 F7.1/02

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