microstructure and corrosion resistance of inorganic–organic (zro2–pmma) hybrid coating on...

Microstructure and corrosion resistance of inorganic±organic(ZrO2±PMMA) hybrid coating on stainless steel 1

S.H. Messaddeq a, S.H. Pulcinelli b, C.V. Santilli b, A.C. Guastaldi b,Y. Messaddeq b,*

a Instituto de F�õsica de S~ao Carlos, USP, P. O. Box 365, 13560-900 S~ao Carlos, SP, Brazilb Instituto de Qu�õmica, UNESP, P. O. Box 355, 14800-900 Araraquara, SP, Brazil

Abstract

Zirconia±polymethylmetacrylate hybrids prepared by a sol±gel method were deposited by dip-coating on stainless

steel to improve the resistance against wet corrosion. The e�ect of the concentration of polymethylmetacrylate and the

number of coating applications on the microstructure and corrosion performance of coated samples was investigated.

The microstructural properties of samples was analyzed by scanning electron and atomic force microscopy, adhesion

tests and pro®lemeter measurements. The electrochemical corrosion was evaluated through potentiodynamic polar-

ization curves at room temperature. Results show that the sample prepared with 17 vol.% of polymethylmethacrylate

has a maximum corrosion resistance, smaller roughness, are hermetic and adherent to the substrate. This ®lm increases

the life time of the stainless steel by a factor 30. Ó 1999 Elsevier Science B.V. All rights reserved.

1. Introduction

The protection of materials against environ-mental attack is generally related to adherent andprotective coatings. Increasing attention has beendone on the corrosion behavior in aqueous, acid orbasic media on steels, metal and alloys protectedby sol±gel coatings, as reviewed by Guglielmi [1].The sol±gel processing technique for coatings ofZrO2, SiO2 and SiO2±TiO2 to prevent chemical

corrosion and gas oxidation has been extensivelystudied [1]. All these ®lms, prepared by dip-coat-ing, show an increasing protection of metal sub-strates against air corrosion and acid attack [2±5].The major drawbacks of the sol±gel process, fromthe standpoint of corrosion resistant layers are:(1) thick coatings (>1 lm) are di�cult to achievewithout cracking [6]; (2) sol±gel ®lms are brittle [6];and (3) relatively high temperatures (600°C) arerequired to achieve good properties [2,3].

The preparation of hybrid organic±inorganicmaterials by the sol±gel method allows the intro-duction of organic molecules into an inorganicnetwork [7]. The presence of organic componentsmakes the gel network more ¯exible and less proneto cracking during further heat-treatment. More-over, the impregnation of open pores by organicmaterials can reduce coating porosity, producing abetter barrier to di�usion [7].

Journal of Non-Crystalline Solids 247 (1999) 164±170

* Corresponding author. Tel.: +55-16 232 3022; fax: +55-16

222 7932; e-mail: [email protected] The results described in this article were judged by an

international committee and were considered to be the best

poster of technological applications presented at the 3rd

Brazilian Symposium on Glasses and Related Materials,

Bonito, Brazil (1998).

0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 0 5 8 - 7

The aim of this work was to evaluate the in¯u-ence of the incorporation of PMMA (polymethyl-methacrylate) in sol±gel ZrO2 coatings prepared ascorrosion protectors to stainless steel in acid media.The in¯uence of di�erent amounts of PMMA(vol.%) and of the number of coating applications(NCA) on the microstructure of these coatingswere also investigated.

2. Experimental

Hybrid ZrO2-PMMA sol was prepared bysonocatalytic sol±gel method using zirconiumpropoxide, Zr(OC3H7)4, diluted in isopropanoland glacial acetic acid. The ®nal molar ratioswere Zr(OC3H7)4:C3H7OH� 2 and CH3COOH:Zr(OC3H7)4� 1. Aiming to improve homogeniza-tion, the mixture was submitted to ultrasonic ir-radiation (20 kHz) produced by a transducer (HeatSystems Ultrasonics W385) and water was addedto complete hydrolysis. PMMA (MW� 400 000)previously dissolved in acetone was introducedinto the sol±gel solution and submitted to an ad-ditional ultrasonic irradiation. The resulting solbecame homogeneous and remain stable at leastfor six months at room temperature when kept in aclosed container.

The substrates used were ¯at sheets of stainlesssteel (AISI 316L) and the surface was degreasedwith neutral detergent, washed in distilled waterunder ultrasound and then rinsed with acetone.Films were deposited by dipping the substrate intothe sol and withdrawing at a constant rate of10 cm sÿ1. The resulting gel ®lms were dried at50°C for 15 min and ®red at 200°C for 30 min.Multicoated layers were obtained by dipping thesubstrate multiple times before ®ring. Thicknessand roughness were measured by pro®lometry(Taylor Hobson Talystep) on similar ®lms depos-ited on glass slabs. The microstructure of coatingsand phase distribution were observed using scan-ning electron microscopy (SEM) (Zeiss 960) cou-pled to a energy dispersed X-ray (EDX).

Electrochemical measurements were carried outto evaluate the potentiodynamic properties of thecoatings with di�ering amounts of PMMA in de-airated H2SO4 (0.5 mol lÿ1) aqueous solution using

a potentiostat (Solartron SF1297). A saturatedcalomel electrode (SCE) was used as reference anda Pt foil served as auxiliary electrode. The po-tentiodynamic measurements were initiated atÿ1.5 V versus SCE and scanned continuously inthe anodic direction at 1 mV sÿ1.

3. Results

Fig. 1(a) and (b) shows the evolution, with in-creasing amounts of PMMA and NCA, of theroot-mean-square deviation of the pro®le (Rq) andthe mean spacing of local peaks of the pro®le (S)respectively. For single coated hybrids containing8 vol.% and 17 vol.% of PMMA both Rq and S aremaximal. Multicoated samples (2 and 3 coatings)have Rq and S greater than single coated samples

Fig. 1. (a) Root-mean-square deviation of the pro®le (Rq), and

(b) mean spacing of local peaks of the pro®le (S), as function of

the amount of PMMA and number of coating applications.

S.H. Messaddeq et al. / Journal of Non-Crystalline Solids 247 (1999) 164±170 165

and these parameters decrease as the amount ofPMMA increases.

The e�ect of NCA on the surface structure of®lms is evidenced by AFM micrographics shownin Fig. 2. The roughness of single layer sample(Fig. 2(a)) is greater than multicoated one(Fig. 2(b)). The former presents stalagmite-likeaspect while the last shows a granular structure.

Potentiodynamic curves for the samples pre-pared with di�erent amounts of PMMA and NCAare shown in Fig. 3(a) and (b), respectively. In theanodic branches, all curves have passivation e�ectin which the current is constant [8]. The passivat-ion current of uncoated stainless steel is(1.19 � 0.01)10ÿ5 A, larger than observed for thesample coated with ZrO2 ((0.65 � 0.09)10ÿ5 A).The ZrO2±PMMA coated sample has a passivat-ion current about one order of magnitude less((0.15 � 0.02)10ÿ5 A) than uncoated stainless steeland is invariant both with PMMA concentrationsand number of coating applications. This data

indicate that ZrO2±PMMA coating increasespassivation. Furthermore, the di�erence betweenthe breakdown and the primary passivationpotentials, called potential of the passive range,DEp, is practically independent of NCA, andincreases from 0.86 � 0.12 to 2.12 � 0.31 V as theamount of PMMA increases from 0 to 25 vol.%.For the coating prepared with the largestconcentration of PMMA (33 vol.%), DEp is inter-mediate between the uncoated stainless steel(DEp� 0.52 � 0.06 V) and that coated exclusivelywith ZrO2 (DEp� 0.86 � 0.12 V).

The dependence of the corrosion rate, deter-mined from potentiodynamic curves [8], on theNCA for samples prepared with di�erent concen-trations of PMMA is displayed in Fig. 4. It isobserved that hybrid coatings are more e�cient

Fig. 2. Atomic force microscopy pictures for the ®lm prepared

with 17 vol.% PMMA with (a) one and (b) two coating appli-

cations.

Fig. 3. Potentiodynamic polarization of (a) 316L stainless steel

and coated with ZrO2±PMMA containing di�erent amount of

PMMA (vol.%), and (b) di�erent number of coating applica-

tions.

166 S.H. Messaddeq et al. / Journal of Non-Crystalline Solids 247 (1999) 164±170

against corrosion than those prepared with oxideonly. Smallest corrosion rate was observed for asample containing 17 vol.% PMMA. Moreover,the corrosion rate decreased as the NCA increasedfrom 1 to 2 layers and then was invariant. Thise�ect is not observed for samples containingsmaller quantities of PMMA (<4 vol.%).

The microstructure of 8 and 33 vol.% ofPMMA coated stainless steel revealed by SEM isshown in Fig. 5(a), (b), (c) and (d), respectively.The comparison between back scattering electronspicture (Fig. 5(a) above) and the EDX mapping ofZr (Fig. 5(a), below, white dots) reveals the pres-ence of areas with greater zirconium concentration(white domains) surrounded by continuousPMMA secondary phase (black domains). Afterexposure to an electrochemical corrosion experi-ment, 8 vol.% PMMA coatings (Fig. 5(b)) showthat regions with larger content of zirconium wereattacked preferentially. Samples with the largestPMMA concentrations show no topographicalneither chemical contrast (Fig. 5(c)), have asmooth surface, and a glass-like microstructure.This coating peeled when submitted to the acidattack (Fig. 5(d)).

Fig. 6(a)±(c) show the optical pictures obtainedafter cut tape adhesion tests (standard NFX 41-022) [9], performed on ZrO2, 17 and 33 vol.% ofPMMA coatings. The sample prepared withoutPMMA had an excellent adherence to thesubstrate, indicating a class 0 material while 60% of

edges of the hybrid coating (33 vol.% PMMA)peeled, indicating a poor adhesion (classi®cation 4).

4. Discussion

The results obtained from di�erent techniquesreveal that the structure of ZrO2±PMMA hybrid®lms depends on both the NCA and the amount ofPMMA. The thickness of coatings increases lin-early from 0.2 to 1.0 lm as the relative volumefraction of PMMA increases from 4 to 33%. Thisdependence is due to increase of sol viscosity [10].For sample prepared with single coated applica-tion and amount of PMMA smaller than 17 vol.%,Rq increases from 0.12 to 0.16 lm as the ®lmthickness increase from 0.2 to 0.4 lm, remainingapproximately half of the ®lm thickness. It indi-cates that ®lms are not continuous presenting de-position faults in which the substrate remainsuncoated. The average distance between these re-gions is almost invariant (S @ 15 lm) and close tothe mean size of the zirconium rich phase observedby SEM (Fig. 5(a)). These isolated rich zirconiadomains may result from the crack propagationduring drying, in which accentuated shrinkage ofsol±gel derived materials is observed [6]. Over to17 vol.%, both Rq and S decrease as the PMMAconcentration increases. In this case the Rq is lessthan thickness (5 times for 17 vol.% and 20 timesfor 33 vol.%), indicating that coatings are crackfree, forming a hermetic coating as evidenced alsoby SEM micrographies (Fig. 5(c)). This suggeststhat these ®lms are ¯exible and less prone tocracking during drying.

The NCA in¯uences distinctly the samplesprepared with PMMA concentration smaller orlarger than 17%. In the former Rq and S increasewith the NCA, Rq staying half of ®lm thickness forNCA� 2, and this ratio is inferior to 0.2 forNCA� 3. This evidences that multicoating appli-cation decreases the amount of defects originatedduring drying of the ®rst deposited layers. Forhigh concentration of PMMA, Rq is minimal andS maximal for NCA� 2 indicating that more ho-mogeneous and cracks free ®lms are obtained un-der these conditions. For NCA� 3 the ®lms arethicker, increasing drying shrinkage and internal

Fig. 4. Evolution of the corrosion rate (CR), with number of

coating applications for samples prepared with di�erent con-

centration of PMMA.


stresses due to the incompliance of the substrate.This e�ect associated to the reduced adherence ofthe ®lms prepared with high concentration ofPMMA (Fig. 6(c)) leads to the increase of rough-ness (Fig. 1).

The corrosion currents of the coated samplesare clearly shifted to smaller values (Fig. 3), indi-cating the ®lms act as a geometric blocking layersagainst the corrosive environment. In this case, thebest corrosion protection performance is expected

Fig. 5. SEM Micrographs of coated stainless steel. (a) back scattering electrons of ZrO2±(8 vol.%) PMMA sample (upper part) and

EDX mapping of Zr (lower part), (b) coating of ZrO2±(8 vol.%) PMMA after corrosion attack, (c) coating of ZrO2±(33 vol.%) PMMA

before and (d) after corrosion attack.


for hermetic, smooth and adherent coatings [8].The pro®lometric and microstructural analyzesindicate that these conditions are obtained for

sample prepared with 17 vol.% PMMA. This iscon®rmed by potentiodynamic corrosion tests,which shows a minimal corrosion rate (0.30 � 0.05mpy) and higher potential of passive range(DEp� 1.00 � 0.01 V) for sample prepared withthis concentration of PMMA. The resistance tocorrosion of stainless steel coated with this hybrid®lm is increased by a factor 30. This performanceis similar to the one obtained in previous study ofthis system [11].

5. Conclusion

The incorporation of PMMA into the ZrO2-coating increases the corrosion resistance ofstainless steel. Maximum corrosion resistance ofthe substrate was observed for the coating con-taining 17 vol.% PMMA. For the smaller con-centrations, the coatings were not hermetic andhad two phase microstructures. In this case do-mains of zirconia rich phase were preferentiallyattacked by acid environment. The coatings pre-pared with the largest amounts of PMMA arethicker presenting a single phase structure at themicrometer scale. Moreover, their adhesion to thesubstrate was worse resulting in the breakdownand the peeling of the coating during the electro-chemical corrosion experiments.

Acknowledgements

This research has been ®nancially supported byFAPESP, CNPq, CAPES. We express specialthanks to Dr P.W. Oliveira (Institut fur NeueMaterialien, Saarbrucken, Germany), for discus-sions on hybrid properties and in adherence mea-surements.

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