Progress in Organic Coatings 53 (2005) 286–291

Silica based organic–inorganic hybrid nanocompositecoatings for corrosion protection

R. Zandi-zanda,b, A. Ershad-langroudia,∗, A. Rahimia

a Iran Polymer and Petrochemical Institute, Tehran 14976/115, Iranb Department of Chemistry, Azad University, Tehran’s North Branch, Iran

Received 3 August 2004; received in revised form 28 February 2005; accepted 1 March 2005

Abstract

Silica based organic–inorganic hybrid nanocomposite coatings have been developed for corrosion protection of 1050 aluminum alloysby dip coatings technique. The hybrid sols were prepared by hydrolysis and condensation of 3-glycidoxypropyl-trimethoxysilane (GPTMS)and tetramethoxy silane (TMOS) in the presence of an acidic catalyst and bisphenol A (BPA) as cross-linking agent. Such prepared hybridcoatings were found to be relatively dense, uniform and defect free. Structural characterization of the hybrid coatings were performedu Corrosionr ethods. Ther e content)o rid coatingsi©

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sing optical microscopy, scanning electron microscopy (SEM) and attenuated total reflectance-infrared (ATR-IR) spectroscopy.esistance properties of the hybrid sol–gel coatings were studied by potentiodynamic scanning (PDS) and salt spray testing mesults indicate excellent barrier protection performance of the coatings. In addition, the effect of molar ratio of GPTMS–BPA (silann corrosion resistance of the coatings was investigated. The PDS results demonstrated that the corrosion resistance of hyb

mproved by decreasing of silane content.2005 Elsevier B.V. All rights reserved.

eywords:Electrochemistry; Corrosion resistance; Hybrid coatings; Sol–gel; Silane content

. Introduction

When an aluminum alloy surface is exposed to the atmo-phere, the natural oxide film forms, which is responsible ofts corrosion resistance. This natural oxide film cannot pro-ect the aluminum substrate in severe corrosion conditions1].

In recent years, a vast amount of researches are inves-igated on corrosion control techniques. One of the mostffective techniques is the electrical isolation of anode from

he cathode[2].Chromatation is the most efficient technique for alu-

inum alloy that typically generated from acidic mixtures ofoluble hexavalent chromium salts; by oxidation–reductioneactions with the metal surface. A continuous layerf insoluble trivalent chromium and soluble hexavalenthromium compounds are formed. But the use of chromates

∗ Corresponding author. Tel.: +98 21 4580000.E-mail address:A.Ershad@ippi.ac.ir (A. Ershad-langroudi).

in the coating has generated serious hazardous profor the coating industry because hexavalent chromatecarcinogenic and highly toxic[3]. For this reason, a widvariety of alternative surface treatments have been deveas potential replacements for hexavalent chromium-bconversion coatings as described in various reviewsrecent years, it has become apparent that silanes caprotect metals against corrosion but single silanes by tselves do not provide any significant corrosion protec[4,5].

Organically modified silicates (Ormosils) are organinorganic hybrid materials formed by hydrolysis and cdensation of organically modified silanes with traditioalkoxide precursors by sol–gel method[6,7]. It is found thathey could provide good corrosion resistance for metalstrates, because they combine the mechanical and checharacteristics of the comprising organic and inorganicworks, producing films that are durable, scratch resistanadherent to metal substrates, flexible, dense, and funally compatible with organic polymer paint systems. Silic

300-9440/$ – see front matter © 2005 Elsevier B.V. All rights reserved.

oi:10.1016/j.porgcoat.2005.03.009

R. Zandi-zand et al. / Progress in Organic Coatings 53 (2005) 286–291 287

modified Ormosils use in different applications that reportedby various researchers. Chu et al.[8] worked on GPTMS andfound that it could be used as a binder in organic–inorganicsilica based systems that leading to increased density andimproved adhesion to polymer substrates. Sol–gel-derivedGPTMS–TEOS hybrid materials have been studied byMetrok et al.[9]. Their results indicate that organic contentand hydrolysis water ratio have a dramatic effect on the cor-rosion resistance of Ormosil films. Failure of these films incorrosion resistance test (i.e., salt spray) is due to localizedpit formation that is likely to form at hydrophilic regions inthe film, such as non-condensed silanol group. To solve thisproblem, it is necessary to produce a dense, continuous filmand impermeable to the transport of corrosion initiator[10].Ooij et al. [5] showed that addition of a cross-linking agentinto the sol–gel hybrid solution results in a denser siloxanefilm that benefits surface protection by decreasing reactionrates with moisture. They also published an overview onthe use of bis-silanes rather than the conventional monosi-lanes for corrosion control of metals and bonding to paintsystems.

In this research, we studied the corrosion resistance of theorganic–inorganic hybrid coatings on 1050 aluminum alloy.These coatings were made from GPTMS and tetramethoxysilane (TMOS) using a two-step acid catalysis process inpresence of BPA as a curing agent. It was demonstrated thats pro-t

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2.3. Substrate preparation and film deposition

The substrates used for the analysis of the sol–gel coatingswere 1050 aluminum alloy that had been prepared as reported[11].

Cleaned aluminum alloy substrates were immersed intothe sol for 1 min. The coating was air-dried onto the substrateand placed in a furnace to cure at temperatures ranging fromambient to 130◦C for 90 min.

2.4. Electrochemical analysis

Electrochemical measurements were performed underextreme environmental conditions, consisting of an aqueous,air exposed sodium chloride (3% NaCl) solution.

Each sample was sealed with waterproof tape in orderto prevent premature corrosion along the edges of the sub-strate. A 1 cm× 1 cm area within the center of each sam-ple was exposed to the solution during testing. Corrosionanalysis of bare and coated substrates was done using anAutolab PGSTAT30 potentiostat system connected to a corro-sion analysis software program. Polarization measurementswere carried out potentiostatically at room temperature usingan Ag/AgCl/Cl− (0.222 V) reference electrode and a plat-inum counter electrode. The potentiodynamic measurementswere taken within the range of−2000 to 2000 mV versusA nts,i ept int

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opy( char-a ticalm g ab tor at1 lly,a -IR)w werer pe,b

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ol–gel-derived hybrid coatings improved the corrosionection of aluminum alloy substrates.

. Experimental

.1. Materials and reagents

Tetramethoxysilane and 3-glycidoxypropyl-trimethoilane (GPTMS) were purchased from Fluka. BisphenBPA) was used as a curing agent and prepared from Mnd was used as received. Sodium chloride and hydrhloride purchased from Merck were used without fururification.

.2. Sol preparation

The hybrid sol was prepared by admixing silica pursors, GPTMS, TMOS, and an organic curing agisphenol A.GPTMS and TMOS were mixed (2:1 molar ratio) in

eaker with 0.01 M HCl and methanol (1:2 H2O/methanoolar ratio) at ambient temperature. The resultant two-p

olution was vigorously stirred at a rate of 240 rpm forydrolysis and condensation of the silane was conducttoichiometric and 50% substoichiometric water/silane rhen BPA was added to the solution and vigorously stigain at a rate of 240 rpm for 4 h at ambient temperafter this step, the sol solution was completely transpand without phase separation.

g/AgCl/Cl− at a rate of 5 mV/s. Prior to the measuremen order to reach steady potential, the electrodes were khe working solutions for at least 30 min.

.5. Corrosion resistance test

Corrosion resistance test of the coated aluminum aubstrates was evaluated by exposure of the samplesog atmosphere generated from 5 wt% aqueous NaClion at 35 + 1◦C for 2000 h in accordance with ASTM B1pecifications.

.6. Characterization

Optical microscopy and scanning electron microscSEM) were performed on the coated substrates tocterize the surface morphology with a Jenavert opicroscope and Cambridge S.360 microscope usinackscattered or a secondary electron image detec0 kV and 2.85 A probe current, respectively. Additionattenuated total reflectance-infrared spectroscopy (ATRas used for morphological analysis. Infrared spectra

ecorded using ATR objective of a Bruker microscoetween 400 and 4000 cm−1.

. Results and discussion

Both optical microscopy and SEM observations shohat the formation of a uniform, homogeneous, crack free

288 R. Zandi-zand et al. / Progress in Organic Coatings 53 (2005) 286–291

Fig. 1. Optical microscopic image of hybrid coated aluminum alloy.

highly adherent protective film on the substrates that leads togood corrosion resistance of aluminum alloys coated with theorganic–inorganic hybrid coatings (seeFigs. 1 and 2).

Silica based organic–inorganic hybrid coating systemshave been prepared by a two-step acid catalysed sol–gelprocess. The first step is the formation of organo-silicanano-particles with peripheral epoxy-functional groups byhydrolysis and condensation of GPTMS and TMOS. Theepoxy rings can be opened and polymerized to form a linearpoly(ethylene oxide) organic network. In the second step,the nano-particles are cross-linked through a conventionalchemical reaction, between –OH functional groups of BPAand epoxy functionalities[12]. The epoxy functions can bereacted with poly(ethylene oxide) chains or can be con-nected by polyaddition reaction with the aromatic diol[7](seeFig. 3).

The cross-linking of organo silica-networks via chemi-cal coupling of aromatic diol and epoxy functionalities was

Fig. 2. SEM image of coated aluminum with organic–inorganic hybrid coat-ing.

observed by ATR-IR spectroscopy.Fig. 4 shows ATR-IRspectra for a series of hybrid films prepared with and withoutaromatic diol as cross-linking agent. The most resolved bandsare attributed to several vibrational frequencies of the epoxyring of organo silica-network of glycidoxypropylsiloxanestructural fragment, including oxirane methylene bending at1480 cm−1, epoxide ring breathing band at 1250 cm−1, andantisymmetric epoxide ring deformation bands at 750 and906 cm−1. The band of the antisymmetric epoxide ring defor-mation at 750 and 906 cm−1 appears to be almost intenseand allows monitoring the chemical reaction of couplingof epoxy functionalities of organo silica-networks with diolcross-linkers. As it can be seen, this band almost disappears inATR-IR spectra of the cross-linked films indicating the chem-ical bonding of organo silica-networks into a cross-linkednetwork[4].

ic hyb

Fig. 3. Reaction scheme of the formation of organic–inorgan rid systems: (I) hydrolysis and condensation and (II) cross-linking[7].

R. Zandi-zand et al. / Progress in Organic Coatings 53 (2005) 286–291 289

Fig. 4. ATR-IR spectra of organic–inorganic hybrid films prepared withcross-linking agent (a), and uncross-linked organic–inorganic hybrid film(b) [11].

Practically, the cross-linking of organo silica-networksoccurs upon application of sol solution on the substrate sur-face simultaneously with the evaporation of the solvent andfilm formation leading to a composite structure of the coating[4]. Aluminum alloy as substrate can form networks with sili-cates, thus enhancing the cross-link density of the silane filmfurther because some SiO Si bonds are replaced by the

Si O Al O Si group [5]. Fig. 5 shows ATR-IR spec-tra for hybrid–aluminum interface. As it can be seen, theAl O Si band appears at 936 cm−1. The presence of thispeak indicates the formation of networks between aluminumsubstrate and silicates[14].

The results of ATR-IR spectroscopy were indirectly sup-ported by the SEM observation on the hybrid–aluminum alloyinterface (as seen inFig. 5). The micrograph shown inFig. 6indicates a good adhesion between coating and aluminumsubstrate that can be due to the formation of SiO Al bond

F alu-m

Fig. 6. SEM micrograph of the hybrid–aluminum interface.

between hybrid coating and aluminum substrate. Corrosionresistance properties of these coatings were studied usingpotentiodynamic scan (PDS) method to provide informationabout electrochemical corrosion in the system.Fig. 7showsthe typical polarization curves of both bare and hybrid sol–gelcoated aluminum substrates.

The polarization curve of the sol–gel coating was appre-ciably different from that of the bare aluminum substrate.First, the open circuit potential,EOC of the sol–gel coatingswas significantly higher than that of the bare aluminum.

Secondly, a passivation region with a rather low passi-vation current density of∼1.19× 10−6 A/cm2 was present,which implied that the sol–gel coating provided a physicalbarrier for blocking the electrochemical process. Such abarrier would fail only at a high potential of∼0.288 V.The bare aluminum alloy substrate exhibited a differentpotentiostat polarization curve that shown smaller passiveregion in comparison with hybrid coated substrate. Thirdly,the open circuit current density of the created sample(3.28× 10−7 A/cm2) was far smaller than that of thebare aluminum alloy substrate (1.694× 10−4 A/cm2). Theprotection efficiency, of the sol–gel coating on the aluminum

F alu-m

ig. 5. ATR IR spectra of organic–inorganic hybrid coating on 1050inum alloy[13].

ig. 7. Polarization curves of bare and GPTMS–BPA sol–gel coatedinum alloy substrates.

290 R. Zandi-zand et al. / Progress in Organic Coatings 53 (2005) 286–291

Fig. 8. 2000 h salt spray tests for bare (a) and hybrid sol–gel coated (b)aluminum alloy.

alloy substrate could be calculated by:

P (%) =(

1 − icorr

iocorr

)× 100

whereiocorr andicorr denoted corrosion current densities of thebare and coated electrodes, respectively. The corrosion cur-rent density was obtained from the intersection of the anodicand cathodic Tafel lines. The corrosion protection efficiencyof the sol–gel-derived coating was found to be 99.8%.

Fig. 8 shows the results of 2000 h salt spray test of bothbare and hybrid sol–gel coated aluminum alloy substrate.These results show that the silane hybrid film is a true barriercoating.

The corrosion protection properties of sol–gel-derivedcoatings are strongly dependent on the processing condi-tions. For example, aging, pH, curing temperature and molarratio of processors. In this research, we study the effect ofGPTMS/TMOS molar ratio or silane content on corrosionresistance of hybrid coatings. For this reason, we preparedthree-hybrid coating (GM1, GM2 and GM3) with 1:0, 1:1and 2:1 molar ratio of GPTMS/TMOS. Silane content wasfound to have a significant influence on the corrosion pro-tection of the sol–gel coatings.Fig. 9shows the polarizationcurves of GM1, GM2 and GM3 hybrid coatings.

F ganich

The sample with the lower silane content had shown bettercorrosion protection properties than the substrate with thehigher silane content. The open circuit potentials and corro-sion resistance decreased from−1.196 V and 2.86× 105 � to−1.522 V and 3.4× 103, respectively. In addition, the cor-rosion current density and corrosion rate was increasedfrom approximately 3.28× 10−7 A/cm2 and 1.072×10−2 mm/year to 2.13× 10−5 and 6.97× 10−1 mm/year,respectively. These results indicate that a coating fromhigher silane content is less effective in corrosion protectionthan a coating from lower silane content. One possibility isthat the coating made from higher silane content had higherporosity than that coatings made from lower silane contentdue to formation of larger linear silica chains and strongergel network. Upon removed of solvent during drying, highersilane content sols with larger polymers and a strong gelnetwork would be more resistance to the capillary-drivencollapse of the gel network, resulting in the formation of amore porous structure.

Electrochemical results of the GPTMS–BPA organic–inorganic films demonstrated the barrier and corrosion resis-tance properties imparted by the hybrid system. The lowcurrent density and high passivation region indicates thatthe film provides an efficient barrier to water and corrosionagents (e.g., chloride and oxygen). In fact, organic group dis-persed throughout the film apparently serve to increase theh ioni

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emsb d bya uni-f ratedt t sys-t alu-m tingso corro-s andc ffec-t lly. Itw owers ilanec dhe-s icalb

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ter-July

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ig. 9. Polarization curves of various silane content organic–inorybrid coated aluminum alloy substrates.

ydrophobicity of the coating, repelling water and corrosnitiators and enhancing corrosion protection properties[9].

. Conclusion

Silica based organic–inorganic hybrid coating systased on Bisphenol A as a cross-linking agent preparetwo-step acid catalysed sol–gel process, were found

orm, defect-free and relatively dense. We have demonsthat the hybrid coatings are promising surface treatmenems, which provide improved corrosion protection forinum alloy. Electrochemical experiments of these coan aluminum alloy substrates demonstrated enhancedion protection by forming an efficient barrier to waterorrosive agents (e.g., chloride ion, oxygen), which eively separated the anode from the cathode electricaas found that the corrosion resistance of the sols with lilane content was better than that of the sols with high sontent. The hybrid coating also demonstrated good aion, which could be attributed to the formation of chemonding at the interface.

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