Ona

HD

a

ARRAA

KSAEN

1

mimmetm[rpembraisd[d

0h

Progress in Organic Coatings 76 (2013) 307– 317

Contents lists available at SciVerse ScienceDirect

Progress in Organic Coatings

jou rn al h om epage: www.elsev ier .com/ locate /porgcoat

ptimization of process factors for the synthesis of advanced chrome-freeanocomposite sol–gel coatings for corrosion protection of marine aluminumlloy AA5083 by design of experiment

amed Rahimi ∗, Reza Mozaffarinia, Akbar Hojjati Najafabadi, Reza Shoja Razavi, Ebrahim Paimozdepartment of Materials Engineering, Malek Ashtar University of Technology, Shahin Shahr, Isfahan, Iran

r t i c l e i n f o

rticle history:eceived 29 June 2012eceived in revised form 9 September 2012ccepted 10 September 2012

a b s t r a c t

Nanocomposite sol–gel coatings based on tetraethylorthosilicate (TEOS) and 3-glycidoxypropyl-trimethoxisilane (GPTMS) have been prepared for corrosion protection of AA5083. Statistical design ofexperimental methodology based on Taguchi orthogonal design has been used to study and optimize var-ious parameters using multifactor analysis of variance (ANOVA). The corrosion current density has been

vailable online 2 November 2012

eywords:ol–gelA5083ISanostructure coating

used as a response. Structure, surface morphology and composition of the hybrid coatings were studiedby using SEM, AFM and ATR/FTIR, respectively. Corrosion performance of coatings was examined usingpotentiodynamic polarization and EIS. The results show that by proper choice of parameters, adherent,dense and protective hybrid coatings can be obtained.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

Aluminum alloys, such as AA5083, are materials of choice forarine and aerospace applications due to their excellent mechan-

cal and physical properties. However, these alloys are reactiveaterials and prone to corrosion, especially in chloride environ-ents [1]. Chromium based-pretreatments are among the most

fficient and successful system for aluminum and its alloys. Fur-hermore, the use of chromate will be totally banded in coating

aterials in the near feature because of their extremely toxic effect2]. In last decade an intensive research is ongoing to develop envi-onmentally friendly alternatives to chromate based-pretreatmentrocesses. The hybrid sol–gel coatings were reported as promisingnvironmentally friendly alternatives for anti-corrosion pretreat-ents on different substrates [3–7]. Hybrid coatings combine

oth the advantages of inorganic polymers (flexibility, lightweight,educes defects, good impact resistance and process ability, etc.)nd inorganic components (high mechanical strength, good chem-cal resistance, high ductility and superior adhesion to the metalurface, etc.) [8]. Good adhesion of sol–gel films to aluminum

erives from the formation of strong covalent Si O Al bands9,10]. Reaction rate for hydrolysis and condensation reactionepends on parameters such as ratio and nature of the organic and

∗ Corresponding author. Tel.: +98 3125225041; fax: +98 3125228530.E-mail addresses: Rahimi@mut-es.ac.ir, hamed83rah@yahoo.com (H. Rahimi).

300-9440/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.porgcoat.2012.09.025

inorganic structural units, solution pH, hydrolysis water ratio andtemperature. Altering these parameters can change the structureand properties of silane layer in a wide range [11,12].

Metroke et al. [13] have shown that organic content and hydrol-ysis water ratio have a dramatic effect on the ormosil structure andin turn the corrosion resistance properties of the ormosil films. vanOoij et al. [14] have indicated that it is important to have highestsilanol group (Si OH) concentration in the sols (hydrolysis step)prior to their application on to the substrate. This would ensuremaximum metallosiloxane bond (Si O Al) formation, which is akey factor governing corrosion performance of the coatings.

As it is well known, corrosion performance of sol–gelorganic–inorganic hybrid system depends upon a number of com-positional and processing factors. The aim of work was to studyand optimize such factors using statistical design of experimentalmethodology (DoE) based on Taguchi method. Thus, in the presentstudy, organic–inorganic hybrid nanocomposite coating based onTEOS and GPTMS, in different proportions, have been derived ontothe aluminum alloy AA5083 surfaces, under varying conditions.Corrosion resistance performance of these systems has been stud-ied and compared. A statistical design of experiment (DoE) has beenused to study systematically vary important compositional andprocess parameters, include GPTMS/TEOS molar ratio, hydrolysis

water content, draying and curing temperatures and curing time,effects on corrosion performance of coatings. Process optimiza-tion has been done using multifactor analysis of variance (ANOVA)analysis method.

308 H. Rahimi et al. / Progress in Organic Coatings 76 (2013) 307– 317

Table 1Experimental factors and their levels.

Factors Level 1 (low) Level 2 Level 3 Level 4 (high) Code

GPTMS/TEOS molar ratio 3/7 5/5 7/3 9/2 AHydrolysis water coefficient (x) 1 2 5 12 B

◦ 30

0

0

2

2

dptomalc5eifeptmwTTa

2

wfpsthT

TB

Curing temp. ( C) 100 1Drying temp. (◦C) 25 6Curing time (min) 30 6

. Materials and methods

.1. Statistical design of experiment (DoE) methodology

Sol–gel hybrid coatings are sufficiently affected by process con-itions such as water hydrolysis content, organic/inorganic ratio,H, synthesis temperature, drying and curing times and tempera-ures, in order to study the effect of the key parameters as a functionf the corrosion resistance performance of coatings, the DoE experi-ental methodology based on Taguchi method has been used. DoE

llows for the multiple factor selection in designing at differentevels, and the response for its interaction can be used as the out-ome to determine the statistically significant factors [15–17]. A-factor–4-level design methodology has been used to study theffects of organic/inorganic ratio, hydrolysis water content, dry-ng and curing temperatures and curing time as input factors atour different levels. The select process parameters and their lev-ls are reported in Table 1. The corrosion current density as perotentiodynamic polarization test has been used as response andhe optimization of factors has been worked out using Taguchi

ethod and Qualitek-4 (Nutek Inc.) software. A standard L16 arrayas obtained from the Qualitek-4 software is shown in Table 2.

he signal-to-noise ratio (S/N) is used as a transformed response inaguchi method to serve as an objective function for optimization,nd the polarization tests were reported two times.

.2. Preparation of hybrid sol

The ormosil solution was prepared by an acid-catalyzed processith TEOS and GPTMS as precursors. The reagents were purchased

rom Merck and were used as received. TEOS and GPTMS werelaced in a beaker with 0.05 M HNO3. The mixture was vigorously

tirred until it turns to a clear sol. Sixteen sols were prepared inhe different GPTMS/TEOS molar ratio (3/7, 5/5, 7/3, and 9/2) andydrolysis water content (x = 1, 2, 5, and 12) according to Table 2.he total amount of hydrolysis water added was calculated using

able 2asic grid design.

Runs A B C

S1a 1 1 1

S2 1 2 2

S3 1 3 3

S4 1 4 4

S5a 2 1 2

S6 2 2 1

S7a 2 3 4

S8 2 4 3

S9a 3 1 3

S10a 3 2 4

S11 3 3 1

S12 3 4 2

S13a 4 1 4

S14a 4 2 3

S15 4 3 2

S16 4 4 1

a Coatings include macrocracks did not tests and Icorr of bare aluminum was reported f

160 200 C80 90 D90 120 E

the equation: x[4 mol TEOS + 3 mol GPTMS]. The sol was allowed tostand for approximately 2 days before film deposition.

2.3. Sample preparation and coating method

Aluminum 5083 alloy sheet (thickness ∼3 mm) as a substratematerial with a composition; 4.9% Mg, 0.4% Fe, 0.4% Si, 0.35% Mn,0.25% Zn, 0.15% Ti, 0.1 Cu, and 0.05% Cr was cut to desired dimen-sions. In order to prepare the specimens for dip coating, they wereground with 280 up to 1000 grit sand papers. This helps in attainingbetter adherence of the coating with the substrate due to strongmechanical interlocking [18]. After bathing with water and soap,there were cleaned by ultrasonic degreasing with acetone, and thenwere washed by distilled water. Organic–inorganic hybrid filmswere produced by a dip-coating procedure conducted by immer-sion of the prepared specimens in the sol for 2 min, followed bycontrolled withdrawal rate of 3 mm/s. After coating application,the specimens were dried at different temperatures for 1 h (72 hfor specimens that dried at the ambient temperature) and thencured at different temperatures for different times according toTable 2.

2.4. Characterization

The surface morphology of the ormosil films were assessedusing an optical microscope (OM) Olympus PMEB, and scanningelectron microscope (SEM) Vega//Tescan was used to examine themicrostructure. Atomic force microscopy (AFM) measurementswere used for ascertaining the surface roughness and morphologyof film surfaces. The surface of the hybrid coatings was charac-terized for functional groups using a Bruker TENSOR 27 infrared

spectrophotometer. The IR absorbance was measured in a rangebetween 4000 and 600 cm−1. Adhesion measurements of coatingto the substrate were evaluated by pull off test using Positest ATaccording to ASTM D4541.

D E Response/results Icorr (�A)

Trial 1 Trial 2

1 1 37.420 37.4202 2 4.224 1.8623 3 3.335 5.2974 4 1.494 2.3283 4 37.420 37.4204 3 9.028 5.7921 2 37.420 37.4202 1 22.020 14.7404 2 37.420 37.4203 1 37.420 37.4202 4 2.648 4.1271 3 7.832 4.9952 3 37.420 37.4201 4 37.420 37.4204 1 2.553 1.3833 2 4.328 2.721

or them.

rganic Coatings 76 (2013) 307– 317 309

2

EpcrsctAstaA1rs

3

3

tpG[lobiarhbciraGoiswOi

3

aoaocbtapfc(w

H. Rahimi et al. / Progress in O

.5. Corrosion tests

Potentiodynamic polarization tests were carried out using anG&G 2273A potentiostat. Polarization curves were obtained byolarizing the specimens from −250 to 450 mV respect to openircuit potential value (EOCP) at scanning rate of 2 mV s−1. The cor-osion current (Icorr) values reported here in correspond to a 50 mVtretch between the cathodic and anodic parts of the polarizationurve. Electrochemical impedance spectroscopy was carried outo assess the corrosion performance of the hybrid sol–gel film onA5083 at open circuit potential (OCP) with applied 5 mV sinu-oidal perturbations in the frequency range of 10−1 to 105 Hz. Ahree-electrode cell equipped with a platinum counter electrode,n Ag/AgCl/Cl− reference electrode and a coated or non-coatedA5083 panel as the working electrode having an exposed area of

cm2. All tests were repeated at least two times in order to assesseproducibility and were performed in an aqueous 3.5 wt.% NaClolution at room temperature.

. Results and discussion

.1. ATR/FTIR analysis

The FTIR spectra of silica-based hybrid films have been inves-igated previously. Infrared band assignment were made asreviously described by Rajamani [19] for hybrid films made ofPTMS and tetramethoxysilane (TMOS), Phanasgaonkar and Raja

20] for hybrid coating made of TEOS and methyltriethoxysi-ane (MTES). Fig. 1 shows ATR-FTIR spectra for the differentrganic–inorganic hybrid films investigated. Strong Si O Si bandsetween 1000 and 1200 cm−1 were observed in all spectra, which

s a structural backbone of the hybrid coatings. The spectra exhibitn O H stretching region around 3200 3500 cm−1 which is rep-esented by a broad band, where freely vibrating OH groups andydrogen bonded OH groups are apparent. As shown in Fig. 1a and

intensity of different bands more depended to hydrolysis waterontent than other variable parameters. As it can be seen withncrease hydrolysis water content at constant GPTMS/TEOS molaratio and other variable parameters (Fig. 1a) important changesre observed in Si O Si band intensity While, with increasePTMS/TEOS molar ratio at constant hydrolysis water content andther variable parameters (Fig. 1b) Significant change are not seenn Si O Si band intensity. It could be attributed to increasingilanol concentration as a result of increasing hydrolysis. As it clearith increasing GPTMS/TEOS molar ratio, because of increasingH and CH2 bands concentrations in the sol, OH and CH2 bands

ntensity increased.

.2. Visual observation and optical microscopy

All coated specimens appeared to be shiny and transparentfter curing. However, some coatings appear crack-free and othernes cracked. The surface morphology of different coatings wasssessed using optical microscopy. Fig. 2 shows optical micrographf different coatings. The cracks morphology was observed in alloatings made of sol with lower hydrolysis water content. It maye attributed to less-well connected network and lower adhesiono substrate. It can be seen with increase hydrolysis water contentt constant GPTMS/TEOS molar ratio (e.g. 3/7) and other variablearameters the tendency for crack formation decreased and crack-

ree coatings observed, While with an increase organic content atonstant hydrolysis water content and other variable parametersx = 1) cracks are seen yet. Also high drying and curing temperatureas could a result for inducing cracking of the coatings.

Fig. 1. ATR-FTIR spectra of different sol–gel hybrid nanocomposite coatings (a) atconstant organic/inorganic ratio and (b) at constant hydrolysis water content.

3.3. Adhesion properties

To study the adhesion property between sol–gel coating and alu-minum substrate conducted a pull off test. The adhesion strengthof different coatings is shown in Fig. 3. As observed, coatingswith highest hydrolysis water and inorganic content had the high-est adhesion to substrate. Increasing hydrolysis water contentand decreasing organic content increased silanol (Si OH) con-centration. Hence, Si O Al formation possibility increased andconsequently, adhesion of coating to the substrate increased.

3.4. Atomic force microscopy (AFM)

Fig. 4 shows AFM topographic images of the TEOS-GPTMS hybridcoatings in scan areas of 5 �m × 5 �m. The topographic imagesshow smooth low roughness films. These AFM micrographs revealthat the compositional and process factors (GPTMS/TEOS molar

ratio, hydrolysis water content, drying and curing temperaturesand curing time) affect the morphology of the film. In order to makea comparison of the film morphology, surface roughness param-eters were calculated for the overall AFM image area. The RMS

310 H. Rahimi et al. / Progress in Organic Coatings 76 (2013) 307– 317

F S3, an

swcpcc

ig. 2. Optical micrograph of (a) samples at constant organic/inorganic ratio S1, S2,

urface roughness values of some samples are shown in Fig. 5. Itas observed that increasing the amount of the hydrolysis water

ontent at constant GPTMS/TEOS molar ratio and other variablearameters results in rougher films and the smooth surface of theoating was observed for the films processed with lower organicontent. It can be attributed to other parameters effects.

d S4 and (b) samples with minimum hydrolysis water content S1, S5, S9, and S13.

3.5. Potentiodynamic polarization analysis

Fig. 6 shows the potentiodynamic curves of different ormosilfilms derived according to Table 2. Corrosion current density (Icorr)values for different samples are shown in Table 2. The Icorr valueof sample S4 is found very low in comparison to all other samples.

H. Rahimi et al. / Progress in Organic Coatings 76 (2013) 307– 317 311

Fig. 3. Pull off adhesion strength of different coatings made of various conditions according to Table 2.

Fig. 4. Atomic force micrograph of (a) S1, (b) S4, (c) S8 and (d) S16.

312 H. Rahimi et al. / Progress in Organic

FS

GbpSS

ig. 5. The RMS surface roughness values of different samples S1, S2, S4, S6, S8, S11,15 and S16.

enerally, lower Icorr values for all sol–gel coatings compare to the

are substrate indicates that sol–gel coatings indeed can provide ahysical barrier for blocking the electrochemical process. Sample4 showed passivation in the range of −821 to −578 mV and sample15 revealed maximum corrosion potential (Ecorr) compare to other

Fig. 6. Potentiodynamic polarization curves recorded in 3.5% NaCl solution of differe

Coatings 76 (2013) 307– 317

samples. The lower Icorr value of sample S4 could be attributed tohigher adhesion of coating to the substrate (ref. Fig. 3).

3.6. Analysis of variance

The analysis of variance (ANOVA) is a powerful technique inTaguchi method that explores the percent contribution of factorsaffecting the response. With the S/N ratio and ANOVA analysis, theoptimal combination of the process parameters can be predicted.The statistical analysis of the results was carried out using Qualitek-4 (Nutek Inc.) software. Table 3 shows the ANOVA statistical termsfor corrosion resistance of ormosil coatings. It can be seen that theimportant contributors to variability of the results are hydrolysiswater content (B), drying temperature (D), curing temperature (C),GPTMS/TEOS molar ratio (A), and curing time (E), respectively. Inthe present study, curing time (E) do not affect the response sig-nificantly, and it was pooled, because its sum of squares was lowerthan ten percent of the highest sum of square (factor E) in ANOVAtable.

Table 4 shows the ANOVA statistical terms for corrosion resis-tance of coatings after omitting factor E. The magnitude of thecalculated other/error term (12.5%) indicates the DoE was accept-able (lower than 15%). Comparison the F-ratio of factors with

nt aluminum dip-coated (a) base, S2, S3, S4, S6, S8 and (b) base, S11, S12,S15, S16.

H. Rahimi et al. / Progress in Organic Coatings 76 (2013) 307– 317 313

Table 3Multifactor ANOVA analysis.

Factors Code Degree of freedom Sum of square Variance F-ratio Contribution %

GPTMS/TEOS molar ratio A 3 271.515 90.505 – 16.474Hydrolysis water coefficient (x) B 3 735.526 245.175 – 44.627Curing temperature (◦C) C 3 279.263 93.087 – 16.944Drying temperature (◦C) D 3 320.624 106.874 – 19.453Curing time (min) E 3 41.199 13.733 – 2.499Other/error – 0 0 0 – 0

sto

3

3

(iIT(aswowlmitdcprh

3

obmaiaiIhGi

The results reveal corrosion current density of samples coated

TM

Total – 15

tandard F-ratio Tables shows that factors are considered statis-ically significant. The main effect of different variable parametersn corrosion current density is shown in Fig. 7.

.7. Effect of different factors on corrosion current density

.7.1. Hydrolysis water content (B)As shown in Fig. 7a with an increase in hydrolysis water content

while the other parameters are variable), there was a decreasen corrosion current density with the best performance at x = 12.t could be attributed to the silanol concentration of the sols.he higher silanol concentration increased metallosiloxane bandsSi O Al), which would enhance corrosion resistance [21], anddhesion of coatings to the substrate as it confirmed by adhe-ion strength results. Under acidic conditions, polymerized speciesith a less cross-linked structure were formed when the amount

f hydrolysis water was small; when the amount of hydrolysisater was large, however, the number of sites hydrolyzed was so

arge, even under acidic conditions, that highly cross-linked poly-ers were generated [13], and as a result corrosion performance

mproved. On the other hand, in low hydrolysis water content,he linear chain condensation and growth mechanism forms aense, microporous silica network. While, in high hydrolysis waterontent, the ring formation condensation and growth mechanismroduces a particulate like silica structure [22], which could be aeason for better corrosion performance of coatings made of higherydrolysis water content.

.7.2. GPTMS/TEOS molar ratio (A)In the hybrid nanocomposite system, the balance between

rganic and inorganic components is important to achieve goodarrier properties while maintaining desirable mechanical perfor-ance. The organic constituent provide flexibility, hydrophobicity

nd reduce defects in the coating matrix, depending upon is chem-cal structure. The inorganic part is responsible for the superiordhesion to the metal surface and the high ductility [3,8]. Thus its important to optimize the ratio of organic/inorganic in coatings.

t can be seen from a Fig. 7b lower organic/inorganic molar ratioad the maximum effect on corrosion current density. The higherPTMS content, the more epoxide ring opening reaction will result

n diol formation, which may add hydrophilicity to the film due to

able 4ultifactor ANOVA analysis after omitting E factor.

Factors Code Degree of freedom

GPTMS/TEOS molar ratio A 3

Hydrolysis water coefficient (x) B 3

Curing temperature (◦C) C 3

Drying temperature (◦C) D 3

Curing time (min) E (3)

Other/error – 3

Total – 15

1648.129 – – 100.00

the higher hydroxyl content, and as a result, make the film moresusceptible to corrosion. An optimum organic/inorganic molar ratiocould provide a good adhesion to substrate and dense films.

3.7.3. Drying and curing temperaturesAs observed from Fig. 7c with increasing drying temperature

(D) corrosion resistance performance increased, which could beattributed to enhance of evaporation of water and alcohol. Asit has shown in Fig. 7d with increase curing temperature (till130 ◦C), increased solvent evaporation, and polymerization andcross-linking of the gel matrix, which densify the coating materi-als hence corrosion resistance performance improved while highercuring temperatures (>130 ◦C), due to decomposition of organicsegments, inducing cracking of the coatings and hence corro-sion performance decreased. These results confirmed by opticalmicroscopy images as shown in Fig. 2.

3.7.4. Curing time (E)The curing time is closely related to the densification kinetic

of ormosil films. Table 3 indicates that curing time did not havea significant effect on corrosion resistance performance. It may bedue to interaction of the curing time and other factors. However, thecorrosion performance could be improved with increasing curingtime (Fig. 7e).

3.8. Optimum conditions

The optimum conditions to attain a hybrid coating with mini-mum corrosion current density were determined from maximumpoints in main effect plots (Fig. 7). Table 5 represents the opti-mum conditions resulting from Taguchi method calculation. Theoptimum condition’s levels are given as A1B4C2D4. This arrange-ment of factor levels does not exist among examined conditionsaccording to Table 2. Coatings were prepared according to opti-mum conditions predicated by Taguchi method was investigated.

with optimum conditions predicated by Taguchi method and usingQualitek-4 software (0.797 �A) was reasonably in agreement withthe experimental data (0.914 �A). The error between theoreticaland experimental data was 12.8%.

Sum of square Variance F-ratio Contribution %

271.515 90.505 6.590 13.974735.526 245.175 17.852 42.128279.263 93.087 6.778 14.444320.624 106.874 7.782 16.954(41.199) Pooled41.198 13.732 – 12.51648.129 – – 100.00

314 H. Rahimi et al. / Progress in Organic Coatings 76 (2013) 307– 317

F is wat e.

3

3

m

ig. 7. Plots showing main effect of the variable study (a) main effect of hydrolysemperature, (d) main effect of curing temperature and (e) main effect of curing tim

.9. Optimum coatings properties

.9.1. Surface characterizationOptical and scanning electron microscopy images of opti-

um coatings show that coatings are crack-free. OM and SEM

ter content, (b) main effect of GPTMS/TEOS molar ratio, (c) main effect of drying

micrographs are shown in Figs. 8 and 9, respectively. Fig. 10

presents AFM topographic images of the optimum coatings inscan areas of 5 �m × 5 �m. Fig. 10 reveals a comparatively smoothnanostructure surface with RMS surface roughness 4.67 nm. Theadhesion stress of optimum coatings to the substrate was 4.74 MPa.

H. Rahimi et al. / Progress in Organic Coatings 76 (2013) 307– 317 315

3

iccoatapanpt(ptp

TO

P

Fig. 8. Optical micrograph of optimum sample (a) 25× and (b) 50×.

.9.2. EIS measurementsGetting deeper in the electrochemical characterization, AC

mpedance was performed for bare substrate and optimumoatings. Fig. 11 displays the Nyquist and Bode plots for bare andoated AA5083 after 30 min of immersion. The phase angle curvef the bare alloy presents two time constants at 72 and 0.28 Hzssign to the electrical double layer and Warburg diffusion, respec-ively. A new time constant at higher frequencies, above 104 Hz,ssociated with the sol–gel coating, appears in the spectra of therotected substrates. The incorporation of this coating producedn increase of impedance modulus at 0. 1 Hz of 1.6 orders of mag-itude as a consequence of the additional barrier provided. Bodelots of coated samples after only 30 min of immersion presentwo time constants at 104 Hz (sol–gel coating) and around 1 Hz

intermediate layer). The interpretation of impedance spectra waserformed using a numerical fitting. The equivalent circuits usedo model all impedance curves are shown in Fig. 12. A constanthase element (CPE) was used instead of an “ideal capacitor” to

able 5ptimum conditions base on the Taguchi method.

Factors Code Level Level description

GPTMS/TEOSmolar ratio A 1 3/7Hydrolysis water coefficient (x) B 4 12Curing temperature (◦C) C 2 130Drying temperature (◦C) D 4 90

redicate Icorr value = 0.797 �A.

Fig. 9. Scanning electron micrograph of optimum sample (a) 29× and (b) 5000×.

explain the deviations from ideal behavior. The impedance of aCPE (ZCPE) can be defined by ZCPE = 1/Y(jω)n. The parameters cor-respond to the frequency (ω), pseudo-capacitance (Y), and theparameter n associated to the system homogeneity. When thisequation describes a capacitor, n = 1 and Y = C (the capacitance).For a CPE, the exponent n is less than one. Rs is the resistanceof the electrolyte and Rcoat the resistance of the sol–gel coating.Ycoat is the pseudo-capacitance of the sol–gel coating, and Raluminaand Yalumina, the resistance and pseudo-capacitance associatedwith the thin natural aluminum oxide layer. Rct is the resis-tance describing the corrosion of the metal substrate, and Ydl thedouble-layer pseudo-capacitance formed in the metal-electrolyteinterface. YWarburg is the pseudo-capacitance associated to War-

burg diffusion. Table 6 shows the fitting of the data for the modelspresented in Fig. 12.

316H

. R

ahimi

et al.

/ Progress

in O

rganic Coatings

76 (2013) 307– 317

Fig. 10.

Atom

ic force

micrograp

h of

optim

um

samp

le.

Fig. 11.

(a) N

yquist

and

(b) B

ode

plots

for bare

and

coated A

A5083

substrates

after30

min

imm

ersion in

3.5% N

aCl

solution

.

Table 6Impedance parameters for bare and coated AA5083 substrates.

Sample Rs (� cm2) Rct (� cm2) Ydl (S cm−2 sn) ndl YWarburg (S cm−2 sn) nWarburg Rcoat (� cm2) Ycoat (S cm−2 sn) ncoat Ral (� cm2) Yal (S cm−2 sn) nal

Bare AA5083 5.801 ± 0.363 400.3 ± 22.5 4.921E−5 ± 3.1E−6 1 0.02116 ± 4.7E−4 0.474 ± 0.013 – – – – – –Coated AA5083 4.328 ± 0.157 – – – – – 4276 ± 214.38 2.640E−7 ± 3.978E-8 0.810 ± 0.013 26521 ± 4608 3.452E−5 ± 4.626E-6 0.578 ± 0.046

H. Rahimi et al. / Progress in Organic

F

4

cpfcasiyctp

A

rfiD

R

[

[

[

[

[

[

[

[

[

[

[

[

ig. 12. Equivalent circuits used for fitting data of (a) bare and (b) coated AA5083.

. Conclusion

The study demonstrates sol–gel coatings provided a physi-al barrier on AA5083 substrate for blocking the electrochemicalrocess. DoE methodology based on Taguchi method has beenound to be a very effective tool in the optimization of key pro-ess parameters. The results show that hydrolysis water contentmong other factors is the most significant one. Minimum corro-ion current density was obtained by using optimum parametersnclude organic/inorganic molar ratio (GPTMS/TEOS = 3/7), hydrol-sis water content (x = 12), drying temperature (D = 90 ◦C), anduring temperature (C = 130 ◦C). Electrochemical impedance spec-roscopy applied in this work allowed estimation of corrosionrotection properties of optimum hybrid sol–gel films.

cknowledgments

The authors would like to acknowledge Department of Mate-ial Engineering of Malek Ashtar University of Technology for thenancial support and also wish to thank Mr. Mohammad Ghanbariorabi for sample preparation.

eferences

[1] E.L. Rooy, in: S.R. Lampman, T.B. Zorc (Eds.), Properties and Selection: Nonfer-rous Alloys and Special-purpose Materials, vol. 2, ASM Handbook Press, 1993,p. 360.

[

Coatings 76 (2013) 307– 317 317

[2] Y. Chen, L. Jin, Y. Xie, Sol–gel processing of organic–inorganicnanocomposite protective coatings, J. Sol–Gel Sci. Technol. 13 (1998)735–738.

[3] T.L. Metroke, R.L. Parkhill, E.T. Knobbe, Passivation of metal alloysusing sol–gel-derived materials – a review, Prog. Org. Coat. 41 (2001)233–238.

[4] M. Niknahad, S. Moradian, S.M. Mirabedini, The adhesion properties and cor-rosion performance of differently pretreated epoxy coatings on an aluminiumalloy, Corros. Sci. 52 (2010) 1948–1957.

[5] D. Raps, T. Hack, J. Wehr, M.L. Zheludkevich, A.C. Bastos, M.G.S. Ferreira,O. Nuyken, Electrochemical study of inhibitor-containing organic–inorganichybrid coatings on AA2024, Corros. Sci. 51 (2009) 1012–1021.

[6] H. Rahimi, R. Mozafarinia, R.S.H. Razavi, E. Paimozd, A. Hojjati Najafabadi,Processing and properties of GPTMS-TEOS hybrid coatings on 5083 aluminumalloy, Adv. Mater. Res. 239-242 (2011) 736–742.

[7] D. Wang, G.P. Bierwagen, Sol–gel coatings on metals for corrosion protection,Prog. Org. Coat. 64 (2009) 327–338.

[8] F. Mammeri, E.L. Bourhis, L. Rozes, C. Sanchez, Mechanical proper-ties of hybrid organic–inorganic materials, J. Mater. Chem. 15 (2005)3787–3811.

[9] Y.J. Du, M. Damron, G. Tang, H. Zheng, C.J. Chu, J.H. Osborne, Inorganic/organichybrid coatings for aircraft aluminum alloy substrates, Prog. Org. Coat. 41(2001) 226–232.

10] B.P. Mosher, Synthesis and Characterization of Sol–gel NanocompositesDemonstrating Enhanced Mechanical Properties, MSc. Thesis, North CarolinaState University, 2006.

11] H. Wang, R. Akid, A room temperature cured sol–gel anticorro-sion pre-treatment for Al 2020-T3 alloys, Corros. Sci. 49 (2007)4491–4503.

12] Y.H. Han, A. Taylor, K.M. Knowles, Characterisation of organic–inorganic hybridcoatings deposited on aluminium substrates, Surf. Coat. Technol. 202 (2008)1859–1868.

13] T.L. Metroke, O. Kachurina, E.T. Knobbe, Specteroscopic and corrosion resis-tance characterization of GLYMO-TEOS Ormosil coatings for aluminum alloycorrosion inhibition, Prog. Org. Coat. 44 (2002) 295–305.

14] W.J. van Ooij, D. Zhu, V. Palanivel, J. Anna Lamar, M. Stacy, Overview, The poten-tial of silanes for chromate replacement in metal finishing industries, SiliconChem. 3 (2006) 11–30.

15] M.D. Soucek, A.H. Johnson, J.M. Wegner, Ternary evaluation of UV-curable seedoil inorganic/organic hybrid coatings using experimental design, Prog. Org.Coat. 51 (2004) 300–311.

16] S.K. Roy, R. Dey, A. Mitra, S. Mukherjee, M.K. Mitra, G.C. Das, Optimization ofprocess parameters for the synthesis of silica gel-WC nanocomposite by designof experiment, Mater. Sci. Eng. C. 27 (2007) 725–728.

17] V. Kakde, V. Mannari, Advanced chrome-free organic–inorganic hybridpretreatments for aerospace aluminum alloy 2024-T3-application ofnovel bis-ureasil sol–gel precursors, J. Coat. Technol. Res. 6 (2) (2009)201–211.

18] M. Zhou, Q. Yang, T. Troczynski, Effect of substrate surface modificationon alumina composite sol–gel coatings, Surf. Coat. Technol. 200 (2006)2800–2804.

19] D. Rajamani, Processing and Properties of Environmentally-friendly CorrosionResistant Hybrid Nanocomposite Coatings for Aluminum Alloy AA2024, MSc.Thesis, University of Cincinnati, 2005.

20] A. Phanasgaonkar, V.S. Raja, Influence of curing temperature, silicananoparticles- and cerium on surface morphology and corrosion behaviourof hybrid silane coatings on mild steel, Surf. Coat. Technol. 203 (2009)2260–2271.

21] R.L. Parkhill, E.T. Knobbe, M.S. Donley, Application and evaluation ofenvironmentally compliant spray-coated ormosil films as corrosion resis-

tant treatments for aluminum 2024-T3, Prog. Org. Coat. 41 (2001)261–265.

22] R.L. Parkhill, Investigation of Water-based Silica and Organically Modified Sil-icate Sol–gel Systems, Ph.D. Thesis, Oklahoma State University, Stillwater, OK,1999.