sio2 based hybrid inorganic–organic films doped with tio2...
TRANSCRIPT
Corrosion Science 51 (2009) 1998–2005
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Corrosion Science
journal homepage: www.elsevier .com/locate /corsc i
SiO2 based hybrid inorganic–organic films doped with TiO2–CeO2 nanoparticlesfor corrosion protection of AA2024 and Mg-AZ31B alloys
M. Zaharescu a, L. Predoana a, A. Barau a, D. Raps b, F. Gammel b, N.C. Rosero-Navarro c, Y. Castro c,A. Durán c, M. Aparicio c,*
a Institute of Physical Chemistry, ‘Ilie Murgulescu’ – Romanian Academy, 202 Splaiul Independentei, 060021 Bucharest, Romaniab EADS Innovation Works, 81663 Munich, Germanyc Instituto de Cerámica y Vidrio (CSIC), Campus de Cantoblanco, 28049 Madrid, Spain
a r t i c l e i n f o a b s t r a c t
Article history:Received 2 April 2009Accepted 15 May 2009Available online 24 May 2009
Keywords:A. AluminiumA. MagnesiumA. Metal coatingsA. Rare earth elementsB. EIS
0010-938X/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.corsci.2009.05.022
* Corresponding author. Tel.: +34 917355840; fax:E-mail address: [email protected] (M. Aparicio
Hybrid sol–gel coatings provide an approach as protective layers on metals. In this work, corrosion pro-tection of aluminium and magnesium alloys by SiO2-methacrylate coatings doped with TiO2–CeO2 nano-particles was studied. The films show an improvement of the barrier properties at initial immersion. Thereactivity of both alloys produces a deterioration of the protection with longer immersion, although TiO2–CeO2 nanoparticles let to observe signals of self-healing effect. Aluminium oxide/sol–gel interface wasfound to be stable. In combination with excellent paint adhesion on sol–gel films, these coatings canbe a promising alternative pre-treatment for high strength aluminium alloys prior to painting.
� 2009 Elsevier Ltd. All rights reserved.
1. Introduction thirds of aluminium. This has been a major factor in the wide-
Aluminium is the most widely used non-ferrous metal and itsproduction exceeded that of any other metal except iron. Relativelypure aluminium is used only when corrosion resistance and work-ability are more important than strength and hardness. Aluminiumreadily forms alloys with many elements like copper, zinc, magne-sium, manganese and silicon, and these alloys are important com-ponents for cars, aircrafts and rockets, especially due to their veryhigh strength to weight ratio. Several methods employing surfacetreatments like organic oxidation, anodic oxidation, organic coat-ings, and combinations thereof have been applied to protect alu-minium against corrosion. Up to now, among the abovementioned methods, the most effective and cost competitive formost applications has been the organic oxidation using chromatesolutions (chromium VI). However, chromate conversion coatingsare environmentally unfriendly and hardly hazardous to humanhealth (e.g., carcinogenic), so it is of high importance to find envi-ronmentally friendly replacements which also offer good corrosionprotection [1–3].
On the other hand, magnesium alloys development has tradi-tionally been driven by the aerospace industry due to the searchfor lightweight materials that can be used under increasinglydemanding conditions. Magnesium alloys have always been attrac-tive to designers due to their low density, which are only two-
ll rights reserved.
+34 917355843.).
spread use of magnesium alloy castings and wrought products.Others interesting properties of particular importance are highthermal conductivity, good dimensional stability, good electro-magnetic shielding characteristics, high damping characteristicand good machineability together with easy recycling. All theseproperties have recommended magnesium for a wide range ofapplications such as automotive and computer parts, aerospacecomponents, sport equipments and household equipment. Theuse of magnesium alloys in automotive industry can decrease thevehicle weight without affecting its strength [4]. However, the im-pact of these advantages is reduced by some major disadvantageswhich should be mentioned: poor corrosion and wear resistance,poor creep resistance and high chemical reactivity. Magnesiumand its alloys are extremely susceptible to galvanic corrosion.The corrosion resistance is much improved by using high purityalloys. Some other downsides are low elastic modulus, limited coldworkability and shrinkage at solidification [5]. A further require-ment in recent years has been acceptable corrosion behaviour,and dramatic improvements have been demonstrated for newmagnesium alloys. Improvements in mechanical properties andcorrosion resistance have led to greater interest in magnesiumalloys [6].
One of the most suitable alternatives for corrosion protectionpre-treatments is offered by the sol–gel technology which is envi-ronmentally compliant and compatible with the organic paintsused in most of the applications. The inorganic sol–gel coatings,which offer excellent barrier properties, unfortunately have to be
M. Zaharescu et al. / Corrosion Science 51 (2009) 1998–2005 1999
excluded in this case due to the requirement of high temperaturecuring for coatings densification, which are not suited for most ofthe aluminium or magnesium alloys. The most promising alterna-tive seems to be the development of inorganic–organic hybridsol–gel coatings, which offer increased flexibility and thicknessand allow curing at lower temperatures. Hydrolysis water ratio,organic content and solvent dilution play an important role influ-encing coating structure and its corrosion resistance [7–16]. Thecorrosion resistance of the hybrid sol–gel coatings is based on itsphysical barrier properties, so a homogenous crack-free materialis required. Small defects like scratches or pores which allow tothe electrolyte to reach the aluminium surface produce localizedcorrosion. A solution to avoid these problems is given by the useof inhibitors which have to protect the damaged zones by enablingthe formation of another protective layer by the so called ‘‘self-healing” effect. New sol–gel processes have been developed inorder to obtain better corrosion protection by including nanoparti-cles in the coating material as well as the use of inhibitors. Theintroduction of inorganic or organic inhibitors in the coating mate-rial can be a successful method of inducing a ‘‘self-healing” abilityto the coating. Derivates of triazole and thiazole have been mostlystudied for the case of cooper and its alloys but recently the studieshave been extended to the aluminium as well [17,18].
Rare earths like Ce and La are also very versatile inhibitors. Cer-ium nitrate proved to be an effective corrosion inhibitor and it isconsidered environmentally acceptable. The corrosion protectionmechanism was widely studied and it is generally accepted thatcerium ions leads to the precipitation of cerium oxides or hydrox-ides which hinder the cathodic reduction reaction. However, it isvery soluble and when it is inappropriately incorporated in thecoating material it can leach from the sol–gel film [19–22] or causeosmotic blistering. On the other hand, the use of nanoparticles asreinforcement material of the hybrid matrix proved to be one veryefficient method to increase the barrier properties and the resis-tance of the films [23,24].
Our objective in this work was to develop hybrid organic–inor-ganic coatings, based on a SiO2-methacrylate matrix with the addi-tion of TiO2–CeO2 nanoparticles with anticorrosive properties,deposited on Al and Mg alloys. The addition of the binary powderin the system should play two important roles: first, the powdernanoparticles, well dispersed in the hybrid matrix, should increasethe barrier properties of the produced coating resulting in a dense,crack free and pore free material. Secondly, cerium oxide from thepowder should play an inhibiting role and hinder the corrosionprocess.
2. Experimental
2.1. Synthesis and characterisation of nanoparticles and suspension
The inhibiting performance of the powders was tested bymeans of a drop test. Therefore, a cavity was mechanical milledinto the surface of an AA2024 sheet material with a cutter to pro-duce a circular cut with 12 mm diameter and 0.2 mm depth. Dis-tilled water with 3 wt% NaCl was used as electrolyte. 1 wt% ofthe nanoparticles were added to the electrolyte. A drop of the sat-urated electrolyte was put on the surface into the recess area of the
Table 1Composition and experimental conditions for the sol preparation.
Reagents Molar ratio
EtOHPPrecursors
H2OPPrecursors
65 mol% (TEOS) + 35 mol% (TSPM) 4 4
circular mill cut. NaCl electrolyte without nanoparticles was alsoused for comparison. Then, the samples were stored in an exsicca-tor with high humidity (a water reservoir was placed inside theexsiccator). After exposure times of 24, 48 and 72 h the electrolytewas removed, the sample was rinsed with pure water and driedwith a bellow. A light microscope was used to determine the num-ber of pits.
The coatings studied were produced using a mixture of tetra-ethyl orthosilicate (TEOS, ABCR, 98%) and 3-methoxysilylpropylmethacrylate (TSPM, ABCR, 98%) with addition of 5% non-function-alized powders (calculated with respect to the SiO2 content of thematrix) in the TiO2–CeO2 system (TiO2:CeO2 = 80:20 wt%), previ-ously prepared and described in another publication [25]. Thecomposition of the matrix and preparation conditions are pre-sented in Table 1. After this procedure, the sol was aged for 7 days.The powder was firstly dispersed in ethanol by alternative mixingand sonication and, then, added to the aged sol. A final aging stepwas applied to the suspension before deposition by stirring it for48 h at room temperature. The viscosity of the hybrid SiO2 sol(without nanoparticles) and the final suspension containing thenanoparticles (average of three measurements) was measuredusing a Brookfield DV-II + Pro viscometer at the moment ofpreparation.
2.2. Preparation of coatings
The deposition of the suspension was performed on AA2024bare and Mg-AZ31B alloys. AA2024 substrates were cleaned usingalkaline etching and an acid desmutting. Mg-AZ31B substrateswere cleaned using an ultrasound bath in three steps, rinsing indetergent solution, distillated water and ethanol. The depositionof single- and bi-layer coatings was realized by dip-coating witha withdrawal rate of 5 cm/min, each layer deposition being fol-lowed by a thermal treatment of 2 h at 120 �C. The sol–gel coatingsfor salt spray test and filiform corrosion test were deposited onAA2024 specimens with standard size of 150 � 80 mm by spraycoating of a single layer followed by the same thermal treatmentas for the dip coated samples. The paint used in this study for stan-dard corrosion tests on painted test specimens was a non-inhibitedwater-based epoxy primer (thickness 25 lm) and a water-basedepoxy topcoat (thickness 30 lm).
2.3. Characterisation of coatings
The textural properties of the sol–gel films deposited by dip-coating on cleaned silicon wafers were investigated by Transmis-sion Electron Microscopy using a TESCAN VEGA II LMU equipmentand by Atomic Force Microscopy based on the Dynamic Force Mod-ule/Intermittent contact mode, using a EasyScan 2 model from aNanosurf� AG Switzerland equipment. The film thickness wasdetermined with a profilometer Talystep (Taylor Hobson, VIC).
Electrochemical techniques, EIS and polarization curves, wereused to study the protection characteristics of the coatings. Theelectrochemical measurements were performed at room tempera-ture in a Faraday cage using a Gamry FAS2 Femtostat. A saturatedcalomel electrode (SCE) was used as the reference electrode, plat-inum as the counter electrode, and the coated alloy as the working
pH Reaction conditions
HNO3PPrecursors
T (�C) t (min)
0.016 3 60 90
Table 2Drop-test results of TiO2–CeO2 powders.
Exposure time 24 h 48 h 72 h
Number of pits 3 3 3
Fig. 2. Microscopic pictures of drop test samples after 72 h exposure to (a) 3 wt%NaCl solution and (b) 3 wt% NaCl solution doped with 1 wt% TiO2–CeO2 (80:20)binary powder.
2000 M. Zaharescu et al. / Corrosion Science 51 (2009) 1998–2005
electrode (with an area of approximately 3.15 cm2) in a conven-tional three-electrode cell. All the spectra were recorded at opencircuit potential with an applied 5 mV sinusoidal perturbationamplitude in a frequency range from 2 � 104 Hz down to 10�2 Hzat different immersion times in 0.3 wt% NaCl solution. Each valuewas obtained as the mean value of five measurements in a loga-rithmic sweep of frequencies (10 points per logarithmic unit).Impedance fitting was performed using appropriate equivalent cir-cuits by means of Gamry Echem Analyst software. The potentiody-namic polarization curves were carried out by applying a0.2 mVs�1 scanning rate. On the other hand, Neutral Salt Spray(NSS) testing was carried out according to ASTM B117 and filiformcorrosion test according to ISO 3665.
3. Results and discussions
3.1. Nanoparticles and suspension characterisation
The previously prepared TiO2–CeO2 (80:20) binary powder usedfor coatings preparation is presented in Fig. 1. The particles have asize of 2–3 nm, but with a high tendency of aggregation. The pow-der is quasi-crystalline and presents a high inhibition actionaccording to the drop-test results presented in Table 2. Fig. 2 showsmicroscopic pictures of the milled surface area after 72 h exposureto the respective electrolyte. Severe pitting corrosion can be foundon the samples exposed to pure 3 wt% NaCl solution (Fig. 2a),whereas only three pits evolve in case of the inhibitor doped elec-trolyte (Fig. 2b).
The viscosity of the hybrid SiO2 sol without and with binarypowder determined at the moment of their preparation is 3.0and 3.2 ± 0.2 cP, respectively. The viscosity of the studied solutionspresented a Newtonian behaviour. One may notice that the addi-tion of the nanopowders to the sol produces only a slight increaseof viscosity.
3.2. Characterisation of hybrid coatings on Mg-AZ31B alloy
In the experimental conditions presented above, homogeneousand defect-free coatings were obtained using both deposition pro-cesses. The presence of the particles was not strongly evidenced inthe film morphology. The thickness of the film prepared by dip-coating, as determined by profilometer using four different pro-files, was 0.88 ± 0.04 lm. The thickness of the sprayed coatingswas 5.0 ± 0.5 lm measured by eddy current. The roughness ofthe coating surface is presented in the AFM image of Fig. 3, show-ing a very low value that could be correlated to the homogenousdispersion of the powder in the bulk of the coating.
Electrochemical impedance spectroscopy measurements allowthe estimation of coating degradation and corrosion kinetic. TheEIS Bode plots at different immersion times of Mg-AZ31B substrateprotected with a two-layer TiO2–CeO2 doped hybrid coating and
Fig. 1. TEM micrograph and SAED image for the TiO2
bare Mg-AZ31B are compared in Fig. 4. The impedance spectra ofbare Mg-AZ31B alloy reveals one time constant around 10 Hzattributed to the charge transfer resistance of corrosion process.Other authors show the presence of one additional time constant
–CeO2 nanoparticles thermally treated at 400 �C.
Fig. 3. AFM image of a coating deposited on Si wafer (roughness of the coating:RMS = 1.39 � 1.40 nm).
Frequency (Hz)10-3 10-2 10-1 100 101 102 103 104 105
Zmod
( ohm
.cm
2 )
101
102
103
104
105
106
0.5 hour3 hours47 hoursBare, 2 hours
Frequency (Hz)10-3 10-2 10-1 100 101 102 103 104 105
Zphz
( º)
-80
-60
-40
-20
0
Fig. 4. EIS Bode plots for Mg-AZ31B alloys protected with a two-layer TiO2–CeO2
doped hybrid coating at different immersion times in 0.3 wt% NaCl, compared withthe bare substrate.
(a)
(b)
Fig. 5. Equivalent circuits used to fit the EIS spectra: (a) Bare Mg-AZ31B after 2 h ofimmersion; and (b) Mg-AZ31B with a two-layer TiO2–CeO2 doped hybrid coatingafter different immersion times.
M. Zaharescu et al. / Corrosion Science 51 (2009) 1998–2005 2001
at medium (or low) frequencies for long immersion times attrib-uted to relaxation of mass transport in the solid phase due to thegrowth of the corrosion product layer [26–29]. The presence ofthe TiO2–CeO2 doped hybrid coating promotes the increasing ofthe low frequency impedance by almost three orders of magnitudecompared to the bare alloy. After 30 min of immersion, the spec-trum shows the presence of two time constants. The time constant
with the maximum at around 1 � 103 Hz in the phase angle plotwas assigned to the hybrid film, and the another one at low fre-quency, 0.5 Hz, to the first signals of a charge transfer controlledprocess at the metal and hybrid coating interface that is not evi-dent yet. However, after only 3 h of immersion, the reduction ofimpedance reveals the deterioration of the protection system.The degradation continues with time up to 47 h. Nevertheless,the time constant at high frequencies associated with the sol–gelcoating remains. The time constant related to the corrosion processmoves to higher frequencies when the immersion time increases,approaching to the bare substrate.
Appropriate equivalent circuits (Fig. 5) based on the physical–chemical model of the corrosion process were used to study thecoating degradation and corrosion phenomena. Constant phaseelements (ZCPE) were included in the fitting instead of an ‘‘ideal”capacitor to simulate the non-ideal response of the EIS spectra.The ZCPE can be defined by ZCPE = (1/Y)/(jw)a which parametersare frequency (w), pseudo-capacitance (Y), and parameter a associ-ated to the system homogeneity. The equivalent circuit used forthe system is composed of two elements in series: a coating resis-tance, Rcoat, due to the formation of ionically conducting paths inthe coating, in parallel with coating pseudo-capacitance, Ycoat;and a pseudo-capacitance assigned to the double layer formed inthe metal–electrolyte interface, Ydl, in parallel with charge transferresistance, Rct. Rs is the resistance of the electrolyte, with very lowvalues being usually ignored. These equivalent circuits are used inseveral papers [28,29] to fit impedance spectra of magnesiumalloys. The results and errors from numerical fitting appear in theTable 3. The resistance and capacitance values associated withthe coating at initial immersion time reflect good barrier proper-ties, associated with the impediment of the electrolyte to reachthe metallic substrate. On the other hand, Rct is two orders of mag-nitude higher compared to bare substrate and Ydl is one order mag-nitude smaller emphasizing the initial barrier properties of thecoating. However, the exposure of coating to the electrolyte (3 h)decreases the barrier properties, reflected in the Rcoat drop in threeorders of magnitude. This behaviour is associated with the struc-ture of the coating, the porosity or cross-linking degree, whichcan promote ‘‘processes of water up-take” [30] and produce waterpaths through the coating with preferential accesses for solvatedions, accelerating the corrosion process. Subsequently, at 48 h ofexposure in the electrolyte, the barrier properties continue deteri-orating, and the resistance and capacitance associated with thecorrosion process are close to the bare substrate. The impedanceresults show good initial barrier properties but a much acceleratedcorrosion kinetic because of the high reactivity of the magnesium
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2002 M. Zaharescu et al. / Corrosion Science 51 (2009) 1998–2005
substrate, preventing the estimation of the possible self-healingproperties of TiO2–CeO2 nanoparticles. The polarization curves atinitial immersion time (Fig. 6) confirms these results showing cur-rent densities two orders of magnitude lower than bare alloy and adisplacement of corrosion potential to more noble values. Thepresence of the coating promotes passive regions of approxi-mately 200 mV (vs. Ref.). On the other hand, the increase in thethickness from one to two layers produces better barrier proper-ties, related with the difficulty of the electrolyte to reach themetallic substrate.
3.3. Characterisation of hybrid coatings on AA2024 alloy
Electrochemical response of AA2024 substrate protected withTiO2–CeO2 doped hybrid coating and bare AA2024 are comparedin Fig. 7. The EIS results of the bare alloy present two time con-stants at 30 and 0.015 Hz, assigned to the intermediate aluminiumoxide layer and the electron charge transfer process from corro-sion, respectively [19,31]. The incorporation of the hybrid coatingproduces an increase of the impedance modulus at 0.01 Hz inthree orders of magnitude as a consequence of the additional bar-rier functionality provided. The phase angle curve shows the pres-ence of a time constant at higher frequencies (104 Hz) associatedwith the hybrid sol–gel layer. The presence of a time constant atlower frequency than 0.01 Hz at initial immersion times indicatesthe first signals of corrosion activity, as a consequence of the por-ous structure of the sol–gel coating [19]. However, this propertycould be adequate for the inhibitors diffusion from nanoparticlesand development of self-healing effect. The increase of the immer-sion time produces a deterioration of the corrosion protection sys-tem. At 28 h of immersion, the total impedance decreases as asignal of degradation. The impedance plot shows additionally aWarburg element at lower frequency, suggesting the presence ofdiffusion processes of ions through the interconnected pores inthe coating [19,32,33].
The reduction of phase angle of the higher frequency time con-stant with immersion time indicates a less capacitive responsedue to the solution permeation through the pores of the hybridcoating and aluminium oxide layer. Above 28 h of immersion,the time constant associated to aluminium oxide (around100 Hz) shows an increase in the phase angle. This behaviourcould be associated to the influence of inhibitors from TiO2 toCeO2 nanoparticles in the formation of a more adherent and stablealuminium oxide layer on the alloy surface by the sealing of theircracks.
Fig. 6. Polarization curves obtained for Mg-AZ31B alloys with one and two-layerTiO2–CeO2 doped hybrid coatings in 0.3 wt% NaCl, after 30 min of immersioncompared with the bare substrate.
(a)
(b)
(c)
(d)
Fig. 8. Equivalent circuits used to fit the EIS spectra: (a) Bare AA2024 after 2 h ofimmersion; (b) AA2024 with a two-layer TiO2–CeO2 doped hybrid coating after0.5 h of immersion; (c) AA2024 with a two-layer TiO2–CeO2 doped hybrid coatingafter 28 and 70 h of immersion and (d) AA2024 with a two-layer TiO2–CeO2 dopedhybrid coating after 142 and 192 h of immersion.
Fig. 7. EIS Bode plots for AA2024 alloys protected with a two-layer TiO2–CeO2
doped hybrid coating at different immersion times in 0.3 wt% NaCl after 30 min ofimmersion, compared with the bare substrate.
M. Zaharescu et al. / Corrosion Science 51 (2009) 1998–2005 2003
In order to confirm this behaviour, the EIS spectra were fitted toequivalent circuits (Fig. 8). In these models, the elements are thesame than those used in the case of Mg-AZ31B substrate. Thenew elements, Rox and Yox, denote the resistance and pseudo-capacitance of the thin natural aluminium oxide, respectively. Also,the Warburg element (W) was used in the equivalent circuits torepresent the ions diffusion under semi-infinite conditions [34].The initial value of Rcoat and Ycoat (Table 4) reflect quite good bar-rier properties. However, after only 28 h, the Rcoat value decreasesin two orders of magnitude and the Ycoat value increases in one or-der of magnitude, probably due to structural changes occurring inthe network structure because of the water uptake [30]. Further in-crease of immersion time leads to a higher deterioration of thecoating, reflected in the drop of Rcoat value and the raise of Ycoat
value. At 142 and 192 h the contribution of the coating is notobserved. On the other hand, Rct after 0.5 h of immersion is threeorders of magnitude higher compared to bare substrate. Theself-healing effect provided by the TiO2–CeO2 nanoparticles is par-tially masked by the high reactivity of the aluminium substrate in aNaCl solution. However, the stability of Yox values at immersiontimes above 28 h and the decreasing of Ydl at 70 and 142 h ofimmersion could be associated with the inhibition activity of thenanoparticles. The diffusion of Ce and Ti ions from the nanoparti-cles to the corrosion sites and the reaction with hydroxyl ions fromthe water reduction in cathode to produce hydroxide–oxides couldbe the base of the inhibition mechanism involved in the process.The precipitation of Ti and Ce hydroxide–oxides combined withthe aluminium oxide from corrosion reaction would be the expla-nation of the Yox values stability. The precipitation on the cathodicsites originates an area decreasing and, consequently, a reductionof Ydl.
The TiO2–CeO2 containing sol–gel coatings deposited onAA2024 test specimens with standard size of 150 � 80 mm bymeans of spray coating were tested using standard tests such asneutral salt spray test (ASTM B117) and filiform corrosion testaccording to ISO 3665. Fig. 9 shows the salt spray test results ofthe inhibitor doped sol–gel coating after 48 h (a) and 168 h (b) testduration (tested without additional organic coat). The coatingdemonstrates reasonable barrier properties with no defects after48 h, however, some pitting after 168 h and a stronger corrosive at-tack after longer test durations can be observed. As describedabove, the sol–gel films are developed as a replacement of chro-mate containing pre-treatments such as chromate conversion coat-ing. Although, the coating does not fulfil the requirement accordingto MIL 5541E, it is a very promising approach towards replacementof chromate conversion coatings.
In addition to the salt spray test on bare samples, acceleratedtests were carried out on fully painted specimens as described inthe experimental section. The adhesion of the organic paint filmon the sol–gel hybrid coating is found to be superior. Cross-cut testresults show excellent paint adhesion and no blistering occurredon the test panels after 1000 h in Q-lab condensation test. Fig. 10shows a picture of a scratched test panel after 1500 h in the saltspray test. It can be observed, that the active corrosion perfor-mance of the inhibited sol–gel coating is not high enough to pro-tect the 1 mm wide scratch from corrosion. Taking into accountthe low film thickness of 5 lm of the sol–gel film and the employ-ment of a non-inhibited primer, this observation is not surprising.The protection of the interface area of the aluminium/sol–gel inter-face however, is adequate. No sub-surface migration is detectedafter 1500 h of salt spray test, revealing good adhesion and inhib-iting action of the sol–gel coating. Furthermore, filiform corrosion
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Fig. 9. Pictures of AA2024 alloy spray coated with TiO2–CeO2 doped hybrid coatingsafter 48 h (a) and 168 h (b) salt spray test according to ASTM B117 (scale in cm).
Fht
Fhc
2004 M. Zaharescu et al. / Corrosion Science 51 (2009) 1998–2005
test was carried out on the same kind of coating systems. Fig. 11shows good results with a maximum filament length of 2 mmmeasured after 960 h test duration. This can be stated as verypromising since a non-inhibited primer and topcoat was usedas organic paint system on top of the sol–gel film.
4. Conclusions
Corrosion protection of aluminium (AA2024) and magnesiumalloy (AZ31B) with hybrid inorganic–organic coatings based on
ig. 10. Picture of a scribed AA2024 test panel spray coated with TiO2–CeO2 dopedybrid coatings and non-inhibited epoxy primer and topcoat after 1500 h salt sprayest according to ASTM B117 (scale in cm).
ig. 11. Picture of a scribed AA2024 test panel spray coated with TiO2–CeO2 dopedybrid coatings and non-inhibited epoxy primer and topcoat after 960 h filiformorrosion test according to ISO 3665 (scale in cm).
M. Zaharescu et al. / Corrosion Science 51 (2009) 1998–2005 2005
SiO2-methacrylate matrix with addition of TiO2–CeO2 nanoparti-cles prepared by sol–gel method was performed. The coatings aredefect-free and show thickness around 0.9 lm (dip-coatings) and5.0 lm (spray coatings). The polarisation curves and Electrochem-ical Impedance Spectroscopy (EIS) results obtained for both alloysindicate the passivation of the substrates at initial immersiontimes providing a barrier, blocking the electrochemical process.The increase of immersion time produces the degradation of thecoating on both substrates because of the presence of small de-fects. However, in the case of AA2024 alloy, the stability of Yox
(constant phase element associated with the thin natural alumin-ium oxide) values at immersion times above 28 h and the decreas-ing of Ydl (constant phase element associated to the double layerformed in the metal–electrolyte interface) at 70 and 142 h ofimmersion could be related with the inhibition activity of thenanoparticles. On the other hand, it is necessary to take into ac-count that these sol–gel coatings would be only a pre-treatmenttrying to provide a self-healing functionality, and a painting willbe crucial to reach an adequate corrosion protection. Corrosiontests on fully painted test samples show a very stable aluminiumoxide/sol–gel interface. In combination with excellent paint adhe-sion on the sol–gel hybrid films, these coatings can be a promisingalternative pre-treatment for high strength aluminium alloys priorto painting.
Acknowledgements
Authors acknowledge the funding provided by the EuropeanCommunity, MULTIPROTECT project: ‘‘Advanced environmentallyfriendly multifunctional corrosion protection by nanotechnology”,Contract No. NMP3-CT-2005-011783. The authors thank LauraPeláez and Eva Peiteado their assistance with the experimentaltechniques.
References
[1] A. Baral, R. Engelken, W. Stephens, J. Farris, R. Hannigan, Arch. Environ.Contam. Toxicol. 50 (2006) 496–502.
[2] A. Caglieri, M. Goldoni, O. Acampa, R. Antreoli, M.V. Vettori, M. Corradi, P.Apostoli, A. Mutti, Environ. Health Persp. 114 (4) (2006) 542–546.
[3] J.H. Duffus, Mineral. Mag. 69 (5) (2005) 557–562.[4] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88–113.[5] B.L. Mordeke, T. Erbert, Mat. Sci. Eng. A 302 (2001) 37–45.[6] L. Duffy, Mater World 4 (1996) 127–130.[7] T.L. Metroke, O. Kachurina, E.T. Knobbe, Prog. Org. Coat. 44 (2002) 295–305.[8] S.K. Poznyak, M.L. Zheludkevich, D. Raps, F. Gammel, K.A. Yasakau, M.G.S.
Ferreira, Prog. Org. Coat. 62 (2008) 226–235.[9] Y. Liu, D. Sun, H. You, J.S. Chung, Appl. Surf. Sci. 246 (2005) 82–89.
[10] M.L. Zheludkevich, I. Miranda Salvado, M.G.S. Ferreira, J. Mater. Chem. 15(2005) 5099–5111.
[11] J. Gallardo, A. Duran, I. Garcia, J.P. Celis, M.A. Arenas, A. Conde, J. Sol-Gel Sci.Technol. 27 (2003) 175–183.
[12] K.H. Wu, M.C. Li, C.C. Yang, G.P. Wang, J. Non-Cryst. Solids 352 (2006) 2897–2904.
[13] R. Zandi-zand, A. Ershad-langroudi, A. Rahimi, J. Non-Cryst. Solids 351 (2005)1307–1311.
[14] M. Fir, B. Orel, A.S. Vuk, A. Vilcnik, R. Jese, V. Francetic, Langmuir 23 (2007)5505–5514.
[15] T.L. Metroke, O. Kachurina, E.T. Knobbe, Prog. Org. Coat. 44 (2002) 185–199.[16] S.S. Pathak, A.S. Khanna, T.J.M. Sinha, Prog. Org. Coat. 60 (2007) 211–218.[17] M.L. Zheludkevich, K.A. Yasakau, S.K. Poznyak, M.G.S. Ferreira, Corros. Sci. 47
(2005) 3368–3383.[18] V. Palanivel, Y. Huang, W.J. van Ooij, Prog. Org. Coat. (2005) 153–163.[19] N.C. Rosero-Navarro, S.A. Pellice, A. Duran, M. Aparicio, Corros. Sci. 50 (2008)
1283–1291.[20] R. Supplit, T. Koch, U. Schubert, Corros. Sci. 49 (2007) 4491–4503.[21] A. Pepe, M. Aparicio, S. Cere, A. Duran, J. Non-Cryst. Solids 348 (2004) 162–171.[22] V. Moutarlier, B. Neveu, M.P. Gigandet, Surf. Coat. Tech. 202 (2008) 2052–2058.[23] Y. Chen, L. Jin, Y. Xie, J. Sol-Gel Sci. Technol. 13 (1998) 735–738.[24] Q. Chen, J.G.H. Tan, T.C. Shen, Y.C. Liu, W.K. Ng, X.T. Zeng, J. Sol-Gel Sci. Technol.
44 (2007) 125–131.[25] M. Zaharescu, V.S. Teodorescu, A. Brau, C. Andronescu, N. Preda, F. Papa, in: J.G.
Heinrich, C. Aneziris (Eds.), Proc. 10th Ecers. Conf, Göller Verlag, Baden-Baden,2007, pp. 1839-1841.
[26] G. Baril, C. Blanc, N. Pébère, J. Electrochem. Soc. 148 (2001) B489–B496.[27] F. Zucchi, A. Frignani, V. Grassi, A. Balbo, G. Trabanelli, Mater. Chem. Phys. 110
(2008) 263–268.[28] M.F. Montemor, M.G.S. Ferreira, Electrochim. Acta 52 (2007) 7486–7495.[29] S.V. Lamaka, M.F. Montemor, A.F. Galio, M.L. Zheludkevich, C. Trindade, L.F.
Dick, M.G.S. Ferreira, Electrochim. Acta 53 (2008) 4773–4783.[30] M. Grosso, J. Vogelsang, L. Fedrizzi, F. Deflorian, Prog. Org. Coat. 37 (1999) 69–
81.[31] M.L. Zheludkevich, R. Serra, M.F. Montemor, I.M. MirandaSalvado, M.G.S.
Ferreira, Surf. Coat. Tech. 200 (2006) 3084–3094.[32] G.W. Walter, Corros. Sci. 26 (1986) 681–703.[33] S.V. Lamaka, M.L. Zheludkevich, K.A. Yasaka, R. Serra, S.K. Poznyak, M.G.S.
Ferreira, Prog. Org. Coat. 58 (2007) 127–135.[34] S. Skale, V. Dolec�ek, M. Slemnik, Corros. Sci. 49 (2007) 1045–1055.