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Surface & Coatings Technology
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Ceria/stannate multilayer coatings on AZ91D Mg alloy
P. Bagalà a,b, F.R. Lamastra b, S. Kaciulis c, A. Mezzi c, G. Montesperelli a,b,⁎a Dipartimento di Ingegneria Industriale, Università di Roma — Tor Vergata, Rome, Italyb Consorzio INSTM — Unità di Ricerca di Roma — Tor Vergata, Italyc ISMN — CNR, Monterotondo Stazione, Rome, Italy
⁎ Corresponding author. Tel.: +39 06 7259 4494; faxE-mail address: [email protected] (G. M
In this work, CeO2/stannate multilayer coatings on AZ91D magnesium alloy were successfully obtained bychemical conversion and sol–gel dip coating. The stannate conversion coatings were prepared from astannate aqueous bath containing Na2SnO3, CH3COONa, Na3PO4 and NaOH at different temperatures and im-mersion times. Ceria films were produced on stannate/AZ91D starting from Ce(III) nitrate solutions in H2O. Insome cases, the PVA was added as chelating agent. Ceria top coatings were fired at 200 °C for 1 h. Coating mi-crostructure was examined by FE-SEM. Finally, the corrosion resistance features of the coatings were testedby the electrochemical impedance spectroscopy (EIS) in 3 wt.% NaCl solution. The effect of PVA addition wasevaluated in terms of microstructure and corrosion resistance features. CeO2/stannate multilayer films, 3 μmthick, uniform, well adherent and nearly crack free were obtained. The formation of CeO2 phase was con-firmed by XRD and XPS analyses. The XPS depth profiles showed a limited diffusion of Mg towards the ceram-ic film. The EIS tests showed a significant improvement of corrosion resistance of the multilayer coatings(~16.6 kΩ after 48 h in NaCl solution) with respect to the blank alloy (~2.4 kΩ after 48 h in NaCl solution).
Magnesium alloys are extremely attractive for aerospace and au-tomotive applications due to their ultra lightness and high strengthto weight ratio. However, the low corrosion resistance in wet envi-ronments is still a limiting factor against their widespread diffusion[1,2].
The corrosion resistance was generally provided by surface treat-ments or ceramic coating. Prior to any surface treatments, it is ex-tremely important to do an appropriate cleaning and pretreatmentof the Mg alloy, because of its high reactivity, that promotes in airthe quick formation of an oxide/hydroxide layer on its surface. Sur-face treatments include electrochemical plating, anodizing andchemical conversion. Electrochemical plating is a simple and lowcost process to produce metallic coatings on Mg alloys, such asCu\Ni\Cr plating, that is employed to protect these alloys in interi-or and mild exterior environments [3]. Anodizing, is an alternativeelectrolytic process to develop stable oxide films. These coatingswere often used as primer for a further protective layer [4]. Chemicalconversion coatings, finally, act as low soluble physical barrier be-tween metal surface and environment. In the past, chromate conver-sion coatings have been used as conventional method to protect Mgalloys [5]. Unfortunately, exavalent chrome is highly toxic and can-cerous and for this reason chromium compounds have been banned
: +39 06 7259 4328.ontesperelli).
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by many applications [6,7]. Some alternative chemical conversioncoatings include phosphate–permanganate [8–11], fluoride [12,13],stannate [14–17], and rare earth [18] compounds. Among them,some are also used as primer for further coatings [12,19,20] andthe stannate seems to be the most promising [21,22].
The coating techniques, mostly used to produce ceramic films, aresputtering [23], physical and chemical vapor deposition [24–26],sol–gel [27,28], etc. [29]. Traditional PVD and CVD usually operateat temperatures greater than those normally considered for the sta-bility of Mg alloys (b280 °C) and cannot be used in their traditionalconfiguration.
The sol–gel coating is an inexpensive and easily exploitable in-dustrial process to prepare thin oxide films at low temperatureswith high purity and homogeneity on large area substrates. Anyway,since sol–gel usually uses water as solvent, its application for coatingdeposition on Mg has strong limitations due to the reactivity of Mg,that could react with the sol to give side reactions. In order to bypassthis problem, some researchers have developed multilayer filmsthrough a combination of sol–gel method and other techniques,such as anodizing [30] and chemical conversion. Zhong et al. coatedsome magnesium alloys with a fluoride conversion coating, beforeapplying the sol gel ceria film [12]. Instead, Chen et al. implementeda multilayer coating composed of a cerium based conversion coatingand a sol gel ceria film [19]. Zhang et al. have rather used as interlayera phytic acid conversion coating [20]. Finally, Qing Li et al. employeda stannate conversion coating as primer for a next ZrO2 sol gel coat-ing [22]. Anyway, so far the values of corrosion resistance of the filmsproduced are not very satisfactory and in addition the heating
Coating Label Dipping conditions in the stannatebath (°C, min)
Ceria precursor sols
Stannate S1 40 °C, 60 min –
S2 60 °C, 15 min –
S3 60 °C, 30 min –
S4 60 °C, 60 min –
S5 80 °C, 15 min –
S6 80 °C, 30 min –
S7 80 °C, 60 min –
Stannate/ceria
S7C1 80 °C, 60 min Sol1: (Ce(NO3)3 in H2O)S7C2 80 °C, 60 min Sol2: (Ce(NO3)3 and PVA
in H2O)
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temperature used is higher (300 °C or more) than the stability limitof many Mg alloys.
On the other hand, to achieve a uniform, well adherent and crack-free coating, an appropriate surface treatment or finishing of the sub-strate, a stable sol and a correct thermal treatment of the sol–gel pre-cursor must be carried out. Particularly, it is well known that uniformfilms can be derived from stable sols containing weakly branched lin-ear polymers [31]. It has been reported that PVA is able to chelatemetal ions due to its hydroxyl groups. Cations may have a directlink to the hydroxyl groups or they may be localized around the poly-mer by the bridging action of the water molecules [32]. During pre-cursor processing, the water evaporates from solution and the freespace between the polymer molecules shrinks. As a result, the cationmobility is greatly reduced and the water remaining in the precursor,keeps all the cations in the entanglement polymer network [31].Moreover, PVA in an aqueous solution, not only acts as chelatingagent but physically prevents precipitation by sterically inhibitingthe interaction among dissolved ions [32].
Among the different ceramics for protective coatings, ceria seemsto have the most promising characteristics. It is reported that ceriahas excellent mechanical and anticorrosion properties, easiness andlow cost of deposition process and is usually made with green precur-sor [33–35]. In this paper, a new multilayer coating, consisting ofstannate conversion film followed by sol–gel deposited ceria layerwas successfully prepared on AZ91D alloy, with the aim to improveits corrosion resistance. In order to enhance the growth and the adhe-sion of the stannate layer, an acid etching was also performed on thealloy. The sol–gel deposition of the ceria layer was carried out usingCe(NO3)3 as precursor. The precursor was selected on the basis ofmain literature [12]. In fact, Zhong et al. obtained sol gel ceria thinfilms on fluorinated AZ91D magnesium alloy, with improved corro-sion resistance, by using Ce(NO3)3 as precursor in ethanol solution.The thickness of a sol–gel coating is a critical process parameter. Itis reported that below a certain critical value, the sol–gel films toler-ate the drying stresses without cracking, but irremediably crack spon-taneously above that thickness [36]. For this reasons, the ceriadepositions were performed starting from the same precursor usingwater as solvent in place of ethanol to verify if its influence on viscos-ity and surface tension, may affect the coating microstructure andsubstrate coverage.
In some case, polyvinyl alcohol (PVA) was added to the precursorsolution as chelating agent. Finally, it is noteworthy that all the pre-cursors used in this innovative method of coating deposition, arecheap and environmentally friendly [37] and that the treatment tem-peratures were low enough (max 200 °C) to avoid alloy instability.Microstructure of the coatings was examined by FE-SEM, their surfacechemical composition was investigated by XPS and phase analysiswas performed by XRD. Finally, the corrosion resistance was testedby the electrochemical impedance spectroscopy (EIS) in 3 wt.% NaClsolution.
2. Materials and methods
2.1. Substrate preparation
AZ91Dmagnesium alloywith a chemical composition of 9.0 wt.% Al;0.7 wt.% Zn; b0.13 wt.% Mn; b0.1 wt.% Si; b0.005 wt.% Fe; b0.002 wt.%Ni; b0.030 wt.% Cuwas used as a substrate. Specimenswith dimensionsof 15×20×1 mm3 were mechanically polished with SiC papers of suc-cessively finer grit down to 2500 and thenwith diamond paste down to3 μm. The polished samples were cleaned in ethanol.
2.2. Stannate conversion coatings
Before deposition, samples were etched in a HCl solution (0.5 wt.%)for 5 s, rinsed with water and immediately immersed in an aqueous
COONa·3H2O (5.98 g/l, Sigma), Na3PO4·12H2O (21.32 g/l, Aldrich)and NaOH (2.00 g/l, Carlo Erba Reagenti) [17]. In order to optimizethe procedure, different temperatures (40, 60 and 80 °C) and immer-sion times (15, 30 and 60 min) were used. The conditions of depositionof stannate layer and the labels used in this paper are summarized inTable 1.
2.2.1. Ceria coatingsDifferent precursor sols were tested. Sol1 was prepared by adding
Ce(NO3)3·6H2O (Aldrich) 13% g/ml in aqueous solution. The solutionwas aged for 3 days, at room temperature and under stirring. Sol2was obtained dissolving Ce(NO3)3·6H2O (13% g/ml, Aldrich) andPVA granules (1 wt.%, Mw 13,000–23,000, Aldrich) in H2O at 80 °Cfor 2 h, under magnetic stirring, and aging the resulting solution for3 days at room temperature. Table 1 summarizes all coatings andtheir labels. Since stannate sample S7 (1 h at 80 °C) gave the mostpromising results, multilayer samples were prepared from this sam-ple. Sample S7 were dipped in the sols, dried in air for 24 h and ther-mal treated in air at 200 °C for 1 h with a heating rate of 2 °C/min anda dwell time at 90 °C of 2 h to remove the solvent. The procedures ofdipping, drying and heating were repeated up to three times in orderto completely cover the stannate layer and reach an appropriatethickness of the ceria coatings.
2.3. Characterizations
2.3.1. SEM/EDSMicrostructure and chemical composition of the coatings were
examined using a Field Emission-Scanning Electron Microscope(FE-SEM, Leo Supra 35) equipped with an energy-dispersive X-rayspectrometer (EDX, INCA Energy 300, Detector Oxford ELXII). ForSEM observations of the cross-sections, the specimens were firstcooled in liquid nitrogen and subsequently fractured. All the sampleswere gold coated.
2.3.2. XPS/XRDSurface chemical composition was investigated by X-ray photo-
electron spectroscopy (XPS). Photoemission spectra were collectedby using an ESCALAB MkII (VG Scientific) spectrometer, equippedwith a standard Al Kα excitation source and a 5-channeltron detec-tion system. All the experiments were performed at a base pressureof about 1×10−8 Pa that has been increased up to 1×10−5 Pa duringthe depth profiling. The energy of Ar+ ion gun has been set to 2.0 keVand sample current density to 2×10−3 mA cm−2 that corresponds toan average sputtering rate of about 0.3 nmmin−1. The binding ener-gy (BE) scale was calibrated by setting the C 1s peak of adventitiouscarbon (surface contamination) to BE=285.0 eV and Au 4f7/2 peak(sample mask) to BE=84.0 eV. More experimental details havebeen reported elsewhere [38].
X-ray diffraction (XRD) (Philips X'Pert 1710, Cu-Kα radiationλ=1.5405600 Å, 2θ=10–80°, step size=0.020°, time per step=2 s,scan speed=0.01°/s)measurementswere performed on both bare sub-strate and thermally treated multilayer coatings.
2.3.3. Scratch testsThe adhesion of coatings to substrates has been evaluated by scratch
test. Tests have been performed on samples using the Revetest ScratchTester (RST) from CSM Instruments. A diamond spherical–conical in-denter 100 μm in diameter was used, applying a progressive loadingfrom 0.9 to 6 N at a loading rate of 10.2 N/min. Two critical loads weredetermined: Lc1, the normal load at which the first adhesion failure(first cracks) appears on the film, and Lc2, the normal load at whichthe first delamination occurs on the coating. Three tests have been per-formed on each sample.
2.3.4. Electrochemical impedance spectroscopy (EIS)EIS was performed in 3 wt.% NaCl aqueous solution at room tem-
perature by using a Solartron SI 1287 Electrochemical Interface anda Solartron 1260 Frequency Response Analyzer at the OCP, in the fre-quency range from 10−2 Hz to 105 Hz, imposing an alternating signalof 10 mV. A standard cell for flat specimen with an O-ring was used.The exposed area was 0.91 cm2. A platinum plate and an Ag/AgCl
Fig. 1. A selection of SEM:micrographs of stannate coatings on AZ91 alloy at different con-ditions. a. Sample S1 (40 °C, 60 min, 50 kx). b. Sample S5 (80 °C, 15 min, 5 kx). c. SampleS7 (80 °C, 60 min, 50 kx).
electrode were used as counter and reference electrode, respectively.EIS spectra were acquired by Zplot and were analyzed by Zview soft-ware (Scribner Associates), using different equivalent circuits and NonLinear Least Squares (NLLS) method and performing chi-squared test(χ2) to evaluate the goodness of fit. Blank tests were also performed.
3. Results and discussion
3.1. Microstructure
A selection of SEMmicrographs of stannate coatings obtained at dif-ferent bath temperatures is shown in Fig. 1. A homogeneous micro-structure, characterized by spherical granules of uniform dimensionwas observed in all cases. Moreover, granule size and coverage in-creased noticeably with temperature and with immersion time. Dip-ping times shorter than 1 h and temperatures lower than 80 °C didnot allow to obtain a total coverage of the surface as shown in Fig. 1aand b. Fig. 1c shows the microstructure of sample prepared at 80 °Cfor 1 h (sample S7). Granule size smaller than 1 μm and total coverageof the surface were observed. As a consequence of this microstructure,the presence of a widespread roughness was observed. Due to thistype of microstructure (i.e. characterized by roughness and porosity)the corrosion resistance provided by stannate filmswas not satisfactory(see Section 3.3) and the need of an additional coating was confirmed.Since sample S7 showed the best coverage, all the multilayer coatingswere deposited starting from this sample. Fig. 2a shows the SEMmicro-graph of the S7C1 multilayer coating. The morphology is fairly uniformafter 3 sol–gel depositions of ceria, but in many places the microstruc-ture of the top layer is heavily influenced by the underlying layer.Fig. 2b shows the microstructure of sample S7C2 in which 1% of PVAwas added for sol preparation (Sol2). This has resulted in a homoge-neous and totally covered surface with reduced roughness and pore-free. Some cracks were observed somewhere. The multilayer coatingsdescribedwere obtained by repeating the procedures of dipping, dryingand heating three times in order to completely cover the substrate. Thetypical microstructure of samples with a single ceria deposition isshown in Fig. 3 for sample S7C2. The EDS analysis revealed the presence
Fig. 2. SEM micrographs of S7C1 (a, 5 kx) and S7C2 (b, 5 kx) multilayer coatings onAZ91 alloy after 3 sol–gel depositions of ceria.
Fig. 3. SEM micrograph of S7C2 multilayer coating after first deposition of ceria (5 kx).The sample shows a partial ceria deposition (white circular spots).
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of somewhite spots with a high concentration of cerium. Outside thesespots, the cerium concentration was negligible. It can be concluded thata partial coverage was obtained.
After the deposition, the samples were heat treated at 200 °C.Some samples were treated at 150 °C for 1 h, but the quality of ceriafilms was not satisfactory because of the poor coverage.
Fig. 4a reports the SEM micrograph of sample S7C2 cross sectionobtained by fragile fracture (substrate on the left and coating on theright). The interface shows a homogeneous, dense, pore-free andwell adherent film with an apparently uniform thickness of about3 μm.
From the analysis of Fig. 4 (and other cross sections carried out)no cracks were detected and it may be concluded that the crack ex-tension is only on the surface.
The EDS line analysis carried out on the interface, allows to deter-mine thicknesses of stannate and ceria of 1.2 and 2.5 μm respectively,with a diffusion layer of about 700 nm (Fig. 4b). However, the thick-ness is very variable due to the high roughness of stannate layer. Any-way, several cross sections were carried out and they confirmed thatthe reported value is significant. Moreover, the EDS spectra evidenced
Fig. 4. SEM micrograph of S7C2 film cross section
a relative low carbon concentration within ceria coating, mainly con-centrated towards the interface between stannate and ceria. EDS datasuggest that a partial decomposition/combustion of PVA occurredduring the thermal treatment at 200 °C. The cross section was alsoperformed on sample S7C1. Except for a difference in the thickness(approximately 2.5 μm for S7C1), no appreciable differences wereobserved.
3.2. Scratch tests
The results of the scratch tests are summarized in Fig. 5, in whichthe critical loads LC1 and LC2 are reported for samples S7C1 and S7C2.The values reported are the average of three tests. The values of crit-ical load for the stannate-coated sample (S7) are also reported for adirect comparison. Sample S7C1 showed the highest critical load.Both multilayer samples (S7C1 and S7C2) showed higher valuesthan S7. Since the critical load is related to coating adhesion, it maybe concluded that the sample S7C1 showed better adhesion andscratch resistance compared to S7C2, for which the addition of PVAhas a negative effect. However, both samples have a better scratch re-sistance of the sample S7.
3.3. XPS/XRD results
The surface chemical composition of the samples S7C1 and S7C2(without and with PVA) is given in Table 2. The components of C 1speak are attributed to C1—aliphatic carbon (C\C and C\H bonds), C2—alcohol or carbonyl bonds and C3—carboxyl bonds of the type O_C\OH.The O 1s peak is composed of two components: O1—oxygen in MgO andadsorbedH2O; O2\CeO and Sn oxides. The BE values ofMg 1s andMg 2ppeaks correspond to MgO [39], whereas the position of Ce 3d5/2 peak atBE=883.5–883.8 eV and the presence of strong shake-up satellites andPr(+5)-like states indicate the chemical state of Ce(+4) [40,41]. A typi-cal spectrum of Ce 3dwith strong Ce(+4) satellites is presented in Fig. 6.It should be noted that the shape of Ce 3d spectrum was not changing
significantly during XPS depth profiling, i.e. the main Ce(+4) state wasmaintained through the thickness of the coating.
The depth profiles of the samples, prepared without and with PVA,are reported in Figs. 7 and 8, respectively. In the first profile (Fig. 7),the concentrations of O1 and O2 (components of O 1s peak) are follow-ing the trend of Mg and Ce concentrations, respectively. The concentra-tion of contaminant carbon goes down to zero at the depth of aboutd=15 nm, at the same depth starts to appear the signal of Sn 3d) atBE=486.7 eV, which corresponds to Sn(+4) [42]. It is observed thatthe segregation ofMgOon the surface layer of the thickness is compara-ble with the thickness of carbon contamination. In the second sample(with PVA), the content of carbon is high (≈35%) and almost constanttill the depth of≈80 nm. This fact is only in apparent contradictionwiththe EDS analysis shown in Fig. 4. EDS and XPS have different sensitivi-ties (in particular for carbon) and very different analysis depth: about1 μm for EDS and only few nm for XPS. Therefore, the EDS results reflecta greater sampling volume in comparisonwith theXPS and aremore af-fected by the dishomogeneity of the sample.
In this sample, also the concentrations of Ce,Mg and total oxygen areconstant through the depth profile. It is worth to note that the O2/Ceratio is equal to the stoichiometric value of CeO2. Moreover, due to thepresence of numerous cracks (see Fig. 2b) the signal of stannate (Sn)was also detected, which is absent in more uniform sample S7C2.
Fig. 9a shows the comparison between stannate film (S7) and basealloy AZ91D XRD patterns. The reflections of the α-phase [43] (Mgsolid solution) and β-phase (Mg17Al12 compound) were observedin both samples, with the Mg solid solution being the principal one.Moreover the main peaks of the MgSn(OH)6 phase are clearly detect-able in the XRD pattern of the coated sample. The formation of thestannate layer modified the substrate X-ray pattern, particularly, re-garding the α phase peaks, the FWHMs decreased and the relative in-tensities changed. Peak broadening mainly depends on grain size andmicrostrains induced by crystalline defects. Since the temperature in-volved in the treatment (80 °C) is below the recrystallization temper-ature, grain size substantially does not change thus the observed peaknarrowing can be ascribed to partial recovery of defective structures,in particular point defects and dislocations. The change of the relativeintensities may be probably attributed to the different reactivity ofsome grain orientations with respect to the others. The stannate coat-ing is a conversion layer, i.e. it is produced by chemical reaction of the
Table 2Surface chemical composition of two samples determined by XPS quantification.
substrate. It is reasonable to think that there is an orientation of thegrains that reacts faster than others. XRD patterns of S7C1 and S7C2multilayer coatings are shown in Fig. 9b, confirming that after thethermal treatment at 200 °C the formation of the cubic phase ofCeO2 was obtained. The addition of PVA does not seem to affect thecrystallization of ceria. The presence of the stannate interlayer was re-vealed only in sample S7C2. Probably the small thickness of thestannate film and therefore the small irradiated volume make it diffi-cult to detect. Moreover for sample S7C1 the magnesium alloy reflec-tions were lower with respect to S7C2, reasonably indicating a higherthickness of such coating.
3.4. Electrochemical measurements
Corrosion resistance tests by means of EIS were performed on allsamples both multilayer and single layer. EIS tests on stannate films(samples S1–S7) did not give satisfactory results. The result is consis-tent with those reported in the literature [16,44] and it is probablydue to the porosity of the conversion coating.
Fig. 10 reports the EIS spectra of the multilayer samples after 24 hin 3 wt.% NaCl aqueous solution in Nyquist and Bode representation.EIS spectra of the blank sample (not coated) are also reported for adirect comparison. An increase of total resistance with respect toblank sample can be easily detected for all coated samples. The cor-rosion resistance grew significantly up to 19,900 Ω for S7C1 sample.EIS data were analyzed with the equivalent circuit procedure. It hasbeen reported the EIS spectrum of a coated metal is given by the
Fig. 7. XPS depth profile of the sample S7C1 (without PVA).
Fig. 8. XPS depth profile of the sample S7C2 (with PVA).
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contribution of the faradaic characteristics (charge transfer resis-tance and double layer capacitance) and coating features (pore resis-tance and coating capacitance) [45]. During the immersion, theweight of the two contributionsmay change depending on film charac-teristics. All the acquired spectra showed only one loop, apart from
Fig. 9. A comparison between XRD patterns of AZ91D substrate with S7 s
sample S7C1 after 24 h of immersion that showed two loops. At immer-sion time longer than 24 h, also sample S7C1 showed a single semicir-cle. In the first moments of immersion of a coating in an electrolyte,thewater adsorption process begins, and the value of the coating capac-itance increases with the uptake of water into the coating. The behaviorof sample S7C1 is most likely due to the slower diffusion rate of waterwithin the defects with respect to other samples.
Fig. 11 reports the equivalent circuits used to fit experimentaldata of all types of samples (single and multilayer) (Fig. 11a) andblanks (Fig. 11b). In the picture, Rs stands for the solution resistance,Rct the charge transfer resistance, CPEdl the double-layer capaci-tance, Rpo the pore resistance that is due to the formation of ionicallyconducting path across the film and CPEf the capacitance of the film[45]. Constant Phase Elements (CPE) have been used instead of a realcapacitance. The CPE parameters were converted into capacitancevalues following the reported procedure [46]. In some cases, onspectra acquired on coated samples, the pore resistance was sohigh as to make the contribution of Rct negligible and the errorvery high. In these cases the circuit 11b was used also for coatedsamples.
As an example, Fig. 12 shows the fitting of experimental data onS7C1 sample after 24 h in NaCl solution, in Bode representation, usingthe equivalent circuit of Fig. 11a. The S7C1 EIS spectrum consisted of
tannate coating (a) and with multilayer coatings S7C1 and S7C2 (b).
two circuits, as easily detectable from phase plot, attributed to chargetransfer reaction and to the presence of a film on the electrode surface.A perfect agreement between experimental and calculated data wasobtained as evidenced by χ2 test that gave values lower than 10−4.
Fig. 13 shows the EIS spectra of the multilayer samples after 3 daysin 3 wt.% NaCl. EIS spectra of the blank test and of the sample S7(stannate single layer) are also reported for a direct comparison.The corrosion resistance of the multilayer samples still maintains ahigh value (between 8000 and 15,000 Ω), higher than the blank(2000 Ω) and the stannate coated samples (4000 Ω).
Finally the overall corrosion resistance of themultilayer samples as afunction of immersion time is summarized in Fig. 14. The resistancevalues, compared to blank ones, demonstrated that all coatings are ef-fective to protect the Mg alloy even after seven days of immersion.However, some differences can be observed in the performance of dif-ferent multi-layer coatings. Sample S7C1 showed the best behavior
especially in the first 48 h. This seems to indicate that, in spite of a pos-itive effect on microstructure, the addition of PVA in sample S7C2 has anegative effect in the corrosion resistance features of CeO2 film. Mostlikely the influence of PVA addition is given by the greatwater solubilityof the polymer. The PVA that was not removed by heat treatment, dis-solves in the first instants of immersion thus producing preferentialsites for water absorption. In addition, PVA has a hydrophilic effectthat may increase the wettability of S7C2 coating, thus affecting its re-sistance in the first hours of immersion with respect to sample S7C1.
4. Conclusion
Three-micron thickmultilayer stannate–CeO2 coatingswere obtainedon AZ91 magnesium alloys by conversion coating and sol–gel tech-niques. Both processes have been optimized by changing the depositionparameters and composition of the precursor solutions. Regarding thestannate primers, immersion times of 1 h and temperatures of 80 °C ofthe treatment bath led to a total coverage. The CeO2 top coatings wereprepared starting from two different sols. Uniform, well adherent and
Fig. 13. EIS spectra of S7C1, S7C2, S7 and blank samples after 3 days in NaCl solution inNyquist (a) and Bode (b) representation.
Fig. 14. Corrosion resistances of ceria samples versus time.
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nearly crack-free multilayer coatings were obtained. The formation ofCeO2 after thermal treatment at 200 °C for 1 h was confirmed by XRDand XPS analyses. Heat treatment at 150 °C did not allow to obtain an ac-ceptable coverage. The XPS depth profiles revealed residual carbon in theceramic film obtained with the aid of PVA. The EIS measurementsshowed a strong improvement of corrosion resistance of the coated sam-ple with respect to the blank alloy. Best results in terms of corrosion re-sistance were obtained with sample S7C1 prepared from sol in aqueoussolution in the absence of PVA. Itwas observed that the use of PVAwithinthe sol precursor solution plays a negative role on the corrosion resis-tance features of the coatings, most likely due to the high solubility ofthe polymer that produces preferential sites of attack on the ceramicfilm.
Acknowledgments
The authors wish to thank Dr. Fanny Ecarla and CSM Instrumentsfor scratch tests.
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