Comparative Investigation of the Adhesion of Ce Conversion Layers and Silane Layers to a AA 2024-T 3 Substrate Through Mechanical and Electrochemical Tests

Cerium conversion layers and silane films are among the potential substitutes for the carcinogenic chromate conversion layers used to protect high-strength Al alloys. In the present work the adhesion of a cerium conversion layer and of a silane film to an aluminium alloy (AA) 2024-T3 substrate was investigated using mechanical and electrochemical tests. Scanning electron microscopy (SEM)X ray energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR) and X ray photoelectron spectroscopy (XPS) were used to characterize the layers prior and after the mechanical test consisting of ultrasonic rinse in deionized water during 30 minutes. Mechanically tested and untested layers were also submitted to electrochemical impedance spectroscopy (EIS) and anodic polarization measurements in 0.1 M NaCl solution. The results of the characterization tests have pointed to a stronger adhesion of the Ce layer to the substrate in comparison with the silane film, which was confirmed by the electrochemical tests. The adhesion between the silane film and the Ce conversion layer was also tested, to evaluate the possibility of using the system as a protective bi-layer in accordance with the new trends being developed to substitute chromate conversion layers.


Introduction
AA 2024-T3 is largely used in aerospace applications.In this alloy, heterogeneous microstructure is intentionally developed to optimise mechanical properties.Unfortunately, the complex microstructure makes it susceptible to localized corrosion during service and complicates aqueous surface finishing processes [1][2][3] .
It is widely known that 2xxx series alloys are among the most corrosion prone commercial materials in common use.The inhomogeneous distribution of Cu in these alloys microstructure has been shown to be a major cause for their low resistance to pitting and stress corrosion cracking.Specifically, the concentration of Cu in secondphase particles and the local depletion of Cu in certain microstructural regions establish local galvanic cell resulting in localized attack 1,2 .Due to this fact, for commercial use, their corrosion resistance is improved by a complete coating system consisting of three individual layers: a conversion layer (pre-treatment), a primer and a topcoat 4 .In this system the substrate pre-treatment plays an important role on the corrosion resistance as it promotes adhesion with the primer and also imparts some corrosion protection [5][6][7] .
For many decades, chromate conversion coatings have been reported as the most efficient pre-treatment used as anti-corrosive inhibitors and adhesion promoters to the surface of AA employed in the aircraft industry 4,[8][9][10][11][12] .The use of chromates is, however, being restricted worldwide, as they are considered highly toxic and carcinogenic [13][14][15][16][17][18][19][20] .Because current environmental legislation is moving towards total exclusion of Cr 6+ of industrial processes, and due to tightening of regulatory pressures in order to reduce the hazardous waste of chromium, many attempts are being made to develop nontoxic alternative methods for corrosion protection of these materials.One of these non-toxic substitute treatments is based on the use of rare earth salts, in particular those of cerium 13,[17][18][19][20][21] .
Based initially on immersion treatments employing cerium salts as corrosion inhibitors, eg.CeCl 3 , Ce ions have shown promising behavior as corrosion inhibitors for AA 17,18 .Later, using conversion baths, Ce ions in solution were reported to form stable hydroxides/ oxides films on the metal surface at locations of increased pH due to local cathodic activity 13,[19][20][21] .The deposition of Ce(OH) 3 and/or CeO 2 gives the films a distinctive yellow colour.Such chemical passivation process produces coatings that are more corrosion resistant than the naturally formed oxide film on the surface of AA 19,20,22,23 , mainly due to polarization of the cathodic reaction.However, these films have been reported to have an inhomogeneous thickness 24 .Recently the introduction of a pre-treatment step that provoked the deposition of a Cu-rich smut on the alloy surface seems to have overcome this latter drawback 22,23 .
Recently pre-treatments based on the use of organofunctional silanes, in this work denominated simply silanes, have attracted the attention of researchers and industries 25,26 .Silanes are known as good coupling agents and several tests have shown that they afford good corrosion protection to the base AA [25][26][27][28][29][30] .Silanes are hybrid organicinorganic compounds with structure X 3 Si(CH 2 ) n Y, where X represents a hydrolysable group such as methoxy or ethoxy, and Y is a functional group 7,25,28 like an amine or a chain of sulphur atoms.When the silane is symmetrical about the functional group Y, e.g., if there are two Si bonded to three alkoxy (X 3 ) groups, these molecules are known as bis-functional silanes, having the structure X 3 Si(CH 2 ) n Y(CH 2 ) n SiX 3 .On the other hand, when Y is just an alkyl chain we have a nonfunctional silane, which can be also mono or bis-functional.When a metallic substrate is immersed in a hydrolysed silane solution, the silanol groups bond to the metal surface through hydrogen bonds.Afterwards, upon heating up (curing) the silane-covered surface, con-densation reactions occur and stable Si-O-Me and Si-O-Si covalent bonds are formed, giving rise to a dense polymeric network.It has been clearly demonstrated that the curing step results in a better corrosion protection of the metallic substrates 7,16,29,31 , however protection degree also depends on such factors like the specific silane structure, the metal substrate 28 , particularly insofar as it influences the nature of the interfacial bonding, as well as on processing details like silane concentration, hydrolysis time, and solution pH 25,32 .
The aim of this study is to indirectly evaluate the bond strength of Ce conversion layers and silane layers to a AA 2024-T3 substrate.With this purpose samples coated with each one of the layers were characterized using SEM/EDS, FTIR and XPS prior and after being submitted to an adherence test consisting of immersion during 30 minutes in an ultrasonic bath 5,6 .The use of such methodology in replacement of the traditionally used ASTM 3359-02 33 seemed to us more adequate as it equally test the coating adhesion all over the surface; however, for the sake of comparison, this test was also performed.Moreover the corrosion properties of samples submitted to the mechanical tests or not were assessed through EIS experiments and anodic polarization curves in 0.1 M NaCl solution.
As recent researches aiming to substitute chromate conversion layers are moving towards layers with more complex structure 29,30,[34][35][36] , in order to evaluate the affinity between Ce and silane layers, the same tests were performed with samples coated with a Ce-silane bi-layer.

Experimental Procedure
The AA 2024-T3 used in this work was supplied by a Brazilian industry (EMBRAER) as sheets with dimensions of (100 x 100 x 0.1) cm.This alloy has a high Cu content, typically between 3.8-4.9%(wt.(%)) 37 .The pre-treatment procedure used to prepare the samples for deposition of the Ce conversion layer has been described elsewhere as well as the conversion bath composition 23 .
To obtain the silane coating the samples were prepared in the same manner as that used to produce the Ce conversion layer 23 with the difference that the Cu-rich smut left at the sample surface after immersion in the acetic acid solution was removed by vigorously washing.
The silane solution was prepared by adding 4% (w.w -1 ) of BTSE (bis-1, 2-(triethoxysilyl) ethane) molecules to a 50%/50% (w.w -1 ) ethanol/water solution, which, afterwards, had its pH adjusted to a value between 3.5 and 4 by the addition of acetic acid, and was left to hydrolyse during 30 minutes, according to the procedure described by Oliveira 38 .It must be emphasized that the silane solution used in the present work was designed specifically to avoid the use of the carcinogenic methanol.To obtain the silane coatings, the pre-treated samples were immersed for 5 minutes in the hydrolysed solution at 25 °C.Next, the samples were left at rest for 2 minutes at room temperature and subsequently cured at 100 °C for 10 minutes which can be considered as a mild curing condition.The same procedure was adopted to obtain the top silane layer in the bi-layer coating, with the difference that the samples had been previously coated with the Ce conversion coating.
All the electrochemical tests were performed using a classical three electrodes cell with Ag/AgCl(sat.)and a platinum grid as reference and counter electrode, respectively, which were always positioned at the same place relatively to the working electrode (WE).Cell construction details can be found elsewhere 22,23 .The experiments were performed in aerated conditions and at room temperature.
The set-up used to control the impedance experiments was composed of a Solartron 1260 frequency response analyzer (FRA) coupled to a Solartron 1287 electrochemical interface, the experiments were controlled by the software Corrware ® .The diagrams were obtained in the 10 kHz to 5 mHz frequency range, with an acquisition rate of 10 points per decade.The ac signal amplitude was 15 mV (rms).All the impedance experiments were performed at the OCP, which was recorded prior and after the measurement in order to verify the stationnarity.
Anodic potentiodynamic polarization curves were obtained at a 0.5 mV.sec -1 scan rate using the same experimental set-up and electrochemical interface employed in the EIS experiments.The curve was initiated after the completion of the impedance measurements, i.e. after 72 hours of contact of the different samples with the test electrolyte, and finished when the total anodic current reached 1 mA.SEM/EDS analyses were performed in all the coated samples and in the bare alloy.The equipment used was a Philips XL -30 scanning microscope equipped with an EDS spectrometer.The acceleration voltage used to carry out the analyses was 20 keV, giving a penetration depth of approximately 1 µm.Semi-quantitative elemental analysis was executed using internal standards of the equipment.The mechanical adhesion test consisted in immersing the coated sample during 30 minutes in DI water containing ultrasonic bath.
The FT-IR technique is well known as a powerful tool in the field of polymer surface characterization.FT-IR measurements were conducted on a Bomem MB 100 spectrophotometer in the mid-IR range from 4000 to 400 cm -1 .All IR spectra were obtained at an incident angle of 75° normal to the surfaces of specimens, with a spectral resolution of 4 cm -1 .
XPS measurements were carried out using an electron spectrometer manufactured by OMICRON Nanotechnology GmbH (Germany).The photoelectrons were excited by AlKα (1486.6 eV) radiation.Spectra were recorded at normal emission in the Constant Analyser Energy mode.

SEM characterization
Figure 1 presents micrographs of the bare sample, Figure 1a, and of the AA 2024-T3 alloy coated with the different treatments: Ce conversion layer (Figure 1b), silane film (Figure 1c) and bi-layer Ce-silane (Figure 1d).These images have already been discussed elsewhere 39 , however their main aspects will be highlighted.The bare sample presents coarse Al-Cu-Mg and Al-Cu-Fe-Mn intermetallics distributed on its whole surface, while, the Ce coated sample presents its usual dry-mud appearance.The presence of the silane layer on the sample surface is evident in Figure 1c, however it has been demonstrated that silanes does not adhere uniformly on Al surface 5,6 the same was verified in the present work.In its turn, comparison of Figures 1b and 1d shows that the curing of the silane layer seems to introduce compressive stress (the cracks are longer and narrower) in the Ce conversion layer.
The micrographs of the coated samples after the mechanical test are presented in Figure 2. From the images is possible to observe that the coatings have been removed from the sample surface at different degrees.For the sample protected with the Ce-conversion layer (Figure 2a) large amounts of the coating has been removed.On the other hand, the detachment of the silane layer can be inferred due to the presence of the intermetallics (Figure 2b).For the last micrograph presented in this series (Figure 2c) it can be observed that the mechanical test removes only small amounts of the protective layer.This seems to indicate a synergetic action between the Ce and silane layers, in accordance with what had already been shown with the untested samples.
Figure 3a presents a comparison of the Si peaks intensities in EDS spectra for samples of AA 2024-T3 alloy coated with the different films before and after the adhesion tests.No difference was found in the analysis before the test, curves A and C, indicating that the silane molecules have equivalent affinity for both the bare alloy surface and for the Ce conversion layer.On the other hand, it is evident that the mechanical test removes almost all the silane molecules, curves B and D. However, the intensity of the Si peak in the Ce coated sample is stronger indicating a better interaction between the silane coating and the Ce conversion layer than with the Al substrate.On the other hand, the intensity of the peaks associated with Ce barely changes prior and after the mechanical tests indicating an excellent adhesion of the conversion layer to the metallic substrate.

FTIR characterization
Figure 4 shows the FTIR spectra of the BTSE silane film on samples submitted to the different treatments.The assignments of the characteristic absorption bands are given in Table 1 7,31,40 .The bands in the SiOSi region, between 1000 to 1250 cm -1 , are clearly more intense in the bi-layer coated samples.According to the literature 40 the band at 1030 cm -1 is indicative of the formation of a long siloxane chain.The higher absorption intensity of this band for the Ce-silane coated sample suggests longer siloxane chains for this particular sample.On the other hand, the absorption intensities of the bands corresponding to silanol (SiOH) at, 3363, 3243 and 870 cm -1 are nearly the same for all the samples, indicating the existence of practically the same amount of non condensed silanol groups in all the samples.Finally, the bands belonging to the C-H stretching (CH 2 and CH 3 ) group: at 2972, 2924, and 2848 cm -1 are also more intense in the Ce-silane coated sample indicating a higher organization degree of the chains.Comparing the spectra obtained for the sonicated samples, (B) and (D), all the adsorption bands are more intense for the Ce-silane coated sample confirming the stronger adhesion of the silane layer to the Ce conversion coating, as already indicated in the EDS results.

XPS results
Survey XPS spectra taken for the silane coated AA samples with or without the Ce conversion layer confirmed the presence of C, Si, O, Al, Cu, Mg and -when appropriate -Ce.The amount of other contaminants was typically below the detection limit.
Figure 5 shows the Si and Al XPS spectra of the silane coated samples with or without the Ce conversion layer after curing, electrochemical and mechanical testing.All spectra were collected after small amount of ion bombardment, which does not seriously affect Wavenumber (cm -1 ) the layer structure, but more or less removes the adventitious hydrocarbon contamination.The analysis of the figure shows that, in all cases, there is a measurable Al signal, most noticeable around 75 eV binding energy.This indicates that the coverage of the substrates by the different layers is never perfect.While the intensity of this signal is relatively weak for the cured (non-tested) and mechanically tested samples, the electrochemical test always significantly reduces the coverage, as indicated by the stronger intensity of the peak.Assuming that the silane layer thickness is much larger than the information depth of the XPS and the Ce conversion layer is completely covered by the silane, the coverage of the substrate by the silane layer was calculated to be around 95-99% in the cases of the cured and mechanically treated samples, while for the electrochemical tested ones the values were around 75-80%.

EIS results
Figure 6 shows the Nyquist plots for AA 2024-T3 samples coated according to the different procedures presented in Figures 1 and 2. All the impedance diagrams were obtained after 72 hours of immersion in the 0.1 M NaCl test solution.The results, before (A) and after (B) the ultrasonic bath test, show that the samples coated with the Ce-silane bi-layer present the best performance in the corrosion test.
Table 2 presents the values of the real impedance at 6 mHz (low frequency limit) extracted from the data presented in Figure 6.The values clearly shows a synergetic effect between the Ce conversion layer and the silane coating, once the low frequency impedance value is higher than the sum of the individual contributions.Moreover, the results also evidences the excellent adhesion of the Ce-conversion layer to the Al substrate, since, proportionally, the decrease in the real impedance value after the mechanical test was lower for this particular sample.

Anodic polarization behavior
The anodic polarization curves for the different samples, without and with the mechanical test, after 72 hours of immersion in the test electrolyte are presented in Figure 7a and b, respectively.The curves were obtained immediately after the completion of the impedance tests depicted in Figure 6.Comparing the response of samples with the same coating, a clear displacement of the currents to higher values was observed after the sonication test; however, the sample coated only with the silane layer suffered the stronger modification as the passive region almost disappeared after the mechanical test.Once more the bi-layer (Ce-silane) coated sample exhibited the best behavior.

Adhesion test according to ASTM D 3359-02 33
To confirm the good adhesion properties of the Ce conversion layer to the Al substrate, samples coated with only with the Ce conversion layer and with the Ce-silane bi-layer were submitted to the tape pull-off test (ASTM D 3359-02) 33 , the results are presented in Figure 8.Unfortunately, due to their transparent feature, the tests performed on samples coated only with the silane layer were inconclusive.The traditional test confirmed the good adhesion properties of the conversion layer to the base metal, as no detachment of the layer was verified after the test.

Conclusions
In this work a comparative investigation of the adhesion of Ce conversion layers and silane layers to a AA 2024-T3 substrate through mechanical and electrochemical tests was performed.The results of the analytical techniques (FT-IR, EDS and XPS) evidenced that silane molecules have equivalent affinity to the Al substrate and to the Ce conversion layer.Moreover, SEM-EDS observations in samples previously submitted to ultrasonic bath testing put in evidence an excellent adhesion of the Ce layer to the surface of the alloy.This was improved in the sample coated with the bi-layer Ce-silane, evidencing a synergistic effect of the Ce conversion layer and the silane coating.
The performance analysis through EIS and anodic polarization curves indicated superiority of the bi-layer, before ultrasonic bath, in comparison with samples protected by only a monolayer of cerium or silane.Additionally, after the application of the mechanical test the impedance values were still higher than the samples coated with only a monolayer not submitted to any type of mechanical test, ratifying the observations of good adhesion properties of the bi-layer to the substrate.
In addition, the results of FT-IR showed a slightly increase of the formation of siloxane structures (Si-O-Si) for the sample with bi-layer (Ce-silane) before ultrasonic bath, which may play a critical role in the corrosion protection process, as evidenced in the results of the electrochemical tests.

3 Figure 7 .
Figure 7. Anodic polarization curves in 0.1 M NaCl solution for AA 2024-T3 with different treatments.Curves obtained after the impedance experiments presented in Figure 6; a) non-sonicated; and b) sonicated samples.

Table 1 .
Absorption region and bond type found in silane films.
OH (from Si-OH group) stretching vibrations

Table 2 .
Real impedance at 6 mHz for AA 2024-T3 coated according to the different procedures prior and after the sonication test.Immersion time 72 hours.