Open Access
Issue
Manufacturing Rev.
Volume 4, 2017
Article Number 2
Number of page(s) 18
DOI https://doi.org/10.1051/mfreview/2017001
Published online 23 February 2017

© I. Kartsonakis et al., Published by EDP Sciences, 2017

Licence Creative Commons
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Aluminium alloy (AA) 2024-T3 is widely used in aerospace industry due to its low cost, lightweight and high mechanical strength. Furthermore, it demonstrates as good fatigue resistance as well high strength to weight ratio. On the contrary, this alloy is highly susceptible to corrosion because of the presence of intermetallic particles at the surface resulting in a need for the development of several methods in order to enhance its resistance against corrosion [1, 2]. During the previous years, chromate based coatings used to be the most effective method, exhibiting excellent anticorrosion properties that are ascribed to the strong oxidation properties of Cr(VI). Unfortunately, Cr(VI) and Cr(III) ions are very toxic and provoke serious human diseases [3]. An alternative, environmentally friendly, protection to metal alloys is based on sol-gel coatings [1]. These coatings protect the metal surface by creating a physical and chemical barrier between the metal and its environment [4]. The accomplishment of improved corrosion protective properties of the sol-gel coatings requires that, not only the synthesized sol-gel coatings have to be homogeneous and crack free but also they should present low porosity [5].

Comparing with chromate based coatings, it may be remarked that sol-gel coatings do not demonstrate self-healing properties when the coating is partially damaged. Nevertheless, this disadvantage can be overcome by the addition of inorganic or organic corrosion inhibitors into the coatings that can provide self-healing properties to sol-gel coatings [68]. Furthermore, the corrosion inhibitors can be encapsulated into containers before their incorporation into the coatings in order to control the release of inhibitor without loss of the coatings coherence [913].

The mechanical integrity of the protective coatings is also an important factor apart from their anticorrosive properties. Nanoindentation and nanoscratch tests are widely performed for the mechanical properties evaluation of thin films [14]. The nanoindentation test can provide information about the mechanical behaviour of the material when it is being deformed at the sub-micron scale. The method developed by Oliver and Pharr [15] allows determining the elastic modulus as well as the hardness from the nanoindentation load-displacement data.

The present work is focused on the combination of nanocontainers with sol-gel coatings for the protection of AA 2024-T3. The study investigates not only the overall corrosion protection of the system coating-containers but also the contribution of each parameter (type of nanocontainers, corrosion inhibitor). The nanocontainers consist of cerium molybdate (CeMo) because cerium cation and molybdate anion are corrosion inhibitors in themselves [1619]. Furthermore, the nanocontainers had been loaded with corrosion inhibitor 2-mercaptobenzothiazole (MBT) prior to their incorporation to the coatings. MBT was selected due to its corrosion inhibitor properties as well as its better corrosion protection properties when compared to other inhibitors [7]. Finally, studies on the nano-mechanical properties of the aforementioned coatings were also conducted.

2. Materials and methods

The chemical composition limits for the used AA 2024-T3 panels is listed in Table 1. All chemicals were of analytical reagent grade. Cerium (III) acetylacetonate (Ce(acac)3, Sigma-Aldrich, St. Louis, USA), MBT (Sigma-Aldrich, St. Louis, USA), sodium molybdate (Na2MoO4, Sigma-Aldrich, St. Louis, USA), acetonitrile (Sigma-Aldrich, St. Louis, USA), potassium persulfate (KPS, Sigma-Aldrich, St. Louis, USA), (3-glycidoxypropyl)trimethoxysilane (GPTMS, Sigma-Aldrich, St. Louis, USA) and absolute ethanol (Sigma-Aldrich, St. Louis, USA), were used without further purification. Methacrylic acid (MAA, Sigma-Aldrich, St. Louis, USA) was double distilled under reduced pressure prior to use.

Table 1.

Chemical composition limits of AA 2024-T3 (wt.%).

The CeMo nanocontainers were synthesized according to the following process. Firstly, cores of CeMo containers were produced using anionic charged polymethacrylic nanospheres as templates [20]. The production of templates was accomplished via radical polymerization using acetonitrile as a solvent, MAA as a monomer and KPS as an initiator. The produced polymethacrylic acid (PMAA) nanospheres were coated with cerium molybdenum oxide using Ce(acac)3 and Na2MoO4 as precursors via the sol-gel process. The CeMo containers were acquired after successive dispersion in ethanol in order to remove the PMAA template, centrifugation, drying in air and calcination at 550 °C for 3 h. The nanocontainers loading was performed via the insertion of a saturated solution of MBT in acetone into the CeMo nanocontainers that had been previously degased under vacuum conditions [21].

The coatings were deposited onto the metal panels via a dip-coating process. Optimum protection of AA 2024-T3 was achieved by a coating formed with a GPTMS/acetic acid/water molar ratio of 3/1/10 solution, a constant withdrawing speed of 5.33 mm/s and curing at 100 °C for 36 h. Each panel was dipped three times and each time it remained in the solution for 1 min. The specimens were withdrawn in a direction parallel to their lengths. Table 2 tabulates the four different types of coatings that were prepared. All the panels were chemically cleaned prior to application of the coatings. The treatment of the panels comprises consecutive submersion in 2 wt.% NaOH solution at 40 °C for 3 min and in 4.33 M HNO3 for 30 s, while being washed in distilled water between the cleaning stages of the process and afterwards. Finally, they were left to dry in air and then transferred to a desiccator.

Table 2.

The produced types of coatings.

Both the average nanocontainer size, the morphology of the coatings as well as their elemental analysis were estimated by scanning electron microscopy (SEM) and energy dispersive X-ray analysis spectroscopy (EDS) using a PHILIPS Quanta Inspect (FEI Company) microscope with W (tungsten) filament 25 KV equipped with EDAX GENESIS (AMETEX PROCESS & ANALYTICAL INSTRUMENTS) and Transmission Electron Microscope (TEM) using a JEM2000 FX (200 KV, resolution 0.28 nm). The chemical analysis of the coatings was conducted via Fourier Transform Infrared Spectroscopy (FT-IR) using a FT-IR attenuated total reflectance Agilent Cary 630 instrument in the range of 400–4000 cm−1. The loading of the inhibitor into the nanocontainers was estimated by thermogravimetric analysis (TGA) using a TA Q50 instrument at the heating rate of 10 °C min−1 in air.

The corrosion behaviour of the coatings was estimated via electrochemical impedance spectroscopy (EIS) using a Princeton Applied Research EG & G 263A potentiostat instrument connected with a SI 1260 Impedance/Phase-Gain analyser. The experiments were conducted at the open circuit potential, in a Faraday cage, at room temperature, using a three-electrode electrochemical cell, consisting of a saturated silver chloride electrode (Ag/AgCl, KCl(sat)) as reference, a working electrode (≈1.0 cm2 of exposed area) and a platinum foil as counter electrode. The root mean square (rms) voltage was 10 mV and the measuring frequency ranged from 50 kHz down to 5 mHz. The Z-view Software applying the adequate equivalent electric circuits was used for the EIS spectra treatment. The corrosive solution that was used was 50 mM NaCl prepared with distilled water.

Nanoindentation testing was performed with a Hysitron TriboLab Nanomechanical Test Instrument, which allows the application of loads from 1 to 30,000 μN and records the displacement as a function of applied loads with a high load resolution (1 nN) and a high displacement resolution. The nanomechanical test instrument employed in this study is equipped with a Scanning Probe Microscope (SPM), in which a sharp probe tip moves in a raster scan pattern across a sample surface using a three-axis piezo positioner. In all depth-sensing tests a total of 10 indents are averaged to determine the mean hardness (H) and elastic modulus (E) values for statistical purposes, in a clean area environment with 45% humidity and 23 °C ambient temperature. In order to operate under closed loop load control, feedback control option was used. All nanoindentation measurements have been performed with the standard three-sided pyramidal Berkovich probe, with an average radius of curvature of about 100 nm [14], with 40 s loading and unloading segment time separately and 3 s of holding time, to avoid residual viscoelasticity. Prior to indentation, the area function of the indenter tip was measured in a fused silica, a standard material for this purpose [22]. The scratch tests performed in this work included three main segments. Firstly, a pre-scan under a very small load (1 μN) was carried out; then, the indenter scraped the sample under a certain force and a scratch would be generated. The normal applied loads were 300 μN, while the length of the scratches was 8 μm.

3. Results and discussion

3.1. Morphology study

The morphology images of the synthesized containers are demonstrated in Figure 1. It is evident from the SEM and TEM images that the fabricated containers are hollow spheres but not uniform in size; their diameter ranges from 340 nm to 460 nm (Figures 1a, 1b). The EDS analysis reveals that the containers consist of cerium, molybdenum and oxygen (Figure 1c).

thumbnail Figure 1.

(a) SEM image, (b) TEM image and (c) EDS analysis of cerium molybdate nanocontainers.

Figure 2 depicts the TGA diagrams of pure MBT and CeMo containers loaded with MBT. It can be seen that there is a retardation in the corrosion inhibitor burn off in the system MBT-CeMo nanocontainers (760 °C, Figure 2b) compared to the burn off of pure MBT (310 °C, Figure 2a). This indication can be assigned to the encapsulation of MBT into the CeMo containers [23]. The thermogravimetric analysis of the spectrum in Figure 2b shows three different sharp weight losses from the low temperature to high temperature attributed to the burn off of the inhibitor that is on the shell (lower temperature range), into the pores of the shell (middle temperature range) and inside the shell of the nanocontainer (higher temperature range), respectively. Taking into account the overall weight loss of the spectrum in Figure 2b between 150 °C and 760 °C (2.27 mg), it may be remarked that the nanocontainers were 24.3 wt.% loaded with MBT. However, not all of the MBT is encapsulated in the nanocontainers. A part of MBT is adsorbed into the shell and into the pores of the shell of the nanocontainers shown by the sharp increase in weight losses.

thumbnail Figure 2.

TGA curves of (a) pure MBT and (b) cerium molybdate nanocontainers loaded with MBT.

The synthesis of the applied coating that consists of GPTMS/acetic acid/water with a molar ratio of 3/1/10 is based to the following reactions. At first, the surface of AA 2024-T3 panels is activated with hydroxyl groups via the cleaning process. Moreover, the trimethoxysilyl- groups of GPTMS molecules are hydrolysed resulting in hydroxylsilyl- group. The resulting hydroxylsilyl- groups of GPTMS are covalently bonded with either the hydroxyl groups of the metal surface or the hydroxylsilyl- groups of other GPTMS molecules. The presence of acetic acid leads to the polymerization between the epoxy groups of the GPTMS molecules through an SN2 mechanism. The epoxy polymerization is initiated by the nucleophile attack of the acetic acid on the epoxy carbon and the nucleophile oxygen of the epoxide ring attacks another epoxy carbon propagating the chain. The whole process results in the formation of a coating onto the metal substrate that is schematically represented in Figure 3.

thumbnail Figure 3.

Schematic representation of the coating formation on AA 2024-T3.

The surface SEM images as well as the corresponding tabulated values of the coatings EDS analyses are shown in Figure 4 and Table 3, respectively. All the coatings are crack free with visible white agglomerates, some of which were identified by EDS as Si-rich. Moreover, the surface of the coatings including nanocontainers loaded or empty (Figures 4a and 4b, respectively) exhibit a large number of white agglomerations that are assigned to the presence of CeMo nanocontainers, after evaluation via EDS analysis. Finally, the image of the coating without nanocontainers demonstrates lots of pin-holes (Figure 4d).

thumbnail Figure 4.

Surface morphology of coatings: (a) AA-coat-CeMo-MBT, (b) AA-coat-CeMo, (c) AA-coat-MBT and (d) AA-coat.

Table 3.

Tabulated values of wt.% element concentration of the produced coatings prior exposure to 50 mM NaCl.

Figure 5 illustrates the cerium elemental mapping analysis and the corresponding SEM micrograph of the surface for the coating including CeMo loaded nanocontainers. It can be seen that some of the nanocontainers have been agglomerated. On the other hand, no part of the surface is free of nanocontainers. This result indicates that the nanocontainers have been distributed into all of the coating. Another important factor is the dispersion of the nanocontainers through the thickness of the coating. The nanocontainers should not be agglomerated on the top or on the interface between the coating and the substrate.

thumbnail Figure 5.

Mapping micrograph (a) and corresponding SEM image (b) of coating AA-coat-CeMo-MBT.

Taking into account Figure 6 that presents the cerium elemental mapping analysis through the thickness of the coating and the corresponding SEM micrograph, it may be remarked that the CeMo nanocontainers have been well dispersed through the coating thickness without aggregations.

thumbnail Figure 6.

Cross-section mapping micrograph (a) and corresponding SEM image (b) of coating AA-coat-CeMo-MBT.

The thickness of each coating is depicted in Figure 7. AA-coat-CeMo-MBT coating has a thickness from 35 nm to 40 nm (Figure 7a) while the coating including empty nanocontainers has a thickness from 31 nm to 35 nm (Figure 7b). The coating including only the inhibitor (AA-coat-MBT) has an estimated thickness range of 36 nm–38 nm (Figure 7c). Finally, the coating AA-coat has a thickness from 28 nm to 33 nm (Figure 7d). Regarding the aforementioned results, it may be remarked that the coating including the nanocontainers loaded with corrosion inhibitor has the maximum thickness whereas the coating without additives has the lowest thickness of the four coatings.

thumbnail Figure 7.

Cross-section SEM images of coatings: (a) AA-coat-CeMo-MBT, (b) AA-coat-CeMo, (c) AA-coat-MBT and (d) AA-coat.

3.2. FT-IR analysis

Analysis of the structure and of the chemical composition of the coatings was evaluated via FT-IR (Figure 8). The identification of the coating peaks is depicted in Table 4 [24, 25]. Considering Table 4, all the characteristic peaks of the vibrations of groups that are ascribed to the inorganic and organic network such as Si-O-Si, epoxy ring, C-H, H-C-H, C=O, C-O-C, C-C and Al-O-Si can be clearly seen. Apart from these peaks, the coatings including nanocontainers reveal additional absorption peak at 895 cm−1 which is ascribed to molybdate ion. The broad peaks observed in this compound at 724 and 828 cm−1 are attributed to the overlapping of the bands of Ce(OH)+2 and molybdate ion in the regions of 450–800 cm−1 [20].

thumbnail Figure 8.

FT-IR spectra of coatings: (a) AA-coat-CeMo-MBT, (b) AA-coat-CeMo, (c) AA-coat-MBT and (d) AA-coat.

Table 4.

FT-IR characteristic peaks of coatings.

3.3. Corrosion study

The coatings produced with and without additives were studied for their protective properties against corrosion of the aluminium substrate. All the electrochemical characterizations include the exposure of the coated substrates to 400 mL of 50 mM NaCl solution prepared with distilled water with pH 5.95 ± 0.05. The SEM images of the surface morphology of the four coatings after their exposure at 50 mM NaCl solution for 72 h are depicted in Figure 9. Regarding the surface morphologies, it may be remarked that the coating including loaded nanocontainers does not show signs of corrosion (Figure 9a). On the other hand, both the coating including empty nanocontainers (Figure 9b) and the coating without nanocontainers present lots of cracks denoting the onset of corrosion (Figure 9d). Moreover, it can be seen that there are areas on the samples where the coating has been delaminated revealing the substrate. Finally, the coating containing only corrosion inhibitor exhibits a few cracks but no delaminated areas are visible (Figure 9c). The EDS analysis confirmed these observations demonstrating higher values of Al and O elements for the coatings AA-coat, AA-coat-MBT and AA-coat-CeMo compared to the coating including loaded nanocontainers (Table 5).

thumbnail Figure 9.

Surface morphology and EDX analysis of coatings: (a) AA-coat-CeMo-MBT, (b) AA-coat-CeMo, (c) AA-coat-MBT and (d) AA-coat, after their exposure at 50 mM NaCl solution for 72 h.

Table 5.

Tabulated values of wt.% element concentration of the produced coatings after 72 h exposure to 50 mM NaCl.

The corrosion resistance of the coatings with or without additives was studied by EIS. Figure 10 depicts the Bode (Figure 10a) and Nyquist (Figure 10b) plots of bare and coated AA 2024-T3 obtained after immersion into 50 mM NaCl solution for 24 h. The Bode diagram for the bare AA 2024-T3 demonstrates one time constant in the middle-low frequency range due to charge transfer processes based on corrosion activity s and a second time constant in the low frequency range that is ascribed to a diffusion limitation of the corrosion process [26]. The Bode plots for both the coatings AA-coat-CeMo and AA-coat-MBT present a relaxation time in the high frequency range that is assigned to the coating properties, a second time constant in the middle frequency domain that is related to the response of both the intermediate oxide layer as well as the Al-O-Si covalent bonds formed due to interaction between interfacial Al-OH and Si-OH groups, and a third time constant in the low frequencies that is attributed to the corrosion onset due to the pits that have been formed into the intermediate oxide layer [8]. The Bode plots for the coatings AA-coat-CeMo-MBT and AA-coat reveal two time constants, one in the high frequency domain due to the coating properties, and a second time constant in the middle frequencies due to the intermediate oxide layer and the Al-O-Si covalent bonds formed via interaction between interfacial Al-OH and SiOH groups. The Nyquist diagrams unequivocally confirm the presence of the aforementioned time constants for the corresponding coatings (Figure 10b). Considering Figure 10a it may be remarked that the coating including loaded nanocontainers (AA-coat-CeMo-MBT) demonstrates the best corrosion protection after exposure to corrosive environment for 24 h as it presents higher impedance values at low frequencies compared to the other three coatings.

thumbnail Figure 10.

EIS (a) Bode plots and (b) Nyquist plots of coatings after exposure to 50 mM NaCl solution for 24 h.

Figure 11 illustrates the Bode and Nyquist plots of the coating after exposure to the aforementioned corrosive environment for 72 h. Taking into account these plots, it can be mentioned that apart from AA-coat-CeMo-MBT, all the other three coatings demonstrate three time constants; one in the high frequency range that is ascribed to the barrier properties of the coatings, a second one in the middle frequencies that is assigned to the layer/substrate interface and a third one in the low frequency range that is attributed to corrosion process. On the other hand, the sample AA-coat-CeMo-MBT illustrates two relaxation times, one in the high frequency domain due to the coating properties, and a second time constant in the middle-low frequencies due to the intermediate oxide layer and the Al-O-Si covalent bonds formed via interaction between interfacial Al-OH and Si-OH groups. Estimating the aforementioned plots, it is evident that the samples AA-coat-CeMo-MBT present the highest impedance value in the low frequency range indicating improved protection against corrosion.

thumbnail Figure 11.

EIS (a) Bode plots and (b) Nyquist plots of coatings after exposure to 50 mM NaCl solution for 72 h.

The EIS plots were fitted using equivalent circuits in order to understand the corrosion protective mechanism (Figure 12) and the obtained values are tabulated in Table 6. Regarding the fitting procedure, constant phase elements (CPE) were used instead of capacitances. This modification is obligatory in the case the phase shift of a capacitor is different from −90° [27]. The impedance of a R-CPE parallel association is given by:(1)where Y0 is the admittance of the CPE and n is the CPE exponent. It should be noted that constant phase elements correspond to a capacitor when the CPE exponent (n) is one. Additionally, when 0.5 < n < 1, CPE represents a non-ideal capacitor. Finally, when (n) equals to 0.5, the CPE describes a diffusion process and represents a Warburg impedance [28]. Using the Cole-Cole approach together with CPE, the capacitance can be calculated from the fittings by [29, 30]:(2)

thumbnail Figure 12.

Equivalent circuits with (a) 5 and (b) 7 components used for numerical simulation of the EIS data of the coatings.

Table 6.

Fitting parameters according to equivalent circuits of Figure 12 after samples exposure to 50 mM NaCl solution.

All the coatings demonstrated two time constants after exposure to corrosive environment for 3 h; one in the high frequency range and a second one in the middle-low frequencies. Consequently, these impedance plots were fitted using the equivalent circuit of Figure 12a that includes the resistance and the capacitance of the coating (Rcoat − CPEcoat) as well as the interface capacitance and the corresponding interface resistance (Rint − CPEint). The resistance of the solution is illustrated as Rsol. As the immersion time elapses, the equivalent circuits for the AA-coat-MBT and AA-coat-CeMo coatings include seven components (Figure 12b) as it describes three time constants; the resistance and the capacitance of the coating (Rcoat − CPEcoat), a second relaxation process containing an interface capacitance and the corresponding interface resistance (Rint − CPEint) and a third time constant including a charge transfer resistance and a double layer capacitance (Rct − CPEdl). The sample AA-coat for the first 24 h is simulated by an equivalent circuit that has the components of Rcoat − CPEcoat and the components of Rint − CPEint, (Figure 12a). As the immersion time elapses, an additional time constant appeared in the low frequency domain and the appropriate equivalent circuit for the fitting is illustrated in Figure 12b. Finally, the sample AA-coat-CeMo-MBT is simulated by the equivalent circuit of Figure 12a for exposure times in corrosive environment up to 96 h and then, as the immersion time elapses, it is simulated by the circuit of Figure 12b.

The evolution of the mean values together with the corresponding error bars of the EIS fitting parameters Rct, Cdl, Rcoat, Ccoat, Rint and Cint of all the coatings after their exposure to 50 mM NaCl solution is illustrated in Figure 13. Taking into account the Rcoat and Ccoat values of all the coatings (Figures 13a, 13b) it is estimated that the coating including loaded nanocontainers (AA-coat-CeMo-MBT) has the best barrier properties as it demonstrates higher Rcoat values and lower Ccoat values compared to the other three coatings (Table 6). The same conclusions come out considering Figure 13c, where it can be seen that the coating including loaded nanocontainers shows the highest Rint values among all the coatings.

thumbnail Figure 13.

The evolution of the EIS fitting parameters of all the coatings after their exposure to 50 mM NaCl solution: (a) Rcoat, (b) Ccoat, (c) Rint (d) Cint, (e) Rct and (f) Cdl.

Regarding the estimating values of the double layer capacitance together with the charge transfer resistance of all the coatings (Figures 13e, 13f), it may be remarked that both the coatings AA-coat and AA-coat-CeMo-MBT reveal neither Cdl nor Rct values for the first 48 h and 96 h, respectively, due to the fact that there is no corrosion process. These results indicate that the two aforementioned coatings exhibit improved protection against corrosion. As the immersion time elapses up to 193 h, the coating including loaded nanocontainers (AA-coat-CeMo-MBT) illustrates the highest Rct value together with lowest Cdl value (Table 6). It is important to mention that high Rct values mean high resistance to the corrosion process and that low Cdl values correspond to low corrosion reaction area [31].

3.4. Corrosion inhibition mechanism

Two main types of corrosion appeared on the AA 2024-T3; pitting and intergranular corrosion.

Pitting corrosion is observed in aerated solutions of halides in the passive region of pH. In aerated NaCl aqueous solutions, the anodic reaction is accelerated by chloride ions, while the cathodic reaction is oxygen reduction. Weak points of the oxide or hydroxide passivating film of the AA2024-T3 are intitiated due to pitting corrosion, according to the following anodic reactions (dissolution of Al):(3)

(4)

Taking into consideration the neutral solution, the consumption of hydroxide ions at the anodic sites can make the pH more acidic, in the range of 3–4, followed by migration of chloride ions [32]. These ions facilitate the anodic reaction and form aluminium chlorides which are hydrolysed and give aluminium hydroxides and acids shifting the pH to acidic values according to the following reaction:(5)

On the other hand, the reduction reactions that occur in acidic aqueous solution are the reduction of water and the reduction of oxygen [33, 34]:(6)

(7)

(8)

It may be remarked that the presence of oxygen and/or another oxidant is essential for pitting and that the cathodic sites are frequently more alkaline due to the local formation of hydroxides.

Regarding an oxygenated chloride solution, it can be mentioned that pit initiation is generally controlled by the cathodic reaction kinetics. Moreover, pit propagation requires a sufficient Cl concentration in the solution contained within the pit, because the formation of a concentrated AlCl3 solution within active pits is assigned to Cl [32]. Furthermore, in aluminium-copper-magnesium alloys (2XXX), thermal treatments can cause selective grain boundary precipitation resulting in the appearance of intergranular corrosion susceptibility. In particular, regarding the AA2024-T3, CuAl2 precipitates accelerate the corrosion of a depleted zone adjacent to the grain boundary due to the fact that these precipitates are nobler than the matrix and act as cathodes [32].

The improved properties of the AA-coat-CeMo-MBT coating against corrosion can be ascribed to the presence of the encapsulated inhibitor into the containers as well as the enhanced coherence of the coating. The encapsulation of the inhibitor into the containers (AA-coat-CeMo-MBT) instead of its addition into the coating (AA-coat-MBT) probably impair the coating coherence to a lesser degree resulting in improved corrosion protection properties. Furthermore, it is assumed that the release of the inhibitor from the containers after the onset of corrosion results, in the case of AA 2024-T3, in the dissolution inhibition of the adjacent anodic materials surrounding the Al (Fe, Mn, Mg/Si) (Table 1) intermetallic particles through the formation of stable complexes between the metals and the thiol groups [35, 36]. Except for the MBT inhibitor, the shell of the CeMo containers acts itself as corrosion inhibitor due to its dissolution to MoO4 −2 and cerium ions in corrosive environment. Furthermore, in aqueous molybdate solution with a pH roughly 6, condensation of MoO4 −2 ions into heptamolybdate (paramolybdate) polyanion occurs [37] according to the following reaction:(9)

Moreover, polyanions such as [(OH)4OMo-O-MoO(OH)4]−2, [Mo8O26]−4, [Mo6O21]−6, [MoO(OH)5] and [MoO3(OH)] can also exist [18, 38]. Subsequently, considering that the aforementioned experiments were performed using corrosive solutions with pH = 5.95 ± 0.05, the polymolybdate ions could be adsorbed onto the positive charged metal surface of AA resulting in its protection of chloride penetration [39].

On the contrary, it may be remarked that the effectiveness of cerium cations as corrosion inhibitors is doubtless. The reason is that the pH of the used corrosive solutions is not alkaline and the formation of Ce(OH)3 film on the cathodic sites of the metal alloy is not feasible [40, 41]. Nevertheless, a film based on Ce(OH)4 is precipitated onto the aluminium alloy, acting as a protective coating against aluminium corrosion. Regarding the reaction (7) the oxygen reduction produces H2O2 that can be further reduced resulting in the local formation of OH as well as to the oxidation of Ce+3 to Ce+4. Consequently, Ce(OH)4 is formed according to the following reactions:(10)

(11)

(12)

As reported in the literature the precipitation of Ce(OH)4 can result even at pH = 2.9 [41, 42]. Thus, it may be mentioned that the cerium ions can inhibit the aluminium corrosion in a NaCl solution.

3.5. Nanomechanical properties

The nanomechanical properties (namely H and E) of the coatings are presented in Figure 14. The incorporation of nanocontainers (with or without inhibitor) affects the mechanical integrity of the coatings, revealing a clear mechanical degradation of sol-gel coating; while H is slightly decreased, samples exhibit greater deviation in E values.

thumbnail Figure 14.

Nanomechanical properties (a) hardness (H) and (b) elastic modulus (E) of coatings.

The ratio of hardness to elastic modulus is of significant interest in tribology. Higher stresses are expected in high H/E, hard materials, and high stress concentrations develop towards the indenter tip, whereas in the case of low H/E, soft materials, the stresses are lower and are distributed more evenly across the cross-section of the material [43, 44]. The high ratio of hardness to elastic modulus (H/E) is indicative of good wear resistance in a disparate range of materials: ceramic, metallic and polymeric (for example: c-BN, tool steel, nylon, respectively), which are equally effective in resisting attrition for their particular intended application. In Figure 15, the change of H/E slope reveals that the addition of nanocontainers empty or loaded with inhibitor strengthens (increase of wear resistance) the coating after ~300 nm of displacement, having no significant impact on surface region (0–300 nm), where all coatings exhibited similar (increased) H/E ratio.

thumbnail Figure 15.

Correlation of H/E ratio to displacement, for all the coatings.

In Figure 16, coefficients of friction for all coatings are presented. Incorporation of nanocontainers decreases the coefficients of friction, for the whole scratch path especially for the case of AA-coat-CeMo-MBT system; however, when the tip further penetrates the sample, the behaviour is almost identical for AA-coat and AA-coat-CeMo samples. In case of AA-coat-CeMo-MBT system, the decrease of coefficient of friction almost at the end of scratch path is well addressed in line with H/E behaviour (Figure 15).

thumbnail Figure 16.

Coefficient of friction for all coatings through nanoscratch testing.

4. Conclusions

A system including sol-gel coating based on GPTMS as well as nanocontainers incorporated into the coating was studied for the protection of AA 2024-T3 against corrosion. The nanocontainers consist of cerium molybdate that were either empty or loaded with corrosion inhibitor MBT. Several coatings were synthesized in the presence or absence of nanocontainers. The results reveal that the produced coatings protect the metal alloy against corrosion, but be that as it may, the presence of nanocontainers loaded with MBT into the coating improves its corrosion protective properties demonstrating higher impedance values at the low frequency range compared to the other coatings. Furthermore, apart from the coating including loaded containers, all the other coatings presented cracks on their surface after their exposure to corrosive environment indicating reduced corrosion properties protection. The evaluation on the nano-mechanical properties of the system proved that the addition of nanocontainers empty or loaded with corrosion inhibitor strengthens the coating and decreases the coefficient of friction which should result in an increase of wear resistance. The enhanced corrosion protection properties of the coatings including nanocontainers loaded with MBT can be attributed to either the increase coherence of the coating or the simultaneously inhibition action of cerium and molybdate ions together with the corrosion inhibitor MBT. However, additional experiments should be conducted in future studies in respect of reducing the time and curing temperature of the coatings production. Moreover, several types of nanocontainers loaded with different corrosion inhibitors will be incorporated into the aforementioned films. Finally, the produced coatings will be applied in various metal alloys in order their protection against corrosion to be further investigated.

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Cite this article as: Kartsonakis IA, Koumoulos EP & Charitidis CA: Advancement in corrosion resistance of AA 2024-T3 through sol-gel coatings including nanocontainers. Manufacturing Rev. 2017, 4, 2.

All Tables

Table 1.

Chemical composition limits of AA 2024-T3 (wt.%).

Table 2.

The produced types of coatings.

Table 3.

Tabulated values of wt.% element concentration of the produced coatings prior exposure to 50 mM NaCl.

Table 4.

FT-IR characteristic peaks of coatings.

Table 5.

Tabulated values of wt.% element concentration of the produced coatings after 72 h exposure to 50 mM NaCl.

Table 6.

Fitting parameters according to equivalent circuits of Figure 12 after samples exposure to 50 mM NaCl solution.

All Figures

thumbnail Figure 1.

(a) SEM image, (b) TEM image and (c) EDS analysis of cerium molybdate nanocontainers.

In the text
thumbnail Figure 2.

TGA curves of (a) pure MBT and (b) cerium molybdate nanocontainers loaded with MBT.

In the text
thumbnail Figure 3.

Schematic representation of the coating formation on AA 2024-T3.

In the text
thumbnail Figure 4.

Surface morphology of coatings: (a) AA-coat-CeMo-MBT, (b) AA-coat-CeMo, (c) AA-coat-MBT and (d) AA-coat.

In the text
thumbnail Figure 5.

Mapping micrograph (a) and corresponding SEM image (b) of coating AA-coat-CeMo-MBT.

In the text
thumbnail Figure 6.

Cross-section mapping micrograph (a) and corresponding SEM image (b) of coating AA-coat-CeMo-MBT.

In the text
thumbnail Figure 7.

Cross-section SEM images of coatings: (a) AA-coat-CeMo-MBT, (b) AA-coat-CeMo, (c) AA-coat-MBT and (d) AA-coat.

In the text
thumbnail Figure 8.

FT-IR spectra of coatings: (a) AA-coat-CeMo-MBT, (b) AA-coat-CeMo, (c) AA-coat-MBT and (d) AA-coat.

In the text
thumbnail Figure 9.

Surface morphology and EDX analysis of coatings: (a) AA-coat-CeMo-MBT, (b) AA-coat-CeMo, (c) AA-coat-MBT and (d) AA-coat, after their exposure at 50 mM NaCl solution for 72 h.

In the text
thumbnail Figure 10.

EIS (a) Bode plots and (b) Nyquist plots of coatings after exposure to 50 mM NaCl solution for 24 h.

In the text
thumbnail Figure 11.

EIS (a) Bode plots and (b) Nyquist plots of coatings after exposure to 50 mM NaCl solution for 72 h.

In the text
thumbnail Figure 12.

Equivalent circuits with (a) 5 and (b) 7 components used for numerical simulation of the EIS data of the coatings.

In the text
thumbnail Figure 13.

The evolution of the EIS fitting parameters of all the coatings after their exposure to 50 mM NaCl solution: (a) Rcoat, (b) Ccoat, (c) Rint (d) Cint, (e) Rct and (f) Cdl.

In the text
thumbnail Figure 14.

Nanomechanical properties (a) hardness (H) and (b) elastic modulus (E) of coatings.

In the text
thumbnail Figure 15.

Correlation of H/E ratio to displacement, for all the coatings.

In the text
thumbnail Figure 16.

Coefficient of friction for all coatings through nanoscratch testing.

In the text

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