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Original Article
DOI: 10.1016/j.jmrt.2018.12.011
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Sol–gel coatings doped with encapsulated silver nanoparticles: inhibition of biocorrosion on 2024-T3 aluminum alloy promoted by Pseudomonas aeruginosa
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E.A. Gonzáleza,
Corresponding author
, N. Leivaa, N. Vejarc, M. Sancyb, M. Gulppia, M.I. Azócara, G. Gomeza, L. Tamayoe, X. Zhoud, G.E. Thompsond, M.A. Páeza,
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maritza.paez@usach.cl

Corresponding authors.
a Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile, Avenida Libertador Bernardo O‘Higgins 3363, Estación Central, Santiago, Chile
b Escuela de Construcción Civil, Facultad de Ingeniería, Pontificia Universidad Católica de Chile. Av. Vicuña Mackenna, Santiago 4860, Chile
c Centro de Investigación y Desarrollo en Ciencias Aeroespaciales (CIDCA), Fuerza Aérea de Chile, Av. José Miguel Carrera, San Bernardo 11087, Chile
d Corrosion and Protection Centre, School of Materials, The University of Manchester, Manchester M13 9PL, England, UK
e Departamento de Química, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Casilla 653, Santiago 8320000, Chile
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Received 25 April 2018, Accepted 04 December 2018
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Tables (3)
Table 1. Sample nomenclature.
Table 2. Contact angles of the uncoated and differently coated AA2024-T3 samples.
Table 3. Variation of open circuit potential of the uncoated and coated AA2024-T3 samples after day 7 of exposure in culture medium inoculated with P. aeruginosa.
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Abstract

Silanol type hybrid polymers modified with silver nanoparticles encapsulated with SiO2 for biocorrosion protection of 2024-T3 aluminum alloy were studied through electrochemical characterization and surface analysis techniques. Two different encapsulated silver nanoparticles were synthesized using tetraethoxysilane as a core shell. The hybrid polymer was prepared by the sol–gel technique by mixing tetraethoxysilane and triethyl(octyl)silane in 1-propanol, followed by the incorporation of silver nanoparticles or encapsulated silver nanoparticles. Relatively uniform coatings were observed by a scanning electron microscopy analysis. Transmission electron microscopy and dynamic light scattering results indicated that the diameter of the silver nanoparticles was around 20nm, whereas the encapsulated silver nanoparticles presented diameters between 24 and 30nm. The viability results showed that polymers modified with encapsulated nanoparticles exhibit higher antibacterial properties than the polymer modified with free silver nanoparticles. This fact may be associated with a higher hydrophobicity of the coatings modified with silver encapsulated nanoparticles. Additionally, impedance measurements revealed a protective effect of all synthesized coatings for 2024-T3 aluminum alloy in chloride media inoculated with Pseudomonas aeruginosa.

Keywords:
Hybrid polymers
Silver nanoparticles
SiO2 nanocapsules
Biocorrosion
Pseudomonas aeruginosa
Full Text
1Introduction

The susceptibility of 2024-T3 aluminum alloy (AA2024-T3) to local corrosion has been attributed to the presence of intermetallic compounds which are copper rich and influence the properties of the passive oxide film on the metal surface [1]. Moreover, the surface treatment of aluminum alloys is an important step to avoid the detrimental effect of corrosion [2]. One of the approaches to protect the metal surface from environmental corrosion is the use of sol–gel type coatings which have been placed as an alternative to chromium-based coatings [3]. Notice that this last type of coating contains chromium species that have been proven both toxic to human health and dangerous as environmental pollutants [4]. The replacement of chromium-based methods has not been easy given their effectiveness against corrosion, be it physical or microbiologically influenced (MIC). The phenomenon of MIC involves the acceleration and/or alteration of corrosion processes resulting from the presence of microorganisms usually forming part of a biofilm on the metal surface [5]. The kinetics of corrosion can be determined by a series of parameters such as oxygen concentration, salt concentration, pH, redox potential, and conductivity, all of which can also influence the bacterial growth [6]. Studies from corrosion sites have placed Pseudomonas aeruginosa as a microorganism associated to aluminum alloy corrosion [7]. P. aeruginosa participates in the initiation development, and stability of the biofilm structure in metallic surfaces whose formation involves the production of alginate (polysaccharide), a fundamental component of the biofilm skeleton [8]. In this context, researches on sol–gel coatings have shown them to have good adhesion to both metallic substrates and organic top coats [9]. However, their protective efficiency may be improved by using dopant agents and can, therefore, offer various ways to prepare functional coatings with different properties [10]. It should be mentioned that the silver nanoparticles (AgNPs) have shown excellent antibacterial properties due to both a silver ion reservoir and direct interaction with microorganisms [11], placing them as promising candidates for their use as biocorrosion inhibitors. In the present work, silver nanoparticles were used to dope hybrid sol–gel coatings in order to improve the corrosion resistance of AA2024-T3 influenced by P. aeruginosa. In addition, since a possibility existed of creating a galvanic cell between aluminum and silver, we studied SiO2 encapsulated nanoparticles in order to prevent this.

2Experimental2.1Substrates

The samples consisted of AA2024-T3 plates, which were provided by the Chilean aerospace company, ENAER. The nominal composition of the alloy was (wt.%): 4.75 Cu, 0.533 Fe, 1.28 Mg, 0.71 Mn, 0.529 Si, 0.254 Zn, 0.16 Ti and the rest is Al. The samples were mechanically polished with SiC paper of 400, 800, 1200 and 2500grit, washed with distilled water, and degreased with acetone. Subsequently, the substrates were etched in 0.01M KOH for 10min and desmutted in 20% v/v HNO3 for 15min [12].

2.2Hybrid polymer preparation

Tetraethoxysilane (>99%, TEOS, Merck), triethoxy(octyl)silane (99%, TEOCS, Merck), HNO3 (69%, Merck, PA) and 1-propanol (Merck, PA) were used in the synthesis of the hybrid polymer. The tetraethoxysilane (TEOS), triethoxy(octyl)silane (TEOCS) monomers, and 1-propanol were mixed with a ratio of 1:1:2 respectively and hydrolyzed with 10% v/v HNO3 to pH 1. The resulting sol–gel system was agitated for 120min and then, aged for 20h at room temperature before deposition on the aluminum alloy. SiO2 nanoparticles (nanopowder, 20–25nm particle size) were purchased in Sigma Aldrich.

2.3Silver nanoparticles and encapsulated silver nanoparticles preparation

The synthesis of silver nanoparticles was performed by the modified method of Wang [13,14], from a 5×10−3M AgNO3 solution (ACS grade, Merck), 0.02M NaBH4 (ACS, Sigma Aldrich) in the presence of oleic acid (C18H34O2) (90%, Sigma Aldrich). Encapsulation of silver nanoparticles (AgNPs) with (TEOS) was performed redispersing AgNPs in 6mL cyclohexane (Winkler, PA) in the presence of KH2PO4 (ACS, JT Baker). Then, to form a micellar system, 400μL of polyoxyethylene(5) nonylphenyl ether (Igepal CO-520) were added into the reactor beaker. Subsequently, and with constant stirring, 90mL of a 25% NH3 solution were included into the reaction mixture. On the nano-encapsulation procedure, the following volumes of TEOS were employed: (i) 50μL and (ii) 100μL. In order to release the encapsulated nanoparticles from the micellar system, methanol was added to the reaction mixture. Encapsulated nanoparticles were separated by gravity from the solvent methanol by centrifugation at 10,000rpm (M24-A, BOECO) and then, re-dispersed by sonication in 1-propanol for subsequent application in the sol–gel coating. AgNPs were characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS), FT-IR spectroscopy and cyclic voltammetry (CV).

2.4Synthesis procedure and application of coatings

Hybrid polymers were aged for 24h and doped with (i) 50μg/mL AgNPs; (ii) 50μg/mL AgNPs encapsulated with 50μL TEOS, (iii) 50μg/mL AgNPs encapsulated with 100μL TEOS and (iv) 50μg/mL SiO2NPs. Polymers were applied to the AA2024-T3 substrate by immersion in the hybrid sol–gel for 15min. Then, they were withdrawn at an average rate of 2mms−1. Afterwards, the coatings were dried at 100°C for 60min. Notice, that from this section, the following nomenclature for samples (as shown in Table 1) will be used in this manuscript.

Table 1.

Sample nomenclature.

Sample  Nomenclature 
AA2024-T3 uncoated  AA2024-T3 
AA2024-T3 coated with hybrid polymer  Hy 
AA2024-T3 coated with hybrid polymer doped with silver nanoparticles  Hy/AgNPs1 
AA2024-T3 coated with hybrid polymer doped with silver nanoparticles encapsulated with 50μL TEOS  Hy/AgNPs2 
AA2024-T3 coated with hybrid polymer doped with silver nanoparticles encapsulated with 100μL TEOS  Hy/AgNPs3 
AA2024-T3 coated with hybrid polymer doped with SiO2 nanoparticles  Hy/SiO2NPs 
2.5Microbiological tests

The samples were exposed for 15min per side to UV-C light in a laminar flow hood in order to be sterilized. The culture medium was prepared using bacteriological peptone, following previously established procedures and protocols [10]. This medium also contained 0.1M NaCl [15]. The Gram negative bacterium P. aeruginosa (ATCC #27853, ISP) was inoculated at 108CFU/mL following the McFarland method [16]. In particular, the antibacterial activity was examined by viability tests performed using a confocal microscope Zeiss LSM 51. The stained samples were examined under a green filter (excitation/emission wavelength, 420–480nm/520/580nm) and a red filter (excitation/emission wavelength 590–800nm/480–550nm). The bacterial viability kit used was Live/Dead BacLight (L7012-Sigma) [17]. Viable bacteria (green) and dead bacteria (red) can be distinguished under a fluorescence microscope [18]. From here, the uncoated and coated AA2024-T3 samples were exposed to the inoculated medium with P. aeruginosa (108CFU/mL) for 16h, and were stained with 0.1mL of Live/Dead solution for 15min.

2.6Electrochemical measurements

A three-electrode electrochemical cell was used for electrochemical impedance measurements (EIS). Coated and uncoated AA2024-T3 samples were employed as working electrodes with an exposed surface area of approximately 1cm2. A graphite rod and a saturated calomel electrode were used as counter and reference electrodes, respectively. The electrochemical cell was kept at room temperature and open to the air. The samples were cleaned with ethanol and exposed to UV light for 15min before the measurements. The electrochemical experiments were performed in a culture medium inoculated with P. aeruginosa (108CFU/mL) and 0.1M NaCl solution, after 7 days of exposure. Open circuit potential and electrochemical impedance measurements were carried out using a potentiostat/galvanostat (Bio-Logic, VSP). The electrochemical cell was placed in a Faraday cage to avoid external interferences. Impedance diagrams were obtained over a frequency range from 10kHz to 10mHz, with eight points per decade using an amplitude of 20mV respect to OCP. On the other hand, cyclic voltammetries were performed in a cell of three-electrodes with a N2 (gas) input. As a working electrode, a disk of ordinary pyrolytic graphite (OPG, Momentive, USA) was mounted on a copper tube, 0.5cm in diameter, insulated with polystyrene. As reference auxiliary electrode were the saturated calomel electrode (SCE) and agglomerated graphite respectively. Electrochemical characterizations were performed according to a previously described method [19,20]. This method consists in placing 5μL of a sample on the working electrode, in this case a colloidal solution of silver nanoparticles or a suspension of encapsulated silver nanoparticles. The solvent was subsequently evaporated at 25°C in a vacuum oven for 10min, leaving the precipitate adhered to the electrode surface. Cyclic voltammograms were carried out at a scan rate of 100mVs−1 using 0.1M NaCl as electrolyte.

2.7Surface morphology

The uncoated and coated AA2024-T3 samples were examined with a scanning electron microscope (JEOL JMS 6010LA). For the local surface analyses, an accelerating voltage of 5kV was used giving a penetration depth of approximately 1μm.

2.8Contact angle measurements

The contact angle measurements of the different samples were analyzed using ImageJ software. For the hydrophobicity analyses a 15μL drop of deionized water was added onto the surface of both coated and uncoated aluminum alloys at room temperature. Each sample was measured in triplicate.

3Results and discussion3.1Determination of particle size distribution

Size distributions of silver nanoparticles were obtained through TEM and DLS techniques. From the TEM images (Fig. 1), AgNPs there are spherical shapes of roughly 20nm in diameter (Fig. 1A1), which is in good agreement with the average size values determined by DLS (Fig. 1A2). It can also be noted that the AgNPs nanoparticles encapsulated with TEOS are spherical, independent of the amount of TEOS added (Fig. 1B1 and C1). The shape of the encapsulated nanoparticles is governed by a pattern associated with the formation of spherical micelles Igepal CO-520, which acts as a microreactor. Furthermore, the thickness of the outer layer of SiO2 in AgNPs encapsulated with TEOS is dependent on the amount of TEOS used in the synthesis; the greater the amount of TEOS, the greater the size of AgNPs encapsulated with TEOS. The average size values are around 23nm and 30nm for encapsulated with 50μL TEOS and encapsulated with 100μL TEOS, respectively (Fig. 1B1, 1B2 and C1, C2).

Fig. 1.
(0.26MB).

TEM images with their respective DLS for: (A1 and A2) AgNPs, (B1 and B2) AgNPs encapsulated with 50μL TEOS and (C1 and C2) AgNPs encapsulated with 100μL TEOS.

3.2Cyclic voltammetry (CV)

In order to demonstrate the effective encapsulation of silver nanoparticles, the cyclic voltammetry of three differently modified pyrolytic graphite electrodes (OPG) were carried out in a 0.1M NaCl solution, as follows: OPG electrode modified with free silver nanoparticles (AgNPs) and OPG electrode modified with silver nanoparticles encapsulated with 50 or 100μL TEOS. In Fig. 2 the first cycle is shown starting in 0.0V vs SCE in the cathodic direction. When 5μL of a colloidal suspension of AgNPs are deposited on the electrode, a voltammogram as shown in red, is observed. The anodic peak around 0.1V indicates oxidation of metallic silver, specifically of AgNPs [20]. Following the same electrode modification procedure, silver nanoparticles encapsulated with 50μL TEOS are deposited on the electrode. The Cyclic voltammogram shows that anodic response associated with silver nanoparticles encapsulated with 50μL TEOS (green line) is markedly lower than the one representing the non-encapsulated AgNPs (red line). However, when 5μL of silver nanoparticles encapsulated with 100μL TEOS are deposited on the electrode, the anodic signal associated with silver oxidation (blue line) is not observed, indicating their electrical isolation. One possible explanation for the differences in the potential current responses of Fig. 2 may be the wall thickness of nanocapsules, which in the case of silver nanoparticles encapsulated with 50μL TEOS would not be sufficiently thick as to avoid the diffusion of the silver ions through the wall of SiO2 toward the electrolyte.

Fig. 2.
(0.09MB).

Cyclic voltammetry of unmodified and differently modified OPG electrodes in 0.1M NaCl solution at a potential sweep rate of 100mVs−1. () OPG electrode; (

) AgNPs; (
) silver nanoparticles encapsulated with 50μL TEOS; (
) silver nanoparticles encapsulated with 100μL TEOS.

3.3Surface and coating characterization

Fig. 3 shows micrographs of the AA2024-T3 modified with different hybrid coatings. In general, the coatings are relatively uniform and without cracks. However, the Hy/SiO2NPs sample, and although in a lesser extent, the Hy/AgNPs3 sample show a more heterogeneous surface. These coatings present this morphology due to the presence of SiO2 nanoparticles or AgNPs encapsulated with 100μL TEOS that were expelled from inside the polymer and deposited on its surface during the drying of the film (Fig. 3D and E).

Fig. 3.
(0.38MB).

SEM images showing the differently coated AA2024-T3 surfaces: (A) Hy; (B) Hy/AgNPs1; (C) Hy/AgNPs2; (D) Hy/AgNPs3; (E) Hy/SiO2NPs.

Fig. 4 shows SEM images of the cross sections of the differently coated AA2024-T3. The thickness for each coating was determined from the cross sections. The order of thickness for the coatings is as follows: Hy/AgNPs2 (1.77μm)Hy/AgNPs1 (1.80μm)<Hy (1.98μm)<Hy/AgNPs3 (2.23μm)Hy/SiO2NPs (2.27μm). The difference in thickness between the coatings might be associated with the degree of interaction between the added nanoparticles and the hybrid. The hybrid polymer by itself has a hydrophobic nature, as it can be seen by the contact angle measurements (Section 3.4). During the drying of the coating, greater crosslinking and shrinkage of the polymer occur due to solvent evaporation and occluded catalyst molecules. In this process the dopant nanoparticles will tend to escape from the polymer if there is no strong interaction between both. In the case of polymers modified with SiO2NPs and AgNPs encapsulated with 100μL TEOS, SEM images show that these nanoparticles, after being expelled, remain deposited on the surface rendering greater thickness and roughness to the coatings. This is observed particularly with Hy/SiO2NPs. However, when it comes to the AgNPs or AgNPs encapsulated with 50μL TEOS, interaction with the hybrid polymer would be minor due to a matter of hydrophobicity. In addition, as was shown by DLS measurements, AgNPs and AgNPs encapsulated with 50μL TEOS are smaller than the AgNPs encapsulated with 100μL TEOS. This fact probably caused the observation of lower observed thickness and roughness in both coatings.

Fig. 4.
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Scanning electron micrographs showing cross-sections of the coated AA2024-T3 samples: (A) Hy; (B) Hy/AgNPs1; (C) Hy/AgNPs2; (D) Hy/AgNPs3; (E) Hy/SiO2NPs.

3.4Contact angle measurements (θ)

Since the substrate surface energy is one of the physical factors which determine biofilm formation, it is fundamental to study the hydrophobicity of the surfaces [21]. Any surface, on which the drop forms a contact angle greater than 90°, is of hydrophobic nature. This condition implies that the wettability, adhesion and surface energy of the solid are low. In contrast, if the surface is hydrophilic, we can observe a contact angle less than 90° and the wettability, adhesion, and surface energy of the solid will be high. Contact angles for each substrate are summarized in Table 2. All studied coatings increase the contact angle (θ) value with respect to the bare aluminum, which is associated with an increase in the hydrophobicity of the surface. Interestingly, when the hybrid polymer is doped with silver nanoparticles its hydrophobicity decreases. On the contrary, when the hybrid polymer is doped with encapsulated silver nanoparticles its hydrophobicity is strongly increased. Moreover, when the hybrid polymer is doped with only SiO2 nanoparticles, its contact angle shows a slight increment compared with the doped hybrid polymers (Fig. 5). This fact might be accounted to the silver isolation when encapsulated placing SiO2 capsules, thus being as responsible for the observed effect, that of which is further confirmed by the similarity of these results to those using SiO2 nanoparticles. Correspondingly, Hy/AgNPs3 presented a higher contact angle than Hy/AgNPs2; since this capsule has a thicker SiO2 core shell, and it achieves better AgNPs insulation. Numerous studies have shown that SiO2 nanoparticles are able to increase the hydrophobicity of different types of polymers, making surfaces highly hydrophobic [22–24]. This fact explains the increase in the contact angle observed when the hybrid polymers are doped with SiO2NPs and AgNPs encapsulated with TEOS.

Table 2.

Contact angles of the uncoated and differently coated AA2024-T3 samples.

Sample  Contact angle, θ (°) 
AA2024-T3  60.5±3.0 
Hy  104.5±2.9 
Hy/AgNPs1  87.4±2.7 
Hy/AgNPs2  106.1±2.8 
Hy/AgNPs3  111.5±2.7 
Hy/SiO2NPs  113.2±3.0 
Fig. 5.
(0.15MB).

Contact angles of: (A) AA2024-T3; (B) Hy; (C) Hy/AgNPs1; (D) Hy/AgNPs2; (E) Hy/AgNPs3; (F) Hy/SiO2NPs.

3.5Antibacterial behavior

The bacterial viability of P. aeruginosa on the studied samples was analyzed by confocal microscopy after exposure to the bacterial strain (108CFU/mL) for 16h. Fig. 6A1 and A2 shows images of the AA2024-T3 after being exposed to the bacterial strain, where we can see a considerable bacterial growth associated with the formation of biofilms. On the contrary, for Hy sample, confocal images show a considerable reduction of bacteria (Fig. 6B1 and B2). From Fig. 6C1 and C2, contrary to what we expected, the presence of AgNPs in the polymer stimulated the development and bacterial death. This is apparently related to two reasons: on one hand, the functionalizing chemical compound, oleic acid, which was employed to avoid nanoparticles agglomeration is possibly a good carbon source for the bacteria to survive, and in addition, the silver nanoparticles in the hybrid polymer increases its hydrophilicity. Some studies have pointed out that P. aeruginosa is a Gram negative bacterium able to grow in culture media with n-alkanes (C11–C40) as a source of carbon and energy [25,26]. In contrast, the presence of encapsulated nanoparticles markedly decreases the presence of P. aeruginosa, particularly with Hy/AgNPs3 sample (Fig. 6E1 and E2). This behavior could be due to the SiO2 capsule completely blocking silver diffusion (as we can see in the voltammetric measurements in Fig. 2). However, the results obtained by cyclic voltammetry indicate that, in the case of Hy/AgNPs2 sample, there is a slow diffusion of metallic Ag through the SiO2 capsule, which in turn would have caused the differences in the hydrophilicity between Hy/AgNPs2 and Hy/AgNPs3 samples.

Fig. 6.
(0.29MB).

Fluorescence images of A1 and A2: AA2024-T3; B1 and B2: Hy; C1 and C2: Hy/AgNPs1; D1 and D2: Hy/AgNPs2; E1 and E2: Hy/AgNPs3; F1 and F2: Hy/SiO2NPs. Green filter (live bacteria) and a red filter (dead bacteria) after exposure to a culture medium inoculated with P. aeruginosa for 16h.

Fig. 6 shows that in general, the antibacterial properties of the polymers modified with encapsulated nanoparticles (Hy/AgNPs2 and Hy/AgNPs3) are similar to the antibacterial capacity of the hybrid polymer host (Hy). This is not surprising since they have very similar hydrophobicity.

In the case of Hy/AgNPs1 sample a strong decrease of the contact angle can be appreciated compared to the Hy sample (Table 2), which can be related to an increase in the hydrophilicity of the coating. This fact would favor the formation of biofilm as it was seen in the bacterial viability results (Fig. 6C1 and C2). On the contrary, by adding silver nanoparticles encapsulated with TEOS to the hybrid polymer, an increase in the contact angle of both coatings (Hy/AgNPs2 and Hy/AgNPs3) was observed, with Hy/AgNPs3 being the most hydrophobic, presumably because of the formation of thicker capsules of SiO2. In addition, Hy/SiO2NPs sample, whose hydrophobicity is the highest among those studied, showed almost zero growth and death of bacteria, confirming the importance of the hydrophobicity of the coatings used to prevent bacterial adhesion (Fig. 6F1 and F2). Studies have demonstrated that nano-silica coatings can control bacterial biofilm formation in cooling tower water systems by reducing its formation. It is known that the preconditioning of surfaces with hydrophobic coatings significantly discourages bacterial attachment and adhesion to these surfaces [27]. It is commonly understood that biofilms promote corrosion. Biofilms are thought to be enhanced by bacterial colony formation tendencies although the evidence is limited because biofilms are thought to require a longer gestation period during which conditions may change dramatically.

3.6Corrosion behavior3.6.1Surface and coating characterization

SEM morphology of uncoated and coated AA2024-T3 samples after 7 days of exposure to culture medium and 0.1M NaCl solution inoculated with P. aeruginosa are presented in Fig. 7. It is clear from the micrographs that surface morphology of AA2024-T3 reveals a multi-pitted appearance as the main feature (Fig. 7A). Many cavities of varying size are observed due to the loss of second phase particles. However, colonies of bacteria are not observed, probably because they were dragged almost completely when washing the sample. On the other hand, it is important to point out that the differently coated AA2024-T3 samples, even after seven days of exposure to the bacterium, did not present predominant cracks. From Fig. 7 it is possible appreciate small clusters of bacteria on the surface of the Hy, Hy/AgNPs2 and principally with the Hy/AgNPs1 sample, even after thoroughly washing the samples. Furthermore, as indicated in Section 3.5, the bacterium P. aeruginosa exhibits a great affinity for hydrocarbons, in this specific case, by the linoleic acid present in the last two coatings above mentioned. However, no notorious damage of these coatings is observed. Several researchers have observed higher bacterial adhesion on rough surfaces and a high ability to retain a greater amount of microorganisms concluding, therefore, that the surface roughness seems to be an important factor in bacterial adhesion [28,29]. Furthermore, the surface of the Hy/AgNPs3 sample and the Hy/SiO2NPs sample were completely free of the biofilm formation (Fig. 7E and F, respectively). This fact would be strongly correlated with the results obtained by confocal microscopy where no biofilm formation was detected after 16h of exposure to the culture media inoculated with P. aeruginosa, which can be associated to the high hydrophobicity of these films. Although the surface of both coatings looks quite intact after seven days of exposure to the culture medium, it is possible to see a couple of small cracks in the Hy/SiO2NPs sample. These cracks may be related to the penetration of water and corrosive ions in areas with lower density of crosslinking in the polymer.

Fig. 7.
(0.41MB).

Scanning electron micrographs after 7 days of exposure to inoculated media with P. aeruginosa (108CFU/mL) of: (A) AA2024-T3; (B) Hy; (C) Hy/AgNPs1; (D) Hy/AgNPs2; (E) Hy/AgNPs3; (F) Hy/SiO2NPs.

3.6.2Electrochemical characterization

Table 3 shows that AA2024-T3 OCP value was slightly displaced toward more positive values with Hy sample and, slightly toward more negative values with Hy/AgNPs1 and Hy/AgNPs2 samples. However, those results did not suggest a significant influence on the corrosion kinetic. On the contrary, the OCP values were modified to more positive values with Hy/AgNPs3 and Hy/SiO2NPs samples, which could be associated to the barrier characteristics that the coating gives to the surface [30].

Table 3.

Variation of open circuit potential of the uncoated and coated AA2024-T3 samples after day 7 of exposure in culture medium inoculated with P. aeruginosa.

System  OCP (V vs. SCE) 
AA2024-T3  −0.854±0.003 
Hy  −0.811±0.004 
Hy/AgNPs1  −0.870±0.005 
Hy/AgNPs2  −0.878±0.005 
Hy/AgNPs3  −0.568±0.008 
Hy/SiO2NPs  −0.737±0.006 

Fig. 8 shows the Bode plots of uncoated AA2024-T3 obtained at E=OCP after 7 days of exposure to culture media sterile and inoculated media with P. aeruginosa with 0.1 NaCl solution. The impedance responses revealed a capacitive behavior that is described by one time constant, which might be associated to the alumina oxide film formation on aluminum surface. As can been seen, a CPE behavior was observed that was associated to the alumina oxide film formed on the surface [31,32]. A CPE parameter named alpha (α) values was derived from the angle phase value in the medium frequency range divided by 90°, and was close to −0.81 in the sterile medium, that is in agreement with previous values reported in Ref. [33]. However, decreased to −0.56 in the inoculated media with P. aeruginosa. Additionally, the modulus of the impedance in the low frequency range decreased significant in inoculated media with P. aeruginosa at OCP condition. This behavior might reveal a corrosive effect and an influence on the protective character of the alumina oxide film by the presence of this microorganism.

Fig. 8.
(0.08MB).

Bode plots of uncoated AA2024T3 obtained at E=OCP and after 7 days of exposure to a culture media (■) sterile and (○) inoculated with P. aeruginosa (108CFU/mL) with 0.1M NaCl solution.

Bode plots of coated AA2024-T3 samples after 7 days of exposure to the bacterial medium with P. aeruginosa are shown in Fig. 9. As can be seen, the modulus of the impedance in all frequency ranges was markedly higher than those of the uncoated samples (see Fig. 8), suggesting the ability of the coatings to protect the aluminum surface. It should be noted that the modulus of impedance of the coated metals at a low frequency range can be attributed to the coating resistance [34]. Therefore, coatings of bare silver nanoparticles (Hy/AgNPs1) revealed a lower coating resistance rather than those coatings containing encapsulated nanoparticles (Hy/AgNPs2, Hy/AgNPs3) and Hy/SiO2NPs sample. Moreover, the impedance response increased in the low frequency range with higher TEOS amounts (Hy/AgNPs2, Hy/AgNPs3), and similar impedance responses are observed for Hy/AgNPs3 and Hy/SiO2NPs samples. Thus, those results follow the next order: Hy<HyAgNPs1<Hy/AgNPs2<Hy/SiO2NPsHy/AgNPs3. The differences in the impedance behavior of the differently coated aluminum alloy samples are possibly the result of variations in thickness and compactness of the polymeric films, as well as, the different hydrophobicity of the coatings. It should be mentioned that no significant damage and cracks in the coatings were revealed by SEM images (see Fig. 8) after a 7-day exposure time, which is in accordance with the impedance results, and could indicate that the coatings exhibit an effective barrier against the penetration of the electrolyte and the biocorrosive action of P. aeruginosa. An important factor to consider regarding the protective efficiency of the coatings is its roughness. Porous and rough coatings are not recommended since regions with low cross-linking density are more susceptible to electrolyte access and subsequent delamination and detachment [35]. As shown in Fig. 7, SEM images of coatings containing SiO2NPs revealed some superficial cracks, which showed a similar protective performance than Hy/AgNPs3, despite being thicker, more hydrophobic, and not allowing biofilm growth.

Fig. 9.
(0.15MB).

Bode plots of AA2024T3 differently coated obtained at E=OCP and after 7 days of exposure to being exposed to 0.1M NaCl solution and inoculated media with P. aeruginosa (108CFU/mL). (♦) Hy; (●) Hy/AgNPs1; (▴) Hy/AgNPs2; (▾) Hy/AgNPs3; (◀) Hy/SiO2NPs samples.

Previous studies have shown that addition of dopants to hybrid polymers, such as nanoparticles, may increase the compactness of the coating [36–38]. In many cases, barrier properties and hydrophobicity are also improved, depending on the degree of affinity with the polymer. Although, the addition of AgNPs to the Hy film decreased the hydrophobicity but increased the compactness. On the contrary, the protective efficiency was lower for HyAgNPs1 than Hy/AgNPs2 and Hy/AgNPs3 samples. This situation might be attributed principally to differences in hydrophobicity since the coating could be less susceptible to water adsorption, and consequently to its diffusion through the film toward the metal-film interface [39–42]. As shown in Fig. 9, the higher impedance was shown with more hydrophobic coatings (Hy/AgNPs3 and Hy/SiO2NPs), which might be associated to a smaller water absorption capacity (WAC) and in consequence, lower tendency to swell and detach. Additionally, WAC increases similar to that of the hydrophilicity of the film due to the absorption of water becoming easier. The WAC is also related with to the compactness and porosity of the films [43]. Thus, a more porous film has a greater WAC, facilitating the entry of water and corrosive ions toward the metal, which might promote its dissolution. As was mentioned before, in all cases, homogeneous films were obtained (see Fig. 3). Hence, the hydrophobicity and the WAC would be related mostly to the type of dopant. For instance, the HyAgNPs1 sample was more hydrophobic than the Hy coating. However, the increase in compactness and the reduction of the porosity in the polymer HyAgNPs1 sample could be associated with a smaller WAC that might explain the lower anodic current densities. Yin et al. [42] have provided evidence that hydrophobicity plays an important role in the corrosion behavior.

On the other hand, it should be noted that the HyAgNPs1 coating showed slight antibacterial effectiveness. This result is not in agreement with numerous studies that have shown the antibacterial ability of the silver nanoparticles, either free or encapsulated. Silver is a well-known biocide component due to the activity of Ag+ ions [44–46]. For this reason, the oleic acid as surfactant employed in the silver nanoparticles synthesis could have played a crucial role in the antibacterial properties (see Section 3.5).

4Conclusions

The encapsulation of silver nanoparticles in silica nanocapsules was performed successfully, showing regular and spherical morphology. It should be mentioned that the size as well as the antibiocorrosive properties of the encapsulated AgNPs were influenced by the ratio between the TEOS and the silver nanoparticles.

Confocal analysis showed that HyAgNPs1 coating has no significant effect on the corrosion of aluminum surface. On the contrary, hybrids polymers doped with encapsulated silver nanoparticles showed a high antibacterial efficacy inhibiting the biofilm growth, where the best performance was obtained with Hy/AgNPs3 coating. No antibacterial effect was observed with free AgNPs, which might be attributed to the use of oleic acid as surfactant that could have served to feed the bacteria.

The impedance results were strongly influenced by the hydrophobicity and compactness of the films, concluding that a more hydrophobic coating considerably improves the corrosion resistance.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

Financial support for this work was provided by CONICYT (PIA, ACT 1412), Dicyt-USACH (project 051742PC-DAS), FONDECYT (Grant 11170419, 1140226, and 1160604), and AFOSR Grant FA9550-146-1-0063.

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