Self-Healing Anticorrosion Coatings Based on pH-Sensitive Polyelectrolyte/Inhibitor Sandwichlike Nanostructures** By Daria V. Andreeva,* Dmitri Fix, Helmuth Mo¨hwald, and Dmitry G. Shchukin High-value materials in, for example, the automotive and aerospace industries require increasingly sophisticated coatings for improved performance, self-healing, and durability, and in this respect recent developments in nanotechnology are most promising.[1,2] This improvement is important for a new generation of protective coating systems, especially when taking into consideration the banning of carcinogenic Cr(VI).[3] Standard anticorrosion coatings developed so far passively prevent the interaction of corrosive species with the metal. In the next generation of protective coatings, it is important to provide several functionalities with the essential possibility to react on external impact (pH, humidity changes, or distortion of the coating integrity), that is, to have self-healing ability. The multilayer structure of a coating, in which the components are integrated and mutually reactive, is a main point in high corrosion protection. Here, we establish a novel smart multilayer anticorrosion system that consists of polyelectrolyte and inhibitor layers deposited on aluminum alloy surfaces pretreated by sonication. Corrosion processes in particular develop fast after disruption of the protective barrier and are accompanied by a number of reactions that change the composition and properties of both the metal surface and the local environment (e.g., formation of oxides, diffusion of metal cations into the coating matrix, local changes of pH and electrochemical potential).[4,5] The selfcuring properties of the novel multilayer system are based on three mechanisms: (i) the polyelectolytes have pH-buffering activity and can stabilize the pH between values of 5 and 7.5 at the metal surface in corrosive media; (ii) the inhibitors are released from polyelectrolyte multilayers only after start of the corrosion process, directly preventing the corrosion propagation in the rusted area; and (iii) polyelectrolytes that form the coating are relatively mobile and have the tendency to seal and eliminate the mechanical cracks of the coating. A very effective solution for the preparation of self-healing anticorrosion coatings is the layer-by-layer (LbL) deposition procedure, which involves the stepwise electrostatic assembly of oppositely charged species (e.g., polyelectrolytes and inhibitors or others: proteins, nanoparticles) on a substrate
¨hwald, Dr. D. G. Shchukin [*] Dr. D. V. Andreeva, D. Fix, Prof. H. Mo Max Planck Institute of Colloids and Interfaces ¨hlenberg 1, Golm/Potsdam, 14476 (Germany) Am Mu E-mail: [email protected] [**] This work was supported by the NanoFutur program of the German Ministry of Science and Education (BMBF). Supporting Information is available online from Wiley InterScience or from the authors.
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DOI: 10.1002/adma.200800705
surface with nanometer-scale precision, and allows the formation of a coating with multiple functionality.[6,7] In addition, polyelectrolytes exhibit very good adhesion to a substrate surface and are able to seal surface defects. Their conformation is mostly dependent on the chosen polyelectrolytes and adsorption conditions and much less dependent on the substrate and charge density of the substrate surface.[8] The design of our novel anticorrosion system is schematically shown in Figure 1. The poly(ethyleneimine) (PEI, Mw 600–1000 kDa, Sigma–Aldrich), poly(styrene sulfonate) (PSS, Mw 70 kDa, Sigma–Aldrich) and 8-hydroxyquinoline (8HQ) nanolayers are deposited on the pretreated aluminum alloy AA2024 by spraying from a 2 mg mL 1 solution of polyelectrolytes in a water/ethanol (1:1, v/v) mixture. The thickness of each layer was about 5–10 nm, as measured by ellipsometry. After each deposition step the samples were washed in a water/ethanol mixture and dried with a nitrogen stream. 8-Hydroxyquinoline (8HQ) was deposited between two PSS layers from a 10 wt % solution of 8HQ in ethanol. Anticorrosion coatings have to be in close molecular contact with the surface, preventing the occlusion of corrosive species at the metal/coating interface.[9] The first polymer layer formed by PEI is positively charged and adheres to the negatively charged Al2O3 layer on the surface of the aluminum alloy. The typical aluminum surface is covered by a 3–7 nm thick natural oxide film. This thin layer is not sufficient to protect against corrosion agents and does not yield good adhesion to subsequent layers of the coating.[10] Therefore, the aluminum surface is always pretreated before use. The most extensively used surface pretreatment procedures are based on aggressive chemicals including chromate solutions.[11] It is generally expected that the surface pretreatment produces a porous oxide layer on the metal surface with roughness sufficient for mechanical interlocking. Here, we applied for the first time intensive sonication in water with an ultrasonic horn for surface pretreatment of aluminum alloy. Aluminum alloy samples AA2024 were degreased in a isopropyl alcohol flow and rinsed in purified water. The surface of the aluminum plates was etched by ultrasound (Ultrasonic Processor CV 33 Sonics Inc., 20 kHz, 500 W) within 10 minutes in pure water. One substantial benefit of ultrasound in pretreatment processing is the partial removal of the natural oxide layer from aluminum surfaces. This is mainly achieved through the large, but localized, forces produced by cavitation. Furthermore, cavitation is an input of energy into the reaction mixture known to be able to change the chemical composition of some of the reagents and of the water itself (through the formation of
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peroxides). Therefore, the new active uniform oxide layer is rapidly formed on the aluminum surface. A highly microrough surface of aluminum oxide prepared by sonication (treatment at 500 W for 10 min) is shown in Figure 2c. The infrared reflection absorption (IRRA) spectrum (IFS 66 FT-IR spectrometer from Bruker, for a more detailed description see the Supporting Information) shows significant increase of the hydroxyl group concentration in the new oxide film. The strong band at 1088 cm 1 increasing with the treatment time is attributed to vibrations of the A1–OH band.[12,13] The highly
developed surface area and the increased concentration of the surface hydroxyl groups provide high capacity and adsorption activity to aluminum alloys. The scanning electron microscopy (SEM) images (Fig. 2, Gemini Leo 1550, for a more detailed description see the Supporting Information) of the modified samples pretreated by sonication of the surface and without pretreatment show that the ultrasonic pretreatment is crucial for formation of a uniform film. The surface of ultrasonically pretreated samples exhibits better wettability, adhesion, and chemical bonding with the polymer layers.[14] It results in a homogeneous distribution of the polymer film on the aluminum surface. Figure 2a and b are SEM images of the aluminum alloys modified by the polymers without sonication. Magnification shows a very smooth surface (Fig. 2a). In this case, the polymers do not form a continuous coating but are randomly bunched on the surface. Aggregates of the PEI/PSS complex can be obviously distinguished on the SEM images (Fig. 2b). Layers of polyelectrolytes and inhibitor were formed by LbL deposition on freshly sonicated aluminum alloys. The formed LbL film is characterized by infrared absorption bands which can be assigned to all film components. The IRRA spectrum of
Figure 2. Scanning electron microscopy images of unmodified Al plates after degreasing and polishing (standard procedure) (a) and covered by polymer/ inhibitor complex (b); unmodified Al plate after sonication (c) and covered by polymer/inhibitor complex (d).
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COMMUNICATION Figure 3. Scanning vibrating electrode measurements of the ionic currents above the surface of the scratched aluminum alloy covered by the standard sol-gel film (scale units: mA cm 2, spatial resolution 150 mm, solution: 0.1 M NaCl) (a); photographs of corrosion degradation of the scratched sol-gel film during the SVET experiment (the scratch is shown by the arrows) (b). The contrast inversion going from 6 h to 16 h may be due to a thickness increase caused by corrosion.
PEI (see the Supporting Information) contains a broad peak with a maximum around 3300 cm 1 and a 3250 cm 1 shoulder that can be attributed to the NH stretching band.[15] It may be assumed that the shoulder at 3250 cm 1 corresponds to the hydrogen bonding of the NH. . .O type in complexes between the PEI film and aluminum oxide. The decrease in the
frequencies of the valence vibrations of the N–H bond in the complex indicates an enhancement in its polarity and, consequently, an increase of the acid properties of the proton. The characteristic bands from both PSS and HQ could be also distinguished in the spectrum.[16–18] Hence, all components of the coating were successfully deposited on the metal substrate.
Figure 4. Scanning vibrating electrode measurements (SVET) of the ionic currents above the surface of the scratched aluminum alloy covered by the polyelectrolyte/inhibitor coating (scale units: mA cm 2, spatial resolution 150 mm, solution: 0.1 M NaCl) (a); photographs of the behavior of the scratched surface during the SVET experiments (the scratch is shown by the arrows) (b).
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The hydrogen bond interactions between PEI and oxide layer of the aluminum alloy as well as mechanical overlapping provide tight contact between the coating and the substrate. The used scanning vibrating electrode technique (SVET, Applicable Electronics MA) allows to measure current density maps over the selected surface of the sample, thus monitoring local cathodic and anodic activity in the corrosion zones.[19] Aluminum plates covered by standard SiO2/ZrO2 sol–gel films were used to compare the efficiency of polyelectrolyte-based coatings. The synthesis and deposition of sol–gel has been described earlier in Ref. [20]. The sol-gel film was mechanically scratched (2 mm long) in order to stimulate the corrosion degradation of the aluminum surface in the aggressive solution. The samples were then introduced into the SVET device and 0.1 M NaCl solution was added immediately before the first scan. Figure 3a shows the local current maps over the unmodified aluminum alloy surface. Well defined anodic activity is observed on the aluminum surface immediately after immersion in 0.1 M NaCl solution. This activity becomes more intense with immersion time, resulting in the development of defects throughout the whole surface of the sample finally leading to total corruption of the Al surface. The product of corrosion degradation can be clearly seen around the defected area of the sol–gel film (Fig. 3b).
Figure 5. Schematic mechanism of corrosion protection.
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Figure 6. Long-term corrosion test: aluminum alloy covered by the polyelectrolyte/inhibitor coating (left) and unmodified aluminum plate (right).
The samples with polymer/inhibitor coating (Fig. 4) exhibit dramatically different behavior. Figure 4a depicts the local current maps over the surface of the aluminum alloy coated by polymer/inhibitor film after immersion in 0.1 M NaCl solution. In this case, the coating was also mechanically damaged. The scratch is highlighted by the arrows on the optical photographs (Fig. 4b). Neither anodic activity nor corrosion products were observed for the experiment time of 16 h (Fig. 4a). The corrosion suppression is also proved by SEM (see Supporting Information). It is amazing that even the nanometer-thick polyelectrolyte/ inhibitor coating provides effective corrosion protection for the aluminum alloy. LbL films consisting of polyelectrolytes and inhibitor can provide three mechanisms of corrosion protection. The schematic representation of the corrosion protection of the polyelectrolyte/inhibitor coating is shown in Figure 5. The approach to prevention of corrosion propagation on metal surfaces achieving the self-healing effect is based on suppression of accompanying physico-chemical reactions. The corrosion processes are followed by changes of the pH value in the corrosive area and metal degradation.[21] Self-healing or self-curing of the areas damaged by corrosion can be performed by three mechanisms: pH neutralization, passivation of the damaged metal surface by inhibitors entrapped between polyelectrolyte layers, and repair of the coating. The corrosion inhibitor incorporated as a component of the LbL film into the protective coating is responsible for the most-effective mechanism of corrosion suppression. Quinolines are environmentally friendly corrosion inhibitors that are attracting more and more attention as alternatives to the harmful chromates. The inhibiting activity of quinolines has been studied for copper and aluminum corrosion.[22–24] 8HQ
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products. Therefore, the smart coating enables prolonged self-healing activity. The nature and properties of the novel anticorrosion coating simultaneously provide three mechanisms of corrosion protection: passivation of the metal degradation by controlled release of inhibitor, buffering of pH changes at the corrosive area by polyelectrolyte layers, and self-curing of the film defects owing to the mobility of the polyelectrolyte constituents in the layer-by-layer assembly. The novel coating exhibits a very high resistance to corrosion attack, long term stability in aggressive media, and an environmentally friendly preparation procedure. The general procedure has been demonstrated for a surface important to the aircraft industry, but is similarly applicable for many types of surfaces, thus enabling many applications in advanced technologies. Also, here we concentrated on corrosion; the method could also be more generally applicable for self-repairing coatings such as antifungal or antifriction materials.
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was found to prevent the adsorption of chloride ions and, thus, improves the corrosion resistance owing to the formation of an insoluble chelate of aluminum that protects the oxide film.[23,24] Either mechanical (mechanical scratch) or chemical (polymer swelling due to changes of local pH) rupture of the polymer film causes release of the encapsulated inhibitor, which occurs in the damaged part of the metal surface. Therefore, the inhibitor release occurs in response to corrosion attack. This results in termination of the corrosion process and prolongation of corrosion protection. Local pH neutralization could be achieved by formation of a coating with pronounced pH-buffering activity on the metal surface, which could stabilize the pH between values of 5 and 7.5 at the metal surface in corrosive media. Considering the high versatility of the LbL assembly approach, polyelectrolyte multilayers consisting of weak polyacids or polybases can be a perspective buffering system for metal protection. Finally, defects formed in the coating by corrosive agents could be covered due to some mobility of the swollen polyelectrolyte complex. One possible explanation is that changing the film environment from strong corrosive media (different concentration of salt solutions) to mild conditions can have an effect similar to annealing. The salt breaks some of the anion–cation bonds during the corrosion attack. The salt removal during the ‘‘rest-time’’ period (pure water, for example) leads to bond reformation in a more equilibrated conformation of the polymer chains. The role of long-range ionic interactions in ‘‘autorecovery’’ of polyelectrolyte multilayers was profoundly studied.[25–27] Owing to the lower activation energy needed to break an ionic bond (in comparison with a covalent bond), partial bond breakage provides some mobility for the polymer chains. Thus, gradual evolution of one set of ionic bonds into the other permits the polymers to relax into a thermodynamically favorable conformation. The long-term coating stability in a corrosive environment was studied by dipping the samples in aqueous NaCl solution at 20 8C. The introduced coatings possess active and passive corrosion protection combining effective barrier properties with the possibility of self-healing of defects in the coating. Figure 6 shows pictures of samples coated by the polymer/ inhibitor complex (left) and without coating (right). Corrosion defects can be observed after 12 hours of immersion in 0.1 M NaCl on the unmodified aluminum, whereas the sample with the polymer/inhibitor complex does not exhibit any visible signs of corrosion attack even after 21 days of immersion. Furthermore, IRRA spectra of the covered samples immersed into NaCl solution for 24 hours exhibit all characteristic bands of the polymers and the inhibitor. Therefore, the complex adhesion is sufficient to resist spontaneous removal of the protective complex from the aluminum surface. In conclusion, we have developed a novel method of anticorrosion protection including the surface pretreatment by sonication and deposition of polyelectrolytes and inhibitors. This method results in the formation of a smart polymer carrier for environmentally friendly organic inhibitors. Release of the inhibitor is stimulated by corrosive species and corrosion
Experimental Materials: Poly(ethyleneimine) (PEI, Mw 600–1000 kDa), poly(styrene sulfonate) (PSS, Mw 70 kDa), ethanol and 8-hydroxyquinoline (8HQ) were purchased from Sigma–Aldrich, Germany. The aluminum alloy AA2024, provided by EADS Deutschland, was used as a model metal substrate. Before corrosion experiments, the surface of the aluminum alloy was pretreated in TURCO 4215 and sonicated in water at 500 W power for 10 min (Ultrasonic Processor CV 33 Sonics Inc., 20 kHz). The water used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18 MV cm. Deposition of the Self-Healing Coating: Aluminum alloy samples AA2024 were degreased in isopropyl alcohol flow and rinsed in purified water. Before polymer deposition the surface of the aluminum plates was etched by ultrasound (Ultrasonic Processor CV 33 Sonics Inc. (Switzerland) operating at 20 kHz with maximal output power of 500 W) within 10 min. The poly(ethyleneimine) and poly(styrene sulfonate) layers were deposited on degreased and etched aluminum alloy AA2024 by spraying from a 2 mg mL 1 solution of polyelectrolytes in a water/ ethanol (1:1, v/v) mixture. After each deposition step the samples were washed in a water/ethanol mixture and dried with a nitrogen stream. 8-Hydroxyquinoline was deposited by the same method as the polyelectrolytes between two PSS layers from 10 wt % solution of 8HQ in ethanol. Characterization: IRRA Spectroscopy: Spectra were acquired with an IFS 66 Fourier transform (FT)-IR spectrometer from Bruker (Ettlingen, Germany) equipped with an external reflectance unit containing a Langmuir trough setup. The infrared beam was directed through the external port of the spectrometer and was subsequently reflected by three mirrors in a rigid mount before being focused on the sample surface. A KRS-5 wire grid polarizer was placed into the optical path directly before the beam hit the sample surface. The reflected light was collected at the same angle as the angle of incidence. The light then followed an equivalent mirror path and was directed onto a narrow band mercury-cadmium-telluride detector, which was cooled by liquid nitrogen. The entire experimental setup was enclosed to reduce relative humidity fluctuations. For all measurements at 40 mN m 1, p-polarized radiation was used at an angle of incidence of 70 8. A total of 128 scans were acquired with a scanner velocity of 20 kHz at a resolution of 8 cm 1.
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The scans were co-added, apodized with the Blackman–Harris three-term function, and fast-Fourier-transformed with one level of zero-filling to produce spectral data encoded at 4 cm 1 intervals. IRRA spectra are presented as absorbance vs. wavenumber. Absorbance, also reflectance-absorbance, was obtained from lg(R/R0), where R is the single-beam reflectance of the sample and R0 the single-beam reflectance of the reference. The spectra were baseline-corrected before peak positions and intensities were determined. Peak heights rather than integrated intensities were used to minimize interference from overlapping spectral features. Microscopy Studies: SEM measurements were conducted with a Gemini Leo 1550 instrument at an operating voltage of 3 keV. Samples were sputtered with gold. Atomic force microscopy (AFM) measurements were performed in air at room temperature using a Nanoscope III Multimode AFM (Digital Instruments Inc., USA) operating in tapping mode. Scanning Vibrating Electrode Technique (SVET): The SVET experiments were performed by using the equipment supplied by Applicable Electronics (Forestdale, MA, USA). Samples were prepared for SVET measurement by cutting into 1 2 cm2 plates. 2 2 mm2 areas were opened for the measurements, other parts of the samples were protected by masking with a Polyester 5 adhesive tape (3M Company). The anticorrosion coating of each sample was scratched to introduce a defect extending to the metal surface; the area of the defect ranged from 0.1 to 0.3 mm2. The sample was mounted in a home-made epoxy-resin cell. The immersion solution was 0.1 M NaCl solution. Scans were initiated within 5 min of immersion and were collected every 2 h for the duration of the experiment, typically 20 h. Each scan consisted of 400 data points obtained on a 20 20 grid, with an integration time of 1 s per point. A complete scan required 10 min. The normal or z component of the measured current density in the plane of the vibrating electrode was plotted in 3D format over the scan area, with positive and negative current densities representing anodic and cathodic regions, respectively. To observe the degradation of the aluminum plates 2 mm long scratches were formed in the coatings. Then the samples were introduced into the SVET device and 0.1 M NaCl solution was added immediately before the first scan. As a reference the aluminum plates covered by standard sol–gel films were used. The synthesis and deposition of sol–gel was as follows: First, sol was synthesized hydrolyzing 70 wt % zirconium n-propoxide precursor in n-propanol mixed with ethylacetoacetate (1:1 volume ratio). The mixture was stirred under ultrasonic agitation at room temperature for 20 min to obtain complexation of precursor. The second organosiloxane sol was prepared hydrolyzing 3-glycidoxypropyltrimethoxysilane in 2-propanol by addition of acidified water in a 1:3:2 molar ratio. The zirconia-based sol was mixed with organosiloxane sol in a 1:2 volume ratio. The final mixture was stirred under ultrasonic agitation for 60 min and then aged for 1 h at room temperature. A more detailed description of the deposition of sol–gel coating can be found in Ref. [20]. Received: March 12, 2008 Revised: March 26, 2008 Published online: June 16, 2008
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[1] R. L. Twite, G. P. Bierwagen, Prog. Org. Coat. 1998, 33, 91. [2] P. R. Roberge, Corrosion Basics, 2nd ed., NACE Press Book, 2005, p. 364. [3] Directive 2000/53/EC of the European Parliament and the council of 18 September 2000 on end-life vehicles, Official Journal of Eur. Communities 2000, L269, 34. [4] a) E. W. Brooman, Met. Finish. 2002, 100, 42. b) J. H. Osborne, Prog. Org. Coat. 2001, 41, 280. c) E. W. Brooman, Met. Finish. 2002, 100, 104. [5] a) S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown, S. Viswanathan, Nature 2001, 409, 794. b) S. H. Cho, H. M. Andersson, S. R. White, N. R. Sottos, P. V. Braun, Adv. Mater. 2006, 18, 997. [6] a) T. R. Farhat, J. B. Schlenoff, Langmuir 2001, 17, 1184. b) D. M. DeLongchamp, M. Kastantin, P. T. Hammond, Chem. Mater. 2003, 15, 1575. [7] a) S. S. Shiratori, M. F. Rubner, Macromolecules 2000, 33, 4213. b) S. T. Dubas, J. B. Schlenoff, Langmuir 2001, 17, 7725. c) D. M. DeLongchamp, P. T. Hammond, Chem. Mater. 2003, 15, 1165. [8] G. Decher, Science 1997, 277, 1232. [9] T. R. Farhat, J. B. Schlenoff, Electrochem. Solid-State Lett. 2002, 5, B13. [10] J. D. Venables, D. K. McNamara, J. M. Chen, T. S. Sun, R. L. Hopping, Appl. Surf. Sci. 1979, 3, 88. [11] C. Spadaro, C. Dispenza, C. Sunseri, J. Phys. Condens. Matter 2006, 18, 2007. [12] E. Wolska, W. Szajda, J. Appl. Spectrosc. 1983, 38, 137. [13] A. N. Rider, D. R. Arnott, Int. J. Adhes. Adhes. 2000, 20, 209. [14] D. Hennemann, W. Brockmann, J. Adhes. 1981, 12, 297. [15] H. C. Haas, N. W. Schuler, N. R. Macdonald, J. Polym. Sci. 1972, 10, 3143. [16] Y. Tran, P. Auroy, J. Am. Chem. Soc. 2001, 123, 3644. [17] J. C. Yang, M. J. Jablonsky, J. W. Mays, Polymer 2002, 43, 5125. [18] M. D. Halls, R. Arosa, Can. J. Chem. 1998, 76, 1730. [19] J. He, V. J. Gelling, D. E. Tallman, G. P. Bierwagen, J. Electrochem. Soc. 2000, 147, 3661. [20] D. G. Shchukin, M. L. Zheludkevich, K. A. Yasakau, S. V. Lamaka, M. G. S. Ferreira, H. Mo¨hwald, Adv. Mater. 2006, 18, 1672. [21] M. L. Zheludkevich, D. G. Shchukin, K. A. Yasakau, H. Mo¨hwald, M. G. S. Ferreira, Chem. Mater. 2007, 19, 402. [22] L. Garriguess, N. Pebere, F. Dabosi, Electrochim. Acta 1996, 41, 1209. [23] G. P. Cicileo, B. M. Rosales, F. E. Farela, J. R. Vilche, Corros. Sci. 1998, 40, 1915. [24] S. Szunerits, D. R. Walt, Anal. Chem. 2002, 74, 886. [25] S. Y. Park, M. F. Rubner, A. M. Mayers, Langmuir 2002, 18, 9600. [26] N. Kotov, Nat. Mater. 2004, 3, 669. [27] C. Jiang, S. Markutsja, Y. Pikis, V. Tsukruk, Nat. Mater. 2004, 3, 721.
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¨hwald, Dr. D. G. Shchukin [*] Dr. D. V. Andreeva, D. Fix, Prof. H. Mo Max Planck Institute of Colloids and Interfaces ¨hlenberg 1, Golm/Potsdam, 14476 (Germany) Am Mu E-mail: [email protected] [**] This work was supported by the NanoFutur program of the German Ministry of Science and Education (BMBF). Supporting Information is available online from Wiley InterScience or from the authors.
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DOI: 10.1002/adma.200800705
surface with nanometer-scale precision, and allows the formation of a coating with multiple functionality.[6,7] In addition, polyelectrolytes exhibit very good adhesion to a substrate surface and are able to seal surface defects. Their conformation is mostly dependent on the chosen polyelectrolytes and adsorption conditions and much less dependent on the substrate and charge density of the substrate surface.[8] The design of our novel anticorrosion system is schematically shown in Figure 1. The poly(ethyleneimine) (PEI, Mw 600–1000 kDa, Sigma–Aldrich), poly(styrene sulfonate) (PSS, Mw 70 kDa, Sigma–Aldrich) and 8-hydroxyquinoline (8HQ) nanolayers are deposited on the pretreated aluminum alloy AA2024 by spraying from a 2 mg mL 1 solution of polyelectrolytes in a water/ethanol (1:1, v/v) mixture. The thickness of each layer was about 5–10 nm, as measured by ellipsometry. After each deposition step the samples were washed in a water/ethanol mixture and dried with a nitrogen stream. 8-Hydroxyquinoline (8HQ) was deposited between two PSS layers from a 10 wt % solution of 8HQ in ethanol. Anticorrosion coatings have to be in close molecular contact with the surface, preventing the occlusion of corrosive species at the metal/coating interface.[9] The first polymer layer formed by PEI is positively charged and adheres to the negatively charged Al2O3 layer on the surface of the aluminum alloy. The typical aluminum surface is covered by a 3–7 nm thick natural oxide film. This thin layer is not sufficient to protect against corrosion agents and does not yield good adhesion to subsequent layers of the coating.[10] Therefore, the aluminum surface is always pretreated before use. The most extensively used surface pretreatment procedures are based on aggressive chemicals including chromate solutions.[11] It is generally expected that the surface pretreatment produces a porous oxide layer on the metal surface with roughness sufficient for mechanical interlocking. Here, we applied for the first time intensive sonication in water with an ultrasonic horn for surface pretreatment of aluminum alloy. Aluminum alloy samples AA2024 were degreased in a isopropyl alcohol flow and rinsed in purified water. The surface of the aluminum plates was etched by ultrasound (Ultrasonic Processor CV 33 Sonics Inc., 20 kHz, 500 W) within 10 minutes in pure water. One substantial benefit of ultrasound in pretreatment processing is the partial removal of the natural oxide layer from aluminum surfaces. This is mainly achieved through the large, but localized, forces produced by cavitation. Furthermore, cavitation is an input of energy into the reaction mixture known to be able to change the chemical composition of some of the reagents and of the water itself (through the formation of
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peroxides). Therefore, the new active uniform oxide layer is rapidly formed on the aluminum surface. A highly microrough surface of aluminum oxide prepared by sonication (treatment at 500 W for 10 min) is shown in Figure 2c. The infrared reflection absorption (IRRA) spectrum (IFS 66 FT-IR spectrometer from Bruker, for a more detailed description see the Supporting Information) shows significant increase of the hydroxyl group concentration in the new oxide film. The strong band at 1088 cm 1 increasing with the treatment time is attributed to vibrations of the A1–OH band.[12,13] The highly
developed surface area and the increased concentration of the surface hydroxyl groups provide high capacity and adsorption activity to aluminum alloys. The scanning electron microscopy (SEM) images (Fig. 2, Gemini Leo 1550, for a more detailed description see the Supporting Information) of the modified samples pretreated by sonication of the surface and without pretreatment show that the ultrasonic pretreatment is crucial for formation of a uniform film. The surface of ultrasonically pretreated samples exhibits better wettability, adhesion, and chemical bonding with the polymer layers.[14] It results in a homogeneous distribution of the polymer film on the aluminum surface. Figure 2a and b are SEM images of the aluminum alloys modified by the polymers without sonication. Magnification shows a very smooth surface (Fig. 2a). In this case, the polymers do not form a continuous coating but are randomly bunched on the surface. Aggregates of the PEI/PSS complex can be obviously distinguished on the SEM images (Fig. 2b). Layers of polyelectrolytes and inhibitor were formed by LbL deposition on freshly sonicated aluminum alloys. The formed LbL film is characterized by infrared absorption bands which can be assigned to all film components. The IRRA spectrum of
Figure 2. Scanning electron microscopy images of unmodified Al plates after degreasing and polishing (standard procedure) (a) and covered by polymer/ inhibitor complex (b); unmodified Al plate after sonication (c) and covered by polymer/inhibitor complex (d).
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COMMUNICATION Figure 3. Scanning vibrating electrode measurements of the ionic currents above the surface of the scratched aluminum alloy covered by the standard sol-gel film (scale units: mA cm 2, spatial resolution 150 mm, solution: 0.1 M NaCl) (a); photographs of corrosion degradation of the scratched sol-gel film during the SVET experiment (the scratch is shown by the arrows) (b). The contrast inversion going from 6 h to 16 h may be due to a thickness increase caused by corrosion.
PEI (see the Supporting Information) contains a broad peak with a maximum around 3300 cm 1 and a 3250 cm 1 shoulder that can be attributed to the NH stretching band.[15] It may be assumed that the shoulder at 3250 cm 1 corresponds to the hydrogen bonding of the NH. . .O type in complexes between the PEI film and aluminum oxide. The decrease in the
frequencies of the valence vibrations of the N–H bond in the complex indicates an enhancement in its polarity and, consequently, an increase of the acid properties of the proton. The characteristic bands from both PSS and HQ could be also distinguished in the spectrum.[16–18] Hence, all components of the coating were successfully deposited on the metal substrate.
Figure 4. Scanning vibrating electrode measurements (SVET) of the ionic currents above the surface of the scratched aluminum alloy covered by the polyelectrolyte/inhibitor coating (scale units: mA cm 2, spatial resolution 150 mm, solution: 0.1 M NaCl) (a); photographs of the behavior of the scratched surface during the SVET experiments (the scratch is shown by the arrows) (b).
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The hydrogen bond interactions between PEI and oxide layer of the aluminum alloy as well as mechanical overlapping provide tight contact between the coating and the substrate. The used scanning vibrating electrode technique (SVET, Applicable Electronics MA) allows to measure current density maps over the selected surface of the sample, thus monitoring local cathodic and anodic activity in the corrosion zones.[19] Aluminum plates covered by standard SiO2/ZrO2 sol–gel films were used to compare the efficiency of polyelectrolyte-based coatings. The synthesis and deposition of sol–gel has been described earlier in Ref. [20]. The sol-gel film was mechanically scratched (2 mm long) in order to stimulate the corrosion degradation of the aluminum surface in the aggressive solution. The samples were then introduced into the SVET device and 0.1 M NaCl solution was added immediately before the first scan. Figure 3a shows the local current maps over the unmodified aluminum alloy surface. Well defined anodic activity is observed on the aluminum surface immediately after immersion in 0.1 M NaCl solution. This activity becomes more intense with immersion time, resulting in the development of defects throughout the whole surface of the sample finally leading to total corruption of the Al surface. The product of corrosion degradation can be clearly seen around the defected area of the sol–gel film (Fig. 3b).
Figure 5. Schematic mechanism of corrosion protection.
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Figure 6. Long-term corrosion test: aluminum alloy covered by the polyelectrolyte/inhibitor coating (left) and unmodified aluminum plate (right).
The samples with polymer/inhibitor coating (Fig. 4) exhibit dramatically different behavior. Figure 4a depicts the local current maps over the surface of the aluminum alloy coated by polymer/inhibitor film after immersion in 0.1 M NaCl solution. In this case, the coating was also mechanically damaged. The scratch is highlighted by the arrows on the optical photographs (Fig. 4b). Neither anodic activity nor corrosion products were observed for the experiment time of 16 h (Fig. 4a). The corrosion suppression is also proved by SEM (see Supporting Information). It is amazing that even the nanometer-thick polyelectrolyte/ inhibitor coating provides effective corrosion protection for the aluminum alloy. LbL films consisting of polyelectrolytes and inhibitor can provide three mechanisms of corrosion protection. The schematic representation of the corrosion protection of the polyelectrolyte/inhibitor coating is shown in Figure 5. The approach to prevention of corrosion propagation on metal surfaces achieving the self-healing effect is based on suppression of accompanying physico-chemical reactions. The corrosion processes are followed by changes of the pH value in the corrosive area and metal degradation.[21] Self-healing or self-curing of the areas damaged by corrosion can be performed by three mechanisms: pH neutralization, passivation of the damaged metal surface by inhibitors entrapped between polyelectrolyte layers, and repair of the coating. The corrosion inhibitor incorporated as a component of the LbL film into the protective coating is responsible for the most-effective mechanism of corrosion suppression. Quinolines are environmentally friendly corrosion inhibitors that are attracting more and more attention as alternatives to the harmful chromates. The inhibiting activity of quinolines has been studied for copper and aluminum corrosion.[22–24] 8HQ
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products. Therefore, the smart coating enables prolonged self-healing activity. The nature and properties of the novel anticorrosion coating simultaneously provide three mechanisms of corrosion protection: passivation of the metal degradation by controlled release of inhibitor, buffering of pH changes at the corrosive area by polyelectrolyte layers, and self-curing of the film defects owing to the mobility of the polyelectrolyte constituents in the layer-by-layer assembly. The novel coating exhibits a very high resistance to corrosion attack, long term stability in aggressive media, and an environmentally friendly preparation procedure. The general procedure has been demonstrated for a surface important to the aircraft industry, but is similarly applicable for many types of surfaces, thus enabling many applications in advanced technologies. Also, here we concentrated on corrosion; the method could also be more generally applicable for self-repairing coatings such as antifungal or antifriction materials.
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was found to prevent the adsorption of chloride ions and, thus, improves the corrosion resistance owing to the formation of an insoluble chelate of aluminum that protects the oxide film.[23,24] Either mechanical (mechanical scratch) or chemical (polymer swelling due to changes of local pH) rupture of the polymer film causes release of the encapsulated inhibitor, which occurs in the damaged part of the metal surface. Therefore, the inhibitor release occurs in response to corrosion attack. This results in termination of the corrosion process and prolongation of corrosion protection. Local pH neutralization could be achieved by formation of a coating with pronounced pH-buffering activity on the metal surface, which could stabilize the pH between values of 5 and 7.5 at the metal surface in corrosive media. Considering the high versatility of the LbL assembly approach, polyelectrolyte multilayers consisting of weak polyacids or polybases can be a perspective buffering system for metal protection. Finally, defects formed in the coating by corrosive agents could be covered due to some mobility of the swollen polyelectrolyte complex. One possible explanation is that changing the film environment from strong corrosive media (different concentration of salt solutions) to mild conditions can have an effect similar to annealing. The salt breaks some of the anion–cation bonds during the corrosion attack. The salt removal during the ‘‘rest-time’’ period (pure water, for example) leads to bond reformation in a more equilibrated conformation of the polymer chains. The role of long-range ionic interactions in ‘‘autorecovery’’ of polyelectrolyte multilayers was profoundly studied.[25–27] Owing to the lower activation energy needed to break an ionic bond (in comparison with a covalent bond), partial bond breakage provides some mobility for the polymer chains. Thus, gradual evolution of one set of ionic bonds into the other permits the polymers to relax into a thermodynamically favorable conformation. The long-term coating stability in a corrosive environment was studied by dipping the samples in aqueous NaCl solution at 20 8C. The introduced coatings possess active and passive corrosion protection combining effective barrier properties with the possibility of self-healing of defects in the coating. Figure 6 shows pictures of samples coated by the polymer/ inhibitor complex (left) and without coating (right). Corrosion defects can be observed after 12 hours of immersion in 0.1 M NaCl on the unmodified aluminum, whereas the sample with the polymer/inhibitor complex does not exhibit any visible signs of corrosion attack even after 21 days of immersion. Furthermore, IRRA spectra of the covered samples immersed into NaCl solution for 24 hours exhibit all characteristic bands of the polymers and the inhibitor. Therefore, the complex adhesion is sufficient to resist spontaneous removal of the protective complex from the aluminum surface. In conclusion, we have developed a novel method of anticorrosion protection including the surface pretreatment by sonication and deposition of polyelectrolytes and inhibitors. This method results in the formation of a smart polymer carrier for environmentally friendly organic inhibitors. Release of the inhibitor is stimulated by corrosive species and corrosion
Experimental Materials: Poly(ethyleneimine) (PEI, Mw 600–1000 kDa), poly(styrene sulfonate) (PSS, Mw 70 kDa), ethanol and 8-hydroxyquinoline (8HQ) were purchased from Sigma–Aldrich, Germany. The aluminum alloy AA2024, provided by EADS Deutschland, was used as a model metal substrate. Before corrosion experiments, the surface of the aluminum alloy was pretreated in TURCO 4215 and sonicated in water at 500 W power for 10 min (Ultrasonic Processor CV 33 Sonics Inc., 20 kHz). The water used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18 MV cm. Deposition of the Self-Healing Coating: Aluminum alloy samples AA2024 were degreased in isopropyl alcohol flow and rinsed in purified water. Before polymer deposition the surface of the aluminum plates was etched by ultrasound (Ultrasonic Processor CV 33 Sonics Inc. (Switzerland) operating at 20 kHz with maximal output power of 500 W) within 10 min. The poly(ethyleneimine) and poly(styrene sulfonate) layers were deposited on degreased and etched aluminum alloy AA2024 by spraying from a 2 mg mL 1 solution of polyelectrolytes in a water/ ethanol (1:1, v/v) mixture. After each deposition step the samples were washed in a water/ethanol mixture and dried with a nitrogen stream. 8-Hydroxyquinoline was deposited by the same method as the polyelectrolytes between two PSS layers from 10 wt % solution of 8HQ in ethanol. Characterization: IRRA Spectroscopy: Spectra were acquired with an IFS 66 Fourier transform (FT)-IR spectrometer from Bruker (Ettlingen, Germany) equipped with an external reflectance unit containing a Langmuir trough setup. The infrared beam was directed through the external port of the spectrometer and was subsequently reflected by three mirrors in a rigid mount before being focused on the sample surface. A KRS-5 wire grid polarizer was placed into the optical path directly before the beam hit the sample surface. The reflected light was collected at the same angle as the angle of incidence. The light then followed an equivalent mirror path and was directed onto a narrow band mercury-cadmium-telluride detector, which was cooled by liquid nitrogen. The entire experimental setup was enclosed to reduce relative humidity fluctuations. For all measurements at 40 mN m 1, p-polarized radiation was used at an angle of incidence of 70 8. A total of 128 scans were acquired with a scanner velocity of 20 kHz at a resolution of 8 cm 1.
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The scans were co-added, apodized with the Blackman–Harris three-term function, and fast-Fourier-transformed with one level of zero-filling to produce spectral data encoded at 4 cm 1 intervals. IRRA spectra are presented as absorbance vs. wavenumber. Absorbance, also reflectance-absorbance, was obtained from lg(R/R0), where R is the single-beam reflectance of the sample and R0 the single-beam reflectance of the reference. The spectra were baseline-corrected before peak positions and intensities were determined. Peak heights rather than integrated intensities were used to minimize interference from overlapping spectral features. Microscopy Studies: SEM measurements were conducted with a Gemini Leo 1550 instrument at an operating voltage of 3 keV. Samples were sputtered with gold. Atomic force microscopy (AFM) measurements were performed in air at room temperature using a Nanoscope III Multimode AFM (Digital Instruments Inc., USA) operating in tapping mode. Scanning Vibrating Electrode Technique (SVET): The SVET experiments were performed by using the equipment supplied by Applicable Electronics (Forestdale, MA, USA). Samples were prepared for SVET measurement by cutting into 1 2 cm2 plates. 2 2 mm2 areas were opened for the measurements, other parts of the samples were protected by masking with a Polyester 5 adhesive tape (3M Company). The anticorrosion coating of each sample was scratched to introduce a defect extending to the metal surface; the area of the defect ranged from 0.1 to 0.3 mm2. The sample was mounted in a home-made epoxy-resin cell. The immersion solution was 0.1 M NaCl solution. Scans were initiated within 5 min of immersion and were collected every 2 h for the duration of the experiment, typically 20 h. Each scan consisted of 400 data points obtained on a 20 20 grid, with an integration time of 1 s per point. A complete scan required 10 min. The normal or z component of the measured current density in the plane of the vibrating electrode was plotted in 3D format over the scan area, with positive and negative current densities representing anodic and cathodic regions, respectively. To observe the degradation of the aluminum plates 2 mm long scratches were formed in the coatings. Then the samples were introduced into the SVET device and 0.1 M NaCl solution was added immediately before the first scan. As a reference the aluminum plates covered by standard sol–gel films were used. The synthesis and deposition of sol–gel was as follows: First, sol was synthesized hydrolyzing 70 wt % zirconium n-propoxide precursor in n-propanol mixed with ethylacetoacetate (1:1 volume ratio). The mixture was stirred under ultrasonic agitation at room temperature for 20 min to obtain complexation of precursor. The second organosiloxane sol was prepared hydrolyzing 3-glycidoxypropyltrimethoxysilane in 2-propanol by addition of acidified water in a 1:3:2 molar ratio. The zirconia-based sol was mixed with organosiloxane sol in a 1:2 volume ratio. The final mixture was stirred under ultrasonic agitation for 60 min and then aged for 1 h at room temperature. A more detailed description of the deposition of sol–gel coating can be found in Ref. [20]. Received: March 12, 2008 Revised: March 26, 2008 Published online: June 16, 2008
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[1] R. L. Twite, G. P. Bierwagen, Prog. Org. Coat. 1998, 33, 91. [2] P. R. Roberge, Corrosion Basics, 2nd ed., NACE Press Book, 2005, p. 364. [3] Directive 2000/53/EC of the European Parliament and the council of 18 September 2000 on end-life vehicles, Official Journal of Eur. Communities 2000, L269, 34. [4] a) E. W. Brooman, Met. Finish. 2002, 100, 42. b) J. H. Osborne, Prog. Org. Coat. 2001, 41, 280. c) E. W. Brooman, Met. Finish. 2002, 100, 104. [5] a) S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown, S. Viswanathan, Nature 2001, 409, 794. b) S. H. Cho, H. M. Andersson, S. R. White, N. R. Sottos, P. V. Braun, Adv. Mater. 2006, 18, 997. [6] a) T. R. Farhat, J. B. Schlenoff, Langmuir 2001, 17, 1184. b) D. M. DeLongchamp, M. Kastantin, P. T. Hammond, Chem. Mater. 2003, 15, 1575. [7] a) S. S. Shiratori, M. F. Rubner, Macromolecules 2000, 33, 4213. b) S. T. Dubas, J. B. Schlenoff, Langmuir 2001, 17, 7725. c) D. M. DeLongchamp, P. T. Hammond, Chem. Mater. 2003, 15, 1165. [8] G. Decher, Science 1997, 277, 1232. [9] T. R. Farhat, J. B. Schlenoff, Electrochem. Solid-State Lett. 2002, 5, B13. [10] J. D. Venables, D. K. McNamara, J. M. Chen, T. S. Sun, R. L. Hopping, Appl. Surf. Sci. 1979, 3, 88. [11] C. Spadaro, C. Dispenza, C. Sunseri, J. Phys. Condens. Matter 2006, 18, 2007. [12] E. Wolska, W. Szajda, J. Appl. Spectrosc. 1983, 38, 137. [13] A. N. Rider, D. R. Arnott, Int. J. Adhes. Adhes. 2000, 20, 209. [14] D. Hennemann, W. Brockmann, J. Adhes. 1981, 12, 297. [15] H. C. Haas, N. W. Schuler, N. R. Macdonald, J. Polym. Sci. 1972, 10, 3143. [16] Y. Tran, P. Auroy, J. Am. Chem. Soc. 2001, 123, 3644. [17] J. C. Yang, M. J. Jablonsky, J. W. Mays, Polymer 2002, 43, 5125. [18] M. D. Halls, R. Arosa, Can. J. Chem. 1998, 76, 1730. [19] J. He, V. J. Gelling, D. E. Tallman, G. P. Bierwagen, J. Electrochem. Soc. 2000, 147, 3661. [20] D. G. Shchukin, M. L. Zheludkevich, K. A. Yasakau, S. V. Lamaka, M. G. S. Ferreira, H. Mo¨hwald, Adv. Mater. 2006, 18, 1672. [21] M. L. Zheludkevich, D. G. Shchukin, K. A. Yasakau, H. Mo¨hwald, M. G. S. Ferreira, Chem. Mater. 2007, 19, 402. [22] L. Garriguess, N. Pebere, F. Dabosi, Electrochim. Acta 1996, 41, 1209. [23] G. P. Cicileo, B. M. Rosales, F. E. Farela, J. R. Vilche, Corros. Sci. 1998, 40, 1915. [24] S. Szunerits, D. R. Walt, Anal. Chem. 2002, 74, 886. [25] S. Y. Park, M. F. Rubner, A. M. Mayers, Langmuir 2002, 18, 9600. [26] N. Kotov, Nat. Mater. 2004, 3, 669. [27] C. Jiang, S. Markutsja, Y. Pikis, V. Tsukruk, Nat. Mater. 2004, 3, 721.
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