pubs.acs.org/Langmuir © 2010 American Chemical Society
Sonochemical Design of Cerium-Rich Anticorrosion Nanonetwork on Metal Surface Ekaterina Skorb,† Dmitry Shchukin,† Helmuth M€ohwald,† and Daria Andreeva*,‡ †
Max Planck Institute of Colloids and Interfaces, Wissenschaftspark Golm, Am M€ uhlenberg 1, Golm 14476, Germany, and ‡Physical Chemistry II, University of Bayreuth, Universit€ atstr. 30, Bayreuth 95440, Germany Received February 16, 2010. Revised Manuscript Received April 11, 2010
As a “green chemistry” tool, ultrasound irradiation of high intensity was successfully applied to formation of cerium/ aluminum oxide anticorrosion protective layers on metal surfaces. The mechanism of metal surface modification in the presence of cerium(III) aqueous solution results from two components: activation of the metal surface by localized heating and activation of cerium ions which are diffused within liquid jets at high velocity to the metal surface. The ultrasonically increased reactivity of cerium and the developed surface of the metal stimulate formation of a novel type of cerium-enriched protective layer: cerium/aluminum oxide nanonetwork, where cerium oxide and aluminum oxide are interlaced in a mixed layer strongly connected to the metal surface. A combination of microscopic and spectroscopic methods was applied to study structure and morphology of the coatings as well as to optimize the ultrasound-assisted preparation method. The anticorrosion activity of the novel cerium/aluminum oxide system was demonstrated by using the scanning vibrating electrode technique.
Introduction Salts of transition and rare-earth metals (Ce, Co, Mo) have been proposed as alternatives to carcinogenic chromate inhibitors1 due to their low toxicity,2 economic efficiency,3 and sufficient cathodic inhibition of metal corrosion.4 However, a number of the developed cerium-containing coatings exhibit moderate anticorrosion activity mostly due to their poor adhesion to the substrate and continuous leakage of the inhibitor from the surface. Recently, several concepts were developed5-9 to incorporate cerium into aluminum alloys and to provide triggered release on demand. A rare-earth metal was coupled with organocarboxylates, an organic inhibitor containing multiple functional groups, in order to produce a multifunctional inhibitor with advanced surface adhesion.5 Skully et al. developed an alloying aluminum with transition metals and lanthanides which provide a reservoir that can serve as a source for active corrosion protection by triggered release of Ce3þ, Co2þ, and MoO42- ions.6 Furthermore, the cerium-containing coating was successfully formed by simultaneous surface etching and deposition.7 Aluminum alloy has been treated by solution of cerium salts in the presence of an oxidant. As a result, a good coating adhesion to the substrate was achieved. The polarization resistance of the modified alloy in 3.5% NaCl is increased by several orders of magnitude. Thus, the most effective anticorrosion coatings with cerium were prepared by a direct incorporation of cerium ions into aluminum substrate and formation of cerium-rich metal oxide protective layer. *Corresponding author. E-mail:
[email protected]. (1) Twite, R. L.; Bierwagen, G. P. Prog. Org. Coat. 1998, 33, 91–100. (2) DHHS-NIOSH, Reg. of Toxic Effects of Chemical Substances; DHHSNIOSH Pub.:1986; Vol. 75, p 103. (3) Patnaik, P. Handbook of Inorganic Chemical Compounds; McGraw-Hill: New York, 2003; pp 199-200. (4) Oleinik, S. V.; Kuznetsov, Yu. I. Prot. Met. 2007, 43, 391–397. (5) Ho, D.; Brack, N.; Scully, J.; Markley, T.; Forsyth, M.; Hinton., B. J. Electrochem. Soc. 2006, 153, B392–B401. (6) Jakab, M. A.; Scully, J. R. Nat. Mater. 2005, 4, 667–670. (7) Xingwen, Y.; Chunan, C.; Zhiming, Y.; Derui, Z.; Zhongda, Y. Corros. Sci. 2001, 43, 1283–1294. (8) Arenas, M. A.; Conde, A.; de Damborenea, J. Corros. Sci. 2002, 44, 511–520. (9) Hamdy, A. S. Mater. Lett. 2006, 60, 2633–2637.
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The extremely high economic and environmental impact of corrosion of metallic structures requires simple and effective solutions. In our previous publications10-12 we demonstrated application of ultrasound of high intensity to interfacial activation of the aluminum alloy 2024. A highly microrough surface of aluminum oxide can be formed by sonochemical surface treatment. The mechanism of surface modification by intense ultrasound treatment based on explosive collapse of acoustic bubbles is followed by formation of a high-speed jet of liquid impinging on the surface and generating free radicals, which oxidize the surface.12-16 Thus, the surface of ultrasonically treated samples induces better wettability, adhesion, and chemical bonding with the polymer layers. Here, we explored a novel approach to formation of a cerium-rich protective layer on the surface of aluminum alloy (AA) 2024 by using the “green chemistry tool” ultrasound irradiation. A cerium/aluminum (Ce/Al) oxide outermost protective layer was formed by intensive ultrasound treatment of the metal substrate in aqueous solution of cerium(III). Ultrasound driven surface modification includes formation of a developed interfacial metal layer, incorporation and homogeneous distribution of cerium ions on the AA2024 substrate followed by surface oxidation, and formation of a Ce/Al oxide nanonetwork. The anticorrosion efficiency of the protective films was tested by the scanning vibrating electrode technique (SVET).
Results and Discussion The SEM images (Figure 1) show that the surface morphology of AA2024 plates can be modified drastically by sonication. (10) Andreeva, D. V.; Fix, D.; Shchukin, D. G.; M€ohwald, H. Adv. Mater. 2008, 20, 2789–2794. (11) Andreeva, D. V.; Fix, D.; Shchukin, D. G.; M€ohwald, H. J. Mater. Chem. 2008, 18, 1738–1740. (12) Skorb, E. V.; Shchukin, D. G.; M€ohwald, H.; Andreeva, D. V. Nanoscale 2010, DOI: 10.1039/C0NR00074D. (13) Suslick, K. S.; Price, G. J. Annu. Rev. Mater. Sci. 1999, 29, 295–326. (14) Doktycz, S. J.; Suslick, K. S. Science 1990, 247, 1067–1069. (15) Mdleleni, M. M.; Heyeon, T.; Suslick, K. S. J. Am. Chem. Soc. 1998, 120, 6189–6190. (16) Suslick, K. S.; Chloe, S.-B.; Cichowlas, A.; Grinstaff, M. W. Nature 1991, 353, 414.
Published on Web 04/21/2010
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Figure 1. Scanning electron microscopy (SEM) images of the initial aluminum alloy 2024 (a) and after 30 min sonication at 30 W/cm2 (b) and at 57 W/cm2 in water (c).
Figure 2. Scanning electron microscopy images of the aluminum alloy 2024 after 5 (a), 20 (b), and 40 min (c) sonication at 30 W/cm2 in 0.5 M cerium(III) nitrate solution.
Figure 1a is the surface of initial aluminum plate 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. Therefore, the aluminum surface is always pretreated before use. It is generally expected that ultrasonic pretreatment of metal plates produces a porous outermost metal layer with roughness sufficient for mechanical interlocking. Intensive sonication (treatment at 57 W/cm2 for 30 min) in water with an ultrasonic horn results in formation of a highly microrough interfacial layer (Figure.1c). The mechanism of the modification of metal structure under ultrasound irradiation is complex and involves a variety of aspects related to localized heating and oxidation of metal surfaces.17 This is mainly achieved through the large, but localized, forces produced by cavitation. Furthermore, cavitation is able to change the chemical composition of some of the reagents and of the water itself (through the formation of peroxides). Therefore, the new active uniform oxide layer is rapidly formed on the developed aluminum structure due to surface oxidation by free radicals. Lower intensity of sonication (30 W/cm2) stimulates the surface oxidation process only. In this case, a new interfacial morphology (Figure 1b) is formed by a rough oxide layer. This contrast ultrasound induced effects from surface heating. Ce/Al oxide protective coating was formed by sonication of 1 2 cm AA2024 plates in 40 mL of 0.5 M aqueous solution of cerium salts. Each plate was irradiated separately from the others for a varying time and at different intensity. The media temperature was kept at about 65 °C by a thermostated cell. Each metal plate was fixed by a homemade holder in the cell in order to control the distance (1 cm) to the sonotrode. It should also noted that shortlived, localized hot spots in a cold liquid produced by cavitation are characterized by temperature of ca. 5000 K, pressure of about 1000 atm, lifetime considerably less than a microsecond, and heating and cooling rates above 1010 K/s. When cavitation occurs in a liquid near a solid surface, the dynamics of the cavity collapse (17) Chemistry with Ultrasound; Mason, T J., Ed.; Elsevier Sci. Pub.: Amsterdam, 1990; Vol. 28, p 195.
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changes dramatically. In pure liquids, the cavity remains spherical during collapse because its surroundings are uniform. However, near a solid surface the cavity collapse is very asymmetric and generates high-speed jets of liquid.12-16 Moreover the highintensity ultrasound irradiation has been used for a long time in metal technology including metal crystallization, forming, and finishing.12,17 The application of ultrasound to a metal melt generally leads to metals with improved grain refinement and homogeneity.18 Thus, after treatment of the samples in cerium salt samples were washed and dried in air condition without calcination steps since proper high-intensity ultrasound treatment could provide stability and crystallization of the formed Ce-Al overlayers. The surface analysis showed that in presence of cerium(III) chloride ultrasound irradiation of the AA2024 stimulates corrosion degradation of the alloy (see Figure S1, Supporting Information). The coatings prepared from cerium(III) nitrate demonstrate the most satisfactory results. The fact that in one case we observed corrosion degradation of the alloy and in another alloy was stable also allows to conclude that pH which changes during the sonication treatment17 does not influence significantly the corrosion of alloy notwithstanding the fact that pH plays a major role in corrosion for the most metal ion systems. Two types of coating can be prepared by ultrasound treatment of aluminum plates in aqueous solution of cerium(III) nitrate varying the intensity of sonication: a micrometer-thick Ce oxide dense coating and a Ce/Al nanonetwork (Figures 2 and 3, respectively). At low intensity of sonication (30 W/cm2, SEM images in Figure 2a) an extremely dense and smooth Ce oxide interfacial layer can be formed within 5 min of sonication. However, it can be seen that a longer sonication (40 min) leads to the rupturing of the coating. The cracks are clearly observed on the SEM images for 40 min of sonication (Figure 2c). Different behavior can be monitored at 57 W/cm2 intensity of ultrasound treatment (Figure 3). The higher intensity results in more (18) Abramov, O. V. Ultrasonics 1987, 25, 73–82.
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Figure 3. Scanning electron microscopy images of the aluminum alloy 2024 after 5 (a), 20 (b), and 40 min (c) sonication at 57 W/cm2 in 0.5 M cerium(III) nitrate solution.
Figure 5. Transmission electron microscopy (TEM) image of the cross section of the aluminum alloy aa2024 covered by a cerium/ aluminum nanonetwork (a) and a dense cerium oxide layer (b). The dense cerium oxide layer is shown by the arrows. Figure 4. Dependence of the atomic concentrations of cerium on sonication time (measured by energy dispersive X-ray spectrometry (EDS). The samples were sonicated at 57 W/cm2 in 0.5 M cerium(III) nitrate solution (a) and at 30 W/cm2 in 0.5 M cerium(III) nitrate solution (b).
pronounced changes in the morphology of the metal surface. In 5 min of sonication a dense Ce oxide layer with huge cracks (Figure 3a) similar to those described above is formed. Further increase of the sonication time (20 min) results in transition into a structured interface where cerium and aluminum oxides form a nanonetwork (Figure 3b). After 30 min of sonication the metal surface is homogeneously covered by the protective Ce/Al oxide nanonetwork layer (Figure 3c). Additional sonication does not cause any further changes in surface morphology. The dependence of the atomic concentration of cerium (Figure 4) on sonication time and intensity was assessed by EDS. The SEM images and EDS spectra demonstrate that activation and modification of the metal surface is strongly influenced by time and intensity of sonication. Ultrasonication at different intensities results in a different mechanism of surface modification. At higher intensity (57 W/cm2) the cerium/aluminum oxide layer is formed very fast in 5 min of sonication (Figure 4a). Further irradiation enhance coating integrity by forming a more compact interfacial layer. At lower intensity (30 W/cm2) the cerium content is exponentially increased and reaches a maximum in 50 min of treatment. Thus, ultrasound treatment at low intensity allows more equilibrium condition during coating formation. Highly concentrated surface layers could be prepared probably due to less destructive influence of cavitation on the surface. There are two possibilities mentioned above assumption that liquid jets are responsible for the transport of cerium ions to the surface or the transport of ions from fluid flow of the liquid around collapsing asymmetric bubbles, such as those near a surface. Intensive in sonification of liquids can generate cavitation bubbles. The (19) Blake, J. R.; Keen, G. S.; Tong, R. P.; Wilson, M. Philos. Trans. R. Soc. London A 1999, 357, 251–267.
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bubbles collapse violently and in doing so generate high temperatures and pressures within the gaseous contents.19 The thickness of the modified layers was estimated by TEM of the cross sections of the plates and is ∼200 nm for a sample prepared at 30 W/cm2 and 20 nm for one prepared at 57 W/cm2 (Figure 5, the dark Ce oxide layers are highlighted by arrows). The TEM image (Figure 5b) clearly demonstrates that a 20 nm thick Ce oxide layer is deposited on a highly developed aluminum structure formed by sonication. Therefore, SEM, EDS, and TEM measurements are in good agreement and demonstrate formation of two different types of coating depending on the intensity of treatment: a 200 nm thick dense Ce oxide coating and a developed aluminum surface covered by 20 nm thick Ce oxide, so-called a Ce/Al oxide nanonetwork. The surface chemistry of the two types of coatings was investigated by ATR IR, XPS, and XRD methods. The ATR IR spectra recorded for the Ce/Al coatings prepared at different intensity and duration of sonication showed that sonication time and intensity do not cause significant changes in the chemical composition of both coatings (see Figure S2, Supporting Information). The infrared band with the maximum at 1300 and 1050 cm-1 was previously analyzed in ref 20 and was assigned to the OH stretching in metal systems. Therefore, formation of a CeO(OH)/Ce(OH)3 mixture on the surface of the alloy can be assumed. Survey XPS spectrum (Figure 6a,d) of samples sonicated in 0.5 M cerium nitrate solution for 40 min under power of 30 differs from the spectrum of 57 W/cm2 treated samples in appearance of a peak at ca. 405 eV in sample sonicated under 30 W/cm2 that is absent in that treated under 57 W/cm2. This emission comes from nitrate groups21 due to the formation in the case of samples sonicated at lower intensity thicker layer and as a result lower film adhesion (see corrosion test). XPS high-resolution core-level spectra of the Ce 3d region can be individually resolved into features grouped as “u” and “v” lines to depict the electronic (20) Kanesaka, I.; Shimizu, R. Spectrochim. Acta, Part A 2003, 59, 569–573. (21) Owens, F. J.; Sharma, J. Appl. Phys. 1980, 51, 1494–1497. (22) Trudeau, M. L.; Tschoepe, A.; Ying, J. Y. Surf. Interface Anal. 1995, 23, 219–226.
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Figure 6. X-ray photoelectron (XPS) spectra of samples after 40 min sonication treatment in 0.5 M cerium(III) nitrate solution at 30 W/cm2 (a, b, c) and 57 W/cm2 (d, e, f) intensity. Survey spectra (a, d), Ce 3d peaks (b, e), and O 1s peaks (c, f).
transitions in Ce3þ and Ce4þ, respectively.22,23 For valence þ4 cerium, v0 and v2 components represent the peaks, and v1 a satellite, in the Ce 3d5/2 spin-orbit split doublet. Correspondingly, v00 and v20 components characterize the Ce 3d3/2 doublet peaks and v00 the associated satellite.22-24 For the valence state þ3, the main components, u0 (u00 ) and associated shake-down peaks u1 (u10 ) characterize the Ce 3d5/2 (Ce 3d3/2) contribution.22,25 The deconvolution of the spectra showed well-defined peaks that enabled the estimation of the relative contribution by the Ce4þ and Ce3þ species. Thus, the peaks at 882.5 and 900.8 eV, 889.1 and 907.5 eV, and 898.3 and 916.6 eV were considered to belong to Ce4þ, while the couples at 886.0 and 904.7 eV and 880.4 and 899.3 eV are due to Ce3þ. The superficial concentrations of the Ce4þ and Ce3þ species were 39 and 61% and 48 and 52% for samples treated at 30 and 57 W/cm2, respectively. In addition, the O 1s spectra shown in Figure 6c,f are composed by two peaks at 529.4 and 531.5 eV, which may be due to oxygen in Ce-O and Ce-OH bonds, respectively. The ratio of each kind of contribution to the total of the two oxygen contributions is ca. 35 and 24% (O2-) and ca. 65 and 74% (OH-) for 30 and 57 W/cm2 treated samples, respectively. Considering the states of both Ce and O, it is proposed that the Ce films are mainly composed of cerium oxides (CeO2 and Ce2O3) and their hydrates such as Ce(OH)4 and Ce(OH)3. Moreover, both Ce coatings have amorphous structure as sharp peaks related to cerium oxides26 in XRD spectra of the Al (23) Laachir, A.; Perrichon, V.; Badri, A.; Lamotte, J.; Catherine, E.; Lavalley, J. C.; El-Fallah, J.; Hilaire, L.; Le Normand, F.; Quemere, E.; Sauvion, G. N.; Touret, O. J. Chem. Soc., Faraday Trans. 1991, 97, 1601–1609. (24) Teterin, Y. A.; Teterin, A. Y.; Lebedev, A. M.; Utkin, I. O. J. Electron Spectrosc. Relat. Phenom. 1998, 88-91, 275–279. (25) Praline, G.; Koel, B. E.; Hance, R. L.; Lee, H.-I.; White, J. M. J. Electron Spectrosc. Relat. Phenom. 1980, 21, 17–30. (26) Dos Santos, M. L.; Lima, R. C.; Riccardi, C. S.; Tranquillin, R. L.; Bueno, P. R.; Varela, J. A.; Longo, E. Mater. Lett. 2008, 62, 4509–4511.
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plates modified in the presence of cerium ions are absent. The peaks assigned to Al oxides are broader after Ce incorporation (Figure S3, Supporting Information). This effect is more pronounced for the samples prepared at higher intensity of ultrasound and characterized by formation of a Al/Ce nanonetwork interfacial layer. Thus, we prove that both physical and chemical aspects are important for surface modification of aluminum plates by ultrasound in the presence of Ce(III) ions. The physical aspect of acoustic cavitation is based on formation of a liquid jet of high velocity27 and mechanical imprinting of the metal surface. This results in removal of the natural oxide layer from AA2024, formation of a developed interfacial layer which is ideal for further modification and incorporation, and homogeneous distribution of Ce ions in the outermost metal layers. Surface oxidation during ultrasound irradiation is a chemical aspect in formation of a Cerich protective layer. Free radicals produced during cavitation28 rapidly oxidize the cerium and the metal surface and provide formation of a novel oxide layer with new morphology and properties. In the presence of cerium ions a coating consisting of a mixture of Ce/Al oxides and hydroxides is formed. At lower intensity of sonication surface oxidation is dominating; sonication at higher intensity allows deeper modification of the metal matrix. The structural and morphological difference of two types of coatings results in dramatically different stability and anticorrosion properties. The scanning vibrating electrode technique (SVET) was applied to measure the current density maps over the selected surface of the sample, thus monitoring local cathodic and anodic activity in the corrosion zones.29,30 For corrosion tests (27) Lindley, J.; Mason, T. J. Chem. Soc. Rev. 1987, 16, 275–311. (28) Weissler, A. J. Am. Chem. Soc. 1959, 81, 1077–1081. (29) Skorb, E. V.; Skirtach, A.; Sviridov, D. V.; Shchukin, D. G.; M€ohwald, H. ACS Nano 2009, 3, 1753–1760.
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Figure 7. Time monitoring of the anodic (curves 1 and 2) and cathodic (curves 3 and 4) activity on the aluminum surface after sonication at 57 W/cm2 (a) and after sonication at 30 W/cm2 (b). Curves 1 and 4 are recorded for the samples sonicated in water. Curves 2 and 3 represent anodic and cathodic activity of the samples sonicated in 0.5 M cerium(III) nitrate solution. The samples were immersed for 9 h in 0.1 M NaCl solution. Right pictures are optical microscopy images of the scratched plates covered covered with a Ce/Al oxide nanonetwork (a) or with a dense cerium rich coating (b) and on the top sol-gel coating after 9 h immersion in 0.1 M NaCl. The scratches are shown by the arrows.
the Ce-modified samples were covered by a silica-zirconia sol-gel film as described elsewhere.31 The sol-gel film was mechanically scratched in order to stimulate the corrosion degradation of the aluminum surface in the aggressive solution. This sol-gel hybrid film is currently used as anticorrosion coating for control and compares corrosion current by SVET.10,11,31 Figure 7a shows the time monitoring of the anodic (curves 1 and 2) and cathodic (curves 3 and 4) activity on the aluminum surface after sonication at 57 W/cm2 in water (Figure 7a, curves 1 and 4) and after sonication at 57 W/cm2 in 0.5 M cerium(III) nitrate solution (Figure 7a, curves 2 and 3) for 30 min. Figure 7b is the time monitoring of the anodic (curves 1 and 2) and cathodic (curves 3 and 4) activity on the aluminum surface after sonication at 30 W/cm2 in water (Figure 7b, curves 1 and 4) and after sonication at 30 W/cm2 in 0.5 M cerium(III) nitrate solution (Figure 7b, curves 2 and 3) for 30 min. The plates were immersed in 0.1 M NaCl solution. The optical images (Figure 7, right) show the scratched plates with a Ce/Al oxide nanonetwork (a) and with a dense cerium rich coating (b) after 9 h of immersion in aggressive solution. The plates sonicated at higher intensity (Figure 7a, curves 2 and 3) demonstrate very high resistance to corrosion. The moderate anodic activity was observed 2 h (scanning time) after defect formation. The development of the defect continued enhancing the anodic activity in the damaged zone. The rest of the surface emits negligible ion flux indicating the absence of the corrosion (30) He, J.; Gelling, V. J.; Tallman, D. E.; Bierwagen, G. P. J. Electrochem. Soc. 2000, 147, 3661–3672. (31) Skorb, E. V.; Fix, D.; Andreeva, D. V.; Shchukin, D. G.; M€ohwald, H. Adv. Funct. Mater. 2009, 19, 2373–2379.
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processes. The defected zone on the ceria-alumina nanonetwork was passivated in time (>2 h), and there was no propagation of the corrosion process detected after 9 h of immersion in aggressive solution. Similar anticorrosion behavior was observed for the AA2024 plates sonicated in water (Figure 7a, curves 1 and 4). An extremely intense anodic corrosion was passivated approximately in 2 h and remained constant. There is no such self-healing effect detected in the case of unmodified aluminum alloy (Figure 7a, curves 1 and 4). The anodic activity demonstrates a tendency to increase in time. The optical microphotograph (Figure 7a) shows that there are no significant changes in optic picture observer before and after the corrosion test in the case of aluminum alloy protection by a ultrasound fabricated Ce/Al oxide nanonetwork. Well-defined anodic activity is observed on the aluminum surface sonicated at an intensity of 30 W/cm2 after 15 min immersion in 0.1 M NaCl solution (Figure 7b). This activity becomes more intense with immersion time, resulting in the development of defects throughout the whole surface of the sample prepared without cerium. For the samples covered by Ce-containing coating a passivation of corrosion degradation is observed with time. However, the passivation process is relatively slow. Finally, the still very high corrosion activity leading to total corruption of the Al surface was obtained in the defected areas after 9 h of immersion of aluminum alloy in 0.1 M NaCl. The products of corrosion degradation in the case of protection by dense cerium-rich coating can be clearly seen in the optical microphotographs (Figure.7b, right). It is also seen from SVET maps (Figure 7b, curves 2 and 3) and optical image (Figure 7b) that the adhesion of sol-gel films is very low in the case of dense DOI: 10.1021/la100677d
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Figure 8. Current density maps recorded by the scanning vibrating electrode technique. The samples are covered with a Ce/Al nanonetwork (a) and with a dense cerium oxide coating (b). The immersion time in 0.1 M NaCl is 15 min (1), 2 h (2), and 9 h (3). Orange-red peak: anodic activity; blue gaps: cathodic activity. The insets are the rescaled (a2) and (a3) maps to demonstrate the weak corrosion of the Ce/Al nanonetwork protected samples.
cerium-rich coating which results in the same character of aluminum protection by dense cerium-rich coating with and without sol-gel on the top. Figure 8 shows the current density maps above the samples covered with a Ce/Al oxide nanonetwork (Figure 8a) and with a dense coating (Figure 8b) and after 15 min (1), 2 h (2), and 9 h (3) exposure to 0.1 M NaCl. Both anodic (orange-red peak) and catholic peaks (blue gaps) appear with experiment time, resulting in defect propagation throughout the whole surface of the sample covered with a dense Ce oxide-rich coating (Figure 8b). For the nanonetwork coating negligible increase of the corrosion process was monitored in 2 h of immersion only (Figure 8a). We showed the rescaled graphs in the insets (Figure 8a) to demonstrate changes in the current density above the nanonetwork coating due to a huge difference in corrosion intensity in the samples protected by these two coatings. Later on the self-healing is observed in the case of the Ce/Al oxide nanonetwork which provides much more effective anticorrosion protection than the Ce oxide coating. Notwithstanding the fact that in 2 h the corrosion activity scientifically decreases (Figures 7b and 8b), this process could not be called self-healing while the corrosion level remains on the high level (∼50 μA/cm2), and the corrosion current decrease could be attributed to the sorption of corrosion products (optic image Figure 7b) onto the surface. The mechanism of anticorrosion protection of the Ce-containing coating is based on pH triggered diffusion of the Ce ions to the defected area. The cerium consumption during the corrosion attack was monitored by EDS analysis of the scratched areas after exposure to 0.1 M NaCl solution. The concentration of the cerium in both coatings was reduced from 4 and 8 atom % recorded for 16978 DOI: 10.1021/la100677d
the nanonetwork and the dense coatings, respectively, to 0.5 atom %. Therefore, cerium can be easily released from an alloy when the coating integrity is broken and corrosion degradation of metal begins. The Ce ions are released into the corroded area and precipitate onto the cathodic sites (cathodic protection). Furthermore, it has been shown32 that cerium oxides/hydroxides were formed preferentially at sites where intermetallic compounds containing copper were located. Therefore, ultrasound driven incorporation of Ce into AA2024 suppresses the formation of local cathodes which is the driving force for pitting corrosion. The difference in anticorrosion properties is probably caused by the poor stability of the Ce oxide dense coating. The dense Ce oxide coating exhibits very poor adhesion which is clearly seen from optic image Figure 7b. The additional protective sol-gel coating which was deposited onto the cerium oxide dense coating during the corrosion prosess is removing the surface and corrosion is seen all over the scanning area. The Ce/Al nanonetwork exhibits a very good stability. Furthermore, the rough surface is a perfect substrate for the deposition of further components (sol-gel coatings, primer, and top coating33) of anticorrosion protection.
Conclusions We propose here a novel ultrasound driven method of incorporation of cerium into interfacial layer of metal plates. The mechanism of modification of metal surfaces by cerium(III) ions (32) Jaffe, L. F.; Nuccitelli, R. J. Cell Biol. 1974, 63, 614–628. (33) Roberge, P. R. Corrosion Basics, 2nd ed.; NACE Press: Bethlehem, PA, 2005; p 364.
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under ultrasound irradiation is based on mechanical imprinting of the surface by high-speed jets of liquid containing cerium(III) ions and surface oxidation by free radicals. Both processes are caused by acoustic cavitation in aqueous solution. At high intensity of sonication (30 min at 57 W/cm2) a 20 nm thick cerium/aluminum nanonetwork deposited on the developed metal surface can be formed. The cerium/aluminum nanonetwork exhibits very good adhesion to both the metal surface and the components of the anticorrosion system (here, silica-zirconia sol-gel film). Corrosion resistance of the metal surface covered by cerium/aluminum nanonetwork is significantly enhanced. Release of cerium ions incorporated into metal interfacial layer is observed after mechanical rapture (cracks) of protective coating. The proposed environmentally friendly method of metal surface modification demonstrated for the aluminum alloy 2024 is universal and can be applied to other metals and alloys in order to supply the surface with anticorrosion, antifouling, etc., properties.
Experimental Section Sonication of AA2024 Plates in the Presence of Cerium Ions. Aluminum alloy AA2024 1 2 cm samples were degreased in isopropanol flow and rinsed in purified water. Each plate was sonicated in 0.5 M aqueous solution of Ce(NO3)3 3 6H2O and CeCl3 3 7H2O (Aldrich) in a thermostated flow cell (FC100L1-1S) (at 65 °C) with the VIP1000hd (Hielscher, Germany) operated at 20 kHz with a maximal output power of 1000 W ultrasonic horn BS 2d22 (head area of 3.8 cm2) and equipped with a booster B21.2. The maximum intensity was calculated to 57 W/cm2 at mechanical amplitude of 81 μm.
Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR). A Bruker Equinox 55 FTIR fitted with a Specac Goldengate ATR stage was used for ATRFTIR measurements. All samples were examined under nitrogen gas and the instrument was controlled by a computer using Opus 2.2 software. X-ray Photoelectron Spectroscopy (XPS). XP spectra were acquired with a SPECS hemispherical energy analyzer (Phoibus 100) and SPECS focus 500 X-ray monochromator using the Al KR with an energy of 1486.74 eV. X-ray Diffraction (XRD). X-ray diffraction of the samples was studied on a Bruker AXS-D8 ADVANCE X-ray diffractometer and on a Nanostar Bruker AXS diffractometer. Microscopy Studies. Scanning electron microscopy (SEM) measurements were conducted with a Gemini Leo 1550 instrument at an operation voltage of 3 keV. Samples were sputtered
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with gold. Microprobe analysis was performed using energy dispersive spectrometry (EDS) model 6587, Pentafet Link, Oxford microanalysis group, UK. Transmission electron microscopy (TEM) images were obtained on a Zeiss EM 912 Omega transmission electron microscope operating at 300 kV. The samples were ultramicrotomed (Leica EM FC6) and placed onto the copper grids coated with carbon film. Scanning Vibrating Electrode Technique (SVET). The SVET experiments were performed by using the equipment supplied by Applicable Electronics (Forestdale, MA).34 Samples were prepared for SVET measurement by cutting into 1 2 cm2 plates. 1 1 mm2 areas were opened for the measurements; other parts of the samples were protected masking with a Polyester 5 adhesive tape (3M Co.). The anticorrosion coating of each sample was scratched to introduce a defect extending to the metal surface; the area of the defect ranges from 0.1 to 0.3 mm2. The sample was mounted in a homemade 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 is 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 aluminum plates covered by standard silica-zirconia sol-gel films were used. The synthesis and deposition of silica-zirconia sol-gel coating onto Al alloy surface were described in ref 27.
Acknowledgment. The authors thank Dmitri Fix for the help with SVET experiments and Sven Kubala and Thomas Hannappel for XPS measurements. The presented research was supported by NanoFutur program of the German Ministry of Education and Research (BMBF), MUST EU FP7 project, and Humboldt Foundation. Supporting Information Available: SEM image of the aluminum alloy sonicated in 0.5 M cerium(III) chloride solution, XRD spectra of initial AA2024 and sonicated at different conditions, ATR-IR spectra of the AA2024 sonicated at different time and intensity. This material is available free of charge via the Internet at http://pubs.acs.org. (34) Mansfeld, F. Russ. J. Electrochem. 2000, 36, 1063–1071.
DOI: 10.1021/la100677d
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