pubs.acs.org/Langmuir © 2009 American Chemical Society
Salt Exclusion in Silane-Laced Epoxy Coatings Peng Wang and Dale W. Schaefer* Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0012 Received June 9, 2009 The corrosion protection mechanism of a one-step silane-laced epoxy coating system was investigated using neutron reflectivity. Pure epoxy and silane-laced epoxy films were examined at equilibrium with saturated NaCl water solution. The results demonstrate that the addition of silane introduces a salt-exclusion effect to epoxy coating. Specifically, the addition of silane densifies the epoxy network, which leads to exclusion of hydrated salt ions by a size effect. The effect is particularly significant at the metal-coating interface. Exclusion of ions improves the corrosion resistance, particularly for metals susceptible to pitting.
1. Introduction Bis-silanes with the general formula of (RO)3Si(CH2)3R’-(CH2)3Si(OR)3, where OR represents an alkoxy group and R’ is an organic functionality, improve the corrosionprotection performance of novolac epoxy coatings compared to neat epoxy.1 Although the addition of silane does decrease the equilibrium water absorption, particularly at the film-metal interface where silane is enriched,2 silane does not make the coating a water barrier. Water penetrates the entire coating and reaches the metal-coating interface.2 Therefore, the protection mechanism must involve other corrosion-current-barrier effects beyond simple hydrophobicity. Here we investigate the influence of silane on electrolyte penetration in epoxy films. We use neutron reflectivity (NR) to investigate the impact of silane on electrolyte distribution in the film. Salt exclusion is particularly important for alloys that are susceptible to pitting corrosion. The high strength aluminum alloy 2024 (AA2024), for example, is susceptible to pitting corrosion because of the inhomogeneous distribution of Cu.3-5 In a pitting scenario,4 anions (typical Cl-) diffuse into an active pit and provide the counterions for the protons produced by the anodic corrosion reaction: Alþ3Cl - þ3H2 O f AlðOHÞ3 þ3HClþ3e Inside the pit,6,7 the concentrated metal ions formed by anodic dissociation lead to the migration of Cl- into the pit for charge neutrality. The process is autocatalytic in the sense that the acid produced by the dissolution enhances the corrosion rate. The *To whom correspondence should be addressed. E-mail: dale.schaefer@ uc.edu. (1) van Ooij, W. J.; Seth, A.; Maguda, T.; Pan, G.; Schaefer, D. W. In A Novel Self-Priming Coating for Corrosion Protection, International Surface Engineering Congress and Exposition, 2005. (2) Wang, P.; Schaefer, D. W. Langmuir 2008, 24(23), 13496–13501. (3) Davis, J. R., Corrosion of Aluminum and Aluminum Alloys, 4th ed.; ASM International: Materials Park, OH, 1999. (4) Jones, D. A., Principles and Prevention of Corrosion. Prentice-Hall Inc.: Upper Saddle River, NJ, 1996. (5) Vargel, C., Corroison of Aluminum, 1st ed.; Elsevier SAS: Paris, France, 2004. (6) Rao, K. S.; Rao, K. P. Transactions of the Indian Institute of Metals 2004, 57 (6), 593–610. (7) Szklarska-Smialowska, Z. Corros. Sci. 1998, 41, 1743–1747.
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electrolyte cation (typically Naþ) plays the analogous role at the cathode (Cu inclusions in AA2024) 4Naþ þ4e - þO2 þ2H2 O f 4NaOH It is reasonable to believe, therefore, that the performance of a coating is closely tied to ion transport, particularly the anion, which has to migrate into the isolated pit. The exclusion of ions from thin films has been studied in depth in the context of filtration and biopermeation.8-14 In those cases, however, the film separates bulk solutions, which makes it easy to determine the species in the solution on both sides of the film. For portective coatings, however, no bulk solution exists on the metal side. In this case, permeation could be studied either by making a free-standing film, which may differ chemically and/or physically from the corresponding coating, or by electrochemical impedance spectroscopy (EIS). EIS probes energy storage and dissipation properties over a range of frequencies. The correlation to salt content is not straightforward since the observed properties are only indirectly related to the ion distribution. Moreover, EIS is sensitive to tiny flaws, which can cause large errors especially when the coating is thin. The special features of NR make it a suitable probe for the salt exclusion study. First, NR is able to examine not only the top coating but also the buried layers nondestructively. Second, the measured reflectivity is determined by the scattering length density (SLD) profile perpendicular to the surface. The SLD profile is determined by the local density and chemical composition. Salt penetration is determined from the observed SLD change when the coating is challenged with salt solution. Third, NR is immune to tiny flaws because the signal collected is an average over a large illuminated area (up to 8 cm diameter). (8) Lyklema, J., Phys. Rev. E 2005, 71, (3). (9) Westreich, P.; Fortier, H.; Flynn, S.; Foster, S.; Dahn, J. R. J. Phys. Chem. C 2007, 111(9), 3680–3684. (10) Bonnet-Gonnet, C.; Leikin, S.; Chi, S.; Rau, D. C.; Parsegian, V. A. J. Phys. Chem. B 2001, 105(9), 1877–1886. (11) Dubois, M.; Zemb, T.; Belloni, L.; Delville, A.; Levitz, P.; Setton, R. J. Chem. Phys. 1992, 96(3), 2278–2286. (12) Cleland, R. L. Macromolecules 1982, 15(2), 382–386. (13) Vlachy, V.; Haymet, A. D. J. J. Electroanal. Chem. 1990, 283(1-2), 77–85. (14) Leung, K.; Rempe, S. B.; Lorenz, C. D. Phys. Rev. Lett. 2006, 96, 9.
Published on Web 08/28/2009
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Figure 1. (a) Neutron reflectivity from a silicon wafer coated with the pure epoxy in as-prepared state. The curve through the data points is the best fit corresponding to the SLD profile in (b).
2. Experimental Section NR was performed on the Surface Profile Analysis Reflectometer (SPEAR) at the Los Alamos National Laboratory at ambient temperature. Neutron reflectivity, R(q), defined as the intensity ratio between reflected and incident neutron beam fluxes, is measured as a function of the scattering vector, q = (4π/λ)sinθ, where θ is the angle of incidence and λ is the neutron wavelength.15-18 For our measurements q was varied by collecting intensity for a range of different wavelengths at a fixed angle of incidence. The incident wavelength distribution ranged from 1.4 to 16 A˚. The wavelength of the detected neutrons is determined by time-of-flight. The reflectivity curves are obtained by merging data from two angles of incidence. The reflectivity curve is inverted to yield the SLD profile normal to the surface. The neutron SLD is a function of density and atomic composition, described as eq 1: SLDneutron ¼ F
X NA atoms bi M i¼1
ð1Þ
where bi is the coherent scattering length of the ith atom, F is mass density, M is molecular weight, NA is Avogadro’s number. Curve fitting of NR data was done using the software “Parratt32,” which is based on recursive Parratt formalism.15 To obtain agreement between the simulated and measured reflectivity, Parratt optimizes the parameters of a candidate real-space SLD profile model by means of nonlinear regression. The simplest reasonable model is accepted if more than one real-space model fit the data. In order to reveal the underlying mechanism of salt partitioning in the film, the coating system was simplified by retaining only key ingredients: novolac epoxy resin, silane, and curing agent in the same ratio as original recipe formulated by van Ooij et al.1 Two kinds of coating films were spin-coated on silicon slabs (one-side-polished, 3 in. diameter single crystal (111) wafers with a thickness of 5 mm obtained from Wafer World, Inc., West Palm Beach, FL) at 2000 rpm and investigated by NR: pure epoxy and epoxy-silane films. The term “pure epoxy film” refers to the films made from simplified recipe without silane. (15) Parratt, L. G. Phys. Rev. Lett. 1954, 95(2), 359–369. (16) Roe, R.-J., Methods of X-Ray and Neutron Scattering in Polymer Science; Oxford University Press: New York, 2000. (17) Russell, T. P. Annu. Rev. Mater. Sci. 1991, 21, 249–268. (18) Russell, T. P. Phys. B 1996, 221, 267–283.
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The precursor solution for epoxy-silane coating film is made by dissolving the epoxy resin (EPON resin SU-8, acquired from Resolution Performance Products, Houston, TX), curing agent (EPIKURE 6870-W-53, a commercial modified polyamine adduct curing agent acquired form Resolution Performance Products) and bis-sulfur silane (bis[3-(triethoxysilyl)propyl]tetrasulfide, provided by Momentive Performance Materials) at weight ratio of 7:2:1 in a mixture of THF and toluene (volume ratio of THF to toluene is 7:3). For the precursor solution of pure epoxy samples, the only difference is the absence of bis-sulfur silane in the recipe (the epoxy resin, curing agent and bis-sulfur silane at weight ratio of 7:2:0). The bis-sulfur silane was prehydrolyzed following the procedures of Pan et al.19 before being mixed with epoxy and curing agent. After spin-coating the samples were dried and cured in an oven at 150 °C for 1 h and then kept in a desiccated environment before measurement. More detailed information about coating formulation and sample preparation procedures can be found elsewhere.2 The original state of the coating was established by measuring the samples in the as-prepared state in a sealed aluminum can with desiccant present. The liquid water (H2O) and saturated-sodiumchloride solution-exposure experiments were performed by mounting the sample with the coating side against the liquid and shooting from the silicon substrate side through the silicon slab. The liquid H2O contact experiments were performed as a baseline to compare with the saltwater-conditioned state. The reason that H2O was used rather than D2O is that the SLD of D2O is too high, which masks the effect of salt absorption. Due to the relatively small absolute SLD value of H2O and saturated NaCl H2O solution, the induced SLD change due to the presence of salt is not large, which compromises data interpretation. Therefore, the redried state was also investigated to verify the conclusions based on the liquid contact experiments by measuring the samples dried in air without rinsing after saltwater exposure. Because the penetrant usually resides in the molecular level free space inside the film,20 the penetrant volume fraction, (φ, can be calculated by the following equation if no swelling occurs: φ¼
SLDsaturated -SLDas -prepared SLDpenetrant
ð2Þ
(19) Pan, G.; Schaefer, D. W.; van Ooij, W. J.; Kent, M. S.; Majewski, J.; Yim, H. Thin Solid Films 2006, 515, 2771–2780. (20) Wang, Y.; Wang, P.; Kohls, D.; Hamilton, W. A.; Schaefer, D. W., Water absorption and transport in bis-amino silane films. In Silanes and Other Coupling Agents; Mittal, K. L., Ed.; VSP: Leiden, Netherlands, 2008; Vol. 5.
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Figure 2. (a) Neutron reflectivity from a silicon wafer coated with the epoxy-silane mixture in the as-prepared state. The curve through the data points is the best fit corresponding to the SLD profile in (b).
Figure 3. (a) Neutron reflectivity from a silicon wafer coated with the pure epoxy under pure H2O (green O) and saturated NaCl H2O solution (blue 4) treatments. The solid curves through the data points are the best fits corresponding to the SLD profiles in (b). where the subscripts “saturated” and “as-prepared” refers to the state of equilibrium with aqueous environment and the virgin dry state respectively. In the presence of swelling or shrinking, eq 2 is modified to eq 3: φ¼
SLDsaturated tsaturated -SLDas -prepared tas -prepared SLDpenetrant tsaturated
ð3Þ
where t is the thickness of the coating.
3. Results and Discussion 3.1. As-Prepared State. The virgin structure of each sample is revealed by the SLD profile in the as-prepared dry state. The reflectivity curves from as-prepared pure epoxy and epoxy-silane coated samples are shown in Figure 1(a) and Figure 2(a) respectively. These results are consistent with our previous measurements.2 For the pure-epoxy sample, a simple one-layer uniform model fits the experimental data well (Figure 1(a)). In the SLD profile (Figure 1(b)), the top surface of silicon slab was picked as the origin. The oxide-covered single crystal silicon substrate is to the 236 DOI: 10.1021/la902066m
left of the origin. The thin surface silicon dioxide layer has an SLD of 3.47510-6 A˚-2. The single crystal silicon layer was treated as infinitely thick with an SLD of 2.07310-6 A˚-2. Roughness of substrate is set to less than 5 A˚, on the assumption of a sharp interface between substrate and coating. To the right-hand of the origin, the epoxy forms a uniform film with a thickness of 1275 ( 5 A˚. The SLD of the epoxy layer is (1.40 ( 0.01)10-6 A˚-2. The roughness of epoxy layer top surface is 20 ( 5 A˚. For the epoxy-silane film, a layered model is required to get a good fit to the reflectivity curve. As shown in Figure 2(b), the structure consists of a bottom layer at the silicon dioxide interface with a thickness of (35 ( 5) A˚ plus a top layer accounting for the rest of thickness of the film (1175 ( 5 A˚). The mean neutron SLD of bottom layer is (1.8 ( 0.1)10-6 A˚-2, while the top layer is (1.60 ( 0.01)10-6 A˚-2. The bottom layer has an interface width comparable to its thickness, which indicates the bottom layer is actually a transitional region gradually changing its composition to the top bulk layer. The air-side surface roughness is 30 ( 5 A˚, which is slightly rougher than the pure epoxy coated sample. 3.2. Saturated Saltwater Treatment. To investigate the salt exclusion properties of pure epoxy and epoxy-silane films, room Langmuir 2010, 26(1), 234–240
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Figure 4. (a) Neutron reflectivity from a silicon wafer coated with the epoxy-silane mixture under pure H2O (green O) and saturated NaCl H2O solution (blue 4) treatments. The solid curves through the data points are the best fits corresponding to the SLD profiles in (b). The solid red curve in (b) corresponds to the as-prepared dry state.
Figure 5. (a) Neutron reflectivity data (blue Δ) from a silicon wafer coated with pure epoxy under saturated NaCl H2O solution conditioning. The solid line through the data points is the closest fit corresponding to the model without interface salt accumulation in (b). The fit is inadequate, showing that the higher SLD region at the interface is necessary to fit the data.
temperature, liquid-H2O contact experiments were performed as a baseline to compare with the saltwater conditioned films. The reflectivity curves from pure epoxy and epoxy-silane coated samples in equilibrium with pure H2O (green O) and saturated NaCl H2O solution (blue Δ) are shown in Figure 3(a) and Figure 4(a) respectively. No critical edge was observed for either sample due to the negative SLD of liquid H2O (-5.61 10-7 A˚-2) and salt water (-6.0210-8 A˚-2), which now act as the substrates. As a result the reflectivity curves cannot be normalized since total reflection is not possible. A scaling factor was utilized to adjust the vertical position of reflectivity curves during the data analysis. The resulting SLD profile is not affected, since the shape and lateral position of the reflection curve is unchanged. 3.2.1. (a) Pure Epoxy Film. When exposed to liquid H2O it is not surprising that the pure epoxy film behaves similarly to the case when exposed to liquid D2O. As shown in Figure 3(b), the H2O penetrates and swells the film. At equilibrium, the total thickness of coating increases from 1275 ( 5 A˚ to 1352 ( 5 A˚, a 6% swelling effect. The SLD of bulk film decreases to (1.34 ( 0.05) 10-6 A˚-2, corresponding to a H2O volume fraction of 11%. A (40 ( 5) A˚ H2O-enriched region appears at the substrate-epoxy interface. The average SLD of this layer is Langmuir 2010, 26(1), 234–240
(1.2 ( 0.1) 10-6 A˚-2, implying the H2O volume fraction is 40%. The roughness of the epoxy coating is (30 ( 5) A˚. When treated with saturated NaCl solution, the reflectivity curve shifts to the right compared to the case of pure H2O contact (Figure 3(a)). According to the best-fit SLD profile in Figure 3(b), the film swells to the same extent as H2O contact. The SLD of bulk film is the same as the as-prepared epoxy ((1.4 ( 0.05)10-6 A˚-2), due to the fact that the absolute value of saturated salt water SLD (-6.02 10-8 A˚-2) is smaller than the SLD of pure epoxy by 2 orders of magnitude. Interestingly, a higher SLD region of thickness (40 ( 5) A˚ was observed at the substrate-epoxy interface. The average SLD of this region is (1.8 ( 0.1) 10-6 A˚-2, which is 64% greater than pure epoxy. Since neither H2O nor salt water is able to increase the SLD of pure epoxy substantially due to their small SLD values, it is reasonable to believe that the excess salt accumulates at the substrate-epoxy interface. The calculated volume fraction of accumulated salt is 0.014 by using eq 4. SLDmeasured -SLDsaltwater-saturated ð4Þ φsaltaccumulation ¼ SLDNaCl where the subscript “saltwater saturated” refers to the calculated SLD of interface layer saturated with salt water (40 vol% as measured above) but without salt enrichment. DOI: 10.1021/la902066m
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Figure 6. (a) Neutron reflectivity data (brown )) from a silicon wafer coated with pure epoxy in redried state after saltwater conditioning. The solid curves through the data points are the best fits corresponding to the SLD profiles in (b). (b) Best-fit SLD profiles. The red curves correspond to the as-prepared dry state.
Figure 7. (a) Neutron reflectivity data (brown )) from a silicon wafer coated with epoxy-silane mixture in redried state after saltwater conditioning. The solid curves through the data points are the best fits corresponding to the SLD profiles in (b). The red curves correspond to the as-prepared dry state.
An attempt was made to fit the saltwater contact data without the salt-enriched interface region. As shown in Figure 5(a), the closest fit diverges from the experimental data except for the first fringe. The corresponding SLD profile is shown in Figure 5(b). The data show that pure epoxy film shows no salt exclusion effect. On the contrary, the interface region actually shows salt enhancement. 3.2.2. (b) Epoxy-Silane Film. When treated with H2O, the data show that epoxy-silane film behaves similarly as treated with liquid D2O. The best-fit SLD profile in Figure 4(b) indicates that H2O penetrates the film without swelling. The SLD of bulk film decreases to (1.58 ( 0.02)10-6 A˚-2, resulting in an H2O volume fraction of 4%, which is consistent with the result from liquid D2O contact. No significant SLD change was observed at the dense interface region. No significant roughness change was observed. When exposed to saturated NaCl water solution, no recognizable shifts in the fringe positions were observed compared to the 238 DOI: 10.1021/la902066m
case of pure H2O contact. The slight offset in reflectivity curves arises from the substrate SLD difference (H2O vs salt water). The best-fit SLD profile in Figure 4(b) indicates the film does not swell when treated with salt water. The SLD of bulk film decreases to (1.58 ( 0.02) 10-6 A˚-2, which is caused by the absorption of 4 vol% H2O. No substantial change is observed in the dense interface region. No salt accumulation occurs at the interface or within the film. The results show that epoxy-silane film excludes salt. 3.3. Redried State. After saltwater treatment the samples were measured in redried state without rinsing to verify the conclusions of saltwater contact experiments. Visible NaCl crystals were observed on the surface of both samples. The reflectivity curves of pure epoxy and epoxy-silane coated samples in redried state after salt water conditioning (black )) are shown in Figure 6(a) and Figure 7(a), respectively. The reflectivity curves for the corresponding as-prepared dry state (red O) are also shown for comparison. Langmuir 2010, 26(1), 234–240
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Figure 8. (a) Neutron reflectivity data (brown )) from a silicon wafer coated with pure epoxy in redried state after saltwater conditioning. The solid line through the data points is the closest fit corresponding to the model without salt penetration in shown in (b). The fit is considered inadequate to explain the observations.
3.3.1. (a) Pure Epoxy Film. As shown in Figure 6(b), the total film thickness recovers as-prepared dry state after redry. The SLD of bulk film decreases to (1.50 ( 0.05)10-6 A˚-2, implying a NaCl volume fraction of 3%. A (45 ( 5) A˚ NaCl-enriched region was observed at the substrate-epoxy interface. The average SLD of this layer is (2.1 ( 0.1)10-6 A˚-2, implying the NaCl volume fraction is 20%. The fact that salt is enriched at the interface is because of the excess free volume at the poorly bonded interface. The peak at the airside surface arises from salt deposition during the redry process. Models without residual salt at the substrate-epoxy interface and within the bulk film failed to fit the experimental data. As shown in Figure 8, the offsets between experimental data and possible closest fit are significant. 3.3.2. (b) Epoxy-Silane Film. As shown in Figure 7(b), the total film thickness and SLDs of both bulk film and substratecoating interface match the as-prepared state. The shift of the reflectivity curve is due to the residual salt at the air-side surface. The fact that the epoxy-silane film excludes salt when treated with salt water and recovers its virgin state after redry demonstrates that the film is a salt barrier, which is critical to prevent pitting corrosion. 3.4. Mechanism of Salt Exclusion. Possible salt exclusion mechanisms are12-14,21-29 (a) Sieving due to fact that the hydrated ion is similar in size-scale to the mesh size of the cross-linked epoxy network; (b) Ion-dipole interactions between the membrane and the ions or between the ions mutually; (c) The combination of the above two.
(21) Beckstein, O.; Biggin, P. C.; Sansom, M. S. P. J. Phys. Chem. B 2001, 105 (51), 12902–12905. (22) Bostrom, M.; Ninham, B. W. Biophys. Chem. 2005, 114(2-3), 95–101. (23) Donnan, F. G. J. Membr. Sci. 1995, 100(1), 45–55. (24) Ikeda, T.; Boero, M.; Terakura, K. J. Chem. Phys. 2007, 126, 3. (25) Jordan, P. C. Biophys. J. 1982, 39(2), 157–164. (26) Nightingale, E. R. J. Phys. Chem. 1959, 63(9), 1381–1387. (27) Rashin, A. A.; Honig, B. J. Phys. Chem. 1985, 89(26), 5588–5593. (28) Schaep, J.; Van der Bruggen, B.; Vandecasteele, C.; Wilms, D. Sep. Purif. Technol. 1998, 14(1-3), 155–162. (29) Beckstein, O.; Tai, K.; Sansom, M. S. P. J. Am. Chem. Soc. 2004, 126(45), 14694–14695.
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Table 1. Comparison of Crystal and Hydrated Radii at 25 °C26 ion
rX (A˚)
rH (A˚)
Naþ Cl-
0.95 1.18
3.58 3.32
The salt ions in the water solution are hydrated. Takashi Ikeda et al. simulated the hydration process of alkali ions and showed that the typical hydration structure for Naþ is a distorted trigonal bipyramid.24 The radii of hydrated ions (rH) are greater compared to the radii measured from crystal X-ray diffraction (rX), due to the water shell associated with the ions. The hydrated radii for Naþ and Cl- are shown in Table 1. Beckstein et al. discuss a more detailed atomistic approach that incorporates both the interaction of the water molecules with the ion (the hydration shell) and the water-pore interaction (hydrophobic effects).21,29 They conclude that the solventmediated wall-ion interaction can extend to about 1 nm. Thus, ions will still be affected by the pore surface even if the pore is 10 times as wide as the bare ion. The barrier is due to the high energetic cost for an ion to shed its first or, in wider pores, its second hydration shell. The barrier relates to the solvation energy for an ion/hydration shell complex in water. It is apparent that the radii of hydrated ions play an important role in the interpretation of salt ion exclusion processes through the size effect. In cross-linked polymer films, salt diffuses through molecular-level free space rather than in distinct channels.2,30,31 Our previous study demonstrated that the epoxy-silane film is denser than pure epoxy film due to the additional cross-linking between epoxy and silane.32,33 The formation of epoxy-silane network not only reduces the total free volume but also reduces the average size of molecular-level free spaces. It is reasonable to believe that the addition of bis-sulfur silane densifies the neat epoxy network by forming an epoxy-silane network. The resulting finer (30) Wang, Y. M.; Wang, P.; Kohls, D.; Hamilton, W. A.; Schaefer, D. W. Phys. Chem. Chem. Phys. 2009, 11(1), 161–166. (31) Wang, Y. M.; Watkins, E.; Ilavsky, J.; Metroke, T. L.; Wang, P.; Lee, B.; Schaefer, D. W. J. Phys. Chem. B 2007, 111(25), 7041–7051. (32) Jensen, R. E.; McKnight, S. H.; Palmese, G. R. Viscoelastic Properties of Alkoxy Silane-Epoxy Interpenetrating Networks; Army Research laboratory: Adelphi, MD, 2003. (33) Tidrick, S. L. Investigations of the silane/epoxy matrix interphase for silane coupling agent blends of varying composition. Dissertation, Case Western Reserve University, 1991.
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molecular-level free space sets a higher diffusion barrier, which leads to the observed salt exclusion effect in epoxy-silane films. At the substrate-coating interface, the epoxy-silane film is well bonded by a SiO2-like interface layer. This layer is denser than the bulk epoxy-silane film, which makes salt exclusion no surprise.
4. Conclusions Pure epoxy film does not show a salt exclusion effect. When treated with salt water, the salt penetrates the film and accumulates at the interface. After redry, the residual salt remains in the bulk film (3 vol%) and interface region (20 vol%) Epoxy-silane film, on the other hand, shows a salt exclusion effect. The cause of salt exclusion effect is the finer molecularlevel free space as a result of the formation of epoxy-silane
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interconnections. The exclusion of salt from the interface eliminates the key mechanism of pitting corrosion. Acknowledgment. We benefited from numerous useful discussions with Professor William van Ooij. We thank Jaraslaw Majewski, Erik Watkins, and Hillary Smith for their effort in collecting the reflectivity data. Work at University of Cincinnati was sponsored by the Strategic Environmental Research and Development Program (www.serdp.org). Work performed at Surface Profile Analysis Reflectometer (SPEAR) at Lujan Neutron Scattering Center at Los Alamos National Laboratory was supported by Los Alamos National Laboratory under DOE contract W7405-ENG-36, and by the DOE Office of Basic Energy Sciences.
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