Polyoligomeric silsesquioxane (POSS)–hydrogenated polybutadiene polyurethane coatings for corrosion inhibition of AA2024

Progress in Organic Coatings 75 (2012) 319–327

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Polyoligomeric silsesquioxane (POSS)–hydrogenated polybutadiene polyurethane coatings for corrosion inhibition of AA2024 G. Markevicius, S. Chaudhuri, C. Bajracharya, R. Rastogi, J. Xiao, C. Burnett, T.Q. Chastek ∗ Washington State University, Applied Sciences Laboratory/Institute for Shock Physics, 412 E. Spokane Falls Blvd, Spokane, WA 99202, United States

a r t i c l e

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Article history: Received 5 March 2012 Received in revised form 25 June 2012 Accepted 4 August 2012 Available online 24 August 2012 Keywords: Corrosion Polymer coating Polyurethane Formulation POSS Aluminum 2024 Electrochemical impedance spectroscopy Salt fog testing

a b s t r a c t A series of coatings were developed that help prevent corrosion of aluminum alloy 2024 (AA2024). The coatings were based on an aliphatic polyurethane–polyoligomeric silsesquioxane (PU–POSS) resin. The materials were selected to exhibit a high level of hydrophobicity, which is expected to increase the moisture barrier properties, and thereby improve corrosion prevention. In addition, corrosion inhibitors (free molecules or encapsulated) were introduced into the coatings to improve corrosion resistance. The performance of the coatings was quantified using electrochemical impedance spectroscopy (EIS) and salt fog testing. Results from various formulations show that the hardness of the coating can be controlled by adjusting the ratio of POSS to hydrogenated hydroxyl terminated polybutadiene in the formulation. The coatings also had remarkable barrier properties, fast curing, and very high adhesion to the treated AA2024 substrate, all of which are expected to improve the anti-corrosion properties of the coatings. The best corrosion protection of AA2024 was observed in a transparent 10 ␮m thick PU–POSS bilayer coating that contained a mixture of sodium-2-mercaptobenzothiazole (i.e., NACAP) and benzotriazoleladen hydrotalcite. It was found that only certain corrosion inhibitors (free molecules or encapsulated) improve the anti-corrosion properties of the coating, whereas other corrosion inhibitors may actually degrade the coating performance. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Inhibiting corrosion of aluminum alloys is a challenge for many industries, especially the aviation industry. Many aircraft components are made from aluminum alloys, especially aluminum alloy 2024 (AA2024). Traditionally, corrosion of aluminum alloys has been inhibited by using chromated conversion coatings. The chromates prevent corrosion by acting as sacrificial anodes. However, hexavalent chromium can leach out of coatings into the environment [1]. Hexavalent chromium is a known carcinogen, and can contaminate drinking water [2,3]. Due to these public health concerns, there is a need for non-chromated alternatives for inhibiting corrosion of AA2024. In addition, transparent coatings are desirable because they allow for visual detection of localized corrosion, which would otherwise be masked by opaque coatings. This work focuses on development of non-chromated transparent coatings that can actively protect AA2024 from corrosion. Several research groups have examined non-chromated corrosion inhibiting coatings [4]. Kinlen and coworkers reported on polyaniline which is an inherently conducting polymer. They found that the conductive nature of the coating passivated exposed metal

∗ Corresponding author. Tel.: +1 509 358 7845; fax: +1 509 335 6115. E-mail address: [email protected] (T.Q. Chastek). 0300-9440/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2012.08.001

areas, thereby making it uniquely tolerant of scratches and pinholes [5]. Several groups have examined sol–gel coatings [6–8]. Metroke et al. examined organically modified silane coatings [7]. They found improved corrosion resistance as the alkyl-silane modifier increased in concentration. This improved performance correlated with increased water contact angle. Voevodin et al. found that a cerium doped zirconium-epoxy sol–gel coating provided good corrosion inhibition of AA2024 [8]. In addition, Ni et al. reported on a polyurethane coating that included TEOS oligomers. They found this coating to perform as well as chromated coatings [9]. Lai et al. examined copolymers of polyurethane–polyoligomeric silsesquioxane used as coatings. Polyoligomeric silsesquioxanes are often abbreviated as POSS, which has been trademarked by Hybrid Plastics. The inclusion of POSS improved the corrosion inhibition of AA2024 [10]. Corrosion inhibiting coatings often include inhibitors embedded within nano- or microscale fillers. These fillers can provide a reservoir of corrosion inhibiting chemicals for active protection. This mode of corrosion inhibition is especially important when the coating is damaged, thereby exposing a bare aluminum alloy. Fillers that have been examined include hydrotalcite [11–14], montmorillonite [15,16], and halloysite [17]. Buchheit et al. found that Al–Zr-decavanadate hydrotalcite performed well when embedded into epoxy. The release of vanadates and Zr2+ inhibited corrosion of bare AA20204 exposed by scratches. Abdullayev et al. examined

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coatings loaded with benzotriazole loaded halloysite, and found reduced corrosion rates. Several reports have also been made on nanocontainers [18–20]. Zheludkevich et al. examined layered double hydroxide nanocontainers and found that they provided better corrosion protection than chromate-based coatings. A variety of small molecule corrosion inhibitors can be used [19,21,22]. Harvey et al. examined 30 different corrosion inhibitors for AA2024 and AA7075, and identified several that performed nearly as well as hexavalent chromium [23]. An important aspect of any coating, especially corrosion inhibiting coatings, is the adhesive strength to the surface. Several surface treatments for AA2024 have been studied [24–26]. Chen et al. described an aminopropylsilane (APS) treatment procedure that significantly improved adhesion of a polyurethane coating to AA2024. Boeing has developed a sol–gel surface treatment (trade names: Boegel, Desogel EAP-12, etc.) that greatly improves the adhesion of various epoxy and polyurethane coatings to aluminum [27]. This surface treatment contains glycidoxyl functional silanes and zirconium alkoxides. Thus, it can both adhere to AA2024 via condensation of hydroxyl groups, and it adheres to coatings via reactions with the glycidyl functional groups. In this work, polyurethane–polyoligomeric silsesquioxane (PU–POSS) coatings were formulated and evaluated in terms of their ability to inhibit corrosion of AA2024. PU–POSS coatings were examined with and without corrosion inhibitors. In some cases, the coatings included corrosion inhibitors loaded into hydrotalcite particles. The coatings were evaluated for hardness, adhesion, and contact angle. In addition they were characterized using electrochemical impedance spectroscopy, infrared spectroscopy, and salt fog testing.

2. Experimental 2.1. Materials AA2024 was obtained from Alcobra Metals, Inc. Desogel EAP12 was obtained from PPG Aerospace and used as directed. Hydrogenated hydroxyl terminated polybutadiene (HHTPB, Nisso GI-3000) was generously provided by the Nisso Corporation. The poly(hexamethylene diisocyanate), dibutyl tin dilaurate (DBTDL), hydrotalcite, benzotriazole, acetone, aminopropylsilane, and chloroform were purchased from Sigma–Aldrich and used as received. The N-phenylaminopropyl polyoligomeric silsesquioxane (POSS – trademark of Hybrid Plastics, AM0281) was purchased from Hybrid Plastics. Tolonate HDB (polymeric aliphatic diisocyanate, trademark of Perstorp) was generously donated by Perstorp. NACAP (sodium-2-mercaptobenzothiazole, 50% in solution, trademark of R.T. Vanderbilt) was generously provided by R.T. Vanderbilt. Inhibitors were embedded into hydrotalcite following the procedure described by Williams and McMurray [14]. 2.2. AA2024 cleaning AA2024 was cleaned for 3 min with 5% (v/v) alkaline New Dim Supreme solution (alkaline degreaser from MacDermid), rinsed with DI water, and washed for 3 min with a 25% (v/v) solution of Isoprep 184 (acidic cleaning solution from MacDermid). A final DI water rinse and acetone rinse were carried out, and the aluminum was allowed to dry. 2.3. Desogel application The two parts were mixed in a 50:1 ratio, and brushed onto the cleaned AA2024. The coating was applied at least 30 min after

mixing, but not later than 24 h after mixing. Excess solution was removed via spin coating after 5 min. 2.4. APS coating on AA2024 APS (3-aminopropyltriethoxysilane) (1.0 g) was dissolved in 500 mL of methanol. A cleaned AA2024 coupon was submersed in the solution for 48 h, while gently stirring. 2.5. Coating formulation (per coat) A typical formulation was mixed in two parts, Part A: 0.43 g N-phenylaminopropyl POSS, 0.110 g Nisso GI-3000 HHTPB, 0.02 g DBTDL catalyst, and 1.5 mL chloroform, and Part B: 0.45 g Tolonate HDB (polyhexamethylene diisoscyanate), and 1.5 mL chloroform. Each part was thoroughly mixed using the vortex mixer. The two parts were combined and mixed thoroughly (30 s of high speed vortex mixing). The mixed formulation was spin coated onto cleaned AA2024 (1500 rpm for 30 s). All of the samples in this study also had a Desogel treatment applied to the surface 1 h before the application of the PU–POSS coat, except as noted in Fig. 2, where different surface treatments were evaluated. The coating was cured at 70 ◦ C for at least 8 h before applying an additional coat. The samples were cooled to room temperature before applying an additional coat. A typical coating thickness was 5 ␮m. 2.6. Electrochemical impedance spectroscopy (EIS) A CH Instruments Chemical Workstation was used with 0.1 M NaCl as electrolyte. Gamry paint test cells were equipped with a saturated calomel reference electrode (SCE) and graphite counter electrode to measure 14.6 cm2 active area of coatings. The EIS data were collected at the open circuit potential using a 0.01 V amplitude through the 0.1 MHz–10 mHz frequency range. The data collection, unless otherwise noted, was performed within 1 h after filling the cell with the electrolyte. The data were analyzed using Bode plots. The measurement cells were housed within a Faraday cage to minimize noise. 2.7. Infrared spectroscopy Measurements were made using an Agilent 680 FT-IR, equipped with a polarization modulation infrared absorption spectrometer (PM-IRRAS). PM-IRRAS was conducted on polished AA2024 surfaces. 2.8. Salt fog testing Measurements were made in an AutoTechnologies model 15A. Tests were carried out according to ASTM B117. Samples were gently rinsed with DI water to remove dried salt and corrosion products from the surface prior to observation. 3. Results and discussion Polyurethane–polyoligomeric silsesquioxane (PU–POSS) was the focus of this work due to its potential to provide excellent performance as a corrosion resistant coating. Furthermore, aliphatic polyurethanes were examined since they are currently used as topcoats in aviation and automobile applications. They offer good resistance to weathering, chemicals, and yellowing [15]. The POSS monomer was selected to increase cross linking, thereby improving hardness and durability. Each POSS molecule had 8 secondary amines (N-phenylpropyl groups) to allow for extensive cross linking, while maintaining the hydrophobic nature of the resin. In

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Table 1 Chemical composition and properties of coatings that were evaluated in this work. Sample A

Sample B

Sample C

Sample D

Sample E

Sample F

0 2.31 52.0 45.7

5.16 2.12 49.4 43.2

10.6 1.96 44.8 42.7

21.1 1.87 41.8 35.2

34.8 1.59 35.8 27.8

52.7 1.18 26.6 19.5

91.4 1.04 4H

98.1 0.46 3H

97.3 0.20 F

95.3 0.06 F

99.8 0.12 HB

105.8 0.61 HB

Sample (wt %) Hydroxy terminated hydrogenated polybutadiene Dibutyl tin dilaurate Poly(hexamethylene diisocyanate) POSS Sample properties Water contact angle Standard deviation of contact angle Hardness (ASTM D3363)

addition, the amine functionality on the POSS made it compatible with both the isocyanate and glycidyl chemistries in the polyurethane and Desogel. Several series of coatings formulations were prepared and evaluated. An optimal isocyanate to hydroxyl/amine ratio of 1:1 was identified and used for all formulations described below. It was found that the main chemical ratio that affected the properties was the ratio of POSS to hydrogenated hydroxy terminated polybutadiene (HHTPB). When this ratio was increased (i.e., more POSS) the coating became harder, whereas lowering the ratio generally increased the hydrophobicity. This is to be expected since HHTPB consists of a soft hydrocarbon polymer chain that is quite hydrophobic, whereas POSS hardens the coating since it possesses 8 secondary amines which highly cross link the material. Given these competing properties, an intermediate ratio, Sample C, was selected for further testing. Table 1 lists representative formulations and associated properties. Hardness was quantified via pencil hardness tests outlined by ASTM D3363. In ASTM D3363, pencils of increasing hardness are drawn across the surface until a pencil of sufficient hardness scratches the coating. The pencil hardness scale goes from 9H (hardest) to 9B (softest). The water contact angles were made at room temperature using a Ramé-Hart 190 goniometer. These samples were also subjected to EIS measurements in

order to evaluate whether different POSS to HHTPB ratios would affect the electrical barrier properties of the coating. As it is evident from Fig. 1, no significant differences were observed among these formulations. 3.1. Adhesion to AA2024 The PU–POSS does not adhere well to bare AA2024, as noted in Fig. 2. It had been expected that use of aminopropylsilane would significantly improve the adhesion, given previous reports. The procedure outlined by Huo and coworkers was followed, and was indeed found to be quite effective [24]. In addition, Vereecken et al., has reported that the thermal curing of organosilane may further affect the thickness and chemistry of the coating [28]. Fig. 2 shows a representative sample that, in addition to the standard APS treatment, was thermally cured for 1 h at 200 ◦ C. This resulted in a slight decrease in adhesion promotion for our PU–POSS formulation when compared to Desogel and uncured aminopropylsilane. Among the surface treatments examined in Fig. 2, it was found that Desogel exhibited the most consistent adhesion promotion, and was the least sensitive to variation in how the PU–POSS coating was applied. In some cases, the aminopropylsilane surface treatment showed slightly lower adhesion promotion, when the

Phase -Θ (degrees)

100 80

Sample A Sample B Sample C Sample D Sample E Sample F

60 40 20 0 10-2

10-1

100

101

102

103

104

105

10-1

100

101

102

103

104

105

1010 109

Impedance (Ω)

108 107 106 105 104 103 102 101 10-2

Frequency (Hz) Fig. 1. EIS Bode plots for the PU–POSS coatings with different POSS to HHTPB ratios.

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Fig. 2. Adhesion testing of PU–POSS coatings to AA2024. Peel tests were conducted in accordance with ASTM D3359-09. Pictured are samples prepared using different surface treatments: (a) Uncoated AA2024 exhibited poor adhesion, 0B. (b) Aminopropylsilane coated AA2024 exhibited excellent adhesion, 4B. (c) Aminopropylsilane coated and cured sample, exhibited very good adhesion, 3B. (d) PU–POSS on Desogel coated AA2024, exhibited excellent adhesion, 4B. It is noted that, in this case, black dye was added to the coating to make the results visually apparent.

PU–POSS was not applied to the substrate quickly enough. The slightly better performance of Desogel is likely attributable to more reactive groups (i.e., glycidyl groups reacting with PU–POSS and zirconium oxide bonds with the aluminum substrate). It is noted that recently developed hybrid coatings, which were not examined in this work, may provide even further adhesion promotion for this PU–POSS coating. For example, Webster and coworkers have recently described hybrid coatings with glycidyl carbamate resins and amino-functional silanes [29]. They found these primers to provide good adhesion promotion, hardness, and solvent resistance properties. 3.2. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) Adhesion results were further examined with PM-IRRAS (Fig. 3). The PM-IRRAS technique takes advantage of the different absorption of s and p polarization at large angles. The materials adsorbed

on reflective metal surfaces interact with the p polarized light much more than the s polarized light. Fir et al. provided a detailed description of using this type of technique to measure bonding to AA2024 [30]. They noted that the Al O Si bonding peaks are located between 800 and 1000 cm−1 . They also noted the challenges associated with making accurate peak assignments given the native oxide background and that the Si O Al modes are not necessarily strong or result in single peaks. Our results indicate that both aminopropylsilane and Desogel treatments resulted in a Si OR functionalized surface which should aid in PU–POSS layer adhesion. The most prominent vibrational bands in APS and Desogel spectra (∼1100 cm−1 ) are typically assigned to the Si O Si vibrations. Compared to the attenuated total reflectance (ATR) spectra of pure Desogel (Fig. 3) these vibrational frequencies shifted to higher wavenumbers. For example, the 1097 cm−1 peak in the ATR spectra, shifted to 1114 cm−1 in the PM-IRRAS spectra. Similarly the 905 cm−1 peak shifted to 916 cm−1 . The shift may be due to the bonding of the Desogel to the surface. However, it is also pos-

Fig. 3. ATR and PM-IRRAS spectra of aminopropylsilane (APS) and Desogel. The effects of the substrate were minimized by subtracting a PM-IRRAS spectra of AA2024 before the surface treatment. All spectra were normalized based on the intensity of the peak at ∼1100 cm−1 .

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Fig. 4. Shown is a Bode plot for PU–POSS coatings of varying thickness. Each layer is 5 ␮m. The thinnest single layer exhibits a moderate impedance. Thicker double (i.e., 10 ␮m) and quadruple (i.e., 20 ␮m) layers exhibit excellent impedance. Based on these results, most of the samples examined were double, 10 ␮m thick layers.

sibly due to the presence of the solvent in the ATR sample, which is evident in the 3500–2900 cm−1 range. 3.3. Electrochemical impedance spectroscopy (EIS) EIS was used to evaluate the coatings properties, such as defects and performance as a water barrier. Frequency scans resulted in Bode plots showing phase and impedance response in the 0.1 MHz–10 mHz frequency range. An excellent coating would typically result in the –90◦ phase values throughout the entire frequency range, and would exhibit a gradual increase in the impedance with the frequency decrease. Decrease in these values, especially in the lower frequency range would be typically associated with corrosion processes. EIS data are often analyzed and modeled with the help of equivalent electrical circuits. Bonora et al. described typical equivalent electrical circuits for characterizing organic coatings on metal substrates [31]. In this paper, we also evaluated the effectiveness of corrosion inhibitors dissolved in the NaCl solution on bare AA2024. Results of this provide insight into how well the inhibitors perform in passivating the bare aluminum. The PU–POSS layers were deposited on AA2024 via spin coating, with chloroform used as a solvent. The spin coating conditions yielded uniform 5 ␮m thick layers. Thicker layers were made by depositing multiple layers on top of each other. The Bode plot in Fig. 4 demonstrates that pure PU–POSS coatings require at least 2 layers (i.e., ≥10 ␮m thickness) for optimal barrier properties. The downward curve of the single layer (i.e., 5 ␮m thickness) data at low frequencies shows a continuous decrease in impedance, indicating non-optimal barrier properties which would increase the likelihood of corrosion. This is likely due to coating defects associated with insufficient coating thickness. Thicker coatings, up to 4 layers thick (i.e., 20 ␮m thickness), provided only limited improvement in barrier properties. Subsequent measurements reported here used 10 ␮m thick bilayer coatings as an optimal choice of coating thickness. It is noted that these coatings perform well even when they are quite thin, as compared to the typical thickness of

3.5–5.5 mil (90–140 ␮m) for paints in commercial aircraft. A thinner coating is desirable since it can lower the aircraft weight and thereby, lower the cost of material and fuel use. For example, a large commercial aircraft (Boeing 747) may have up to 500 lbs (227 kg) of coatings with a typical thickness of 100 ␮m. Reducing the coating thickness could reduce the aircraft weight by up to 450 lbs (204 kg), which could yield significant cost savings over the aircraft lifetime. In addition, since it is transparent, potential threats from undercoating corrosion can be easily detected and addressed using various manual and automated inspection techniques. When making these coatings, it was found that it was important to fully cure each coating layer prior to adding additional layers. Without full curing, the chloroform solvent would swell and roughen the underlying layer. It is expected that alternative coating deposition methods would eliminate this concern. Ongoing work is focused on developing coatings formulations that can be sprayed on, which are more compatible with production facilities. 3.4. Corrosion inhibitors The PU–POSS coatings described above exhibit excellent barrier properties and good adhesion, which will inherently inhibit corrosion. However, active corrosion protection, via embedded corrosion inhibitors, is required to protect bare aluminum that is exposed through scratches or other defects in the coating. Various corrosion inhibitors have been evaluated for AA2024. One of the most common inhibitors is benzotriazole. The interaction of benzotriazole with copper was described by Allam et al. [21]. They noted that benzotriazole generally acts as an anodic corrosion inhibitor through physisorption onto the copper surface. It should also be noted that AA2024 contains approximate 4.4% of copper in the form of intermetallic particles. Bucheit et al. described that 60% of intermetallic particles in AA2024 were anodic S phase particles (Al2 CuMg). The other particles were a complex mixture [12]. Additional work by Zheludkevich et al. demonstrated that both benzotriazole and 2-mercaptobenzothiazole were effective at long term protection from corrosion of AA2024 [32]. They found

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Fig. 5. Shown is a Bode plot for AA2024 monitored in the presence of NaCl and corrosion inhibitors. The results indicate that benzotriazole (BTA) and NACAP (i.e., sodium2-mercaptobenzothiazole) reduce the corrosion rate of bare AA2024.

that 2-mercaptobenzothiazole precipitated as a deposit on the copper-rich intermetallic particles. They determined that these two inhibitors outperformed other triazoles. In this work, initial examination of corrosion inhibitors was performed by conducting EIS on bare AA2024 in aqueous 0.1 M NaCl solutions with 0.001 M of corrosion inhibitors. The Bode plot in Fig. 5 shows that two time constants are evident in

the spectra when no corrosion inhibitors were present. The peak spanning 1–100 Hz range is resulting from the capacitance of the oxide layer formed on the aluminum surface, while the low frequency time constant corresponds to the corrosion activity. This corrosion activity is modified with the addition of benzotriazole and NACAP (sodium-2-mercaptobenzothiazole, 50% in solution), which also exhibit higher impedance values

Fig. 6. Shown is a Bode plot for PU–POSS coatings directly loaded with corrosion inhibitors. A single inhibitor laden layer was coated with a pure PU–POSS layer, yielding at 10 ␮m thick coating. Measurements were taken 1 h after filling the cell with electrolyte. The NACAP corrosion inhibitor increased the electron permeability of the coating, but the benzotriazole (BTA) had minimal impact.

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Fig. 7. Shown is a Bode plot for PU–POSS coatings with hydrotalcite particles soaked with corrosion inhibitors. A hydrotalcite layer was coated with a pure PU–POSS layer, yielding a 10 ␮m thick bilayer coating. Measurements were taken 1 h after filling the cell with electrolyte. The hydrotalcite with NACAP (HT-NACAP) notably lowered the resistivity of the coating, and the hydrotalcite with benzotriazole (HT-BTA) lowered the resistivity even further. It is believed that the large hydrotalcite agglomerates completely spanned the thickness of the coating, thereby providing an electrical pathway.

indicating enhanced corrosion inhibition. This agrees with the findings of Harvey et al. [23] who determined that benzotriazole and 2-mercaptobenzothiazole had an inhibitor efficiency of 98 and 85, respectively, when compared to hexavalent chromium. PU–POSS coatings containing benzotriazole and NACAP were formulated and tested with EIS and salt fog exposure. One of the reasons for selecting these two inhibitors was their solubility. Benzotriazole readily dissolved in the uncured PU–POSS resin, whereas NACAP was insoluble and formed an emulsion. These solubility properties greatly impacted the performance of the coatings. EIS testing in Fig. 6 shows that including NACAP directly into the coating greatly reduced its barrier properties. This may be expected since the insoluble NACAP droplets could provide a pathway for electrochemical conductivity. On the other hand, incorporating benzotriazole directly into the coating had no noticeable impact on the barrier properties of the coating. However, the salt fog testing (Fig. 8) found that after 24 h of exposure, NACAP was more effective than benzotriazole (when added directly into the PU–POSS) in protecting bare AA2024 exposed by a scribe. It is noted that the surface of the NACAP laden coating was inherently rough, due to the presence of some emulsified NACAP droplets. Since this surface roughness was observed prior to salt fog testing, it was not attributed to corrosion. It may be that the benzotriazole is strongly bound in the PU–POSS and therefore does not easily migrate to the exposed AA2024, whereas the insoluble NACAP domains are able to readily migrate. 3.5. Hydrotalcite inhibition In addition to embedding benzotriazole and NACAP directly into the PU–POSS, these inhibitors were also embedded into hydrotalcite particles that were in turn incorporated into the PU–POSS. Williams and McMurray described the role of hydrotalcite as rapidly exchanging aggressive (e.g., chloride anions) for

non-aggressive or corrosion-inhibiting anions [14]. This is important since the chloride anion is known to induce corrosion in copper containing aluminum alloys such as AA2024. The hydrotalcite loaded coatings were evaluated using EIS and salt fog testing. Fig. 7 demonstrates that the hydrotalcite additives had a significant impact on EIS results. This was likely caused by the large size of agglomerated hydrotalcite particles. That is, individual hydrotalcite particles were quite small (<5 ␮m), but they readily agglomerated. Mechanical grinding was used to reduce the size of agglomerates. However, there were a notable number of agglomerates that were up to 300 ␮m in diameter. Thus, at least some of the agglomerated particles were sufficiently large to impede completely smooth coatings from being deposited via spin coating. Ongoing work is focusing on whether use of hydrotalcite with better dispersion of particles can improve the coating barrier properties, as observed by EIS. In the case of salt fog testing, the results were opposite to the free inhibitors. That is, hydrotalcite loaded with benzotriazole performed better than hydrotalcite loaded with NACAP. This may be due to the relatively slow anion exchange rate associated with NACAP (i.e., sodium-2-mercaptobenzothiazole) due to associated electrostatic attractions. In comparison, BTA may provide a faster exchange with chloride ions thereby allowing it to diffuse to the exposed aluminum surface and provide corrosion protection. 3.6. Salt fog exposure The best way to estimate the performance of corrosion inhibiting properties of a coating is to test purposely damaged coatings under corrosive environment. The performance of various coatings was evaluated by using scribed test coupons (i.e., coated AA2024 coupons that have been purposely scribed). Coated coupons with carefully calibrated scribes of equal depth were exposed to a salt fog (ASM B117) environment, as shown in Fig. 8. It is notable that the PU–POSS coating without inhibitors had only a limited degree

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Fig. 8. Salt fog testing was conducted for various PU–POSS coatings after exposing (top) 24 h and (bottom) 100 h. It was found that the best performance was from NACAP and hydrotalcite loaded with benzotriazole (HT-BTA). After 100 h this sample exhibited only minor corrosion at the scratch site. It is noted that the surface of the NACAP sample was inherently rough. At 100 h the NACAP sample appears spotted throughout, due to this inherent surface roughness.

of corrosion, even after 100 h of exposure. The addition of NACAP proved to be effective in the short term corrosion protection. The coatings which contained encapsulated NACAP in their formulations exhibited an entirely corrosion free scribe after 24 h of salt fog testing. In the 100 h test, however, the pure NACAP was not as effective as the hydrotalcite loaded with benzotriazole. These additives were further examined in combination, with good success. After 100 h of salt fog testing, only a very minimal amount of corrosion was observed in this sample. As the current generation coatings are not fully doped with reservoirs of corrosion inhibitors, longer duration ASTM B117 tests were not performed. However, in this limited context, the corrosion inhibition capabilities are comparable to other non-chromated coatings. 4. Summary Several PU–POSS coatings were developed and evaluated in terms of their ability to inhibit corrosion of AA2024. It was found that the hardness and hydrophobicity of the coating could be notably affected by varying the POSS to hydrogenated polybutadiene ratio. The resulting coatings exhibited excellent barrier properties, as measured with electrochemical impedance spectroscopy, even when as thin as 10 ␮m. Benzotriazole and NACAP (sodium-2-mercaptobenzothiazole, 50% in solution) were included in the PU–POSS formulation, directly and also embedded in hydrotalcite. AA2024 coupons with scribed coatings were subjected to salt fog testing, and the best performing coatings were those that

included NACAP and benzotriazole-loaded hydrotalcite. A coating that included both NACAP and benzotriazole-loaded hydrotalcite was found to greatly inhibit corrosion of AA2024 in this type of scribe/salt fog test. Ongoing efforts are aimed at better understanding this performance using computational modeling, and formulating a sprayable composition that is suitable for industrial applications. Acknowledgements The authors wish to thank Perstorp, R.T. Vanderbilt, and Nisso Corp for generously providing materials for testing. Emilio Tedeschi, Erik Pihl, and Lauren Bergam are greatly thanked for providing valuable assistance in laboratory work. This work was supported by the Office of Naval Research under Grant No. N0001404-1-0688. References [1] S.M. Cohen, Corrosion 51 (1995) 71–78. [2] M.D. Cohen, B. Kargacin, C.B. Klein, M. Costa, Crit. Rev. Toxicol. 23 (1993) 255–281. [3] T. Davidson, T. Kluz, F. Burns, T. Rossman, Q. Zhang, A. Uddin, A. Nadas, M. Costa, Toxicol. Appl. Pharm. 196 (2004) 431–437. [4] R.L. Twite, G.P. Bierwagen, Prog. Org. Coat. 33 (1998) 91–100. [5] P.J. Kinlen, D.C. Silverman, C.R. Jeffreys, Synth. Met. 85 (1997) 1327–1332. [6] A.N. Khramov, N.N. Voevodin, V.N. Balbyshev, R.A. Mantz, Thin Solid Films 483 (2005) 191–196. [7] T.L. Metroke, J.S. Gandhi, A. Apblett, Prog. Org. Coat. 50 (2004) 231–246.

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