A Neutral Salt - American Chemical Society

Chapter 4

Downloaded by JOHNS HOPKINS UNIV on October 2, 2013 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0962.ch004

Investigation of Electroactive Polymers and Other Pretreatments as Replacements for Chromate Conversion Coatings: A Neutral Salt Fog and Electrochemical Impedance Spectroscopy Study 2

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P. Zarras1 , N . Prokopuk , N . Anderson , and J. D. Stenger-Smith

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1

Polymer Science and Engineering Branch (Code 498200D), Naval Air Warfare Center Weapons Division, Department of the Navy, 1900 North Knox Road (Stop 6303), China Lake, CA 93555-6106 MateriaIs Chemistry Branch (Code 498210D), Naval Air Warfare Center Weapons Division, Department of the Navy, 1900 North Knox Road (Stop 6303), China Lake, C A 93555-6106

2

A series of coatings were examined as replacements for chromate conversion coating (CCC) pretreatments on aluminum alloy (Al 2024-T3). The pretreatment coatings that were studied include: Trivalent chromium pretreatment (TCP), poly(2,5-bis(N-methyl-N-hexylamino)phenylene vinylene), (BAM-PPV) and poly((2-(2-ethylhexyl)oxy-5-methoxy-pphenylene)vinylene), (MEH-PPV). The pretreatments were coated onto Al 2024-T3 and subjected to neutral salt fog spray until failure. The B A M - P P V , M E H - P P V and TCP coated Al 2024-T3 panels each passed the minimum requirement (military pretreatment specification) of 336 hours exposure to neutral salt fog spray. Both the TCP coating and C C C showed no corrosion at 1000 hours in the neutral salt fog chamber. B A M - P P V coatings and C C C (as control) on A l 2024-T3 panels were also studied using EIS for a six month immersion study in 0.5 N NaCl. In both cases, the impedance values at low frequencies did not change over the exposure time. BAM– PPV shows both capacitive and resistive properties at high frequency, with the capacitive nature diminishing over time. The C C C showed purely resistive properties over time.

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© 2007 American Chemical Society

In New Developments in Coatings Technology; Zarras, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Introduction Aerospace and Department of Defense (DoD) currently use chromate conversion coatings (CCC) to inhibit corrosion of aluminum alloys (/). In addition these coatings provide excellent paint adhesion to the metal surface (2). The CCCs are applied via immersion or spraying onto both aluminum and steel substrates (3). Several recent studies have shown that residual hexavalent chromium (Cr(VI)) in chromate conversion coatings provide corrosion protection via a self-healing mechanism (4-8). However, Cr(VI) is a known carcinogen (9-12) and is highly regulated by the Environmental Protection Agency (EPA) and Occupational Safety and Health Agency (OSHA) (13). Any viable alternative to Cr(VI) coatings must meet or exceed the performance of Cr(VI) (1, 2, 14). Ideally, these alternative coatings must be able to passivate the metal surface (15) and several alternatives to C C C have been investigated. One example is the trivalent chromium pretreatment (TCP). This coating was developed by the Naval Air Warfare Center Aircraft Division ( N A W C A D ) as an acceptable alternative to C C C on aluminum alloys (16-18). Other pretreatment coatings such as cerium films protect the metal alloy through a passivation mechanism (19-21). During the past decade electroactive polymers (ΕΑΡ) have received considerable interest as corrosion inhibiting coatings (22-25). Most of these studies have focused on polyaniline (PANI) applied as a primer onto steel substrates (26-29). More recent studies have focused on PANI and derivatives of PANI as replacements for chromated pretreatments (30-32). Additional ΕΑΡ materials have also been prepared based on derivatives of poly(/?-phenylene vinylene) (PPV). Poly(2,5-bis(N-methyl-N-hexylamino)phenylene vinylene, B A M - P P V and poly((2-(2-ethylhexyl)oxy-5-methoxy-p-phenylene)vinylene), (MEH-PPV) were prepared as described in the literature (33, 34). B A M - P P V coated onto aluminum alloys has shown corrosion inhibition in simulated seawater and exposure to neutral salt fog spray (35-40).

Experimental B A M - P P V solutions were prepared with p-xylene as the solvent and were stirred for 2 days at 50°C. The solutions were filtered prior to use. The filtered solution was applied via spray gun onto A l 2024-T3 panels (.032 χ 3 χ 6"obtained from Q-Panel Lab Products Inc). After spraying, the B A M - P P V panels were placed in a vacuum oven and dried at 60°C for 2 hours. The average film thicknesses of the B A M - P P V coated panels were - 2 microns. M E H - P P V was obtained from Aldrich Chemical Co and used without further purification. M E H - P P V was also synthesized at the Naval A i r Warfare Center Weapons



42 Division ( N A W C W D ) and solutions were prepared from cyclopentanone solvent. These solutions were stirred for 2 days at 50°C and filtered prior to use. The filtered solution was applied via spray gun onto A l 2024-T3 substrates and dried as described for the B A M - P P V panels. The average film thicknesses for the M E H - P P V coatings (Aldrich) were between 0.3-0.4 microns and -4.0 microns ( N A W C W D ) . T C P coated A l 2024-T3 panels were supplied by the N A W C A D . For each pretreatment system, 3 panels of each coating were placed in a neutral salt fog chamber in racks at a 6° angle. The panels were removed at selected time intervals for visual inspection. The coupons were examined for any delamination of the coating, blistering or corrosion. The pretreatments were tested with the current standard C C C as the control. B A M - P P V and C C C pretreatments (CCC coupons obtained from Q-Panel Lab Products Inc.) on A l 2024-T3 were examined via electrochemical impedance spectroscopy (EIS). Each surface (12.56 cm ) was exposed to 0.5 Ν NaCl (aq) solutions and impedance spectra were acquired over six months. Aluminum/liquid contacts were kept at room temperature and solutions were exposed to the ambient environment and nominal light. Impedance spectra were acquired with a Princeton Applied Research Model 2273 potentiostat/galvanostat. The frequency range extended from 2 M H z to 0.005 Hz with an rms amplitude of 20 mV. Two-electrode cells were employed with a platinum counter electrode. Data were fit using E Q U I V C R T software. 2

Results and Discussion Neutral Salt Fog Exposure of Alternative Pretreatment Coatings on AI 2024-T3 Several alternative pretreatment systems coated onto A l 2024-T3 panels were tested against C C C in a neutral salt fog chamber (see Table 1). The testing of these panels follows the guidelines found in A S T M Β117 (41). These unpainted coatings were tested as an alternative to C C C (Type 1A) (42). The evaluation of these unpainted coatings follows a specific military specification (mil spec) for alternative coatings that contain hexavalent chromium. The mil spec requires that the unpainted coating pass 336 hours of neutral salt fog exposure. The accept/reject criteria from the mil spec is no corrosion of the underlying aluminum and that areas within % inch from the edges are accepted. Any staining of the coating is not considered a failure and the coating must show no blistering, delamination or evidence of corrosion (2, 42). During the neutral salt fog exposure tests for each pretreatment coating, C C C controls lasted well over 1000 hours showing no signs of corrosion. The TCP treated samples passed the minimum requirement set by the mil spec (Figures 1-4). At 1000 hours no delamination or visible corrosion was evident


43 Table 1. Pretreatment Alternatives to C C C Pretreatment Method Alodine 1200S (CCC) (2)


N A V A I R - A D Trivalent chromium pretreatment (TCP) (2) BAM-PPV(74, 15) MEH-PPV

Chemical Composition Chromic acid, hexavalent and trivalent chromium complexes Chromium III sulfate basic, potassium hexafluorozirconate Organic compound (C, H , N ) Organic compound (C, H , 0 )

Application Immersion, spray or wipe Immersion, wipe or spray Immersion or spray Spray

on the TCP panels. At 1500 hours there were signs of corrosion along the edges of the panels and within the panels. In addition some discoloration of the T C P coating was also apparent. Electroactive polymers, B A M - P P V and M E H - P P V were coated onto A l 2024-T3 panels and tested in the neutral salt fog chamber. B A M - P P V at various film thicknesses (0.5 -1.6 microns) have repeatedly passed 336 hours exposure to neutral salt fog (14) but failure before 500 hours was evident with these thinner coatings. In this study, the B A M - P P V coating thickness was increased to 2 microns to maximize its performance in the neutral salt fog chamber. At 336 and 840 hours of neutral salt fog exposure, the B A M - P P V coating shows similar corrosion resistance (Figures 5-8). The bulk of the panels at 840 hours did not show any blisters, corrosion or delamination of the coating. A t 1304 hours, corrosion was evident along both the edges and in the bulk of the coating. The testing was stopped at this point due to failure of the B A M - P P V coating. The next set of panels to be tested consisted of M E H - P P V coated onto A l 2024-T3. M E H - P P V has been extensively studied for applications in photovoltaics (43-45), optical memory (46), light-emitting devices (47-49) and lasers (50). Since B A M - P P V shows promise as an alternative to C C C , a study into whether M E H - P P V could provide similar corrosion inhibition for A l 2024T3 was investigated. Our initial studies focused on a thin film of M E H - P P V (film thickness -0.3 micron). These M E H - P P V coated panels did pass the 336 hours for neutral salt fog exposure (Figure 9). In addition, M E H - P P V (film thickness 4.0 micron) also passed the 336 hours neutral salt fog exposure (Figure 10). In both cases, the M E H - P P V coatings showed no difference in corrosion performance at 336 hours. However, at this time, the optimal thickness for M E H - P P V has not been established for improved corrosion resistance in neutral salt fog chamber.



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Figure 1. TCP on Al 2024-T3; Time = 0 hours (See page 1 of color inserts.)

Figure 2. TCP on Al 2024-T3; Time = 336 hours (See page 1 of color inserts.)



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Figure 3. TCP on Al 2024-T; Time = 1000 hours (See page 2 of color inserts.)

Figure 4. TCP on AÎ 2024-T; Time = 1500 hours (See page 2 of color inserts.)



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Figure 5. BAM-PPV Coated A12024-T3, Time = 0 hours (See page 3 of color inserts.)

Figure 6. BAM-PPV Coated Al 2024-T3, Τ = 336 hours (See page 3 of color inserts.)



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Figure 7. BAM-PPV Coated Al 2024-T3; Τ = 840 hours (See page 4 of color inserts.)

Figure 8. BAM-PPV Coated Al 2024-T3, Time = 1304 hours (See page 4 of color inserts.)



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Figure 9. MEH-PPV Coated Al 2024-T3; Time = 336 hours (See page 5 of color inserts.)

Figure 10. MEH-PPV Coated onto Al 2024-T3; Time = 336 hours (See page 5 of color inserts.)


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Electrochemical Impedance Studies (EIS) of C C C and BAM-PPV Coated Al 2024-T3 Coupons C C C coated A l 2024-T3 were compared to B A M - P P V coated A l 2024-T3 (~1.5 microns) using EIS. The EIS measurements of the C C C shows low values for the impedance (Bode plot) which is consistent with published results (57-53). A similar result was obtained with the B A M - P P V coated panels. The Bode plots for A l treated with C C C and B A M - P P V exhibit significant changes over time (Figure 11). However, the impedances at low frequencies do not significantly change within the first six months of exposure to the salt solution. Specifically, the total resistance does not deviate significant from 10 -10 ohms regardless of the coating (Figure 12). There are different frequency dependent processes occurring with the two surfaces. For example, at high frequencies (10 -10 Hz) the total impedance of the BAM-PPV-aluminum surface initially has both resistive and capacitive elements. But over four months, the capacitive nature of this high-frequency process diminishes. B y contrast the high-frequency impedance of the C C C treated aluminum is purely resistive at these frequencies. These results suggest that the two coatings are behaving differently yet still produce similar impedance and salt fog results. 4

5

4

6

Conclusions A l l the pretreatment systems studied, TCP, B A M - P P V and M E H - P P V show promise as alternatives to C C C . The B A M - P P V has repeatedly passed 336 hours neutral salt fog exposure and this represents a potential non-chromium based alternative military pretreatment. A n increase of the B A M - P P V coating thickness did extend the lifetime of the panels in neutral salt fog chamber. However, both the B A M - P P V and the M E H - P P V coatings did not match the performance of the C C C or T C P coating systems. These coatings ( B A M - P P V and M E H - P P V ) do offer a step in eliminating hexavalent chromium use in military pretreatment coatings. The optimal coating thickness for B A M - P P V and M E H - P P V has not been established at this time. The EIS study on the B A M - P P V coating shows similar performance to the C C C in long term immersion studies. The B A M - P P V coating performed as well as the C C C in providing the minimum barrier protection in 0.5 Ν NaCl solutions. Both systems C C C and B A M - P P V as measured by EIS are inhibiting corrosion by two distinct mechanisms. Future work will include B A M - P P V and M E H - P P V pretreatments in full military coating systems with and without chromium (Cr (VI)). These systems will be studied using a combination of neutral salt fog and EIS.



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Figure 11. Bode plots of CCC on Al 2024-T3 panel (left) and BAM-PPV on Al 2024-T3 (right) in 0.5 Ν NaCl solution. (See page 6 of color inserts.)

• '

•

• Δ Δ

• Δ

• •Δ

Δ

_ 3 Ε

• Δ

-20

0

20 40 60

AI/CCC ΑΙ/ΒΑΜ

80 100 120 140 160 180

Time (/days)

Figure 12. Total impedance obtained at low frequencies for CCC on Al 2024-T3 (black squares) and BAM-PPV coated Al 2024-T3 (white triangles) in 0.5 Ν NaCl solution


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Acknowledgements


The authors would like to acknowledge the continuing support of the Office of Naval Research (ONR), Dr. A . Perez and the Strategic Environmental Research and Development Program (SERDP), Mr. C. Pellerin/Program Manager, Pollution Prevention. M r . Craig Matzdorf ( N A W C A D ) for T C P coated aluminum panels, Mr. Bradley Douglas ( N A W C W D ) for the synthesis of the M E H - P P V compound and Ms. Cindy Webber ( N A W C W D ) for the coating of B A M - P P V and M E H - P P V onto A l 2024-T3 panels.

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A Neutral Salt - American Chemical Society

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