Extraordinary Corrosion Protection from PolymerâClay Nanobrick Wall

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Extraordinary Corrosion Protection from Polymer-Clay Nanobrick Wall Thin Films Eric J. Schindelholz, Erik David Spoerke, Hai-Duy Nguyen, Jaime C. Grunlan, Shuang Qin, and Daniel C. Bufford ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05865 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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ACS Applied Materials & Interfaces

Extraordinary Corrosion Protection from Polymer-Clay Nanobrick Wall Thin Films Eric J. Schindelholz1*, Erik D. Spoerke1, Hai-Duy Nguyen1, Jaime C. Grunlan2, Shuang Qin2, Daniel C. Bufford1 1

Materials Science and Engineering, Sandia National Laboratories, Albuquerque, NM, 87185, United States; 2Department of Materials Science and Engineering, Texas A&M University, College Station, Texas, 77843, United States *corresponding author

Abstract Metals across all industries demand anti-corrosion surface treatments and drive a continual need for high-performing and low-cost coatings. Here we demonstrate polymer-clay nanocomposite thin films as a new class of transparent conformal barrier coatings for protection in corrosive atmospheres. Films assembled via layer-by-layer deposition, as thin as 90 nm, are shown to reduce copper corrosion rates by >1000x in an aggressive H2S atmosphere. These multilayer nanobrick wall coatings hold promise as high-performing anti-corrosion treatment alternatives to costlier, more toxic, and less scalable thin films, such as graphene, hexavalent chromium, or atomic layer deposited metal oxides. Keywords: layer-by-layer assembly; copper; corrosion resistant; anti-corrosion barrier; coating; inhibitor (or inhibition); nanocomposite; oxidation

Atmospheric corrosion remains a persistent and evasive challenge to the functional lifetime and performance of structural, decorative, electronic and high-tech materials.1 Films and coatings have long been used to combat corrosion by physically separating metals from the corrosive medium, or by inhibiting electrochemical reactions that drive corrosion. Ultrathin nanocomposite films fabricated by layer-by-layer (LbL) assembly form an emerging class of coatings with remarkable barrier performance.23

These properties hold considerable promise for corrosion protection, yet remain largely underexplored

in this aspect. LbL films are constructed by alternating deposition of complementary components by simple and scalable aqueous dip-coating or spraying processes. The components sequentially bind 1 ACS Paragon Plus Environment

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together with attractive forces (e.g., electrostatic interactions or hydrogen bonding).4 Adjustments to component solution chemistry and process parameters provide precise control over film composition and architecture. The resulting versatility, conformal nature, gas impermeability, and thinness make LbL films attractive candidates for anti-corrosion coatings. Recent work found that polyelectrolyte multilayer films considerably improved corrosion resistance of steel and light alloys by charge and mass transfer inhibition during immersion in salt water solutions, and even demonstrated self-healing properties when constructed with weak, mobile polyelectrolyte layers.5-8 Atmospheric corrosion concerns arise in applications ranging from architecture to electronics, where thin, conformal and transparent coatings are often desired. Hydrogen sulfide is one of several sulfurcontaining gases and air pollutants that are particularly corrosive to copper, with even ppb levels in the air causing breakdown of electronics.9 Recent studies attempted to address atmospheric corrosion of copper using graphene and graphene oxide as gas barriers.10-14 While these materials provide short term (hours) corrosion protection at ambient and elevated temperatures, their effectiveness diminishes over time due to infiltration by defects and, with humidity, catalysis of corrosion by the coating itself.10, 12, 15 In addition to improved stability, PCNs offer several potential advantages as anti-corrosion coatings over graphene and other thin films (e.g., atomic layer deposited oxides), such as minimization of through-film defects via a multi-step coating process, incorporation of electrically insulating clay constituents, and a simple and scalable deposition process. The particular PCN film architecture applied to the copper plates in this study comprises highly organized, alternating layers of cationic branched polyethyleneimine (PEI) and anionic poly(acrylic acid) (PAA) and exfoliated montmorillonite clay (MMT). Clay platelets are effectively bricks held in place by polyelectrolyte mortar, forming what may be considered a nanoscale brick wall. Here, we refer to one complete deposition cycle (PEI/PAA/PEI/MMT) as a quadlayer (QL); several QL depositions are used to build up the described barrier films. Films as thin 4QL on polymers exhibit oxygen permeability orders of

2 ACS Paragon Plus Environment

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magnitude below that of SiOx and parylene (poly(chloro-p-xylylene)) coatings, rivalling that of metallized films.2 The slightly larger kinetic diameter of H2S versus O2 (3.6 and 3.5 Å, respectively) and exceptional impermeability of these QL PCN films,16-17 suggest these films could also serve as H2S barriers. Our results reveal that PCN films as thin as 90 nm can indeed reduce copper corrosion rates by >1000X in a highly corrosive H2S atmosphere. Their performance exceeds that of conformal coatings of microns-thick conventional vapor deposited parylene commonly used in industry today. LbL coatings provide other notable practical benefits like low-cost, a simple and scalable deposition process, and avoidance of highly toxic constituents (e.g., hexavalent chromium). The growth and structure of QL films on polished copper coupons is shown in Figure 1. Figure 1a shows film thickness exhibits exponential-type growth with increasing QLs, similar to trends observed in previous studies for deposition of QL films on non-metal substrates.2-3 The surface topography of the 2QL and 6QL coatings was relatively smooth, with a few particulate features evident in the AFM image in Figure 1a. In contrast, the 10 QL coatings exhibited a strong texture with a wavelike pattern. We observed a similar pattern on 20 bilayer (BL) coatings of PEI/PAA (10 QL without clay) on copper. This

Figure 1. PCN coating thickness as a function of the number of PEI/PAA/PEI/MMT QL deposited (insets are AFM topography maps of 6 and 10 QL coatings on copper) (a). TEM cross-sections of 2 (b) and 10 QL (c) coatings on copper (arrows designate the number of layers). (d) Schematic representation of the quadlayer thin film architecture.



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texturing is notably different than the smooth morphologies reported in previous studies for comparable film thicknesses on non-metal substrates.2-3 It is, however, consistent with results of Cao and coworkers, who showed similar wave-like patterns could be achieved for PEI/PAA films of 6 to 12 BL on silicon by immersion in 10 mM Cu2+.18 They attributed this to coordination of Cu2+ with carboxylate groups of the PAA and chelation by amine groups of BPEI, causing coating restructuring. A similar mechanism is likely operative for the QL and BL films, where some amount of Cu2+ would be expected to be present when applying the PCN to a copper substrate. Cross-sectional scanning transmission electron micrographs (STEM) of the films on copper, shown in Figure 1b and c, reveal layers of exfoliated clay platelets intercalated within compact polymer layers through 6QL. Above 6QL, there is an evident change in the film texture, showing significantly thicker polymer layers and a less ordered composite structure above 6 QL. The trend of increasing polymer layer thickness with increasing QL is a phenomenon observed in previous studies of this PCN architecture and is attributed to the interdiffusion of PEI and PAA during deposition.2 The thicker and less aligned clay layers above 6 QL along with the wavy topography may be related to the copper ion effect discussed above.18 An idealized schematic of the QL structure and the variation in polymer layer thickness is shown in Figure 1d. Importantly, these results verify our ability to create highly ordered PEI/PAA/PEI/MMT films on a technologically-important metal, and the evident organized, oriented composite structure is expected to create a highly tortuous pathway for corrosive gas species. Furthermore, based on previous studies using similar deposition parameters and thicknesses, clay loadings in these films are an order of magnitude higher than have been achieved in conventional, bulk nanocomposites, including clay- and graphene-based, used as corrosion barrier coatings (e.g., 26 wt % for 5 QL).2, 19-20 Increased particle loading and degree of orientation are known to increase the effectiveness of conventional nanocomposite anti-corrosion coatings in serving as mass transport barriers to corrosive species.


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To assess the protective character of these nanobrick walls, coated and uncoated samples were exposed to flowing air at 75 %RH and 30°C with 10 ppb H2S, based on standard accelerated corrosion tests for copper electronic components.21 Increases in sample mass resulting from the formation of sulfidation products provide a reliable metric for the rates of corrosion of the coated and uncoated samples. The mass gain of copper coupons seen in Figure 2a (Δmtotal) is primarily the result of reaction of copper with sulfur species to form Cu2S (Δmsubstrate) and, for coated samples, uptake of gaseous species (e.g., water or H2S) by the coating itself (Δmfilm ); where Δmtotal = Δmsubstrate + Δmfilm. To determine the relative contribution of Δmfilm, we also exposed coated and uncoated glass plates, which were not expected to substantially react with the H2S and gain mass (Δmsubstrate = 0), as shown in Figure 2b. The mass gain seen here is the result of gases chemically or physically sorbed to the PCN itself. From Figure 2, substrate mass gain rate is fast at the initial stage of exposure (~10 to 200 h), but then slows and becomes relatively linear with time after (> 200 h). This is qualitatively consistent with power-law type behavior of copper sulfidation in similar laboratory environments and reflective of the initially fast uptake of gas species by the films themselves.22

Figure 2. Mass gain of bare and coated copper (a) and glass (b) plates during exposure to hydrogen sulfide atmosphere, with linear fits of the 200 – 800 h exposure period. (c) Mass gain rates of copper and glass substrates with different treatments derived from linear regressions in (a) and (b). Error bars in (c) are the standard error of the regression slopes. Linear fits of mass gain versus time over the 200 to 800 h exposure range in Figure 2a, b were used to estimate steady-state mass gain rates on the copper and glass samples. Given the relatively negligible



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mass gain of the bare glass samples, values for the coated glass samples in Figure 2c are indicative of Δmfilm/Δt. These values show increasing mass gain rate with number of QL. Subtracting the coated glass values (Δmfilm/Δt) from the corresponding coated copper values (Δmtotal/߂‫ )ݐ‬reveals of the mass gain associated with copper corrosion (Δmsubstrate/߂‫)ݐ‬, in other words, the copper corrosion rate. It is clear in Figure 2c that 6 and 10QL coatings reduced corrosion rate by a factor of >1000X compared to the bare copper plates. These rates are on par with samples coated with much thicker (2.6 µm) Parylene-C, a vapor-deposited conformal coating commonly used in industry. Coated copper mass gain rates for these cases were sufficiently low as to not be distinguishable from film mass gain using the microbalance method employed here. The mass gain of copper coated with 10 BL of PEI/PAA (1.8 µm thick), which is essentially 10 QL without clay, was only ~3x less than that of the uncoated copper coupons, showing that incorporation of clay in these films is vital to performance.


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The qualitative visual appearance and quantitative optical reflectance measurements show similar trends in corrosion performance, but further distinguish 10QL as the top performer, maintaining the brightest visual appearance and least change in reflectivity, as shown in Figure 3. In reflectance measurements, the degree of sulfidation is revealed by decreases in light reflected from the highly polished metal surface as dark, broadly absorbing copper sulfide grows over the sample surface. In Figure 3a reflectance intensity of bare copper clearly decreases with corrosive exposure time across the measured wavelength range (400 to 1000 nm), which is attributed to absorbance of thickening copper sulfide over time. 23 The PCN coated samples exhibited considerably suppressed decreases in reflectivity with extended H2S exposure time as exemplified by the data for 2QL-coated copper in Figure 3b. The difference in reflectivity is more evident in Figure 3c, which compares the relative reflectance of the samples versus the unexposed condition, calculated as the integration of each spectrum at a given timestep divided by the initial reflectance of each sample. From this plot, 10QL is the best performer, maintaining 93± 1% of its original reflectance after 800 hours. These results are also visually apparent, with the 10 QL maintaining a bright, untarnished appearance after 800 h exposure (d). This is in stark contrast to the bare plate, which is covered in black, spalling corrosion product, indicative of the highly aggressive nature of this test. Images of the 10BL (no clay) samples show attack occurred around the edges and discrete points across the coating surface, again demonstrating the importance of clay in this coating. It is notable that reflectance measurements of the 10BL samples were taken away from the highly corroded edges. It is also notable that the 10QL sample is pinker in color than the other samples. The QL coatings were found to reflect strongest in the redder end of the visible spectrum with reflectance decreasing with thickness (see Supplemental Information for further details).



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Figure 3. Reflectance of uncoated (a) and 2 QL-coated (b) copper during exposure to hydrogen sulfide atmosphere. Change in reflectance integrated across 400 – 1000 nm for all copper treatments during exposure (c). Images of an uncoated (bare) plate before exposure and all plate types after 800 h exposure (d). Plate size is 1.9 x 2.5 mm.

The QL films likely protects the copper by both decreasing the diffusion rate of H2S to the copper surface and slowing the electrochemical kinetics of copper sulfidation. Assuming all H2S that permeated through the 6 and 10 QL films reacted with the copper to form Cu2S, permeation rates would be < 0.5 cc·m-2day-1atm-1 according to our mass gain results. On a per-thickness basis, this rate is over three orders of magnitude lower than H2S permeation reported for all-polymer films.24-26 By the same reasoning, however, the parylene coating used in this study would have a similarly low permeation rate, which is four orders of magnitude higher than H2S permeability of parylene reported elsewhere.24 As with parylene and other anti-corrosion polymer coatings, the polymer components (PEI and PAA) of the QL films likely suppress the corrosion electrochemical kinetics. The amine groups in PEI may serve to form strong coordinate bonds with copper as electron donors.27-28 Moreover, PAA is known to strongly coordinate with copper and immobilize Cu(II) ions through interaction with its -COOH groups. These characteristics are generally known to contribute to corrosion inhibition effectiveness.18, 27-28 The ability 8 ACS Paragon Plus Environment

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of the PEI/PAA BL films to maintain some discrete highly reflective areas, as seen in Figure 3, is suggestive of their effectiveness as inhibiting polymers. Clearly, though, the integrity and efficacy of the films is enhanced in the clay-containing composite structures. Although further investigation is required, this highlights an important feature of LbL deposition over other ultrathin coating systems as a barrier against atmospheric oxidation- the ability to design a system that inhibits both diffusion of corrosive gases and electrochemical corrosion kinetics. This study has demonstrated polymer-clay nanocomposite thin films as a new class of low-cost, conformal barrier coatings for protection of metals against corrosive atmospheres. We have demonstrated the ability to deposit successive PEI/PAA/PEI/MMT quadlayer PCN films on copper, an important industrial metal, using a relatively simple, scalable dip-coat process. Evidenced by mass gain and optical properties of copper samples, PCN films as thin as 90 nm effectively reduced the copper corrosion rate by more than 1000x when exposed to an aggressive H2S-containing atmosphere. Mechanisms for this impressive performance are likely a combination of the PCN films acting as a gas permeation barrier and by protective chemical interactions with the copper. These PCN films hold promise as higher performing alternatives to currently costlier and less scalable ultrathin films, such as graphene or atomic layer deposited oxides, to protect metal against oxidizing atmospheres.

Supporting Information Experimental methods, reflectance curves of coated copper.

Acknowledgements We gratefully acknowledge Ana Baca, Ron Goeke and Paul Kotula of Sandia for aid in sample characterization. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-mission laboratory



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managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

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Extraordinary Corrosion Protection from PolymerâClay Nanobrick Wall

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