Air-Impregnated Nanoporous Anodic Aluminum Oxide Layers for

Sep 22, 2015 - The hydrophobic nanoporous anodic aluminum oxide layer with the largest pore diameter and the thickest oxide layer (i.e., the maximized...
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Air-Impregnated Nanoporous Anodic Aluminum Oxide Layers for Enhancing the Corrosion Resistance of Aluminum Chanyoung Jeong,† Junghoon Lee,† Keith Sheppard,‡ and Chang-Hwan Choi*,† †

Department of Mechanical Engineering and ‡Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, New Jersey 07030, United States S Supporting Information *

ABSTRACT: Nanoporous anodic aluminum oxide layers were fabricated on aluminum substrates with systematically varied pore diameters (20−80 nm) and oxide thicknesses (150−500 nm) by controlling the anodizing voltage and time and subsequent porewidening process conditions. The porous nanostructures were then coated with a thin (only a couple of nanometers thick) Teflon film to make the surface hydrophobic and trap air in the pores. The corrosion resistance of the aluminum substrate was evaluated by a potentiodynamic polarization measurement in 3.5 wt % NaCl solution (saltwater). Results showed that the hydrophobic nanoporous anodic aluminum oxide layer significantly enhanced the corrosion resistance of the aluminum substrate compared to a hydrophilic oxide layer of the same nanostructures, to bare (nonanodized) aluminum with only a natural oxide layer on top, and to the latter coated with a thin Teflon film. The hydrophobic nanoporous anodic aluminum oxide layer with the largest pore diameter and the thickest oxide layer (i.e., the maximized air fraction) resulted in the best corrosion resistance with a corrosion inhibition efficiency of up to 99% for up to 7 days. The results demonstrate that the air impregnating the hydrophobic nanopores can effectively inhibit the penetration of corrosive media into the pores, leading to a significant improvement in corrosion resistance.

1. INTRODUCTION Metal corrosion is a serious problem, having deleterious effects on the economy,1 the environment,2 human health,3 and many engineering systems such as aircraft,4 automobiles,5 pipelines,6 and naval vessels.7 Aluminum is one of the primary metallic materials used in such systems. The major incentive for employing aluminum is its weight saving compared to steel. The use of aluminum results in an initial cost premium, which is justified over the life of the application by the light weight and low maintenance cost. However, because of its susceptibility to corrosion in saltwater,8 a protective measure such as painting or cathodic protection9 must be used for a satisfactory life in service. Unfortunately, such anticorrosion methods have associated issues, such as environmental regulation, reduced hydrodynamic efficiency, and durability, as summarized in Table 1. Chromatebased coatings10−12 provide highly effective corrosion protection, but environmental regulations are increasingly restricting their use. Anodization13,14 increases the thickness of the protective oxide layer, but it retains its porous nature,15 requiring a sealing process to achieve effective corrosion resistance. Other oxidation techniques such as microarc oxidation15,16 induce significant surface roughness, resulting in poor hydrodynamic efficiency for pipeline systems and ships. Layered materials such as anionic clays (e.g., layered double hydroxides)17,18 and cationic clays (e.g., montmorillonite)19 have been widely © 2015 American Chemical Society

investigated as additives in organic anticorrosion coatings or as polymer−clay nanocomposite corrosion-resistant coatings, but their effectiveness is restricted by their sizes and charges. Zeolites have also been explored as corrosion-resistant coating materials,20,21 but their microporous surface structures reduced their effectiveness. Hydrophobic self-assembled monolayers of surfactant molecules have also been employed as corrosion inhibitors,22 but the layers have limited stability and moleculesized defects that allow water to reach the underlying surface. Recently, a novel approach using superhydrophobic coatings has emerged for the prevention of light metal corrosion. A superhydrophobic surface can create a composite interface with liquid by retaining air within the hydrophobic surface structures, minimizing the contact area with the liquid, resulting in highly nonwetting and anticorrosive surface properties. Table 2 summarizes the current state of the art of studies on anticorrosion using superhydrophobic coatings23−30 to compare with this study. Although the current state of the art23−30 indicates that the use of superhydrophobic surfaces for the prevention of corrosion offers significant advantages over conventional anticorrosion methods, it should be noted that Received: June 29, 2015 Revised: September 22, 2015 Published: September 22, 2015 11040

DOI: 10.1021/acs.langmuir.5b02392 Langmuir 2015, 31, 11040−11050

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Langmuir Table 1. Conventional Anticorrosion Methods for Light Metal techniques chromatebased10−12 anodization13,14 microarc oxidation15,16 anionic clays17,18 cationic clays19 zeolite20,21

self-assembled monolayer22

key idea or example chromate itself is an effective corrosion inhibitor increase oxide passivation layer thickness increase oxide passivation layer thickness e.g., layered double hydroxides e.g., montmorillonite zeolite is used as a corrosion inhibitor

hydrophobic coating

advantages

issues

highly effective

environmental regulations

better corrosion passivation qualities than for thermally grown oxide films increase corrosion resistance to be superior to that of stainless steel and hard anodized coatings increase wear resistance homogeneous cationic clay can be pillared thermally and chemically stable

porous surface structures

short crystallization time more uniform than chromate coatings thin monolayer coating corrosion resistance increases as the monolayer thickness increases

poor hydrodynamic efficiency due to increased surface roughness exclusively restricted by size and charge interlayer can be easily exchanged microporous surface structures

limited stability and molecule-sized defects, resulting in pitting become disordered and oriented parallel to the surface

higher pore aspect ratio, ϕ/t, where t represents the oxide layer thickness or a pore depth) would enhance the corrosion resistance better because of the larger area and volume fractions of entrained air which would work as a barrier layer effectively along with the oxide layer while still remaining in the same wetting regime. In this study, we fabricate the nanoporous oxide layers with systematically varied pore sizes and oxide layer thicknesses on aluminum substrates by employing anodizing processes with a modulated anodizing time and pore-widening procedure.40,41 After the anodization, the surface is coated with Teflon (only a couple of nanometers thick) to make the surface hydrophobic and trap air in the pores. We evaluate the effects of pore size (20− 80 nm) and oxide layer thickness (150−500 nm) on the corrosion prevention in saltwater (3.5 wt % NaCl solution) at room temperature for varying durations (1 h to up to 7 days) using potentiodynamic polarization techniques and derive an estimate of the corrosion inhibition efficiency compared to that of a bare (nonanodized) aluminum substrate as well as the bare aluminum just coated with Teflon.

most superhydrophobic surfaces evaluated in previous studies were based on irregularly structured coatings, resulting in random surface roughness. Such random surface roughness, with the lack of controllability of the structural dimensions and shapes, has been a critical drawback, precluding the systematic understanding of the effects of the superhydrophobic surface parameters on the corrosion resistance as well as the practical application in a controllable way. For practical applications, one of the critical challenges using air as a corrosion passivation layer engineered by surface superhydrophobicity is how to maintain an air-retained (i.e., Cassie−Baxter) state31 under harsh conditions such as high hydraulic pressure underwater, as opposed to an airdepleted (i.e., Wenzel) state.32 Theoretically, superhydrophobic surface parameters required to sustain a liquid−air interface against external forces (e.g., hydraulic pressure) can be analyzed by minimizing the surface free energy33 or the force balance between surface tension and external forces across the interface (Figure S1 in the Supporting Information).34 Although discontinuous pillared structures can allow better superhydrophobicity than continuous porous structures because of their lower solid fraction and contact line boundary,35 the porous structures are more effective at retaining the air than the pillared structures because the air is exclusively trapped in individual pores with dead ends,36,37 which would help to retain the anticorrosion properties of the superhydrophobic surfaces with greater durability. The main objective of this study is to investigate how the structural parameters of hydrophobic porous surfaces, such as a pore size and a porous layer thickness (i.e., the area and the volume fractions of the air impregnating into the pores), would affect the corrosion resistance of a light metal such as aluminum. Figure 1 illustrates the concept for the prevention of corrosion of an aluminum substrate using a hydrophobic nanoporous oxide layer. The penetration of corrosive media (e.g., saltwater) into the oxide (hydrophilic) pores is the main reason for the corrosion of conventional anodized aluminum.38,39 In contrast, the hydrophobized nanoporous surface can trap or retain air in individual pores underwater due to the low surface tension. The impregnated air then works as a corrosion barrier layer by preventing water (e.g., chloride ions in saltwater) from reaching the bottom metallic surface. It is hypothesized in this study that the hydrophobic nanoporous surface with larger pores (i.e., high porosity, ϕ/Dint, where ϕ and Dint represent a pore diameter and an interpore distance, respectively) and a thicker oxide layer (i.e.,

2. EXPERIMENTAL SECTION 2.1. Fabrication of Nanoporous Anodic Aluminum Oxide Layers. In the case of aluminum alloys, alloying elements form intermetallic compounds or precipitates that are hard to anodize, remaining in the nanoporous anodic film and degrading its homogeneity.42−45 The inhomogeneous parts of the anodic film can be an easier path for the penetration of corrosive media toward the aluminum substrate than the path via pores.45 In addition, if the thickness of an anodic aluminum oxide layer is too high, then any irregular parts of the anodic aluminum oxide layer can be a critical location for corrosion. If so, the corrosion resistance of anodized aluminum is not determined by the penetration of corrosive media into the pores but by the irregular parts in the anodic aluminum oxide layer. In this study, so as to consider only the corrosion inhibition mechanism via the pores, a high-purity aluminum substrate was used, and relatively thin oxide layers (150−500 nm) were fabricated for testing. The schematic of overall fabrication processes and the representative fabrication results of nanoporous anodic aluminum oxide layers are shown in Figure 2. High-purity (99.9995%) aluminum foils (Goodfellow, 2 cm × 3 cm × 0.05 cm) were first degreased in acetone and ethanol by ultrasonication for 10 min and rinsed in deionized water (Figure 2a). Subsequently, each specimen was electropolished to reduce surface irregularities (both sides without using any protective coating or tape) in a mixture of perchloric acid and ethanol (HClO4/C2H5OH = 1:4 in volumetric ratio) under an applied potential of 20 V (∼120 mA/ 11041

DOI: 10.1021/acs.langmuir.5b02392 Langmuir 2015, 31, 11040−11050

a

aluminum

Zhang27

aluminum

this work

Teflon

polythiophene nano, uniform

nano, random

nano, random

micro, random

micro, random

hydroxide films

1H,1H,2H,2H perfluorooctyltrichlorosilane (PFTS) fluoroalkylsilane (FAS)

micro, random

micro, random

micro, random

micro, random

organosilica aerogel coating

myristic acid

layer by layer (LbL) of epoxyfunctionalized ZrO2/SiO2 myristic acid

surface treatment

surface pattern (scale, regularity)

3.5% NaCl 3.5% NaCl

5% NaCl

3.5% NaCl 3% NaCl

5% NaCl

seawater

seawater

5% NaCl

test solution

EIS: electrochemical impedance spectroscopy. bOCP: open-circuit potential.

Leon30

magnesium alloy steel

Ishizaki29

Liu

zinc

aluminum

Barkhudarov26

28

aluminum

aluminum alloy copper

Yin25

Liu

24

Shchukin

23

reference

substrate material

polarization

polarization, EIS polarization

polarization, EIS polarization, EIS neutron reflectivity OCP, polarizationb polarization

EIS

a

test method

main results and/or issues

superhydrophobic surface would be an effective strategy for improving the anticorrosion performance of various engineering materials fabrication of the dual properties of the superhydrophobic anticorrosion nanostructured conducting polymer coating follows a two-step coating procedure that is very simple and can be used to coat any metallic surface excellent corrosion inhibition as a function of increasing air volume (i.e., pore size and oxide layer thickness) with customized nanostructures

higher corrosion-resistance properties

surface prevents infiltration of water into the superhydrophobic porous film and limits the exposure of the metal surface to corrosive elements good film adhesion and mechanical stability

film stability should be improved further

nanoreservoirs increase long-term corrosion protection of the substrate and provide effective inhibitor storage and its prolonged release on demand corrosion resistance of the material was improved remarkably

Table 2. Current State-of-the-Art Studies on Anticorrosion Using Superhydrophobic Coatings56

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samples were degreased in acetone and methanol for 15 min, rinsed with deionized water in an ultrasonic cleaner for 15 min, and dried on a hot plate at 150 °C for 10 min. Each sample was then coated with a 0.2 wt % Teflon solution (a mixture of Teflon AF1600 powder in perfluoro compound FC-75, Fisher Scientific) by spin-coating at 1000 rpm for 30 s at room temperature. Each sample was then baked on a hot plate at 112 °C for 10 min to evaporate the solvent and then baked again at 165 °C for 5 min and at 330 °C for 15 min to improve adhesion. The spincoating procedure resulted in a uniform film thickness of only a couple of nanometers over the surface, which was measured with a high-resolution field-emission scanning electron microscope (Zeiss Auriga small dualbeam FIB-SEM). Each sample was dried in air for 1 day before it was tested for wettability and corrosion resistance. For the estimation of surface hydrophobicity after the coating, the apparent contact angle of a sessile droplet (∼3 μL) of deionized water was measured on each sample at room temperature with a goniometer system (model 500, ramé-hart). The average and standard deviation values of the contact angles were obtained with more than three measurements conducted at different locations on each specimen. 2.3. Potentiodynamic Polarization Test. Corrosion resistances of the prepared samples were evaluated using an electrochemical method. In particular, to determine the corrosion potential and the corrosion current density for each specimen, a potentiodynamic polarization test was conducted with the Tafel fitting method.49,50 The working cell was a standard three-electrode cell having platinum as a counter electrode and Ag/AgCl as a reference electrode (Figure S2). The back side and the outer area of each sample were insulated with polyester masking tape so that only the middle area of 1 cm × 1 cm of the sample was exposed to electrolyte. All tests were performed at ambient temperature (25 °C) in a 3.5 wt % NaCl solution using a Solatron moduLab system (Solatron Analytical Company, USA). Prior to the measurement, each sample was immersed in the electrolyte for an hour to approach a steady state. For the potentiodynamic polarization experiments, the potential was scanned from −300 to +400 mV versus open circuit potential (OCP) at a scan rate of 2 mV/s.

Figure 1. Schematic of corrosion prevention using an air-impregnated hydrophobic nanoporous oxide layer on a metallic substrate. Dint = interpore distance, ϕ = pore diameter, and t = oxide layer thickness (or pore depth). cm2) for 3 min at 15 °C with strong and uniform agitation of the electrolyte using a magnetic stirrer. The polished specimen was used as a working electrode (anode) in the electrochemical anodization process with a platinum counter electrode. The two electrodes were separated at a distance of 5 cm. In this work, a two-step anodizing process was employed to create uniform and well-ordered nanoporous structures of precise dimensions.46,47 The first anodizing step employed 0.3 M oxalic acid (H2C2O4) for 10 h at 40 V with 20 °C (Figure 2b). During the anodization, the solution was agitated with a magnetic stirrer to help maintain uniform anodization over the sample surface. During the anodizing, a nanoporous anodic aluminum oxide layer is grown by a locally concentrated potential, which is the net result of competition between a field-assisted dissolution mechanism and the simultaneous galvanically driven oxide growth.48 In the initial stage of anodizing, the applied potential distribution is affected by the surface morphology, typically resulting in unordered and irregular pore patterns on the top surface. However, if the anodic potential is maintained at a constant value, then the pore arrangement becomes ordered and regular, especially at the bottom interface between aluminum and oxide. The initial irregular nanoporous anodic aluminum oxide layer grown in the first anodizing process was then removed to obtain the ordered and regular prepattern of aluminum which would lead to well-ordered and uniform oxide pore patterns in the second anodizing step. For the removal of the initial less-ordered oxide layer, the specimen was submerged in an aqueous solution of 1.8 wt % chromic acid and 6 wt % phosphoric acid at 65 °C for approximately 10 h (Figure 2c). The second anodizing step was applied to the aluminum surface having the prepattern under the same conditions as the first anodizing step but with different anodizing times to get the oxide layers with different thicknesses (Figure 2d). In order to widen the pore diameter, a porewidening process was further applied in 0.1 M phosphoric acid at 30 °C with a modulated immersion time (Figure 2e), dissolving the cell wall of the nanoporous anodic aluminum oxide layer. 2.2. Hydrophobic Surface Treatment and Characterization of Hydrophobicity. The fabricated samples were first cleaned by O2 plasma (Harrick plasma) for 15 min in order to remove organic residues and make the surfaces hydrophilic. After the plasma treatment, the

3. RESULTS AND DISCUSSION 3.1. Nanoporous Anodic Aluminum Oxide Layers with Varying Pore Sizes and Oxide Layer Thicknesses. Figure 3 shows the scanning electron microscope (SEM) images of the nanoporous anodic aluminum oxide layers with a Teflon coating (named TAAO), fabricated on top of the aluminum substrates, with varying pore sizes (ϕ = 20 and 80 nm) and oxide layer thicknesses (t = 150 and 500 nm) but a constant interpore distance (Dint = 100 nm). The result shows that the well-ordered and uniform nanopore structures of the varying pore sizes and oxide layer thicknesses were successfully formed by employing the two-step anodizing process incorporated with the porewidening step. The SEM images also show the negligible thickness (not more than a couple of nanometers) of the Teflon film which is uniformly covering the nanoporous anodic aluminum oxide (AAO) surface (Figures S3 and S4). Table 3

Figure 2. Schematics of the two-step anodizing process of aluminum and the scanning electron microscope (SEM) images of the fabricated surfaces after each step. (a) Bare aluminum (BA) substrate before anodizing. (b) Nanoporous anodic aluminum oxide (AAO) layer created by the first anodizing step. (c) Prepattern of aluminum left after the removal of the initial AAO layer. (d) Well-ordered nanoporous AAO layer formed after the second anodizing step. (e) Nanoporous AAO layer with an increased pore size after a pore-widening step. 11043

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Figure 3. SEM images of nanoporous, Teflon-coated anodic aluminum oxide (TAAO) layers fabricated on aluminum substrates. (a) TAAO-20-150 (pore diameter, 20 nm; oxide layer thickness, 150 nm). (b) TAAO-80-150 (pore diameter, 80 nm; oxide layer thickness, 150 nm). (c) TAAO-20-500 (pore diameter, 20 nm; oxide layer thickness, 500 nm). (d) TAAO-80-500 (pore diameter, 80 nm; oxide layer thickness, 500 nm).

Table 3. Summary of Anodizing Conditions and Geometric Dimensions of Fabricated Samples second anodizing (s)

pore widening (min)

pore diam (ϕ, nm)

BA

N/A

N/A

N/A

TBA

N/A

N/A

N/A

AAO-20-150 TAAO-20-150 TAAO-80-150 TAAO-20-500 TAAO-80-500

50 50 60 150 160

0 0 10 0 10

20 20 80 20 80

sample name

pore depth (t, nm) N/A (only native oxide) N/A (only native oxide) 150 150 150 500 500

summarizes the structural dimensions for each surface sample, including TAAO-20-150 (pore diameter of 20 nm and oxide layer thickness of 150 nm), TAAO-80-150 (pore diameter of 80 nm and oxide layer thickness of 150 nm), TAAO-20-500 (pore diameter of 20 nm and oxide layer thickness of 500 nm), and TAAO-80-500 (pore diameter of 80 nm and oxide layer thickness of 500 nm), all Teflon-coated. Anodizing and pore widening-times applied for each sample are also summarized in Table 3. 3.2. Hydrophobicity of Teflon-Coated Nanoporous Anodic Aluminum Oxide Surfaces. Figure 4 shows the apparent contact angles of a sessile droplet of deionized water (∼3 μL) on a bare aluminum (BA), a Teflon-coated bare aluminum (TBA), a hydrophilic (i.e., no Teflon coating) nanoporous anodic aluminum oxide surface with a pore diameter of 20 nm and an oxide layer thickness of 150 nm (AAO-20-150) and Teflon-coated (i.e., hydrophobic) nanoporous anodic aluminum oxide surfaces with varying pore dimensions (TAAO-20-150, TAAO-80-150, TAAO-20-500, and TAAO-80500). The contact angle hysteresis on each surface is also summarized in Table S1. Whereas the contact angle on the BA surface was 70 ± 0.5°, that on TBA was 107 ± 0.5°. The contact

hydrophobic treatment

estimated air volume impregnated (μL/cm2)

none

0

Teflon coating

0

none Teflon coating Teflon coating Teflon coating Teflon coating

0 5.4 × 10−4 8.7 × 10−3 1.8 × 10−3 2.9 × 10−2

angle on the hydrophilic nanoporous oxide surface (AAO-20150) was 8 ± 0.1°, which is significantly lower than that on BA (70 ± 0.5°). It indicates that the hydrophilic nanopores are fully wetted by water, following the Wenzel state.32 In contrast, the contact angle on TAAO-20-150 was 122 ± 0.5°, which is significantly higher than that on TBA (107 ± 0.5°). The contact angle on TAAO-20-500 was 121 ± 0.5°, which is almost the same as that on TAAO-20-150 (122 ± 0.5°). The contact angles on TAAO-80-150 and TAAO-80-500 were 140 ± 2.0 and 139 ± 1.5°, respectively, being alike. The results show that the contact angles do not depend on the oxide layer thickness (i.e., the depth of nanopore) but depend only on the pore size (i.e., surface air fraction). This suggestes that air should be trapped within the hydrophobic nanopores and that water wets the top solid surface only partially following the Cassie−Baxter state.31 Agreeing with the Cassie−Baxter model,31 the larger contact angles on the surfaces with a larger pore size (80 nm for TAAO-80-150 and TAAO-80-500) than those on surfaces with a smaller pore size (20 nm for TAAO-20-150 and TAAO-20-500) should result from the reduced liquid−solid contact area (solid fraction) or the increased liquid−air contact area (air fraction) on the top surface. On the basis of the geometric dimensions (i.e., interpore 11044

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corrosion current density (Icorr) for each specimen were estimated from the Tafel fitting of the polarization curves, as summarized in Table S2. Ecorr was shifted slightly to be more positive for AAO-20-150 (−1.627 V), TBA (−1.596 V), and TAAO-20-150 (−1.593 V) samples, compared to BA (−1.682 V). Meanwhile, Icorr decreased significantly for AAO-20-150 (6.5 × 10−6 A/cm2), TBA (3.6 × 10−6 A/cm2), and TAAO-20-150 (9.7 × 10−7 A/cm2) samples compared to BA (8.5 × 10−6 A/ cm2). The result indicates the improvement in the corrosion protection properties of the surfaces in that order. The decrease in the Icorr of AAO-20-150 compared to that of BA should be due to the passivating oxide layer. However, the effect is of limited extent since the relative decrease (from 8.5 × 10−6 to 6.5 × 10−6 A/cm2) is only 23.5%. It should be due to the porous nature of the AAO surface which should be wetted by water due to the surface hydrophilicity of anodic aluminum oxide. The result that TBA reduces the Icorr more than AAO-20-150 indicates that a simple hydrophobic treatment of Teflon (even only a couple of nanometers thick) would be more efficient than using the hydrophilic nanoporous oxide layer (as thick as 150 nm) in protecting corrosion. In contrast, TAAO-20-150 shows a significant reduction of Icorr, which is more than 1 order of magnitude lower than that of other samples. The significant reduction of Icorr compared to both the hydrophilic nanoporous oxide layer and the Teflon-coated bare aluminum indicates that the air trapped in the hydrophobic nanopores effectively separates and prevents the corrosive media (saltwater) from reaching and reacting to the bottom metal (aluminum) surface. In order to obtain a more quantitative comparison of the enhanced corrosion resistance of each fabricated specimen to the bare aluminum (BA), the corrosion inhibition efficiency (IE) of each sample was calculated using the equation51

Figure 4. Contact angles of a sessile droplet of water on BA (bare aluminum), TBA (Teflon-coated bare aluminum), AAO-20-150 (AAO with pore diameter 20 nm and oxide layer thickness 150 nm), TAAO20-150 (Teflon-coated AAO with pore diameter 20 nm and oxide layer thickness 150 nm), TAAO-80-150 (Teflon-coated AAO with pore diameter 80 nm and oxide layer thickness 150 nm), TAAO-20-500 (Teflon-coated AAO with pore diameter 20 nm and oxide layer thickness 500 nm), and TAAO-80-500 (Teflon-coated AAO with pore diameter 80 nm and oxide layer thickness 500 nm).

distance, pore diameter, and pore depth) of the periodic hexagonal pore array and the assumption that the pore is fully occupied with air up to the top surface of the TAAO layer with a flat meniscus of water, the estimated air volume impregnated within the hydrophobic nanopores per unit surface area of each TAAO sample is 5.4 × 10−4, 8.7 × 10−3, 1.8 × 10−3, and 2.9 × 10−2 μL/cm2 for TAAO-20-150, TAAO-80-150, TAAO-20-500, and TAAO-80-500, respectively, which is also summarized in Table 3. 3.3. Corrosion Resistance of Air-Impregnated Hydrophobic Nanoporous Oxide Layers. Figure 5a shows the potentiodynamic polarization curves for the hydrophilic nanoporous oxide layer (AAO-20-150), the Teflon-coated hydrophobic bare aluminum (TBA), and the Teflon-coated hydrophobic nanoporous oxide layer (TAAO-20-150) compared to the bare aluminum (BA), measured in 3.5 wt % NaCl solution after immersion for 1 h. The corrosion potential (Ecorr) and the

IE =

Icorr,bare − Icorr,coated Icorr,bare

× 100% (1)

where Icorr,bare and Icorr,coated represent the corrosion current density for the bare aluminum and the fabricated specimen, respectively. Since the corrosion current is directly related to the mass loss reaction by corrosion, the Icorr value was used for the estimation of the corrosion inhibition efficiency instead of the

Figure 5. (a) Potentiodynamic polarization data measured in 3.5 wt % NaCl solution at room temperature after immersion for 1 h and (b) the corrosion inhibition efficiency (IE) of the hydrophilic nanoporous anodic aluminum oxide layer (AAO-20-150), Teflon-coated hydrophobic bare aluminum (TBA), and Teflon-coated hydrophobic nanoporous anodic aluminum oxide layer (TAAO-20-150) compared to the hydrophilic bare aluminum (BA). 11045

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Figure 6. (a) Potentiodynamic polarization data measured in 3.5 wt % NaCl solution at room temperature after immersion for 1 h and (b) the corrosion inhibition efficiency (IE) of the Teflon-coated hydrophobic nanoporous anodic aluminum oxide layers with varying pore diameters and oxide layer thicknesses, compared to the hydrophilic bare aluminum (BA).

TAAO-80-150 and comparatively from 1.0 × 10−7 to 9.8 × 10−9 A/cm2 for TAAO-20-500 to TAAO-80-500). For the same pore size, Icorr also decreased with the increase in the oxide layer thickness, which was more pronounced with a larger pore size (i.e., from 9.7 × 10−7 to 1.0 × 10−7 A/cm2 for TAAO-20-150 to TAAO-20-500 and comparatively from 9.7 × 10−8 to 9.8 × 10−9 A/cm2 for TAAO-80-150 to TAAO-80-500). The result that Icorr decreases with the increase in pore size at the same given oxide layer thickness suggests that the impregnated air layer is more efficient than the oxide layer in protecting the surface from corrosion. The estimated IE values of the Teflon-coated hydrophobic nanoporous oxide layers are 88, 98, 98, and 99% for the TAAO-20-150, TAAO-80-150, TAAO-20-500, and TAAO-80-500 samples, respectively, also showing the enhanced corrosion resistance with the increase in pore size and oxide layer thickness. The TAAO-80-500 sample with the largest pore size and the thickest oxide layer (i.e., the deepest pore) shows the greatest IE. The increase in pore size and oxide layer thickness (i.e., pore depth) represents the larger air fraction in terms of both the surface area and the volume. Thus, the results eventually mean that the TAAO sample with the larger air volume impregnated within the nanopores (i.e., larger pore diameter with a thicker oxide layer) exhibits a higher corrosion inhibition efficiency with a lower corrosion current. Although TAAO-80150 is expected to possess a slightly larger air volume than TAAO-20-500 (Table 3), their values are effectively the same order of magnitude, showing similar corrosion current and inhibition efficiency values. Therefore, the results indicate the Teflon-coated hydrophobic nanoporous oxide layer with a larger pore size and a greater oxide layer thickness is more efficient at inhibiting corrosion due to a greater availability of the air within the pores to separate and prevent the corrosive medium (saltwater) from reaching and reacting to the bottom metal (aluminum) surface. 3.4. Durability of Corrosion Resistance of Air-Impregnated Hydrophobic Nanoporous Oxide Layers. In order to examine the durability of the corrosion inhibition of the Tefloncoated hydrophobic nanoporous oxide layer, potentiodynamic polarization tests were also performed for varying immersion times of up to 72 h (3 days) and then 168 h (7 days) in a 3.5 wt % NaCl solution at room temperature. Figures 7a−e show the potentiodynamic polarization curves measured for different

corrosion potential which does not involve any information related to the reaction rate. Figure 5b shows the IE values of the fabricated specimens with respect to the bare aluminum which are also summarized in Table S2. The estimated IE values of the fabricated specimens are 24, 57, and 88% for the AAO-20-150, TBA, and TAAO-20-150 samples, respectively, compared to BA, also illustrating the significantly enhanced corrosion resistance for the Teflon-coated hydrophobic nanoporous oxide layer. The results shown in Figure 5 indicate that the Teflon coating on a hydrophilic nanoporous oxide layer enhances the corrosion resistance due to the impregnated air in the hydrophobized pores. This suggests that further enhancement of corrosion resistance would be enabled by enlarged air volume in the pore, which can be represented by a larger pore diameter and a thicker oxide layer (i.e., a deeper pore). Figure 6 shows the potentiodynamic polarization curves and the estimated IE values for the Teflon-coated hydrophobic nanoporous oxide layers with varying pore diameters and oxide layer thicknesses, including TAAO-20-150, TAAO-80-150, TAAO-20-500, and TAAO-80500, all measured in 3.5 wt % NaCl solution after immersion for 1 h. The Ecorr, Icorr, and IE values for the samples are also summarized in Table S2. The Ecorr values for TAAO-20-150, TAAO-80-150, TAAO-20-500, and TAAO-80-500 were −1.593, −1.592, −1.474, and −1.360 V, respectively. This shows that for the thin oxide layer (t = 150 nm) Ecorr does not change with the pore size (from −1.593 to −1.592 V only, within experimental error). For the thick oxide layer (t = 500 nm), a significant change occurs, from −1.474 V (for TAAO-20-500) to −1.360 V (for TAAO-80-500) with the increase in pore size. For the same pore size, Ecorr was also inclined to shift in a positive direction with the increase in the oxide layer thickness, which was more pronounced with a larger pore size (i.e., from −1.592 V (for TAAO-80-150) to −1.360 V (for TAAO-80-500) for a pore size of 80 nm compared to from −1.593 V (for TAAO-20-150) to −1.474 V (for TAAO-20-500) for a pore size of 20 nm). Meanwhile, the reduction of Icorr was more significant than the change in Ecorr. The Icorr values for TAAO-20-150, TAAO-80150, TAAO-20-500, and TAAO-80-500 were 9.7 × 10−7, 9.7 × 10−8, 1.0 × 10−7, and 9.8 × 10−9 A/cm2, respectively. For the same oxide layer thickness, Icorr decreased with the increase in pore size, which was more pronounced at the thicker oxide layer (i.e., from 9.7 × 10−7 to 9.7 × 10−8 A/cm2 for TAAO-20-150 to 11046

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Figure 7. (a−e) Potentiodynamic polarization measurement data for elongated immersion times from 1 h to 72 and 168 h in 3.5 wt % NaCl solution at room temperature for the Teflon-coated hydrophobic samples, including TBA, TAAO-20-150, TAAO-80-150, TAAO-20-500, and TAAO-80-500. (f) Change in the corrosion inhibition efficiency (IE) of the samples with respect to the immersing times on the basis of the hydrophilic bare aluminum (BA) immersed for 1 h.

immersion times for the TBA, TAAO-20-150, TAAO-80-150, TAAO-20-500, and TAAO-80-500 samples, and Figure 7f shows the changes in IE of the samples with respect to the immersion times. The Ecorr, Icorr, and IE values for the Teflon-coated hydrophobic samples for immersion times of 72 and 168 h are also summarized in Tables S3 and S4. The IE values of the

samples were estimated by using eq 1, where the corrosion current density of the bare aluminum (BA) sample immersed for an hour was used for the Icorr,bare value. The results show that Ecorr values were slightly shifted in a negative direction for all samples with an increase in immersion time from 1 to 72 to 168 h (Figures 7a−e), such as from −1.596 11047

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Langmuir Table 4. Relative Change (%) in IE Values over Time Compared to the IE Value in 1 h immersed time (h)

TBA (%)

72 168

−40 −70

TAAO-20-150 (%)

TAAO-80-150 (%)

TAAO-20-500 (%)

TAAO-80-500 (%)

−1 −8

0 0

0 0

0 −5

to −1.639 to −1.662 V for TBA, from −1.593 to −1.619 to −1.647 V for T20-150, from −1.592 to −1.627 to −1.655 V for TAAO-80-150, from −1.474 to −1.490 to −1.525 V for TAAO20-500, and from −1.360 to −1.411 to −1.462 V for TAAO-80500. Although the change in Ecorr with immersion time was not significantly different among the samples, the change in Icorr was significantly affected by the pore size and the oxide layer thickness, such as from 3.6 × 10−6 to 5.5 × 10−6 to 7.0 × 10−6 A/ cm2 for TBA, from 9.7 × 10−7 to 9.9 × 10−7 to 1.3 × 10−6 A/cm2 for TAAO-20-150, from 9.7 × 10−8 to 2.3 × 10−7 to 8.0 × 10−7 A/ cm2 for TAAO-80-150, from 1.0 × 10−7 to 1.2 × 10−7 to 1.3 × 10−7 A/cm2 for TAAO-20-500, and from 9.8 × 10−9 to 1.5 × 10−8 to 1.7 × 10−8 A/cm2 for TAAO-80-500 for an immersion time from 1 to 72 to 168 h. Overall, Icorr increased with the immersion time. However, the change was less in the case of the hydrophobic (Teflon-coated) nanoporous oxide layers (TAAO samples) than the hydrophobic bare aluminum (TBA). The result shows that for the same oxide layer thickness the increase in Icorr with the immersion time was more pronounced with a larger pore size. However, the increase became less significant with the thicker oxide layer. For the same pore size, the increase in Icorr with the immersion time was also less pronounced with a thicker oxide layer. The change in IE shows a trend similar to that of the corrosion current density (Figure 7f), such as from 57 to 35 to 17% for TBA, from 88 to 88 to 84% for TAAO-20-150, from 98 to 97 to 90% for TAAO-80-150, from 98 to 98 to 98% for TAAO-20-500, and from 99 to 99 to 99% for TAAO-80-500 for an immersion time from 1 to 72 to 168 h. The relative changes (%) in the IE values over time, compared to the initial IE value estimated in 1 h, are also summarized in Table 4. The decrease in IE with the increase in immersion time is less significant in the hydrophobic nanoporous oxide layers (TAAO samples) than in the hydrophobic bare aluminum (TBA). In the case of the Teflon-coated nanoporous oxide layers with a thin oxide layer (TAAO-20-150 and TAAO-80-150), the decrease in IE with the immersion time was more pronounced with a larger pore size. However, in the case of the Teflon-coated nanoporous oxide layers with a thick oxide layer (TAAO-20-500 and TAAO-80-500), no noticeable change in IE over time was observed even with different pore sizes. The significant increase in Icorr (or the significant decrease in IE) in the case of the Teflon-coated bare aluminum sample indicates that the thin (only a couple of nanometers thick) Teflon film is not robust enough to protect the corrosion over time. In the case of the Teflon-coated nanoporous oxide layers, both the oxide layer and the air layer impregnating the hydrophobic pores can further protect the corrosion although the thin Teflon film fails to protect the corrosion for itself. However, the result also suggests that if the oxide layer is thin or the pore depth is shallow (e.g., 150 nm), both the oxide layer and the air layer impregnating the hydrophobic pores would be less durable over an elongated time. In the potentiodynamic polarization measurement for the 1 h immersion (Figure 6), it was found that the air layer should be more efficient than the oxide layer to prevent corrosion. However, in the case of the thin oxide layer (t = 150 nm), it was found that the increase in Icorr (or

the decrease in IE) with the increase in immersion time was more pronounced with a larger pore size. These results suggest that the air impregnating the low-aspect-ratio pores (i.e., pores with a large pore diameter and a shallow depth) should be less durable to be retained in the pores. For example, the air layer can be replaced by the corrosive medium (saltwater) by capillary pressure or the dissolution of air into saltwater, which would be more effective in the low-aspect-ratio pore with a large pore diameter and a shallow depth. Then, it would allow the corrosive medium in the saltwater to contact the oxide layer more, resulting in an increase in the corrosion current density or a decrease in the corrosion inhibition efficiency more eminently. However, if the pore has a high aspect ratio (i.e., small and deep), then capillary filling in such a closed-end nanopore is difficult once it is filled with air.52−54 Then, the only way for the corrosive medium to penetrate or fill the pores is the dissolution of air into the liquid.52,53 According to Henry’s law, Henry’s constant of oxygen and nitrogen in water is very small.55 Thus, as long as the pores are deep (i.e., thick oxide layer), a larger pore size would be more advantageous to minimize the contact of the corrosive media with the bottom metal surface and keep a greater volume of air in the pores for a longer time, as shown in the result with the sample of TAAO-80-500.

4. CONCLUSIONS In this study, it has been demonstrated that air-impregnated hydrophobic nanoporous anodic aluminum oxide layers directly grown on aluminum substrates can significantly enhance the corrosion resistance of the aluminum substrates, compared to hydrophilic nanoporous anodic aluminum oxide layers or the hydrophobic coating itself. The results show that the hydrophobic nanoporous oxide layer coated with a thin film of Teflon can trap air within pores. Then the air impregnating the porous oxide layer can function as a barrier to inhibit the penetration of corrosive media toward an aluminum substrate and hence improve the corrosion resistance. The results show that the air impregnating the nanoporous structures is more effective than the oxide layer at preventing corrosion. The hydrophobic nanoporous oxide layer with the largest pore diameter and the thickest oxide layer (i.e., the deepest pore), having the lowest liquid contact with the top solid surface and the greatest amount of entrained air, shows not only the best corrosion resistance but also the best durability in protecting corrosion under saltwater (3.5 wt % NaCl solution) over time, with the corrosion inhibition efficiency as high as 99%. Enabled by testing the nanoporous oxide layers with systematically controlled pore diameters and oxide layer thicknesses, a systematic understanding of the effect of the air-impregnated hydrophobic oxide nanostructures on the corrosion inhibition has been achieved in this study. Such advanced understanding will be of a great significance in the design and practical applications of hydrophobic or superhydrophobic surfaces/coatings for corrosion protection in an efficient way. 11048

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02392. Superhydrophobic surface parameters for air impregnation; potentiodynamic polarization test setup; surface and cross-sectional SEM images of AAO without and with Teflon coatings; EDS mapping/spectra for Teflon-coated AAO; contact angle hysteresis of a sessile droplet of water on Teflon-coated AAO surfaces; and tables for the summary of the corrosion potential, corrosion current density, and corrosion inhibition efficiency obtained from potentiodynamic polarization tests (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 201-216-5579. Fax: 201-216-8315. E-mail: cchoi@ stevens.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Office of Naval Research (ONR) under the Young Investigator Program (award N0001410-1-0751). We thank Dr. Airan Perez at the ONR for motivation for the idea investigated in this work. The research effort used microscope resources partially funded by the National Science Foundation through grant DMR-0922522 and the ONR Defense University Research Instrumentation Program (award N0001411-1-0841).

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