Electroactive Polymer-Based Anticorrosive Coatings
Kung-Chin Chang; Jui-Ming Yeh Department of Chemistry, Center for Nanotechnology and Biomedical Technology at Chung-Yuan Christian University (CYCU), Chung Li, Taiwan, ROC
Abstract
Use of electroactive polymer (EAP) coatings for corrosion protection has been discussed for many years. Quite a number of possible protection mechanisms are proposed, of which the possible passivation of the metal through the high potential of redox EAP such as polyaniline is maybe the most frequently stated. However, introducing the conjugated oligoaniline into the EAP backbones for using anticorrosive coatings are rarely discussed.
This chapter focuses on the introduction to the anticorrosive properties of EAP-based coatings. The polymer coatings include EAP, electroactive polymer nanocomposites, and electroactive polymer with hydrophobic/superhydrophobic surfaces. The synthesis and mechanism for metal corrosion of the EAP-based coatings are also be discussed.
16.1 Introduction
Traditionally, polymeric materials have been used as insulators. However, in the early 1960s, Pohl and Katon et al. synthesized and characterized some conjugated polymers with conductivities in the semiconductor range.1,2 Until 1977, the first polymer shown to conduct electricity was iodine-doped poly(acetylene). This discovery was published by Hideki Shirakawa et al. As a result of this pioneering work,3,4 they received the 2000 Nobel Prize in Chemistry. The development of this new class of polymeric materials continues to offer the promise of a wide range of novel applications, including molecular electronics, actuators, electrochromic windows/displays, supercapacitors, transistors, photovoltaics, and corrosion protection. This discovery opened up new areas of research, with many commercial products now incorporating polymers as electrical conductors.
For application of the conducting polymer to corrosion protection, DeBerry in 1985 was first to report that stainless steel covered by polyaniline (PANI) remained in the passive state for a relatively long period in sulfuric acid solution.5 Wessling then pointed out that the conducting polymer coating of PANI and polypyrrole (PPy) possibly possessed self-healing properties, in which the passive oxide between the substrate metal and the conducting polymer could be spontaneously reformed at a flawed site by oxidative capability of the conducting polymer.6
Electroactive polymers (EAPs) are composed of conjugated chains containing π-electrons delocalized along the polymer backbone. They are a new class of materials that received recent attention from academia and industry due to their potential for applications such as biosensors, artificial muscles, actuators, corrosion protection, electronic shield shielding, environmentally sensitive membranes, visual displays, solar materials, and components in high-energy batteries.
In this chapter, we present the EAP-based materials composed of conjugated oligoaniline into the polymer backbones (including EAP, electroactive polymer nanocomposites (EAPNs), and electroactive polymer with hydrophobic/superhydrophobic surfaces (HEAPs/SEAPs)) as model coatings to demonstrate the advanced anticorrosive properties by performing a series of electrochemical corrosion measurements.
16.2 Corrosion
Corrosion is the deterioration of materials by chemical interaction with their surrounding environment. The consequences of corrosion are many and varied and the effects of these on the safe, reliable, and efficient operation of equipment or structures are often more serious than simple loss of a mass of a metal. Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small. Some of the disastrous effects of corrosion can be summarized below:
(1) Hazards or injuries to people arising from structural failure or breakdown (e.g., bridges, cars, aircrafts)
(2) Reduced value of goods due to deterioration of appearance
(3) Contamination of fluids in vessels and pipes (for instance, beer goes cloudy when small quantities of heavy metals are released by corrosion)
(4) Loss of technically important surface properties of a metallic component. These could include frictional and bearing properties, ease of fluid flow over a pipe surface, electrical conductivity of contacts, surface reflectivity, or heat transfer across a surface
(5) Perforation of vessel and pipes allowing escape of their contents and possible harm to the surroundings
(6) Loss of time availability of profile-making industrial equipment
(7) Reduction of metal thickness leading to loss of mechanical strength and structural failure or breakdown. When the metal is lost in localized zones so as to present a cracklike structure, very considerable weakening may result from quite a small amount of metal loss
(8) Added complexity and expense of equipment that needs to be designed to withstand a certain amount of corrosion and to allow corroded components to be conveniently replaced
(9) Mechanical damage to valves, pumps, and so on, or blockage of pipes by solid corrosion products
16.3 Measures of Corrosion Prevention
There are several methods of corrosion control, such as inhibitors, cathodic protection,7 anodic protection,8 coating9 and alloying. The next section briefly describes a few of these methods.
16.3.1 Inhibitors
An inhibitor is a substance that, when added in small concentrations to an environment, decreases the corrosion rate. In a sense, an inhibitor can be considered as a retarding catalyst. There are numerous inhibitor types and compositions. Most inhibitors have been developed by empirical experimentation, and many inhibitors are proprietary in nature and thus their composition is not disclosed. Inhibition is not completely understood because of these reasons, but it is possible to classify inhibitors according to their mechanism and composition.10,11 Figure 16.1 is the schematic diagram of adsorption inhibitors. Adsorption inhibitors form a chemisorptive bond with the metal surface and impede ongoing electrochemical dissolution reactions.12
16.3.2 Cathodic protection
Cathodic protection was employed before electrochemistry had been developed. Humphrey Davy used cathodic protection on British naval ships in 1824. The principles of cathodic protection may be explained by considering the corrosion of a typical metal in an acid environment. Electrochemical reactions occurring are the dissolution of the metal and the evolution of hydrogen gas:
16.3.3 Anodic protection
In contrast to cathodic protection, anodic protection is relatively new; it was first suggested by Edeleanu in 1954. This technique was developed using electrode kinetics principles and is somewhat difficult to describe without introducing advanced concepts of electrochemical theory. Simply, anodic protection is based on the formation of a protective film on metals by externally applied anodic currents.13
16.3.4 Coatings
16.3.4.1 Metallic and other inorganic coatings
Relatively thin coatings of metallic and inorganic materials can provide a satisfactory barrier between metal and its environment. The chief function of such coating (aside from sacrificial coatings such as zinc) is to provide an effective barrier. Metal coatings are applied by electrodeposition, flame spraying, cladding hot dipping, and vapor deposition. Inorganics are applied or formed by spraying, diffusion, or chemical conversion. Spraying is usually followed by baking or firing at elevated temperatures. Metal coatings usually exhibit some formability, whereas the inorganics are brittle. In both cases a complete barrier must be provided. Porosity or other defects can result in accelerated localized attack on the basic metal because of two-metal effects.14 There are two types of metallic coatings used to protect underlying metal substrates. These are sacrificial and noble metal coatings. The first of these types, sacrificial coatings, functions by cathodic protection of the substrate. Figure 16.2 is the schematic diagram of sacrificial coating. The second type of metallic coating, the noble metal coating, is illustrated by a coating of nickel on steel. Figure 16.3 is the schematic diagram of noble metal coating.12
16.3.4.2 Organic coatings
These involve a relatively thin barrier between substrate material and the environment. Paints, varnishes, lacquers, and similar coatings doubtless protect more metal on a tonnage basis than any other method for combating corrosion. Exterior surfaces are most familiar, but inner coatings or linings are also widely utilized. Approximately $2 billion per year are expended in the United States on organic coatings. A myriad of types and products are involved, and some are accompanied by outlandish claims. Substantial knowledge of this complex field is required for successful performance. The best procedure for the uninitiated is to consult with a reputable producer of organic coatings. As a general rule, these coatings should not be used where the environment would rapidly attack the substrate material.15 Figure 16.4 is the schematic diagram of the deterioration of an organic coating. The sequence of events which occurs in the breakdown of an organic coating in the subcoating microenvironment is illustrated by the work of Ritter and Rodriguez.16 The sequence of events is as follows12:
(a) The organic coating first becomes penetrated by water and oxygen molecules, as discussed above.
(b) Corrosion is initiated due to several possible causes, such as defects in the coating, mechanical rupture of the coating, or chemical rupture through generation of osmotic pressure at sites containing soluble salts.
(c) The substrate iron then goes into a solution at a local anodic site:
16.4 Polymer Coatings
Polymeric (or organic) coatings have been employed to protect metals against corrosion for a long time. The primary effect of a polymeric coating is to function as a physical barrier against aggressive species such as O2 and H+. However, all polymeric coatings are not permanently impenetrable, and once there are defects in the coatings, pathways will be formed for the corrosive species to attack the metallic substrate, and localized corrosion will occur. Therefore, as a second line of defense against corrosion, various pigments with a lamellar or platelike shape such as micaceous iron oxide and aluminum flakes have been introduced into the polymeric coating to effectively increase the length of the diffusion pathways for oxygen and water, as well as to decrease the permeability of the coatings. A number of electrochemical measurements have been used as a tool to evaluate the anticorrosion performance of polymeric coatings, including electroactive (e.g., polyaniline) or nonelectroactive (e.g., polystyrene) polymers. Wei et al.17 demonstrated the anticorrosive performance of electroactive polyaniline and nonelectroactive polystyrene by performing a series of electrochemical measurements of corrosion potential and corrosion current on the sample-coated cold-rolled steel (CRS) electrode under various conditions. Lee and coworkers18 investigated the corrosion-resistance properties of PANI-coated mild steel in saline and acid by electrochemical impedance spectroscopy (EIS).
Recently, EAPs, EAPNs, and HEAPs/SEAPs used as enhanced anticorrosion coatings have been reported by Yeh et al.19–30 For example, EAPs (e.g., polyimide,19 epoxy,20 polyurethane,21 polyamide,22 and polyurea23), EAPNs (e.g., polyimide-clay,24 polyimide-TiO2,25 epoxy-SiO2,26 and polyanilie-graphene27), and HEAPs/SEAPs (e.g., epoxy,28–30 PANI,31 and polyimide32) for making a series of novel advanced anticorrosion coatings based on a series of electrochemical corrosion parameter measurements of corrosion potential, polarization resistance, corrosion current, and EIS at room temperature.
16.4.1 EAP-based coatings
Corrosion protection using EAPs was first suggested by MacDiarmid in 1985.33 EAPs can be synthesized both chemically and electrochemically. It has been observed that most EAPs can be electrochemically produced by anodic oxidation, enabling one to obtain a conducting film directly on a surface. EAPs can go from the insulating to the conducting state through several doping techniques, such as (1) chemical doping by charge transfer, (2) electrochemical doping, (3) doping by acid-base chemistry (only PANI undergoes this form of doping), (4) photodoping, and (5) charge injection at a metal-semiconducting polymer interface.34
Among those EAPs, PANI is a potential material for commercial applications due to its environmental stability, good processability, and relatively low cost.35,36 However, PANI prepared by chemical and/or electrochemical means usually exhibits structural defects and has limited solubility in many solvents. These shortcomings would impede the better understanding of the structure-property correlations and the conducting mechanism and also restrict the practical applications of PANI. One possible approach to alleviate the problems is the incorporation of well-defined and conjugated oligoaniline37,38 into the copolymer backbones, which could combine the properties of the specific oligoaniline and desirable polymer properties such as mechanical strength and film-forming ability.
In 2009 and 2012, Yeh et al.19–21 described the first evaluations for the effect of amine-capped aniline trimer (AT) on the corrosion protection efficiency of as-prepared electroactive polyimide, epoxy, and polyurethane. First of all, the amine-capped ATs were synthesized by a one-step method from the oxidative coupling reactions of p-phenylenediamine with aniline in an acidic aqueous medium. Subsequently, a series of polyimides, epoxy, and polyurethane with different content of AT molecules were obtained by thermal imidization, thermal curing, and prepolymerization. The synthesis procedure of electroactive polyimide, epoxy, and polyurethane is depicted in Figures 16.5–16.8. The as-prepared electroactive polyimide, epoxy, and polyurethane were further identified by a series of electrochemical measurements, such as corrosion potential (Ecorr), polarization resistance (Rp), corrosion current (Icorr), and EIS studied in 5 wt.% NaCl electrolyte.
Figures 16.8–16.10 and Table 16.1 represent the results of electrochemical measurements of potentiodynamic (e.g., Tafel plots) and EIS (e.g., Nyquist and Bode plots) of electroactive polyimide at room temperature.
Table 16.1
Feed composition ratio of electroactive polyimide, electroactive copolyimide, and non-electroactive polyimide with the Ecorr, Rp, Icorr, and Rcorr measured with electrochemical method.19
| Compound Code | Feed Composition (M) | ||||||||
| Diamine | Dianhydride | Ecorr (mV) | Electrochemical Corrosion Measurements | PEF (%) | |||||
| ODA | ATs | BSAA | Rp (kΩ cm2) | Icorr (μA/cm2) | Rcorr (MPY) | Thickness (μm) | |||
| Barea | - | - | - | − 720 | 0.018 | 18.900 | 8.820 | - | |
| Electroactive polyimide | 0 | 1 | 1 | − 486.5 | 0.560 | 0.103 | 0.048 | 22 | 30.0 |
| Electroactive copolyimide | 0.5 | 0.5 | 1 | − 592.2 | 0.246 | 0.400 | 0.187 | 20 | 12.6 |
| Nonelectroactive polyimide | 1 | 0 | 1 | − 647.8 | 0.074 | 0.700 | 0.327 | 19 | 3.1 |
a Pristine CRS used for test.
The as-prepared polyimide with higher content of ATs exhibited obviously enhanced corrosion protection efficiency on CRS electrodes. The significant enhancement of corrosion protection on CRS electrodes probably is attributed to the redox catalytic property of electroactive aniline trimer in the formation of a passive layer of metal oxide, which was investigated by SEM and ESCA and is shown in Figures 16.11 and 16.12. The iron oxide of the CRS surfaces were studied and found to have a thin layer containing Fe2O3 and Fe3O4. The passivation mechanism of electroactive polyimide on the CRS surface is similar to that of PANI. The ATs show the excellent corrosion inhibition and act as a redox catalyst to increase the corrosion protecting ability.
16.4.2 EAP-based nanocomposite coatings
In the past 20 years, polymer nanocomposites used as enhanced anticorrosion coatings have been reported by Yeh et al.,39–47 for example, polymer-clay nanocomposites39–45 and polymer-silica/titanium nanocomposites46,47 through different preparative routes such as in situ polymerization or a solution dispersion approach for making a series of advanced anticorrosion coatings based on a series of electrochemical corrosion parameter measurements at room temperature. Enhanced corrosion protection effect of polymer nanocomposites compared to bulk polymer might be a result from dispersing silicate nanolayers of clay and silica/titanium nanoparticles in polymer matrix to increase the tortousity of diffusion pathway of oxygen gas.48,49
Currently, the research interests on developing EAPNs as anticorrosion coatings, and polymers with different types of inorganic fillers, were employed and listed as follows:
16.4.2.1 EAP/silicate nanolayer (clay) nanocomposites
Yeh et al. investigated the effect of organophilic-MMT clay on the anticorrosive properties of EAPNs (e.g., polyimide-clay24). The electroactive PI-clay (EAC) nanocomposite materials were prepared by performing the chemical imidization of EPI in the presence of organophilic clay platelets. Dispersion capability of organophilic clay in EPI matrix was further identified by wide-angle powder X-ray diffraction (WAXRD; Figure 16.13) pattern and transmission electron microscopy (TEM; Figure 16.14). To study the effect of organophilic clay on the anticorrosive properties of EAC nanocomposite coatings, a series of electrochemical (e.g., corrosion potential, polarization resistance, corrosion current, corrosion rate, and protection efficiency) measurements were studied; the results are shown in Figure 16.15 and Table 16.2. The as-prepared EAC nanocomposite coatings were found to exhibit advanced corrosion protection efficiency as compared to that of NEPI, EPI, and bare CRS electrodes based on the sequential electrochemical corrosion measurements in saline condition. The significant enhancement in corrosion protection of EACN coatings on CRS electrodes might probably be attributed to the redox catalytic property of organic EPI inducing the formation of a passive metal oxide layer and to the fact that the barrier property of well-dispersed organophilic clay platelets existed in EPI matrix, as evidenced by the investigations by the SEM/ESCA and GPA studies, respectively. The analytical results of SEM and ESCA were similar to Figures 16.11 and 16.12, and the results of gas barrier properties are shown in Figure 16.16.
Table 16.2
Feed composition ratio and electrochemical corrosion measurements of the prepared materials.24
| Sample Code | Feed Composition (g) | Electrochemical Corrosion Measurements | ||||||||
| ODA | ACAT | BSAA | Organophilic clay | Ecorr (mV) | Rp (kΩ cm2) | Icorr (μA/cm)2 | Rcorr (mm/year) | Thickness (μm) | PEF (%) | |
| Barea | - | - | - | - | − 1020.9 | 2.47 | 9.14 | 1.07 × 10− 1 | - | - |
| NEPI | 0.1 | - | 0.26 | - | − 900.2 | 5.09 | 4.02 | 4.71 × 10− 2 | 25 | 1.06 |
| EPI | - | 0.145 | 0.26 | - | − 807.7 | 17.30 | 2.53 | 2.96 × 10− 2 | 25 | 6.00 |
| EPC01 | - | 0.145 | 0.26 | 0.004 | − 664.9 | 36.67 | 1.89 | 2.21 × 10− 2 | 24 | 13.85 |
| EPC03 | - | 0.145 | 0.26 | 0.012 | − 528.5 | 69.99 | 1.09 | 1.28 × 10− 2 | 27 | 27.34 |
a Pristine CRS used for test.
16.4.2.2 EAP/nanoparticle (TiO2, SiO2) nanocomposites
Yeh et al. described the anticorrosive properties of a series of EAP-TiO2/SiO2 nanocomposites (e.g., polyimide-TiO225 and epoxy-SiO226). The electroactive PI-TiO2 (EPT) nanocomposite materials were prepared by performing the chemical imidization. The dispersion capability of TiO2 nanoparticles in EPI matrix was further observed by TEM studies (Figure 16.17). It should be noted that the EPT even adding 10% content of TiO2 still revealed a redox current as compared to that of EPI. According to electrochemical measurement, EPT exhibited a good anticorrosion property; the results are shown in Figures 16.18–16.20 and Table 16.3. The mechanism of anticorrosion property for EPT could be attributed the a synergistic effect of formation of passive metal oxide layers induced from redox catalytic capabilities of ACAT units, and a gas barrier property induced from well-dispersed TiO2 nanoparticles in its matrix. The analytical results of passive metal oxide layers were similar to Figure 16.11, and the results of gas barrier properties were shown in Figure 16.21. The synergistic effects cause the EPT coatings to reveal advanced corrosion protection efficiency as compared to that of NEPI, EPI, and bare CRS electrodes based on the sequential electrochemical corrosion measurements in saline condition.
Table 16.3
Feed composition ratio and electrochemical corrosion measurements of the prepared materials.25
| Sample Code | Feed Composition (g) | Electrochemical Corrosion Measurements | ||||||||
| ODA | ATs | BSAA | Ti(OBu)4 | Ecorr (mV) | Rp (kΩ cm2) | Icorr (μA/cm2) | Rcorr (mm/year) | Thickness (μm) | PEF (%) | |
| Barea | - | - | - | - | − 976.9 | 0.011 | 5.275 | 6.2 × 10− 2 | - | - |
| NEPI | 0.1 | - | 0.26 | - | − 647.8 | 0.074 | 0.700 | 8.2 × 10− 3 | 20 | 5.98 |
| EPI | - | 0.145 | 0.26 | - | − 469.5 | 0.112 | 0.598 | 7.0 × 10− 3 | 20 | 9.56 |
| EPT05 | - | 0.145 | 0.26 | 0.203 | − 330.4 | 0.271 | 0.333 | 3.9 × 10− 3 | 22 | 24.56 |
| EPT10 | - | 0.145 | 0.26 | 0.405 | − 179.0 | 0.439 | 0.164 | 1.9 × 10− 3 | 21 | 40.37 |
a Pristine CRS used for test.
16.4.2.3 EAP/graphene nanocomposites
Yeh et al. reported the anticorrosive properties of EAP-graphene nanocomposites (e.g., polyaniline-graphene27). The polyaniline/graphene composites (PAGCs) were prepared by in situ oxidative polymerization. The as-prepared PAGCs were subsequently characterized by TEM (Figure 16.22). From the results of electrochemical measurements, gas permeability, and vapor permeability rates (Figures 16.23 and 16.24), the PAGCs coatings were shown to effectively protect steel because of the good O2 and H2O gas barrier. Well-dispersed graphene, with a relatively high aspect ratio compared with clay, in a polymer matrix enhances the gas barrier and is responsible for the highly desirable anticorrosion properties that make PAGCs coatings much more effective than polyaniline-clay composite (PACCs) coatings.
16.4.2.4 EAP-based hydrophobic/superhydrophobic coatings
Over the past few years, the remarkable surface structures of superhydrophobic plant leaves, characterized by the so-called lotus effect, have attracted the attention of many researchers. In particular, the superhydrophobic property induced by these structures has become quite an important issue because there are a variety of practical applications utilizing their self-cleaning, drag reduction, antifogging and anticorrosion effects, and so on. However, reports on the corrosion protection studies associated with hydrophobic/superhydrophobic surfaces using EAP coatings are limited. Recently, Yeh et al. demonstrated a feasible strategy to develop advanced anticorrosion coatings by combining the crucial benefits of electroactivity and hydrophobicity/superhydrophobicity in corrosion protection.
A photograph of natural, fresh Xanthosoma sagittifolium leaves is shown in Figure 16.25a. Figure 16.25b is the high magnification SEM image of the X. sagittifolium leaf. The average contact angle formed on the fresh X. sagittifolium leaves is ca. 146°, as shown in the illustration in Figure 16.25b. From Figure 16.25b, many small papillary hills are clearly visible on the natural X. sagittifolium leaf. The diameters of the small papillary hills are between 7 and 9 μm.
The hydrophobic/superhydrophobic polymer coating was fabricated through a nanocasting technique using the PDMS templates. Figures 16.26 and 16.27 shows the structures on the surfaces of the nanocasted layers on the CRS slides observed using SEM. Numerous papillary microstructures whose average diameter is about 7-9 μm are formed on the surfaces. The papillary microstructures are replicas of the surface patterns of the X. sagittifolium leaves.
Figure 16.28 shows the schematic diagram for the fabrication of hydrophobic/superhydrophobic electroactive polymer coating materials. First, a PDMS prepolymer is cast against a fresh X. sagittifolium leaf surface and then cured under proper conditions. The PDMS template prepared has negative X. sagittifolium leaf surface structures and is obtained after peeling the leaf off. Second, the substrate is covered with the EAP solution, and the template is pressed against the CRS. After the process and peeling off the PDMS template, a X. sagittifolium leaflike surface is formed on the CRS.
The resulting hydrophobic electroactive epoxy (HEE)29, superhydrophobic electroactive epoxy (SEE),28,30 polyaniline (SH-PANI),31 and polyimide (SEPI)32 with the replicated nanostructured surface showed a hydrophobic and superhydrophobic characteristic with a water contact angle of ~ 120° and ~ 155°. The significant increase in the contact angle indicated that the biomimetic morphology effectively repelled water. The results of potentiodynamic and electrochemical impedance spectroscopic measurements in saline conditions (Figure 16.29 and Table 16.4) indicated that the electroactive epoxy coating with superhydrophobic surfaces (SEE) offered better protection against corrosion than the electroactive epoxy (EE) coating did.
Table 16.4
Electrochemical corrosion measurements for EE and SEE.28
| Sample Code | Immersion Perioda (day) | pH valuesb | Electrochemical Corrosion Measurements | |||
| Ecorr (mV) | Icorr (μA/cm2) | Protection Efficiency (%) | Coating Thickness (μm) | |||
| Barec | 0 | 7 | − 839 | 32.73 | ||
| EE | 0 | 7 | − 564 | 10.11 | 69.11 | 31 |
| SEE | 0 | 7 | − 244 | 4.33 | 86.77 | 30 |
| SEE | 0 | 5 | − 246 | 4.39 | 86.58 | 30 |
| SEE | 0 | 3 | − 253 | 4.60 | 85.94 | 29 |
| SEE | 0 | 1 | − 259 | 5.02 | 84.66 | 32 |
| bare | 7 | 7 | − 1047 | 46.88 | − 43.23 | - |
| EE | 7 | 7 | − 628 | 15.58 | 52.39 | 31 |
| SEE | 7 | 7 | − 253 | 4.48 | 86.31 | 31 |
a 3.5% aqueous NaCl electrolyte as an immersion solution.
b pH values control by 1 M HCI (aq).
c Pristine CRS used for test.
16.5 Conclusions
EAPs have been shown to offer many potential advantages as corrosion protection coatings. This chapter is intended to provide the reader with an understanding of synthesis and mechanism of corrosion in metal with protective EAP-based coatings.
Enhancement of corrosion protection using EAP-based coatings might be attributed to the following three reasons: (1) the redox catalytic capabilities of oligoaniline units in EAP may induce the formation of passive metal oxide layers on CRS electrode, (2) the well-dispersed inorganic fillers embedded in EAP matrix could function as an impervious layer to effectively enhance the oxygen barrier property, and (3) the hydrophobicity/superhydrophobicity of the coating repelled the moisture and further reduced the water/corrosive media adsorption on the EAP surface, preventing the underlying metals from corrosion attack.