Encapsulation of aliphatic amines into nanoparticles for self-healing corrosion protection of steel sheets

Progress in Organic Coatings 76 (2013) 1316–1324

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Encapsulation of aliphatic amines into nanoparticles for self-healing corrosion protection of steel sheets Hana Choi, Kyoo Young Kim, Jong Myung Park ∗ Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea

a r t i c l e

i n f o

Article history: Received 6 July 2012 Received in revised form 21 February 2013 Accepted 4 April 2013 Available online 15 May 2013 Keywords: Polymer capsules Encapsulation Amine corrosion inhibitor Self-healing

a b s t r a c t A noble approach based on the encapsulation of corrosion inhibitors has been presented, which are capable of improving the active corrosion protection without negatively influencing the barrier properties of the coating layers. Polymeric nanocapsules loaded with six types of amine corrosion inhibitors were synthesized by multi-stage emulsion polymerization. Depending on the basicity and water solubility of amines, different amounts of releasable corrosion inhibitors were encapsulated into the polymer capsules. Encapsulated organic amines were generally well released under alkaline conditions, and linear amines were more easily released from inside capsules than branched ones. The nanocapsules were incorporated into the coating resin and were coated on cold-rolled steel sheets to investigate corrosion protection efficiencies. The corrosion inhibitive efficiencies of the nanocapsule-containing coating layers were evaluated by electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET). In this study, it was revealed that the intrinsic properties of the amines as well as their encapsulation/release behaviors determined the barrier property and self-healing protection capability of the coating layer. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Organic coating is an economical and effective means of protecting steel against corrosion in both wet and dry environments. For corrosion protection of metallic substrates, the protective efficiency of the coating layer depends on its own intrinsic barrier properties, such as durability and resistance to water penetration, and its interfacial adhesion to metal surfaces. In corrosive environments, the metal substrate beneath the coating film starts to corrode due to the penetration of water and corrosive ions weakening the durability of the coating layer. Moreover, improper pretreatment can make the coating layer fall off the substrate exposing the metal to corrosive environments. Metal substrates begin to corrode under severe conditions. Therefore, bulky corrosion products are accumulated at cathodic sites, which cause coating delamination or blistering. One potential problem is coating damage from external impact while stacking coated steel sheets or during the forming process. The metal substrates can be exposed when the coating film is scratched or a metal sheet is cut or deformed to create structures or shapes. Once a metal substrate is exposed to the environment, corrosion of the exposed area is accelerated due to the relatively small anode and the rapid diminishment of the barrier protection of the organic coating layer. This

∗ Corresponding author. Tel.: +82 1050601559. E-mail addresses: [email protected], [email protected] (J.M. Park). 0300-9440/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2013.04.005

problem has compromised the long-term corrosion protection of organic coating layers. Smart coating systems, such as self-healing protection, have been developed to compensate for the drawbacks of the organic coating layers. The primary aim of the self-healing concept is to provide active, healing agents autonomously to the damaged matrix without any external trigger [1,2]. The self-healing coatings enable rapid healing after changes in the coating integrity and the surrounding environment [1]. In general, released healing agents are introduced in nano-/microcontainers [3–6], nanoporous materials [7,8], multi-layer systems [9] and ion-exchanged particles [10]. Among the various strategies for selfhealing coatings, the use of nanocontainers has been the most widely employed due to the versatility of nanocontainer fabrication and the variety of applicable healing agents. We have verified the feasibility of the polystyrene-based nanocapsules as reservoirs for storing a corrosion inhibitor [11]. Triethanolamine (TEA), which is a corrosion inhibitor, was introduced into the nanocapsules, and the encapsulated TEA showed excellent anticorrosive properties compared to an organic coating without an encapsulated corrosion inhibitor. The nanoencapsulated corrosion inhibitor system retards steel corrosion by releasing the inhibitor for a long time. In addition, the encapsulated corrosion inhibitors do not aggravate the durability of the coating layer in the electrolyte, and prevent coating delamination during the initial stage of corrosion by retarding water penetration.

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Fig. 1. List of the amine groups with/without hydroxyl groups including the chemical structures and properties.

The penetration of the electrolyte through the coating film is one of the coating problems that arise when a water-soluble corrosion inhibitor is directly added to the coating layer. A reasonable range of inhibitor solubility is critical to determine the efficiencies of inhibition. However, excessive water solubility can cause excessive leaching of the inhibitor to the electrolyte and thus decreasing the effective inhibition time. In addition, when the soluble corrosion inhibitor is directly added to the coating layer, the barrier protection and durability of the coating layer can be weakened because it can be readily released to the electrolyte [12,13] and attract water to the coating layer causing osmotic blistering of the coating film [14]. The storage of the corrosion inhibitor in nanocontainers prevents the encountered problems when adding corrosion inhibitors directly into the coating layer, such as uncontrollable release, fast exhaustion of corrosion inhibitor and coating failure like osmotic blistering. It also increases the corrosion resistance of the organic coating layer with self-healing protection in terms of longterm corrosion protection. In this system, the inhibition efficiency is dependent on the encapsulation quantities of the corrosion inhibitor, the controllable release on demands and the inhibition power of the encapsulated inhibitor. According to Bhajwala and Vashi [15], the addition of TEA to the acidic electrolyte decreased the corrosion rate of zinc, but TEA was less powerful inhibitor compared to ethanolamine or diethanolamine. The inhibition efficiency of ethanolamine was much higher than that of TEA, and the inhibition was found to be a function of the adsorption ability, which was dependent on the electron donating ability of the alkyl group and steric hindrance of the nitrogen atom. In this study, the effects of the intrinsic properties of amines were investigated on the corrosion inhibition ability and encapsulation/release properties using six types of amine-type corrosion inhibitors.

particle size was 100 nm. Potassium persulfate (Samchun Chemical, South Korea) was used as a free radical initiator, and Rhodapex® CO-436 (ammonium nonylphenol ether sulfate, from Rhodia Inc.) was used as an anionic emulsifier in the emulsion polymerization. Six types of amines with different molecular weights, functional groups and basicity were utilized in the encapsulation: diethanolamine (DEA, 98%, Samchun Chemical), ethanolamine (ETA, 99%, Junsei Chemicals), propylamine (PPA, 98%, Junsei Chemicals), triethanolamine (TEA, 99%, Sigma–Aldrich), dipopylamine (DPA, 99%, Sigma–Aldrich) and 5-amino-1-pentanol (5AP, 95%, Tokyo Chemical). The polymerization process was performed under a nitrogen inlet, and deionized water (DI water) was used for all of the experimental processes. All of chemicals were used as received. Details of the six types of amine species are given in Fig. 1. The polymerization process was performed according to a previously published protocol [11]. For neutralization of the core materials, the amine solutions were simultaneously fed into the reactor. The amount of injected amine was 10 g, except for DPA. Due to poor water solubility, only 5 g was added to the reactor to prevent coagulation in the emulsion. In conventional hollow core–shell synthesis, a base is typically introduced after all of the shell layers are synthesized. The stable polystyrene shell prevented the amine from penetrating into the core through the bilayer, and only a small portion of the amine could reach to the acidic portion at the center. Instead of adding the amine after the formation of the outermost polystyrene shell, the amine solutions were simultaneously fed at 87 ◦ C during the polymerization of the styrene to increase encapsulation efficiency. The feeding time of the amine solution was 15–20 min to maximize the loading quantities. The detailed formulation for the preparation of the nanocapsules is described in Table 1.

2. Experimental procedure

2.2. Characterization

2.1. Encapsulation of corrosion inhibitors into nanoparticles

The synthesized nanocapsules were collected for further analysis by filtering the obtained latex with 0.2 ␮m cellulose filter paper and rinsing with DI water and ethyl alcohol to remove the remaining amines and unreacted monomers on the surface of the capsules. Then, the filtered capsules were dried under ambient conditions for several hours. Transmission electron microscopy (TEM, Philips CM 200) was employed to visualize the synthesized nanocapsules and determine the size and shell shape. The dried

The hollow core–shell nanoparticles were fabricated using multi-stage emulsion polymerization to entrap the amine corrosion inhibitors. The synthesis was initiated by seed latex obtained from KCC Co. (South Korea). The seed latex was prepared by co-polymerization of methyl methacrylate, butyl acrylate and methacrylic acid in a ratio of 63:9:28, and the average

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Table 1 Synthesis of the amine capsule with six types of aliphatic amines. Step

Chemicals

1

DI water Core latex DI water Sodium persulfate Butyl acrylate Methyl methacrylate Methacrylic acid DI water Styrene Sodium persulfate Surfactant Amines

2 3 (1st shell formation) 4 (2nd shell formation)

5 (Injection of base/corrosion inhibitor) a

435 g 60 g 15 g 0.5 g 4.2 g 51 g 1.8 g 76 g 196 g 0.7 g 1.0 g 10 ga

DPA(6): 5 g.

nanocapsules were dispersed in ethyl alcohol and dropped on a carbon-coated cupper grid for TEM analysis. Quantitative analysis was performed using gas chromatography-mass spectrometry (GC–MS, Thermoelectro Polaris Q). To determine the contents of the encapsulated amines using GC–MS, the nanoparticles were dissolved in tetrahydrofuran (THF, Sigma–Aldrich) at approximately 5000 ppm, and 25 ␮l of N,O-bis(trimethylsilyl)trifluoroacetamide (Fluka) was added to 1 ml of this solution for silylation. Silylation is a common and versatile method for derivatizing polar organic amines. N,Obis(trimethylsilyl)trifluoroacetamide was used as the silylation reagent, which yielded the trimethylsilyl (TMS) derivatives. In the silylation procedures, the reactions were maintained at 60 ◦ C for 60 min [16,17]. The amines in nanocapsules were characterized in terms of mass and retention times of species. An Agilent 6890 N GC/5975 MSD equipped with a 60 m DB-5 capillary column was employed to obtain the mass spectra using a He carrier gas at a flow ratio of 1 ml/min. The initial oven temperature was maintained at 40 ◦ C for 3 min and then increased to 250 ◦ C at a rate of 10 ◦ C/min and maintained at this temperature for 5 min. The calibration samples were prepared in a 1–50 ppm concentration range in THF. 2.3. Release behavior of encapsulated amines depending on pH In the course of surface-metal corrosion, electrochemical reactions occur. As a result of these anodic and cathodic reactions, the pH distribution of the metal surface is changed. To suppress the corrosion process, the corrosion inhibitor should be easily releasable into the electrolytes when the pH shifts due to anodic and cathodic reactions or environmental changes. In the present study, the release behaviors of the encapsulated amine species in different pH environments were investigated. An ammonia (NH3 ) solution was used to simulate alkaline conditions (pH 12), and DI water at a pH 6.53 was used to simulate a neutral environment. The nanoparticles were rinsed with DI water and ethyl alcohol prior to use and were subsequently packed in molecular porous membrane tubing (Spectra/Por® 3 dialysis membrane). The dried nanoparticles were sealed in the membrane tubes with clips and submerged in a pH-adjusted solution for 3 d with continuous stirring. The obtained solutions were analyzed by liquid chromatography and mass spectrometry (LC–MS, 1220 Infinity LC System).

Fig. 2. Equivalent electric circuit for the organic coating/metal specimens with two time constants.

nanocapsules in aqueous dispersion form were directly added to the coating resin with a curing agent (Cymel 324, Cytec Industries) and a small amount of a wetting agent (BYK 348, BYK). The concentration of the incorporated capsules was 33.3 wt% of the total solid in the coating system. The coating process was performed using a bar coater (No. 20). Before curing, the specimens were prebaked at 105 ◦ C using an induction oven to avoid severe popping of the film due to a rapid evaporation of water. After flashing off the water, the samples were cured at the peak metal temperature of 170 ◦ C in the induction oven. The thickness of the coating layer was measured around 12 ␮m. 2.5. Corrosion tests For comparison of the corrosion resistance, electrochemical tests were performed in a 0.05 M NaCl solution. The electrochemical impedance spectroscopy (EIS) measurements were performed using a Gamry reference 600. The specimens were fixed in a commercial flat cell with a saturated calomel reference electrode and a platinum mesh counter electrode, and the working electrode area was 1 cm2 . The EIS measurements were performed at an open circuit potential with a 10 mV amplitude sinusoidal voltage in the frequency range of 0.01–10,000 Hz. Bode diagrams were plotted and fitted using an equivalent circuit, as shown in Fig. 2. The circuit consists of the electrolyte resistance (Rs ), the pore resistance (Rc ), the coating capacitance (Cc ), the charge-transfer resistance (Rt ), and the double layer capacitance (Cdl ). The values of the elements in the equivalent circuit were determined by fitting the EIS data using the Echem Analyst program from Gamry Instruments. The scanning vibrating electrode technique (SVET, Applicable Electronics Inc.) employing a microelectrode (MicroProbes Inc.) with a black Pt coating on the tip was conducted in a 0.05 M NaCl solution (conductivity < 196 ␮S/cm). The microelectrode was scanned 150 ␮m above the specimens, and the scanning process was controlled by the ASET software. For the semi-immersion tests and SVET measurements, the samples were cut into a 20 mm × 20 mm size, and all of the edges were sealed with nonconductive tape to prevent cut-edge corrosion. An X-shaped scribe (∼300 ␮m width) was created with a ceramic tip to investigate the inhibition and self-healing protection in a small active area. 3. Results and discussion 3.1. Nanocapsule morphologies with different amine species

2.4. Coating process To evaluate the corrosion inhibition efficiencies of encapsulated amines, the synthesized nanocapsules were included in the coating solution and coated on commercial cold-rolled steel sheets that were 0.783 mm thick and supplied by POSCO. A polyurethane resin (UD-300H, HuChem, 1 K, thermosetting, isophorone polyisocyanate) was used as the main binder, and the synthesized

For the controlled release of corrosion inhibitors on demand, the nanocapsules made by emulsion polymerization are effective nano-scale containers due to the easy manipulability and feasibility of mass production. The desired characteristics of the nanocapsules include nano-size, specifically between 300 and 600 nm for use in thin organic coating systems, adequate synthesis stability after confinement of the amines, pH-sensitive permeability for release

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Fig. 3. Core–shell particle structure of the amine capsule system.

of the corrosion inhibitors and compatibility with organic coating networks. In addition, they should be highly effective at encapsulating corrosion inhibitors without deteriorating the particle’s properties. Hollow latex particles loaded with the various types of amines were successfully fabricated using multi-stage emulsion polymerization. Amines with different molecular weights, functional group number and chemical structure were selected and categorized according to functional group type as amines with hydroxyl groups and without hydroxyl groups. In this study, three steps were required to synthesize the bilayer polymeric particles. For better understanding of the bilayer capsule system, the nanocapsule structures are schematically represented in Fig. 3. The synthesis procedure started with the formation of a poly(MMA-co-MA-co-BA) shell on the carboxylated seed materials. Due to the hydrophilic characteristics of the seed materials, it is hard to fabricate the hydrophobic polystyrene shell with concentric core–shell particle morphology. Therefore, the intermediate shell, which exhibits an amphiphilic property, was forced to increase the affinity between the hydrophilic core and the hydrophobic polystyrene shell. The next step is the formation of the outermost shell, which is composed of polystyrene. The prepared nanoparticles have enough physical stability only after the complete formation of the styrene shell and can be used as additives in the coating resin. The last step is the swelling process by the neutralization of the core with amine species, as shown in Fig. 4. The swelling process is simultaneously conducted with the formation of the second shell. The swelling depends entirely on

the neutralization of the core materials with the amine. Water intake due to osmotic pressure during neutralization process is the primary driving force for the swelling. For the nanocapsules neutralized with soluble amines, such as ETA, 5AP, DEA, TEA and PPA, the diameter of the core significantly increased from 100 nm to 250–400 nm. The swollen core shrinks after drying, and the black dot from the shrunken trace was observed in the TEM image of ETA, 5AP, DEA and PPA-containing nanocapsules. When an immiscible amine is employed as a neutralization agent (i.e., DPA), the core size are not dramatically changed due to insufficient swelling. Therefore, the core appears darker in TEM image compared to the bright core in the case of soluble amines. Hydrophobic amines that are lacking hydroxyl groups are easy to localize and concentrate near particles via hydrophobic attraction. The swelling of the particles is suppressed by hydrophobic attraction when the neutralization process is performed with amines that lack hydroxyl groups [18,19]. 3.2. Encapsulation behaviors of amines in the nanocapsules For quantitative analysis of encapsulated amines, GC/MS was performed. In general, the amines tend to be adsorbed and decomposed on the columns and readily yield tailed elution peaks, ghosting phenomena and low detector sensitivity [20,21]. To prevent the adsorption of amine onto the column and to circumvent the low detector sensitivity, amines that contain polar species were silylated using N,O-bis(trimethylsilyl)trifluoroacetamide. The hydroxyl group is easily silylated with a silylating agent and

Fig. 4. Transmission electron micrographs of the nanocapsules neutralized by (a) ETA, (b) 5AP, (c) DEA, (d) TEA, (e) PPA and (f) DPA.

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Fig. 5. Quantitative amount of encapsulated amines contained in the nanocapsules measured by GC–MS.

carboxylic, amino and amide groups are known to be silylated under anhydrous reaction conditions by replacing the reacting functional groups with O Si (CH3 )3 [16]. In Fig. 5, the encapsulation concentration for the six amines is plotted in terms of the quantitative amounts. For the encapsulation, the base molecules diffused into the carboxylated particle cores, and the diffused base ions and molecules interacted with the carboxylic groups of those cores [19]. The encapsulated amounts of the amine increased in the following order: PPA (0.20) < ETA (0.90) < DPA (1.25) < DEA (1.40) < TEA (1.79) < 5AP (2.00). 5AP showed the maximum encapsulation behavior among the six types of amine, and the amines with a low molecular weight (i.e., ETA and PPA) were encapsulated with poor efficiencies. PPA exhibited the lowest encapsulation efficiency, which may be due to its low boiling point below the neutralizing temperature. During the emulsion polymerization, the temperature of the reactor was maintained around 87 ◦ C, which was well above the melting temperature of PPA (48 ◦ C). Most of the PPA evaporated during the neutralization process and only a limited amount could be entrapped in the latex particles resulting in a low encapsulation efficiency. The encapsulation efficiencies were found to be dependent on the basicity of amine species. The amine reaction with the core polymer was based solely on the acid–base interaction. Therefore, the stronger the amine basicity is, the more favorable the acid–base interaction. For 5AP, its high dissociation activity dictated that the reaction sites could be easily protonated. Because it had a longer carbon chain between the hydroxyl and amino groups, the negative logarithm of the base dissociation constant (pKb ) was smaller than that of amines with less carbon chain, and the acid–base reaction more easily proceeded. In the same manner, DPA, which was the strongest base among amines used in this study, showed good encapsulation efficiency even though input amount was only 5 g. 3.3. Release behaviors of encapsulated amine species under different pH conditions The external stimulus for the release of encapsulated amines is a pH change in the environment. The encapsulated inhibitor can be released by a local pH change near an anodic/cathodic site. The inhibition efficiencies of the coating layer with the encapsulated corrosion inhibitors are dependent on the corrosion inhibition properties of the inhibitors themselves as well as the timely release

Fig. 6. Released amounts of the six types of amines under neutral and alkaline conditions.

of a reasonable inhibitor concentration. The release behaviors were simulated both in neutral and alkaline conditions at a pH of 6.53 and 12, respectively. Fig. 6 plots the quantitative concentrations of the pH-dependent release of amines measured by LC–MS. Based on the quantitative analysis, the linear amines were released by diffusion through polymer wall with higher efficiencies than the branched amine, such as TEA. The amines form a carboxylated salt as a result of the acid–base interaction with the carboxylic acid in the core materials. As the pH of the environment increases, the dissociation of amine becomes more facile freeing them from the latex core and releasing them into the environment. The release mechanisms should be primarily dependent on the ionization of the amine salt. In addition, the diffusion flux of the amines to the environment was correlated with the distribution coefficient and the aqueous solubility of amines. A computational analysis of the solubility and partition coefficient of the amines was conducted, and the results are listed in Table 2. The distribution coefficient (log D) is an ionization-corrected partition coefficient, which includes a pKa and pH terms, as follows: log D = log P − log(1 + 10pH−pKa ) where this expression is for weak acids [22,23]. The log D value of the hydrophilic amines was more negative than that of hydrophobic amines (log DETA = −1.37, log D5AP = −0.36, log DDEA = −1.56, log DTEA = −2.01 at pH 12), whereas the log D of DPA was 1.76 at the same pH. The amines with a smaller distribution coefficient were more likely to be released from the nanocapsules. However, relatively poor release behaviors were found for TEA despite its tendency to remain in the water rather than in the organic solvent. TEA is a tertiary amine, and its bulky structure hampers its escape through the polymer wall. This structural effect also determined

Table 2 Aqueous solubility, partition coefficients and distribution coefficients of the amine species. Chemicals

Aqueous solubility

log S

log P

log D (pH 12)

Ethanolamine (ETA) 5-Amino-1-pentanol (5AP) Diethanolamine (DEA) Triethanolamine (TEA) Propylamine (PPA) Dipropylamine (DPA)

0.85 kg/L 0.22 kg/L 0.47 kg/L 0.50 kg/L 0.23 kg/L 44.4 g/L

1.14 0.32 0.65 0.52 0.59 -0.36

−1.31 −0.09 −1.43 −1.00 0.48 1.74

−1.37 −0.36 −1.56 −2.01 0.30 1.76

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Fig. 7. Bode plot (a and b) and phase angle (c) of specimens coated with nanocapsules loaded with various types of corrosion inhibitors. (a) A: 5AP, DEA, TEA and (b) B: ETA, PPA, DPA.

the enhanced release behaviors observed for the smaller molecules (i.e., ETA and PPA). When painted steel corrodes locally, the metal substrate beneath the coating layer is exposed, which becomes partially anodic and cathodic with a variation in the pH along the exposed area. Near the cathode region, the electrolytes become alkaline, and ionizable compounds can be released into the electrolytes. The interaction between the amines and the ions in the electrolyte as a function of the pH variation enables the release of the inhibition amines from the nanoparticles into the corrosive environment. Also, corrosion inhitors can release by rupture of polymeric wall induced by mechanical impact or damage of metal substrate and coating layer. Due to the mechanical burst of nanocapsules, rest of amines which are not released from capsules inside can react with exposed metal surface and it enables the effective self-healing protection of metal against corrosion even though the released amount of amine through polymeric wall is only 1 to 18% as shown in Fig. 6. The released amines can be adsorbed on the surface of the exposed metals to form passive and protective layers against corrosion with newly formed N H stretching bond on metal surface as verified by FTIR (not included in this paper). In addition, the adsorbed amines subsequently retard the corrosion of steel maybe by forming complex with metal cation due to the presence of lone pair electrons on nitrogen atom.

3.4. Corrosion performance of developed organic coating with encapsulated corrosion inhibitors Organic amine compounds are introduced in the different components of the coating systems to afford long-term corrosion protection by providing inhibitors to the metal surface when the barrier layer is damaged [12,24]. However, the direct addition of amines can cause blistering and delamination of the protective coating film due to the osmotic pressure. In addition, an uncontrolled release can accelerate the consumption of the entrapped corrosion inhibitors [5]. A smart approach using nanoreservoirs and controlled, on demand release can prolong the protection longevity of the organic coating layers. The organic coating film protects a metal substrate by blocking the transportation of water and corrosive ions. Therefore, the interfacial adhesion between the coating layer and the metal is important. When nanoparticles are added to the coating matrix, the barrier protection of the polymer film is diminished. The protection performance can deteriorate as a result of too high a concentration of nanocapsules. Therefore, nanocapsules must be introduced into the coating layer in the correct concentration range.

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Fig. 8. Changes in the coating resistance of the coated specimens over 7 days: (a) pore resistance (Rc ), (b) charge-transfer resistance (Rt ) and (c) double-layer capacitance (Cdl ) of group A (DEA) and group B (ETA).

3.4.1. Electrochemical corrosion test: electrochemical impedance spectroscopy (EIS) Though nanocapsule without corrosion inhibitor can decrease long-term barrier property of organic coating, the encapsulated corrosion inhibitor, TEA, was revealed to effectively protect steel immersed in thin electrodes as well as bulk electrodes [11]. For the determination of the inhibitive effectiveness of the different types of inhibitors, the non-quantitative immersion test is not suitable because the inhibition protection results from a local electrochemical reaction. Thus, EIS measurements were employed to verify the corrosion performance difference of the specimens coated with six types of encapsulated inhibitors. The Bode plots of each group are shown in Fig. 7, and the pore resistance (Rc ), charge-transfer

resistance (Rt ) and double-layer capacitance (Cdl ) of the representative corrosion inhibitors, DEA and ETA from each group were plotted in Fig. 8. According to the observed corrosion protection properties of amines, the aliphatic amines used in this study were classified in two groups that included the amines with self-healing protection (group A: 5AP, DEA and TEA) and without self-healing protection (group B: ETA, PPA and DPA). The coating resistance of group A initially decreased as a result of the absorptions of water and ionic species in the coating film and then increased with repassivation of the corroded area by the corrosion inhibition action of leached amines. The corrosion resistance of the specimen coated with encapsulated DEA after 6 days was even higher than that of the intact coating layer at the early stage of immersion. In addition,

Fig. 9. Surface morphologies of the specimen surfaces after 1 week of completed experiments.

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Fig. 10. SVET maps measured on the surface of the coated steel sheets with a defect: (a) group A (DEA), (b) group B (ETA) for 2 h and (c) group A (DEA), (d) group B (ETA) for 5 h after reaction in a 0.05 M NaCl solution. Surface morphology of the specimens coated with (e) DEA and (f) ETA after SVET measurement.

the recovery of coating resistance confirmed the effectiveness of the encapsulated amines with self-healing corrosion protection. For TEA, the corrosion resistance of the coating layer was relatively low compared to those of other amines in group A due to the unfavorable release of the bulky TEA, as described in the previous section. The corrosion resistance of the coating layers in group B decreased over the course of the experiments due to water and corrosive ions migrating into the coating film. In general, when the resistance of the coating is less than 107  cm2 , then such coating no longer offers adequate corrosion protection to the surface [25]. The acceptable protectiveness of the coating layer in group B decreased within 3 days, and the coating resistance was not restored. Moreover, the second time constant appeared after one week in group B showing poor corrosion protective behavior. The capacitance value was also calculated by evaluation of Bode plots. From the definition of capacitance, the Cdl value reflects surface coverage ratio () and electroactive state of the surface [26,27]. In the case of group B, the Cdl value was gradually increased with experimental time due to active corrosion reaction on the surface and increase of the interaction area between steel substrate and electrolyte. The lowest corrosion resistance was observed for the specimen coated with

PPA due to insufficient amount of encapsulated amine. On the other hand, poor corrosion resistance of the specimen coated with ETA was originated from the high water solubility and fast exhaustion of ETA causing water uptake into coating layer. The optical micrographs of the specimens, as shown in Fig. 9, supported the EIS results. Red rust was formed on the surface of the steel substrate with following coating delamination. The amines exhibited high corrosion resistance in the EIS measurements, such as DEA, showed no trace of corrosion products on the surface. However, the surfaces of the specimens coated with ETA or PPA capsules, which did not exhibit sufficient corrosion protection of the substrates, were covered by red rust. 3.4.2. Electrochemical corrosion test: scanning vibrating electrode technique (SVET) In the EIS measurements, the intrinsic properties of and the changes in the barrier protection of the polymer films were evaluated by comparing polarization resistance values. For a smart coating system, organic coating films should protect the substrates when the barrier coating layer is degraded due to the corrosion process or external impact. To investigate active corrosion

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protection, specimens were artificially defected, and the current density distribution over the defected area was observed using a SVET system. The current density maps of the specimens coated with representative amines, DEA and ETA from groups A and B were three-dimensionally plotted in Fig. 10 as a function of time. Metal dissolution reactions were active during the first 2 h after contact with the electrolytes induced electrochemical activity. Except for highly corroded specimens, such as ETA, PPA, and DPA in group B, the anodic reactions primarily occurred at the X-defected area due to the higher electrochemical activity of the carbon steel compared with that of the polymer film. In the defected region, the current density was the highest after 2 h, but the active electrochemical reactions diminished after a few hours. In the early stages of the experiment, the steel substrate was actively dissolved into the electrolytes at the anode, and the rate of inhibition action was slower than that of steel dissolution. As the reaction time increased, the exposed metal surface was repassivated by the released corrosion inhibitors, and the electrochemical activity decreased. Self-healing protection was clearly evident for group A (5AP, DEA and TEA), and the results coincided with the recovery of the coating impedance in EIS plots, as shown in Fig. 7. The specimens coated with group B, which exhibited a continuous drop in the coating resistance, were in a very electrochemically active state during the entire period of SVET measurements. It has been confirmed that the use of nanocapsules to store corrosion inhibitors is a smart method that guarantees the long-term corrosion protection of metal substrates with self-healing protection when the corrosion inhibitors have sufficient efficiencies of encapsulation and release. Encapsulation method suppresses the easy consumption of the corrosion inhibitors with high water solubility and increases the surface coverage area by the corrosion inhibitors with poor water solubility. This enables the non-soluble amines in water to have good corrosion inhibition power to protect metal substrates. 4. Conclusions Six types of aliphatic amines with different molecular weights, structures, solubilities in water and basicities were encapsulated into nano-sized polymeric particles by multi-stage emulsion polymerization. The bilayer nanocapsules, which have an intermediate hydrophilic shell and a hydrophobic outermost shell, were capable of loading amine-type corrosion inhibitors by interaction of the carboxylic acid in the core polymer and the amines. The amines with high water solubility were more efficient in both swelling and encapsulation than the amines with low water solubility. The encapsulation behaviors were also affected by the basicity of the amines. The amines that are strongly basic were more effectively encapsulated due to higher dissociation activity than the weak bases. The release of the encapsulated amines was pH dependent regardless of the exposure time. In addition, the molecular structure and aqueous solubility were critical factors in the release

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