Encapsulation of triethanolamine as organic corrosion inhibitor into nanoparticles and its active corrosion protection for steel sheets

Surface & Coatings Technology 206 (2012) 2354–2362

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Encapsulation of triethanolamine as organic corrosion inhibitor into nanoparticles and its active corrosion protection for steel sheets Hana Choi a, Yon Kyun Song b, Kyoo Young Kim a, Jong Myung Park a,⁎ a b

Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea Surface Technology Research Group, POSCO Technical Research Laboratories, Gwangyang 545-090, Republic of Korea

a r t i c l e

i n f o

Article history: Received 11 July 2011 Accepted in revised form 11 October 2011 Available online 19 October 2011 Keywords: Self-healing Hollow core–shell Encapsulation Corrosion inhibitor Triethanolamine

a b s t r a c t Triethanolamine (TEA), a corrosion inhibitor for zinc and steel, was introduced into nano-sized particles as nanoreservoirs to increase longevity of inhibitive property and prevent degradation caused by direct addition of corrosion inhibitor into coating layer. TEA-incorporated nanoparticles with average particle size around 400–450 nm were successfully synthesized by sequential emulsion polymerization, occupying around 5% of total solid weight of particles during a neutralization process. Encapsulated TEA was released from the capsule inside when the pH level of environment became acidic or alkaline due to an acid–base interaction or ionization of seed material in specific conditions. In the corrosion tests, the encapsulated TEA decreased the corrosion rate of steel substrate owing to its adsorption on steel surface and the resistance of coating layer against corrosive environment was much higher and remained its resistance as immersion time increased when TEA was incorporated in coating layer in the encapsulated form. Based on the scanning vibrating electrode technique (SVET) result, anticorrosive ability of the encapsulated TEA seemed to improve due to the spontaneous passivation of exposed metal on the defected region of coated steel. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Organic amine corrosion inhibitors have been widely used for corrosion protection of metal from aggressive environments due to their economic and effective corrosion retarding capability in various industrial fields. They are added in small concentrations into corrosive electrolyte to form a thin passive film on the surface of the metal substrate, retarding the access of the corrosive species to the metal. The main inhibitive mechanism is considered to be adsorption on metal surface followed by the formation of a passive and protective layer [1,2]. It is well known that organic amines with low molecular mass and high water solubility have enhanced adsorption and corrosion inhibition properties [3–5]. Corrosion inhibitors are usually included in pickling or cooling fluids, antifreezing liquids and concretes to retard the corrosion process of the metal substrate. They are generally incorporated into the environment, but cannot be used directly to metals themselves. In the case of steel applications such as an automotive body, corrosion inhibitors are not considered as fundamental solution for steel corrosion protection except for the surface treatment with inhibitors. Because of the restrictions of a corrosion inhibitors available environment, organic coating is more widely used for the protection of steel products in both wet and dry conditions with an excellent

⁎ Corresponding author. Tel.: + 82 54 279 9017; fax: + 82 54 279 9299. E-mail address: [email protected] (J.M. Park). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.10.030

insulated film on the surface which blocks the penetration of corrosive ions, water and oxygen. However, the organic layer also obstructs the path of electron and causes several problems in post-processes such as welding. In addition, when coated steel substrates with defects meet corrosive environments, the organic coating is unable to serve as a protective film for the metal and thus causes coating delamination or blistering. Eventually, the corrosion of metal substrate accelerates due to the decrease of the anodic-to-cathode area ratio. In order to compensate drawbacks of the organic coating system, extensive efforts have been devoted to the development of self-healing coating systems which can recover coating defects by themselves on demand. One simple approach is the addition of corrosion inhibitors directly into the polymer coating binder to provide additional active corrosion protection and to hinder the corrosion activity in defect sites. The coatings can release active and healing species after certain changes in the coating integrity. The active agents such as corrosion inhibitors can be introduced into the different parts of the coating layer, such as pretreatment, primer and topcoat layers. Healing agents are effective only if their solubility in the defect environment is in the right range. Very low solubility can cause a lack of the active healing agent at the substrate interface and as a result, the healing effect will be weak. On the contrary, if the solubility is too high, the active agents will be available too short of a time span to be effective. Moreover, high solubility leads to blistering and delamination of the active surface due to osmotic pressure causing water to be transported through the coating. Additionally, the release of inhibitors from coatings will be

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relatively fast and uncontrollable [6–8]. Therefore, direct addition of corrosion inhibitor into coating formulations can cause uncontrollable release of corrosion inhibitors leading to the fast exhaustion of the active anticorrosion potential and osmotic blistering of the polymer film [9]. An active protection or self-healing system is considered as new coating system to extend lifetime of steel products because of their ability to activate the corrosion inhibiting action only when necessary. Self-healing is defined as the healing ability of damaged materials autonomously without any external trigger [10]. Currently, self-healing systems have been developed to recover mechanical strength of cracked barrier coating or release active inhibitive agent by stabilizing corrosion activity of defected area on metal surface [10,11]. In new advanced corrosion protection coating systems, various methods to store and discharge active agents are employed; for instance, nanoporous metal oxide multilayer materials like titania [12], ion-exchanged particles [13] and nanocontainers [14,15] to ensure good barrier properties and an effective self-healing mechanism [16]. In this paper, we stored organic corrosion inhibitors in modified hollow latex nano-reservoirs which have been mainly used in coating applications to enhancing the coating opacity [17–19]. One of the definite advantages of our system is that it makes better use of application by replacing any other corrosion inhibitors with needs. We focused on corrosion prevention of cold-rolled steel with both barrier protection and inhibitive protection. In order to incorporate the selfhealing properties into the developed coating system, triethanolamine was used as the corrosion inhibitor for steel.

Table 1 Synthesis recipe for standard polymeric capsule (REF1) and neutralized capsule with ammonia (REF2) and TEA (TEA capsule).

2. Experimental procedure

2.2. Characterization

2.1. Capsule synthesis

Polymeric capsules were collected for further analysis by filtering the obtained latex with 0.2 μm cellulose filter paper and rinsed with DI water and ethyl alcohol to remove the remained TEA and unreacted monomers on the surface of the capsules. The filtered capsules were then dried in ambient condition for several hours. Transmission electron microscope (TEM, Philips CM 200) and scanning electron microscope (SEM, Hitachi SU-6600) were used for visualization of the synthesized nanocapsules and comparison of size and shell shape. Samples were coated in a few nanometers of Pt sputtering method to prevent the charge-up of the specimen surface. Quantitative analysis was done using thermogravimetric analysis (TGA, TA instruments) and gas chromatography–mass spectrometry (GC–MS, Thermoelectro Polaris Q). In the case of TGA, a sample amount of 10–30 mg was put in an alumina pan and the thermograms were recorded from 25 °C to 600 °C with a heating rate of 5 °C/min in nitrogen atmosphere. For the determination of TEA contents with GC–MS, nanoparticles were dissolved in tetrahydrofuran (THF, Sigma Aldrich) at about 5000 ppm, and 25 μl of N,O-bis(trimethylsilyl)trifluoroacetamide (Fluka) was added into 1 ml of this solution for silylation. Silylation is a common and versatile method to derivatize polar organic amines. N, O-bis(trimethylsilyl)trifluoroacetamide was used as the silylation reagent, which lead to the formation of trimethylsilyl (TMS) derivatives. In the silylation procedures, reactions took place at 60 °C for 60 min [20,21]. TEA concentration in nanocapsules was identified by comparison of retention time, and a Trace GC 2000 was used to obtain mass spectra equipped with a 60 m DB-5 capillary column (Agilent, Palo Alto, CA) and a Polaris Q Ion Trap MS (Thermoquest, San Jose, CA). The initial oven temperature was 50 °C and the temperature was maintained at 50 °C for 5 min, and then increased to 250 °C with 10 °C/min of heating rate and held for 5 min. Calibration samples were prepared in the concentration range from 25 to 300 ppm in THF, and the working sample was 10,000 ppm.

Nano-sized polymeric capsules as nanoreservoirs for carrying triethanolamine (TEA) as a corrosion inhibitor were fabricated by sequential emulsion polymerization. The core latex was obtained from KCC Co. (South Korea), and the average particle diameter was 100 nm. The main components of the core latex were methyl methacrylate, butyl acrylate and methacrylic acid with the ratio of 63:9:28. Potassium persulfate (Samchun Chemical) was used as a free radical initiator and Rhodapex® CO-436 (ammonium nonylphenol ether sulfate) from Rhodia Inc. was used as an anionic emulsifier for emulsion polymerization. The polymerization process was carried out under nitrogen atmosphere, and deionized water (DI water) was used throughout all of the experimental processes. All chemicals were used without further purification. Firstly, 60 g of the core latex particles was dispersed in 430 g of DI water in a round-bottom flask at 80 °C. Then, 0.5 g of sodium persulfate dissolved in 15 g of DI water was fed into the flask to initiate radical reaction within 5 min. The first shell layer with intermediate hydrophilicity was synthesized on the hydrophilic core latex particles, and the main components of this layer and seed materials were same but different in ratio as described in Table 1. This intermediate layer was employed for better encapsulation of hydrophobic polymer shell on the hydrophilic core polymer. After holding at 80 °C for 1 h, a stable monomer preemulsion containing 98 g of styrene was dropped into the reactor at the rate of 2.27 g/min. Once the formation of the polystyrene outermost shell is completed, penetration of TEA into the core through the shell is difficult since the stable and rigid polystyrene shell prevents the amine from reaching into the acidic center part. Therefore, TEA was simultaneously fed with styrene pre-emulsion mixture for first 15 min to maximize the neutralization degree (TEA capsule). Standard blank nanoparticles with the same physical property as TEA capsule except for the absence of the basic corrosion inhibitor were also fabricated as reference (REF1) to compare the active anticorrosion performance of encapsulated corrosion inhibitor. REF1 particles were solid compared to the hollow structure of TEA capsules. Another reference capsules were prepared to study the effect of the hollow structure on the

Step 1 2

3

4

5

Chemicals

DI water Core latex DI water Sodium persulfate Butyl acrylate (1st shell formation) Methyl methacrylate Methacrylic acid DI water (2nd shell formation) Styrene Sodium persulfate Surfactant Aqueous NH3 (Injection of base/corrosion inhibitor) TEA

REF1 REF2 TEA capsule 435 g 60 g 15 g 0.5 g 4.2 g 51 g 1.8 g 38 g 98 g 0.5 g 0.33 g – 20 g – –

– 0.15 M

corrosion behavior of the capsule-containing coating layer. In this case, ammonia solution (28%, Samchun Chemical) was used instead of TEA to make hollow polymer capsules (REF2). Because, unlike TEA, ammonia was very volatile at the high baking temperature, as described in the Section 2.3, the REF2 capsules embedded in the coating layer remained hollow without any active agents inside.

2.3. Release behavior of corrosion inhibitor depending on pH Corrosion inhibitor should be easily released into electrolytes to protect the metal substrate against corrosion when pH is shifted

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due to anodic and cathodic reactions when corrosion proceeds or the environment changes. Release behaviors in different pH environments were investigated. To simulate the different electrolyte conditions, hydrochloric acid solution (0.1 M HCl), potassium hydroxide solution (0.1 M KOH) and neutral DI water were used to manipulate the pH of the solution to 1.71, 5.63 and 10.85. Nanoparticles in powder form were rinsed with DI water and ethyl alcohol before use and then packed in the molecular porous membrane tubing (Spectra/ Por®3 dialysis membrane). Dried nanoparticles were sealed in membrane tubes with clips, submerged in a pH adjusted solution, and collected 0.5, 1, 12, 18, 24, 72, 96 and 120 h after submersion. The obtained solutions were analyzed by liquid chromatography and mass spectrometry (LC–MS). 2.4. Coating process The corrosion inhibitive efficiency of encapsulated TEA was evaluated. Commercial cold-rolled steel (CRS) sheets with thickness of 0.783 mm were supplied from POSCO. The specific chemical composition of specimen is detailed in Table 2. Synthesized nanocapsules in emulsion state were added in polyurethane resin with curing agent (Cymel 325, Cytec Industries) and wetting agent (Byk-348, BYK chemie). Since total solid content affects the coating thickness after curing, solid volume ratio of the coating solution was maintained at a similar level. In the case of REF1, total shell thickness and average particle size of unneutralized capsules were much smaller than those of neutralized ones (TEA capsule), and the core latex was different from that of TEA capsule. When the core–shell latex was swollen by base, the inside of the capsule became hollow and the particle diameter increased. During this process, capsule properties are totally changed with formation of micro-/nanopores on the shell and the decrease of the shell thickness. It degraded the barrier property of coating layer to supply more penetration path of corrosive ions when capsules are employed in organic coating. For offsetting condition differences, reference nanoparticles for coating were neutralized with ammonia solution (REF2) with same method as TEA capsule fabrication. Concentration of incorporated capsules was 33.3 wt.% of total solid in the coating system. The coating process was performed by using a bar coater (no. 20). Before curing, specimens were first baked at 105 °C using an induction oven to prevent coating failure caused by the difference of boiling temperature between water and resin. After evaporating water, samples were cured at the peak metal temperature of 170 °C in the induction oven. The coating thickness and the cross-section of the specimens were observed by scanning transmission electron microscopy (STEM, Jeol 2100F) after making specimens by focused ion beam (FIB, Seiko SMI 3050 SE). 2.5. Corrosion tests Conventional corrosion evaluation of coated steel has been conducted using salt spray test, cyclic corrosion test and immersion test. However, in our coating system, excess electrolytes or sever rinsing disturb the action of inhibition due to low concentration or loss of corrosion inhibitors from coating layer. Quantity of TEA in organic coating is few ppm compared to total solid in the coating layer. In order to observe effect of encapsulated corrosion inhibitor clearly, semi-immersion tests were performed by forming a thin film with 10 ml of electrolytes (3.5% NaCl solution) on the specimens in a petri dish. In this case, the penetration of oxygen was much easier and partial pressure of oxygen in electrolyte was higher than that of Table 2 Chemical composition of cold-rolled coil sheet.

(%)

C

P

S

Si

Mn

Ni

Cr

Mo

0.0022

0.0062

0.0040

0.004

0.088

0.01

0.01

0.00

the conventional immersion test. At the end of the experiments, the coating layer was peeled off and the scribed zone was analyzed with a micro-image analyzer (Sometech videoscope). The samples were cut into a size of 50 mm × 50 mm and all the edges were sealed with non-conductive tape to prevent cut edge corrosion. Scanning vibrating electrode technique (SVET, Applicable Electronics Inc) employing microelectrode (MicroProbes Inc.) with a black Pt coating on tip was conducted in 0.05 M NaCl solution (conductivity b 196 μS/cm). The microelectrode was scanned with a distance of 150 μm above the specimens and all processes of the experiments were controlled by the ASET software. Electrochemical impedance spectroscopy (EIS) measurements were performed using a Gamry reference 600 in 0.05 M NaCl solution. Specimens were fixed in a commercial flat cell with a saturated calomel reference electrode and a platinum mesh counter electrode, and working electrode area was 1 cm2. EIS measurements were carried out at the open circuit potential with a 10 mV amplitude sinusoidal voltage in the frequency range of 0.01–10,000 Hz and Bode diagrams were plotted. For semi-immersion tests and SVET measurements, artificial defects were created by a ceramic tip to investigate the inhibitive and self-healing protection at the small active area. 3. Results and discussion 3.1. Synthesis of latex polymeric capsules Hollow latex particles loaded with TEA were formed by the following three stages: Stage 1. Fabrication of amphiphilic first shell on the carboxylated seed materials Stage 2. Synthesis of hydrophobic second shell to give stability on the nanoparticles Stage 3. Capsule swelling by neutralization of seed materials with basic amine corrosion inhibitor (TEA) Our capsulated corrosion inhibitor system composes of a soft inner seed core material and hard outer thermoplastic polymer layer. The seed latex formed by copolymerization of MMA and MA with small amount of BA (63:28:9, w/w) is hydrophilic, and the average size is 100 nm as shown in Fig. 1. The center of each capsule is brighter than the surrounding sheaths due to different chemical composition of each part because styrene has darker contrast due to high electron density of benzene ring [22]. It is surrounded by about 20 nm of the first shell which is amphiphilic and act as the buffer layer between hydrophilic inner seed and hydrophobic outer shell for the concentric core–shell growth. The same chemicals of the seed materials were used in the first shell layer, but the relative ratio of each chemical was adjusted in a proper way. It is for next multi-stage polymerization to synthesize the hydrophobic polymer onto the hydrophilic acid-containing core. The concentric core–shell morphology is not the thermodynamically preferred one and thus the kinetics and thermodynamic factors are optimized by changing compositions and polymerization conditions to obtain the desired morphology [23,24]. The outermost hydrophobic layer is composed of polystyrene and the shell thickness is 100–130 nm. Fig. 1 (a) represents the standard nano-sized particles (REF1) not containing any corrosion inhibitor. The most definite change after the addition of TEA as the basic corrosion inhibitor is the increase of seed and shell layer size as shown in Fig. 1 (b). In REF1, the total size of the capsule is 300–350 nm, while TEA capsule shows 400–450 nm in size. The seed layer is increased by 100 nm to 300 nm because of the osmotical swelling caused by the diffusion of water with TEA through the outer polymeric layer. The attraction derived from acid-basic neutralization was used as the driving force to diffuse TEA into the core of the capsule. The chemical composition of the core polymer is designed to be ionizable when pH of solution changes to alkaline. Therefore, the

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a

b

c

d

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Fig. 1. Transmission electron micrograph of reference polystyrene nanoparticles without corrosion inhibitor (REF1 capsule) (a) and nanoparticles after injecting of corrosion inhibitor (TEA capsule) (b). Scanning electron micrograph of REF1 (c) and TEA capsule (d).

subsequent swelling process imbibes water into the center of the particle. The water leaves a void upon evaporation during the drying of the coating formulation and less volatile TEA stays inside of the capsule [23]. Fig. 1 (c) and (d) show the morphologies of REA1 capsule and TEA capsule. REF1 capsule has a smooth surface and the size of each particle is quite uniform. 3.2. Characterization of encapsulated TEA in the nanocapsules inside In order to confirm the presence of TEA, thermogravimetric analysis was performed and TGA weight loss curve for polymeric capsule without/with corrosion inhibitor is presented in Fig. 2. When comparing the thermal degradation behavior of two types of polymeric nanoparticles, TEA capsule showed different weight loss characteristics at 180 °C and 400 °C, and 2.35 wt.% of TEA capsule was degraded between 180 °C and 250 °C while 0.85% of REF1 was degraded. Because the boiling point of TEA is 330–335 °C, most of TEA evaporates

Fig. 2. TGA curve of REF1 capsule and TEA capsule.

through the pores at 335 °C. When assuming that the properties of polymeric outer shell layer and the thermal property of TEA are not changed by the incorporation of TEA, the degradation before 335 °C is mainly due to the evaporation of encapsulated TEA. At 335 °C, the TEA capsules showed a 6 wt.% weight loss, while REF1 showed 3% weight loss. Thus, the total amount of TEA deposit in the capsules was estimated to be about 3 wt.% of the powdered capsule. For more precise quantitative analysis of encapsulated TEA, the analysis by using GC/MS was carried out. In general, amines tend to be adsorbed and decomposed on the columns, and readily give tailed elution peaks, ghosting phenomena and low detector sensitivity [25,26]. Derivatization is a popular method for overcoming this problem. N,O-bis(trimethylsilyl)trifluoroacetamide is a powerful silylating reagent and reacts not only with amino groups but also with hydroxyl and carboxyl groups under anhydrous reaction conditions. The byproduct of the reagent is volatile and thus does not interfere in analysis. In general, the silylation reaction occurs in the following order, alcohols > phenols > carboxylic acids > amines > amides [20]. In the case of TEA, three hydroxyl groups can be trimethylsilated by replacing \OH with \O\Si\(CH3)3. Dissolved TEA capsules in THF and standard solutions of TEA were prepared with concentration of 10,000 ppm for sample and 25, 50, 100, 200 and 300 ppm for calibration. Loaded amount of TEA in nanoparticles was calculated by peak area of the characteristic peak of TEA which was confirmed by mass spectrometry with comparison of mass/charge (m/z) and checked again by retention time of standard and working sample. Peak area of standard samples showed linear distribution and linear trendline was obtained by linear fitting function of Origin 8.0. From the trendline, the quantitative concentration of encapsulated TEA depending on different input was calculated and the result was shown in Fig. 3. When 0.08 M TEA was introduced to the synthesized reactor, total encapsulated TEA was 219.02 ppm in 10,000 ppm of powdered capsule/THF, indicating that TEA occupied 2.19% in total weight of one capsule. TEA ratio in capsule increased from 2.19% to 5.04% as the introduced amount of TEA increased from 0.08 M to 0.15 M. However,

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Fig. 3. Quantitative analysis of encapsulated TEA in nanoparticles depending on different input of TEA with GC–MS.

encapsulated TEA did not increase more than 5.22% even though 0.2 M TEA was added during neutralization process, meaning maximum incorporation of TEA was between 0.15 M and 0.2 M. 3.3. Release behavior of encapsulated TEA depending on different pH condition When localized corrosion occurs, reduction and oxidation reactions can occur to cause pH distribution on the cathode/anode areas. In the case of local corrosion in an organic coating system, the defected region of coating layer can be the local anode, whereas the undamaged region is the local cathode. Oxygen reduction in the cathodic area makes cathode alkaline usually above pH 8 and forms reactive species which deteriorates adhesion between metal and polymer layer [27,28]. Due to pH variation in defected or corroded area, the corrosion inhibitor incorporated into nanocontainers is released through microchannels formed on the surface of polymeric sheath by osmotic swelling. The ability to release the healing agent in a target region is one of the most required properties in the encapsulated corrosion inhibitor system since it is directly related to the anticorrosion efficiency of the system. In order to investigate the release behavior of the corrosion inhibitor during the corrosion process, each local corrosion system was simulated by submerging nanoparticles into solutions of different pH as a function of time. Acidic and alkaline solutions respectively represent localized corrosion environments in anode and cathode. As shown in Fig. 4, TEA incorporated into nanocontainers was readily released from the capsule inside in both acidic and alkaline condition, whereas relatively more

Fig. 4. pH-dependent release behavior of TEA capsule in different pH range.

corrosion inhibitor remained inside the nanoreservoirs in the neutral environment. The amount of released TEA did not depend on reaction time in the solution, meaning that the release of corrosion inhibitor instantly occurred when pH of the surrounding condition was changed. In the study on a core–shell polymeric nano- or microcapsule fabrication for drug delivery, it was reported that carboxylic groups and amine species are not ionizable in a neutral condition, but they can be charged and then dissolve in aqueous phase if pH level of outside solution becomes acidic or alkaline [29–32]. According to the result of LC–MS, TEA was released in an alkaline condition as well as in an acidic environment. Initially, TEA exists inside nanocapsules in a salt form with the latex core. The higher pH of the environment, the stronger the degree of hydrolysis of the salt. Thus, TEA with weak basicity would be free from the interaction with core material and finally released into an electrolyte. When the environment was changed to a low pH condition, strong attraction between amine and acid in the electrolyte mainly affected the release of amine species. Therefore, release efficiency of encapsulated amine would have a minimum near neutral conditions. When painted steel locally corrodes, the metal substrate beneath the coating layer is exposed and becomes partially anode and cathode with a different pH distribution along the exposed area. Near the anode/cathode region, electrolytes become acidic and alkaline, and ionizable groups can release into electrolytes. The interaction between TEA and ions in electrolyte with pH variation makes it possible to release TEA from nanoparticles into the corrosive environment. Released TEA is adsorbed on the surface of exposed metals to form the passive and protective layer against corrosion, and it retards corrosion of steel by repairing damaged regions in the coating layer. 3.4. Inhibitive behavior of encapsulated TEA on steel The inhibitive ability of aliphatic and aromatic amines is due to the donation of unshared π-electron pair on the nitrogen atom [33,34]. TEA can act as a corrosion inhibitor in many metals such as zinc and steel. However, its high solubility in water hinders long-term corrosion protection, especially when it is added into coating systems directly. Encapsulated corrosion inhibitor can guarantee the longevity of corrosion protection by controlling the release of corrosion inhibitor on demand. Synthesized nanocapsules with TEA were incorporated into a polyurethane resin and then coated on CRS at a dry film thickness of about 10–12 μm. The coated specimen for cross-section analysis was prepared by FIB and observed using STEM as shown in Fig. 5. Only a part of the total coating layer is shown in Fig. 5 since FIB could not treat the thick organic coating layer due to the depth limitation of the equipment during preparation for the STEM sample. The total coating layer was analyzed by SEM attached with FIB and the upper part of coating layer of about 5 μm thickness was sliced and analyzed with STEM. It was shown that nanocapsules were well dispersed through the coating layer and not transformed during curing process in high temperature.

Fig. 5. Cross-sectional view of coated specimen.

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The anticorrosive effect of encapsulated TEA was evaluated using semi-immersion tests. In the coated specimens, total amount of TEA was expected to be less than 1% and in the conventional immersion test, it was not adequate for inhibitive action because of its diluted concentration in large electrolyte. To avoid concentration limitations, a small amount of electrolyte was used for corrosion test. In this semi-immersion tests, very thin electrolyte film with 10 ml of NaCl solution was formed on the surface of each specimen in a petri dish, and specimens were sealed to prevent evaporation of electrolyte to keep the constant volume of electrolyte and electrolyte level. Metal substrates were artificially exposed to observe the corrosion process at the scribed region. The corrosion process and surface change of coated specimens containing REF1, REF 2 and TEA capsules are shown in Fig. 6. All specimens started to corrode right after immersion and the corrosion rate of REF1- and REF2-contailing specimens was much faster than that of TEA capsules-containing specimen in the early stage of the immersion (Fig. 6 (b)). Red rust was formed at the scribed region and also at the non-scribed zone. In a day, the specimens without TEA were corroded over almost the whole surface, while the TEA capsule-containing specimen was corroded only at the X-cut region (Fig. 6 (c)). After 3 days of immersion, a much larger amount of rust was observed in the specimens with REF1 and REF2 than in the TEA capsule-containing specimen (Fig. 6 (d)). After removing the remaining coating layers, the scribed area was observed using an optical microscope (Fig. 6 (e)). Corrosion reaction dominantly occurred under the coating nearby the scribed region. In the specimens with REF1 and REF2, red rust was observed in many places while it was rarely observed in the specimen with TEA capsule, indicating better corrosion protection due to encapsulated TEA, as can be seen in Fig. 6 (e). Corrosion products were analyzed with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). However, any difference in corrosion products between specimens with and without TEA was not detected by two analysis methods. It seemed that none or very small amount of TEA took part in the reaction between the metal and ions, or the differences were not detected due to the shift in energy state being too small to distinguish the TEAadsorbed peak from non-adsorbed one or its insufficient concentration below detection resolution of the equipment. In an organic coating system, corrosion protection of the initial stage is important because once metal corrosion begins, corrosion at this point is accelerated causing catastrophic degradation of the coated steel. Self-healing efficiency determines corrosion rate of early stage of

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corrosion. For the investigation of corrosion behaviors in the initial points of corrosion by using SVET, specimens with an artificial defect were cut in the same size, and the edges were sealed and then immersed in 0.05 M NaCl solution for 30 min. After 30 min, the first measurement of current density over the surface of the substrate was conducted, and the second experiment followed after 3 h of immersion. In the case of REF2 capsule, further SVET measurement was not carried out because the bulky corrosion products formed on the surface can cause damage on the microtip probe. For the TEA-containing specimen, additional SVET measurement was performed 2 h after the first measurement. At the scribed zone, the current density along the defected region was increased due to the corrosion process in the specimen without corrosion inhibitor (REF2), resulting active anode and cathode on the surface of exposed metal to appear as shown in Fig. 7. In the case of the specimen with TEA capsules, the current distribution was relatively even on the entire surface of the sample and the highest current density at the locally corroded region was much smaller than that of the REF2-containing specimen. The suppression of corrosion activity in the TEA-containing system occurred within a fairly short amount of time after immersion into the electrolytes. This spontaneous passivation and suppression of corrosion activity on the defect region are well in accord with the reduction of corrosion process in the previous semi-immersion result of the specimen containing TEA capsules. In order to corroborate the results of the semi-immersion and SVET tests, EIS experiments were performed on coated specimens in a 0.05 M NaCl solution. Fig. 8 shows the Bode plots of the EIS tests after 2, 12, 24, 36 and 48 h (for the REF2-containing specimen) and 2, 12, 24, 48, 96 and 144 h (for the TEA-containing specimen) of immersion in the electrochemical test cell. At the initial state of immersion within 2 h of interaction between organic coating layer and corrosive species in electrolyte, the total impedance (Zmod) of both specimens showed similar values with around order of 6. However, after 1 day, the resistance of coating layer for the specimen containing TEA dramatically degraded below 10 5, and constantly decreased with respect to time. In TEA-containing specimen, the total impedance was gradually increased from 8 × 10 5 to 5 × 10 6 for 2 days. This may indicate that there was a self-healing action of encapsulated TEA released from nanocapsules by forming passive films on exposed metal substrate. After 2 days in electrolyte, the value of Zmod started to decrease but remained higher than that of the initial experiment (after 2 h from immersion). As confirmed in the SVET measurement, TEA released in the corrosive environment passivated the damaged metal

Fig. 6. a) Experimental setup of semi-immersion test in 3.5% NaCl solution. Top-view of specimens containing 33.3% of REF1, REF2 and TEA capsule b), c) and d) after 12, 24 and 72 h after immersion and e) magnified images of scribed region after peeling off the coating layer.

2360 H. Choi et al. / Surface & Coatings Technology 206 (2012) 2354–2362 Fig. 7. Current density maps taken in the same area of specimens with REF2 capsule after (a) 30 min and (b) 3 h of immersion, and TEA capsule after (c) 30 min, (d) 3 h and (e) 5 h of immersion in 0.05 M NaCl solution. Scanned area: 6.0 × 6.0 mm2. Current density legend: μA/cm2.

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Fig. 8. Bode plot of coated specimens containing a) REF2 and b) TEA capsule exposed to 0.05 M NaCl solution (solid: impedance, open : phase angle).

surface and retarded further corrosion by adsorbing on the metal surface. In the very early stage of corrosion process within 30 min, the total impedance (Zmod) of the TEA-containing specimen was not much lower than that of the specimens without neutralization. When nanoparticles were swollen after neutralization, shell thickness decreased due to an increase of core of the particle and characteristics of the shell polymer such as porosity were changed. As a result of acid–base interaction, highly carboxylated polymers in the center of the particles formed polyelectrolyte salts resulting in hollow core– shell structures. Accordingly, it would provide a bad influence on the barrier property of the coating layer and in the initial stage of EIS tests, specimens with neutralized nanocapsules (REF2 and TEA capsule) showed lower total impedance value compared with unswollen particles. In addition, amine species attracted water into the organic coating layer and deteriorated barrier properties of the coating. In REF2, ammonia mostly evaporated during the baking process in the induction oven and did not have a bad influence on the coating quality any more. TEA, on the other hand, when added directly to the coating solution, will remain in the dried film even after curing at over 170 °C, and had a chance to deteriorate the coating layer, degrading barrier properties. On the contrary to this, nanoparticles neutralized with TEA did not deteriorate the stability of the coating layer, and the coating resistance had even increased with the increase of immersion time. TEA was expected to compensate the decrease of barrier protection caused by the hollow capsule structure because it acts as the corrosion inhibitive agent to retard a steel corrosion by adsorbing on the steel surface. SVET and EIS tests results revealed TEA showed the rapid passivation on the metal surface in the early stage of corrosion and it was mainly contributed to the better corrosion protection of TEA capsules-containing specimens in all corrosion tests. However, the inhibitive mechanism of TEA is not completely understood yet and a further investigation will be followed. 4. Conclusions Nanocapsules were used as nanocarriers of amine-type corrosion inhibitor TEA for long-term and self-healing corrosion protection of a steel substrate. • Nano-polymer capsules with corrosion inhibitor, TEA, were successfully fabricated using multi-stage emulsion polymerization. After neutralization with TEA, the average size of nanocontainers increased from 350 nm to 450 nm due to osmotic swelling. Approximately, 5 wt.% of TEA was stored in the center of nanocapsules when 0.15 M of TEA was fed during fabrication.

• TEA was released more rapidly in both low and high pH environments than a neutral environment, which demonstrates higher possibility of its effective adsorption on anodic and cathodic sites generated by the onset of corrosion reaction of defected metal substrate. • In the semi-immersion corrosion tests and electrochemical tests such as EIS and SVET measurement, specimens with encapsulated TEA showed outstanding corrosion inhibition behavior. The noticeable corrosion protection ability of nanocapsules loaded with TEA was thought to be due to the passivation of metal surface. Acknowledgments This work was financially supported by POSCO. We appreciated the assistance of Ki Hwan Kim in the operation of TEM and Eul Ho Shin in the analysis of LC–MS at Research Institute of Industrial Science and Technology (RIST). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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