Polyamide Protective Coating

Article pubs.acs.org/IECR

High Performance Self-Healing Epoxy/Polyamide Protective Coating Containing Epoxy Microcapsules and Polyaniline Nanofibers for Mild Carbon Steel Hairui Zhang, Jixiao Wang,* Xiuxiu Liu, Zhi Wang, and Shichang Wang Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Tianjin Key Laboratory of Membrane Science and Desalination Technology, State Key Laboratory of Chemical Engineering and Synergetic Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300072, PR China ABSTRACT: Epoxy microcapsules and polyaniline nanofibers are incorporated into epoxy/polyamide coatings to enhance the protective and self-healing performance of the coatings for mild carbon steel. Epoxy microcapsules are prepared through the wellknown interfacial polymerization process. Sodium chloride (NaCl, 12 wt %) solution is used as the model corrosion media. Protective and self-healing performance of the coatings is characterized by the electrochemical impedance spectroscopy (EIS) technique, scanning electron microscope (SEM) image analysis, and X-ray photoelectron spectroscopy characterization. The results demonstrate that the coatings containing epoxy microcapsules and polyaniline nanofibers have almost no deterioration in the 100-day testing period under the condition of being immersed in 12 wt % NaCl solutions at room temperature. The excellent protective performance and self-healing behavior are ascribed to the controlling release of epoxy which is encapsulated and the passive property of PANI nanofibers. The results might give some insights on the preparation of high performance protective coatings.

1. INTRODUCTION Corrosion of metals is not only a serious waste of resources but also a reason for some disasters. The corrosion process relates to the oxidation and the function loss of metals. The protection of metals can be realized by coatings, cathodic protection, passive film formation methods, and so on. It is estimated that approximately 90% of corrosion protective methods and service costs are related to protective coatings.1 Among the widely used protective coatings, organic coatings can provide very good corrosion protection for different metals and alloys.2−6 Although organic coatings can provide good corrosion protection for metals, their long-term durability and reliability are still problematic due to aging and inevitable collisions or scratches induced by external stresses, which cause various types of defects in their structure. These defects, which could subsequently propagate and lead to accelerated failure of the coatings, will significantly decrease their service life and even cause catastrophic consequences. This will severely restrict their applications in the field of heavy-duty corrosion environment. Hence, new technologies and proper or optimized protective organic coatings are needed to improve coating quality and at the same time cut the costs. In recent years, the self-healing process based on microcapsule containing reactive core materials has attracted increasing attention,7 because the microcapsules can be easily prepared and incorporated into the matrix materials. The microcapsules embedded in the coatings will release the encapsulated active and repairing material when the coating integrity is broken by the outer impact and heal the damaged region through the following polymerization reactions.8 Among a variety of core materials, epoxy resin has the healing ability without need of additional catalysts9,10 and should be a good © 2013 American Chemical Society

choice as healing agent for the fabrication of self-healing composites, usually epoxy-amine or epoxy-amide systems which are slightly alkaline. In these systems, the polymerization processes related to the epoxy released from ruptured microcapsules would occur to heal microcracks caused by external stress. Chemically, the healing process will be achieved through the polymerization processes below: one is the curing reaction of epoxy with amine curing agent, which is based on the polymerization of epoxy groups and amino groups, and the other is the polymerization between epoxy molecules under the alkaline condition. Another shortcoming of organic protective coatings is that the pinholes in the coatings cannot be completely eliminated up to now. The corrosion media might transport to the substrate metals through the pinholes, and thus, corrosion happens there. Thus, improving the density of the coatings and reducing the pinholes of the coatings will improve their corrosion protective performance. Forming a dense passive film on the metal surface is an effective method to eliminate the effect of pinholes. Adding inhibitors into the coatings can facilitate the formation of passive films on the metal surface. Unfortunately, nearly all powerful corrosion inhibitors (usually formulations containing Cr6+) may have detrimental effects on both environment and human health due to their toxic and carcinogenic properties.11,12 Thus, the development of novel approaches is required to replace the conventional inhibitors. Recently, conducting polymers (CPs), including polyaniline Received: Revised: Accepted: Published: 10172

March 4, 2013 June 24, 2013 July 3, 2013 July 3, 2013 dx.doi.org/10.1021/ie400666a | Ind. Eng. Chem. Res. 2013, 52, 10172−10180

Industrial & Engineering Chemistry Research

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carried out just as described in our published paper.36 In detail, 0.80 g of Arabic gum, 0.60 g of Tween-80, 20.0 g of epoxy, and 40.0 mL of deionized water were added into a 100 mL threenecked flask and emulsified mechanically under a stirring rate of 1000 rpm at 40 °C for 30 min and then followed by 2 h (hour) at 500 rpm and 60 °C to obtain a uniform and stable oil-inwater emulsion. After that, 3.50 g of EDA was dropped slowly into the mixture and maintained stirring at the mentioned temperature for 2 h. Encapsulation of epoxy took place with reaction of emulsified epoxy with EDA readily dissolved in water at the interface of epoxy droplets, which further condensed to form the capsule wall. The microcapsules from the suspension were recovered by filtration and thoroughly rinsed with deionized water and finally dried in the air for 24 h before going to the vacuum oven at 30 °C for 24 h. The dry microcapsules were free-flowing solid particles. 2.3. Fabrication of Self-Healing Epoxy/Polyamide Coatings. The self-healing coatings were prepared by dispersing the obtained epoxy microcapsules and PANI nanofibers into conventional epoxy/polyamide coatings at ambient temperature. The process was described in detail just as below: 12.0 g of epoxy and 8.0 g of anhydrous alcohol were mixed to form an epoxy solution; 3.54 g of epoxy microcapsules and 0.43 g of PANI nanofibers were added into the obtained epoxy solution, and then, 5.76 g of polyamide (diethylenetriamine condensate) was added as the curing agent. After the mixture was homogenized under ultrasonic condition for 30 min, it was rolled on the pretreated mild steel plates and left to level automatically. The thicknesses of different coatings were controlled similarly to each other by controlling the amount of the applied coatings. After 3 days of curing in the air, solid coatings were obtained. A coating without either microcapsules or PANI nanofibers and a coating only without PANI nanofibers were also prepared as control samples. The three kinds of coatings prepared were named as coating I (neat epoxy/polyamide coating without either microcapsules or PANI), coating II (epoxy/polyamide coatings containing microcapsules), and coating III (epoxy/polyamide coatings containing both microcapsules and PANI), respectively. 2.4. Characterization of Prepared Coatings. 2.4.1. The Characterization of the Surface Morphology and Thickness of the Coating. The surface morphology of the prepared coatings was characterized by scanning electron microscopy (SEM) with a JEOL JSM-6700F microscope. The thickness of the coating was measured by a Coating Thickness Guage (HT200F, Tianjin Sansi Test Instrument Manufacturing Co., LTD). The thickness of the coatings was about 150 (±5) μm. 2.4.2. Contact Angle Measurements. All contact angle measurements were performed on a Dataphysics OCA15EC contact-angle goniometer at room temperature using the Sessile Drop Technique. The coated steel specimens were used for the tests. Contact angles were determined within 1 min by applying a droplet of water (2 μL) to the surface at ambient temperature. The tangent to the drop at its intersection with the surface was estimated visually. All the angles reported here were the averages of at least ten measurements. 2.4.3. Water Absorption Measurements. The water absorption measurement was conducted as follows. The dry samples (6 cm × 8 cm) prepared just as described in Section 2.3 were weighted and then immersed in 400 mL of distilled water at room temperature. After regular time intervals (24 h), the samples were reweighed after removing the surface water by a blotting paper. The above process was repeated until the

(PANI), polypyrrole, and polythiophene, have been successfully used as inhibitive additives to improve the corrosion protective properties of conventional organic coatings.13−20 Because of its simple synthesis, low cost, good environmental stability, and relatively high electrical conductivity,21 PANI is the most studied one among them. Organic coatings containing PANI have been increasingly applied for corrosion protection because of their excellent corrosion protection performances.22−31 PANI could induce a passivation oxide layer between the metal and the coating to protect the metal beneath the oxide layer from further corrosive attack. Recent studies show that PANI dispersed at the concentration of 1− 3% by weight in various coatings can cause significant improvement in metal protection, and usage of a higher percentage of PANI has no beneficial effect.32−34 It is also reported that the coatings containing PANI nanofibers have superior protective performance to that of coatings containing conventional PANI.35 Consequently, the combination of the self-healing effect of microcapsules and the high protective performance of PANI nanofibers will improve the corrosion protection performance of the coatings significantly. In this work, epoxy/polyamide coatings containing epoxy microcapsules and PANI nanofibers were applied to protect mild carbon steel. Microcapsules with epoxy as the core healant and epoxy-amine polymer as the shell material were prepared by interfacial polymerization. In order to measure self-healing ability and corrosion protective performance of the fabricated coatings, defects were made by scratching the coatings by a blade-edge to expose the metal substrates. Electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM) images analysis as well as surface appearance observation were used to characterize the self-healing ability and corrosion protective performance of the coatings. To strengthen the corrosion process, the samples were immersed in 12 wt % NaCl solution for a long time (100 days). Experimental results show that the coatings exhibit high self-healing effect and protective performance, and the “healing” efficiency is about 500%.

2. EXPERIMENTAL SECTION 2.1. Materials. An epoxy (diglycidyl ether of bisphenol A, 0.41−0.47 eq/100 g) serving as both the healing agent and the matrix component was purchased from Wuxi Blue Star Ltd. Co., China. Ethylenediamine (EDA, Aladdin) was applied as the curing agent for epoxy monomer to form the shell material. Tween-80 and Arabic gum used as emulsifiers were purchased from Tianjin Kewei Reagent Ltd. Co., China. Polyamide resin (diethylenetriamine condensate, 180−220 mg KOH/g) used as the curing agent and anhydrous alcohol applied as the solvent in the coating matrix solution were purchased from Tianjin Yanhai Reagent Ltd. Co., China. PANI of nanofibers applied as additive was purchased from Tianjin Advanced materials Ltd. Co., China, which was prepared by an electrochemical process. Mild steel plates (Q235, C: 0.17−0.24%; Si: 0.17−0.37%; Mn: 0.35−0.65%; S: < 0.030%; P: < 0.030%) were used as substrate for the corrosion test. Prior to application, the plates were grinded successively with 400#, 800#, 1200#, and 2000# abrasive papers, followed by a thorough rinse with deionized water and anhydrous ethanol until free of emery particles, and then air dried. All chemicals are analytical and used without any further purification. 2.2. Preparation of Epoxy Microcapsules. The preparation and the characterization of epoxy microcapsule were 10173

dx.doi.org/10.1021/ie400666a | Ind. Eng. Chem. Res. 2013, 52, 10172−10180


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Figure 1. Overview of (a) coating I and (b) coating II and (c) the schematic overview of the coating II structure.

relative to open circuit potential (OCP). Each sample was measured at three different points, and the final result is the average of these three ones. The frequency values were spaced logarithmically, and EIS data were plotted in terms of Bode magnitude (logarithm of the impedance modulus |Z| as a function of the logarithm of the frequency) plots. In addition, the recorded impedance spectra were modeled into equivalent electrical circuits (EEC), and the relevant parameters were used to evaluate the healing effect and corrosion protective performance of the coatings. 2.4.6. X-ray Photoelectron Spectroscopy (XPS) Measurements. The passivation oxide layer on mild steel was characterized using an X-ray photoelectron spectroscopy (XPS, PHI-1600). XPS analysis was performed using Mg Kα as the radiation source, and the spectra were taken with the electron emission angle at 45°. Spectra of O 1s and Fe 2p were recorded.

samples’ weights had no change. The accuracy of the weight measurement is in the order of ±0.01 mg. 2.4.4. Adhesion Strength Tests. The adhesion strength of the coatings was determined by mechanical pull-off tests using a PosiTest Adhesion Tester (PosiTest AT-M, Defelsko, USA). First, clean up the dolly and the coating surface. Then, fix the dolly vertically to the testing part with special adhesive. After the adhesive was dried completely, cut away the excess adhesive from the dolly application process. Lastly, the dolly was lassoed and stretched slowly until the testing coating was destroyed. 2.4.5. Evaluation of Self-Healing and Corrosion Protection Performance of Coatings. Electrochemical impedance spectroscopy (EIS) was employed to characterize the self-healing and corrosion protection performance of the prepared coatings. Artificial cross-cutting defects were created on the coated steel specimens using a blade-edge to expose the substrates (about 100 μm in width). Throughout the scratch process, the blade was held to be vertical to coating surface. In order to ensure the same gap width, all cross-cuts should be made at one time. All specimens were kept in the air (25 °C) for 24 h after being scratched for their healing process. Then, long-term salt solution immersion tests were conducted at room temperature. The impedance of the scratched specimens were measured and recorded at intervals of several days during the immersion period of 100 days. The impedance of an unscratched specimen coated with neat epoxy/polyamide coating (coating I) was also measured for comparison. After the EIS spectra were recorded, the specimens were returned back into the corrosive medium. Here, in order to simulate the corrosion process which occurs under the condition of desalination (such as desalination in membrane distillation) or concentration under some chemical process, the corrosion media was strengthened to 12 wt % NaCl solution. All EIS measurements were performed on an electrochemical workstation (PAR 273A, USA) at ambient temperature in a classic one-compartment cell, a three-electrode system. The test cell consisted of a glass cylinder clamped with an O-ring seal at one end to the coating specimen surface. A platinum wire dipped into the electrolyte solution was employed as the counter electrode (CE); a saturated calomel electrode (SCE) was used as the reference electrode (RE), and the specimen under study with an exposure area of approximately 1.00 cm2 was used as the working electrode (WE). The cell was placed in a faraday cage to avoid Coulombs fields. The electrolyte used in the cell was also 12 wt % NaCl solution which was quiescent and remained in equilibrium with air. All the EIS spectra were recorded in the frequency range of 10 mHz and 100 kHz. The amplitude of the sinusoidal perturbation signal was 10 mV

3. RESULTS AND DISCUSSION 3.1. Surface Morphology and Thickness of the Coatings. Figure 1 shows the coating surface morphology and schematic overview of the coating structure. Figure 1a clearly demonstrates that the neat epoxy/polyamide coating has a smooth surface. When epoxy microcapsules were added into the epoxy/polyamide coating, some bumps appear at the surface (Figure 1b) and thus cause coating II to have a rougher surface than coating I. The characterization of the epoxy microcapsules has been reported in detail in our previous report.36 As shown in the schematic overview (Figure 1c), the microcapsules were dispersed uniformly in the coating matrix. When the coating is scratched, the microcapsules are ruptured under local stress and release the encapsulated epoxy. The released epoxy meets with the polyamide curing agent contained in the matrix and causes the scratch repair by polymerization. 3.2. Wettability of the Prepared Coatings. Surface wettability is one of the important characteristics of materials and has been widely applied to study self-cleaning and waterproof materials. Poor water proofing property of the coatings will lead to fading, gloss, chalking, cracking, flaking, etc., which would reduce their protective performance and cause their failure prematurely.37 Generally, the less hydrophilic the coating, the better is its water proofing property. The contact angle test is the most used means to characterize the hydrophilic/hydrophobic property of the coatings. Here, the determination of contact angle is indispensable to the study of 10174



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penetration of the outside electrolyte solution attained at the coating/metal interface, and thus, the formation of corrosion battery at the coating/metal interface was inhibitive. The pulloff test results show that the adhesion strengths of all the three kinds of coatings are in the range from 4.0 to 5.0 MPa, despite a certain degree of gradual increase from coating I to coating II, and further to coating III, as shown in Table 1. This is because

coating performance. The contact angle measurements were performed using the Sessile Drop Method with water. As shown in Figure 2, the contact angles of coating I, coating II, and coating III are 65°, 85°, and 90°, respectively. The

Table 1. Adhesion Strength of the Three Kinds of Coatings

Figure 2. Contact angles of (a) coating I, (b) coating II, and (c) coating III.

results demonstrate that the addition of microcapsules and PANI nanofibers can increase the contact angle of coatings and accordingly decrease their hydrophilic behavior. The hydrophilic/hydrophobic property of a surface is determined by both its chemical composition and surface topology. When microcapsules were prepared, the bulk hydrophobic alkyl groups (−R) and other small hydrophobic groups of the epoxy-amine polymer make the microcapsules relatively hydrophobic. Therefore, the addition of microcapsules could decrease the hydrophilic behavior of epoxy/polyamide coating. Another reason is that the addition of microcapsules and PANI can increase the surface roughness of the coatings just as shown in Figure 1b. These two factors will increase the apparent contact angle of the coatings. The increase of contact angle from coating I to coating II, and further to coating III, indicates that the addition of microcapsules and PANI could decrease their hydrophilic behavior and accordingly improve their water proofing properties, which implies the improvement of corrosion protection performance to some extent. In addition to contact angles of the coatings, water absorption was also used to characterize the coatings, as shown in Figure 3. The experimental results show that coating I

coatings

adhesion strength 1(MPa)



I II III

4.15 4.67 4.83

4.01 4.59 4.97

4.04 4.61 4.78

the adhesion strength of a coating relates not only to the coating composition and structure but also to the roughness of the steel substrate. The pretreatment of the steel plates successively with 400#, 800#, 1200#, and 2000# abrasive papers, followed by a thorough rinse with deionized water and ethanol until free of emery particles, results in the small surface roughness of the substrates. This may lead to the weak and even the loss of the anchoring interaction between coatings and the substrates. Consequently, there is little difference between the adhesion strengths of the three kinds of coatings. 3.4. Self-Healing and Corrosion Protective Properties of Coatings from Bode Plots. A series of experiments were carried to determine which coating system has the best selfhealing performance and the longest corrosion protective lifetime. The impedance of the coated specimens was measured during the whole immersion period of 100 days, and the Bode plots of the results are presented in Figure 4. As shown in Figure 4, scratching of coating I led to a sharp decline in the impedance and the impedance decreased continuously over the immersion time. It can be concluded that coating I nearly lost its protection effect only after being immersed in 12 wt % NaCl solutions for 1 day (Figure 4b) and further decline after being immersed for 5 days, which indicates that coating I deteriorates rapidly and has hardly any selfhealing ability. Figure 4 shows that coatings II and III have much better protective behavior than coating I in their self-healing behavior. The EIS curves in Figure 4 indicate that the impedances of coatings II and III decreased in the initial stage (from Figure 4b,c) and then increased (from Figure 4c,d) with the immersion time getting longer. In the initial stage, the extent of the impedance decrease of coating III was smaller than that of coatings II. The recovery amplitude of the impedance of coating III was higher than that of coatings II. Figure 4 also demonstrates that the impedance of coating III was higher than that of coating II throughout the whole immersion period. The protective performance deterioration of coating II started after being immersed in 12 wt % NaCl solutions for 50 days shown in Figure 4e,f, while coating III has no obvious deterioration in the period of 100 days immersion, as shown in Figure 4f. These phenomena may be related to the self-healing process of the prepared coatings. It is well-known that two polymerization processes occur in the alkaline coating system that contains epoxy and polyamide curing agent. The first one is the curing reaction of epoxy with polyamide resin, which is based on the reaction between epoxy groups and amino groups, and the second one is the

Figure 3. Water adsorption behavior of the coatings.

absorbs more water than coating II and coating III, which is consistent with the contact angles results. Therefore, coating II and coating III may perform better protective performance than coating I. 3.3. Adhesion Strength of the Coatings. In the real application process, adhesion strength of the coatings is a quite important parameter for measuring their protective performance. Coatings of good adhesion can effectively restrain the 10175



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Figure 4. Bode plots of EIS data of coated specimens (a) before being scratched, immersed for (b) 1 day and (c) 5, (d) 25, (e) 50, and (f) 100 days in 12 wt % NaCl solution after being scratched.

Figure 5. XPS spectra of coating III removing its surface coating after a 100 day immersion in 12% NaCl solution: (A) O 1s; (B) Fe 2p.

polymerization between epoxy resin molecules under the alkaline condition. Thus, although the polyamide curing agent was used approximately in a stoichiometric amount for curing

epoxy in coating matrix, in fact it was in excess compared with the amount of epoxy. The released epoxy will be cured by the residual polyamide in the matrix and caused the scratch to be 10176



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matrix and intertwined with each other to form a dense network, which allows the distribution of PANI nanofibers to have excellent continuity (Figure 7). This will really be beneficial to the formation of the dense passive layer on the steel surface to protect the metal beneath the oxide layer from further corrosive attack.

repaired by the two polymerization processes stated above. As it is known, however, the fluidity of epoxy resin is bad at room temperature due to its high viscosity. Therefore, in the initial stage of scratching, only part of the epoxy released from ruptured microcapsules flowed to the scratched region and formed a new barrier layer, which was relatively thin and might be unable to cover the exposed substrate completely. This would allow a small amount of corrosive medium to penetrate to the coating/steel interface, leading to the decrease of impedance in the early stage. With time going on, much more epoxy reached the scratched region and formed a thicker protective barrier, resulting in the increase of impedance, but for coating II, with the exposure time getting longer than 50 days, corrosive medium gradually penetrated the barrier and its corrosion protective effect weakened. The smaller decrease and quicker restoration of the impedance coating III than that of coating II is attributed to the presence of PANI. PANI, an electroactive polymer, can passively catalyze the steel surface and produce a dense passive film between the steel surface and the coating which protects the steel beneath the passive layer from further corrosive attack.38−43 This can be proved by the XPS results of the mild steel surface under the coating, which was presented in Figure 5. The O1s spectra of the surface oxides can be fitted with two peaks corresponding to the total oxide. The corresponding binding energies of the two peaks are 529.84 and 530.84 eV, respectively (Figure 5A), suggesting that the oxides are Fe2O3 and Fe3O4. In Figure 5B, the peaks related to Fe2O3 at 710.81 and 724.37 eV and the satellite feature emerging at 718.73 eV demonstrate the presence of an oxide layer of Fe3O4.44 Therefore, the coating III does have an inert passive oxide layer (Fe2O3/Fe3O4) and thus has higher impedance and can be maintained effectively for longer time than coating II. The concentrations of the microcapsules and PANI in coatings have profound effect on the coating protective performance. Here, 20 wt % microcapsules36 and 2.0 wt % PANI were adopted. For microcapsules, this may be explained by the fact that the healing agent delivered to the damaged region is inadequate when the concentration of microcapsules is too low, while the coating porosity increases and density reduces when the concentration of microcapsules is too high. For PANI, it can cause significant effect in corrosion protection at low concentration of 1.0−3.0% by weight and it is able to offer sufficient protection for steel.32−34 This may be because PANI nanofibers have small size and uniform morphology, as shown in Figure 6. They can be dispersed uniformly in coating

Figure 7. PANI nanofibers in coating III.

3.5. Self-Healing and Corrosion Protective Properties of Coatings from EEC Results. Figure 8 shows the Nyquist plots corresponding to Figure 4 and their fitted results of the three coatings at different stages. The initial stage refers to immersion for 1 day for coating I and 5 days for coating II and coating III. The end stage refers to immersion for 5 days for coating I and 25 days for coating II and coating III. The circuits at the top-right corner are the corresponding EEC models, in which Rs, Rc, CPEc, CPEd, Rct, and W are solution resistance, coating resistance, coating capacitance, double layer capacitance, charge transfer resistance, and Warburg impedance, respectively.45,46 The values of Rc, Rct, and impedance at 10 mHz are shown in Table 2. There are two semicircles in the Nyquist plot of coating I(a), and the appearance of the second one indicates that corrosion processes had happened at the interface of the coating/metal.45 The Warburg resistance is attributed to the deposition of the corrosion products on the electrode surface,45 which results in the charge diffusion resistance. In the late stage, only one depressed semicircle appears which is related to the double layer capacitance instead of the coating capacitance. This phenomenon suggests that serious corrosion has occurred on the steel surface. The values of Rc, Rct, and impedance was shown in Table 2, indicating that the coating I loses its protection completely after 5 days of immersion. The Nyquist plot of coating II(a) shows two semicircles as well, meaning that slight corrosion processes had happened under coating II after immersion for 5 days, but at the late stage, only one semicircle remains and the impedance is presented in Table 2, indicating that coating II regains its protection capacity. The semicircle refers to the coating capacitance. The Nyquist plots of coating III are the same as those for coating II except for the presence of Warburg component (W) in the initial stage. This is attributed to the formation of dense passive film due to the existence of PANI, which restricts the charge diffusion and displays Warburg impedance in the EIS

Figure 6. Size of polyaniline nanofibers. 10177



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Figure 8. Nyquist plots and their corresponding fitted results of the three coatings at (a) initial stage and (b) late stage. The circuits at the top-right corner are the corresponding EEC models.

morphologies of the scratched region can also reflect the selfhealing behavior of the coatings. As shown in Figure 9, the

Table 2. Data Obtained from the Fitted Nyquist and Bode Plots, Using the Electrical Model Shown in Figure 8a Rc (MΩ cm2)

a

sample

a

coating I coating II coating III

0.47 14.26 8.87

Rct (MΩ cm2)

log(Z/Ω cm2) (at 10 mHz)

b

a

b

a

b

0.32

112.70 2717

0.55 9.87 21.26

6.10 7.34 7.50

5.90 8.05 9.79

(a) Initial stage; (b) late stage. Figure 9. SEM images of scratched regions of (a) coating I, (b) coating II, and (c) coating III, with the left part in each image being the scratched region newly made and the right being the scratched region after the probable healing process.

spectra. Although the corrosion happens at the initial stage with the appearance of the second semicircle, the values of the Rc increase by more than two orders magnitude at the late stage. Thus, coating III not only recovers its protective performance at the late stage but also performs better than ever. The good self-healing and protective properties are attributed to the epoxy microcapsules and passive layer promoted by PANI, as discussed in Section 3.4. 3.6. Self-Healing and Corrosion Protective Properties of Coatings from SEM and Optical Graphs. SEM

cross-section of the coating without either epoxy microcapsules or PANI nanofibers (coating I) was in a brittle fracture feature and both the width and depth of the breach was almost unchanged (Figure 9a) even 2 days after it was made. However, for coating II containing only epoxy microcapsules, it can be seen in Figure 9b that the cross-section presented a tear aspect and the breach has been partly filled. This is attributed to the 10178


Industrial & Engineering Chemistry Research healing effect of the released epoxy resin from ruptured microcapsules. As shown in Figure 9c, similar to coating II, the cross-section of coating III also presented a tear aspect. Even better than this, both the width and depth of the breach was reduced significantly. This may be interpreted as the PANI nanofibers in the matrix being intertwined with each other to form a dense network, which enables the coating to have high scratch resistance. Furthermore, the swelling of PANI also contributes to the narrower and shallower breach. Optical photographs of the steel surface under the tested coating specimens are shown in Figure 10. It can be seen in

ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the project SKL-ChE-12T12, the Program of Introducing Talents of Discipline to Universities (No. B06006), National Natural Science Foundation of China (20836006), the National Basic Research Program (No. 2009CB623405), and National High Technology Research and Development Program of [2012AA03A611].

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Figure 10. Optical photographs of the steel specimens under (a) coating I without scratch, (b) scratched coating I, (c) scratched coating II, and (d) scratched coating III after 100 days of immersion in 12 wt % NaCl.

Figure 10a that, even without a scratch, coating I was corroded at some points. When it was scratched, serious corrosion occurred (Figure 10b), but for coating II, only pitting corrosion occurred (Figure 10c); for coating III, there was not any corrosion. Even after 100 days of immersion in 12 wt % NaCl solution, there was no apparent corrosion found on the surface, as shown in Figure 10d. Consequently, the prepared epoxy/ polyamide protective coatings containing both epoxy microcapsules and PANI nanofibers perform excellent self-healing and protective performance for mild carbon steel.

4. CONCLUSIONS In summary, epoxy/polyamide coating with high corrosion protective performance and the “self-healing” effect was fabricated. The EIS data and visual examination reveal that epoxy can be released successfully from the ruptured microcapsules and polymerization reaction occurs to form a new protective layer at the damaged region. The experimental results also demonstrate that the epoxy/polyamide protective coatings containing epoxy microcapsules and PANI nanofiber can be used for heavy-duty corrosion protection. However, the sensitivity of the coatings to damage needs to be improved for the poor fluidity of epoxy resin used in this paper. The combination of the use of different types of epoxy resins may be a solution worthy of a try.

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Polyamide Protective Coating

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