Corrosion performance of epoxy coatings containing

Corrosion Science 53 (2011) 89–98

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Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Corrosion performance of epoxy coatings containing silane treated ZrO2 nanoparticles on mild steel in 3.5% NaCl solution M. Behzadnasab a, S.M. Mirabedini a,⇑, K. Kabiri a, S. Jamali b a b

Colour, Resin & Surface Coatings Dept., Iran Polymer and Petrochemical Institute, P.O. Box 14965-115, Tehran, Iran School of Applied Sciences, University of Northampton, St. George’s Avenue, Northampton NN2 6JD, UK

a r t i c l e

i n f o

Article history: Received 29 March 2010 Accepted 2 September 2010 Available online 16 September 2010 Keywords: A. Mild steel B. EIS B. IR spectroscopy C. Polymer coatings

a b s t r a c t Clear epoxy coatings were modified by adding various levels of ZrO2 nanoparticles. In order to achieve proper dispersion of nanoparticles in the epoxy-based coating and making possible chemical interactions between nanoparticles and polymeric coating, the surface of the nanoparticles was treated with amino propyl trimethoxy silane (APS). Corrosion performance of mild steel coated specimens was investigated employing EIS, electrochemical noise (ECN) techniques and salt spray test. Coatings with 2–3 wt% ZrO2 nanoparticles possessed the best corrosion performance among the coating specimens. Possible chemical interactions between polymeric matrix and treated nanoparticles in nanocomposites cause high barrier properties and ionic resistances. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, steel has become an important part of our life due to its extensively applications in automotive, household appliances, business machine and heavy construction such as marine and chemical industries. Mild steel is selected for construction because of its mechanical properties and machine-ability at a low price, while at the same time; they have to be resisted against corrosion phenomena. This fact is one of the major reasons for industrial accidents and consuming of material resources [1,2]. Cathodic protection and polymeric coatings are usually employed to protect metallic constructions against the corrosion [1], but isolating the metal from corrosive agents is still the most conventional method to prevent corrosion [3]. In this regard, polymeric coatings can provide protection either by a barrier action from the layer or from active corrosion inhibition supplied by pigments in the coating, which give protection to the underlying substrate [4]. However, in practice, all polymeric coatings are permeable to corrosive species such as oxygen, water and ions to some extent [5–8]. Water molecules at the metal/coating interface may reduce the coating adhesion, thus favouring corrosion of the metal underneath the film. In recent years, polymeric nanocomposites have been attracted a lot of focused due to their outstanding properties, i.e. elastic stiffness, excellent barrier resistance, flame retardancy, scratch/wear resistance, optical characteristics and electrical properties [9]. The barrier properties of the organic coatings can be improved ⇑ Corresponding author. Tel.: +98 21 4458 0040; fax: +98 21 4458 0023. E-mail address: [email protected] (S.M. Mirabedini). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.09.026

by inclusion of proper fillers [10,11]. In this regards, nanosized fillers have superior barrier properties than conventional fillers even at low concentrations [12,13]. Homogenous dispersion of nanoparticles is a key factor in preparation of polymeric nanocomposites. Nanoparticles have high tendency to interact with each other to form agglomeration [9]. Their unique properties are mainly due higher surface area of the nanosized particles in compare to microsized fillers caused by their large surface area to volume ratio [14]. There are various reports concerning improving corrosion resistance of coatings using nanoparticles such as; TiO2 [10,15,16], ZnO [17], SiO2 [18] and organoclay [19]. Zirconium dioxide, ZrO2, is one of the most promising nanoparticles employed in anticorrosion coatings [20–22]. ZrO2 reveals excellent properties such as; high strength, high fracture toughness, excellent wear resistance, high hardness and excellent chemical resistance. Although, the preparation of zirconia nanocomposites through sol–gel routs have been extensively studied [15,23–28], however, a few published works on the preparation of epoxy–zirconia nanocomposites using up to down approach could be found in the literature [29]. In this study, corrosion performance of zirconia–epoxy nanocomposites, prepared by up to down approach, on mild steel in 3.5% NaCl solution was studied. To ensure appropriate dispersion in nanosized scales and enhance possible chemical interactions between zirconia powder and epoxy matrix, surface of nanoparticles was treated with amino propyl trimethoxy silane (APS) coupling agent. Modified nanoparticles were then dispersed in epoxy matrix and the effect of modified zirconia on the corrosion performance of the resultant nanocomposite coatings was investigated using various techniques.

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2. Experimental 2.1. Materials Elemental compositions of the mild steel specimens are listed in Table 1. Epoxy resin, Epon 828, based on diglycidyl ether of bisphenol A (DGEBA) and the amine containing hardener (Epikure F205) were purchased from Shell chemicals. Zirconium dioxide nanoparticles (Zircox™ 15) with an average particle size of 15 nm were provided by IBU-tec advanced materials AG. APS (99.5%) and acetone (98.0%) were purchased from Merck Chemicals. 2.2. Surface modification of ZrO2 nanoparticles In the first stage, 10 g of ZrO2 nanoparticles were kept in a vacuum chamber for 1 h at 120 °C and then dispersed in 30 ml acetone via stirring at 300 rpm for 1 h at ambient temperature and sonicated (Bandelin, SONO PULS-UW 2200) for 20 min under power of 70 W, 20 kHz frequency, 0.7 s pulse on and 0.3 s pulse off. In the second step, 50 wt% (5 g) APS was gradually added to the dispersion and stirred for further 24 h in ambient temperature. Finally, it was centrifuged (6000 rpm) and the residue washed with acetone. The washing procedure was repeated for three times and the remained precipitate was dried in a vacuum oven at 50 °C for 48 h [9]. The grafting of APS on the ZrO2 nanoparticles was evaluated by FTIR and TGA techniques. FTIR spectroscopy was carried out in KBr pellet on a Bruker EQUINOX 55 FTIR spectrometer, Ettlingen, collecting 35 scans in the 400–4000 cm1 range with 4 cm1 resolution. The thermal behaviour of the untreated and treated nanoparticles (after drying in a vacuum oven at 80 °C for 24 h) was evaluated with TGA, PL-1500, under O2 atmosphere from room temperature up to 600 °C with the heating rate 10 °C min1. 2.3. Dispersion stability test Dispersion stability of nanoparticles (untreated and treated), was evaluated in an organic solvent (acetone). Dispersions of 0.1 g L1 nanoparticles in acetone were prepared and sonicated for 10 min and stirred further 1 h. The dispersion was then allowed to stand for at least 5 days and sedimentation behaviour of nanoparticles was visually evaluated. 2.4. Preparation of epoxy nanocomposite coatings ZrO2 nanoparticles (1, 2 and 3 wt%) were kept in a vacuum oven at 80 °C for 1 h to remove physically absorbed moisture and then directly added to the acetone with concentration of 30 ml g1, dispersed by ultrasonically irradiating for 10 min. Exterior cooling was employed by surrounding the sample’s container in an ice and water bath to avoid heat rising during sonication process. The dispersion was added slowly to the epoxy resin and stirred with Heidolph-RZR 2102 mechanical stirrer (2000 rpm) for 12 h at 70 °C and degassed for further 3 h in a vacuum oven at 70 °C, to remove remained acetone and trapped air during the mixing steps. Once the appropriate dispersion was achieved, a stoichiometric amount of curing agent, Epikure F205, with weight ratio of 58:100, was added to the dispersion, and stirred for further

Table 1 Elemental composition of mild steel specimens. Elements wt%

Fe Balance

C 0.17

Mn 1.24

Si 0.41

P 0.01

S 0.02

Cr 0.03

Cu 0.10

5 min. Various formulations were employed for preparation of nanocomposite coatings are listed in Table 2. Epoxy coatings with desired thickness were then applied on the grinded and degreased mild steel (Q-Panel size) substrates using a film applicator (Model 352, Erichsen Co.). 2.5. Transmission electron microscopy (TEM) Transmission electron micrographs of the nanocomposite films containing 2 wt% of untreated and APS-treated ZrO2 nanoparticles were obtained using TEM apparatus (Model EM 208, Philips Co.). Thin sections ca. 70 nm of the specimen were gained by microtome with diamond knife and the filament voltage was set at 100 kV. 2.6. Corrosion performance tests To verify the effect of ZrO2 nanoparticles on the corrosion performance of epoxy coating on the mild steel substrate, EIS, electrochemical noise (ECN) techniques and natural salt spray test were carried out. In order to prepare epoxy coated mild steel panels for electrochemical tests, a defined area of each specimen was exposed to the electrolytes and the backs and the edges of the specimen were sealed by hot melt mixture of beeswax and colophony resin. EIS measurements were performed with a PGSTAT 30 Autolab (Metrohm) using FRA software. A three-electrode arrangement was used, consisting of an Ag/AgCl reference electrode, a platinum counter electrode and the exposed sample (4 cm2 and with a coating thickness of 45 ± 5 lm) as a working electrode, immersed in a 3.5% NaCl solution. All EIS measurements were conducted during 120 days of immersion at open circuit potential at the frequency range of 10 mHz to 10 kHz. Three replications were performed to ensure repeatability. EIS spectrum was also collected for bare mild steel specimen within the exposure time of 30 min. ECN data were obtained for nanocomposite specimens containing different levels of ZrO2 nanoparticles using a PGSTAT 30 Autolab (Metrohm) using EnAnaize software. A three-electrode cell arrangement including of two nominally identical panels with an exposed area of 1 cm2 for each specimens as dual working electrodes (with a film thickness of 45 ± 5 lm) and a saturated reference Ag/AgCl electrode was used [30,31]. The working electrodes were prepared in a similar method explanation for EIS. Electrochemical current noise was collected between the dual working electrodes and simultaneously, the potential fluctuations of two short circuited working electrodes were measured with respect to reference electrode during a time period of 1024 s with sampling rate of 1 point/s. The frequency domain corresponding to the sampling conditions was evaluated to be between fmax = 1/2Dt and fmin = 1/N Dt, where Dt and N are the sampling interval and the total number of data points, respectively [31,32]. Finally, the corrosion performance of the coated specimens was evaluated in a neutral salt spray test, following the procedure of ASTM B 117 and employing 5% (50 g L1) sodium chloride solution at 35 ± 1 °C. Prior to exposure, the backs and the edges of the

Table 2 Formulation of various samples for preparation of nanocomposite coatings. Sample codinga

ZrO2 (wt%)

Epon 828 (wt%)

Epikure F205 (wt%)

NE EZ-1 EZ-2 EZ-3

0 1 2 3

66.70 66.00 65.33 64.67

33.30 33.00 32.67 32.33

a NE and EZ represent neat epoxy and zirconia containing nanocomposites, respectively. The number shows the wt% of ZrO2 nanoparticles embedded in epoxy coating.

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specimens were covered with hot melt mixture of beeswax and colophony resin. The specimens were removed from the salt spray chamber after 2000 h and representative areas were imaged with a digital camera. The images were then used to evaluate the corrosion performance of the coated specimens.

Table 3 Characteristic absorption peaks obtained from FTIR spectrum of APS-treated ZrO2 nanoparticles.

3. Results and discussion 3.1. FTIR spectroscopy Fig. 1 shows the FTIR spectra of treated and untreated ZrO2, as well as APS coupling agent. The assignments for the main FTIR bands of the APS-treated nanoparticles are listed in Table 3. The bands at around 2930 cm1 and 2850 cm1 observed in FTIR spectrum of treated nanoparticles, are attributed to the alkyl groups [–(CH2)n–] [9]. Furthermore, the band at around 964 cm1 reconfirms condensation reaction between methoxy groups of APS and hydroxyl groups on the surface of ZrO2 nanoparticles. These absorption bands can be related to the grafting of APS on the nanoparticles. It could be concluded that hydroxyl groups, on the surface of ZrO2 nanoparticles (ZrOH) are reacted with alkoxy groups of silane compounds [34,36].

No.

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Functionality

1 2 3 4 5 6

490 510 964 1000–1300 1570 2870

7

2930

8

3430

Stretch vibration band of Si–O [33] Stretch vibration band of Zr–O–Zr [33] Stretch vibration band of Zr–O–Si [33] O–Si asymmetric flexible vibration [35] Main characteristic peaks of Si–O bonds [35] Symmetric stretch vibration band of methylene [–(CH2)n–] and methyl –(CH3) [36] Asymmetric stretch vibration band of methylene [–(CH2)n–] and methyl –(CH3) [36] Vibration band of –OH [36]

100 Untreated Zirconia Nanoparticle APS Treated Zirconia Nanoparticle

Weight Lost (%)

99

3.2. TGA analysis

a

98

97

b

96

TGA thermographs of untreated and treated ZrO2 nanoparticles are displayed in Fig. 2. The weight loss happens in two stages, which the first stage occurs in the range of 40–180 °C, and is related to the removal of physically absorbed water from the nanoparticles. This is about 0.5 and 1 wt% for untreated and treated nanoparticles, respectively. This indicates that treated nanoparticles tend to absorb more water as the result of APS grafting. The second stage of weight loss happens in the range of 200–560 °C. For untreated nanoparticles it is about 1.2 wt% and is attributed to water formed from condensation of hydroxyl groups on the particle’s surface (ZrOH) [35]. For treated nanoparticles (Fig. 2b), the weight loss is about 2.8 wt% and is caused by the condensation of zirconia groups (on the ZrO2 nanoparticles) or/and thermal decomposition of APS chains and removal of the resultant water.

95 0

100

200

300

400

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3.3. Dispersion stability test in an organic solvent The results for sedimentation test on various samples are shown in Figs. 3 and 4. As it is shown, for sample without treatment and regular mixing with magnetic stirrer, sedimentation

Absorbance/%

(b)

(a)

3000

600

Fig. 2. TGA curves of (a) untreated and (b) APS-treated ZrO2 nanoparticles.

(c)

4000

500

Temperature (C)

2000

1000

400

Wavelength /cm-1 Fig. 1. FTIR spectra of (a) APS, (b) untreated ZrO2 nanoparticles, and (c) APS-treated ZrO2 nanoparticles.

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(a) 3 min after mixing

(b) after Sonication

(c) 10 min

(d) 1 h

(e) 24 h

Fig. 3. Sedimentation behaviour of 2 wt% suspensions of untreated ZrO2 nanoparticles in acetone.

(a) after Sonication

(b) 24 h

(c) 48 h

(d) 72 h

(e) 120 h

Fig. 4. Sedimentation behaviour of 2 wt% suspensions of APS-treated ZrO2 nanoparticles in acetone.

occurs in less than 3 min; the supernatant was a bit turbid and turn into a clear supernatant after further 10 min. The sedimentation speed and mechanism are changed after sonication. The sedimentation mostly happened by flocculation mechanism. The suspensions separated very quickly (less than 10 min) into sediments and a clear supernatant on top of the sediment was observed. The separation interfaces between the sediment and liquid were sharp and moved downward with time. But the rate of sedimentation decrease with the time as seen in the Fig. 3c–e. This sedimentation behaviour is typical of flocculated suspensions. Two possible types of sedimentation mechanisms have been reported by Xu et al. [37], i.e. flocculation and accumulation. In contrast, the treated nanoparticle dispersion is much more stable and depending on the APS content grafted on the ZrO2 surface, reveals different stabilities. For APS-treated sample (Fig. 4),

the sedimentation performance was due to accumulation at the bottom, while columns of cloudy supernatant suspensions still remained after 5 days of settling. This sedimentation behaviour is typical of well-dispersed suspensions and smaller particles have much slower settling rates, which might be counter balanced by Brownian motion, they will remain in the supernatant for long times. Even after 5 days the solution remained turbid. It indicates that APS modification can lead to increase stability of nanoparticles in the polar organic media. 3.4. TEM results TEM images of nanocomposite coatings containing 2 wt% of untreated and APS-treated ZrO2 nanoparticles are displayed in Fig. 5. The size aggregations varied between 40 and 300 nm with an

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Fig. 5. Transmission electron micrographs of nanocomposite coatings containing 2 wt%: (a) untreated; (b) APS-treated ZrO2 nanoparticles.

3.5. Corrosion performance tests 3.5.1. EIS study EIS plays an important role to monitor and predict degradation of organic coatings [39–41]. The Nyquist plot obtained from EIS measurements for bare mild steel specimen after 30 min immersion in 3.5% NaCl solution is shown in Fig. 6. The plot is characterized by a depressed semicircle from high to medium frequencies and an inductive loop at low frequencies. The appearance of the inductive loop in the Nyquist plot is attributed to the adsorption of an intermediate product in the corrosion reaction [42]. Four different coatings, with various contents of zirconia nanoparticles, were examined during 120 days immersion in chloride electrolyte. Nyquist plots are displayed in Fig. 7 for the coatings with 0, 1, 2 and 3 wt% nanoparticles after various immersion times in 3.5% NaCl electrolyte. For the coated substrate in the absence of nanoparticles, after 1 day immersion in the electrolyte (Fig. 7a), the impedance response reveals initial behaviour that is dominated by

10

2

8 Z" /Ohm.cm

average of 150–200 nm. Images indicate that most of the treated ZrO2 nanoparticles are relatively dispersed at the scale of 100– 170 nm in the nanocomposite epoxy film, but some aggregates still can be observed. However, this is because ZrO2 nanoparticles have much high surface area, strong hydrogen bonding through OH groups and high surface free energy, and thus tend to be aggregated. More aggregations and poorer dispersion of untreated nanoparticles can be observed from Fig. 5. Also the size of agglomerations of untreated nanosized zirconia is bigger than treated ones (200– 350 nm). The TEM images reveal that surface modification of ZrO2 nanoparticles with APS somewhat improved the dispersability of the nanoparticles in the polymeric matrix. The surface treatment of nanoparticles probably increases the steric hindrance between the nanoparticles and also improves their wettability and compatibility in the polymeric matrix [38].

6 4 2 0 0 -2

2

4

6

8

10 Z'/Ohm.cm

12

14

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20

2

Fig. 6. Nyquist plot for mild steel after 30 min immersion in 3.5% NaCl electrolyte.

the coating capacitance at high frequencies and coating resistance in the low frequency region with a resistive component greater than 3 109 X cm2. With increasing immersion time (5 days), the resistance decreased, due to the penetration of water and movement of ionic species through the coating layer, increasing the coating conductivity. Initially, the electrolyte penetrates through the coating film, and sets up conducting pathways at various depths within the coating [43–45]. Further, a second semicircle at low frequencies immerged in the EIS spectra, suggesting that electrochemical reactions at the metal/coating interface are progressing [46]. At this stage, diffusion of electrolyte inside the coating is completed and the electrolyte phase meets the metal/oxide interface and corrosion reactions are occurred. With additional immersion time up to 90 days, the barrier properties of the coating decreased further. It can be ascribed to increasing of corrosion rate, possibly through the presence of further pores in the coating or an increase in the area exposed at the base of the existing pores or flaws [44]. An increasing in the coating resistance was observed with further increasing immersion time (120 days). This behaviour suggesting that pores within the coating layer were blocked with corrosion products, and ionic movement in the coating layer was impeded [43].

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(d) EZ -3 Fig. 7. Nyquist plots of nanocomposite coatings containing various levels of APS-treated ZrO2 nanoparticles, during 120 days immersion in 3.5% NaCl electrolyte.

EIS spectra of epoxy coating containing 1 wt% ZrO2 nanoparticles, (Fig. 7b) are slightly different from that of the clear coating and the magnitude of decrease of impedance value was minor. In

the immersion time of 10 days, the resistance value remained above 1.2 109 X cm2, about two orders higher than that of clear epoxy coating. However, after 10 days immersion, there is only one

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The calculated coating resistance and coating capacitance are shown in Fig. 9. It is clearly observed that the coating resistance of epoxy coatings containing ZrO2 nanoparticles is higher than for the neat epoxy coated specimen. This fact may be attributed to the higher barrier properties and ionic resistance of zirconia nanoparticles embedded in the epoxy coating samples. Coating resistance value may be decreased due to the diffusion of water and movement of ionic species in coating film which causes to increase coating conductivity. The electrolyte diffuses in the coating film, arranges conducting paths at various depths through the coating film [48]. The dielectric constants of polymeric coatings and water are about 4–8 and 80, respectively, at ambient temperature [43,44]. Consequently, a small amount of water uptake within the coating film, can donate to a relatively large change in the coating capacitance. It is clear that penetration of electrolyte and ions through the nanosized zirconia containing epoxy coated samples is much lower than the neat epoxy coatings. Since coating capacitance is directly affected by ingress of electrolyte and ions into coating pores, improvement of coating characteristics such as ionic resistance and crosslink density has dramatically decreased the coating capacitance [44]. Increase of nanoparticle content causes to increase corrosion resistance based on EIS findings due to higher coating resistance in the immersion time period. Increase corrosion resistance for ZrO2 nanoparticles/epoxy coating, could be attributed to the bonding between OH groups on the surface of ZrO2 nanoparticles and APS coupling agent. Thus, APS surface treatment is appropriate for ZrO2 nanoparticles, which is abundant in active hydroxyls [9].

(a) 10000 EZ-3M

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capacitive arc, which implies that the coating acts as an intact capacitor prohibiting permeation of corrosive species such as water, oxygen, and other ions towards the surface of the metal substrate. After 30–60 days immersion, the capacitive arc changes as a semicircle, and the resistance of the coating is detected. At the same time, there is an arc appearing at the end of the semicircle, which implies that there are two time constants, suggesting reactions on the metal surface are progressing. After 90 days, a short line at the end of the semicircle, transformed to a small arc, suggesting the corrosion of the substrate is occurring. EIS spectra of the coating containing 2 or 3 wt% nanoparticles (Fig. 7c and d), the reveal capacitive behaviour at high frequencies and coating resistance in the low frequency region with a resistive component greater than 14 109 X cm2 and 20 109 X cm2, respectively. The coating resistance decreased to some extent with increasing exposure time, and reached a constant value after 60 and 90 days for coatings containing 2 or 3 wt% nanoparticles, respectively. The impedance values recorded for these coatings are clearly higher than impedance data obtained for 0 and 1 wt% containing nanoparticles epoxy coatings, indicating the barrier properties and high ohmic resistance of ZrO2 nanoparticles [47]. For the systems included with 2 or 3 wt% ZrO2 nanoparticles, it was clearly seen that only one apparent time constant was observed for 120 days of immersion as shown in Fig. 6c and d. It was characterized by a single capacitive loop representative of resistance of coating. By contrast with clear epoxy coating, although, the change trend of resistance values decreased with the immersion time, but the resistance values were much higher than those of clear epoxy coating. The relatively high resistances of the coating film and its stability over the long immersion period confirm the barrier protection. This indicated effectiveness of ZrO2 nanoparticles for improving barrier properties of coating layer. Corrosion resistance of treated ZrO2 nanoparticles embedded epoxy coating was significantly improved in comparison with clear coating. The results show that the dispersed and agglomerated ZrO2 nanoparticles counteracted the barrier effect. Analyses of Nyquist plots suggest that different equivalent circuit models are required to fit the results (Fig. 8), which represent initial times of corrosion process, water saturated epoxy coating on the mild steel substrate, and finally accumulation of corrosion products at metal/coating interface (diffusion process). These models reveal electrolyte resistance, Rs, coating capacitance, Cc, coating resistance, Rpf, charge transfer resistance, Rct, double layer capacitance, Cdl, and Warburg resistance, Zw.

60 80 Time (Days)

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CC RS

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Rct ZW Fig. 8. Equivalent electrical circuits.

Fig. 9. (a) Time dependence of coating resistance and (b) time dependence of coating capacitance, for epoxy coating containing various levels of ZrO2 nanoparticles on decreased mild steel during 120 days immersion in 3.5% NaCl solution.

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3.5.2. Electrochemical noise analysis In order to verify the results obtained from EIS, electrochemical noise analysis was carried out. Fig. 10 shows typical noise data recorded in time domain over different immersion times for neat and 3 wt% zirconia nanocomposite coating. As, it can be seen the potential noise values are more noble at the beginning of the exposure and decreases with the increasing of immersion time. Furthermore, the noise data are nobler for 3 wt% nanoparticles containing sample even after 30 days of immersion. The amplitude of potential noise is decreasing with the time as well. It can be seen that signal fluctuations are increased by increasing exposure time for neat epoxy coating [49]. Noise resistance, Rn, is one of the most commonly used for the interpretation of electrochemical noise data. In several cases, Rn is equivalent to the polarization resistance, Rp, measured with linear polarization; therefore, it is proportionally inversed to the corrosion rate. Rn can be estimated by dividing the standard deviation of the potential by the standard deviation of the current [50]:

-0.5875

Rn ¼

rE rI

The Rn values obtained from ECN measurements for various coating specimens are shown in Fig. 11. As, it can be seen, with increasing immersion time (7 days), the resistance values decreased rapidly, due to uptake of the test solution and movement of ionic species through the coating layer. After this stage, the values level-off to greater than 1 106 X cm2 and 1 108 X cm2 for neat epoxy and 3 wt% nanoparticles containing coatings, respectively, indicating nanoparticles containing coatings affording good corrosion protection. Rn values are significantly increased with the increasing level of zirconia nanoparticles in coating matrix, due to the higher barrier properties and ionic resistance of zirconia nanoparticles embedded in the coating specimens. Being consistent with EIS data, trend and level of Rn values confirmed that the epoxy coatings containing modified zirconia nanoparticles provided enhanced corrosion protection compared with neat epoxy coating. These findings clearly support EIS results.

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(b) Epoxy coating containing 3 wt % nanoparticles Fig. 10. Typical electrochemical noise obtained data during different exposure times.

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M. Behzadnasab et al. / Corrosion Science 53 (2011) 89–98 10000 NE

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Rn (MOhm.cm )

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ZrO2 nanoparticles incorporated nanoparticles. The visual observation of the coatings containing different amounts of ZrO2 nanoparticles after 2000 h exposure in salt fog is shown in Fig. 12. Serious rusting appeared along the surface for neat epoxy coating with diameter of rusts more than 5 mm was observed after salt spray for 2000 h. However, no apparent rusting along the surface was observed on the coatings including 2 or 3 wt% ZrO2 nanoparticles. However, small a few rusts with diameter of 1–2 mm could be observed for 1 wt% ZrO2 nanoparticles incorporated epoxy resin. A high concentration of nanoparticles leads to improved barrier properties and, therefore, to an improved protection against general corrosion. The salt spray results coincided with the impedance spectra, implying that the addition of APS-treated ZrO2 nanoparticles can effectively improve corrosion performance via increasing barrier properties of the epoxy coatings. 4. Conclusions

3.5.3. Salt spray test results The corrosion resistance of various epoxy coating specimens was also evaluated by the rusts and blistering along the coating’s surface on the mild steel substrate. For the coated specimen without ZrO2 nanoparticles, corrosion was visible to the naked eye after exposure for 72 h. Corrosion of the coating containing 1 wt% ZrO2 nanoparticles was detected after 480 h and, for the coating with 2 or 3 wt% ZrO2 nanoparticles, corrosion was not evident even after 2000 h. The best performance in the salt spray test, with respect to general corrosion, was displayed for the coating with 2 or 3 wt%

This work describes an effective approach to improve corrosion resistance of an epoxy-based coating with employing APS-treated ZrO2 nanoparticles. APS coupling agent employed to improve the dispersing of ZrO2 nanoparticles in epoxy coatings, and increasing possible chemical interactions of APS functional groups on nanoparticle and polymeric matrix. By incorporation of treated ZrO2 nanoparticles, the corrosion rate is reduced significantly compared with the neat epoxy. EIS and ECN results revealed that 2–3 wt% incorporation of ZrO2 nanoparticles considerably improved the corrosion resistance of

Fig. 12. Surface appearance of the epoxy coated specimens with different APS-treated ZrO2 nanoparticle contents after salt spray testing for 2000 h: (a) without nanoparticles; (b) 1 wt% ZrO2; (c) 2 wt% ZrO2; (d) 3 wt% ZrO2.

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