carbon steel interfacial delamination behavior: Electrochemical impedance and X-ray spectroscopic analyses

Corrosion Science 102 (2016) 326–337

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

Demonstration of epoxy/carbon steel interfacial delamination behavior: Electrochemical impedance and X-ray spectroscopic analyses Mehdi Ghaffari a,∗ , Mohammad Reza Saeb b , B. Ramezanzadeh c , Peyman Taheri d a

Polymer Group, Golestan University, P.O. Box 155, Gorgan, Golestan, Iran Department of Resins and Additives, Institute for Color Science and Technology, P.O. Box 16765-654, Tehran, Iran c Department of Surface Coatings and Corrosion, Institute for Color Science and Technology, P.O. Box 16765-654, Tehran, Iran d Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720, United States b

a r t i c l e

i n f o

Article history: Received 16 June 2015 Received in revised form 16 October 2015 Accepted 17 October 2015 Available online 21 October 2015 Keywords: A. Organic coatings B. XPS B. EIS C. Interfaces

a b s t r a c t In this work, we introduce an efficient methodology to assess the delamination mechanism of epoxy coating from steel substrate combining spectroscopic and analytical techniques. To reflect the competence of this approach, different samples were prepared by applying epoxy/polyaminoamide layers on untreated, acid and alkaline pretreated substrates. The results obtained from XPS and energy dispersive X-ray analyses describe delamination of the coatings on differently pretreated surfaces during the pulloff test. EIS measurements demonstrated that acid treatment improves the resistance against cathodic delamination, while XPS analysis provided useful data about coating delamination from the substrate. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction For many years, it was, and still is, the concern of engineers to protect metal surfaces against corrosive media using organic coatings [1,2]. The protection efficiency of a metal/coating system can be regulated by manipulating the chemical composition and constitution of the organic coatings [3,4], and surface characteristics of the metal substrate affecting the coating adhesion to the metal surfaces [5,6]. Among various organic coatings, epoxy-based coatings provide adequate chemical, water, solvent and abrasion resistance for industrial, marine, and packaging applications [7–11]. In addition, epoxy/polyaminoamide (PAA) systems are generally suggested for steel protection owing to their desirable adhesive and barrier properties [12–15]. However, the epoxy coating suffers from a degradation process after exposure to the corrosive electrolyte leading to the decrease of coating barrier properties as well as creating a path to the underlying surface [16,17]. As a consequence, corrosive electrolyte reaches the coating/metal interface and corrosion starts

∗ Corresponding author. Fax: +98 1732430516. E-mail address: [email protected] (M. Ghaffari). http://dx.doi.org/10.1016/j.corsci.2015.10.024 0010-938X/© 2015 Elsevier Ltd. All rights reserved.

at anodic and cathodic sites leading to hydroxyl (OH− ) creation (2H2 O + O2 + 4e− → 4OH− ) underneath the coating. The abundance of pH breaks interfacial bonds followed by coating delamination from the substrate [18]. The interfacial bonding properties between the coating and metal substrate are important factors determining the extent of corrosion resistance. It is well-documented that the acid–base properties of the metal surface alter the adhesion of organic compounds and coatings [19–25]. Principally, different kinds of intermolecular bonding can be involved in the adhesion including dispersion forces, acid-base interactions, covalent bonding and electrostatic contributions. Covalent bonding and acid–base interactions impose more effects on the adhesion strength among the above-mentioned parameters [26–28]. To capture the effect of chemistry of the steel/epoxy interfaces, it is of prime importance to reach the metal/polymer interface. Nevertheless, this is indeed hard to reach the interface through the conventional analytical techniques due to the relatively high thickness of polymer coatings. To evaluate the metal and polymer interfaces, ATR-FTIR in Kretschmann geometry can be used in which the thin metal film is subsequently coated with a polymer film. Infrared beam passing through the thin metal layer provides the opportunity to probe the changes of interfacial bondings insitu [29,30]. Some other methods have been employed to access the

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interface, e.g., application of Ar+ ion beam to sputter the coating [31] or breakage of the metal/coating samples in liquid nitrogen [32,33]. However, the aforementioned methods may damage the interfacial bonding due to the destructive nature of techniques utilized. It has been shown that adhesion of organic coatings to the metals is a important characteristic of the coatings to retard corrosion. Therefore, significant efforts have been conducted to investigate the mechanisms involved in polymer–metal adhesion and to develop methods to study adhesion. There are large numbers of experimental methods to determine the coating adhesion indicating the importance of this research field. Pull-off test can provide information about the interaction strength between the coating and metal [34–36]. Nonetheless, the nature of bonding, i.e., the contribution of chemical, mechanical or physical bonding to delamination prevention under corrosive solution exposure cannot be well detected and/or described based on pull-off measurements [37]. As an alternative to the traditional approaches, electrochemical impedance spectroscopy (EIS) has been utilized for different cases resulted in promising and reliable results. In a work done by Deflorian and Fedrizzi [39], it has been shown that EIS provides valuable information on the delamination of organic coatings on aluminium, galvanized steel, and stainless steel substrates with different pretreatments. Attar et al. characterized the degradation and delamination processes of polyester powder coating on the steel panels with different pretreatments by EIS technique [40]. Dietmar Schachinger et al. [41] showed that EIS is an appropriate technique for blister growth kinetics evaluation of polymer coated galvanized steel sheet in alkaline solutions. Khun and Frankel [42] studied the effects of surface roughness, texture and polymer degradation on cathodic delamination of epoxy coating from steel substrate by scanning kelvin probe (SKP) technique. The effect of each mentioned parameter on the coating delamination was clearly investigated by SKP technique. Dong and Zhou [43] investigated the failure behavior of epoxy coatings in 5 wt.% KCl solutions. They could find out the relationship between the ion transport and the failure of the coating by EIS analysis. EIS has been also extensively used to evaluate the extent of organic coating delamination when exposed to corrosive electrolyte [44–47]. Various parameters including the breakpoint frequency (fb ), the frequency of the phaseangle minimum (fmin ) at high frequencies (min ), and the ratio of the impedance recorded at two frequencies can be extracted from impedance data for prediction of coating delamination. The authors of this paper utilized XPS technique to obtain information about the nature of interactions between coating and metal substrate, thereby coating delamination during EIS experiment would be well described. Moreover, we disclose the idea of combining XPS and EIS results to obtain a comprehensive picture of delamination at the vicinity of epoxy/carbon steel (CS) interface. The electrochemical and adhesion performances are correlated to the surface compositional and morphological aspects.

2. Experimental 2.1. Materials and sample preparation The epoxy resin used in this study was Epikote1001-X75 provided by Hexion Specialty Chemicals Co., Netherlands. Epikote1001-X-75 consists of diglycidyl ether of bisphenol-A (DGEBA) with an epoxide equivalent weight of 450–500 g equiv−1 . The resin was cured with a liquid polyaminoamide resin, Crayamid 115, supplied by Cray Valley Co., UK. The substrate used in this work was carbon steel supplied by Qpanel Company (composition wt.%: C: 0.13, P: 0.04, Mn: 0.5, S: 0.05 and Fe: balance). The samples were mechanically abraded with SiC-

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paper (80, 120, 220, 500, 800, 1200, 4000 ␮m grid) in subsequence steps to obtain a mirror-like appearance. Subsequently, the samples were cleaned ultrasonically in ethanol and thoroughly rinsed for 10 s using deionized water and blown dry using compressed clean air. Prior to the application of the epoxy coating, the carbon steel substrates were treated using alkaline and acidic solution. Borate buffer solution containing 0.15 M Na2 B4 O7 ·10H2 O + 0.6 M H3 BO3 was used for the alkaline treatment. In this case, the samples were immersed in the solution for 30 min at room temperature (25 ± 5 ◦ C). Subsequently, they were thoroughly rinsed with deionized water for 10 s to remove the remaining treatment solutions from the surface. Another set of carbon steel samples was treated in acidic solution containing 0.1 M HCl in the same conditions as described for the alkaline treatment. A thin layer (10 ± 2 ␮m) of epoxy/polyaminoamide coating was applied on the set of differently treated samples using spin coater. The coated substrates were cured at 70 ◦ C for 6 h. In a previous work, it was shown that this curing procedure results in a well-cured epoxy coating [48,49].

2.2. Characterization 2.2.1. Surface studies An atomic force microscope (AFM), Asylum Research MFP-3D AFM; was used to study the surface roughness at room temperature in tapping mode (TM). To characterize the surface composition, XPS analysis was conducted with a PHI 1600/3057 instrument using Mg K␣ incident X-ray radiation (energy = 1253.6 eV). The vacuum pressure was approximately 5 × 10−9 Torr. Narrow multiplex scans were recorded with a 29.35 eV pass energy and a 0.1 eV step size. C1s and O1s peak fittings were carried out using the PHI Multipak V8.0 software. A constrained fitting procedure was used in which the mixed Gauss–Lorentz shapes for the different fit components in the peaks were allowed to change in the 80–100% region. Only small variations in peak position and full widths at half-maximum (FWHM) were permitted. The 2 value of the evaluated peaks after fitting was lower than 1.5 for all of the measurements.

2.2.2. Adhesion properties measurements Pull-off measurements were performed on the epoxy/polyaminoamide coatings applied on carbon steel with an Instron 1122 testing machine operating at a crosshead speed of 0.05 mm/min. The size of coating specimens and dollies was 80 mm × 80 mm × 2 mm and 20 mm, respectively. The experimental setup allowed the alignment of the jaws to impede any bending of the substrate during the test. The measurements were performed at the air-conditioned room (45% RH and 21 ◦ C).

2.2.3. Electrochemical measurements An artificial hole of 1 cm in diameter and 10 ␮m in depth was made on the coatings by an electrical drill in order to promote the cathodic disbonding during the electrochemical impedance spectroscopy (EIS) measurements. The impedance measurements were carried out using Autolab PGSTAT 30 at open circuit potential (OCP) in the frequency range of 10 mHz to 10 kHz with 15 mV perturbation. The experiments were performed in a conventional three electrode cell where the coated sample, Ag/AgCl (saturated KCl) and platinum grid were the working, reference and counter electrodes, respectively. The electrochemical measurements were performed on 9 cm2 working electrode including an artificial hole exposed in 3.5 wt.% NaCl solution. The results obtained through the EIS measurements were analyzed using Z-view software. Three parallel measurements were carried out for each coating system to obtain the repeatability of the experiments.

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Fig. 1. AFM micrographs from the surface of (a) untreated CS, (b) alkaline treated and (c) acid treated sample.

3. Results and discussion 3.1. Surface characterization of alkaline and acid treated samples AFM measurements are conducted on the untreated and chemical treated surfaces to investigate the morphological aspects of the carbon steel (CS) surface. Figs. 1 and 2 show AFM micrographs and root-mean-square (RMS) roughness values of the carbon steel surfaces. It can be seen that the surface of untreated sample is approximately smooth covered with an oxide layer. The surface treatment of the carbon steel substrate in the borate buffer solution leads to a decrease of roughness compared to the untreated sample indicating the oxide layer removal in the alkaline medium. However, the surface treatment of carbon steel caused a significant increase of surface roughness. The increase in the surface roughness in the acid solution can be attributed to the growth of oxide layer in the acid medium as well as the formation of deep and partly close

pits due to the Cl− attack during metal dissolution process [50]. The oxide formation and removal upon exposures to aqueous solutions can be correlated to low and high pH values of the acid and borate buffer solutions, respectively. According to the Pourbaix [51] diagram, low pH values stabilize oxide layer on iron surfaces, while high pH values lead to oxide removal and hydroxyl formation. 3.2. Pull-off studies The adhesion strength of the epoxy/polyaminoamide coatings on the untreated and treated carbon steel samples was determined through pull-off procedure. Fig. 3 shows the pull-off strength obtained for the samples. It can be seen that the adhesion strength values of the both treated samples are higher than that of the untreated one. Additionally, the acid treatment results in a higher adhesion strength than that of treated in the alkaline solution. This high adhesion promotion obtained for this sample may be

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80

RMS Roughness (nm)

60

40

20

0 Untreated sample Untreated

AcidAlkaline treated Treated sample

Alkaline treated sample Acid treated

Fig. 2. RMS roughness of carbon steel before and after the treatments.

sity. On the other hand, the most significant element detected on the carbon steel surface is Fe. This confirms that the coating detachment after pull-off test is mostly in the form of adhesive failure. The EDX analyses confirm the absence of bulk coatings on the carbon steel surfaces after pull-off. At the same time, the EDX results show lower Fe peak intensity of the acid treated sample than others indicating more coating residue on the metal surface which is attributed to the greater coating adhesion on this sample.

20

Pull off strength (MPa)

15

10

3.3. XPS analysis 5

0 Untreated sample

Alkaline treated

Acid treated sample

Fig. 3. Pull-off adhesion strength of the coating of the untreated and differently treated samples.

attributed to two main reasons. First, surface treatment in acidic solution resulted in the increase of surface roughness providing higher surface area for the coating interaction to the steel surface and mechanical interlocking. Second, the surface compositions of the steel surface vary after treatment affecting the surface free energy and therefore the wettability of the surface. These two possible mechanisms are expected to affect the adhesion of polymer coatings on metal surface [52–54]. These require further characterizations which will be disused later. 3.2.1. Morphology of coating failure after pull-off test The energy dispersive X-ray (EDX) analysis was carried out on the carbon steel surfaces before and after the polymer coating removal through pull-off test. This enables a better understanding of the coating detachment mechanism from the steel substrate during pull-off experiment. Fig. 4 presents the SEM spectra of different samples after pull-off test for carbon steel side and epoxy side of samples. Micrographs shown in Fig. 4 clearly demonstrate a significant roughness irrespective of treatment mode. It is also apparent that failure mechanism is deadhesion in regard to inhomogeneous surface roughness after pull-off test, particularly in case of alkaline treated specimen. EDX Analyses suggest that the steel surface coated with epoxy/polyaminoamide consists of Fe, O, N and C, where C appears to be the main element with the highest inten-

The chemical composition of the carbon steel treated samples was studied by XPS analysis. Fig. 5 shows typical C1s and O1s peak fittings of the untreated sample. Upon the exposure of the carbon steel samples in ambient air, carbonates are formed on the oxide surface, giving rise to the “carbonaceous contamination” [55–57]. Consequently, C1s signal (Fig. 3a) resolved into four different components, i.e., C C/C H, C COOX, C O and O C O/O C O species located around 283.75 eV, 284.75 eV, 286.06 eV and 288.4 eV respectively [57–59]. Table 1 shows the binding energy parameters corresponding to the subpeaks obtained through the fitting procedure. O1s peak (Fig. 5b) is deconvoluted into three individual subpeaks, corresponding to O2− , OH− , and H2 O components. A detailed overview of the fitting results of O1s peak for the different samples is listed in Table 2. The contributions of the metal oxides, hydrated films, and water molecules bonded to the surface are roughly located at 529.4 eV, 531.06 eV and 532.4 eV, respectively. The results are in good agreement with other works indicating 1–1.5 eV difference in bonding energy of OH− and O2− [50,55]. To calculate the surface hydroxyl, it should be noted that most of the oxygen functional groups such as C O and O C O species in polymers give O1s binding energies around 532 eV or approximately at the same location of the OH− peak in the O1s photopeak. As a results, to obtain a reliable hydroxyl content, it is necessary to correct the O1s photopeak form the contributions of C O and O C O species. The correction of the hydroxyl fraction in this study is conducted according to the procedure described by Wielant et al. [55]. Fig. 6 shows the hydroxyl fractions of the untreated and differently treated carbon steel samples obtained from O1s peak deconvolutions (Fig. 5b). It can be observed that hydroxyl fraction increases after the alkaline treatment while it dramatically reduces due to the acid treatment. There are still ongoing studies concerning the structure of the oxide layer formed in alkaline

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Fig. 4. SEM micrographs from the surface of (a1 ) untreated CS, (b1 ) alkaline treated and (c1 ) acid treated samples, and SEM micrographs after pull-off test from the epoxy side (a2 ) untreated CS, (b2 ) alkaline treated and (c2 ) acid treated samples.

solutions. The studies suggested that oxide layers created in alkaline solution mainly consist of a ␣-Fe2 O3 or Fe3 O4 covered by a hydroxide deposition layer [58,59]. Consequently, the treatment in the borate alkaline solution is expected to result in the formation of a thin hematite and magnetite film covered by a hydroxide-rich surface film [60], whereas the outer scale of acid oxides consists of thicker hematite and magnetite films covered by thin layer of hydroxides. The decrease of hydroxyl content after acid treatment confirms that the procedure successfully removed the surface oxide layer. The correction of the hydroxyl fraction was done according the procedure described by Wielant et al. [55]. The above findings declare that the surface hydroxyl fraction has no effect on the adsorbed coating and adhesion strength. Similar results have been observed by Wielant et al. [55].

In order to investigate, the effects of surface treatment on the interfacial bonding properties, the metal and coating surfaces obtained after the pull-off test were examined using XPS. Cls peak of the steel side surface is deconvoluted to four subpeaks, i.e., C C/C H, C C O/C N, C O and O C O/N C O species located around 284.26 eV, 285.55 eV, 287.35 eV and 289.07 eV, respectively. The presence of these peaks is expected to be originated from the interfacial remained bondings after the pull-off test, the amount of which represents the interfacial stability. As a results, any change in the intensity of these peaks on steel side surface after the pulloff test can be associated with the interaction of amine and amide groups of coating with surface oxide. Table 3 shows the C1s peak deconvolution collected from the surface of bare metal and those after pulling-off the coatings. C1s

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35

30

OH fraction (%)

25

20

15

10

5

0 Untreated Unt reated

Alkali ne Alkaline treated

Acitreated dic Acid

Fig. 6. Hydroxyl fractions of the untreated and differently treated carbon steel samples.

Fig. 5. Typical fittings of (a) C1s and (b) O1s peaks of carbon steel samples.

peak fitting exhibits an increase in the C C O/C N peak intensity of untreated and alkaline treated sample, while this value reduces for the sample treated in the acid solution. This shows a higher interaction level of amine groups originating from the curing agent of the coating with surface oxides of the untreated and alkaline treated samples. It was shown in Fig. 6 that the hydroxyl fractions of the untreated and alkaline treated samples are higher than that of acid treated one. Therefore, it can be concluded that Brønsted-like interactions between hydroxyl hydrogens and amine nitrogenous (nitrogen protonation) take place. Table 3 shows that C O peak intensities of both of the treated samples increase compared to the untreated one that may be correlated to an increase in the amount of the remained epoxy (C O) on

the carbon steel surfaces. Moreover, the results show that the intensity of the O C O/N C O peak decreases for both of the untreated and alkaline treated samples, while this value increases for the acid treated sample. The presence of O C O/N C O components can be due to the amide group of the curing agent. Therefore, it can be inferred that the acid treated steel adsorbs more amide than alkaline and untreated samples do. Since, Brønsted-like acid–base interaction depends to the metal surface hydroxyl fraction, Lewislike acid–base interaction between basic oxygen of amide and metal cation mainly occurs between the amide group and carbon steel surface oxide. Another possibility is the occurrence of Lewis-like acid–base interaction between the nitrogen proton of the amide and iron oxide surface. In this case, hydrogen bonding can take place between the nitrogen proton of amide and oxygen atom in the proximity of the metal cation site [55]. The XPS analysis conducted on C1s peaks collected from the coatings after the pull-off test showed that this peak is composed of three components: the main C C/C H peak at 285.1 eV, C N peak at 285.5 and O C N at 287.2 eV [61]. Intensities of C1s peak deconvolution of the coating sides are summarized in Table 4. It can be seen that the intensity of N C O peak in coating side of acid

Table 1 Peak deconvolution of C1s peaks corresponding to C C/C H, C COOX, C O and O C O/O C O samples. Sample

Untreated CS CS in borate buffer CS in HCl solution

C C/C H

C COOX

components of the untreated and differently treated carbon steel

C O

O C O/O C O

BE (eV)

FWHM (eV)

BE (eV)

FWHM (eV)

BE (eV)

FWHM (eV)

BE (eV)

FWHM (eV)

283.75 284.80 283.75

2.20 1.29 1.74

284.75 285.31 284.58

1.42 2.00 1.47

286.06 286.63 285.88

1.88 1.70 2.20

288.4 288.31 288.34

1.79 1.65 1.45

Table 2 Peak deconvolution of O1s peaks corresponding to O2− , OH− and H2 O components of the untreated and differently treated CS samples. Sample

Untreated CS CS in borate buffer CS in HCl solution

OH−

O2−

H2 O

BE (eV)

FWHM (eV)

BE (eV)

FWHM (eV)

BE (eV)

FWHM (eV)

529.43 530.01 529.59

1.60 1.46 1.58

531.06 532.03 531.23

1.67 2.20 1.70

532.48 533.16 532.71

1.55 2.00 1.42

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Table 3 Cls, XPS subpeak intensitiesa obtained from the peak deconvolution of the carbon steel (CS) side surfaces. Treatment

C C/C H (284.26 eV)

C C O/C N (285.55 eV)

C O (287.35 eV)

O C O/N C O (289.07 eV)

Untreated CS 1 Untreated CS 2 CS-alkaline treated 1 CS-alkaline treated 2 CS-acid treated 1 CS-acid treated 2

57.69 50.68 72.08 12.16 4.25 3.28

5.40 37.68 9.94 57.71 72.76 60.71

26.01 10.94 3.75 19.92 18.64 27.32

10.83 0.69 14.23 10.21 4.36 8.69

1: Before applying coating; 2: After pulling-off the coatings. a Area under the peak.

Table 4 Cls, XPS subpeak intensitiesa obtained from the peak deconvolution of the coating sides. Treatment

C C/C H (285.1 eV)

C N (285.5 eV)

O C N (287.2 eV)

Coating-untreated CS Coating-alkaline treated CS Coating-acid treated CS

24.81 27.22 22.51

61.41 54.44 68.62

13.78 18.34 8.44

a

Area under the peak.

Table 5 Ols, XPS subpeak intensitiesa obtained from the peak deconvolution of the steel side surfaces. Treatment

O2− (529.43 eV)

OH− (531.20 eV)

¨ H2 align=¨leftO (532.48 ¨ align=¨lefteV)

Untreated CS 1 Untreated CS 2 CS-alkaline treated 1 CS-alkaline treated 2 CS-acid treated 1 CS-acid treated 2

43.19 32.38 57.47 1.88 60.07 19.40

41.77 50.06 33.17 65.92 34.82 21.50

15.04 17.55 9.37 32.20 5.11 59.10

1: Before applying coating; 2: After coating separated by pull-off. a Area under the peak.

treated is less than those of the rest. This confirms the presence of less amide group on the coating surface pulling off the acid treated sample and more adsorbed amide group by the surface of the acid treated sample as noted before. O C O/N C O peak intensity of the steel side acid treated sample less than that of alkaline treated and untreated samples while it was expected to be vice versa. It can be associated with the presence of a higher O C O/N C O peak intensity in alkaline treated steel (14.23) compared to the acid treated one (4.36) before application of the coating as shown in Table 3. Additionally, it is probable that the applied coating removed a part of the initial contamination presented on sample and replaced the amide molecules during bonding [62,63]. Table 5 shows the subpeak intensity of O1s of the bare carbon steel surfaces and those of the pulled-off coatings. It is known that the hydroxyl group is one of the main products of epoxy curing process using polyaminoamide [55,64]. Thus, the changes in the intensity of OH− in steel side can be associated to the cross-linking density of the coating. Table 5 shows that the OH− peak intensities increase for the untreated and alkaline treated samples. However, it decreases for acid treatment. Therefore, it can be concluded that the coating surface obtained from the untreated and alkaline treated samples possess a higher cross-linking density compared to the acid treated one. These could be attributed to the occurrence of a higher interaction of amine and amide curing agents with acid treated carbon steel in comparison to the untreated and alkaline treated samples. As shown the intensity of amide group reduces on

the coating surface so that it can be concluded that a reduction in crosslink density on the surface coating of acid treated is associated to the reduction of amide group on the surface. Moreover, it was reported that the amide oxygen contributes to the high energy part of O1s peak, which can be due to the fact that these amide oxygen atoms attach to the oxide surface through a hydrogen bond [50,65]. The sharp increase in H2 O peak intensity of the sample treated in the acid solution (Table 5) verifies a higher interaction level of the coating amide groups with surface oxides. These prove the C1s results (Table 3) regarding the higher interaction of amid group with the surface oxide of the acid treated sample. 3.4. EIS measurements Electrochemical impedance spectroscopy is a rapid and convenient way to investigate the organic coating delamination as well as corrosion products development at the metal/coating interface during exposure to a corroding solution like 3.5 wt.% NaCl solution. The electrolyte diffusion through the coating porosities and/or defects leads to the metal substrate oxidation at anodic sites and oxygen reduction at cathods. The increase of pH at cathodic regions is responsible for the cathodic delamination of the coating from the steel substrate. Also, the corrosion products accumulation results in the coating delamination progress causing coating blistering. Therefore, the EIS measurements are conducted on the untreated and differently treated samples with artificial defects where the OCP was held potentiostatically through the course of a single frequency scan. The OCP changes for various samples as a function of immersion time are depicted in Fig. 7. According to Fig. 7, a descending trend can be seen for the OCPAg/AgCl(saturatedKCl) versus immerstion time for all samples. More cathodic OCPAg/AgCl values are detected for the untreated sample. The acid treated one shows the most anodic values among the samples. At each immersion time the OCPAg/AgCl was held potentiostatically and the EIS measurements were conducted at constant potential. Figs. 8 and 9 depict typical Bode and Nyquist diagrams obtained from the EIS measurements of the untreated, alkaline and acid treated samples after 3 days immersion in 3.5 wt.% NaCl solution. Results show just one relaxation time in the spectra of the untreated and alkaline treated samples. This may indicate that the electrochemical processes on these samples are mainly under control of charge transfer process. This observation obviously shows that there is no remained coating on the carbon steel substrate after artificial defect for the untreated and alkaline treated samples which is in agreement with EDX and XPS results. However, two relaxation times are observed in the Bode phase diagram of the acid treated sample. These two relaxation times are attributed to the charge transfer resistance (Rct ) at high frequency region and the remained (undetached coating) coating film (Rf ) on the steel surface at artificial defect. The same results are obtained from the EDX and XPS experiments confirming good adhesion of the epoxy coating to the acid treated sample. Looking at Bode modulus diagrams it can be

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-0.35 Untreated sample Alkaline treated sample

-0.4

Acid treated sample -0.45

OCP Ag/AgCl (V)

-0.5 -0.55 -0.6

-0.65 -0.7 -0.75 0

10

20

30

40

50

60

70

80

Immersion me (h) Fig. 7. Variations of OCP versus immersions times for different samples during EIS measurements.

3500

3000

Untreated

500

Alkaline treated

400

Acid treated 300 2500

-Z''(ohm cm2)

200 0.01 Hz

2000

100 0.01 Hz

0

1500

0

1000

100

200

300

500

0.12 Hz

0.69 Hz

0.29 Hz

500

400

0.01 Hz

1.62 Hz 0.01 Hz

0 0

500

1000

1500

2000

Z' (ohm

2500

3000

3500

cm2)

Fig. 8. Typical Nyquist plots of the coatings with 9 cm2 area and an artificial hole (1 cm in diameter and 10 ␮m in depth) subjected to 3.5 wt.% NaCl solution for 3 days.

seen that the coating applied on the acid treated sample reveals a higher impedance value at low frequency limit (10 mHz) than that of the untreated and the one treated in alkaline solution. The ionic mobility in an interlayer region depends on the value of the attractive forces between the layers and the untreated carbon steel. A strong field of high attractive force between the layers leads to a higher adhesion strength and a lower ionic mobility. Consequently,

it can be assumed that the low impedance values correspond to a higher disbonded surface area [65,66]. The impedance data were fitted by the suitable equivalent circuits shown in Fig. 10. where Rs , Rct , Rf , CPEdl , and CPEf are the solution resistance, charge transfer resistance, film resistance, constant phase element of double layer and constant phase element of the film, respec-

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Fig. 9. Typical (a) bode modulus and (b) bode phase diagrams of the coatings with 9 cm2 area and an artificial hole (1 cm in diameter and 10 ␮m in depth) subjected to 3.5 wt.% NaCl solution for 3 days.

tively. Double layer capacitance (Cdl ) values were obtained from Eq. (1) [67]. C dl = (Q dl × Rct 1−n )1/n

(1)

where Cdl , Qdl , Rct and n show double layer capacitance, admittance constant, charge transfer resistance and the empirical exponent, respectively. The polarization resistance, which is the sum of Rct and Rf , and capacitance values extracted from impedance data fitting within different test periods are presented in Fig. 11. The values altered continuously as time elapsed, which proves the hypothesis that a growth in the delaminated area may result in changes of resistance and capacitance values. The increase in the capacitance with exposure time may imply that water and oxygen diffuse beneath

the coating across the artificial hole resulting in the growth of disbonded area [45,68]. From the results it can be seen that, the acid treated carbon steel sample exhibits lower increase of Cdl than untreated and alkaline treated samples indicating a smaller disbonded area, while the untreated sample possesses higher increase of Cdl showing a larger disbonded area. Fig. 11 also shows that the acid treated carbon steel possesses the highest polarization resistance (Rp ) among the studied samples. According to the pull-off and XPS results, a stronger interaction between the acid treated carbon steel with epoxy/polyaminoamide coating is expected compared to those of untreated and alkaline treated samples. Hence, acid treated carbon steel with epoxy coating on top is likely to have a limited active zone leading to the significant increase of the charge transfer resistance. One should

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Fig. 10. Electrochemical equivalent circuits proposed to model impedance data; (a) steel sample treated in alkaline solution and (b) steel sample treated in the acid solution.

notice that the principle role of the adhesive bonds lies in reducing the exposed metal surface and active sites available for electrochemical reaction. The lower disbonded area detected for the acid treated carbon steel can be attributed to the interaction of the carbon steel with amide and amine groups. Basically, adhesion strength is taken into account as a key parameter to probe the occurrence of the cathodic disbondment. From Fig. 9 it is clear that the maximum phase angle shifted to lower frequencies region after acid treatment compared to the untreated and alkaline treated samples. These demonstrated higher disbonded area of the untreated and alkaline treated samples when exposed to corrosive environment. Moreover, the sudden drop of the phase angle is correlated to a growing tendency of AC current to pass through the resistor in the circuit. This means that the system exhibiting a higher resistance is characterized by the higher phase angle. Bode plots shown in Fig. 9 illustrate phase angle values () of 8.32, 9.59 and 18.52◦ at 10 kHz for the untreated, alkaline treated and acid treated samples, respectively. In the case of the acid treated carbon steel, a higher value of phase angle at 10 kHz could reflect

the reduced disbonded area as well as the presence of undetached coating from the steel surface when producing artificial defect. Breakpoint frequency (fb ) is another useful parameter to describe the coating disbondment from the steel surface when exposed to corrosive electrolyte. There is a close relationship between the coating delamination area and breakpoint frequency. It can be seen from Fig. 8 that fb on the untrtreaed and alkaline treated samples occurred at higher frequencies indicating greater coating disbondment than acid treated sample. All of these indicated a relatively poor interaction of the epoxy coating to the untreated steel surface, and coating delamination occurred after short immersion time due to solution/substrate electrochemical reaction interface. Thereafter, the coating delamination gradually increased as immersion continued and the penetration of the medium had become more easily. The better interaction of the coating to the acid treated surface prevents the corrosive electrolyte diffusion across the coating/metal interface through artificial defect and as a result lower solution/substrate electrochemical reaction occurred at the interface.

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0.004

40000 Untreated Alkaline treated

30000

0.002

20000

0.001

10000

0

Rp (ohm cm2)

Cdl (nF cm-2)

Acid treated

0.003

0 0

10

20

30

40

50

60

70

80

Exposure time (h) Fig. 11. The Rp (Rct + Rf ) and Cdl parameters extracted from EIS data of the coatings with artificial hole subjected to 3.5 wt.% NaCl solution for 3 days.

The results obtained from EIS measurements are in good agreement with XPS and EDX results indicating good correlation between the surface characteristics and adhesion properties of the coating on the steel surface. 4. Conclusion This work investigates the interfacial bonding properties of the metal surfaces with an epoxy/polyaminoamide coating via XPS, SEM/EDX and EIS analyses. It was also found that hydroxyl fraction of the sample treated in the acid solution is less than those of treated in the alkaline solution one. However the adhesion strength does not significantly increase with an increase in surface hydroxyl fraction. The results showed that the oxide layer of the sample treated in the acid solution interacted with more amide groups than the rest of the samples, while the acid treatment reduces the disbonded area in the presence of the NaCl solution. On the other hand, AFM measurements showed a greater roughness of the sample treated in the acid solution. Therefore, it can be inferred that although the hydroxyl fraction is an important factor, the roughness plays an important role in metal/coating adhesion. Increasing the roughness can promote the metal/coating adhesion by two factor: (1) increasing the contact surface between metal and coating and (2) increasing mechanical inter-locking. Therefore the roughness plays more important role in epoxy/carbon steel adhesion. References [1] M. Behzadnasab, S.M. Mirabedini, M. Esfandeh, Corrosion protection of steel by epoxy nanocomposite coatings containing various combinations of clay and nanoparticulate zirconia, Corros. Sci. 75 (2013) 134–141. [2] W. Funke, ACS Symposium Series, Polymeric Materials for Corrosion Control, American Chemical Society, Washington, DC, 1986, p. 222. [3] X. Liu, J. Xiong, Y. Lv, Y. Zuo, Study on corrosion electrochemical behavior of several different coating systems by EIS, Prog. Org. Coat. 64 (2009) 497–503. [4] B. Ramezanzadeh, M. Khazaei, A. Rajabi, G. Heidari, D. Khazaei, Corrosion resistance and cathodic delamination of an epoxy/polyamide coating on milled steel, Corrosion 70 (1) (2014) 56–65.

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