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The mechanism of inhibition by zinc phosphate in an epoxy coating
Corrosion Science 69 (2013) 77–86
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
The mechanism of inhibition by zinc phosphate in an epoxy coating Yongsheng Hao a, Fuchun Liu a,⇑, En-Hou Han a, Saima Anjum b, Guobao Xu b a b
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 29 August 2012 Accepted 19 November 2012 Available online 29 November 2012 Keywords: A. Mild steel B. EIS B. Polarization B. XPS
a b s t r a c t Epoxy coatings containing different volume fractions of zinc phosphate have been successfully prepared and their inhibitive properties have been studied by electrochemical impedance spectroscopy (EIS) and immersion tests. The results show that zinc phosphate can improve the protection ability of epoxy coatings and its best volume fraction is 30%. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) results indicate that the presence of zinc phosphate can form an inhibiting film which is composed of the phosphating film of FePO4, Fe2O3, and FeO, as well as the shielding film of zinc phosphate on the steel surface. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Organic coatings are the most widely used methods in metal protection and they act as a physical barrier between corrosive electrolyte and the metal substrate. However, the number of pores, the ionic resistance of coating against electrolyte diffusion, and the cross-linking density of coating can affect the coating against electrolyte diffusion. Epoxy coating is used extensively in metal protection owing to its good adhesion, good acid/alkali and solvent resistance, high mechanical properties and cross-linking density. Epoxy coating can protect the metal substrate by releasing inhibiting chemicals from the pigment to form a strong passive or barrier layer that inhibits the corrosive medium contact with the metal substrate. Based on the inhibition mechanism, pigment can be categorized into two main groups of active (such as chromate and polyaniline) and barrier (such as glass flake) pigments. Though pigments containing lead and chromium have excellent inhibition properties, they are toxic and their applications are limited due to the environmental requirements. Therefore, many kinds of eco-friendly inhibition pigments have been developed to replace these toxic pigments, such as polyphosphate-based pigments [1], ion-exchanged pigments [2–4], ferrite pigments [5–8], conductive polymers [9–15], and so on. Zinc phosphate (ZP) is an eco-friendly inhibition pigment. However, there are many arguments about its inhibition mechanism. Several researchers [16–20] considered that its corrosion inhibition effect was due to phosphatization or passivation function which can form the complex compounds on the steel surface. Amo et al. [21] and Beiro et al. [22] regarded that the corrosion ⇑ Corresponding author. Tel.: +86 24 23915895; fax: +86 24 23894149. E-mail address: [email protected] (F. Liu). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.11.025
inhibition mechanism of ZP was attributed to its shielding ability. Rossenbeck et al. [23,24] regarded that ZP not only had shielding ability, but also showed the anti-cathodic delamination ability. Shao et al. [25] found that epoxy coating containing ZP had a pronounced self-healing function. Naderi and Attar [26] studied the protective performance and anti-cathodic disbonding of epoxy coating containing ZP and found that ZP had an anti-cathodic disbonding property. Perrin0 s [27] study indicated that ZP had an inhibition effect. Recently, Valcarce0 s research [28] disclosed that PO43had an inhibition effect in brass protection in tap water which was rich in chloride and carbonated ions. In contrast, Zubielewicz and Gnot [4] and Bastos et al. [29] thought that ZP did not have any positive functions for the coating. Therefore, it is very important to reveal the actual inhibition mechanism of ZP. Epoxy coating containing ZP can be used to protect the metal structures in ship, such as ballast tank, owing to its non-toxic characteristic. The temperature usually can reach 40 °C in ballast tank, indicating that the epoxy coating has to be immersed in 40 °C artificial sea water for a long term. Nevertheless, there are no relative reports in this aspect. Therefore, it is very important to study the protection ability of epoxy coating containing ZP in high temperature sea water and the inhibition mechanism of ZP in an epoxy coating. In the present work, the inhibition mechanism of ZP is evaluated by polarization curves, open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and immersion tests. 2. Experimental 2.1. Materials ZP was obtained from Wonder Technology (Wuxi) Co Ltd., China. Fig. 1 shows the XRD spectrum of ZP. The main composition
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Y. Hao et al. / Corrosion Science 69 (2013) 77–86 Table 2 The composition of different coating systems.
14000 ♦
Zn3(PO4)2• 4H2O
♦
12000
♦
Intensity (cps)
10000 8000 6000 4000 2000
♦
♦
♦
♦ ♦
♦
♦ ♦ ♦ ♦ ♦
♦♦ ♦ ♦ ♦♦
♦
♦
50
60
Coating number
ZP-10
ZP-20
ZP-30
ZP-40
NPEl-128 (g) Petroleum resin (g) Zinc phosphate(g) Bentone-27 (g) Anti-terra U (g) BYK 057 (g) BYK 320 (g) Xylene (g) 1-Butanol (g) Total mass (g)
27 13.5 42.8 1.5 0.4 0.2 0.4 12.2 2 100
18 9 55 1.8 0.4 0.1 0.2 10 5.5 100
11 5.5 59 1.8 0.4 0.1 0.2 15 7 100
7 3.5 62 1.8 0.4 0.1 0.2 15 10 100
0 10
20
30
40
70
80
90
2θ Fig. 1. XRD pattern of ZP.
of ZP discussed in this paper is Zn3(PO4)24H2O [30]. NPEL-128 epoxy resin (epoxy equivalent 180–190 g/eq) and curing agent polyamide 8115 (amine value 230–246 mgKOH/g) were obtained from Yexuya Electronic Chemistry (Kunshan) Co. Ltd., and Yuanda Chemicals Co. Ltd., China, respectively. Petroleum resin was obtained from Shandong Qibang Chemical Co. Ltd., Defoaming agent (BYK 057), flatting agent (BYK 320), and dispersion agent (Anti-terra U) were obtained from BYK company. Rheological agent (Bentone-27) was obtained from Elementis company. Xylene, 1butanol, acetone, and ethanol were obtained from Sinopharm Chemical Regent Co., Ltd. 2.2. Preparation of pigment extract Artificial sea water is prepared according to the ASTM standard D-1141 without incorporating heavy metals [31]. The composition of artificial sea water is shown in Table 1. To prepare pigment extract, 2 g ZP was stirred in 1 L of artificial sea water for 24 h, and then filtered to achieve a saturated solution. 2.3. Preparation of coating systems Petroleum resin was dissolved with xylene by a mass ratio of 1:1, subsequently blended with NPEL-128 epoxy resin by a mass ratio of 2:1. Then, ZP was added into the mixture of petroleum resin and NPEL-128 epoxy resin. The volume fractions of ZP in epoxy coatings were 10%, 20%, 30%, 40% and they were named ZP-10, ZP20, ZP-30 and ZP-40, respectively (Table 2). All the compositions were added into an agate jar according to the required proportion, then the mixture was dispersed using a ball-milling machine for 8 h in order to get the paint matrix. The solvent mixture of xylene and 1-butanol was used to adjust the viscosity to wet the pigment particles completely by epoxy resin. A pure epoxy coating was used as a reference and it was named ZP-0. Q235 mild steel panels (50 mm 50 mm 3 mm) were used as working electrodes for electrochemical impedance spectroscopy (EIS) measurements. The sample surface was successively abraded with 150, 240, 400, and 800 grit emery papers, and then cleaned thoroughly with deionized water and acetone, respectively. Finally,
the samples were dried by cold air. All the paints were coated twice on the steel surface by air spraying in order to avoid pinholes and get a dense coating. The overcoating interval was 24 h at room temperature. The coated panels were dried at room temperature for 24 h, and then dried in an air-circulating oven at 60 ± 2 °C for 8 h to ensure that the coating cured completely without any residual solvent. The dry film thickness of each coating was 50 ± 5 lm. Q235 mild steel panels (150 mm 75 mm 2 mm) were used as the substrates for immersion tests. The sample surface was sandblasted to Sa 2.5, degreased with ethanol, and painted twice by air spraying. The overcoating interval was 24 h at room temperature. The coated panels were dried at room temperature for 24 h and then dried in an air-circulating oven at 60 ± 2 °C for 12 h. The total dry film thickness of each coating was 320 ± 30 lm. 2.4. Electrochemical measurements The electrochemical measurement system M388 (EG&G) was composed of a 273A potentiostat and a 5210 lock-in amplifier. Electrochemical tests were performed in a three-electrode corrosion cell. A saturated calomel reference electrode (SCE) and platinum foil were used as reference and counter electrodes, respectively. PMMA tubes were attached to the coated panels to form the working electrode. The measurement area was 12.56 cm2. The cell was filled with artificial sea water and then it was submerged in a 40 ± 2 °C water bath. The test solution was refreshed regularly during the whole test period. The real equipment of EIS measurement cell with water bath is shown in Fig. 2. Impedance measurements were carried out at a frequency range of 102–105 Hz with an AC amplitude of 20 mV. This AC amplitude was used to improve the detecting current because the epoxy coating has weak conductivity and the AC amplitude cannot destroy the coating system. Polariza-
Table 1 Concentrations of inorganic salts in artificial sea water. Composition
NaCl
MgCl26H2O
CaCl22H2O
Na2SO4
KCl
NaHCO3
Concentration (g L1)
24.54
11.10
1.54
4.09
0.69
0.23 Fig. 2. The real equipment of EIS measurement cell with water bath.
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Y. Hao et al. / Corrosion Science 69 (2013) 77–86
2.5. Immersion test A ‘‘’’ scratch with a width of 1 mm exposing to the substrate was produced on the coating surface. The coated panels were immersed in 40 °C artificial sea water for 60 days. The panels were observed and recorded carefully by digital camera. Simultaneously, artificial sea water was changed at intervals. Three specimens from each series were measured to check the reproducibility.
ture of Gaussian and Lorentzian functions on a Shirley-type background [32–34]. The surface was sputtered for 120 s in order to remove the adsorbed air and impurity. Sputter profiles were obtained with a 2 2 mm2 Ar+ beam at 2 keV. The binding energies for the spectra were referenced to the hydrocarbon line at 285.0 eV. ESEM XL30 FEG field emission scanning electron microscopy (ESEM) was used to characterize micro-morphology. Typically, the acceleration voltage was set at 10 kV. All the samples were treated by gold spraying before analysis.
0 ZP-10 ZP-20 ZP-30 ZP-40 ZP-0
-200
Ecorr / mV vs. SCE
tion measurements of the bare mild steel in pigment extract were also evaluated. Square Q235 steel samples (10 mm 10 mm) were fixed at the end of Teflon tube that sealed with epoxy adhesive. The samples were connected with copper wire and adopted as working electrodes for polarization measurements. The sample surface was successively abraded with 150, 240, 400, 800, and 1200 grit emery paper, and degreased with deionized water and acetone, respectively. Finally, the samples for electrochemical measurements were dried by cold air and immediately used. The polarization curves were conducted at the scan rate of 0.250 mV/s from 250 mV to +250 mV relative to the open circuit potential (OCP). The results of the polarization curves and EIS were fitted by the commercial software packages CorrView 3.0 and ZsimpWin 3.2, respectively. All the measurements were carried out inside a Faraday cage in order to minimize the external interference on the system. Three specimens from each series were measured to check the reproducibility.
-400
-600
-800 0.0
2.6. Measurements
0.4
0.8
1.2
1.6
2.0
log (Time) / day X-ray diffraction (XRD) spectrum were obtained using a DMAX/ 2400 instrument (Rigaku Denki) equipped with Ni filtered Cu Ka radiation (40 kV, 100 mA). X-ray photoelectron spectroscopy (XPS) analysis of the steel surface beneath the coating was performed by ESCALAB250 spectrometer (Thermal VG), with an unmonochromatized Al Ka X-ray source. The source was operated at 15 kV and 20 mA. Curve fitting of the spectra was done by a mix-
Fig. 4. The variation of OCP for the coated panels in 40 °C artificial sea water.
Qc Qdl
Rs Rpore
0 -200
Potential / mV vs.SCE
Rct
0h in extract 18h in extract 36h in extract 0h in artificial sea water 18h in artificial sea water 36h in artificial sea water
-400
(a)
Qc
-600
Rs
-800
Rpore
-1000 -9
10
-8
10
-7
10
-6
10
-5
-4
10
10
-3
10
-2
10
-1
10
−2
Icorr / A cm
(b)
Fig. 3. Polarization curves of the bare mild steel at different immersion time in artificial sea water with and without ZP.
Fig. 5. The equivalent electric circuits, (a) two time constant model, (b) one time constant model.
Table 3 0 0 E0corr , E0corr , icorr , icorr , Tafel slope, and calculated protection efficiency (P) values for mild steel samples at different immersion time in extract solution. Time (h)
E0corr (mV vs. SCE)
i0corr (lA cm2)
-bc (mV dec1)
ba (mV dec1)
E0corr (mV vs. SCE)
i0corr (lA cm2)
bc (mV dec1)
ba (mV dec1)
P (%)
0 18 36
729 747 732
15.4 4.1 13.0
189 303 117
52 85 57
710 674 734
14.5 2.3 0.7
323 147 164
63 87 88
6 83 94
0
0
Y. Hao et al. / Corrosion Science 69 (2013) 77–86
Three parallel test results show that the experimental data has little scatter and good reproducibility. The corrosion current density decreased and the corrosion potential increased with the elapsed immersion time in pigment extract. However, the current density and corrosion potential changed little during the whole immersion time in artificial sea water. In addition, P increased from initial 6% to 94% after 36 h immersion in pigment extract. The electrochemical measurement results of the bare mild steel in artificial sea water with and without ZP confirm that ZP has an obvious corrosion inhibition effect.
3. Results 3.1. Polarization curves Fig. 3 shows the polarization curves of the bare mild steel at different immersion time in artificial sea water with and without ZP. The corrosion parameters determined from these curves are listed in Table 3. E0corr and E0corr are the corrosion potential in the absence 0 and presence of aqueous extract of the pigment, respectively. icorr 0 and icorr are the corrosion current densities in the absence and presence of aqueous extract of the pigment, respectively. The protection efficiency (P) of ZP is calculated by the Eq. (1) [35].
100
0
ð1Þ
6
The variation of OCP with immersion time is shown in Fig. 4. The OCP of ZP-0 reduced continuously to 570 mV vs. SCE after
10
100
10
ZP-0
5
10
ZP-10
0d 3d 14d
10
100
9
10 80
80
8
10 60
2
⏐Ζ ⏐/ Ω cm
3.2. Open circuit potential (OCP)
!
4
10
40 3
10
Phase / degree 2 ⏐Z ⏐/ Ω cm
0 icorr
icorr
60
7
10
od 3d 14d 26d 41d 60d
6
10
5
10
20
40 20
4
10
0 2
-1
10
0
10
1
10
2
3
10
10
3
0
4
10 -2 10
5
10
10
0
10
Frequency / Hz
10
4
10
Frequency / Hz
10
10
10
ZP-30
100
10
9
10
100
80
8
10
od 3d 14d 26d 41d 60d
6
10
5
10
40 20
2
10
10 ⏐Ζ ⏐ / Ω cm
60
Phase / degree
8
7
80 od 3d 14d 26d 41d 60d
7
10
6
10
5
10
3
2
3
10
4
10
10
-2
0
10
2
10
Frequency / Hz
10
Frequency / Hz 10
120
10
ZP-40 9
10
100
2
10
80 od 3d 14d 26d 41d 60d
7
10
6
10
5
60 40
10
20
4
10
3
10 -2 10
Phase / degree
8
⏐Ζ ⏐/ Ω cm
0
10
0
10
40 20
4
0
60
10
4
10
10 -2 10
120
10
ZP-20 9
2
2
2
10
4
0
10
Frequency / Hz Fig. 6. Bode plots of the coated panels at different immersion time in 40 °C artificial sea water.
4
10
0
Phase / degree
10 -2 10
⏐Ζ ⏐ / Ω cm
P¼
0 icorr
Phase / degree
80
81
Y. Hao et al. / Corrosion Science 69 (2013) 77–86
3.3. EIS
14-day immersion because it has no inhibition pigment. In contrast, the OCP of coatings containing ZP increased gradually to a different degree after the fluctuation at initial time. The OCP of ZP-30 dramatically increased to 51 mV vs. SCE after 60-day immersion. The OCP of ZP-10 decreased to a minimum value of 623 mV vs. SCE after 3-day immersion, and then slightly shifted to a maximum value of 278 mV vs. SCE after 60-day immersion. The OCP of ZP-20 decreased from 351 mV vs. SCE to a minimum value of 669 mV vs. SCE after 50-day immersion, and then slowly increased to 559 mV vs. SCE after 60-day immersion. The OCP of ZP-40 decreased from 304 mV vs. SCE to a minimum value of 587 mV vs. SCE after 35-day immersion, and then increased moderately to 186 mV vs. SCE after 60-day immersion. The positive shift of OCP values indicates that epoxy coating containing ZP has an obvious corrosion inhibition function and this similar phenomenon has been reported by other researchers [13,36,37].
3.3.1. Equivalent electric circuit (EEC) model Fig. 5 shows the EEC models of the coatings at different immersion time. Rs, Rpore, Rct, Qc, and Qdl are solution resistance, pore resistance, charge transfer resistance, coating capacitance, and double layer capacitance, respectively. The use of constant phase element (CPE) in the equivalent circuit of the impedance not only minimizes the systematical error but also provides more detailed information about the non-ideal dielectrical properties of the coating polymer. The CPE is defined by Eq. (2)
Z¼
ðjxÞn Y0
Where Y0 and n are the admittance and empirical exponent of the CPE, respectively, j is an imaginary number and x is the angle
60
0.09
50
0d 3d 14d
40
2
0.06 2
− Zim / ΜΩ cm
0.03
3.0
0d 3d 14d 26d 41d 60d
ZP-0
− Zim / ΜΩ cm
ð2Þ
ZP-10
2.5 2.0 1.5 1.0 0.5
30
0.0 -0.5 -0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
20 10 0
0.00 0.00
0.03
0.06
-10 -10
0.09
0
10
2
− Zim / ΜΩ cm
0.6
10 0.4
0.2
1200 800 400
0.0
0 -5 -5
0.0
0
5
10
15
0.2
0.4
20
0.6
25
0.8
30
0 35
0
400
2
Zre /Ω cm
800
1200
1600
2
Zre / ΜΩ cm
100
2
ZP-40
0d 3d 14d 26d 41d 60d
80
− Zim / ΜΩ cm
2
− Zim / ΜΩ cm
0.8
5
60
ZP-30
0d 3d 14d 26d 41d 60d
1600
15
50
2000
ZP-20
0d 3d 14d 26d 41d 60d
20
40
re
35
25
30
Z / ΜΩ cm
Zre / MΩ cm
30
20
2
2
60 40 20 0 0
20
40
60
80
100
2
Zre / ΜΩ cm
Fig. 7. Nyquist plots of the coated panels at different immersion time in 40 °C artificial sea water.
2000
Y. Hao et al. / Corrosion Science 69 (2013) 77–86
3.3.2. Charge transfer resistance (Rct) Fig. 9 shows the variation of Rct with immersion time. The Rct value for coating ZP-0 decreased with increasing immersion period from 0 to 14 days. The Rct value for coating ZP-0 has reduced to 2.4 103 O cm2 after 14-day immersion. The Rct values for coatings ZP-10 and ZP-20 decreased gradually at the initial time, and then increased gradually with the elapsed immersion time. The Rct values of coatings ZP-10 and ZP-20 are maintained at 2.9 106 and 4.6 105 O cm2 after 60-day immersion, respectively. The decrease in Rct indicates that the breakdown of coating film leads to ingress of corrosive species. This indicates that these coatings are not suitable for corrosion protection. The Rct value of coating ZP-40 changes a little and its value is maintained at 107 O cm2 during the whole immersion period. In contrast, the Rct value of coating ZP-30 increases gradually with increasing immersion period and it increases from initial 3.1 108 to 1.6 109 O cm2 after 41day immersion. At the initial immersion period, impedance of coat-
5
ZP-10 ZP-20 ZP-30 ZP-40 ZP-0
0
10
20
30
40
50
60
70
Time / day Fig. 9. Time dependence of the charge transfer resistance (Rct) of different coatings exposed to 40 °C artificial sea water.
ing ZP-30 presented two time constant characteristic, and then it exhibited one time constant characteristic at the last immersion stage, indicating high protective nature of the coating against corrosion and a certain degree of self-healing effect. 3.3.3. Coating capacitance (Qc) The coating capacitance is a measure of water permeation into the coating. The variation of Qc with immersion time is shown in Fig. 10. The Qc of coating ZP-30 changes negligibly during the entire immersion period. In contrast, the Qc values of the other coatings fluctuate obviously during the entire immersion period. The water uptake can result in the swelling of the coating and blisters may subsequently occur at some thin or defect areas in the coating. The osmotic pressure due to the swelling of the coating might break through the blisters to form direct paths for corrosion species to access the metal substrate, resulting in corrosion reaction. In the case of coating ZP-30, the change in Qc is negligible, which indicates that coating ZP-30 has stable coating/metal interface without any indication of corrosion. This is due to the fact that there is just sufficient polymer/resin available to wet the pigment completely and fill the voids between the particles. The Qc value of coating ZP-0 increases gradually during the whole immersion time and it is several orders of magnitude higher than initial value, indicating that water permeates into coating ZP-0 easily and coating ZP-0 has lost its protection ability completely. The values of other three coatings have an obvious fluctuation during the whole
-3 -4
Qdl
Rs
6
2 -10
Qc
3
8
4
-5
(b)
ZP-10 ZP-20 ZP-30 ZP-40 ZP-0
−2
Rpore Rct
log (Qc) / F cm
2
10
experimental fitted
4
− Zim / MΩ cm
12
2
frequency. For n = 1, an ideal capacitor is defined. For n = 0, the CPE represents an ideal resistor. For n = 1, the CPE is equivalent with an inductance. Using a CPE instead of a capacitor provides the deviation from ideal capacitive behavior [38,39]. The faradic elements are in series with the pore resistance because the corrosion process is presumed to occur at the base of the pores in the coatings. In this condition, two time constant equivalent circuit (Model (a)) was used to fit the experimental data. Model (b) was used to fit the impedance of coating ZP-30 at the last immersion stage because the impedance of coating ZP-30 presented one time constant characteristic at the last immersion stage. Figs. 6 and 7 show the variations of the Bode and Nyquist plots with immersion time in 40 °C artificial sea water. Fig. 8 shows the fitted and experimental data for coating ZP-20. The EEC fitted well with the experimental results over a wide frequency range. The decrease in impedance for coatings ZP-0, ZP-10 and ZP-20 with immersion time indicates an increase in delaminated area where corrosion occurs. In contrast, the impedance values for coatings ZP-30 and ZP-40 have exceeded their initial values after 60-day immersion in 40 °C artificial sea water, which indicates that coatings ZP-30 and ZP-40 have better protection ability for the substrate than other coatings. The protection behaviors of all the coatings are discussed in terms of charge transfer resistance (Rct), pore resistance (Rpore), coating capacitance (Qc), and double layer coating capacitance (Qdl) below.
log(Rct ) / Ω cm
82
2
1
-6 -7 -8 -9
0
-10
0
1
2
3
4
5
2
Zre / MΩ cm
Fig. 8. Experimental and fitted data of coating ZP-20 after 6-day immersion.
-10
0
10
20
30
40
50
60
70
Time / day Fig. 10. Time dependence of the coating capacitance (Qc) of different coatings exposed to 40 °C artificial sea water.
Y. Hao et al. / Corrosion Science 69 (2013) 77–86
83
immersion time, which indicates that they are not stable and cannot provide effective protection for the substrate. 3.3.4. Pore resistance (Rpore) The pore resistances (Rpore) for different coatings with respect to immersion period are shown in Fig 11. The Rpore values of coatings ZP-0, ZP-10, and ZP-20 decrease with increasing exposure period. However, the Rpore values of coatings ZP-30 and ZP-40 increase gradually with increasing immersion time and their values have been higher than initial values after 60-day immersion. The Rpore value of coating ZP-30 increases gradually from 4.6 106 to 3.9 109 O cm2 after 60-day immersion, indicating excellent protective nature of this coating. The decrease in Rpore value with immersion time indicates that pores are opened and electrolyte permeates the coating. The trends of decreasing the Rpore values with respect to immersion period are observed in coatings ZP-0, ZP-10, and ZP-20. The Rpore values for these coatings are less than 106 O cm2, indicating poor protective nature of these coatings. 3.3.5. Double layer coating capacitance (Qdl) The Qdl values of various coatings are shown in Fig. 12. The double layer coating capacitance (Qdl) is a measure of the area over which the coating has been deteriorated. The increase in Qdl value is associated with water spreading at the interface and increase in delaminated area. A stable Qdl is an indication of a stable interface. By comparing the Qdl values of different coatings, coating ZP-30
11 ZP-10 ZP-20 ZP-30 ZP-40 ZP-0
10
log(Rpore) / Ω cm
2
9 8
Fig. 13. Photos of the coated panels after 60-day immersion in 40 °C artificial sea water, (a) ZP-10; (b) ZP-20; (c) ZP-30; (d) ZP-40; (e) ZP-0.
7
exhibits not only the lowest Qdl values but also remain constant over longer exposure. This indicates that this coating has stable coating/metal interface without any indication of corrosion.
6 5 4 3
3.4. Immersion test
2 1 -10
0
10
20
30
40
50
60
70
Time / day Fig. 11. Time dependence of the pore resistance (Rpore) of different coatings exposed to 40 °C artificial sea water.
-2 ZP-10 ZP-20 ZP-30 ZP-40 ZP-0
-3
log (Qdl) / F cm
−2
-4
Fig. 13 shows the images of coated panels after 60-day immersion in 40 °C artificial sea water. There are no rust extension and blisters at the scratched areas of coating ZP-30. However, the scratched areas of coatings ZP-0, ZP-10, and ZP-20 have a large amount of rust and considerable blistering, which indicates that they have poor adhesion strength to the substrate after 60-day immersion. A few blisters and slight rust extension can also be observed at the scratched areas of coating ZP-40. The immersion test confirms that epoxy coating containing 30% volume fraction of ZP gets the best protection ability. 4. Inhibition mechanism
-5 -6 -7 -8 -9 -10 -10
0
10
20
30
40
50
60
70
Time / day Fig. 12. Time dependence of the double layer coating capacitance (Qdl) of different coatings exposed to 40 °C artificial sea water.
The previous experimental results have confirmed that ZP can improve the anticorrosion properties of the epoxy coatings and its best volume fraction is 30%. How does ZP play a positive role in an epoxy coating? What is the inhibiting film on the steel surface? These questions are needed to be explained clearly. In this section, a series of analytical measurements are used to solve these questions. Fig. 14 shows the XPS spectra of the inhibiting film beneath coating ZP-30. Three peaks at 705.8, 707.8, and 719.0 eV in Fig. 14(a) belong to metallic iron (Fe0), FeO, and Fe3+, respectively [40,41]. The O1s spectrum yielded two peaks at binding energies of 529.8 and 532.1 eV. The peak of the O 1s spectrum centered at
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Y. Hao et al. / Corrosion Science 69 (2013) 77–86
40000
20000
35000
−1
21000
19000 18000 17000
30000 25000 20000 15000
16000 15000 695
(b)
45000
Contents / s
−1
Contents / s
50000
(a)
22000
10000 700
705
710
715
720
725
730
735
5000 520
740
525
Binding energy / eV
530
535
540
545
Binding energy / eV
9000
(c)
(d)
80000
8000 70000 −1
60000
Contents / s
Contents / s
−1
7000 6000 5000 4000
50000 40000 30000
3000 20000 2000 120
125
130
135
140
145
150
10000 1000
Binding energy / eV
1010
1020
1030
1040
1050
1060
Binding energy / eV
Fig. 14. XPS spectra of the inhibiting film beneath coating ZP-30 after 60-day immersion in 40 °C artificial sea water, (a) Fe, (b) O, (c) P, (d) Zn.
binding energy of 532.0 eV is most likely a convolution of a hydroxide component and a phosphate component [42,43]. A binding energy of 532.1 eV has been reported for FePO4. The peaks at 132.8 and 138.7 eV (Fig. 14c) belong to PO43 [44,45]. The peaks at 1045 eV and 1022 eV (Fig. 14d) belong to Zn 2p3/2 and Zn 2p1/2, respectively. The peaks assigned to a non-conductive form of Zn, indicating that ZP incorporated into the film [46]. SEM image and EDS spectrum of the inhibiting film beneath coating ZP-30 are shown in Fig. 15. The inhibiting film exhibits a cluster shape with main elements of P, O, Zn, and Fe. The XRD pattern of this inhibiting film is shown in Fig. 16. The peaks of a-FeOOH, c-FeOOH, Zn3(PO4)2, FePO4, Fe2O3, and FeO [34,47,48] are observed. a-FeOOH and c-FeOOH are typically the compositions of iron rust [47] formed at initial immersion time. With the elapsed immersion time, ZP released PO43 with the help of the endosmotic water. Then, steel reacted with PO43 in the presence of dissolved oxygen and formed a phosphating film which was composed of FePO4, Fe2O3, and FeO. Therefore, ZP has a phosphatization function to a certain degree. But, the phosphating products of ZP are different from the compositions in the previous reports [16,19,25]. Meanwhile, the peaks of ZP can be detcted in Fig. 16, indicating that ZP can also form a shielding film on the steel surface which is in accordance with Amo et al. [21] and Beiro et al. [22] reports. It can be concluded from the XPS and XRD data that the inhibiting film is mainly composed of the phosphating film of FePO4, Fe2O3, and FeO, as well as the shielding film of ZP. The formed inhibiting film can cover the corrosion products formed at initial immersion time and prevent from corrosion. Therefore, the inhibition mechanism of ZP includes two aspects: one is the phosphatization function; the other is the shielding effect. The self-healing function of epoxy coating containing ZP is due to the formation of the inhibiting film on the steel surface.
5. Discussion EIS and immersion results show that pigment volume concentration has an important effect on the protection ability of epoxy coating. Epoxy coating containing high concentration of ZP has better protection ability. The coating resistance values of coatings ZP30 and ZP-40 are higher than their initial values after 60-day immersion in 40 °C artificial sea water. In contrast, the coating resistance values of coatings ZP-10 and ZP-20 are less than their initial values. Epoxy coating containing low ratio of ZP to resin, such as coatings ZP-10 and ZP-20, releases less inhibitive species. In contrast, epoxy coating containing high ratio of ZP to resin can release enough inhibitive species and block pores within the coating film. The distance between pigment particles decreases when pigment concentrations increase further. The water films around the pigment particles may touch each other and form a capillary system between the pigment particles. The capillary system may lead to an increase in coating permeability [26]. Therefore, the impedance value of coating ZP-40 cannot exceed the value of coating ZP-30. Concerning protective properties of organic coatings, it is generally accepted that the electrical resistance of good coatings is larger than 108 O cm2, whereas poor coatings is less than 106 O cm2. The impedance of coating ZP-30 is one order of magnitude than its initial value, which indicates that coating ZP-30 has better protection ability than other coatings after 60-day immersion in 40 °C artificial sea water. For all the coatings, the protection sequence is as follows: ZP-30 > ZP-40 > ZP-10 ZP-20 ZP-0. As shown in Fig. 12, Qdl shows a fluctuating trend for epoxy coating containing ZP. This is the result of both the delamination effect caused by corrosive attack and the healing effect by the formation of inhibiting film during the immersion [49]. All the epoxy coatings containing ZP have lower Qdl than the pure epoxy coating,
Y. Hao et al. / Corrosion Science 69 (2013) 77–86
85
(1) Polarization curves of the mild steel in pigment extract confirm that the dissolved ZP in solution has an obvious inhibition effect. (2) EIS and immersion tests illustrate that ZP can improve the protection ability of epoxy coating and its best volume concentration is 30% in epoxy coating. Epoxy coating containing ZP has a self-healing function to a certain degree. (3) The inhibition mechanism of ZP in an epoxy coating is attributed to the synergistic effect of the shielding and phosphatization functions of ZP. Acknowledgements The authors thank the National Key Technology R&D Program (Grant Nos. 2009BAE70B02 and 2012BAB15B00), China Postdoctoral Science Foundation funded project, for supporting these studies. References
Fig. 15. SEM image and EDS spectrum of the inhibiting film beneath coating ZP-30 after 60-day immersion in 40 °C artificial sea water, (a) SEM image, (b) EDS spectrum.
2500 • α-FeOOH ♦ γ−FeOOH ο Fe O
2000 Intensity (cps)
2
∗
3
♥ Zn (PO ) 3 4 2 ∗ FeO
1500
♣ FePO 4
1000 500
♣ ♦♥
• ο
♦
•
ο ο♥ ♥
0 10
20
30
40 2θ
50
60
70
Fig. 16. XRD pattern of the inhibiting film beneath coating ZP-30 after 60-day immersion in 40 °C artificial sea water.
indicating that ZP has an inhibition effect. Qdl values of epoxy coating containing ZP are close to Qc values, which is due to the formation of an inhibiting film on the steel surface beneath the epoxy coating. In general, the epoxy coatings with lower capacitances Qc and Qdl performs well in corrosive media [50]. 6. Conclusions In the present work, the inhibition mechanism of ZP and its optimized concentration in an epoxy coating have been discussed. The main conclusions can be summarized as follows:
[1] R. Naderi, M.M. Attar, The inhibitive performance of polyphosphate-based anticorrosion pigments using electrochemical techniques, Dyes Pigm. 80 (2009) 349–354. [2] B. Chico, J. Simancas, J.M. Vega, N. Granizo, I. Díaz, D. de la Fuente, M. Morcillo, Anticorrosive behaviour of alkyd paints formulated with ion-exchange pigments, Prog. Org. Coat. 61 (2008) 283–290. [3] L.W. Vasconcelos, I.C.P. Margarit, O.R. Mattos, F.L. Fragata, A.S.B. Sombra, Inhibitory properties of calcium exchanged silica epoxy paintings, Corros. Sci. 43 (2001) 2291–2303. [4] M. Zubielewicz, W. Gnot, Mechanisms of non-toxic anticorrosive pigments in organic waterborne coatings, Prog. Org. Coat. 49 (2004) 358–371. [5] A. Kalendová, Anticorrosive spinel-type pigments of the second generation, Pigm. Resin Technol. 27 (1998) 225–230. [6] Y.M.A. Ayana, S.M. El-Sawy, S.H. Salah, Zinc–ferrite pigment for corrosion protection, Anti-Corros. Methods Mater. 44 (1997) 381–388. [7] A. Kalendová, J. Snupárek, P. Kalenda, Nontoxic anticorrosion pigments of the spinel type compared with condensed phosphates, Dyes Pigm. 30 (1996) 129– 140. [8] Y. Hao, F. Liu, E. Han, Inhibitive behavior and mechanism of a ferrite inhibition pigment in epoxy paints, J. Electrochem. Soc. 159 (2012) C403–C410. [9] E. Armelin, R. Pla, F. Liesa, X. Ramis, J.I. Iribarren, C. Alemán, Corrosion protection with polyaniline and polypyrrole as anticorrosive additives for epoxy paint, Corros. Sci. 50 (2008) 721–728. [10] S. Sathiyanarayanan, S. Muthkrishnan, G. Venkatachari, Corrosion protection of steel by polyaniline blended coating, Electrochim. Acta 51 (2006) 6313– 6319. [11] A. Adhikari, P. Claesson, J. Pan, C. Leygraf, A. Deı˘dinaitı˘, E. Blomberg, Electrochemical behavior and anticorrosion properties of modified polyaniline dispersed in polyvinylacetate coating on carbon steel, Electrochim. Acta 53 (2008) 4239–4247. [12] A. Kalendová, I. Sapurina, J. Stejskal, D. Vesel, Anticorrosion properties of polyaniline-coated pigments in organic coatings, Corros. Sci. 50 (2008) 3549– 3560. [13] S. Sathiyanarayanan, S.S. Azim, G. Venkatachari, Corrosion protection coating containing polyaniline glass flake composite for steel, Electrochim. Acta 53 (2008) 2087–2094. [14] S. Radhakrishnan, C.R. Siju, D. Mahanta, S. Patil, G. Madras, Conducting polyaniline-nano-TiO2 composites for smart corrosion resistant coatings, Electrochim. Acta 54 (2009) 1249–1254. [15] B.N. Grgur, M.M. Gvozdenovic´, V.B. Miškovic´-Stankovic´, Z. Kac´arevic´-Popovic´, Corrosion behavior and thermal stability of electrodeposited PANI/epoxy coating system on mild steel in sodium chloride solution, Prog. Org. Coat. 56 (2006) 214–219. [16] G. Blustein, M.C. Deyá, R. Romagnoli, B.d. Amo, Three generations of inorganic phosphates in solvent and water-borne paints: a synergism case, Appl. Surf. Sci. 252 (2005) 1386–1397. [17] W. Kozlowski, J. Flis, An ellipsometric study of the effect of phosphate anions in borate solution on anodic films grown on iron, Corros. Sci. 32 (1991) 861– 875. [18] M. Bethencourt, F.J. Botana, M. Marcos, R.M. Osuna, J.M. Sánchez-Amaya, Inhibitor properties of ‘‘green’’ pigments for paints, Prog. Org. Coat. 46 (2003) 280–287. [19] J. Sinko, Challenges of chromate inhibitor pigments replacement in organic coatings, Prog. Org. Coat. 42 (2001) 267–282. [20] L. Chromy, E. Kaminska, Non-toxic anticorrosive pigments, Prog. Org. Coat. 18 (1990) 319–324. [21] B. del Amo, R. Romagnoli, V.F. Vetere, L.S. Hernández, Study of the anticorrosive properties of zinc phosphate in vinyl paints, Prog. Org. Coat. 33 (1998) 28–35.
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Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
The mechanism of inhibition by zinc phosphate in an epoxy coating Yongsheng Hao a, Fuchun Liu a,⇑, En-Hou Han a, Saima Anjum b, Guobao Xu b a b
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 29 August 2012 Accepted 19 November 2012 Available online 29 November 2012 Keywords: A. Mild steel B. EIS B. Polarization B. XPS
a b s t r a c t Epoxy coatings containing different volume fractions of zinc phosphate have been successfully prepared and their inhibitive properties have been studied by electrochemical impedance spectroscopy (EIS) and immersion tests. The results show that zinc phosphate can improve the protection ability of epoxy coatings and its best volume fraction is 30%. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) results indicate that the presence of zinc phosphate can form an inhibiting film which is composed of the phosphating film of FePO4, Fe2O3, and FeO, as well as the shielding film of zinc phosphate on the steel surface. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Organic coatings are the most widely used methods in metal protection and they act as a physical barrier between corrosive electrolyte and the metal substrate. However, the number of pores, the ionic resistance of coating against electrolyte diffusion, and the cross-linking density of coating can affect the coating against electrolyte diffusion. Epoxy coating is used extensively in metal protection owing to its good adhesion, good acid/alkali and solvent resistance, high mechanical properties and cross-linking density. Epoxy coating can protect the metal substrate by releasing inhibiting chemicals from the pigment to form a strong passive or barrier layer that inhibits the corrosive medium contact with the metal substrate. Based on the inhibition mechanism, pigment can be categorized into two main groups of active (such as chromate and polyaniline) and barrier (such as glass flake) pigments. Though pigments containing lead and chromium have excellent inhibition properties, they are toxic and their applications are limited due to the environmental requirements. Therefore, many kinds of eco-friendly inhibition pigments have been developed to replace these toxic pigments, such as polyphosphate-based pigments [1], ion-exchanged pigments [2–4], ferrite pigments [5–8], conductive polymers [9–15], and so on. Zinc phosphate (ZP) is an eco-friendly inhibition pigment. However, there are many arguments about its inhibition mechanism. Several researchers [16–20] considered that its corrosion inhibition effect was due to phosphatization or passivation function which can form the complex compounds on the steel surface. Amo et al. [21] and Beiro et al. [22] regarded that the corrosion ⇑ Corresponding author. Tel.: +86 24 23915895; fax: +86 24 23894149. E-mail address: [email protected] (F. Liu). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.11.025
inhibition mechanism of ZP was attributed to its shielding ability. Rossenbeck et al. [23,24] regarded that ZP not only had shielding ability, but also showed the anti-cathodic delamination ability. Shao et al. [25] found that epoxy coating containing ZP had a pronounced self-healing function. Naderi and Attar [26] studied the protective performance and anti-cathodic disbonding of epoxy coating containing ZP and found that ZP had an anti-cathodic disbonding property. Perrin0 s [27] study indicated that ZP had an inhibition effect. Recently, Valcarce0 s research [28] disclosed that PO43had an inhibition effect in brass protection in tap water which was rich in chloride and carbonated ions. In contrast, Zubielewicz and Gnot [4] and Bastos et al. [29] thought that ZP did not have any positive functions for the coating. Therefore, it is very important to reveal the actual inhibition mechanism of ZP. Epoxy coating containing ZP can be used to protect the metal structures in ship, such as ballast tank, owing to its non-toxic characteristic. The temperature usually can reach 40 °C in ballast tank, indicating that the epoxy coating has to be immersed in 40 °C artificial sea water for a long term. Nevertheless, there are no relative reports in this aspect. Therefore, it is very important to study the protection ability of epoxy coating containing ZP in high temperature sea water and the inhibition mechanism of ZP in an epoxy coating. In the present work, the inhibition mechanism of ZP is evaluated by polarization curves, open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and immersion tests. 2. Experimental 2.1. Materials ZP was obtained from Wonder Technology (Wuxi) Co Ltd., China. Fig. 1 shows the XRD spectrum of ZP. The main composition
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Y. Hao et al. / Corrosion Science 69 (2013) 77–86 Table 2 The composition of different coating systems.
14000 ♦
Zn3(PO4)2• 4H2O
♦
12000
♦
Intensity (cps)
10000 8000 6000 4000 2000
♦
♦
♦
♦ ♦
♦
♦ ♦ ♦ ♦ ♦
♦♦ ♦ ♦ ♦♦
♦
♦
50
60
Coating number
ZP-10
ZP-20
ZP-30
ZP-40
NPEl-128 (g) Petroleum resin (g) Zinc phosphate(g) Bentone-27 (g) Anti-terra U (g) BYK 057 (g) BYK 320 (g) Xylene (g) 1-Butanol (g) Total mass (g)
27 13.5 42.8 1.5 0.4 0.2 0.4 12.2 2 100
18 9 55 1.8 0.4 0.1 0.2 10 5.5 100
11 5.5 59 1.8 0.4 0.1 0.2 15 7 100
7 3.5 62 1.8 0.4 0.1 0.2 15 10 100
0 10
20
30
40
70
80
90
2θ Fig. 1. XRD pattern of ZP.
of ZP discussed in this paper is Zn3(PO4)24H2O [30]. NPEL-128 epoxy resin (epoxy equivalent 180–190 g/eq) and curing agent polyamide 8115 (amine value 230–246 mgKOH/g) were obtained from Yexuya Electronic Chemistry (Kunshan) Co. Ltd., and Yuanda Chemicals Co. Ltd., China, respectively. Petroleum resin was obtained from Shandong Qibang Chemical Co. Ltd., Defoaming agent (BYK 057), flatting agent (BYK 320), and dispersion agent (Anti-terra U) were obtained from BYK company. Rheological agent (Bentone-27) was obtained from Elementis company. Xylene, 1butanol, acetone, and ethanol were obtained from Sinopharm Chemical Regent Co., Ltd. 2.2. Preparation of pigment extract Artificial sea water is prepared according to the ASTM standard D-1141 without incorporating heavy metals [31]. The composition of artificial sea water is shown in Table 1. To prepare pigment extract, 2 g ZP was stirred in 1 L of artificial sea water for 24 h, and then filtered to achieve a saturated solution. 2.3. Preparation of coating systems Petroleum resin was dissolved with xylene by a mass ratio of 1:1, subsequently blended with NPEL-128 epoxy resin by a mass ratio of 2:1. Then, ZP was added into the mixture of petroleum resin and NPEL-128 epoxy resin. The volume fractions of ZP in epoxy coatings were 10%, 20%, 30%, 40% and they were named ZP-10, ZP20, ZP-30 and ZP-40, respectively (Table 2). All the compositions were added into an agate jar according to the required proportion, then the mixture was dispersed using a ball-milling machine for 8 h in order to get the paint matrix. The solvent mixture of xylene and 1-butanol was used to adjust the viscosity to wet the pigment particles completely by epoxy resin. A pure epoxy coating was used as a reference and it was named ZP-0. Q235 mild steel panels (50 mm 50 mm 3 mm) were used as working electrodes for electrochemical impedance spectroscopy (EIS) measurements. The sample surface was successively abraded with 150, 240, 400, and 800 grit emery papers, and then cleaned thoroughly with deionized water and acetone, respectively. Finally,
the samples were dried by cold air. All the paints were coated twice on the steel surface by air spraying in order to avoid pinholes and get a dense coating. The overcoating interval was 24 h at room temperature. The coated panels were dried at room temperature for 24 h, and then dried in an air-circulating oven at 60 ± 2 °C for 8 h to ensure that the coating cured completely without any residual solvent. The dry film thickness of each coating was 50 ± 5 lm. Q235 mild steel panels (150 mm 75 mm 2 mm) were used as the substrates for immersion tests. The sample surface was sandblasted to Sa 2.5, degreased with ethanol, and painted twice by air spraying. The overcoating interval was 24 h at room temperature. The coated panels were dried at room temperature for 24 h and then dried in an air-circulating oven at 60 ± 2 °C for 12 h. The total dry film thickness of each coating was 320 ± 30 lm. 2.4. Electrochemical measurements The electrochemical measurement system M388 (EG&G) was composed of a 273A potentiostat and a 5210 lock-in amplifier. Electrochemical tests were performed in a three-electrode corrosion cell. A saturated calomel reference electrode (SCE) and platinum foil were used as reference and counter electrodes, respectively. PMMA tubes were attached to the coated panels to form the working electrode. The measurement area was 12.56 cm2. The cell was filled with artificial sea water and then it was submerged in a 40 ± 2 °C water bath. The test solution was refreshed regularly during the whole test period. The real equipment of EIS measurement cell with water bath is shown in Fig. 2. Impedance measurements were carried out at a frequency range of 102–105 Hz with an AC amplitude of 20 mV. This AC amplitude was used to improve the detecting current because the epoxy coating has weak conductivity and the AC amplitude cannot destroy the coating system. Polariza-
Table 1 Concentrations of inorganic salts in artificial sea water. Composition
NaCl
MgCl26H2O
CaCl22H2O
Na2SO4
KCl
NaHCO3
Concentration (g L1)
24.54
11.10
1.54
4.09
0.69
0.23 Fig. 2. The real equipment of EIS measurement cell with water bath.
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Y. Hao et al. / Corrosion Science 69 (2013) 77–86
2.5. Immersion test A ‘‘’’ scratch with a width of 1 mm exposing to the substrate was produced on the coating surface. The coated panels were immersed in 40 °C artificial sea water for 60 days. The panels were observed and recorded carefully by digital camera. Simultaneously, artificial sea water was changed at intervals. Three specimens from each series were measured to check the reproducibility.
ture of Gaussian and Lorentzian functions on a Shirley-type background [32–34]. The surface was sputtered for 120 s in order to remove the adsorbed air and impurity. Sputter profiles were obtained with a 2 2 mm2 Ar+ beam at 2 keV. The binding energies for the spectra were referenced to the hydrocarbon line at 285.0 eV. ESEM XL30 FEG field emission scanning electron microscopy (ESEM) was used to characterize micro-morphology. Typically, the acceleration voltage was set at 10 kV. All the samples were treated by gold spraying before analysis.
0 ZP-10 ZP-20 ZP-30 ZP-40 ZP-0
-200
Ecorr / mV vs. SCE
tion measurements of the bare mild steel in pigment extract were also evaluated. Square Q235 steel samples (10 mm 10 mm) were fixed at the end of Teflon tube that sealed with epoxy adhesive. The samples were connected with copper wire and adopted as working electrodes for polarization measurements. The sample surface was successively abraded with 150, 240, 400, 800, and 1200 grit emery paper, and degreased with deionized water and acetone, respectively. Finally, the samples for electrochemical measurements were dried by cold air and immediately used. The polarization curves were conducted at the scan rate of 0.250 mV/s from 250 mV to +250 mV relative to the open circuit potential (OCP). The results of the polarization curves and EIS were fitted by the commercial software packages CorrView 3.0 and ZsimpWin 3.2, respectively. All the measurements were carried out inside a Faraday cage in order to minimize the external interference on the system. Three specimens from each series were measured to check the reproducibility.
-400
-600
-800 0.0
2.6. Measurements
0.4
0.8
1.2
1.6
2.0
log (Time) / day X-ray diffraction (XRD) spectrum were obtained using a DMAX/ 2400 instrument (Rigaku Denki) equipped with Ni filtered Cu Ka radiation (40 kV, 100 mA). X-ray photoelectron spectroscopy (XPS) analysis of the steel surface beneath the coating was performed by ESCALAB250 spectrometer (Thermal VG), with an unmonochromatized Al Ka X-ray source. The source was operated at 15 kV and 20 mA. Curve fitting of the spectra was done by a mix-
Fig. 4. The variation of OCP for the coated panels in 40 °C artificial sea water.
Qc Qdl
Rs Rpore
0 -200
Potential / mV vs.SCE
Rct
0h in extract 18h in extract 36h in extract 0h in artificial sea water 18h in artificial sea water 36h in artificial sea water
-400
(a)
Qc
-600
Rs
-800
Rpore
-1000 -9
10
-8
10
-7
10
-6
10
-5
-4
10
10
-3
10
-2
10
-1
10
−2
Icorr / A cm
(b)
Fig. 3. Polarization curves of the bare mild steel at different immersion time in artificial sea water with and without ZP.
Fig. 5. The equivalent electric circuits, (a) two time constant model, (b) one time constant model.
Table 3 0 0 E0corr , E0corr , icorr , icorr , Tafel slope, and calculated protection efficiency (P) values for mild steel samples at different immersion time in extract solution. Time (h)
E0corr (mV vs. SCE)
i0corr (lA cm2)
-bc (mV dec1)
ba (mV dec1)
E0corr (mV vs. SCE)
i0corr (lA cm2)
bc (mV dec1)
ba (mV dec1)
P (%)
0 18 36
729 747 732
15.4 4.1 13.0
189 303 117
52 85 57
710 674 734
14.5 2.3 0.7
323 147 164
63 87 88
6 83 94
0
0
Y. Hao et al. / Corrosion Science 69 (2013) 77–86
Three parallel test results show that the experimental data has little scatter and good reproducibility. The corrosion current density decreased and the corrosion potential increased with the elapsed immersion time in pigment extract. However, the current density and corrosion potential changed little during the whole immersion time in artificial sea water. In addition, P increased from initial 6% to 94% after 36 h immersion in pigment extract. The electrochemical measurement results of the bare mild steel in artificial sea water with and without ZP confirm that ZP has an obvious corrosion inhibition effect.
3. Results 3.1. Polarization curves Fig. 3 shows the polarization curves of the bare mild steel at different immersion time in artificial sea water with and without ZP. The corrosion parameters determined from these curves are listed in Table 3. E0corr and E0corr are the corrosion potential in the absence 0 and presence of aqueous extract of the pigment, respectively. icorr 0 and icorr are the corrosion current densities in the absence and presence of aqueous extract of the pigment, respectively. The protection efficiency (P) of ZP is calculated by the Eq. (1) [35].
100
0
ð1Þ
6
The variation of OCP with immersion time is shown in Fig. 4. The OCP of ZP-0 reduced continuously to 570 mV vs. SCE after
10
100
10
ZP-0
5
10
ZP-10
0d 3d 14d
10
100
9
10 80
80
8
10 60
2
⏐Ζ ⏐/ Ω cm
3.2. Open circuit potential (OCP)
!
4
10
40 3
10
Phase / degree 2 ⏐Z ⏐/ Ω cm
0 icorr
icorr
60
7
10
od 3d 14d 26d 41d 60d
6
10
5
10
20
40 20
4
10
0 2
-1
10
0
10
1
10
2
3
10
10
3
0
4
10 -2 10
5
10
10
0
10
Frequency / Hz
10
4
10
Frequency / Hz
10
10
10
ZP-30
100
10
9
10
100
80
8
10
od 3d 14d 26d 41d 60d
6
10
5
10
40 20
2
10
10 ⏐Ζ ⏐ / Ω cm
60
Phase / degree
8
7
80 od 3d 14d 26d 41d 60d
7
10
6
10
5
10
3
2
3
10
4
10
10
-2
0
10
2
10
Frequency / Hz
10
Frequency / Hz 10
120
10
ZP-40 9
10
100
2
10
80 od 3d 14d 26d 41d 60d
7
10
6
10
5
60 40
10
20
4
10
3
10 -2 10
Phase / degree
8
⏐Ζ ⏐/ Ω cm
0
10
0
10
40 20
4
0
60
10
4
10
10 -2 10
120
10
ZP-20 9
2
2
2
10
4
0
10
Frequency / Hz Fig. 6. Bode plots of the coated panels at different immersion time in 40 °C artificial sea water.
4
10
0
Phase / degree
10 -2 10
⏐Ζ ⏐ / Ω cm
P¼
0 icorr
Phase / degree
80
81
Y. Hao et al. / Corrosion Science 69 (2013) 77–86
3.3. EIS
14-day immersion because it has no inhibition pigment. In contrast, the OCP of coatings containing ZP increased gradually to a different degree after the fluctuation at initial time. The OCP of ZP-30 dramatically increased to 51 mV vs. SCE after 60-day immersion. The OCP of ZP-10 decreased to a minimum value of 623 mV vs. SCE after 3-day immersion, and then slightly shifted to a maximum value of 278 mV vs. SCE after 60-day immersion. The OCP of ZP-20 decreased from 351 mV vs. SCE to a minimum value of 669 mV vs. SCE after 50-day immersion, and then slowly increased to 559 mV vs. SCE after 60-day immersion. The OCP of ZP-40 decreased from 304 mV vs. SCE to a minimum value of 587 mV vs. SCE after 35-day immersion, and then increased moderately to 186 mV vs. SCE after 60-day immersion. The positive shift of OCP values indicates that epoxy coating containing ZP has an obvious corrosion inhibition function and this similar phenomenon has been reported by other researchers [13,36,37].
3.3.1. Equivalent electric circuit (EEC) model Fig. 5 shows the EEC models of the coatings at different immersion time. Rs, Rpore, Rct, Qc, and Qdl are solution resistance, pore resistance, charge transfer resistance, coating capacitance, and double layer capacitance, respectively. The use of constant phase element (CPE) in the equivalent circuit of the impedance not only minimizes the systematical error but also provides more detailed information about the non-ideal dielectrical properties of the coating polymer. The CPE is defined by Eq. (2)
Z¼
ðjxÞn Y0
Where Y0 and n are the admittance and empirical exponent of the CPE, respectively, j is an imaginary number and x is the angle
60
0.09
50
0d 3d 14d
40
2
0.06 2
− Zim / ΜΩ cm
0.03
3.0
0d 3d 14d 26d 41d 60d
ZP-0
− Zim / ΜΩ cm
ð2Þ
ZP-10
2.5 2.0 1.5 1.0 0.5
30
0.0 -0.5 -0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
20 10 0
0.00 0.00
0.03
0.06
-10 -10
0.09
0
10
2
− Zim / ΜΩ cm
0.6
10 0.4
0.2
1200 800 400
0.0
0 -5 -5
0.0
0
5
10
15
0.2
0.4
20
0.6
25
0.8
30
0 35
0
400
2
Zre /Ω cm
800
1200
1600
2
Zre / ΜΩ cm
100
2
ZP-40
0d 3d 14d 26d 41d 60d
80
− Zim / ΜΩ cm
2
− Zim / ΜΩ cm
0.8
5
60
ZP-30
0d 3d 14d 26d 41d 60d
1600
15
50
2000
ZP-20
0d 3d 14d 26d 41d 60d
20
40
re
35
25
30
Z / ΜΩ cm
Zre / MΩ cm
30
20
2
2
60 40 20 0 0
20
40
60
80
100
2
Zre / ΜΩ cm
Fig. 7. Nyquist plots of the coated panels at different immersion time in 40 °C artificial sea water.
2000
Y. Hao et al. / Corrosion Science 69 (2013) 77–86
3.3.2. Charge transfer resistance (Rct) Fig. 9 shows the variation of Rct with immersion time. The Rct value for coating ZP-0 decreased with increasing immersion period from 0 to 14 days. The Rct value for coating ZP-0 has reduced to 2.4 103 O cm2 after 14-day immersion. The Rct values for coatings ZP-10 and ZP-20 decreased gradually at the initial time, and then increased gradually with the elapsed immersion time. The Rct values of coatings ZP-10 and ZP-20 are maintained at 2.9 106 and 4.6 105 O cm2 after 60-day immersion, respectively. The decrease in Rct indicates that the breakdown of coating film leads to ingress of corrosive species. This indicates that these coatings are not suitable for corrosion protection. The Rct value of coating ZP-40 changes a little and its value is maintained at 107 O cm2 during the whole immersion period. In contrast, the Rct value of coating ZP-30 increases gradually with increasing immersion period and it increases from initial 3.1 108 to 1.6 109 O cm2 after 41day immersion. At the initial immersion period, impedance of coat-
5
ZP-10 ZP-20 ZP-30 ZP-40 ZP-0
0
10
20
30
40
50
60
70
Time / day Fig. 9. Time dependence of the charge transfer resistance (Rct) of different coatings exposed to 40 °C artificial sea water.
ing ZP-30 presented two time constant characteristic, and then it exhibited one time constant characteristic at the last immersion stage, indicating high protective nature of the coating against corrosion and a certain degree of self-healing effect. 3.3.3. Coating capacitance (Qc) The coating capacitance is a measure of water permeation into the coating. The variation of Qc with immersion time is shown in Fig. 10. The Qc of coating ZP-30 changes negligibly during the entire immersion period. In contrast, the Qc values of the other coatings fluctuate obviously during the entire immersion period. The water uptake can result in the swelling of the coating and blisters may subsequently occur at some thin or defect areas in the coating. The osmotic pressure due to the swelling of the coating might break through the blisters to form direct paths for corrosion species to access the metal substrate, resulting in corrosion reaction. In the case of coating ZP-30, the change in Qc is negligible, which indicates that coating ZP-30 has stable coating/metal interface without any indication of corrosion. This is due to the fact that there is just sufficient polymer/resin available to wet the pigment completely and fill the voids between the particles. The Qc value of coating ZP-0 increases gradually during the whole immersion time and it is several orders of magnitude higher than initial value, indicating that water permeates into coating ZP-0 easily and coating ZP-0 has lost its protection ability completely. The values of other three coatings have an obvious fluctuation during the whole
-3 -4
Qdl
Rs
6
2 -10
Qc
3
8
4
-5
(b)
ZP-10 ZP-20 ZP-30 ZP-40 ZP-0
−2
Rpore Rct
log (Qc) / F cm
2
10
experimental fitted
4
− Zim / MΩ cm
12
2
frequency. For n = 1, an ideal capacitor is defined. For n = 0, the CPE represents an ideal resistor. For n = 1, the CPE is equivalent with an inductance. Using a CPE instead of a capacitor provides the deviation from ideal capacitive behavior [38,39]. The faradic elements are in series with the pore resistance because the corrosion process is presumed to occur at the base of the pores in the coatings. In this condition, two time constant equivalent circuit (Model (a)) was used to fit the experimental data. Model (b) was used to fit the impedance of coating ZP-30 at the last immersion stage because the impedance of coating ZP-30 presented one time constant characteristic at the last immersion stage. Figs. 6 and 7 show the variations of the Bode and Nyquist plots with immersion time in 40 °C artificial sea water. Fig. 8 shows the fitted and experimental data for coating ZP-20. The EEC fitted well with the experimental results over a wide frequency range. The decrease in impedance for coatings ZP-0, ZP-10 and ZP-20 with immersion time indicates an increase in delaminated area where corrosion occurs. In contrast, the impedance values for coatings ZP-30 and ZP-40 have exceeded their initial values after 60-day immersion in 40 °C artificial sea water, which indicates that coatings ZP-30 and ZP-40 have better protection ability for the substrate than other coatings. The protection behaviors of all the coatings are discussed in terms of charge transfer resistance (Rct), pore resistance (Rpore), coating capacitance (Qc), and double layer coating capacitance (Qdl) below.
log(Rct ) / Ω cm
82
2
1
-6 -7 -8 -9
0
-10
0
1
2
3
4
5
2
Zre / MΩ cm
Fig. 8. Experimental and fitted data of coating ZP-20 after 6-day immersion.
-10
0
10
20
30
40
50
60
70
Time / day Fig. 10. Time dependence of the coating capacitance (Qc) of different coatings exposed to 40 °C artificial sea water.
Y. Hao et al. / Corrosion Science 69 (2013) 77–86
83
immersion time, which indicates that they are not stable and cannot provide effective protection for the substrate. 3.3.4. Pore resistance (Rpore) The pore resistances (Rpore) for different coatings with respect to immersion period are shown in Fig 11. The Rpore values of coatings ZP-0, ZP-10, and ZP-20 decrease with increasing exposure period. However, the Rpore values of coatings ZP-30 and ZP-40 increase gradually with increasing immersion time and their values have been higher than initial values after 60-day immersion. The Rpore value of coating ZP-30 increases gradually from 4.6 106 to 3.9 109 O cm2 after 60-day immersion, indicating excellent protective nature of this coating. The decrease in Rpore value with immersion time indicates that pores are opened and electrolyte permeates the coating. The trends of decreasing the Rpore values with respect to immersion period are observed in coatings ZP-0, ZP-10, and ZP-20. The Rpore values for these coatings are less than 106 O cm2, indicating poor protective nature of these coatings. 3.3.5. Double layer coating capacitance (Qdl) The Qdl values of various coatings are shown in Fig. 12. The double layer coating capacitance (Qdl) is a measure of the area over which the coating has been deteriorated. The increase in Qdl value is associated with water spreading at the interface and increase in delaminated area. A stable Qdl is an indication of a stable interface. By comparing the Qdl values of different coatings, coating ZP-30
11 ZP-10 ZP-20 ZP-30 ZP-40 ZP-0
10
log(Rpore) / Ω cm
2
9 8
Fig. 13. Photos of the coated panels after 60-day immersion in 40 °C artificial sea water, (a) ZP-10; (b) ZP-20; (c) ZP-30; (d) ZP-40; (e) ZP-0.
7
exhibits not only the lowest Qdl values but also remain constant over longer exposure. This indicates that this coating has stable coating/metal interface without any indication of corrosion.
6 5 4 3
3.4. Immersion test
2 1 -10
0
10
20
30
40
50
60
70
Time / day Fig. 11. Time dependence of the pore resistance (Rpore) of different coatings exposed to 40 °C artificial sea water.
-2 ZP-10 ZP-20 ZP-30 ZP-40 ZP-0
-3
log (Qdl) / F cm
−2
-4
Fig. 13 shows the images of coated panels after 60-day immersion in 40 °C artificial sea water. There are no rust extension and blisters at the scratched areas of coating ZP-30. However, the scratched areas of coatings ZP-0, ZP-10, and ZP-20 have a large amount of rust and considerable blistering, which indicates that they have poor adhesion strength to the substrate after 60-day immersion. A few blisters and slight rust extension can also be observed at the scratched areas of coating ZP-40. The immersion test confirms that epoxy coating containing 30% volume fraction of ZP gets the best protection ability. 4. Inhibition mechanism
-5 -6 -7 -8 -9 -10 -10
0
10
20
30
40
50
60
70
Time / day Fig. 12. Time dependence of the double layer coating capacitance (Qdl) of different coatings exposed to 40 °C artificial sea water.
The previous experimental results have confirmed that ZP can improve the anticorrosion properties of the epoxy coatings and its best volume fraction is 30%. How does ZP play a positive role in an epoxy coating? What is the inhibiting film on the steel surface? These questions are needed to be explained clearly. In this section, a series of analytical measurements are used to solve these questions. Fig. 14 shows the XPS spectra of the inhibiting film beneath coating ZP-30. Three peaks at 705.8, 707.8, and 719.0 eV in Fig. 14(a) belong to metallic iron (Fe0), FeO, and Fe3+, respectively [40,41]. The O1s spectrum yielded two peaks at binding energies of 529.8 and 532.1 eV. The peak of the O 1s spectrum centered at
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Y. Hao et al. / Corrosion Science 69 (2013) 77–86
40000
20000
35000
−1
21000
19000 18000 17000
30000 25000 20000 15000
16000 15000 695
(b)
45000
Contents / s
−1
Contents / s
50000
(a)
22000
10000 700
705
710
715
720
725
730
735
5000 520
740
525
Binding energy / eV
530
535
540
545
Binding energy / eV
9000
(c)
(d)
80000
8000 70000 −1
60000
Contents / s
Contents / s
−1
7000 6000 5000 4000
50000 40000 30000
3000 20000 2000 120
125
130
135
140
145
150
10000 1000
Binding energy / eV
1010
1020
1030
1040
1050
1060
Binding energy / eV
Fig. 14. XPS spectra of the inhibiting film beneath coating ZP-30 after 60-day immersion in 40 °C artificial sea water, (a) Fe, (b) O, (c) P, (d) Zn.
binding energy of 532.0 eV is most likely a convolution of a hydroxide component and a phosphate component [42,43]. A binding energy of 532.1 eV has been reported for FePO4. The peaks at 132.8 and 138.7 eV (Fig. 14c) belong to PO43 [44,45]. The peaks at 1045 eV and 1022 eV (Fig. 14d) belong to Zn 2p3/2 and Zn 2p1/2, respectively. The peaks assigned to a non-conductive form of Zn, indicating that ZP incorporated into the film [46]. SEM image and EDS spectrum of the inhibiting film beneath coating ZP-30 are shown in Fig. 15. The inhibiting film exhibits a cluster shape with main elements of P, O, Zn, and Fe. The XRD pattern of this inhibiting film is shown in Fig. 16. The peaks of a-FeOOH, c-FeOOH, Zn3(PO4)2, FePO4, Fe2O3, and FeO [34,47,48] are observed. a-FeOOH and c-FeOOH are typically the compositions of iron rust [47] formed at initial immersion time. With the elapsed immersion time, ZP released PO43 with the help of the endosmotic water. Then, steel reacted with PO43 in the presence of dissolved oxygen and formed a phosphating film which was composed of FePO4, Fe2O3, and FeO. Therefore, ZP has a phosphatization function to a certain degree. But, the phosphating products of ZP are different from the compositions in the previous reports [16,19,25]. Meanwhile, the peaks of ZP can be detcted in Fig. 16, indicating that ZP can also form a shielding film on the steel surface which is in accordance with Amo et al. [21] and Beiro et al. [22] reports. It can be concluded from the XPS and XRD data that the inhibiting film is mainly composed of the phosphating film of FePO4, Fe2O3, and FeO, as well as the shielding film of ZP. The formed inhibiting film can cover the corrosion products formed at initial immersion time and prevent from corrosion. Therefore, the inhibition mechanism of ZP includes two aspects: one is the phosphatization function; the other is the shielding effect. The self-healing function of epoxy coating containing ZP is due to the formation of the inhibiting film on the steel surface.
5. Discussion EIS and immersion results show that pigment volume concentration has an important effect on the protection ability of epoxy coating. Epoxy coating containing high concentration of ZP has better protection ability. The coating resistance values of coatings ZP30 and ZP-40 are higher than their initial values after 60-day immersion in 40 °C artificial sea water. In contrast, the coating resistance values of coatings ZP-10 and ZP-20 are less than their initial values. Epoxy coating containing low ratio of ZP to resin, such as coatings ZP-10 and ZP-20, releases less inhibitive species. In contrast, epoxy coating containing high ratio of ZP to resin can release enough inhibitive species and block pores within the coating film. The distance between pigment particles decreases when pigment concentrations increase further. The water films around the pigment particles may touch each other and form a capillary system between the pigment particles. The capillary system may lead to an increase in coating permeability [26]. Therefore, the impedance value of coating ZP-40 cannot exceed the value of coating ZP-30. Concerning protective properties of organic coatings, it is generally accepted that the electrical resistance of good coatings is larger than 108 O cm2, whereas poor coatings is less than 106 O cm2. The impedance of coating ZP-30 is one order of magnitude than its initial value, which indicates that coating ZP-30 has better protection ability than other coatings after 60-day immersion in 40 °C artificial sea water. For all the coatings, the protection sequence is as follows: ZP-30 > ZP-40 > ZP-10 ZP-20 ZP-0. As shown in Fig. 12, Qdl shows a fluctuating trend for epoxy coating containing ZP. This is the result of both the delamination effect caused by corrosive attack and the healing effect by the formation of inhibiting film during the immersion [49]. All the epoxy coatings containing ZP have lower Qdl than the pure epoxy coating,
Y. Hao et al. / Corrosion Science 69 (2013) 77–86
85
(1) Polarization curves of the mild steel in pigment extract confirm that the dissolved ZP in solution has an obvious inhibition effect. (2) EIS and immersion tests illustrate that ZP can improve the protection ability of epoxy coating and its best volume concentration is 30% in epoxy coating. Epoxy coating containing ZP has a self-healing function to a certain degree. (3) The inhibition mechanism of ZP in an epoxy coating is attributed to the synergistic effect of the shielding and phosphatization functions of ZP. Acknowledgements The authors thank the National Key Technology R&D Program (Grant Nos. 2009BAE70B02 and 2012BAB15B00), China Postdoctoral Science Foundation funded project, for supporting these studies. References
Fig. 15. SEM image and EDS spectrum of the inhibiting film beneath coating ZP-30 after 60-day immersion in 40 °C artificial sea water, (a) SEM image, (b) EDS spectrum.
2500 • α-FeOOH ♦ γ−FeOOH ο Fe O
2000 Intensity (cps)
2
∗
3
♥ Zn (PO ) 3 4 2 ∗ FeO
1500
♣ FePO 4
1000 500
♣ ♦♥
• ο
♦
•
ο ο♥ ♥
0 10
20
30
40 2θ
50
60
70
Fig. 16. XRD pattern of the inhibiting film beneath coating ZP-30 after 60-day immersion in 40 °C artificial sea water.
indicating that ZP has an inhibition effect. Qdl values of epoxy coating containing ZP are close to Qc values, which is due to the formation of an inhibiting film on the steel surface beneath the epoxy coating. In general, the epoxy coatings with lower capacitances Qc and Qdl performs well in corrosive media [50]. 6. Conclusions In the present work, the inhibition mechanism of ZP and its optimized concentration in an epoxy coating have been discussed. The main conclusions can be summarized as follows:
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