Studies of the impedance models and water transport behaviors of cathodically polarized coating

Electrochimica Acta 56 (2011) 5828–5835

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Studies of the impedance models and water transport behaviors of cathodically polarized coating Chengfei Zhu ∗ , Rui Xie, Jinhua Xue, Linlin Song College of Materials Science & Engineering, Nanjing University of Technology, Nanjing 210009, China

a r t i c l e

i n f o

Article history: Received 12 March 2011 Received in revised form 9 April 2011 Accepted 13 April 2011 Available online 28 April 2011 Keywords: Cathodic protection Chlorinated rubber coating Electrochemical impedance spectroscopy Electrical equivalent circuit Water diffusion

a b s t r a c t Cathodic protection (CP) is usually combined with organic coatings to protect metallic structures exposed to seawater. However, the application of CP would enhance coating failure, such as cathodic delamination. To date, there has been few works characterizing the impedance models and water transport behaviors of cathodically polarized coating. In the present article, the analyses of impedance models and water uptake processes of chlorinated rubber coating subjected to various levels of cathodic protection were studied during coatings aging process by electrochemical impedance spectroscopy (EIS). Four distinguished electrical equivalent circuits (EEC) were used to fit the EIS plots of coatings without CP, while only two were employed for samples with CP. Since no corrosion was expected to take place at the metal/coating interface for sample which was polarized cathodically. Coating capacitance was used to investigate the sorption characteristic of water in coating since the increase of Cc was associated with water penetration into the coating. Compared with the sample without CP, those coating systems under CP have a smaller water diffusion coefficient and a further water uptake process after the saturation period. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved.

1. Introduction Organic coatings are often associated with cathodic protection (CP) in order to protect metallic structures exposed to seawater [1–10]. With such association, coatings defects and damages produced during the degradation of coating can be protected by cathodic current and further delamination of coating can be avoided, which finally prolonged the lifetime of the coated metallic structural equipments. Meanwhile, the current demand for CP decreases and a substantial power economy can be expected [4,8,9]. During the past decades electrochemical impedance spectroscopy (EIS) has been extensively used to monitor coating degradation, the reactants (such as water, oxygen and so on) transport and the charge transfer characteristic [3–5,11–17]. By using EIS, significant progress has been made in the mechanistic understanding organic coatings aging process and water uptake behavior [3–6,8,11–14,18–22]. Several valuable equivalent electric circuits (EECs) have been proposed to fit the EIS data during coatings aging process [12–14]. However, little work has been conducted to reveal the evolution of coatings impedance models with applying CP. The CP prevents corrosion but causes the cathodic

∗ Corresponding author at: 5 Xinmofan Road, Nanjing 210009, Jiangsu, China. Tel.: +86 25 83172117. E-mail address: [email protected] (C. Zhu).

surface to become strongly alkaline and generates hydrogen at exposed metal surfaces. Alkalization is the predominant reason for cathodic disbonding which proceeds either through hydrolysis of the interfacial bonds that attach the coating to the substrate resulting in direct disbonding, or through hydrolysis of the coating itself de-polymerization [1,7–9,11,23–25]. The main reason, which causes the increase of alkalinity is the water reduction under cathodic polarization. Many studies have been carried out on coatings water uptake process, and there has been a significant progress in the mechanistic understanding of water diffusion behavior [12–14,18–22,26–29]. Nevertheless, almost no studies have been conducted to reveal the water uptake process of organic coatings with CP, the influence of CP on coatings barrier properties is still unknown. In the present article, we report an attempt to evolve impedance models of cathodically polarized coated metals in seawater by continuous equivalent electric circuit (EEC) simulation of EIS plots and to determine diffusion coefficient of water in organic coatings from the initiation time of the species related circuit element(s) appeared in EEC.

2. Experimental The chemical composition of type A3 mild steels was 0.20% C, 0.30% Si, 0.60% Mn, 0.015% P, 0.009% S, and bal. Fe (weight fraction). The size of A3 mild steel was 100 mm × 100 mm × 3 mm. They were abraded by abrasive papers (No. 220#), washed with ethanol

0013-4686/$ – see front matter. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.04.068

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Fig. 1. The schematic diagram for the principle of immersion tests under CP and EIS measurements.

and acetone, and then coated by brushing at the room temperature (∼298 K). The paint used in this study was a commercial chlorinated rubber (type 968 from Jiangsu Changjiang Paint Company Limited, Nanjing, China). The coated samples were kept in a desiccator for three weeks to guarantee complete solidification and then electrochemical tests were conducted. The coatings thickness was measured by using a German FISCHER-MPOR equipment with a precision, in the thickness range of studied materials, of ±5 ␮m. The average value of coating thickness was 100 ␮m. Coated samples with different CP potential were immersed in seawater. The seawater was lab-made in the light of the Moclendon’s prescription which had been widely used to simulate natural seawater, and the chemical position is (g/L): CaCl2 1.220, MgCl2 ·H2 O 5.105, MgSO4 ·7H2 O 7.035, KCl 0.763, NaBr·2H2 O 0.082, NaCl 28.270, NaHCO3 0.210, Na2 SiO3 0.003, HNO3 0.062, Al2 Cl6 ·12H2 O 0.026, LiNO3 0.001. The solution was made from analytic grade reagents (Fisher Scientific) and ultra-pure water (18 M cm). The immersion solutions were renewed every two weeks to prevent corrosion products from falling into the solution. A plastic tube was attached on the coatings surface using a twocomponent epoxy adhesive. The schematic diagram of immersion test and EIS measurement is shown in Fig. 1. EIS measurements proceeded in a classic three-electrode system. Taking into account the adhesive joint, the inner diameter of the plastic tube is 20 mm (tested area is 314 mm2 ). Four different continuous immersion conditions were chosen: at free corrosion potential and at three cathodic protection potentials −0.85 vs. SCE/V, −0.95 vs. SCE/V and −1.1 vs. SCE/V. The solution was directly contacted with the air and the temperature of the solution was set at 313 K both for experiment and EIS measurement. In Fig. 1, the reference electrode is a saturated calomel and the counter electrode is a stainless steel disk. EIS measurements were performed with a frequency response analyzer (Solartron 1260) and an electrochemical interface (SI 1287) at the free corrosion potential. The applied frequency range was 105 to 10−2 Hz with signal amplitude of 20 mV. Data analysis was performed by the software ZView using classical electrical equivalent circuits. For each coating system, three parallel experiments were carried out, and the middle one was selected among the three curves obtained. 3. Results and discussion 3.1. Effect of CP on the evolution of coatings impedance spectra In the present article, an A3 steel/chlorinated rubber system with a thickness of 100 ␮m and an A3 steel/chlorinated rubber/CP (−0.85 vs. SCE/V) system with the same thickness were selected as examples to demonstrate the effect of CP on the evolution of coat-

ings impedance. The data analysis of the contribution of capacitive elements to the total impedance was generally obtained by using a constant phase element (CPE) instead of a pure capacitance considering the non-ideal behavior of the organic coatings [12,14,18,30]. All the data presented are therefore obtained as CPE, but also called capacitance for simplicity. The impedance of CPE is defined by the following equation [12,14,30]: Z(jω) = (Y0 )−1 (jω)

−n

(3.1)

where Y0 is the CPE-constant, j is the imaginary unit, n the CPEpower (0 ≤ n ≤ 1), and ω is the angular frequency (ω = 2␲f, f is the frequency). For n = 1 the CPE is a pure capacitance. Prior to the measurements on coated metals under CP, the EIS measurements on coated samples under the open circuit condition in seawater were carried out. Fig. 2 shows the impedance spectra of coated metal in seawater at the free corrosion potential immersed for different time. Coatings presented a typical capacitive behavior and can be treated as pure electric capacitor when immersed for 0 h [12]. After a short time of immersion, the impedance spectra began to deviate from the purely capacitive behavior and a resistive behavior at low frequency was visible, indicating that the coating resistance decreased and the coating capacitance increased as a consequence of water uptake in the painted film [12–14]. The impedance spectra of the sample immersed for 30 min is shown in Fig. 2a. Then the coating was equivalent to a barrier layer with a high-value coating resistance in parallel with a low-value coating capacitance, which is presented by the EEC (model A) in Fig. 3a. Rs was the solution resistance; Cc and Rc were the coating capacitance and the coating resistance, respectively. The diagram of the sample immersed in seawater for 28 h is displayed in Fig. 2b. Model B (see Fig. 3b) was introduced to fit the data considering that water and oxygen molecules reached the substrate surface and the electrochemical reactions at the metal/coating interface may take place. Cdl was the double-layer capacitance and Rct was the charge-transfer resistance. However, only one capacitive loop is phenomenally observed in Fig. 2b. The reason was probably that the electrochemical reaction area at metal/coating interface was yet small in this immersion time, which led to difficulty in distinguishing the time relaxation of coating’s physical impedance from that of electrochemical reaction impedance at metal/coating interface [12,13]. After being immersed for 28 h, model B was constantly applied to fit the impedance spectra as the immersion time increased until the immersion time arrived at 216 h (see Fig. 2c). Prolonging the immersion time to about 240 h produced a small tail at low frequencies in complex plan (see Fig. 2d), and the spectra can no longer be satisfactorily fitted to model B. This tail can be correlated to diffusion processes caused by the presence of corrosion

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Fig. 2. EIS plots (left: Nyquist, right: Bode) and fitting results of chlorinated rubber/A3 steel system without CP immersed in seawater for different time: scatters: experimental data; solid line: fitting results.

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Fig. 3. Electrical equivalent circuits (EEC) used to fit the EIS plot.

products on the surface of electrochemically active site [12–14]. Therefore model C (see Fig. 3c) was introduced to fit the spectra, of which the element Warburg impedance (W1) was introduced to characterize the corrosion products. The simulation result is also shown in Fig. 2d. However, the diffusion behavior was not usually an ideal Warburg impedance, due to a deviation of dispersive number (n) from 0.5. The non-ideal Warburg impedance behavior was commonly associated with “finite-layer diffusion” or the tangential penetration of electrolyte or heterogeneous penetration of an electrolyte [12–14]. After 288 h of immersion the low-frequency tail disappears and is displaced by a loop as shown in Fig. 2e, which indicates that the diffusion behavior can no longer be characterized by Warburg impedance. J.T. Zhang suggested that the coating had a barrier performance against the diffusion of corrosion products, and that the diffusion process could become a control procedure in Faraday processes [13]. Therefore, model D (see Fig. 3d) which contained the diffusion capacitance (Cdiff ) and the diffusion resistance (Rdiff ) was introduced to fit the impedance spectra [12–14]. Fig. 2e shows the EIS data fitted to model D and the simulation results. Fig. 4 presents the impedance spectrums and fitting results of chlorinated rubber/A3 steel system under cathodic potential of −0.85 vs. SCE/V immersed in seawater for different time. As

indicated in Fig. 4a, the coating system presented a high barrier performance at the beginning of immersion (30 min). Model A in Fig. 3a is used to fit the impedance spectra (see Fig. 4a), and the fitting result is also shown in Fig. 4a. After being immersed for 32 h, the anti-corrosion performance decreased and the EIS of coating system with CP is presented in Fig. 4b. Considering that water and oxygen molecules reach the substrate surface and the electrochemical reactions at the metal/coating interface may take place, model B (see Fig. 3b) was introduced to fit the experimental data. After 32 h immersion, model B was constantly used to fit the impedance spectra as shown in Fig. 4c and d, which indicated that no sensible corrosion products were produced for samples with CP during the coatings aging process. Table 1 illustrates the quality of the fitting procedure. By comparing Fig. 2a and b with Fig. 4a and b, it’s not difficult to discover that no obvious differences exist between coatings system with and without CP at the initial stage of immersion. Coating without any defect revealed a high barrier property, making a separation between the corrosive media and the substrate. Thus, the imposed voltage almost had no influence on coatings impedance spectra at the initial stage. With the increasing of the immersion time, models C and D were respectively introduced to fit the EIS of coatings

Table 1 Parameters and error values in the EIS analysis of coating. Coating without CP 0.5 h 2

Rc ( cm ) Error% n1 Error% Cc (F) Error% Rct ( cm2 ) Error% n2 Error% Cdl (F) Error% Rdiff ( cm2 )/W1-R Error% n3 (W1-P) Error% Cdiff (F)/W1-T Error%

Coating with CP

28 h

1.08 × 10 4.38 0.94 0.13 2.43 × 10−10 0.84 10

216 h

9.42 × 10 4.50 0.93 0.38 4.51 × 10−10 2.67 6.14 × 108 8.08 0.66 5.07 3.86 × 10−9 6.81 8

240 h

9.46 × 10 1.06 0.93 0.13 5.83 × 10−10 0.97 1.72 × 107 7.03 0.87 3.97 6.41 × 10−8 4.78 7

288 h

2.67 × 10 3.03 0.94 0.30 1.39 × 10−9 2.78 9.67 × 107 5.98 0.93 4.24 9.21 × 10−10 4.78 8.14 × 107 6.90 0.30 4.98 157.23 5.80 7

0.5 h

4.86 × 10 1.09 0.95 0.09 8.37 × 10−10 0.78 3.32 × 107 4.83 0.95 2.37 4.84 × 10−10 3.20 1.39 × 107 1.29 0.84 0.089 4.23 × 10−8 0.84 6

32 h

5.01 × 10 3.98 0.96 0.04 2.96 × 10−10 1.30 9

216 h

4.39 × 10 5.02 0.92 2.95 5.82 × 10−10 4.33 6.12 × 108 4.10 0.93 0.32 4.43 × 10−10 1.54 8

312 h

8.65 × 10 2.18 0.93 0.04 5.67 × 10−10 0.78 1.87 × 107 5.40 0.89 2.73 1.94 × 10−8 3.45 7

1.96 × 107 4.32 0.82 0.98 7.86 × 10−10 1.33 5.76 × 107 2.84 0.93 0.12 5.21 × 10−8 1.16

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⋅ ⋅

⋅ ⋅ ⋅ ⋅

7

⋅

⋅ ⋅ ⋅ ⋅

Fig. 4. EIS plots (left: Nyquist, right: Bode) and fitting results of chlorinated rubber/A3 steel system with CP (−0.85 vs. SCE/V) immersed in seawater for different time: scatters: experimental data; solid line: fitting results.

without CP as the corrosion products were continually produced and accumulated at metal/coating interface. However, model B was constantly applied to fit the impedance spectra of coatings with CP, indicating that almost no corrosion products were produced and accumulated at the metal/coating interface during the degradation of coatings system with CP. After the immersion test, organic coating was respectively removed from the metal surface for samples under different CP conditions. It was found that there existed obvious corrosion products on the metal surfaces of samples without CP. This was because seawater and reactants gradually diffused into

the intact coating-steel interface which directly led to the corrosion reactive of steel. However, no corrosion product was observed for samples with CP. Since there was a cathodic current and the steel surface was polarized cathodically, no corrosion was expected to take place and no corrosion product was expected to accumulate at the metal/coating interface. Same thing went for the other two samples with CP (−0.95 vs. SCE/V, −1.1 vs. SCE/V). It can be concluded that, for samples with CP (minus than −0.8 vs. SCE/V), the imposed cathodic potential was supposed to protect steel from corrosion during coatings degradation process since the

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(a)

(b)

-22.38

-22.38 Experimental Fitted

-22.40 -22.41

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ln(Cc/F)

ln(Cc/F)

-22.42 Experimental

-22.44

-22.44

Fitted

-22.46 -22.47

-22.48 -22.50

0

5

10 1/2 1/2 t (h )

15

20

-22.50

0.5

1.0

1.5 2.0 1/2 1/2 t (h )

2.5

3.0

Fig. 5. ln Cc –t1/2 curve of chlorinated rubber/A3 steel system without CP current immersed in seawater (a) and the linear region in the initial period of immersion (b). Scatters: experimental data; solid lines: linear fitting.

sample was polarized cathodically and no corrosion was expected to occur. It’s thus logical that only two EECs were employed to fit EIS plots of coatings with CP. 3.2. Effect of CP on the diffusion behavior of water through organic coatings The anticorrosive performance of organic protective coatings was controlled by a number of critical factors with one of the most important being the migration of water from an external environment to the coating-substrate interface [22]. It was therefore of great importance to understand the kinetics and the nature of diffusion from aqueous solutions into organic coatings when different CP conditions applied. Coating capacitance had been widely used to investigate the sorption characteristic of water in coating since the increase of Cc was associated with water penetration into the coating [12–15,18–22], and the capacitance–time curve (ln Cc –t1/2 ) was widely used for the determination of water uptake behavior [12–14]. Figs. 5 and 6 account for the effect of CP on coatings water transport behavior, respectively. The ln Cc –t1/2 curve for specimen without CP shown in Fig. 5 indicates that the coating capacitance presented two typical stages as the immersion time increased. The coating capacitance increased rapidly in the beginning time of immersion (0–8.7 h) and then tended to be a steady value after ∼8.7 h. The rapid increase of the coating capacitance in the initial period of immersion suggested that the water could permeate into the coatings through the micro-pores formed by solvent volatilization. The linear increase of ln Cc –t1/2 curve meant that water diffuses in a homogeneous way and the water transport behavior followed the Fickian law [12–14]. After a certain time of immersion, the coating capacitance reached a steady value, indicating that the water uptake of the coating attained a saturation state. The ln Cc –t1/2 curve in Fig. 6 presents the water uptake process for coating with CP. Fig. 6a indicates that the coating capacitances had three stages as the immersion time increased. During the first two stages, the evolution of ln Cc –t1/2 curve was generally similar to the curve of sample without CP and the difference would be discussed detailed in the following text. Generally, the first stage of the ln Cc –t1/2 curve can be fitted by the linear equation (3.2): y = a + bx,

x = root(t)

(3.2)

y is the coating capacitance at time t1/2 , a is the coating capacitance at time zero, b is the increasing slope of coating capacitance and t is the immersion time. Parameters for all of the four samples in the first stage are listed in Table 2, of which R2 is the fitting deviation. Obviously, Eq. (3.2) could simulate the first stage of ln Cc –t1/2 curve well for all four samples according to the value of R2 . In Table 2,

Table 2 Parameters in the first stage of ln Cc –t1/2 curve for samples under different CP conditions. CP imposed vs. SCE/V

a

102 × b (s−1/2 )

R2 (%)

0 −0.85 −0.95 −1.1

−22.53 −22.51 −22.53 −22.53

5.02 3.26 3.49 3.70

95.65 97.24 95.32 95.67

all of the four samples had a similar a because all the four samples had a similar barrier performance before immersion which was in accordance with the fact. However, the increasing rate of coating capacitance was different as can be seen from the value b, which indicated that cathodic current had effect on coatings water uptake process. In order to know much more about the effect of CP on coatings water uptake process, the water diffusion coefficient D was therefore introduced for a further interpreting of the water amount taken up by the coating. The water amount taken up by the coating could be calculated from the capacitance values using the following relationship [14]: M(t) =

LSw ln ln εw

Ct C0

(3.3)

Ct and C0 are the coating capacitance at time t and zero, respectively. The value of capacitance Ct is normally obtained from impedance measurements at high frequencies using the Eq. (3.4) [13,16]: Cc =

1 2fZ 

(3.4)

where f is selected as 10 kHz and Z is the imaginary value when frequency reached 10 kHz. The value of C0 is extrapolated from the values of capacitance determined from the initial stages of immersion. S is the surface area of coating, L is the coating thickness, w is the density of water absorbed into the polymer, and εw is the dielectric constant of pure water (∼80). Eq. (3.3) is usually employed as [14,19,20]: M(t) log Ct − log C0 = M∞ log C∞ − log C0

(3.5)

C∞ is the coating capacitance at saturation, and M∞ is the amount of absorbed water at equilibrium. Eqs. (3.3) and (3.5) were deduced assuming that [14,19,20]: (1) a linear relationship between the permittivity of the polymer–water system and those of the pure components, (2) a random distribution of water, and (3) a linear relationship between permittivity and capacitance.

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(a)

(b)

-22.41

-22.36

Experimental Fitted -22.44 ln(Cc/F)

ln(Cc/F)

-22.40 -22.44 Experimental

-22.48

-22.47

Fitted

-22.50

-22.52 0

5

10 15 1/2 1/2 t (h )

20

25

0

1

2 1/2 1/2 t (h )

3

Fig. 6. ln Cc –t1/2 curve of chlorinated rubber/A3 steel system with CP (−0.85 vs. SCE/V) immersed in seawater (a) and the linear region in the initial period of immersion (b). Scatters: experimental data; solid lines: linear fitting. Table 3 Analysis results of saturation capacitance and water diffusion coefficient for samples under different CP conditions. CP imposed vs. SCE/V

D/×10−13 m2 s−1

ln C∞ /F

OCP −0.85 −0.95 −1.1

2.52 1.82 1.83 2.18

−22.38 −22.41 −22.43 −22.41

For an ideal Fickian behavior, the water diffusion coefficient is constant, and can be derived from [14,19,20]: 1 M(t) 8  =1− 2 exp M∞  (2n + 1)2 ∞

n=0



−(2n + 1)2 D2 t L2



(3.6)

Deduced from Eqs. (3.5) and (3.6), Eq. (3.7) was obtained, which was most extensively used in calculating the water diffusion coefficient [12–14,19–22]: √ log Ct − log C0 2 t√ = √ D (3.7) log C∞ − log C0 L  Using Eq. (3.7), the water diffusion coefficient (D) for coating system with different CP were obtained and listed in Table 2. As shown in Table 3, the water diffusion coefficient varied widely for samples with different CP conditions. It was visible that the diffusion coefficient of samples with CP (−0.85 vs. SCE/V, −0.95 vs. SCE/V, −1.1 vs. SCE/V) was smaller than the sample without CP. Generally speaking, the exposed metal area increased as the immersion test continued for coating without CP, which caused the electrochemical reaction and the decrease of the coating performance. However, application of CP would protect the exposed active area and finally prolong the water uptake process. It can be seen from Table 3 that the water diffusion coefficient increased with the increasing of the imposed cathodical potential. Cathodic delamination was one of the main degradation processes for organic coatings on steel structures under CP [1–11,23–25]. It’s generally believed that the major driving force for cathodic delamination in corrosion processes in the presence of air was the cathodic reaction [1–5,7,10,23,24,30,31]: H2 O +

1 O2 + 2e− = 2OH− 2

(3.8)

when an applied potential is used, the important reaction may be: 2H+ + 2e− = H2

the water transport pathway increased. Consequently, for samples with CP, the water diffusion coefficient increased with the increasing of the imposed cathodic potential. For coating system with different CP conditions, the saturation capacitance (ln C∞ ) was almost the same, which indicated that the imposed CP almost had no influence on coatings saturation capacitance in the stable stage. Since the saturation stage was the saturation of the polymeric matrix [18] which was only concerned with microstructure and properties of the coating itself. After the saturation state there was a further increase in the water content for samples with CP due to more water accumulation in the coating, which could be explained as to supply more water for cathodic delamination process. After an intensive immersion, there was sufficient water and oxygen accumulated at the coating/metal interface and cathodic delamination started to play an important role for coatings with CP. As the cathodic delamination increased, more water was needed to supply cathodic reactions (3.8) and (3.9) as mentioned above. Consequently, a further increase stage of water appeared for samples with CP. 4. Conclusions By using EIS, the effect of cathodic protection on coatings impedance models and water transport behavior were studied. Four distinguishing impedance models were used to fit the spectra of the sample without CP. In the initial time of immersion, an EEC containing the coating capacitance and the coating resistance was used to fit the EIS data. Along with the immersion time, corrosion reaction took place, led to the forming of corrosion products at metal/coating interface which resulted in the appearance of diffusion elements. However, for samples with CP, only two models were employed to fit the EIS data. Since no corrosion reaction was expected to take place on the metal substrate and no corrosion product was expected to accumulate at the metal/coating interface. A “two-stage sorption” for water in coatings without CP was observed from the time dependence of coating capacitance. However, the ln Cc –t1/2 curves of samples with CP demonstrated a further water uptake process after saturation period in contrast to those without CP. The water diffusion coefficient of samples with CP was smaller than the sample without CP. Generally, the water diffusion coefficient for samples with CP increased with the increasing of the imposed cathodic potential. References

(3.9)

Based on the reaction formulas (3.8) and (3.9), it was found that the driving force for cathodic delamination was enhanced as the imposed cathodic potential increased. It’s therefore necessary that more water was needed for reaction formulas (3.8) and (3.9) and

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