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Effect of Polyacrylic Acid on the Corrosion Behaviour of Carbon Steel and Magnetite in a Simulated Wet layup Condition Paper Number: 142 Geun-Dong Song1,2, Soon-Hyeok Jeon1, and Do Haeng Hur1 1

Nuclear Materials Safety Research Division, Korea Atomic Energy Research Institute, Republic of Korea 2 Department of Advanced Materials Science and Engineering, Sungkyunkwan University, Republic of Korea ABSTRACT Polyacrylic acid (PAA) has been considered as a potential dispersant to mitigate corrosion product accumulation in steam generators of pressurized water reactors. In this work, the effect of polyacrylic acid on the corrosion behaviour of carbon steel and magnetite was investigated in alkaline aqueous solutions at 25 oC using potentiodynamic polarization tests and zero resistance ammetry (ZRA). Magnetite specimens were prepared by electrodeposition on the carbon steel substrate in a solution consisting of 2 M sodium hydroxide, 0.1 M triethanolamine and 0.043 M Fe(III) sulfate hydrate. The

corrosion current density of carbon steel was increased by the addition of 100 ppm polyacrylic acid, whereas that of magnetite was decreased. Carbon steel acted as the anode of the galvanic couple between carbon steel and magnetite, regardless of the presence of polyacrylic acid. In this couple, the galvanic corrosion current density of carbon steel decreased by the addition of 100 ppm polyacrylic acid. INTRODUCTION Magnetite particles formed on the surface of secondary feed water pipe in pressurized water reactors (PWRs) are carried with the feed water flow to the secondary side of a steam generator. They are deposited on heat transfer tubes and accumulated on the tube sheet and support plates in the secondary side of a steam generator. Aggressive chemical impurities may become concentrated in the magnetite deposits, thereby accelerating corrosion degradation of steam generator tubes [Abellà et al. (1998), Odar (2004), Prusek et al. (2013)]. In addition, the magnetite adhering to steam generator tubes not only reduces heat transfer from the primary to the secondary side but also distorts eddy current signals from the tubes during in-service inspection [Srikantiah and Chappidi (2000), Hur et al. (2003), Bakhtiari et al. (2009)]. To mitigate these problems, a method using a polymeric dispersant for the removal of magnetite has recently been proposed [Fruzzetti (2009), Roy et al. (2014)]. It was reported that a polymeric dispersant could be physically adsorbed on the surface of various oxide particles including magnetite [Viota et al. (2005), Hajdù et al. (2012)], alumina [Das and Somasundaran (2003)], titanium oxide [Strauss et al. (1993), De Laat et al. (1995)] and zirconium oxide [Chibowski et al. (2004)]. This adsorption influences the dispersion stability of the oxide particles because the zeta potential of the oxide particles is affected by the adsorption of dispersants [Holmberg and Jonsson (2002)]. The zeta potential of magnetite particles is negative under alkalized reducing conditions of pH 9 or higher, which is typical secondary water chemistry of PWRs [Song et al. (2014)]. Thus, an anionic polymer such as polyacrylic acid (PAA) should be selected to increase the zeta potential of magnetite particles. When PAA is physically adsorbed on the surface of magnetite particles, it increases the steric and electrical repulsion forces between magnetite particles [Hajdú et al. (2009), Tombácz et al. (2013)].

20th NPC International Conference Brighton, United Kingdom - October 2-7, 2016 Paper Number: 142

Therefore, magnetite particles do not aggregate and can be stably dispersed in the secondary water. As a consequence of this dispersion, the deposition of magnetite particles on the surface of the steam generator tubes is minimized. Consequently, the particles can be easily removed from the steam generator through blowdown.

If PAA is used as a polymeric dispersant to mitigate fouling phenomena in a steam generator, it is necessary to evaluate the effects of PAA on the corrosion behavior of carbon steel in secondary environments of PWRs. Some authors have reported that the addition of PAA inhibits the dissolution of aluminium alloy [Amin et al. (2009), Umoren et al. (2010)] and carbon steel [Umoren et al. (2011), Zhang et al. (2015)] in different media owing to the adsorption of PAA on their surfaces. In addition, it was reported that the corrosion rate of mild steel increased with increasing anionic polymer concentration by 50 or 100 ppm in a solution with higher calcium ion concentrations [Sekine et al. (1992)]. However, the effects of PAA on the corrosion of carbon steel in simulated secondary water of PWRs are unclear. The addition of PAA increased the corrosion rate of carbon and low-alloy steels in simulated secondary system environment [Fruzzetti (2005), Song et al. (2014)]. By contrast, the presence of PAA did not affect the corrosion rate of carbon and low-alloy steels that had been preconditioned to form an oxide layer on their surfaces similar to the oxide layer formed under operating conditions in a secondary system [Fruzzetti (2011), Joshi et al. (2013)]. Furthermore, the mechanism of interaction between PAA and carbon steel has not been elucidated. There are also very few papers that address the electrochemical behavior of magnetite specimens in a simulated secondary environment. Although magnetite is the root cause of steam generator fouling, it protects carbon steel piping from general corrosion and from flow-accelerated corrosion in secondary system environments [Laronge and Ward (1999), Robinson and Drews (1999)]. The surface of carbon steel is typically covered with magnetite under reducing operation conditions. Consequently, the galvanic cell between carbon steel and magnetite can be formed because they are electrically contacted. Some authors have reported that the corrosion rate of carbon steel is increased by the galvanic coupling with magnetite [Fushimi et al. (2002), Al-Mayouf (2006), Jeon et al. (2015)]. Therefore, it is important to evaluate the effect of PAA on the corrosion behavior of magnetite as well as that of carbon steel. In order to evaluate the electrochemical behavior of magnetite, it is necessary to simulate magnetite deposited on the surface of carbon steel in a secondary circuit system. Electrodeposition is one method that can be used to produce a magnetite layer on a carbon steel substrate [Kothari et al. (2006), Kulp et al. (2009), Goujon et al. (2013)]. In this study, magnetite specimens were prepared using the electrodeposition method to evaluate corrosion behavior of magnetite. The effect of PAA on the corrosion behavior of carbon steel and magnetite was investigated using electrochemical techniques under simulated wet layup conditions of the steam generator. The mechanism of interaction of PAA with carbon steel and magnetite is also discussed. EXPERIMENTAL Preparation of carbon steel and magnetite specimens Carbon steel specimens were machined from SA106Gr.B pipe material into a size of 10 mm x 5 mm x 1 mm. The specimens were wet-ground with silicon carbide papers down to # 1000 grit and then ultrasonically cleaned in ethanol for 5 min. Magnetite specimens were prepared by the electrodeposition of magnetite on the carbon steel substrate in a solution consisting of 2 M sodium hydroxide, 0.1 M triethanolamine and 0.043 M Fe(III) sulfate hydrate. The electrodeposition was conducted using a PAR273A potentiostat and a three-electrode cell. A saturated calomel electrode (SCE) and a graphite rod were used as a reference and counter electrodes, respectively. Magnetite layer was electrodeposited on the carbon steel substrate at an


applied potential of -1.05 VSCE for 1800 s at 80 oC. After the electrodeposition of magnetite was completed, magnetite specimens were carefully rinsed with deionized water and dried in a desiccator. The detailed electrodeposition process is given in the previous studies [Kothari et al. (2006), Kulp et al. (2009), Goujon et al. (2013), Jeon et al. (2016)]. The morphology of magnetite layer electrodeposited on the carbon steel substrate was analysed using a QUANTA 3D FEG focused ion beam-scanning electron microscope (FIB-SEM). The magnetite layer was milled using FIB in the vertical direction of magnetite to observe a cross section of magnetite on the carbon steel substrate. X-ray diffraction (XRD) pattern of magnetite layer was obtained using a D/Max-2500 X-ray diffractometer with Cu-K radiation (λ=1.5406 Å ). XRD scan was run from 2θ values of 10o to 100o. The lattice parameter of magnetite layer was determined using JADE version 9.0 software. Electrochemical corrosion tests Electrochemical corrosion tests were conducted to evaluate the effects of PAA on the corrosion behavior of carbon steel and magnetite in alkaline aqueous solutions. The base test solution was deionized water with a pH of 9.5. To study the effect of PAA, PAA with a molecular weight of 100,000 g/mol was added to the base solution. Under wet layup conditions of steam generators, the use of higher PAA concentrations on the order of a few hundred ppm than qualified for online addition on the order of a few ppb may be acceptable. This is because wet layup applications would be much shorter time and at lower temperature compared to online addition [Fruzzetti (2011)]. Accordingly, PAA concentration of 100 ppm (wt.) was chosen to investigate wet layup applications. Regardless of PAA addition, the final pH of all test solutions at 25 oC was adjusted to 9.5 with ethanolamine, which is an organic chemical agent used to control the pH of secondary water in PWRs. All electrochemical corrosion tests were carried out under a deaerated condition at 25 oC. For a deaerated condition, the test solutions were continuously purged with high-purity nitrogen gas (99.98 %) at a rate of 600 cm3/min during testing. This test environment was designed to simulate wet layup conditions in steam generators. The detailed parameters for wet layup are given elsewhere [Fruzzetti (2009)]. Polarization tests were conducted using a PAR273A potentiostat and a three-electrode cell. A SCE and platinum wire were used as a reference and counter electrode, respectively. The open circuit potential (OCP) of each working electrode was stabilized within 1 h. After that, a polarization scan was started from 10 mV below the OCP to the anodic direction or from 10 mV above the OCP to the cathodic direction. The scan rate was 1 mV/s. Each anodic and cathodic polarization curve was finally combined in one graph. In the case of magnetite, each cathodic and anodic polarization curve was obtained from a newly prepared sample because magnetite can be reductively dissolved by cathodic reaction [Lister and Lang (2002), Vepsäläinen and Saario (2010)]. The corrosion current density (icorr) of carbon steel and magnetite were calculated by using the cathodic Tafel-extrapolation of polarization curves. The galvanic corrosion potential (Ecouple) and the galvanic current density (icouple) between carbon steel and magnetite were predicted by the application of the mixed potential theory. In addition, a Gamry Reference 600 equipped with a zero resistance ammeter (ZRA) was used to measure the actual Ecouple and icouple between carbon steel and magnetite. The area ratio of carbon steel and magnetite was 1:1. After the OCP of carbon steel and magnetite were stabilized, the actual Ecouple and icouple of the couple were measured for 3600 s. All electrochemical corrosion tests were conducted at least three times to confirm the reproducibility. The corrosion potential (Ecorr), icorr, Ecouple and icouple were presented by the average with a standard error, and good reproducibility was confirmed. Potentiostatic tests were also conducted to verify the effects of PAA on the corrosion morphologies of both carbon steel and magnetite electrodeposited on the carbon steel substrate using a PAR273A potentiostat. Carbon steel and magnetite specimens were polarized at potentials of -0.65 VSCE and 0.30 VSCE, respectively, in the same environment as that used in the polarization test for 7 days. After the potentiostatic test was completed, the corrosion morphologies and the oxide layers were observed with FIB-SEM.


RESULTS Characteristics of electrodeposited magnetite Figure 1 shows the SEM image of magnetite layer electrodeposited on the carbon steel substrate. The surface of the electrodeposited magnetite had a dense and polyhedral morphology, which was homogeneous on the entire surface of the specimen. The average thickness of the electrodeposited magnetite layer was approximately 4.7 μm. No defects, such as holes or cracks, could be observed at the interface between magnetite and the carbon steel substrate, confirming that magnetite was tightly bonded to the carbon steel substrate. The XRD patterns of the electrodeposited layers corresponded to crystalline magnetite (JCPDS card no.19-0629). Pt coating

Magnetite

Substrate 2 m

2 m

(a) (b) Figure 1. SEM images of magnetite layer electrodeposited on carbon steel at the applied potential of -1.05 VSCE in Fe(III)-TEA solution with pH 12.5 at 80 oC: (a) top view and (b) cross section. Electrochemical corrosion behavior of carbon steel and magnetite Figure 2 shows the polarization curves of carbon steel and magnetite in alkaline aqueous solutions with pH 9.5 at 25 oC. The Ecorr and icorr calculated by cathodic Tafel-extrapolation of polarization curves in Figure 2 are presented in Table 1. The addition of 100 ppm PAA to the test solution shifted the Ecorr of carbon steel from -0.769 VSCE to -0.816 VSCE. The overall polarization curves were also shifted in the direction of higher current density. The icorr of carbon steel at the OCP increased from 1.4 μA/cm2 to 4.1 μA/cm2 with the addition of 100 ppm PAA. This result indicates that the addition of 100 ppm PAA increases the corrosion rate of carbon steel by approximately 2.9 times. The Ecorr of magnetite was lowered from -0.424 VSCE to -0.494 VSCE with the addition of 100 ppm PAA to the test solution. However, the overall polarization curves were shifted in the direction of lower current density. The icorr of magnetite at the OCP decreased slightly from 1.3 μA/cm2 to 0.8 μA/cm2 with the addition of 100 ppm PAA. This result indicates that the addition of 100 ppm PAA decreases the corrosion rate of magnetite by approximately 1.6 times. 0.0

0.0 No PAA 100 ppm PAA

No PAA 100 ppm PAA

-0.2

Potential (VSCE)

Potential (VSCE)

-0.2

-0.4

-0.6

-0.8

-0.6

-0.8

-1.0 10-8

-0.4

10-7

10-6

10-5

Current density (A/cm

(a)

2

)

10-4

-1.0 10-8

10-7

10-6

10-5

Current density (A/cm

2

10-4

)

(b)

Figure 2. Potentiodynamic polarization curves of (a) carbon steel and (b) magnetite in alkaline aqueous solutions of pH 9.5 with and without 100 ppm PAA at 25 oC.


Table 1. Electrochemical corrosion parameters of carbon steel and magnetite obtained from the cathodic Tafel-extrapolation and the mixed potential theory in alkaline aqueous solutions of pH 9.5 with and without 100 ppm PAA at 25 oC. PAA concentration (ppm, wt.)

Materials

Ecorr (VSCE)

icorr (μA/cm2)

Carbon steel

-0.769 ± 0.004

1.4 ± 0.1

Magnetite

-0.424 ± 0.006

1.3 ± 0.2

Carbon steel

-0.816 ± 0.011

4.1 ± 0.1

Magnetite

-0.494 ± 0.005

0.8 ± 0.1

0

100

Ecouple (VSCE)

icouple (μA/cm2)

-0.652 ± 0.006

8.3 ± 0.1

-0.796 ± 0.010

4.8 ± 0.4

Figure 3 shows the corrosion morphology of carbon steel after potentiostatic testing. A dense oxide layer was formed uniformly on the entire surface of carbon steel in alkaline aqueous solutions without PAA. However, the addition of 100 ppm PAA to the test solution greatly influenced the growth of an oxide layer on the carbon steel surface. The top view shows that the surface of the oxide layer is rough and irregularly crazed. The oxide layer was about 5 times thicker than that formed in alkaline aqueous solution without PAA. Numerous cavities and defects were also observed on the cross section of the oxide layer.

No PAA

100 ppm PAA

Top view

Cross section

10 m

10 m

Pt coating

Pt coating

Oxide layer

Oxide layer

Carbon steel

Carbon steel

300 nm

1 m

Figure 3. SEM images showing the corrosion morphology of carbon steel after potentiostatic testing in alkaline aqueous solutions of pH 9.5 with and without 100 ppm PAA at 25 oC.


Figure 4 shows the corrosion morphology of magnetite after potentiostatic testing. The magnetite particles appeared to be preferentially dissolved along a crystallographic facet in the test solution without PAA, comparing to the original feature shown in Figure 1(a). The dissolved magnetite particles in the test solution without PAA were less faceting and thinner than that in the test solution with 100 ppm PAA. This result indicates that the preferential dissolution of magnetite particles is greater in the test solution without PAA. Consequently, many crevices were observed between the dissolved magnetite particles on both the surface and the cross section of the specimen in the test solution without PAA. However, this phenomenon was not observed in the test solution with 100 ppm PAA.

No PAA

100 ppm PAA

Top view

2 m

2 m

Pt coating

Cross section

Magnetite

Substrate

Pt coating

Magnetite

1 m

Substrate

1 m

Figure 4. SEM images showing the corrosion morphology of magnetite after potentiostatic testing in alkaline aqueous solutions of pH 9.5 with and without 100 ppm PAA at 25 oC.

Galvanic corrosion behavior between carbon steel and magnetite Magnetite layers are formed or deposited on carbon steel in reducing operation conditions of PWRs. When the magnetite layers on carbon steel are partially removed by flow-accelerated corrosion or erosion corrosion, galvanic corrosion will occur between carbon steel and magnetite. Therefore, the polarization curves of Figure 2 were rearranged in Figure 5 to predict the galvanic corrosion behavior between carbon steel and magnetite using the mixed potential theory. Figure 5 shows that the Ecorr of carbon steel is lower than that of magnetite in the test solution both with and without PAA. This means that carbon steel behaves as an anode, if carbon steel and magnetite are in electrical contact. In this couple, the Ecouple and icouple are determined by the intersection of the anodic curve of carbon steel and the cathodic curve of magnetite, according to the mixed potential theory. These electrochemical corrosion parameters are also presented in Table 1.

20th NPC International Conference Brighton, United Kingdom - October 2-7, 2016 Paper Number: 142 0.0

-0.2

-0.4

-0.4

-0.6

-0.6

-0.8

-0.8

-1.0 10-8

100 ppm PAA Carbon steel Magnetite

Potential (VSCE)

Potential (VSCE)

-0.2

0.0 No PAA Carbon steel Magnetite

-1.0 10-7

10-6

10-5

Current density (A/cm2)

10-4

10-8

(a)

10-7

10-6

10-5

10-4

Current density (A/cm2)

(b)

Figure 5. Galvanic corrosion behavior between carbon steel and magnetite in alkaline aqueous solutions of pH 9.5 with and without 100 ppm PAA at 25 oC: (a) no PAA and (b) 100 ppm PAA. As shown in Figure 5 and Table 1, when carbon steel and magnetite are galvanically coupled in equal area, the Ecouple of the couple is expected to shift in the negative direction with the addition of 100 ppm PAA. Furthermore, this galvanic coupling increases the corrosion rate of carbon steel from 1.4 μA/cm2 to 8.3 μA/cm2 in the test solution without PAA. The corrosion rate of carbon steel is also increased slightly from 4.1 μA/cm2 to 4.8 μA/cm2 by the coupling in the test solution with 100 ppm PAA. That is, the presence of 100 ppm PAA reduces the extent of the galvanic corrosion of carbon steel from 8.3 μA/cm2 to 4.8 μA/cm2. In addition, the cathodic current density for magnetite, which is the cathode in the couple, is expected to increase in the test solution both with and without PAA. In the case of magnetite, the cathodic reactions involve both the hydrogen evolution on magnetite and reductive dissolution of magnetite. Consequently, the cathodic reaction of magnetite increases when carbon steel and magnetite are galvanically contacted in equal area, regardless of PAA addition. Figure 6 shows the actual Ecouple and icouple between carbon steel and magnetite, obtained from ZRA measurements in alkaline aqueous solutions with pH 9.5 at 25 oC. The actual icouple of carbon steel was the positive value in the oxidation reaction, indicating that carbon steel was the anode of the couple. The addition of 100 ppm PAA shifted the actual Ecouple in the negative direction. The actual icouple of carbon steel was also shifted in the direction of lower current density. In addition, the actual icouple of carbon steel decreased slightly over time, which can be attributed to the formation of oxide layers on the carbon steel surface that grows with time and that partially protects the surface of carbon steel [Burstein et al. (2005), Blasco-Tamarit et al. (2008), Sánchez-Tovar et al. (2013)]. The Ecouple and the icouple determined by the mixed potential theory and ZRA are quite similar, regardless of the presence of PAA. Therefore, the galvanic corrosion behavior between carbon steel and magnetite can be evaluated quantitatively using both the mixed potential theory and ZRA measurements. DISCUSSION The results obtained in this study can be summarized as follows: the addition of 100 ppm PAA increased the corrosion rate of carbon steel under simulated steam generator wet layup conditions, whereas PAA slightly decreased the corrosion rate of magnetite. These differences in behavior can be discussed in light of the interaction mechanism of PAA with carbon steel and magnetite. Under alkalized reducing conditions, a protective magnetite layer is formed on carbon steel through reactions 1 and 2 [Laronge and Ward (1999), Robinson and Drews (1999)]. Fe + 2H2O → Fe(OH)2 + H2

(1)

3Fe(OH)2 → Fe3O4 + 2H2O + H2

(2)

-0.5 No PAA 100 ppm PAA -0.6

-0.7

-0.8

-0.9 0

600

1200

1800

2400

3000

3600

Galvanic current density ( A/cm2)

Galvanic corrosion potential (VSCE)

20th NPC International Conference Brighton, United Kingdom - October 2-7, 2016 Paper Number: 142 8 No PAA 100 ppm PAA 6

4

2

0 0

600

Time (s)

(a)

1200

1800

2400

3000

3600

Time (s)

(b)

Figure 6. (a) Galvanic corrosion potential and (b) galvanic current density of carbon steel coupled to magnetite obtained from ZRA measurement in alkaline aqueous solutions of pH 9.5 with and without 100 ppm PAA at 25 oC. According to previous studies, the presence of 1 ppm PAA does not preclude the formation of a stable protective oxide on carbon and low-alloy steels.24 However, in the presence of high concentrations of PAA, ferrous species might not form a uniform oxide layer on the surface of carbon steel owing to the adsorption or a dispersion effect of PAA [Sekine et al. (1992), Song et al. (2014)]. Figure 7 shows a schematic mechanism of the effect of PAA on the formation of an oxide layer on carbon steel in a solution with a high concentration of PAA. In the early stage, when magnetite particles begin to be formed on the surface according to the corrosion processes described above, PAA adsorbs on the surface of Fe3O4 and/or Fe(OH)2 as well as the surface of carbon steel. As the corrosion of carbon steel proceeds, the magnetite particles are surrounded by PAA. Thus, the high molecular weight PAA is included into the oxide layer. This result is supported by the XPS analysis that PAA molecules are present on the outer surface and interior of oxide layer formed on carbon steel (not shown in this paper). Consequently, the oxide layer grows non-uniformly and contains numerous defects, as shown in Figure 3. This oxide layer is much thicker and less protective than the stable oxide layer formed in PAA-free solution. Therefore, the addition of 100 ppm PAA accelerated the corrosion of carbon steel. In contrast to carbon steel, the corrosion rate of magnetite was slightly decreased in the presence of 100 ppm PAA. Under alkalized reducing conditions, the dissolution of magnetite and the formation of dissolved ferrous species is described by the following equation 3 (reductive dissolution) [Lister and Lang (2002), Vepsäläinen and Saario (2010)]: Fe3O4 + 2H2O + 2H+ + 2e- → 3Fe(OH)2

Figure 7. Schematic of mechanism for the effect of PAA on the formation of oxide layer on the surface of carbon steel.

(3)


In a solution with PAA, the adsorption of PAA on a metal surface would replace water molecules and other ions acting as the medium of electrochemical reaction at the metal/electrolyte interface [Grchev et al. (1991), Umoren (2009), Umoren et al. (2010)]. Therefore, the addition of 100 ppm PAA inhibits reductive dissolution of magnetite, most likely due to a decrease in the number of dissolution sites because of the adsorption of PAA on the magnetite surface, as shown in Figure 4. Although the presence of PAA increases the corrosion rate of bare carbon steel, most of the carbon steel surface is covered with a magnetite layer under operating conditions. The result summarized in Table 1 shows that when carbon steel and magnetite are in electrical contact in equivalent area ratio, the corrosion rate of carbon steel increases. This type of galvanic corrosion occurs when the magnetite layer on carbon steel is partially removed. However, the extent of galvanic corrosion between carbon steel and magnetite is reduced in the presence of 100 ppm PAA. Therefore, it is expected that the use of PAA dispersant during wet layup will be beneficial in reducing magnetite deposits and will not cause corrosion-related problems. CONCLUSIONS (1) The presence of 100 ppm PAA led to the formation of a defective oxide layer having numerous cavities on the surface of carbon steel owing to the adsorption effect of PAA. Consequently, this phenomenon can account for an increase in the corrosion rate of carbon steel. However, the presence of 100 ppm PAA slightly decreased the corrosion rate of magnetite, most likely due to a reduction in the number of dissolution sites owing to the adsorption of PAA on the magnetite surface. (2) When carbon steel and magnetite were in electrical contact, carbon steel was the anode, regardless of the presence of PAA. The galvanic coupling with magnetite increased the corrosion rate of carbon steel in the test solution both with and without PAA. In this couple, the extent of the galvanic corrosion of carbon steel was reduced by the addition of 100 ppm PAA. These galvanic corrosion behaviors were predicted using the mixed potential theory and verified using the ZRA technique. ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2012M2A8A4025888). REFERENCES Abellà, J., Barcelò, J., Victori, L. (1998). “Evaluation by electrochemical impedance spectroscopy of a process of removal of iron oxides deposited on a heat exchanger tubing,” Corrosion Science, UK, 40 1561-1574. Al-Mayouf, A. M. (2006). “Dissolution of magnetite coupled galvanically with iron in environmentally friendly chelant solutions,” Corrosion Science, UK, 48 898-912. Amin, M. A., Abd El-Rehim, S. S., El-Sherbini, E. E. F., Hazzazi, O. A., Abbas, M. N. (2009). “Polyacrylic acid as a corrosion inhibitor for aluminium in weakly alkaline solutions. Part I: Weight loss, polarization, impedance EFM and EDX studies,” Corrosion Science, UK, 51 658667. Bakhtiari, S., Kupperman, D. S., Shack, W. J. (2009) “Assessment of Noise Level for Eddy Current Inspection of Steam Generator Tubes,” U.S. Nuclear Regulatory Commission, Argonne National Laboratory, NUREG/CR-6982. Blasco-Tamarit, E., Igual-Munoz, A., García-Antón, J., García-García, D. (2008). “Comparison between open circuit and imposed potential measurements to evaluate the effect of temperature on galvanic corrosion of the pair alloy 31-welded alloy 31 in LiBr solutions,” Corrosion Science, UK, 50 3590-3598. Burstein, G. T., Liu, C., Souto, R. M. (2005). “The effect of temperature on the nucleation of corrosion pits on titanium in Ringer’s physiological solution,” Biomaterials, NLD, 26 245–256.


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