Corrosion behavior of coupled active and passive reinforcing steels in simulated concrete pore solution

Construction and Building Materials 240 (2020) 117955

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Corrosion behavior of coupled active and passive reinforcing steels in simulated concrete pore solution Zheng Dong a,b,⇑, Amir Poursaee b,c a

Department of Structural Engineering, College of Civil Engineering, Tongji University, 1239 Siping Rd, Shanghai 200092, China Glenn Department of Civil Engineering, Clemson University, Clemson, SC, United States c Department of Civil Engineering, Department of Materials Science and Engineering, Clemson University, Clemson, SC, United States b

h i g h l i g h t s
The ratio of galvanic current to total current of coupled steel was correlated to A/P ratio.
As active area decreased, a more general and severe corrosion was observed on the active surface.
Galvanic coupling changed the Tafel slopes of coupled active and passive specimens.

a r t i c l e

i n f o

Article history: Received 23 October 2019 Received in revised form 16 December 2019 Accepted 24 December 2019

Keywords: Corrosion Reinforcing steel Concrete Pore solution Galvanic corrosion Coupling

a b s t r a c t The present study aimed to investigate the corrosion behavior of coupled active and passive steels in concrete simulated environment. Three different active-to-passive area (A/P) ratios (i.e. A/P = 1/1, 1/3, 1/20) were studied by changing the exposed area of the active steel while keeping the passive steel constant. For each group of A/P ratio, three cells were prepared: individual cell with active steel specimens, individual cell with passive steel specimens, and a coupled cell which connected active and passive steels. All specimens were immersed in a simulated concrete pore solution for 14 days. Afterwards, 3 wt% NaCl was added to the cells. Different electrochemical measurement techniques were used to assess the influence of galvanic coupling and A/P ratio on the corrosion behavior of the steel specimens. Results indicated that coupling changed the corrosion behavior of the active steel. In the case of A/P = 1/1, the corrosion was mostly observed in the form of localized corrosion, whereas in the case of A/P = 1/20, a more general and severe corrosion was observed. In addition, coupling increased the anodic and cathodic Tafel slopes which are closely related to the iron oxidation rate (i.e. corrosion rate). Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Galvanic corrosion of reinforcing steels is a frequent and detrimental type of corrosion in reinforced concrete (RC) structures. Some examples of galvanic corrosion in RC structures are: the upper reinforcement in chloride-contaminated concrete in contact with the bottom reinforcement in chloride-free concrete [1,2], the corroding steel in old concrete in contact with the repassivated steel in repaired concrete [3–7], steels with different corrosion activities [8–12] or microstructures [13], as well as between pits and adjacent passive areas [14,15]. Numerous studies have been carried out to investigate the galvanic corrosion between active and passive steels [1,2,11,16–26]. ⇑ Corresponding author. E-mail address: [email protected] (Z. Dong). https://doi.org/10.1016/j.conbuildmat.2019.117955 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

In general, the active and passive steels involved in a galvanic cell form a mutually polarized system, where the active steel is anodically polarized and the passive steel is cathodically polarized. It is assumed that the iron oxidation and oxygen reduction reactions occur at both surfaces of the active and passive steels [3,4,27– 29]. However, the iron oxidation current produced by the passive area is usually negligible [5,29,30]. Additionally, it is believed that the oxygen reduction current at the active surface is also negligible [31–33]. Fig. 1 presents a schematic illustration of the potentialcurrent relationship for the aforementioned scenarios. When both the iron oxidation and oxygen reduction reactions occur at the active surface, as Fig. 1a shows, the anodic current of the active steel is increased from Icorra to Ia_a, i.e., the total anodic current which should be considered in predicting the corrosion rate [4,18,25,34]. The difference between anodic current (Ia_a) and cathodic current (Ic_a) at the active area equals the galvanic current

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Z. Dong, A. Poursaee / Construction and Building Materials 240 (2020) 117955

(a)

E

(b)

Cathodic polarization curve of active steel

Cathodic polarization curve of passive steel

Ea

Ep

Ecorra

Ea Anodic polarization curve of active steel

Ig

Ic_a Icorra

Ohmic resistance Anodic polarization curve of active steel Ia_a=Ic_p=Icorr=Ig

Ia_a

Icorr

|I|

Fig. 1. Schematic illustration of the active steel involved in galvanic corrosion when (a) both the iron oxidation and oxygen reduction reactions occur on the active surface, and (b) iron oxidation reaction occurs at the active surface while oxygen reduction reaction takes place at the passive surface.

(Ig). When the oxygen reduction current on the active steel is negligible, as Fig. 1b shows, the anodic current gives the value of galvanic current, which is considered to be the corrosion rate in terms of the active area [30,35]. The difference between the potentials of the anode and cathode (Ea and Ep) in Fig. 1b is correlated to the ohmic resistance in-between. Anode-to-cathode area ratio is known to be one of the most significant factors in galvanic corrosion [14,36–40]. It is generally acknowledged that larger cathode area provides more surface area for the reduction reaction, which acts as the driving force and in return increases the anodic dissolution current. In this regard, the severity of the galvanic corrosion is inversely proportional to the anode-to-cathode ratio and is influenced by other parameters such as the anodic and cathodic Tafel slopes [37]. Nevertheless, there is no single relationship between galvanic current density and the anode-to-cathode area ratio. It was shown that as the area ratio reaches a small value (i.e., less than 0.01), the rate of increase in galvanic current density decreases [39]. If the total area of anode and cathode is kept constant, a maximum value of galvanic current may exist [37]. The galvanic corrosion between active and passive steels in concrete environment is relatively complex due to various geometries and active-to-passive area (A/P) ratios. As aforementioned, both the active and passive areas can possibly generate currents due to iron oxidation and oxygen reduction. In this regard, the magnitude of iron oxidation and oxygen reduction currents on the active area may be correlated to the A/P ratio [16]. Despite the general recognition that the smaller the A/P the more severe the galvanic corrosion, such influence may not be critical compared to other factors like the geometry [24]. On the other hand, other studies showed that the galvanic current density could be in the order of 31 times the residual microcell current density in case of coupled steel [12,25]. As such, the total current density of active steel could be ~2 times the microcell current density without considering galvanic corrosion, and should be taken into account when predicting the service life of steel reinforced concrete structures [18,25]. Such differences from various studies may be attributed to the different methods to induce corrosion (i.e., by adding salt during mixing or by contaminating concrete with salt solution after curing), as well as different periods for galvanic coupling (i.e., start coupling before or after the onset of corrosion in the active area). Although there is still uncertainty in how severe could galvanic corrosion be with respect to different A/P ratios, several methods have been used to take area relationship into account to calculate the total corrosion current on active steel in concrete environment. Suzuki et al. [38] multiplied the increment of anodic current density (i.e., (Ia_a  Icorra) in Fig. 1 per unit active area) by a factor which

was inversely proportional to the A/P ratio. In the case of juxtaposed active and passive steels, transmission line model was used to mathematically solve the distribution of galvanic current and potential, by dividing the large passive area into infinitesimal sections [22,41]. Basically, the total anodic current of the active steel (Ia_a) involved in a galvanic system is modeled and calculated by summation of current obtained from the polarization curves of steel together with the value of the galvanic current [4,18,25]. In most cases, the polarization curves of the active and passive steels are obtained separately before coupling or after disconnecting the galvanic couple [18,25,30]. Specifically, when using the polarization curves of individual active and passive steels obtained before coupling, their polarization behaviors (i.e., Tafel slopes) are assumed to be unchanged during coupling [25,38]. Nevertheless, whether and how the galvanic coupling affects the polarization behavior of steel are still in question. Therefore, it is imperative to study the galvanic corrosion of reinforcing steels in concrete environment under different A/P ratios, which was the objective of this research. The present study investigated the corrosion behavior of coupled active and passive steels with respect to three different area ratios (A/P = 1/1, 1/3, and 1/20), which were achieved by changing the area of active steel while keeping the area of the passive steel constant.

2. Experimental procedures #3 (/ = ~9.5 mm) ribbed steel bars were used in this study. 190mm-long pieces were cut from identical steel. Specimens were sandblasted and cleaned by alcohol, and dried with air. Afterwards, each steel specimen was coated with 4 layers of UV cure vinyl ester epoxy resin, except a certain exposed length of the midsection, and 10 mm of one end of each steel specimen for electrical connection, as shown in Fig. 2. Three groups of steel specimens were prepared in this experiment, to simulate different active-to-passive area (A/P) ratios. As depicted in Fig. 2, active steels with three different exposed lengths, i.e., 90 mm, 30 mm, and 4.5 mm, were prepared while the exposed lengths of the passive steels were kept constant as 90 mm. Active steels with different exposed lengths were denoted as AI, AII, AIII, and the passive steel was denoted as P. This configuration provided three A/P ratios: A/P = 1/1, 1/3, and 1/20. For each set of A/P ratio, three cells were prepared: (i) individual cell with three identical active steel specimens, (ii) individual cell with three identical passive steel specimens, and (iii) one coupled cell with an active cell and a passive cell connected using a salt bridge, as

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Z. Dong, A. Poursaee / Construction and Building Materials 240 (2020) 117955 Table 1 Chemical compositions and pH of the simulated concrete pore solution.

Fig. 2. Illustration of steel specimens.

shown in Fig. 3. By using the salt bridge, the galvanic coupling between an active and a passive steel can be simulated in the pore solution without contaminating the passive cell after addition of the chlorides to the active cell. The salt bridge was made of a Ushaped borosilicate glass tube with an internal diameter of 13 mm. The two ends of the U-shaped glass tube were sealed by sintered discs. The glass tube was filled with 1 mol/L KOH. The coupled cell provided A/P ratio of 1/1 was denoted as CI, and corresponding individual active cell was AI. In the case of A/P = 1/3, the coupled cell and individual active steel were denoted as CII and AII. Similarly, the coupled cell and corresponding individual cell with A/P ratio of 1/20 were denoted as CIII and AIII, respectively. Simulated concrete pore solution with the chemical compositions shown in Table 1, was used as the electrolyte in each cell to simulate the concrete environment [42]. The advantage of performing the experiments in the solution rather than in the concrete is that the surface of steel bars can be visually examined during the test period, and the results can be obtained in a reasonable time frame. Additionally, concrete resistance, as one of the limiting factors for the current flow is eliminated by using solution rather than the actual concrete. While this is not the case in a real concrete structure, it is beneficial to focus on fundamental aspect of galvanic coupling. All steel specimens were immersed in chloride-free pore solution for 14 days. Afterwards, 3 wt% of laboratory grade NaCl was added to the individual cells for active steel specimens and the side with active steel specimens in the coupled cells in each group. To further accelerate the corrosion activity, the percentage of the NaCl was increased to

Compound

Mol/L

NaOH KOH Ca(OH)2 CaSO4H2O (gypsum) pH

0.1 0.3 0.03 0.002 13.1

10 wt% 90 days after starting the immersion. During the experiment, the pH of the pore solution in each cell was periodically measured to ascertain the value was around 13.1. The cells were sealed during the experiment to minimize the atmospheric carbonation effect and evaporation. Corrosion potential measurement, linear polarization resistance (LPR), cyclic polarization (CP), zero resistance ammetry (ZRA) and electrochemical impedance spectroscopy (EIS) techniques were used to assess the corrosion activity of the specimens. A threeelectrode measurement setup, including a steel specimen as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a 316L stainless steel as the counter electrode, was used for the LPR, CP and EIS tests. SCE was used to measure the corrosion potential. All measurements were conducted at the ambient temperature (~23 °C). Corrosion potentials of specimens in coupled and individual cells were measured every 3 days. To measure the galvanic current flow between the active and passive steel specimens in the coupled cells, ZRA test was carried out every 3 days. The test was conducted for 1 h during each measurement. To determine the electrochemical polarization resistance of specimens, LPR within the range of ± 10 mV vs. corrosion potential was conducted every 3 days, with a scan rate of 0.166 mV/s [43]. The CP technique was employed to evaluate the susceptibility to pitting corrosion, as well as determining the Tafel slopes, ba, bc which were used to calculate the Stern-Geary constant, B, using Eq. (1), and the corrosion current, using Eq. (2).

B ¼ ðba  bc Þ=ðba þ bc Þ Icorr ¼

ð1Þ

B Rp

ð2Þ

where B (V) is the Stern-Geary constant (Tafel slopes ba and bc are obtained in the form of natural logarithm), Rp (X) is the polarization resistance obtained through LPR technique, and Icorr (A) is the corrosion current. Wires

Chloridecontaminated solution

Salt bridge (1 mol/L KOH)

90 mm

90 mm

Reference electrode

Counter electrode

Fig. 3. Schematic illustration of the coupled CI measurement cell.

Chloride-free solution

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Z. Dong, A. Poursaee / Construction and Building Materials 240 (2020) 117955

All the CP tests started at 200 mV vs. the corrosion potential to +500 mV vs. SCE and then decreased to 200 mV vs. the corrosion potential, at a scan rate of 0.166 mV/s. To further investigate the influence of the galvanic coupling on the corrosion of the active steel, EIS tests were conducted on both the individual active specimens and the active steels involved in the coupled specimens after disconnecting for 1 h. A 10 mV alternating sinusoidal potential perturbation over the frequency range of 105 Hz to 102 Hz was used for the EIS tests. At the end of the experiment, the specimens (not the ones underwent CP tests) were removed from the solution and their surface examined using optical microscopy.

90

CI CII CIII

Ig (μA)

60

30

0

0

30

60

90

120

Time (days) 3. Results and discussion

Fig. 5. Galvanic current versus time, obtained from the ZRA test. The first and second vertical dashed lines represent the time of addition of 3 wt% and 10 wt% NaCl, respectively.

Fig. 4 presented the potentials of active steel in all individual and coupled cells. The potential of the coupled specimens CI was more positive than that of the individual specimens AI. This observation was attributed to that the active steel in a coupled specimen was anodically polarized by the passive steel [3,8,18,27]. However, in the cases of groups II and III, no significant difference was observed between the potentials of individual and coupled specimens. In general, passive area is considerably polarized under galvanic coupling, while there is only slight polarizing effect on the active area [44]. On the other hand, galvanic coupling aggravates the corrosion activity of active area, which may lead to more negative potential [13,45,46]. Fig. 4 indicated that, although theoretically active steel was anodically polarized to a more positive potential under galvanic coupling, as shown in Fig. 1, its corrosion behavior could be changed, possibly reducing the potential difference between individual active steel and coupled steel. The values of galvanic current measured through ZRA were demonstrated in Fig. 5. As it was expected, after addition of NaCl in one side of each coupled cell, corrosion initiated on the specimens in that side while specimens in the other side remained passive and electrons flowed from active steel to passive steel. As can be seen in Fig. 5, after addition of 3 wt% NaCl, galvanic current in group CI gradually became lower than the other two groups as corrosion propagated. After increasing the concentration of NaCl to 10 wt%, the galvanic currents of three groups considerably increased, whereas no significant difference was observed between them. Fig. 6 showed the results of corrosion current (Icorr) measured through LPR tests. In all cases, the current of the coupled specimen was greater than that of the individual active specimen. In terms of the coupled specimens, assuming iron oxidation and oxygen reduction reactions occurred on the surfaces of both active and passive steels, the galvanic current (Ig) was the net ionic current flow circulated the coupled active and passive steels, as expressed

in Eq. (3). The current measured through LPR was the total anodic or cathodic current (Icorr) of the coupled specimen, as expressed in Eq. (4). As shown in Figs. 5 and 6, the galvanic current was not greater than the total current of the coupled specimen.

Ig ¼ Ia a  Ic

Potential (V) vs. SCE

¼ Ia

Icorr ¼ Ia a þ Ia

p

p

 Ic p

¼ I c a þ I c p

0

II

III

-0.2

-0.2

-0.2

-0.4

-0.4

-0.4

0

30

60

90

Time (days)

120

ð4Þ

C

0

I

-0.6

ð3Þ

where Ia_a and Ic_a are the iron oxidation current and oxygen reduction current on the surface of active steel; Ia_p and Ic_p are the iron oxidation current and oxygen reduction current on the surface of passive steel. Fig. 7 presented the results of Ig/Icorr ratio. Theoretically, when the ratio was equal to 1, the only significant process on the active steel is iron oxidation while the only significant process on the passive steel is oxygen reduction, whose E-I relationship was demonstrated in Fig. 1b. As can be seen in Fig. 7, the Ig/Icorr ratios of groups CI and CII were generally in the range of 0.2 to 0.4. This ratio for group CIII, approximately reached 0.8 in the end, indicating a situation towards Fig. 1b. Fig. 8 demonstrated the cyclic polarization curves of all the individual and coupled specimens at 60 days of exposure to 3 wt% of NaCl. In terms of individual specimens, the current of group AI was larger than the other two groups as a result of larger surface area (Fig. 8a). All the individual specimens, though with different surface areas, displayed hysteresis, indicating the susceptibility to localized corrosion [47,48]. Whereas, for the coupled specimens shown in Fig. 8b, in the case of CIII which consisted of relatively small active area with the A/P ratio of 1/20, very small hysteresis

A 0

a

-0.6

0

30

60

90

Time (days)

120

-0.6

0

30

60

90

120

Time (days)

Fig. 4. Corrosion potential values versus time. The first and second vertical dashed lines represent the time of addition of 3 wt% and 10 wt% NaCl, respectively.

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Z. Dong, A. Poursaee / Construction and Building Materials 240 (2020) 117955

Fig. 6. Corrosion current versus time, obtained from the LPR test. The first and second vertical dashed lines represent the time of addition of 3 wt% and 10 wt% NaCl, respectively.

1.0

CI

CII

CIII

Ig/ Icorr

0.8 0.6 0.4 0.2 0.0

0

30

60

90

120

Time (days) Fig. 7. The value of Ig/Icorr versus time under different A/P ratios. The first and second vertical dashed lines represent the time of addition of 3 wt% and 10 wt% NaCl, respectively.

was exhibited, indicating rather general and severe corrosion than localized corrosion [47]. Tafel slopes of the coupled specimens, C, and the individual active steels, A, were extracted from the results of the CP tests, and listed in Table 2. Both the anodic and cathodic Tafel slopes of coupled specimens were greater than that of the individual active specimens, leading to a larger Stern-Geary constant, B. Thus, the galvanic coupling affected the polarization behavior of steel. In this regard, when modeling macro-cell corrosion on the basis of polar-

0.6

AI AII AIII

0.3

Potential (V, vs. SCE)

Potential (V, vs. SCE)

0.6

ization curves [18,27], the influence of galvanic coupling on the polarization behavior needs to be considered. To evaluate the effect of galvanic coupling on the corrosion of active steel and not to destroy the surface condition as a result of polarization, EIS tests were performed on the individual active steel specimens as well as the disconnected active steels in coupled specimens (denoted as A-disconnected). As the bode plots in Fig. 9 demonstrated, galvanic coupling decreased the total impedance of the active steel, which became more distinguishable as the area of active steel decreased. In the range of intermediate and low frequencies, the values of phase angle (phase) of A-disconnected specimens were lower than the individual A specimens, indicating an increase in the current flowing through resistance [49,50]. Noticeably, the drop in the maximum phase angle gradually increased with decreasing the active area in coupled specimens. At the end of the experiment, specimens were removed from the solution, and optical microscopy was carried out on the specimens to study the surface condition of the corroding steels. As shown in Fig. 10, generally, active steels in coupled specimens exhibited more corrosion activity than the individual active specimens. On the one hand, as the active area decreased with respect to the passive area, the corrosion on the active steel became more severe. On the other hand, while the corrosion was mostly observed in the form of localized corrosion in group CI, as the active area decreased, a more general corrosion was exhibited in group CIII. This observation agreed with the results from the CP tests.

0.0 -0.3 -0.6

0.3

CI CII CIII

0.0 -0.3 -0.6

(a)

(b)

-0.9 10-8

10-6

10-4

Current (A)

10-2

100

-0.9 -8 10

10-6

10-4

10-2

100

Current (A)

Fig. 8. Results of the cyclic polarization (CP) tests at 60 days of exposure to 3 wt% NaCl for steel specimens in (a) individual active cells and (b) coupled cells.

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Z. Dong, A. Poursaee / Construction and Building Materials 240 (2020) 117955

Table 2 Tafel slopes, ba and bc, and Stern-Geary constant B, obtained from cyclic polarization test. A/P ratio

1/1 1/3 1/20

ba (V/decade)

bc (V/decade)

B

C

A

C

A

C

A

0.314 0.216 0.322

0.130 0.193 0.120

0.047 0.039 0.032

0.018 0.016 0.016

0.041 0.033 0.029

0.016 0.017 0.014

Fig. 9. Bode plots of the individual active steel specimens and disconnected active steels in coupled specimens. (a) 90 mm-length active steel. (b) 30 mm-length active steel. (c) 4.5 mm-length active steel.

AI

AII

AIII

CI

CII

CIII

Fig. 10. Optical micrograph of the active steel specimen in each cell.

4. Conclusions This study attempted to investigate the corrosion behavior of the coupled active and passive steels in concrete environment. Three different active-to-passive area (A/P) ratios were designed by changing the area of active steel while keeping the passive steel constant. During the whole period of experiment, the active and passive steels involved in coupled specimens were electrically connected exclusive of the time of measuring. In the cases of A/P = 1/3 and 1/20, the potentials of coupled specimens were similar to that of the individual active specimens. Noticeably, in the case of A/P = 1/20, the ratio between the galvanic

current circulated the coupled cell (Ig) and the total corrosion current of the coupled specimen (Icorr) was approximate 0.8 in the end. In this regard, the oxygen reduction current on the active area and the iron oxidation current on the passive area could possibly be neglected. For A/P ratio which is greater than 1/20, it is recommended to consider both the iron oxidation and oxygen reduction reactions to be occurred on the surfaces of steels involved in a galvanic coupled system. Galvanic coupling changed the corrosion behavior of active steel. In the case of A/P = 1/1, the corrosion was mostly observed in the form of localized corrosion, whereas in the case of A/ P = 1/20, a more general and severe corrosion was observed.

Z. Dong, A. Poursaee / Construction and Building Materials 240 (2020) 117955

Galvanic coupling also increased the anodic and cathodic Tafel slopes which are closely related to the iron oxidation rate (i.e. corrosion rate), compared to that of the individual active steels. CRediT authorship contribution statement Zheng Dong: Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Amir Poursaee: Writing - original draft, Writing - review & editing, Supervision.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was conducted in the Corrosion Research Laboratory (CorRLab) at Clemson University. Zheng Dong would like to express her sincere gratitude to Prof. Xiang-Lin Gu from Tongji University and the support from China Scholarship Council (CSC). References [1] J. Warkus, M. Raupach, Modelling of reinforcement corrosion – corrosion with extensive cathodes, Mater. Corros. 57 (2006) 920–925, https://doi.org/ 10.1002/maco.200604032. [2] C.M. Hansson, A. Poursaee, A. Laurent, Macrocell and microcell corrosion of steel in ordinary Portland cement and high performance concretes, Cem. Concr. Res. 36 (2006) 2098–2102, https://doi.org/10.1016/j.cemconres.2006. 07.005. [3] S. Soleimani, P. Ghods, O.B. Isgor, J. Zhang, Modeling the kinetics of corrosion in concrete patch repairs and identification of governing parameters, Cem. Concr. Compos. 32 (2010) 360–368, https://doi.org/10.1016/j.cemconcomp.2010. 02.001. [4] S. Qian, J. Zhang, D. Qu, Theoretical and experimental study of microcell and macrocell corrosion in patch repairs of concrete structures, Cem. Concr. Compos. 28 (2006) 685–695, https://doi.org/10.1016/j.cemconcomp.2006. 05.010. [5] E. Lozinguez, J.F. Barthélémy, V. Bouteiller, T. Desbois, Contribution of Sacrificial Anode in reinforced concrete patch repair: results of numerical simulations, Constr. Build. Mater. 178 (2018) 405–417, https://doi.org/ 10.1016/j.conbuildmat.2018.05.063. [6] J.L.S. Ribeiro, Z. Panossian, S.M.S. Selmo, Proposed criterion to assess the electrochemical behavior of carbon steel reinforcements under corrosion in carbonated concrete structures after patch repairs, Constr. Build. Mater. 40 (2013) 40–49, https://doi.org/10.1016/j.conbuildmat.2012.09.097. [7] M. Raupach, Patch repairs on reinforced concrete structures – model investigations on the required size and practical consequences, Cem. Concr. Compos. 28 (2006) 679–684, https://doi.org/10.1016/j.cemconcomp.2006. 05.016. [8] X.-L. Gu, Z. Dong, Q. Yuan, W.-P. Zhang, Corrosion of stirrups under different relative humidity conditions in concrete exposed to chloride environment, J. Mater. Civ. Eng. 32 (2019) 1–8, https://doi.org/10.1061/(ASCE)MT.19435533.0003001. [9] X.-L. Gu, Z. Dong, Z.-H. Jin, Macrocell corrosion between crossed steel rebars embedded in concrete under chloride environments, in: Proc. 5th Int. Conf. Concr. Repair, Rehabil. Retrofit., 2018. https://doi.org/10.1051/matecconf/ 201819904005. [10] C. Fu, N. Jin, H. Ye, X. Jin, W. Dai, Corrosion characteristics of a 4-year naturally corroded reinforced concrete beam with load-induced transverse cracks, Corros. Sci. 117 (2017) 11–23, https://doi.org/10.1016/j.corsci.2017.01.002. [11] S.I. Miyazato, N. Otsuki, Steel corrosion induced by chloride or carbonation in mortar with bending cracks or joints, J. Adv. Concr. Technol. 8 (2010) 135–144, https://doi.org/10.3151/jact.8.135. [12] N. Otsuki, S.I. Miyazato, N.B. Diola, H. Suzuki, Influences of bending crack and water-cement ratio on chloride-induced corrosion of main reinforcing bars and stirrups, ACI Mater. J. 97 (2000) 454–464, https://doi.org/10.14359/7410. [13] H. Torbati-Sarraf, A. Poursaee, Corrosion of coupled steels with different microstructures in concrete environment, Constr. Build. Mater. 167 (2018) 680–687, https://doi.org/10.1016/j.conbuildmat.2018.02.083. [14] C. Cao, 3D simulation of localized steel corrosion in chloride contaminated reinforced concrete, Constr. Build. Mater. 72 (2014) 434–443, https://doi.org/ 10.1016/j.conbuildmat.2014.09.030. [15] U. Angst, B. Elsener, C.K. Larsen, Ø. Vennesland, Chloride induced reinforcement corrosion: rate limiting step of early pitting corrosion,

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