Realkalization of corroded rebar in mortar with
monoethanolamine as a corrosion inhibitor: influence of
total charge and applied electric field on repair
effectiveness

Yunyun Tong, Ziwei Li, Mengya Li, Jinjian Ye, Jiong Wang, Jianqiao Chen, Wei Mao, Wei Zeng

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Yunyun Tong, Ziwei Li, Mengya Li, Jinjian Ye, Jiong Wang, et al.. Realkalization of corroded rebar in mortar with monoethanolamine as a corrosion inhibitor: influence of total charge and applied electric field on repair effectiveness. Journal of Asian Architecture and Building Engineering, 2025, pp.1 - 12. 10.1080/13467581.2025.2492905. hal-05155914

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# Realkalization of corroded rebar in mortar with monoethanolamine as a corrosion inhibitor: influence of total charge and applied electric field on repair effectiveness

Yunyun Tong, Ziwei Li, Mengya Li, Jinjian Ye, Jiong Wang, Jianqiao Chen, Wei Mao & Wei Zeng

**To cite this article:** Yunyun Tong, Ziwei Li, Mengya Li, Jinjian Ye, Jiong Wang, Jianqiao Chen, Wei Mao & Wei Zeng (16 Apr 2025): Realkalization of corroded rebar in mortar with monoethanolamine as a corrosion inhibitor: influence of total charge and applied electric field on repair effectiveness, Journal of Asian Architecture and Building Engineering, DOI: 10.1080/13467581.2025.2492905

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# Realkalization of corroded rebar in mortar with monoethanolamine as a corrosion inhibitor: influence of total charge and applied electric field on repair effectiveness

Yunyun Tong<sup>a,b</sup>, Ziwei Li<sup>a,b</sup>, Mengya Li<sup>c</sup>, Jinjian Ye<sup>a</sup>, Jiong Wang<sup>a</sup>, Jianqiao Chen<sup>d</sup>, Wei Mao<sup>e</sup> and Wei Zeng<sup>a,b,f</sup>

## ABSTRACT

The realkalization process is an electrochemical technique to restore and enhance the pH level in carbonated concrete. This study investigated the effectiveness of realkalization using an alkaline solution ($K_2CO_3$) with monoethanolamine (MEA) as a corrosion inhibitor. Reinforced mortar specimens were subjected to accelerated carbonation, followed by accelerated corrosion of the embedded steel rebars. During the repair process, MEA was used to increase the polarization resistance of the rebar and provided surface protection. The effectiveness was evaluated by monitoring the corrosion potential and corrosion current density at various total electric charges. Comparative experiments without the electric field were also conducted. After repair, the pH values, potassium ions ($K^+$) concentrations, and MEA contents of the mortar cover were measured. The results showed that the corrosion potential shifted positively, while the corrosion current density decreased, indicating a reduction in corrosion activity. Electrochemical and analytic results exhibited that a total charge of at least 150 $A·h/m^2$ was required for effective repair. However, the long-time effectiveness was limited, as the steel rebars remained in a moderate corrosion state.

## ARTICLE HISTORY

Received 26 November 2024

Accepted 11 March 2025

## KEYWORDS

Carbonated reinforced concrete; corrosion inhibitor; electrochemical realkalization; ion migration behavior; corroded rebar repair

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<figure><img src="image_11.png" /></figure>

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<footnote><sup>a</sup>School of Civil Engineering and Architecture, Zhejiang University of Science and Technology, Zhejiang, China; <sup>b</sup>Zhejiang International Science & Technology Cooperation Base for Low-carbon Building Material Technology with Recyclable Waste Resource, Zhejiang University of Science and Technology, Hangzhou, China; <sup>c</sup>L2MGC, CY Cergy Paris Université, Cergy, France; <sup>d</sup>Department of Civil and Environmental Engineering, Imperial College London, London, UK; <sup>e</sup>Hangzhou Judai Cultural Heritage Protection Technology, Co. Ltd, Hangzhou, China; <sup>f</sup>Zhejiang Construction Investment Group Co. Ltd, Hangzhou, China</footnote>
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# 1. Introduction

In reinforced concrete structures, the corrosion of steel rebars is a critical issue that significantly reduces their service life. One of the primary causes of this corrosion is concrete carbonation. As atmospheric carbon dioxide ($CO_2$) penetrates the concrete, it reacts with calcium hydroxide in the pore water to form carbonic acid. This process neutralizes the alkaline environment ($pH > 12.5$), which normally protects the embedded steel rebar from corrosion (Chen et al. 2020; Subbaram and Komarasamy 2021; Tang et al. 2015). This reduction in alkalinity lowers the pH of the concrete, leading to the breakdown of the passive oxide layer on the steel surface, ultimately initiating corrosion.

Several chemical techniques have been developed to repair and protect reinforcement concrete from corrosion. Electrochemical techniques such as cathodic protection, electrochemical chloride extraction, and electrochemical realkalization have been employed. Cathodic protection involves applying an external current to neutralize the corrosive current, thus slowing down the corrosion process (Redaelli and Bertolini 2011b). Electrochemical chloride extraction is primarily used in chloride-contaminated concrete, where an electrical current is applied to remove chloride ions, which are also a major cause of the steel rebar corrosion (Karthick et al. 2016; Sánchez and Alonso 2011). Electrochemical realkalization specifically targets the effects of concrete carbonation by restoring the alkalinity of the concrete. In this process, an anode is placed on the concrete surface, while the steel rebars serve as the cathode, forming a closed circuit with an external electrical supply. Through the electrolysis of water and the diffusion of alkaline substances, the pH value of concrete could be increased, and the passivation of the steel rebar can be appropriately rehabilitated (Réus and Medeiros 2020; Tong et al. 2012; Zou et al. 2017, 2022). However, while electrochemical realkalization can temporarily restore the pH, some studies reported that its long-term effectiveness was limited (Al-Otaibi et al. 2014; Redaelli and Bertolini 2011a; Tissier et al. 2019). Over time, the repair effect would gradually disappear, and the corrosion of steel rebars may became active again.

Zhang et al. (2013) investigated the composition and morphology of the rust layer on the surface of steel rebar before and after electrochemical realkalization, and the results demonstrated that the introduction of corrosion inhibitors during the realkalization process in reinforced concrete effectively limited the further corrosion. Traditional corrosion inhibitors can be chemically classified into inorganic and organic types. Organic corrosion inhibitors were heterocyclic compounds with polar functional groups (-OH, -COOH, -OCH₃, -COOC₂H₅, -NO₂, -NH₂, -CONH₂) (Munis et al. 2020). The corrosion inhibition mechanism was closely associated with physical and chemical adsorption

process. Chemical absorption occurred through the formation of stable covalent bonds between polar groups (e.g., N, O, and P) or heterocyclic groups and the vacant d-orbitals of the steel rebar, through the sharing of lone-pairs of electrons or π-electrons. In addition, physical adsorption occured through electrostatic attraction (Van der Waals forces) (Pradipta, Kong, and Tan 2019; Wang and Zuo 2017). Amines and amino alcohols were widely employed as corrosion inhibitors due to their water solubility and inert characteristics. Amino alcohols demonstrated inhibition efficiency by adsorbing $OH^-$ onto the surface of steel rebar, thereby forming a thin protective film, and thus efficiently prevent the adsorption and contact of $O_2$ and chlorides, increasing the critical chloride concentration required for corrosion initiation and reducing the corrosion damage rate of the steel rebar (Ryu et al. 2016). Shen et al. (2019) demonstrated that increasing the concentration of corrosion inhibitor in alkaline solution raised the pitting potential and improved the stability of the passive state.

The effectiveness of different inhibitors can vary. Our previous study assessed the repair efficiency by incorporating three organic inhibitors: monoethanolamine (MEA), dimethylethanolamine (DMEA), and triethylenetetramine (TETA) during the realkalization process (Tong et al. 2023). Microscopic analysis revealed that MEA provided superior protection, as the surfaces of steel rebars treated with MEA remained relatively clean and smooth, and only minor corrosion, primarily in the form of pitting, was observed (Figure 1). Similar results were reported in (Agrawal, Patil, and Muhammad 2024), where MEA could form a denser protective film on the steel surface, enhancing chelation activity and improving repair effectiveness.

Under the influence of an applied electric field, organic cationic substances migrate through the carbonated concrete and accumulate near the embedded steel cathode. This accumulation creates a localized region of higher alkalinity, providing protection to the steel rebar. Additionally, the combination of realkalization solution and corrosion inhibitors accelerated the inhibitor migration from the surface to the interior of the reinforced concrete through the applied electric field, thereby improving the overall repair effectiveness (Kubo et al. 2013). Yeih and Chang (2005) found that the repair efficiency of realkalization depended on both the applied current density and the duration of the process. As the parameters increased, the pH value increased and the depth of realkalization expanded. However, few studies have investigated the combined effects of current density and realkalization duration influence when the corrosion inhibitor was incorporated into the process.
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Figure 1. SEM microscopic analysis of steel rebar after realkalization repair (a) without inhibitor (b) with MEA (Tong et al. 2023).</figcaption></figure>

This study aimed to evaluate the realkalization
process using MEA as a corrosion inhibitor, with
a specific focus on the influence of total charge
and applied electric field on repair effectiveness.
Electrochemical and analytical methods were
employed to assess the corrosion state of the steel
rebars and the ion migration phenomenon in the
carbonated mortar during the realkalization with
MEA at varying total charges. Additionally,
a control group without electric field was also set
up in order to better understand the importance of
electric field on the repair mechanism.

**2. Materials and methods**

**2.1. Materials and specimen preparation**

The mortar specimens were prepared using an ordin-
ary Portland cement (P.O 42.5), sand with a maximum

particle size of 4 mm, and water. The mix design for the
mortar specimens followed a ratio of cement, water
and sand of 1:0.65:4 by mass. Each specimen was cast
as a prismatic block with dimensions of 40 mm ×
40 mm × 80 mm, embedding a plain HPB300 round
steel rebar with a diameter of 6 mm and a length of
60 mm (Figure 2(a)). To ensure precise control of the
working area, the exposed ends of the steel rebar
(20 mm) were insulated with a high-performance
epoxy coating, leaving only the 40 mm in direct con-
tact with the mortar.

After demolding, the sides of the specimens were
sealed with sealing tape, and the top and bottom
were exposed to the air to ensure the unidirectional
migration. A wire was welded to one end of the
embedded steel rebar for use in treatment and
measurement (Figure 2(b)). The specimens were
cured for 28 days at 20 °C and relative humidity
above 90%.

<figure><img src="image_16.png" /><figcaption>Figure 2. Illustration of specimen (a) dimensions (b) front view with welded wire for measurements.</figcaption></figure>
<figure><img src="image_17.png" /></figure>

## 2.2. Accelerated carbonation and corrosion

After curing, the specimens were subjected to accelerated carbonation and corrosion to simulate the corrosion of steel rebars in carbonated mortar. The accelerated carbonation process was carried out in an HC-HTX12 concrete carbonation chamber. In accordance with (2010), the CO₂ concentration, relative humidity, and temperature were maintained at 20 ± 0.5%, 70%, and 20°C, respectively. The phenolphthalein was used to monitor the carbonation process. The 1% phenolphthalein solution was sprayed onto the fractured surface of carbonated mortar specimens. Since no color change occurred, indicating a drop in pH below 9, the specimens were considered fully carbonated.

To accelerate the corrosion of the steel rebars, an anodic current method was applied (Raffaini et al. 2021; Zhang et al. 2013). The embedded steel rebar served as the anode, while a stainless-steel mesh acted as the cathode. A current density of 1 A/m² was imposed, resulting in a current of 0.75 mA through the steel rebar. A total charge of 180 A·h/m² was applied over a period of 7.5 days.

## 2.3. Realkalization treatment incorporating MEA

After the ageing process, the specimens underwent realkalization using a solution containing 40 g/L potassium carbonate (K₂CO₃) and 0.5 mol/L monoethanolamine (MEA) as the electrolyte. The wire connected to the embedded steel rebar was attached to the cathode of the power supply, while the stainless-steel mesh was immersed in the electrolyte solution and connected to the anode, as illustrated in Figure 3.

Electrochemical parameters are crucial in determining the effectiveness of the repair process (Liu et al. 2024). The current density during electrochemical treatment using in previous studies was usually maintained between 1 and 3 A/m², with treatment durations ranging from 7 to 28 days (Fei et al. 2015; Shan et al. 2016). In this study, a current density of 1 A/m² was used according to EN 14,038-1-2016 (2016), with the treatment duration gradually increased from 25 h to 200 h. This corresponds to total charges of 25, 50, 75, 100, 125, 150, 175, and 200 A·h/m². Parallel experiments, conducted without the application of electric field, were also performed, as summarized in Table 1.

## 2.4. Morphology

To compare the corrosion morphology of the steel rebars in the mortar before and after repair, the

specimens were split, and the steel rebar was removed from the mortar. Hitachi TM3000 scanning electron microscope (SEM) was used to analyze the morphology.

## 2.5. Electrochemical measurements

Electrochemical measurements were performed using an electrochemical workstation (Shanghai Chenhua CHI660d) to analyze and evaluate the corrosion resistance of specimens under varying total electrical charges. The potential and polarization resistance were monitored, and the corrosion current density was calculated. The electrochemical setup consisted of a three-electrode system: the steel rebars served as the working electrode, a titanium/platinum-based grid was used as the counter electrode, and a KCl saturated calomel electrode (E vs NHE=+242) functioned as the reference electrode.

### 2.5.1. Corrosion potential

Before conducting electrochemical measurements, the corrosion potential was tested to stabilize the electrode potential. The scanning duration was 600 s, with data recorded at intervals of 0.1 s.

### 2.5.2. Corrosion current density

The quantification of steel rebar corrosion was determined based on its corrosion current density ($I_{corr}$) values (Faritov et al. 2016; Nguyen et al. 2018). Linear

<figure><img src="image_18.png" /><figcaption>Figure 3. Realkalization process set-up.</figcaption></figure>

<table><caption>Table 1. Parameters of realkalization process.</caption><thead><tr><td>Electric field mode*</td><td>Current [A/m²]</td><td>Treatment duration [h]</td><td>Total charge [A·h/m²]</td></tr></thead><tbody><tr><td>None</td><td>0</td><td>25,50,75,100,125,150,175,200</td><td>0</td></tr><tr><td>Under</td><td>1</td><td>25,50,75,100,125,150,175,200</td><td>25,50,75,100,125,150,175,200</td></tr></tbody></table>
<figure><img src="image_19.png" /></figure>

sweep voltammetry (LSV) was used to measure the polarization resistance of the steel rebar R<sub>p</sub> (Ω), with a scanning rate of 0.01 mV/s. The scan was carried out at a potential range of ±150 mV relative to the corrosion potential to obtain the Tafel polarization curve. The current density I<sub>corr</sub> (μA/cm<sup>2</sup> of the steel rebars) was calculated using the Stern-Geary equation (Equation (1)):

$$I_{corr} = \frac{B}{R_{p} \times S} \qquad (1)$$

where B is a constant (26 mV, the value when the steel rebar is in an active state) (Andrade and Alonso 2004), and S is the area of the polarized steel rebar (7.54 cm<sup>2</sup> in this study).

## 2.6. Analytical characterization

After the realkalization treatment, the specimens were sectioned and analyzed at different depths. The mortar specimens were taken every 10 mm, up to a depth of 40 mm, using a precision cutting machine (Figure 4). The pH values, potassium ions (K<sup>+</sup>) concentrations, and MEA contents were measured for each layer of the sliced specimens.

### 2.6.1. pH value

The pH values were measured using a SMART SENSOR pH acid-base meter (Figure 5), which had a resolution of 0.01 following accurate calibration. This instrument allowed for precise and rapid measurement of pH within the specimens, providing insights into the pH changes throughout the material.

### 2.6.2. Potassium ion (K<sup>+</sup>) concentration

The concentration of K<sup>+</sup> was determined using the tetraphenylboron sodium titration method. In this process, potassium chloride reacted with tetraphenylboron sodium, producing tetraphenylboron potassium precipitate and sodium chloride. Excess tetraphenylboron sodium then reacted with quaternary

ammonium salts, such as cetyltrimethylammonium bromide, forming insoluble tetraphenylboron complex salts. The endpoint of the titration was indicated by a color change from bromothymol blue to blue, from which the K<sup>+</sup> concentrations in the solution were calculated based on the titration volume.

<figure><img src="image_20.png" /><figcaption>Figure 5. pH acid-base meter.</figcaption></figure>

### 2.6.3. Monoethanolamine (MEA) content

The MEA content was determined by the chemical oxygen demand (COD) method. When the organic content in the solution was relatively low, strong oxidants such as potassium dichromate and potassium permanganate were used to oxidize the reducing substances, primarily organic matter. The higher the chemical oxygen demand, the greater the amounts of organic matter in the solution, allowing for a quantitative estimation of the MEA contents.

## 3. Results and discussions

### 3.1. Surface morphology

The microstructure of the mortar surrounding the steel rebar before and after repair with an electric field is

<figure><img src="image_21.png" /><figcaption>Figure 4. Sampling positions for analytical characterization at various depths.</figcaption></figure>

<figure><img src="image_22.png" /></figure>
<figure><img src="image_23.png" /></figure>

shown in Figure 6. Before repair, the mortar exhibited carbonation-induction damage, characterized by fragmented particles and limited hydration products (Figure 6(a)). After repair, the mortar featured a denser and more compact microstructure, with dense needle-like and fibrous formations representing new hydration products formed during the realkalization process (Figure 6(b)). These observations indicated that realkalization with MEA effectively modified the morphology of the cement hydration products. Similar findings have been reported in (Zhang, Zhu, and Qu 2021), where the addition of corrosion inhibitors during realkalization resulted in improved repair, with surface covered by crystalline and flaky substances. The corrosion inhibitors were shown to successfully penetrate and adsorb onto the surface of the steel rebars, contributing to enhanced repair efficiency.

## 3.2. Electrochemical parameters

### 3.2.1. Corrosion potential

The corrosion potentials measured before and after realkalization treatment are presented in Figure 7. Before the repair, the specimens exhibited a significantly negative corrosion potential of around -600 mV. This corresponded to a corrosion probability of greater than 90% for the steel rebars. After the repair, the corrosion potential values steadily increased over time, indicating the effectiveness of the treatment. By 150 h, the corrosion potential had improved to -220.83 mV, reducing the corrosion risk to 50%. However, it should be noted that the rate of repair began to slow after this point. Although the potential continued to rise as the repair duration increased, the rate of improvement gradually decreased.

### 3.2.2. Corrosion current density

The corrosion current density of steel rebar before and after repair is illustrated in Figure 8. Before the repair, the corrosion current density reached a high corrosion

<figure><img src="image_24.png" /><figcaption>**Figure 7.** Corrosion potential before and after realkalization treatment over time. (dashed lines represent corrosion probability thresholds as defined in (ASTM C876-91 1999): low probability (< 10%) when the potential is above -120 mV/SCE, and high probability (> 90%) when the potential is below -276 mV/SCE.).</figcaption></figure>

rate of 1.3 µA/cm². Due to variability among the specimens at different time intervals, the corrosion current density values before repair fluctuated, but as the repair time increased, a general trend of reduction in corrosion current density was observed.

According to the RILEM technical report (Andrade and Alonso 2004), the corrosion activity of steel rebar can be categorized into four ranges based three corrosion current density thresholds:

- below 0.1 µA/cm²: negligible corrosion state;

- 0.1 to 0.5 µA/cm²: low corrosion state;

- 0.5 to 1 µA/cm²: moderate corrosion state;

- Above 1 µA/cm²: high corrosion state.

After 150-h treatment, the corrosion current density decreased to below 1 µA/cm², indicating that the corrosion state had reduced from high to a moderate

<figure><img src="image_25.png" /><figcaption>**Figure 6.** SEM images of mortar surrounding the steel rebars (a) before repair (b) after repair.</figcaption></figure>
<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /><figcaption>**Figure 8.** Corrosion current density before and after realkalization treatment over time. (dashed lines indicate corrosion severity: values above 1 µA/cm² represent a high level of corrosion, while values above 0.5 µA/cm² indicate a moderate level).</figcaption></figure>

level. By 200 h, the corrosion current density had further dropped to 0.8 µA/cm², representing a reduction of 50% compared to the pre-repair condition. However, the post-repair steel rebars in this study had not yet reached a negligible or low corrosion state.

Similar to the corrosion current density results, corrosion potential measurements indicated that a total charge of at least 150 A·h/m² was sufficient for an effective treatment, which is lower than the European standard of 200 A·h/m² (EN 14038-1, 2016). This improvement can be attributed to the enhanced effectiveness of the realkalization process facilitated by the incorporation of MEA. The repair efficiency continued to improve with higher total charges, further demonstrating the effectiveness of the realkalization with MEA, although full repassivation of the steel rebars was not achieved.

### 3.3. Characterizations of substances within mortar after realkalization

#### 3.3.1. pH value

The pH values within the specimens were measured under two different conditions: without and with the application of an electric field. These values are represented as a function of depth (0–40 mm) and treatment duration (Figure 9), with the experimental results are summarized in Table 2.

Before the repair, the pH values across the specimens were maintained at a low alkalinity level, around 8.0 at varying depth. During the treatment, an increase in pH was observed for two scenarios. Over time, the pH tended to stabilize, but without the influence of an electric field, the penetration of alkalinity into the deeper layers of the mortar was limited. In this condition,

<figure><img src="image_28.png" /><figcaption>**Figure 9.** Comparison of pH values in mortar at varying depths and treatment duration with and without an electric field.</figcaption></figure>

<table><caption>**Table 2.** The pH values in mortar.</caption><thead><tr><th rowspan="2">Treatment duration/h</th><th colspan="4">Non-electric field</th><th colspan="4">Under electric field</th></tr><tr><th>10</th><th>20</th><th>30</th><th>40</th><th>10</th><th>20</th><th>30</th><th>40</th></tr></thead><tbody><tr><td>0</td><td>8.25</td><td>8.09</td><td>8.11</td><td>8.1</td><td>8.36</td><td>8.15</td><td>7.97</td><td>8.08</td></tr><tr><td>25</td><td>8.55<br>(+3.6%)*</td><td>8.95<br>(+10.6%)</td><td>8.93<br>(+10.1%)</td><td>8.88<br>(+9.6%)</td><td>8.91<br>(+6.6%)</td><td>8.75<br>(+7.4%)</td><td>8.66<br>(+8.7%)</td><td>8.5<br>(+5.2%)</td></tr><tr><td>50</td><td>9.85<br>(+19.4%)</td><td>9.78<br>(+20.9%)</td><td>9.62<br>(+18.6%)</td><td>9.51<br>(+17.4%)</td><td>9.53<br>(+14.0%)</td><td>9.28<br>(+13.9%)</td><td>9.33<br>(+17.1%)</td><td>9.15<br>(+13.2%)</td></tr><tr><td>75</td><td>9.91<br>(+20.1%)</td><td>9.76<br>(+20.6%)</td><td>9.63<br>(+18.7%)</td><td>9.5<br>(+17.3%)</td><td>9.68<br>(+15.8%)</td><td>9.55<br>(+17.2%)</td><td>9.43<br>(+18.3%)</td><td>9.41<br>(+16.5%)</td></tr><tr><td>100</td><td>9.93<br>(+20.4%)</td><td>9.81<br>(+21.3%)</td><td>9.56<br>(+17.9%)</td><td>9.32<br>(+15.1%)</td><td>9.66<br>(+15.6%)</td><td>10.13<br>(+24.3%)</td><td>10.09<br>(+26.6%)</td><td>10.11<br>(+25.1%)</td></tr><tr><td>125</td><td>10.03<br>(+21.6%)</td><td>9.9<br>(+22.4%)</td><td>9.61<br>(+18.5%)</td><td>9.42<br>(+16.3%)</td><td>10.03<br>(+20.0%)</td><td>10.11<br>(+24.0%)</td><td>10.13<br>(+27.1%)</td><td>10.15<br>(+25.6%)</td></tr><tr><td>150</td><td>10.18<br>(+23.4%)</td><td>9.92<br>(+22.6%)</td><td>9.64<br>(+18.9%)</td><td>9.48<br>(+17.0%)</td><td>10.16<br>(+21.5%)</td><td>10.25<br>(+25.8%)</td><td>10.2<br>(+28.0%)</td><td>10.23<br>(+26.6%)</td></tr><tr><td>175</td><td>10.23<br>(+24.0%)</td><td>9.97<br>(+23.2%)</td><td>9.69<br>(+19.5%)</td><td>9.58<br>(+18.3%)</td><td>10.33<br>(+23.6%)</td><td>10.2<br>(+25.2%)</td><td>10.21<br>(+28.1%)</td><td>10.25<br>(+26.9%)</td></tr><tr><td>200</td><td>10.25<br>(+24.2%)</td><td>9.98<br>(+23.4%)</td><td>9.75<br>(+20.2%)</td><td>9.61<br>(+18.6%)</td><td>10.49<br>(+25.5%)</td><td>10.38<br>(+27.4%)</td><td>10.29<br>(+29.1%)</td><td>10.39<br>(+28.6%)</td></tr></tbody></table>

<footnote>* The percentage value represents the improvement compared to the value before repair at t=0.</footnote>
<figure><img src="image_29.png" /></figure>

the migration of alkaline electrolytes without the electric field relied solely on gravity and capillary force to move through the specimens. The pH values increased relatively quickly at shallow depth (10 and 20 mm), after which the increase slowed down. The pH values around the steel rebars (at 40 mm depth) had only risen to 9.61 by 200 h, indicating that the realkalization was less effective at greater depth without the application of an electric field.

In contrast, the pH values increased steadily over time under the electric field. A clear improvement in pH was observed, especially at deeper levels of the specimens. The results showed that the external alkaline electrolyte successfully penetrated the mortar and reached the embedded steel rebars through diffusion, capillary absorption, and electromigration. Additionally, the electrolytic reaction occurring in the vicinity, producing a large amount of $OH^-$, which further increased the alkalinity of the mortar. By 200 h, the pH values at 40 mm depth had risen to 10.39. Previous studies have suggested that when the pH dropped below 9.5, the passive film on steel rebar broke down, leading to the initiation of corrosion (Neves, Branco, and de Brito 2013; Pu et al. 2012). Compared to the conventional realkalization treatment, which increased the alkalinity of carbonated concrete around the steel rebar from a pH of less than 9 to about 10.3 (Tong et al. 2012), the realkalization with MEA proved to be more effective. The application of the electric field enhanced the alkalinity of carbonated mortar at 40 mm depth by 28.6%, highlighting its importance in improving the treatment's effectiveness and allowing for better penetration into the deeper layers over time.

### 3.3.2. $K^+$ concentrations

Potassium ions ($K^+$), as part of the electrolyte solution ($K_2CO_3$), contribute indirectly to the alkalinity restoration during the realkalization process. Under the electric field, negative ions (e.g. $OH^-$, $Cl^-$ ions) moved towards the anode and positive ions (e.g. $Na^+$ or $K^+$ ions) moved

towards the cathode (Tong et al. 2012). Measuring $K^+$ concentrations at different depths helped to evaluate the efficiency of ion migration driven by the electric field.

The $K^+$ concentrations in the mortar are illustrated in Figure 10, and the experimental values are summarized in Table 3. Before the repair, the $K^+$ concentration in the mortar was very low, essentially negligible, as the potassium content originated from cement. During the repair, it can be noted that the $K^+$ concentrations increased, with higher levels near the surface, then gradually decreased with depth. Without the application of an electric field, the migration of $K^+$ was slower, and the concentration decreased more significantly at deeper levels. Even after 100 h, only a small amount of $K^+$ was detected in the deeper mortar layers, which indicating limited penetration of substances in the absence of an electric field.

<figure><img src="image_30.png" /><figcaption>Figure 10. Comparison of $K^+$ concentrations in mortar at varying depths and treatment duration with and without an electric field.</figcaption></figure>

<table><caption>Table 3. Distribution of the $K^+$ concentrations in mortar.</caption><thead><tr><th rowspan="2">Treatment duration/h</th><th colspan="4">Depth in mortar (mortar/mm)</th><th colspan="4">Non-electric field/wt.%</th><th colspan="4">Under electric field/wt.%</th></tr><tr><th>10</th><th>20</th><th>30</th><th>40</th><th>10</th><th>20</th><th>30</th><th>40</th><th>10</th><th>20</th><th>30</th><th>40</th></tr></thead><tbody><tr><td>0</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>25</td><td>0.401</td><td>0.103</td><td>0.034</td><td>0.022</td><td>0.452<br>(+12.7%)*</td><td>0.155<br>(+50.5%)</td><td>0.104<br>(+205.9%)</td><td>0.103<br>(+368.2%)</td><td>0.505</td><td>0.154</td><td>0.053</td><td>0.035</td></tr><tr><td>50</td><td>0.505</td><td>0.154</td><td>0.053</td><td>0.035</td><td>0.759<br>(+50.3%)</td><td>0.256<br>(+66.2%)</td><td>0.207<br>(+290.6%)</td><td>0.155<br>(+342.9%)</td><td>0.651</td><td>0.402</td><td>0.254</td><td>0.108</td></tr><tr><td>75</td><td>0.651</td><td>0.402</td><td>0.254</td><td>0.108</td><td>0.902<br>(+38.6%)</td><td>0.354<br>(+15.7%)</td><td>0.308<br>(+21.3%)</td><td>0.252<br>(+133.3%)</td><td>0.953</td><td>0.606</td><td>0.357</td><td>0.155</td></tr><tr><td>100</td><td>0.953</td><td>0.606</td><td>0.357</td><td>0.155</td><td>1.057<br>(+10.9%)</td><td>0.558<br>(+13.4%)</td><td>0.453<br>(+26.9%)</td><td>0.452<br>(+191.6%)</td><td>1.154</td><td>0.753</td><td>0.502</td><td>0.307</td></tr><tr><td>125</td><td>1.154</td><td>0.753</td><td>0.502</td><td>0.307</td><td>1.206<br>(+4.5%)</td><td>0.755<br>(+10.2%)</td><td>0.706<br>(+40.6%)</td><td>0.602<br>(+96.1%)</td><td>1.321</td><td>0.808</td><td>0.455</td><td>0.351</td></tr><tr><td>150</td><td>1.321</td><td>0.808</td><td>0.455</td><td>0.351</td><td>1.303<br>(+1.4%)</td><td>0.955<br>(+18.2%)</td><td>0.853<br>(+87.5%)</td><td>0.802<br>(+128.5%)</td><td>1.402</td><td>0.954</td><td>0.603</td><td>0.405</td></tr><tr><td>175</td><td>1.402</td><td>0.954</td><td>0.603</td><td>0.405</td><td>1.458<br>(+4.0%)</td><td>1.105<br>(+15.8%)</td><td>1.007<br>(+67.0%)</td><td>0.652<br>(+61.0%)</td><td>1.452</td><td>1.053</td><td>0.654</td><td>0.462</td></tr><tr><td>200</td><td>1.452</td><td>1.053</td><td>0.654</td><td>0.462</td><td>1.515<br>(+4.3%)</td><td>1.153<br>(+9.5%)</td><td>1.051<br>(+60.7%)</td><td>0.853<br>(+84.6%)</td><td></td><td></td><td></td><td></td></tr></tbody></table>

<footnote>* The percentage value represents the enhancement of $K^+$ ions migration due to the electric field.</footnote>
<figure><img src="image_31.png" /></figure>

When the electric field was applied, the K⁺ concentrations increased significantly, reaching up to 1.515 wt.‰ at the surface, higher than in the non-electric field scenario. Furthermore, the concentration remained elevated (about 0.853 wt.‰) even at greater depth, suggesting that the electric field enhanced the migration of K⁺ ions throughout the mortar. The carbonated mortar exhibited better ion penetration and a more uniform concentration throughout the depth under the influence of the electric field. By 200 h, the application of the electric field enhanced K⁺ migration by 4.3% near the surface and by 84.6% at a depth of 40 mm. Similar results were reported in Jaśniok and Zybura (2009), indicating that the distribution of ion concentrations was time-related. Despite the improved migration, the penetration of K⁺ around the steel rebar remained limited.

However, while the electric field enhanced the migration and flow of ions, caution should be exercised regarding the excessive entry of K⁺ into the mortar. An overabundance of Na⁺ or K⁺ could trigger an alkali-aggregate reaction, which can affect the durability of concrete (Leemann et al. 2014; Wang, Noguchi, and Maruyama 2022).

### 3.3.3. MEA contents

Variation in MEA concentration indicated the extent of adsorption and the distribution of inhibitor along the steel surface. Higher concentrations near the steel rebar suggest effective transport, enhancing corrosion inhibition and reducing the corrosion current density. It can be obviously noted in Figure 11 that without the application of an electric field, the migration of MEA in the mortar was hindered. Similar to the migration behavior of K⁺, no MEA was detected before the repair process. During the treatment, higher concentrations were observed near the concrete surface and decreased sharply as the depth increased. The slope of MEA content in each layer was flatter compared to the scenario with an electric field, and the growth rate

<figure><img src="image_32.png" /><figcaption>Figure 11. Comparison of MEA contents in mortar at varying depths and treatment duration with and without an electric field.</figcaption></figure>

slowed down after 100 h. After 150 h, the MEA content around the steel rebars reached only 0.305 wt.‰ at a depth of 40 mm without electric field, whereas it reached 0.444 wt.‰ when the electric field was applied, representing an increase of 45.6%.

Under the electric field, MEA contents were more evenly distributed throughout the mortar depth, demonstrating that the electric field significantly enhanced the migration of the corrosion inhibitor into the deeper layers. After 200 h, the MEA contents around the steel rebars reached 0.569 wt.‰, with the electric field contributing 70.9% to the MEA migration. The applied electric field significantly enhanced the penetration of MEA, allowing the inhibitor to effectively reach the steel rebars. Similar results using TETA as corrosion inhibitor were reported in (Xu et al. 2016), while the inhibitor continued to migrate from electrolyte solution to carbonated concrete, but the quantity monitored decreased in terms of depth from surface. The experimental results are summarized in Table 4.

<table><caption>Table 4. Distribution of MEA contents in mortar.</caption><thead><tr><th rowspan="2">Treatment duration/h</th><th colspan="4">Depth in mortar mortar/mm</th><th colspan="4">Under electric field/wt.‰</th></tr><tr><th>10</th><th>20</th><th>30</th><th>40</th><th>10</th><th>20</th><th>30</th><th>40</th></tr></thead><tbody><tr><td>0</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>25</td><td>0.097</td><td>0.077</td><td>0.062</td><td>0.028</td><td>0.222<br>(+128.9%)</td><td>0.208<br>(+170.1%)</td><td>0.111<br>(+79.0%)</td><td>0.083<br>(+196.4%)</td></tr><tr><td>0</td><td>0.166</td><td>0.111</td><td>0.083</td><td>0.041</td><td>0.389<br>(+134.3%)</td><td>0.361<br>(+225.2%)</td><td>0.278<br>(+234.9%)</td><td>0.167<br>(+307.3%)</td></tr><tr><td>75</td><td>0.213</td><td>0.239</td><td>0.208</td><td>0.113</td><td>0.444<br>(+108.5%)</td><td>0.430<br>(+79.9%)</td><td>0.361<br>(+73.6%)</td><td>0.278<br>(+146.0%)</td></tr><tr><td>100</td><td>0.305</td><td>0.291</td><td>0.277</td><td>0.249</td><td>0.500<br>(+63.9%)</td><td>0.458<br>(+57.4%)</td><td>0.416<br>(+50.2%)</td><td>0.333<br>(+33.7%)</td></tr><tr><td>125</td><td>0.347</td><td>0.305</td><td>0.305</td><td>0.291</td><td>0.583<br>(+68.0%)</td><td>0.514<br>(+68.5%)</td><td>0.444<br>(+45.6%)</td><td>0.375<br>(+28.9%)</td></tr><tr><td>150</td><td>0.374</td><td>0.333</td><td>0.319</td><td>0.305</td><td>0.611<br>(+63.4%)</td><td>0.605<br>(+81.7%)</td><td>0.527<br>(+65.2%)</td><td>0.444<br>(+45.6%)</td></tr><tr><td>175</td><td>0.388</td><td>0.374</td><td>0.305</td><td>0.319</td><td>0.680<br>(+75.3%)</td><td>0.638<br>(+70.6%)</td><td>0.583<br>(+91.1%)</td><td>0.527<br>(+65.2%)</td></tr><tr><td>200</td><td>0.405</td><td>0.405</td><td>0.341</td><td>0.333</td><td>0.775<br>(+91.4%)</td><td>0.722<br>(+78.3%)</td><td>0.694<br>(+103.5%)</td><td>0.569<br>(+70.9%)</td></tr></tbody></table>

<footnote>* The percentage value represents the enhancement of MEA migration due to the electric field.</footnote>
<figure><img src="image_33.png" /></figure>

## 4. Conclusion

In this paper, the efficiency of the realkalization process incorporating the corrosion inhibitor MEA was investigated by electrochemical and analytical methods. To better understand the repair mechanism, the corrosion state of steel rebars at varying total charges, as well as the migration of ions in the carbonated mortar, were assessed. Additionally, comparative tests were conducted with and without the application of the electric field. Based on the experimental results, following conclusions can be drawn:

(1) Electrochemical results indicated that a total charge of at least 150 A·h/m² with a current density of 1 A/m², was required for effective treatment. This charge requirement was lower than the standard for realkalization without corrosion inhibitor (2016), confirming the effectiveness of MEA in the process. However, as the total charges continued to increase, the improvement in the repair became less pronounced. After a total charge of 200 A·h/m², the corrosion potential and corrosion current density stabilized at -210 mV and 0.8 µA/cm², respectively, significantly reducing the corrosion risk of the steel rebars;

(2) After the repair, the pH around steel rebars increased from 8.08 to 10.39, and the concentrations of K⁺ and MEA exceeded 0.853 wt.‰ and 0.569 wt.‰, respectively. The increasing alkalinity in the mortar, along with the successful migration of ions from the electrolyte solution to the steel rebar, demonstrated the effectiveness of realkalization process with MEA;

(3) The application of the electric field enhanced the restoration of alkalinity in the mortar at greater depths and accelerated the migration of the corrosion inhibitor MEA to the steel rebars, increasing the corrosion inhibition efficiency and contributed to the re-establishment of the passivation layer.

Further research should focus on optimizing electric field design to improve MEA migration efficiency, ensuring more effective repair and wider practical applicability. Additionally, the potential for recarbonation and long-term corrosion in repaired concrete requires further investigation to better understand the durability and performance of the treatment.

## Disclosure statement

No potential conflict of interest was reported by the author(s).

## Funding

This work was supported by the National Science Foundation of China [Grant No. 51408546], Natural Science Foundation

of Zhejiang Province [LY19E080002] and Zhejiang Science and Technology Project of Cultural Relics Preservation (2019014), and Zhejiang University of Science and Technology Scientific Research Fund Project [2023QN025].

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