Concrete corrosion in nuclear power plants and other nuclear installations and its mitigation techniques: a review

2.1 Corrosion initiation

In recent decades, several researchers have focused their studies on evaluating the initiation and propagation of corrosion processes in reinforced concrete structures (Bastidas-Arteaga et al. 2011; Marques and Costa 2010). The alkalinity of the pore solution protects the reinforcement in concrete structures by forming a stable passive layer of γ-Fe₂O₃ at the steel surface (Pradhan and Bhattacharjee 2011; Takaya et al. 2021). However, the free chlorides in the pore solution of concrete or diffused from outside environment and possible carbonation can cause a significant reduction in the pH of the pore solution. The lower pH will lead to deactivation of the passive layer which results in corrosion initiation (Fu et al. 2017; Shaheen and Pradhan 2017). Occurrence of crack due to any cause can lead to easy ingress of chloride ions, oxygen and water to the steel surface which results in faster corrosion. Additionally, cracks introduce discontinuity in the material medium which in combination with the ionic concentration leads to a spatial inhomogeneity in the electrochemical state of steel leading to accentuated corrosion (Subramaniam and Bi 2010).

The region where the passive layer in the reinforced steel surface is disturbed becomes active initially and serves as anode, while the undisturbed reinforcement steel acts as cathode (Ji et al. 2016). The released electrons from the metal reacts with existing water and air (inside the concrete matrix) to form hydroxyl ion (OH⁻) (Shayanfar et al. 2016). This process continues and the produced OH⁻ reacts with Fe²⁺ ion and produces iron hydroxide Fe(OH)₂ as represented in Figure 1 (Cao et al. 2013). The Fe(OH)₂ is the common rust which can react with dissolved oxygen and form different products such as hydrated ferric oxide (Fe₂O₃·H₂O), black magnetite (Fe₃O₄), and green hydrated magnetite (Fe₃O₄·H₂O) etc. These corrosion products slowly fills any empty space between concrete and steel as well as create expansive internal pressure and forms cracks and spalls in the concrete. Based on this understanding of the corrosion initiation process, the detailed mechanism of corrosion is presented in the following sections.

Figure 1:

The process of steel reinforcement corrosion in concrete. Reprinted with permission from (Cao et al. 2013). Copyright @ Elsevier 2013.

2.2 Corrosion mechanism

The corrosion mechanism consists of anodic iron dissolution reaction and cathodic oxygen reduction. The difference in the electrochemical potential between anode and cathode produces a driving voltage in the system. During the anodic and cathodic reaction, the ionic current flows through the surrounding concrete and an electric current through the reinforcing steel. The majority of the studies propose that the prediction of propagation of reinforcement corrosion require electrochemical input parameters such as anodic and cathodic Tafel constants and exchange current densities (Andrade 2018; Warkus and Raupach 2008). In addition, the parameters such as surface state of the electrode, temperature, moisture content, and geometry also have a significant impact on reinforcement corrosion processes (Andrade 2018; Michel et al. 2016).

In the corrosion propagation stage, researchers propose the possibility of two scales of electrochemical corrosion cells called as microcells and macrocells (Andrade et al. 2008; Kim and Kim 2008; Subramaniam and Bi 2010). Microcell corrosion involves a pair of immediately adjacent anodes and cathodes, whereas macrocell corrosion consists of spatially isolated anodes and cathodes as represented in Figure 2 (Cao et al. 2013). Microcell corrosion produces uniform removal of the steel whereas the formation of a macrocell has been shown to result in intense localized metal loss at the anode. An Evans diagram analysis of macrocell corrosion system indicates that an increase in the macrocell current density results in a decreasing contribution from the local microcell at the macrocell anode (Subramaniam and Bi 2010). In both cases, the corrosion initiates as pitting due to the depassivation caused by local contamination with chlorides. The pits on the metal surface forms an anode and the area around it acts as a cathode which may be a large or small area depending upon resistivity of the concrete matrix. Due to the H⁺ ions in the solution around the pits, hydrolysis of the dissolved metal ions takes place, which reduces the local pH. This process can accelerate the dissolution of Fe at pits which increases the rate of corrosion (Warkus and Raupach 2008).

Figure 2:

Representation of microcell corrosion and macrocell corrosion. Reprinted with permission from (Cao et al. 2013). Copyright @ Elsevier 2013.

In concrete structures, electrolytic corrosion may also occur when an external current (from cathodic protection systems and high voltage power supplies) enters into the reinforcing steel and this is called stray current induced corrosion. The currents from the external source can deviate from their intended path if they find an alternative route to flow such as metallic pipe buried in soil. During this process, cathodic reaction occurs at the portion where current enters the structure and an anodic reaction occurs where the current leaves the structure. This process results in metal loss at the anodic site (Goyal et al. 2018).

The potential affected areas of corrosion in concrete structures are steel reinforcement, pre-stressing steel, liner plate and structural steel. The corrosion process of these steel structures in concrete starts with anodic iron dissolution reaction and cathodic oxygen reduction. The parameters such as surface state of the electrode, temperature, moisture content, and geometry also have a significant impact on reinforcement corrosion processes. Similarly, macro cell corrosion and microcell corrosion can be occurred and this corrosion can be initiated by de-passivation caused by local contamination with chlorides. Additionally, the sulfate attack and carbonation induces the corrosion of steel structures in the concrete.

The preceding discussion provides a detailed understanding about the mechanisms and kinetics of corrosion. The specific effect due to various causes of corrosion is discussed in the following section so as to lead to the understanding of the mitigation mechanisms discussed in the latter part of the paper.

2.3 Effect of chloride ions

Chloride ions are the primary source of corrosion of reinforcement steel bars in concrete, which enters into concrete either from external source or from internal source (Khan et al. 2017). Chloride ions from external sources such as sea water and deicing salts penetrate into the concrete during its hardened state. Internally, chloride may be mixed into concrete from the ingredients used for making concrete such as mixing water and aggregates (Al-Attar and Abdul-Kareem 2011; Anacta 2013). Chlorides diffusion depends on many factors such as composition and microstructure of concrete and exposure conditions (like temperature, relative humidity and presence of chlorides (El Hassan et al. 2010)). In addition, intermitted drying-wetting cycles also accelerate the entry of chloride into the concrete structures (Ye et al. 2013, 2015). When chloride attack is accompanied with carbonation under wetting–drying cycles, it may enhance the risk of corrosion in reinforcement concrete structures (Ye et al. 2016). It has been also shown that there is an increased risk of chloride-induced corrosion of steel reinforcements, if alkali–silica reactions occurring in concretes contaminated with NaCl (Sergi and Page 2000).

The chloride induced reinforcement corrosion results in the formation of pits causing loss in cross section leading to reduced bearing capability and ductility of concrete structures (Andrade 2018). In addition, the rust formed around corroded steel generate extensive tensile stresses in the surrounding concrete cover due to volumetric expansion, which leads to longitudinal cracks (corrosion-induced cracks) (Michel et al. 2011; Zhao et al. 2014). Presences of cracks have a significant impact on the mechanisms and kinetics of corrosion in reinforcement concrete structures regardless of its origins and forms (Liang and Wang 2020). The corrosion rate of the reinforcing steel increases with crack-width, and decreases with higher quality and depth of the concrete cover under natural environmental conditions (Cao et al. 2013; Otieno et al. 2010, 2016). It was found that the presence of longitudinal cracks along with mechanical load can accelerate the entry of chloride, water, and oxygen into concrete (Fu et al. 2016; Ye et al. 2013). However, there are some contradictory studies which suggest that the transverse cracks and their width have no impact on initial stage of corrosion (initiation and propagation) of reinforcement structures (Berrocal et al. 2015). Longitudinal cracks generally formed after the corrosion of steel and are considered more dangerous than transverse cracks for corrosion since it has more exposure area of steel to the aggressive environment (Shaikh 2018). The presence of longitudinal cracks are the evidence of the critical development of corrosion on the reinforcing steel and the chloride diffusion increases with crack depth and w/c ratios (Shaikh 2018).

2.4 Effect of sulfate ions

Sulfate ions can easily enter into any concrete structure from external sources or internal sources such as aggregates, curing water and admixtures (Alum et al. 2008). The degradation of concrete due to sulfate attack is very prominent in NPPs due to the exposure to alkali and sulfates present in surrounding environment of NPPs. The degree of sulfate attack by sulfate-containing water or soil is dependent on the concentration of sulfate ions, available cations (sodium, potassium, ammonium or magnesium), C3A content of the cement, and the permeability and quality of the concrete (Krauss and Naus 1998). In presence of both chloride and sulfate ions, these ions interact with hydrated cement phases simultaneously and hence the mechanism of corrosion of concrete becomes complex. In addition, the cations associated with chloride and sulfate makes the corrosion mechanism more complex (Shaheen and Pradhan 2017). Zuquan et al. (2007) found that presence of sulfate in the sulfate-chloride composite solution prevents the entry of chloride into the concrete at early exposure period; however, it enhances the entry of chloride at latter exposure period. In addition, the formation of gypsum and ettringite were higher in the concrete exposed to sulfate solution in comparison with sulfate-chloride composite solution. It was also found that the exposure to only sulfate ions cannot initiate reinforcement corrosion while the mixed chloride-sulfate solution can induce considerable reinforcement corrosion (Shaheen and Pradhan 2015). Another study found that the presence of sulfate ions in a chloride environment has no effect in the initiation time of reinforcement corrosion, however the corrosion current density was enhanced with increase in the concentration of magnesium sulfate and sodium sulfate (Dehwah et al. 2002). Shaheen and Pradhan (2017) found that the presence of sodium sulfate in a chloride environment has diminished the effect of chloride ions while the magnesium sulfate in a chloride environment has enhanced the effect of chloride ions which results in the reduction of passivity of steel reinforcement. Another form of sulfate attack on concrete structure is by microbiological sulfate attack and this is described as microbially influenced concrete corrosion. Thus it maybe concluded that for NPPs located in areas prone to chloride exposure, the inevitable presence of sulfate ions creates a condition favorable for accelerated corrosion and so appropriate mitigation strategies are imperative in such situations.

2.5 Microbially influenced concrete corrosion

This microbial induced corrosion process of concrete structures is known as microbially influenced concrete corrosion (MICC). It is already mentioned that the concrete is typically highly alkaline and at this pH range, normal microbes cannot survive. However, the pH of the concrete may be reduced to pH 9.5 by atmospheric CO₂ and adhesion of several microbes is possible at this condition (Wei et al. 2013). As a result of microbial reaction, sulfur compounds oxidize to sulfuric acid, which leads to corrosion and degradation of concrete structures. The sulfuric acid reacts with the free lime, which produces calcium sulfate and it forms a corroding layer on the concrete surface as well as it penetrates into the concrete which leads to the degradation of the concrete structures (Aviam et al. 2004). In a similar way, nitrifying bacteria produces nitric acid that also causes deterioration of concrete structures. In short, microbial metabolites like organic acids, sulfuric acid and nitric acid etc. can cause biodeterioration of concrete by reacting with the different components of concrete (Bertron 2014; Magniont et al. 2011). Corrosion failures due to microbial activity in NPPs can occur in different systems, which are normally stagnant or which experience continuous flow at low fluid velocities. MICC is also considered as a potential problem in long-term nuclear waste storage as well. Contradictorily, microorganisms can also be used to protect or repair the concrete structures such as bacteria-based self-healing systems. Selected bacteria along with suitable chemical precursors have been found to be used to fill the micro-cracks in concrete so that the durability increases (Jonkers et al. 2010; Wiktor and Jonkers 2011). In addition, some microbial biofilm on the surface of concrete materials can act as a protective layer against biological damage (Decho 2010).

Although all these components are relevant in the possible corrosion initiation in the RCC structures of NPPs, the specific occurrence of such degradation reported for NPPS are discussed in the following section.

4.1 Corrosion inhibitors

Corrosion inhibitors are chemical substances that are added to the matrix to reduce corrosion rate of a material, typically a metal or an alloy that comes into contact with the fluids (Raja et al. 2016). The advantages of corrosion inhibitors are that they are less costly, excellent corrosion resistance effect and easy to handle compared to other preventive methods for corrosion protection (Królikowski and Kuziak 2011). Corrosion inhibitors are commonly used as admixtures in concrete at the time construction; however, they can also be employed as repair patches by mixing with concrete and can be sprayed onto the surface of the concrete. Corrosion inhibitors generally reduce the corrosion process by increasing the anodic/cathodic polarization behavior, by reducing the ions movement of the metallic surface and by increasing the electrical resistance of the metallic surface. A corrosion inhibitor should be able to protect the various construction materials during all phases of plant operation. This means the inhibitor has to be chemically stable throughout the operating temperature and radiation exposure ranges in NPPs. The cooling system of NPPs contains specific types of metallic materials; the corrosion inhibitor should be able to adequately protect them. In addition, the corrosion inhibitor should have temperature resistance and resistance to high energy gamma radiations (Chajduk and Bojanowska-Czajka 2016).

Corrosion inhibitors can be anodic, cathodic or mixed inhibitors depending on their mode of action. Anodic inhibitors suppress the anodic corrosion reaction by increasing the electric potential needed for corrosion initiation of steel which results in reduction of corrosion rate. Anodic corrosion inhibitor passivates the anodic surface of steel by forming an insoluble protective film (Söylev and Richardson 2008). Calcium nitrite is the most widely used anodic inhibitor for concrete protection (Ann et al. 2006). Cathodic inhibitors (e.g. Sodium hydroxide) suppresses the cathodic corrosion reaction by decreasing the potential needed for corrosion of steel and resulting in precipitation of insoluble compounds as a barrier film on the cathodic sites. The mixed inhibitors suppress both anodic and cathodic reactions without changing the corrosion potential. Here, there is a formation of a thin protective hydrophobic film caused by surface adsorption over steel bars which reduces the corrosion rate (Söylev and Richardson 2008). Several studies suggested that the use of mixed inhibitor results in longer time to cracking, lower coulomb values, higher compressive strength, and reduced corrosion rate (Saraswathy and Song 2007; Song and Saraswathy 2006). It was reported that the phosphate ions can be used as a mixed type inhibitor and it reduces the effect of chloride ions in localized corrosion (Nahali et al. 2015; Yohai del Cerro et al. 2013).

Recently, the use of organic inhibitors has gained more attention and it can be used as admixed corrosion inhibitor (Angst et al. 2016) or migrating corrosion inhibitors (MCI) (Ormellese et al. 2011; Shi et al. 2017). Both admixed and migrating corrosion inhibitors inhibit the steel corrosion through the formation of a thin protective barrier film on the steel surface by adsorption mechanism. MCI are applied as liquid and it migrates through concrete by capillary infiltration and vapour diffusion. After reaching the steel surface, the inhibitor gets deposited on the surface (Shen et al. 2014). It was found that carboxylate and amino alcohol-based corrosion inhibitors display dual actions in concrete as these inhibitors form a protective film against chloride after adsorbing on the metal surface, while the carboxylate ester compound precipitates and blocks the pores of the concrete (Söylev et al. 2007). The pore blocking provides additional protection against the corrosion of reinforcements (Yohai et al. 2016). The major problem with MCI is that it may not penetrate deep enough to reach the steel surface and it depends on the porosity and quality of the concrete. In addition, the depth of penetration varies with different parts of the concrete structure which results in non-uniform protection. Since most of the corrosion inhibitors are toxic in nature, there is a need for non-toxic, cost-effective and environmentally friendly substitutes (Pei et al. 2017). Towards this aspect, agro-waste/natural products and medical waste (henna, neem, bamboo, penicillins, cefatrexyl etc.) with negligible environmental impact can be used to replace traditional corrosion inhibitors (Etteyeb and Nóvoa 2016; Gece 2011; Raja and Sethuraman 2008).

Corrosion inhibitors can be electrochemically injected by applying a current density of 1–5 A m⁻² between externally placed anode and the embedded steel cathode (Sawada et al. 2007). This process is called as electrochemical injection of corrosion inhibitors (EICI) and is found to be an effective in preventing the corrosion for carbonated and chloride-contaminated reinforced concrete structures. During the electrochemical injection process, the cationic species migrate to the cathode inside the concrete cover and chloride ions migrates out of the concrete towards external anode as shown in Figure 3 (Lee et al. 2017). Mangayarkarasi and Muralidharan (2014) evaluated the efficiency of the electrochemical injection of inhibitors in the presence of chloride in the concrete by using electrochemical techniques. The results showed that the multi-component inhibitor injection (consist of guanidine, thiosemicarbazide, triethanolamine and ethyl acetate) exhibited more than 95 % efficiency in corrosion rate reduction irrespective of chloride levels in the concretes. It was found that tetrabutylammonium bromide (TBA-B) and tetramethylammonium chloride (TMA-C) can be efficiently used as corrosion inhibitors in electrochemical injection with 85 and 75 %, reduction in the corrosion rate, respectively, along with improvement in the chloride penetration resistance and compressive strength (Nguyen et al. 2015; Pan et al. 2008).

Figure 3:

Schematic representation of electrochemical EICI process in concrete. Reprinted with permission from (Lee et al. 2017). 2023 Intech Open under CC BY license.

4.2 Alternative reinforcement

The corrosion resistant steel bars such as epoxy coated steel, high chromium steel and stainless steel with higher chloride threshold can be used to delay the initiation and progress of corrosion. The epoxy coating protects the steel against the chloride ions by acting as a physical barrier and hinders the corrosion initiation. In addition, the higher resistance of epoxy coating to the electric charge eliminates the formation of macro-cells unless defects are generated within the rebar (Keßler et al. 2016). However, the formation of cracks termed as “holidays” on the coating surface may act as potential sites for accelerated corrosion and thereby become detrimental to the corrosion mitigation process. Hence, the sequence of application process should be done with utmost care and avoid any post application cracking (California test 685).

Carbon steel with higher content of chromium shows good resistance against corrosion since the formation of surface oxide FeCr₂O₄ instead of Fe₃O₄ in presence of chromium (Chajduk and Bojanowska-Czajka 2016). The stainless steel exhibits higher corrosion resistance than carbon steel or mild steel in alkaline solutions with Cl⁻ concentrations from 0 to 0.25 M (Moser et al. 2012). However, the cost of the stainless steel limits their application in reinforcement structures and the cost issue can be minimized by making an outer layer of stainless steel over carbon steel. However, there is chance of galvanic corrosion of carbon steel occurs as a result of coupling between carbon steel and stainless steel if used simultaneously in an applications.

Fibre reinforced plastics (FRP) are the composite materials consisting of fibre phase in a matrix phase with high corrosion resistance, light weight and high tensile strength; it can be used as the alternative reinforcement in concrete structures mostly as a repair strategy (Masuelli 2013; Rajak et al. 2019). The use of FRP is recommended only after arresting the ongoing corrosion and repairing any degradation to the substrate, since it is not capable to stop the ongoing corrosion.

4.3 Mineral admixtures

In concrete design, mineral admixtures can have significant effects on performance, quality, service life, workability and protection from environmental effects (Barbudo et al. 2013; Shi et al. 2012). Even though a small amount of admixture is added, it will actively participate in the hydration reaction mostly through alteration of hydration kinetics of cement resulting in improvement of the properties of hardened concrete and steel-mortar interfacial region (Shi et al. 2009). The admixtures such as fly ash, silica fume and blast furnace slag reduce the chloride penetration and hence to reduce the corrosion as a consequence of the densification of the concrete matrix due to the lime consuming reactions (Berndt 2009; Gesoğlu et al. 2009). The addition of silica fume prevents chloride ingress to the reinforcement, salt induced corrosion of steel and it enhances the electrical resistance of concrete to corrosion (Samad and Shah 2017). The blended cements made by combining portland cement(PC)/pulverized fly ash (PFA) and PC/ground granulated blast furnace slag (GGBS) can reduce the chloride penetration significantly (Ayano and Fujii 2021; Samad and Shah 2017). However, the chance of carbonation will increase if these materials are not cured properly. It was found that Type I cement concrete with 40 % or more ground granulate blast-furnace slag (GGBS) by binder weight can enhance the corrosion resistance of a steel bar two times compared to cement without GGBS (Yeau and Kim 2005). The effect of marble and granite waste dust as partial cement replacement or as inert fillers on the corrosion behavior of reinforced concrete structures was evaluated by Ghorbani et al. (2018). They found that 20 % replacement of marble and granite waste dust in the concrete displayed negligible deterioration and significant improvement in corrosion resistance of reinforcing steel without any significant loss in compressive strength. Ming et al. (2020) evaluated the effect of blast furnace slag (BFS) and carbon nanotubes (CNTs) admixtures in the portland cement on the electrochemical corrosion resistance of carbon steel reinforced concrete. They found that the BFS and CNTs admixtures can be used to prevent corrosion of the steel rebar due to the reduction in the chloride ion permeability and water absorptivity.

4.4 Coatings

Coatings act as a physical barrier against corrosion in reinforcement steel of concrete structures. Generally, coatings used for the protection of reinforcement steel bars can be metallic, organic or cementitious coatings. Two families of metallic coatings are available, the sacrificial coating which are made up of less noble metals such as zinc and cadmium while non-sacrificial or noble coatings uses Ag, Ti, Ni, and Cr for protecting the steel by forming a passive layer. Sacrificial coating can be applied by dipping, electroplating, spraying, cementation and diffusion, and it sacrifices itself to protect the steel cathode (Revie and Uhlig 2008). The formation of hydrogen gas is the main problem with zinc coating (galvanizing), which results in loss of bond between the coating and the cement paste. In non-sacrificial coatings, the parent steel acts as an anode and the passive film acts as cathode.

Organic coatings such as epoxy coating, poly-propylene, polyvinyl chloride, polyurethane etc. isolate the steel from aggressive agents and provide excellent corrosion protection (Pan et al. 2017b). Organic coatings block the penetration of carbon dioxide and chloride ions since it form a continuous polymeric film on the concrete surface with 100–300 μm thickness (Pan et al. 2017b). However, the epoxy coated steel bars are less efficient in long term protection owing to their hydrophilic and porous nature (Pei et al. 2017; Pour-Ali et al. 2015).

Cementitious coatings can provide promising resistance against carbonation and chloride penetration due to the formation of continuous polymer film which acts as a physical barrier to prevent the penetration of corrosive substances into cementitious substrate (Diamanti et al. 2013). Cementitious coating acts by forming a low permeability layer with a thickness of about 2–10 mm. Polymer modified cementitious coatings are made with polymers along with cement and aggregates; and the addition of polymer significantly enhances the properties of cement paste such as strength, resilience, adhesion, chemical resistance and impermeability (Pan et al. 2017b). However, this coating will affect future inspection and testing of concrete structures. Hydrophobic impregnation materials such as silanes, siloxanes and silicate-based compounds can be used to prevent the penetration of chloride and other aggressive ions (Pan et al. 2017a; Zhang et al. 2017). These materials can penetrate into the concrete surface and form a water repellent lining on the pore walls. Sivasankar et al. (2013) found that silane coating can delay the reinforcement corrosion time by at least 4 times, which depends on the molecular size of the coating agent. Even though the silane coating can significantly reduce the corrosion rate in un-cracked concrete specimens, it can accelerate the corrosion on steel bars in cracked concrete (Tittarelli and Moriconi 2010). Afshar et al. (2020) found that the best coating system to prevent corrosion of steel reinforcement bars in concrete are polyurethane, epoxy and alkyd top coatings. Binici et al. (2012) evaluated the effect of different coatings such as colemanite, basaltic pumice, barite and ground granulated blast furnace slag on reinforcement bars against corrosion. They found that pre-coating of steel bar delays the corrosion process and colemanite was more efficient coating materials compared to others.

4.5 Cathodic protection

Cathodic protection is an effective and efficient method to prevent corrosion of reinforcement steel in concrete structures and it can be used in NPPs as either sacrificial anode cathodic protection (SACP), or impressed current cathodic protection (ICCP), or both. The cathodic protection also prevents hydrogen embrittlement, anode degradation or loss of adhesion between anode and concrete and the chance of alkali-silica reaction (Peelen et al. 2008; Zhang et al. 2016). However, the high cost of anode materials, complex monitoring system and short service life are the drawbacks of the cathodic protection. To overcome the cost related issues, polymer composite anodes, especially carbon polymer composites can be used as anode materials with affordable costs, good electrical properties and manufacturing flexibility. Zhang et al. (2016) evaluated the possibility of carbon fiber mesh as anode material for long-term cathodic prevention system and found that it is appropriate for the application as anode in long-term cathodic prevention systems (Zhang et al. 2016).

Sacrificial cathodic protection is commonly used for the protection of submerged structures and underground pipelines (Ameh and Ikpeseni 2017). This method uses zinc or aluminium (less noble metals than steel) to connect with the steel bar and the dissolution of this anode metal delivers DC current due to the potential difference between the steel cathode and the sacrificed metal anode (Figure 4A). However, the main disadvantages of these techniques are the periodic replacements of anode due its dissolution, limited control over the system and low driving voltage which may be insufficient to provide protection.

Figure 4:

Schematics of cathodic protection systems. (A) Sacrificial anodic cathodic protection system. (B) Impressed current cathodic protection system. Reprinted with permission from (Goyal et al. 2018). Copyright 2023 Springer Nature.

In impressed current cathodic protection (ICCP), a small DC current is supplied from an external power source from the permanent anode to the steel bar through the concrete electrolyte as shown in Figure 4B (Leng 2017). The current produced is sufficient to produce hydroxyl ions as a result of cathodic reaction and this will increase the alkalinity and re-passivation of the steel bar followed by strengthening of passive layer. Recently, Su et al. (2019) introduced impressed current cathodic protection in combination with structural strengthening (ICCP-SS) for repairing reinforced concrete which has been subjected to chloride induced corrosion. This technique involves the application of an external current through concrete to steel and cathodic polarization of steel to a more negative potential. In this condition, corrosion is unlikely to occur thermodynamically and steel enters in the immunity zone once the potential of steel is reduced sufficiently by cathodic polarization. In addition, the structural strengthening by applying fiber reinforced polymer is done which enhances the performance of existing degraded structures under loading. Here, they have used carbon fiber reinforced cementitious matrix to embed with carbon fiber mesh. This will assure the accurate bonding between strengthening material and the surface of reinforced concrete structures as well as the existence of electrical conductivity path between corroded reinforcement bars and anode. The service life of various cathodic protection systems have been evaluated across the world and found that any cathodic protection system can work for 15 years without any major issues. A recent update says that 90 % of the cathodic protection systems performed well beyond at least 13 years based on anode life as the main criterion (Polder et al. 2011).

Hybrid cathodic protection system uses a temporary current in combination with low maintenance galvanic system to maintain alkalinity (Byrne et al. 2016). It uses discrete anode as sacrificial system which connected to titanium wire for impressing the current. The steel passivity can be restored by maintaining high pH and by re-alkalization of acidic sites. Here, the sacrificial anode system delivers both the pit re-alkalization and supplementary galvanic treatments (Glass et al. 2008). The treatment was capable of reducing the extreme corrosion with low maintenance requirements.

Cathodic prevention is a special type of cathodic protection which requires only 1/10th of the energy compared to cathodic protection and it is applied to passivate the reinforcement in new constructions before chloride penetration occurs (Zhang et al. 2018). This technique avoids the initiation of pitting corrosion even the subsequent chloride penetration occurs. Cathodic prevention cannot be appropriate if pitting corrosion has already initiated since the potential should be lowered to the minimum value to protect the steel at this stage.

4.6 Electrochemical realkalization

Electrochemical re-alkalization provides long term corrosion protection to steel in the carbonated concrete structures. In this method, a small electric field is applied between steel in concrete and an alkaline electrolyte solution (containing carbonate or hydroxyl ions) and a temporary external anode similar to the ICCP system (Marques and Costa 2010). Platinized titanium mesh and steel mesh anodes are usually used along with electrolyte solutions of sodium, potassium and lithium (Ribeiro et al. 2013). During the re-alkalization process, hydroxyl ions are formed by the reduction of oxygen at the steel, and this helps to increase the alkalinity of the concrete as well as to restore and maintain the passive protective layer around the reinforcing steel. However, high current densities and voltages may results in the hydrogen embrittlement, alkali-aggregate reaction, bond degradation and anodic acidification (Goyal et al. 2018).

The corrosion protection layer cannot be formed and active dissolution occurs when the pH of the pore solution is decreased. The decrease in pH of the carbonated concrete also increases the risk of corrosion because the concentration of chlorides necessary to initiate corrosion, the threshold value, decreases with the pH. This is occurring due to breakdown of chloroaluminates to bound chorides when the pH decreases. At elevated temperature (>60 C), the formation of FeOOH can occurs by the reaction between Fe(OH)₂ and O₂ even at normal concrete pH (Townsend Jr 1970).

4.7 Electrochemical chloride extraction (ECE)

Electrochemical chloride extraction is a non-destructive and cost-effective restoration method used for treating heavily chloride contaminated and corroded concrete structures (Du Fengyin et al. 2018). Here, an electric field is applied for a short period between the steel bar and the externally deposited anode which is surrounded by alkaline electrolyte solution. As a result of this, the negatively charged chloride ions present at the steel surface are removed and migrates towards the external anode so that the chances of corrosion initiation is minimized (Yeih and Chang 2005). In addition, the production of hydroxyl ions by the reduction of oxygen and water maintains the alkalinity of concrete near to steel bar (Sánchez and Alonso 2011). Catalyzed titanium mesh and steel mesh are commonly used as anodes while water, calcium hydroxide solution and lithium borate solution are the commonly used electrolytes. The disadvantages of this system are the requirement of 50–500 times higher charge than cathodic protection systems, possibility for hydrogen embrittlement and alkali–silica reaction as a result of increased hydroxyl ion concentrations near the steel surface during the process (Huang and Wang 2016).

4.8 Mitigation methods of MICC

The main method for the mitigation of MICC is to increase the alkalinity of the concrete by modifying the concrete mix design with admixtures (Silva and Nail 2013). By preventing the favourable growth conditions of microbes, the biodeterioration of concrete structures can be controlled. Using expensive steel alloys with microbial corrosion resistance, chemical biocides and mechanical removal of formed biofilms can be introduced to mitigate MICC. Concrete with a water-to-cement ratio less than 0.45, and with special additives like polypropylene and biocides can able to resist biological attack (Carse 2002). It was found that the thickness and interface properties of the concrete significantly affect the penetration of liquids (Vaidya and Allouche 2010). Nasrazadani and Sudoi (2010) proposed some methods to mitigate microbial corrosion damage such as surface cleaning using biocides, surface painting using admixtures with biocides and replacement of the concrete cover depending upon the type of organism detected and the degree of damage of the structure. It was found that epoxy coating and polyurea lining can provide promising performance against biogenic sulfuric acid corrosion (Berndt 2011; De Muynck et al. 2009). Vincke et al. (2002) found that the addition of the styrene acrylic ester polymer leads to enhanced corrosion resistance against biogenic sulfuric acid attack. Another study found that concrete mixtures containing 10 % ZnO, and photocatalytic titanium dioxide can inhibit fungal colonization and biofouling (Alum et al. 2008). Berndet (2011) found that enhanced microbial corrosion resistance occurs when the cement is partially replaced with 5–10 % silica fume or 40 % blast furnace slag. It was also found that the addition of 4 % by weight of nano TiO₂ in the cement mixture provide antibacterial properties to the concrete (Nazari and Riahi 2010). Since the microbes can easily adapt to the changing environment and the lack of knowledge of natural microbial growth, it is very challenging to protect the concrete structures from microbial corrosion. Regenerative biofilms have been applied for inhibiting the SRB corrosion at the Three Mile Island NPP (Arps et al. 2003). Here, biofilms were used to produce gramicidin-S (a cyclic decapeptide) towards inhibiting the corrosion-causing SRB. In addition, they found that these biofilms were able to protect carbon steel continuously from corrosion in the aggressive, cooling service water of the Three-Mile-Island NPP. The SRB growth was inhibited by supernatants of the gramicidin-S-producing bacteria as well as by purified gramicidin S.