Anticorrosion properties of flavonoids for rust-free building materials: a review

1.1 Corrosion and its implications

Corrosion is a natural process that occurs when metals are exposed to environmental factors like oxygen, water, and certain chemicals. The process of corrosion can be accelerated in harsh environments like marine environments due to the presence of saltwater and other corrosive substances. The combination of high salt concentrations, humidity, and temperature variations can cause corrosion to occur more quickly and severely, leading to the deterioration of metal structures and components. Additionally, radiation exposure can also cause corrosion in certain materials. Effective corrosion prevention and control measures are essential to prolonging the lifespan of metal structures and components, especially in harsh environments (Harsimran et al. 2021). Corrosion is a significant problem for industrial economies, and its direct costs can be substantial (Figure 1). According to various studies conducted over the past 30 years, the annual direct cost of corrosion to an industrial economy is approximately 3.1 % of the country’s gross domestic product (GDP). In the case of the United States, this translates to over $276 billion per year (Koch 2017).

Figure 1:

Total percentage cost of corrosion in USD billion (a); total GDP percentage of economic regions in USD billion (b).

Corrosion of building materials is caused by the reaction of the material with its environment, such as air, water, or chemicals), which can lead to the degradation of the material over time. Steel is particularly susceptible to corrosion due to its iron content, which can react with oxygen and water to form iron oxide, or rust (Roberge 2019). The presence of salts or acids can accelerate the corrosion process, leading to structural damage and potentially compromising the safety and integrity of steel structures (Chen et al. 2017). Copper, while more resistant to corrosion than steel, can still be affected by environmental factors such as air pollutants, moisture, and acids. Over time, copper can develop a patina or greenish coating, which can protect the underlying metal from further corrosion, but this protection can be compromised if the patina is damaged or worn away (Prosek et al. 2014). Concrete, while not a metal, can also be affected by corrosion due to the presence of reinforcing steel bars, or rebar, which can rust and expand, causing cracks and weakening the structure. Additionally, exposure to chemicals or high levels of moisture can also lead to corrosion of the concrete itself, leading to degradation and potentially compromising the safety and durability of the structure (Fuhaid and Niaz 2022).

Moisture can contribute to the corrosion of cement mortar and concrete. When water comes into contact with cement, it can cause a chemical reaction that leads to the formation of new compounds, including calcium hydroxide. This reaction is necessary for the cement to harden and gain strength (Kumar et al. 2009). However, if there is too much moisture in the concrete or if it is stored in a wet environment, it can lead to excessive hydration, which can cause the concrete to weaken and deteriorate over time. The presence of moisture can also promote the growth of bacteria and fungi, which can further contribute to the corrosion of the material.

In addition to moisture, other factors that can contribute to corrosion in cement mortar and concrete include exposure to acidic or alkaline substances, high temperatures, and the presence of certain chemicals or pollutants. To minimize the risk of corrosion, it is important to store and maintain cement mortar and concrete in a dry, controlled environment and to avoid exposing it to potentially damaging substances or conditions (El Gamal et al. 2017). The excess water in fresh concrete can take some time to evaporate, which can lead to a prolonged period of high moisture content and an increased risk of corrosion. Therefore, it is important to carefully monitor the curing process and ensure that the concrete is properly dried out before it is exposed to external conditions. In addition to monitoring moisture levels, it is also important to use high-quality materials and ensure proper construction techniques to minimize the risk of corrosion in cement-based materials.

The corrosion rate in Portland cement mortar can vary depending on the level of moisture in the material. The corrosion rate can be roughly estimated to be 1:6:25 for dry, moist, and wet states, respectively (Kim et al. 2019). This means that the corrosion rate is highest when the material is wet, and lowest when it is dry. However, it is important to note that even in the dry state, there is still a risk of corrosion if the material is exposed to certain chemicals or pollutants.

1.2 Corrosion inhibitors

Corrosion of metals such as steel, copper, and concrete is inevitable, but it can be controlled by using corrosion inhibitors. For improved corrosion safety of steel incorporated in reinforced concrete structures, a corrosion-inhibiting coating process creates a secure, adherent coating film that is precisely coated onto metal surface. The electrochemical interaction of metals with the corrosive atmosphere is the source of this physical phenomenon. Interactions between the metallic surface and the corrosive environment produce sulfides, oxides, and other compounds (Leygraf et al. 2016).

Corrosion inhibitors can be used to control and slow down the corrosion process in metals such as steel and copper. These inhibitors work by either forming a protective layer on the metal surface or by reducing the rate of the corrosion reaction (Palanisamy 2019). In the case of reinforced concrete structures, corrosion inhibitors can be used to protect the embedded steel from corrosion. This can be done through a variety of methods, such as the use of corrosion-inhibiting coatings, which create a secure, adherent coating film that is precisely applied to the metal surface. This coating can protect the steel from exposure to moisture and other corrosive agents, helping to extend the lifespan of the structure (Thomas et al. 2022).

While corrosion inhibitors can be effective in slowing down the corrosion process, they are not a complete solution and must be used in conjunction with other measures, such as regular inspection and maintenance, to ensure the long-term durability and safety of the structure (Goyal et al. 2018). It is important to note that while corrosion is a common and prevalent process that can cause materials to deteriorate over time, not all materials are equally susceptible to corrosion. Materials selection, proper design, and environmental factors can all play a role in the extent and rate of corrosion in a given application. By understanding the causes and effects of corrosion and taking appropriate measures to prevent or mitigate it, materials can be designed and maintained to ensure long-term performance and safety (Wang et al. 2020).

1.3 Chemistry of flavonoids

The natural products chemistry is important for understanding the natural products isolation and their pharmaceutical importance (Najmi et al. 2022). Similarly, synthetic organic chemistry is another dynamic field that concerned with the design and synthesis of organic compounds or modifications in natural products (alkaloids, amino acids, flavonoids, terpenoids, fatty acids, steroids, etc.) using different conventional methods, microwave method, green concept of synthesis, and solid phase reactions (Mitra et al. 2021). Among them, flavonoids are one of the most fascinating areas of the plant chemistry, and organic synthesis has many challenges in the transformation of these molecules to obtain higher level of complexity for enhanced medicinal values as compared to natural flavonoids and bi-flavonoids. Isolation and identification of unknown natural products are among the big tasks (Marsafari et al. 2020). Therefore, synthetic organic chemistry specifically becomes center of attraction for many organic chemists due to the capability to produce valuable products such as natural products, pharmaceuticals, medicinal/drugs, agricultural, and materials during the organic modification of flavonoids.

Flavonoids are bioactive compounds that prevent growth of microorganisms and widely distributed phytochemicals that play a role in cellular functions, plant development, and reproduction, among other things (Shrinet et al. 2021). Two ortho hydroxyl group on the bearing (catechol) containing flavone derivatives are especially impressive among this heterogeneous group of compounds because of their ability to deactivate reactive oxygen species (ROS) and activate siderophores, both of which are essential in plant immune mechanisms (Bailly 2021; Sheikh et al. 2020).

2.1 Chalcone

The name “chalcone” was derived from the Greek word “chalkos,” which means copper, because of its color (Elkanzi et al. 2022). Chalcones have been found to have various biological activities and have been studied extensively in medicinal chemistry (Al-Ostoot et al. 2021). Chalcone is a type of open-chain flavonoid that is composed of two aromatic rings, which are referred to as ring A and ring B. These rings are connected by a highly electrophilic α, β-unsaturated carbonyl group, which consists of a carbon atom double-bonded to an oxygen atom (forming a carbonyl group) and a carbon atom that is also double-bonded to an adjacent carbon atom (forming an unsaturated bond) (Thapa et al. 2021). The α, β-unsaturated carbonyl group is highly reactive due to the presence of both a carbonyl and an unsaturated bond, which makes it a common target for chemical reactions (Figure 3). The linear or nearly planar structure of the chalcone molecule is due to the delocalization of electrons between the carbonyl and unsaturated bonds, which results in a conjugated system. This conjugation system gives chalcone its unique electronic properties, which make it useful in various applications such as organic synthesis and medicinal chemistry (Rudrapal et al. 2021). The double bonds in the chalcone unit and the phenyl rings are conjugated, which means that the π-electrons in these double bonds are delocalized across the entire molecule. This delocalization of π-electrons creates a planar structure and gives chalcones their characteristic yellow color. It also makes them interesting from a chemical perspective, as they exhibit a range of biological and pharmacological activities such as antioxidant, anti-inflammatory, and anticancer properties (Ullah et al. 2020).

Figure 3:

General structure of chalcone and some examples of its derivatives.

They are important intermediates in the biosynthesis of other flavonoids, including flavones, flavonols, and anthocyanins, which are responsible for the pigmentation of many fruits, vegetables, and flowers. Chalcones are known to act as self-protective compounds in plants, helping to protect them from environmental stressors such as UV radiation, pathogens, and herbivores (Saddique et al. 2018). They are also involved in plant–insect interactions, serving as attractants, repellents, or feeding stimulants for insects, depending on the specific chalcone and the insect species involved. In addition to their role in plant biology, chalcones have been found to have a variety of medicinal properties, such as antioxidant, anti-inflammatory, antimicrobial, and anticancer activities. Many herbs and medicinal plants contain chalcones, which may contribute to their therapeutic value (Pratyusha 2022).

Chalcones are indeed versatile molecules that have been extensively used in the synthesis of various biologically important compounds (Marotta et al. 2022). Due to the presence of these functional groups, chalcones can undergo a wide range of chemical reactions, making them useful building blocks for the synthesis of many different types of compounds. One important class of compounds that can be synthesized from chalcones is pyrans, which are six-membered rings that contain one oxygen atom (Asif and Imran 2019).

2.2 Flavone

Flavones are the subclass of flavonoids, which are based on the backbone of 2-phenylchromen-4-one, which consists of a benzene ring fused to a pyran ring with a ketone group at the 4-position. Flavones are distinguished from other flavonoids by the absence of a hydroxyl group at the 3-position of the C-ring (Figure 4). Flavones are widely distributed in the plant kingdom and can be found in many fruits, vegetables, and medicinal plants (Roy et al. 2022).

Figure 4:

Basic structural unit of flavone and its derivative.

Flavones are the type of flavonoid that is commonly found in a variety of plants, including cereal grains and herbs. Flavonoids are a class of plant pigments that have a wide range of biological activities and are believed to play a role in protecting plants from environmental stressors such as UV radiation and pests (Hostetler et al. 2017). Flavones are specifically characterized by their chemical structure, which includes two benzene rings connected by a three-carbon bridge. They are often found in the leaves, flowers, and seeds of plants and are known for their antioxidant and anti-inflammatory properties (Rana et al. 2022).

2.3 Flavonols

Flavonols are the class of flavonoids, which belong to a diverse group of plant compounds known for their antioxidant and health-promoting properties. Flavonoids are found in various fruits, vegetables, herbs, and other plant-based foods. Flavonols, specifically, are a subclass of flavonoids that have a certain chemical structure characterized by a 3-hydroxyflavone backbone. One of the most well-known flavonols is quercetin, which is widely studied for its potential health benefits. Flavonols, including quercetin, are known for their antioxidant and anti-inflammatory properties, which can help protect cells from oxidative stress and reduce inflammation in the body.

2.4 Flavonones

Flavonones are another subclass of flavonoids, which are a group of polyphenolic compounds commonly found in various fruits and vegetables. Like other flavonoids, flavonones possess antioxidant and anti-inflammatory properties, making them potentially beneficial for health. Flavonones are structurally characterized by a 3-hydroxyflavone backbone with a ketone group at the C-4 position. The most well-known flavonone is hesperidin, found in citrus fruits, especially in the peel and membranes of oranges and lemons. Other flavonones include naringenin, found in grapefruits and tomatoes, and eriodictyol, found in lemons and berries.

2.5 Isoflavones

Isoflavones are the class of phytoestrogens, which are naturally occurring plant compounds that have a similar structure to the hormone estrogen. They are primarily found in legumes, particularly in soybeans and soy products. Isoflavones have gained considerable attention due to their potential health effects and their ability to interact with estrogen receptors in the human body.

2.6 Flavan-3-ols

Flavan-3-ols are a class of flavonoids, which are a group of polyphenolic compounds found in various plant foods. Flavan-3-ols are characterized by their chemical structure containing a 2-phenyl-3,4-dihydro-2H-chromen-3-ol backbone. The flavan-3-ol subgroup includes several compounds such as catechin, epicatechin gallate, epigallocatechin, epigallocatechin gallate, etc. Flavan-3-ols and their derivatives are known for their antioxidant properties and have been associated with various health benefits, including cardiovascular health and potential protective effects against certain diseases.

2.7 Anthocyanins

Anthocyanins are another class of flavonoids, which are water-soluble pigments responsible for the red, purple, blue, and magenta colors found in many fruits, vegetables, flowers, and other plants. These compounds play important roles in plant biology, acting as antioxidants, UV protectants, and contributing to pollination by attracting insects and animals. The chemical structure of anthocyanins consists of a 2-phenylbenzopyrylium (flavylium) cation that can undergo structural modifications, leading to different anthocyanin pigments with varying colors. The color of anthocyanins can change depending on the pH of the environment they are in. In acidic conditions, they appear red, while in more neutral or alkaline conditions, they exhibit blue or purple hues.

4.1 Iron/steel

In the presence of oxygen and water, iron undergoes a series of reactions that lead to the formation of rust, which is primarily composed of iron (III) hydroxide. The process starts with the conversion of iron metal into iron (II) hydroxide (Fe(OH)₂), which is then oxidized by oxygen to form FeOOH. This process is known as the rusting of iron or corrosion (Tahawy et al. 2021). The formation of rust on iron or steel surfaces is a major concern as it weakens the material and can lead to structural failures. The pH of the aqueous medium plays a critical role in the corrosion process. In a nearly neutral to weakly basic medium, the corrosion rate of iron is relatively high. This is because the formation of a protective oxide layer is hindered in such an environment. Reaction of formation of corrosion products of iron in neutral or basic medium and in acidic medium is presented in Figure 7.

Figure 7:

Reaction of formation of corrosion products of iron in neutral or basic medium and in acidic medium.

However, in an acidic environment, the corrosion rate is even higher as the acid reacts with the oxide layer, leaving the metal surface exposed to further corrosion. On the other hand, in a strongly basic environment, the corrosion rate is reduced as the alkaline medium promotes the formation of a protective oxide layer on the metal surface (Tamura 2008). Therefore, it is important to consider the pH of the environment when designing materials for use in corrosive environments. Materials such as stainless steel are often used in such applications as they have a high resistance to corrosion due to the formation of a stable and protective oxide layer on their surface.

In Portland cement-based concrete, steel is protected against corrosion by the highly alkaline pore water in the concrete. When Portland cement hydrates, it produces calcium hydroxide (Ca(OH)₂), which is highly alkaline and contributes to the high pH of the concrete pore water. The high pH of the concrete pore water creates a passivating layer of hydrated iron oxide (Fe₂O₃.nH₂O) on the surface of the steel (Plusquellec et al. 2017). This passivating layer acts as a barrier to prevent the diffusion of oxygen, moisture, and other corrosive agents to the steel surface, thereby protecting it from corrosion.

The thickness of the passivating layer depends on several factors, including the pH of the concrete pore water, the composition of the concrete, and the exposure conditions. In general, the passivating layer can range from 2 to 20 μm in thickness (Angst et al. 2017). However, it is important to note that the presence of aggressive ions, such as chloride or sulfate ions, can break down the passivating layer and cause pitting corrosion on the steel surface. Therefore, it is important to limit the amount of these ions in the concrete mix and to provide adequate concrete cover to the steel reinforcement to prevent exposure to aggressive agents.

The corrosion-protective effect of concrete on embedded steel can be compromised if the pH of the concrete pore water falls below a certain threshold, which is typically around pH 9.0. At lower pH levels, the passive layer of hydrated iron oxide on the steel surface breaks down, and corrosion can occur (Huet et al. 2005). Carbon dioxide from the air reacts with the calcium hydroxide in the concrete to form calcium carbonate, which is a neutral compound. This reaction reduces the amount of alkaline ingredients in the concrete and decreases the pH of the pore water, which can lead to the breakdown of the passive layer on the steel surface and subsequent corrosion.

Another important factor that affects the corrosion of embedded steel in concrete is the availability of oxygen. The passive layer on the steel surface can only form and be maintained if there is enough oxygen available to facilitate the formation of the hydrated iron oxide. If oxygen is limited or prevented from reaching the steel surface, the passive layer can break down and corrosion can occur (Alhozaimy et al. 2016). The corrosion protection of steel in concrete is dependent on several factors, including the pH of the pore water, the presence of aggressive ions, the amount of free water in the cement stone, and the availability of oxygen. Proper design, material selection, and construction practices can help minimize the risk of corrosion in reinforced concrete structures (Cwalina 2014).

HCl is commonly found in industrial settings and can be present in building materials due to pollution or chemical processes. The performance of inhibitors in HCl solutions provides valuable insights into their effectiveness in environments where acidic substances may come into contact with building materials. Abdelaziz et al. investigated the use of leaves extract from the Arbutus unedo leaf plant as a green corrosion inhibitor for mild steel (MS) in HCl medium (Abdelaziz et al. 2021). The aqueous extract derived from the leaves of A. unedo leaf plant was found to be abundant in polyphenols, specifically flavonoid compounds, which were quantitatively determined using the aluminum chloride colorimetric method. The aqueous extract derived from the leaves of A. unedo leaf plant was found to be abundant in polyphenols, specifically flavonoid compounds, which were quantitatively determined using the aluminum chloride colorimetric method. Among the various extracts tested, the n-butanol extract exhibited the highest concentration of total phenols and total flavonoid contents, measuring 219.46 gallic acid equivalent (GAE) mg/g extract and 174.66 mg quercetin equivalents (QE)/g of dry extract, respectively. The initial corrosion current density (I _corr) and corrosion potential (E _corr) measured without the presence of leaves extract from A. unedo leaf extract are 348.98 μA/cm² and −500.79 mV/SCE, respectively. Furthermore, the introduction of different concentrations of A. unedo leaves extract results in a reduction in both cathodic and anodic current densities. The findings indicated that as the concentration of leaves extract increases, the corrosion current density decreases (reaching 41.55 μA/cm² at a concentration of 0.5 g/L). Clearly, even at low concentrations, the addition of the plant extract leads to a significant decrease in current density values, thereby enhancing the IE up to a maximum of 88.09 % (0.5 g/L). The effectiveness of A. unedo L. leaves extract as a corrosion inhibitor is influenced by its organic constituents, particularly the phenol and flavonoid contents. The total flavonoid content of the extract varies from 9.10 ± 7.70 to 174.66 ± 35.11 mg QE/g of dry extract, with the n-butanol extract exhibiting the highest concentration.

Meng et al. investigated the corrosion inhibitory properties of sugarcane purple rind ethanolic extract (SPRE) for carbon steel in a 1 M HCl solution (Meng et al. 2021). The results revealed that the IE of C-steel in the HCl solution increased with higher concentrations of SPRE. However, elevated temperatures moderately decreased the anticorrosive efficacy of SPRE. The maximum IE of 96.2 % was achieved at 298 K using 800 mg/L of SPRE. The polarization curves indicated that SPRE suppressed both the anodic and cathodic reactions, classifying it as a mixed-type corrosion inhibitor with a predominant anodic effect. The corrosion current density (icorr-P) decreased as the concentration of SPRE increased, while the charge transfer resistance (Rct) increased, indicating enhanced inhibitory properties due to SPRE adsorption. At higher temperatures, partial desorption of SPRE occurred, resulting in a slight increase in icorr-P and a decrease in Rct. However, even at 328 K, SPRE maintained its morphology and wettability.

Two different alkaloids were extracted from plants Artemisia vulgaris and Solanum tuberosum. These alkaloids were then tested as corrosion inhibitors for MS samples in 1 M H₂SO₄ (Parajuli et al. 2022). The corrosion inhibition potential of the extracted alkaloids was evaluated using weight loss and potentiodynamic polarization measurement methods. The results showed that the corrosion IE of A. vulgaris alkaloid was 92.58 %, while that of S. tuberosum alkaloid was 90.79 %, based on the weight loss measurement after immersing the samples for 6 h in a 1,000 ppm inhibitor solution. Furthermore, the potentiodynamic polarization measurement revealed that the corrosion IE of A. vulgaris alkaloid was 88.06 %, and for S. tuberosum alkaloid, it was 83.22 % for a sample immersed for 1 h in a 1,000 ppm inhibitor solution. These findings suggest that the alkaloids extracted from A. vulgaris and S. tuberosum plants have promising efficiency as corrosion inhibitors for MS in H₂SO₄.

Vu et al. presented a study on the corrosion inhibition properties of Houttuynia cordata leaf extract for steel in an aqueous 0.1 M HCl solution (Vu et al. 2020). The research findings reveal that the H. cordata leaf extract acts as a mixed-type inhibitor for steel, effectively inhibiting the corrosion process. It exhibits a high level of inhibition performance, with up to 98.3 % corrosion inhibition achieved by adding 1,500 ppm of the inhibitor. Dehghani and coworkers explored the potential of Chinese gooseberry fruit shell extract as a green and cost-effective corrosion inhibitor for MS in acidic solutions, especially in HCl environments (Dehghani et al. 2019a). The extract contains soluble biologically active compounds that effectively inhibit MS corrosion. Electrochemical tests showed up to 92 % IE with 1,000 ppm extract after 2.5 h of metal immersion. Weight loss experiments also revealed 94 % efficiency after 5 h at 25 °C. The extract primarily exhibits cathodic inhibition with minimal impact on the hydrogen evolution reaction.

The study by Dehghani and coworkers investigated the corrosion inhibition impact of Eucalyptus leaf extract (ELE) on mild steel (MS) in HCl solution through experimental and computational analyses (Dehghani et al. 2019b). Electrochemical impedance spectroscopy (EIS) revealed that higher ELE concentrations led to a significant increase in charge transfer resistance, resulting in an IE of approximately 88 % with 800 ppm ELE after 5 h of exposure. Polarization tests showed mixed inhibition effects of ELE with a slight cathodic prevalence, reducing the corrosion current density (icorr) from 0.93 μA/cm² for the uninhibited sample to 0.25 μA/cm² for the inhibited sample (800 ppm ELE).

Ogunleye and coworkers investigated the green corrosion inhibition properties of Luffa cylindrica leaf extract (LCLE) on MS in a HCl environment (Ogunleye et al. 2020). Various techniques were employed, including gravimetric, depth of attack, and surface analysis. The study examined the effect of inhibitor concentrations (0.50–1.00 g/L), temperatures (30–60 °C), and immersion time (4–12 h) on the IE of the LCLE on MS immersed in a 0.5 M HCl solution. The optimum IE of 87.89 % was achieved.

4.2 Aluminum

The good corrosion resistance of aluminum is due to the formation of a passive oxide or hydroxide layer on its surface. This layer acts as a barrier that protects the underlying metal from further corrosion (Jagtap et al. 2022). The passive layer on aluminum is formed naturally in the presence of oxygen and water, and it is stable in a pH range between 4 and 9. In this pH range, the passive layer is largely insoluble and prevents further corrosion of the metal. As a result, aluminum materials are resistant to corrosion in nearly neutral to weakly acidic aqueous media, as well as in humid air and atmospheric corrosion conditions.

The passive layer on aluminum is also self-renewing, meaning that if it is damaged or scratched, it will quickly reform in the presence of oxygen and water. This property of self-renewal further enhances the corrosion resistance of aluminum (Chen et al. 2020). The good corrosion resistance of aluminum makes it a popular material in constructional engineering, particularly in applications where it is exposed to outdoor or corrosive environments. Additionally, aluminum is lightweight, strong, and easily fabricated, making it an attractive material for a wide range of structural and decorative applications.

In strongly acidic solutions, the protective layer on aluminum can be dissolved, leaving the underlying metal vulnerable to general corrosion. Similarly, in strongly alkaline solutions, the protective layer can be converted into a more soluble form, which also leaves the underlying metal vulnerable to corrosion (Green 2020). The disintegration of the protective layer on aluminum in more strongly alkaline media can begin at a pH as low as 9.0, which is why the application of aluminum and its alloys is not recommended in strongly alkaline environments. In such environments, aluminum is susceptible to general corrosion, which can lead to a loss of strength and durability in structural applications. Therefore, when selecting materials for use in environments with extreme pH levels, it is important to consider the amphoteric nature of aluminum and its susceptibility to corrosion in strongly acidic or alkaline solutions. Other materials with better resistance to such environments, such as stainless steel or plastics, may be more suitable for these applications.

Aluminum materials are highly reactive, as you would expect from their position in the electrochemical series. Even in the absence of oxygen, aluminum can react with acids or bases to produce hydrogen gas and soluble aluminum salts, such as aluminates (Zhang et al. 2009). For example, in strongly acidic solutions, aluminum reacts with hydrogen ions (H⁺) to form aluminum ions (Al³⁺) and hydrogen gas (H₂):

In strongly alkaline solutions, aluminum reacts with hydroxide ions (OH⁻) to form soluble aluminum hydroxide species and hydrogen gas:

These reactions can contribute to the corrosion of aluminum in acidic or alkaline environments and can also result in the evolution of hydrogen gas, which can be hazardous in certain applications. Therefore, it is important to consider the reactivity of aluminum when selecting materials for use in corrosive environments, and to take appropriate precautions to prevent or mitigate corrosion and hydrogen gas evolution (Ibrahimi et al. 2021).

In the presence of free alkali hydroxides in the pore solution of Portland cement building materials, aluminum and its alloys can undergo general corrosion, which can result in the formation of soluble aluminum hydroxide species and the loss of material. This can occur through a process called alkaline attack, which is a type of chemical corrosion that is driven by the highly alkaline environment of the concrete (Hansson et al. 2012). Alkaline attack can occur when aluminum is in direct contact with fresh concrete or with concrete that has not yet fully cured. During the early stages of curing, the pore solution of the concrete is highly alkaline due to the presence of free alkali hydroxides, such as sodium hydroxide and potassium hydroxide. These hydroxides can react with the surface of the aluminum to form soluble aluminum hydroxide species, which can then be washed away by the alkaline pore solution (Wei et al. 2022).

To prevent alkaline attack of aluminum in Portland cement building materials, it is important to use appropriate coatings or barrier materials to protect the aluminum surface from contact with the alkaline environment. Additionally, aluminum alloys that are specifically designed for use in corrosive environments, such as those with high levels of chromium or other corrosion-resistant elements, may be used to mitigate the effects of alkaline attack (Quiambao et al. 2019).

Anodization is an electrochemical process that forms an oxide layer on the surface of aluminum, which provides protection against corrosion and enhances the material’s appearance. However, this anodization layer produced anodically, also known as Eloxal, can still be vulnerable to attack by moist alkaline-reacting building materials. This can occur when the anodized aluminum comes into contact with such materials, causing the protective layer to break down and exposing the underlying metal to corrosion (Lee and Park 2014). To prevent corrosion damage caused by contact with moist alkaline building materials, it is essential to apply additional protection to the aluminum.

Metals like aluminum and its alloys can be susceptible to corrosion when they come into contact with alkaline materials, and the risk of corrosion increases as the pH value of the material increases. One way to limit the corrosion of aluminum is to choose an adequate binding agent that can help protect the metal surface from the corrosive effects of the alkaline material (Nazeer and Madkour 2018). In the case of concrete, not all types of concrete are equally aggressive toward aluminum. Some types of concrete, such as autoclaved aerated concrete (gas concrete), have a relatively low alkalinity and are not considered aggressive toward aluminum. Therefore, the risk of corrosion in aluminum materials can be minimized by choosing the appropriate type of concrete for the construction project (Herting and Odnevall 2021).

The study investigates the influence of garlic extract as a green inhibitor on the uniform and localized corrosion of aluminum alloy in aerated 3.5 wt% NaCl solution (Hajsafari et al. 2021). The electrochemical noise analysis revealed that the garlic extract initially increases the pit initiation rate by forming an antioxidant layer on the aluminum surface. However, with time, the chemical adsorption of the extract strengthens, leading to a significant reduction in the propagation of corrosion damage by up to one-tenth. The EIS results indicated that a garlic extract concentration of 8 mL L⁻¹ induced the most effective inhibition, enhancing the corrosion resistance from 2,917 Ω cm² to 14,557 Ω cm².

Udensi et al. evaluated the corrosion inhibition performance of a low-cost and eco-friendly extract obtained from Treculia africana leaves on AA7075-T7351 aluminum alloy in 2.86 % NaCl solutions (Udensi et al. 2021). The results of various characterization techniques showed comparable IE of T. africana. Increasing the T. africana concentration and temperature of the environment led to an increase and decrease in η, respectively. The suitability of T. africana as an eco-friendly inhibitor for the corrosion of AA7075-T7351 aluminum alloy in 2.86 % NaCl solution was investigated. The IE obtained from gravimetric, EIS, and Tafel polarization (TP) techniques were in agreement, particularly for T. africana concentrations of 0.6 g/L and above. Higher inhibition efficiencies were observed at lower temperatures in the environment. Moreover, increasing T. africana concentrations raised the activation barrier for corrosion initiation, indicating the inhibitory effect of T. africana on the corrosion process. These findings provide strong justification for the use of T. africana leaves extract as an eco-friendly and effective corrosion inhibitor for AA7075-T7351 aluminum alloy in chloride-containing environments.

Another study by Udensi et al. explored the corrosion inhibition properties of Newbouldia laevis leaves extract on aluminum alloy AA7075-T7351 in a 1.0 M HCl environment (Udensi et al. 2020). The extract contains biologically active and water-soluble compounds such as luteolin 7-glucoside and phenolic polymeric compounds, which act as organic corrosion inhibitors for aluminum. Gravimetric analysis revealed that the optimal IE was 86.1 % at room temperature (298 ± 1 K) and 67.5 % at 338 K for the maximum concentration of 0.6 g/L of N. laevis extract. The results indicated that the N. laevis extract’s inhibitory effect was influenced by temperature, and the inhibition process was likely achieved through an electrostatic attraction of the polymeric components of the N. laevis extract onto AA7075-T7351 coupons or physical adsorption.

4.3 Copper

The excellent corrosion resistance of copper materials is due to their “noble” character, which means they have a positive standard electrode potential of +0.34 VH. This makes them less likely to undergo oxidation and corrosion compared to other metals with lower electrode potentials (Jing et al. 2008). Copper materials also have the ability to form protective layers in many normal media, chemicals, and on contact with building materials. In water and neutral salt solutions, copper materials have excellent corrosion resistance in a wide pH range. In diluted (nonoxidizing) acids and in the alkaline region, copper is particularly superior to other noniron metals.

However, copper and its alloys can become inapplicable if the formation of protective layers is hampered, and the material is heavily attacked through the formation of complex salts, such as when in contact with ammonia and ammoni-alkaline solutions. In these cases, the copper material may corrode more rapidly, and additional measures such as protective coatings or selection of alternative materials may be necessary to prevent corrosion damage (Gurrappa 2005). Copper and its alloys are resistant to uniform corrosion when embedded in moist concrete or cement mortar because of the protective layer of slightly soluble copper (I) oxide that forms on the surface when exposed to air. This layer is virtually insoluble in alkalis and helps to protect the metal from corrosion (Bacelis et al. 2020). As a result, copper and its alloys are often used in construction for their corrosion resistance and durability, particularly in environments where they may be exposed to moisture or other corrosive agents.

When cements with higher alkalinity (pH value of the cement stone pores solution > 13.3) are used, the protective layer of copper (I) oxide on the surface of copper and its alloys may be disrupted, leading to accelerated corrosion. Brasses, which are copper alloys that contain significant amounts of zinc, are especially susceptible to corrosion in these conditions (Megahed et al. 2020). Zinc is more reactive than copper and can form complex salts with alkalis, leading to the breakdown of the protective layer on the surface of the brass. Therefore, in situations where cements with high alkalinity are used, it may be necessary to consider alternative materials or protective coatings to prevent corrosion damage to copper and its alloys, especially brasses (Assad et al. 2023).

Artocarpus heterophyllus Lam. leaves extract was used as the corrosion inhibitor for copper in H₂SO₄ solution (He et al. 2021). Scanning electron microscope (SEM) and atomic force microscope (AFM) results showed that when A. heterophyllus Lam. leaves extract was added to a H₂SO₄ environment, the copper surface remained flat. Electrochemical experiments revealed that at 500 ppm concentration of A. heterophyllus Lam. leaves extract, the corrosion IE obtained from EIS data was 97.3 %. Even at a temperature of 313 K, the corrosion inhibition nature of A. heterophyllus Lam. leaves extract remained at 97 %. The A. heterophyllus Lam. leaves extract was found to be adsorbed onto the copper surface in a Langmuir monolayer adsorption manner. These findings indicate the potential of A. heterophyllus Lam. leaves extract as an effective corrosion inhibitor for copper in a H₂SO₄ environment.

Xiang and He studied the investigation of the anticorrosion properties of Citrus reticulata leaves extract on copper in a sulfuric acid environment using both theoretical and experimental approaches (Xiang and He 2021). One of the key findings of their research is that when the C. reticulata leaves extract concentration is 500 mg/L, its IE can be as high as 97.3 %, indicating that it is effective in inhibiting the corrosion of Cu in the presence of sulfuric acid. Furthermore, the researchers investigated the effect of soak time on the corrosion inhibition nature of C. reticulata leaves extract. They found that even after 24 h of immersion in a sulfuric acid solution containing 500 mg/L C reticulata leaves extract, the IE of C. reticulata leaves extract remains relatively high at over 93 %. This suggests that C. reticulata leaves extract is capable of forming a stable barrier film on the surface of the copper electrode, providing sustained protection against corrosion over an extended period.

Tan et al. investigated the potential of Papaya leaves extract (PLE) as an eco-friendly corrosion inhibitor for copper in a sulfuric acid medium (Tan et al. 2021). The study demonstrated that PLE exhibits excellent anticorrosion properties over a specific temperature range. Morphological analysis tests conducted at different temperatures provided strong evidence of PLE’s ability to inhibit corrosion. Additionally, X-ray photoelectron spectroscopy (XPS) results showed the formation of an adsorption film on the copper surface, specifically consisting of Cu–S bonds and Cu–N bonds. These bonds contribute to the anticorrosion mechanism of PLE. The research also found that PLE performs remarkably well in a sulfuric acid corrosion solution with a concentration of 0.5 mol/L. Even with increasing temperatures within a specific temperature range (298–308 K), PLE continues to demonstrate excellent anticorrosion performance for copper.

Feng and coworkers explored the potential of Veratrum root extract as an eco-friendly corrosion inhibitor for copper in a H₂SO₄ solution (Feng et al. 2021). It was found that at a concentration of 200 ppm, Veratrum root extract achieved an impressive IE of 97 %, signifying its high effectiveness as an inhibitor. Zhang et al. explored the potential of Rosa laevigata extract as an environmentally friendly corrosion inhibitor for copper in a 0.5 M sulfuric acid solution (Zhang et al. 2022). The electrochemical tests demonstrated that R. laevigata extract functions as a mixed-type inhibitor, with a corrosion IE reaching 89.8 % at a concentration of 300 mg/L. The inhibitive ability of R. laevigata extract remained significant within a certain temperature range. The adsorption of R. laevigata extract on the copper surface followed the Langmuir adsorption model, with K_ads (adsorption equilibrium constant) and ΔG₀ads (standard free energy of adsorption) values at 298 K being 36.97 L/g and −26.07 kJ/mol, respectively. This indicates that the adsorption process is spontaneous and involves both chemisorption and physisorption. AFM results revealed that the immersion of the copper sample in R. laevigata extract solution led to a decrease in the average roughness value from 38.9 nm to 13.2 nm, suggesting a positive effect on the surface morphology of copper. Additionally, the theoretical calculations provided insight into the inhibitory properties of specific molecules present in the R. laevigata extract. The E_binding values (binding energies) for 6,7-dimethoxycoumarin, catechin, kaempferol, and loliolide were determined to be 341.5 kJ/mol, 514.1 kJ/mol, 500.3 kJ/mol, and 316.0 kJ/mol, respectively. These values indicate that these molecules exhibit a certain degree of inhibition on copper corrosion in sulfuric acid solution.

Kusumaningrum investigated the potential of A. heterophyllus peel extract, a nontoxic fruit waste containing antioxidants, as a corrosion inhibitor for protecting pure copper in nitric acid (HNO₃) environments (Kusumaningrum et al. 2022). The study explored the corrosion inhibition properties of the peel extract at varying concentrations (0–800 ppm) through potentiodynamic polarization methods. The results indicated that the A. heterophyllus peel extract acts as a mixed inhibitor, primarily of the anodic type. The highest IE observed was 98 % at a concentration of 800 ppm and a temperature of 25 °C. The adsorption of inhibitor molecules on the pure copper surface followed the Frumkin adsorption isotherm equation, with physical adsorption being the dominant mechanism.

4.4 Zinc

Zinc has a negative standard electrode potential (−0.76VH) and is, therefore, thermodynamically susceptible to corrosion (Hoang Huy et al. 2021). However, like aluminum, it also has the ability to form protective coatings made of solid corrosion products in many normal environments and building materials by reacting with its surroundings (Popoola et al. 2014). These protective coatings, which are primarily composed of zinc oxide and zinc hydroxychloride, help to prevent further corrosion of the underlying zinc material by acting as a physical barrier and inhibiting the movement of ions and other corrosive agents. As a result, zinc is often used in construction for its corrosion resistance, durability, and aesthetic qualities.

Zinc is an amphoteric metal, which means it can react with both acids and bases. As such, it is not resistant in both acidic environments with a pH below 5 and alkaline environments with a pH above 12. In more alkaline solutions, zinc hydroxide can form and react with the alkaline compound to create readily soluble and nonprotective zincates, while also producing hydrogen gas. This reaction can lead to the breakdown of any protective coatings on the surface of zinc, increasing the susceptibility of the metal to further corrosion. Therefore, it is important to consider the pH of the surrounding environment when using zinc in construction and to implement appropriate protective measures, such as coatings or galvanization, to prevent corrosion damage (Hale et al. 2012).

In alkaline concrete with high pH values of the pore’s solution (between 12.6 and 13.8), zinc can be susceptible to corrosion due to its amphoteric reaction. However, it has been observed that for pH values of the pores solution ≤ 13.3, the dissolution rate of zinc under the formation of hydrogen quickly diminishes (Andrade and Alonso 2004). This can be explained by the passivation of the zinc surface, which occurs when a thin and stable layer of corrosion products forms on the surface of the metal. This layer acts as a barrier and prevents further corrosion of the zinc substrate.

Therefore, when using zinc in construction applications where it will be exposed to alkaline environments, it is important to consider the pH of the environment and ensure that appropriate measures, such as passivation or galvanization, are taken to prevent corrosion damage (Pokorný et al. 2017). The formation of the slightly soluble calcium hydroxozinkat Ca[Zn(OH)₃]₂·2H₂O in the presence of Ca(OH)₂ and Zn(OH)₂ is believed to be responsible for the passivation of zinc in concrete. This corrosion product forms a protective layer on the surface of the zinc, which slows down the corrosion process (Pokorný et al. 2019). While zinc is still susceptible to corrosion in alkaline building materials, it is less susceptible than aluminum and lead and slightly more susceptible than copper materials. The critical pH value of 13.3, the passivatability of zinc is more restricted with increasing alkalinity, and zinc corrosion increases significantly. However, in carbonated concrete, the corrosion rate of zinc can be slightly higher than in alkaline concrete, but it is still considerably lower than that of steel. Therefore, galvanized reinforcing steels are often used if premature carbonation is expected.

Chromium in cement can form a passive layer on the surface of the zinc, which can protect it from further corrosion. The passivation process is accelerated in the presence of chromium, and the resulting protective layer is more stable and durable. Chromium can also react with other components in the cement to form insoluble compounds that further protect the zinc from corrosion. However, it is important to note that excessive amounts of chromium can have negative environmental impacts and may lead to health concerns, so the amount of chromium in cement must be carefully controlled (Ai et al. 2016).

Lebrini et al. focused on evaluating the effect of Bagassa guianensis extract on the corrosion behavior of zinc in a chloride medium (3 % NaCl) (Lebrini et al. 2020). The results of the study demonstrated that the plant extract from B. guianensis served as a sustainable and environmentally friendly inhibitor for zinc corrosion in a 3 % NaCl solution. At a concentration of 100 ppm, the extract exhibited an impressive IE of approximately 97 %. The presence of the green inhibitor influenced the electrochemical reactions, as observed from the polarization curves. In the presence of the extract in the 3 % NaCl solution, a shift toward more positive potentials was detected.

In a study conducted by Fouda et al., the corrosion inhibition properties of Ailanthus altissima extract were investigated for Zn in a 0.5 M HCl solution (Fouda et al. 2018). The researchers found that the extract from A. altissima demonstrated effective corrosion inhibition for Zn in the acidic solution. Chemical and electrochemical measurements provided evidence supporting the inhibitive nature of the A. altissima extract. The IE increased with higher doses of the extract and decreased with elevated temperatures. The maximum IE value recorded was 77.5 % at a concentration of 500 ppm.

Fouda et al. reported the use of Ferula hermonis plant extract as a corrosion inhibitor for Zn in HCl solution (Fouda et al. 2021). The results of the study indicated that the F. hermonis extract exhibited good efficiency in preventing zinc corrosion and displayed high inhibition efficiencies. The maximum IE was observed to be approximately 90.6 % at a concentration of 300 ppm of the extract. This suggests that the extract has a significant inhibiting effect on zinc corrosion in HCl solution. Furthermore, the study revealed that the %IE increased with an increase in the concentration of the F. hermonis extract, indicating a concentration-dependent inhibition effect. However, the IE was found to decrease with increasing temperature, implying that higher temperatures might weaken the inhibiting properties of the extract.

Ramezanzadeh investigated the combined corrosion inhibition properties of different compounds found in Mangifera indica leaves extract and zinc ions on MS in simulated seawater (Ramezanzadeh et al. 2019). The corrosion inhibitory capability of M. indica extract in saline solution was evaluated using electrochemical techniques. The results from polarization tests revealed that the highest corrosion inhibition power, reaching 91 %, was achieved when a combination of 400 mg/L of M. indica extract and 400 mg/L of zinc cations was used. This indicates that the presence of both M. indica extract and zinc ions together provides a strong synergistic effect, leading to enhanced corrosion inhibition of MS in the simulated seawater environment.

Loto et al. investigated the corrosion inhibition and surface protection properties of a combined admixture of Rosmarinus officinalis (a plant extract) and zinc oxide on low carbon steel in 1 M HCl and H₂SO₄ solutions (Loto 2018). The results obtained from the study confirmed that the plant extract showed higher effectiveness in the presence of HCl solution compared to H₂SO₄ solution. In 1 M HCl, the optimal IE reached 93.26 %, while in H₂SO₄, it was 87.7 %. The compound exhibited mixed-type inhibition behavior in both acids, indicating that it can inhibit the corrosion of low carbon steel through multiple mechanisms. The plant extract caused a shift in the corrosion potential values of the low carbon steel, which indicates specific corrosion inhibition behavior without the application of an external potential. In HCl, the corrosion potential shifted cathodically, while in H₂SO₄, it shifted anodically.

Sameh investigated the potential of Pyracantha coccinea phenolic extracts as eco-friendly plating additives in zinc electroplating processes (Sameh et al. 2021). The study explores the effect of these extracts on the electrodeposition process, coating morphology, and corrosion resistance of zinc-plated substrates. The results demonstrated that the electrodeposition process was significantly influenced by both the concentration of the additives and the specific type of extract used. Moreover, the study showed that the presence of the P. coccinea extracts during the plating process enhanced the corrosion resistance of the zinc-coated substrates compared to coatings formed without the extracts. Specifically, when 1.2 g/L of EAE (presumably one of the P. coccinea extracts) was added, it led to a significant decrease in the corrosion rate and current density. The corrosion rate was measured at 6.2 × 10⁻⁴ mg/cm² h, while the current density was reported to be 6.6 × 10⁻³ mA/cm². These values indicate a considerable enhancement in the corrosion protection properties of the zinc coatings due to the presence of the P. coccinea extracts.