Green nanomaterials and nanocomposites for corrosion inhibition applications

3.1 Green nanomaterial characterization methods

Numerous techniques are used to investigate nanomaterials. The choice of characterization methods depends on the investigated properties of the nanomaterials. Commonly, nanomaterials characterization is used to investigate the surface, porosity, aggregation, size, composition, and shape beside many other properties. Various characterization techniques are applied including thermogravimeter analysis (TGA), Fourier transform infrared spectroscopy (FTIR), transition electron microscopy (TEM), scanning electron microscopy (SEM), dynamic light scattering (DLS), atomic force microscopy (AFM), ultraviolet and visible spectroscopy (UV–VIS), X-ray photoelectron spectroscopy (XPS), etc.

3.2 Corrosion inhibition measurements

There is a wide range of corrosion measurements that are employed to determine the inhibition efficiency of an inhibitor. These various measurements use different characteristics of the corroded metal like weight, conductivity, hydrogen evolution, and surface changes. These measurements are described in detail in many references. They include the weight loss method (WLM) which depends on measuring the weight difference with and without the inhibitor in different intervals of time or different temperature (Usman and Okoro 2015). Another method depends on the reaction of metal in acids that releases hydrogen gas. The amount of the released gas depends on the corrosion reaction. This method is known as the hydrogen evolution method (HEM) (Safizadeh et al. 2015). Water contact angle (WCA) is another method to investigate the effect of adding the inhibitor on the interaction between the medium and the metal surface.

Electrochemical measurements for corrosion inhibition are fast and reliable (Mansfeld and Little 1991; Xia et al. 2021). These measurements include various methods that are very similar for green nanomaterials to other types of inhibitors. Among these methods, is linear polarization resistance (LPR) which focuses on measuring current versus potential or vice versa. The measurements are based on applying a small potential ±10 mV as a constant pulse or dynamic pulse (PDP) and mapping the change in the current. It can also be performed by applying current and mapping the change in the potential. Since corrosion is associated with current resulting from the redox reaction, the amount of corrosion or the lack of it can be assessed using this method. Half-cell potential (HCP) is the potential created by the oxidation or reduction process at the electrode of a half-cell. This potential, which is used to indicate corrosion activity, measures the propensity of one reaction, such as oxidation, to continue at its one half-cell. Tafel analysis is a technique that depends on using DC current and applies larger potentials than LPR. This results in larger, measurable currents, but because of the nonlinearity in the potential-current relationship, the log of the current is used instead of the actual values in a plot called Tafel plots (TP) (Fajardo et al. 2018).

Linear sweep voltammetry (LSV) is a method that sheds light on the passivation process and indicates how well a material will withstand corrosion in a particular setting by forming a film that serves as a barrier on the material’s surface. Another electrochemical technique called cyclic voltammeter (CV) is used to pinpoint the mechanism of the electrochemical reactions that occur at an electrode surface. The potential is linearly scanned from the initial potential to the second potential value, then the scan is reversed to return to the initial potential value (Xia et al. 2021).

Electrochemical impedance spectroscopy (EIS) is another method that has been getting popularity in corrosion investigation. EIS is based on measuring the AC current versus an applied AC potential at various frequencies. A lot of information about the electrochemical cell (the corrosion cell) can be determined using EIS (Ma et al. 2021). Some of the limitations associated with EIS measurements were excluded by the dynamic electrochemical impedance spectroscopy (DEIS) method. A collection of elementary signals, each of which has its own amplitude, frequency, and phase shift, are excited at the same time as the object under investigation. The main goal of corrosion inhibitor studies is to establish the manner, duration, and extent of the corrosion inhibitor’s efficacy in the corrosion cell (Gerengi 2018; Wysocka et al. 2017).

Recently, corrosion scientists have become interested in electrochemical frequency modulation (EFM), a quick and nondestructive method for determining the corrosion rate and polarization resistance of metals and alloys without having to know the Tafel constants beforehand. The method resembles a modification of the LPR and EIS methods in that the corroding metal is simultaneously subjected to two frequencies and nondestructive potential wave perturbation with an amplitude of between 10 and 20 mV around the corrosion potential, while higher frequencies—known as harmonic and intermodulation frequencies—are used to measure the responses of the alternating current density (Obot and Onyeachu 2018).

In the case of biocorrosion, the degradation of metal caused by microorganisms, some biological tests are employed to investigate the effect of the corrosion inhibitor on the microorganisms. One of the basic methods used is the disk diffusion method (DDM) which is a technique for determining microbial resistance to inhibitors. The antibiotic or inhibitor carried on filter paper disks with known concentrations will be put on top of an agar surface that has already been exposed to the target bacteria. Measuring the disk’s surrounding area that is devoid of microbial growth allows for the evaluation of the microbial susceptibility to certain inhibitor (Biemer 1973).

3.3 Corrosion mechanism investigation

There are several mechanisms by which corrosion inhibitors function. Based on these mechanisms, an inhibitor can be classified as an anodic, cathodic, or mixed inhibitor. An inhibitor may function through the adsorption principle, whereby the inhibitor is chemically adhered to the metal surface due to adsorption, which leads to the formation of a protective film that shields the metal from the corrosive medium. The inhibitor may also function through surface layer formation, where it promotes the formation of an oxide film on the metal. Inhibition could also happen through the interaction of the inhibitor with the corrosive medium, which leads to the formation of precipitates that protect the metal from further corrosion attacks (Taghavikish et al. 2017).

Corrosion inhibition through nanomaterials is usually explained based on adsorption. The high surface area of nanomaterials makes their adsorption more effective than other bulk materials inhibitors. The adsorption could happen through physical adsorption, chemical adsorption, and film formation. Electrostatic attraction between the inhibitor and the metal surface leads to physical (or electrostatic) adsorption. Physically adsorbable inhibitors interact quickly, but the protective adsorbed layer can easily detach from the surface as well. Chemically adsorbed inhibitors are the most potent inhibitors (chemisorbed) where a charge transfer process between the inhibitor and the metal surface causes is responsible for the occurrence of chemical adsorption. Although not entirely reversible, chemisorption is slower than physical adsorption (Jain et al. 2020).

The effectiveness of organic inhibitors is due to the presence of heteroatoms like sulfur, oxygen, and nitrogen in these inhibitors. As a result of the high electron density on these atoms, there are active centers for the adsorption process. In green nanomaterials, some organic compounds from green sources are loaded onto the synthesized nanoparticles. The combination of a high surface area due to the nanomaterials and the existence of heteroatoms increases their adsorption efficiency, which gives them their inhibition efficiency (Dariva and Galio 2014).

Several techniques are used to investigate the mechanism that certain inhibitors work with. Usually, surface monitoring methods can provide information about the inhibitor adhesion to the metal surface. Among these methods are field emission scanning electron microscope (FESEM), atomic force microscopy (AFM), SEM, and TEM. These methods provide an image of the metal surface which provides information about the effect of corrosion on the surface, the adhesion of the inhibitors, and the change of corrosion effect on the surface when inhibitors are added.

An examination of the inhibitor composition before and after adding it to the corrosion medium could also provide information about how it worked. Some of the methods that are used for such identification are energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR). Furthermore, analysis of the corrosion product could give a hint about the corrosion mechanism. Such analysis can be performed using X-ray diffraction (XRD) and thermal gravimetric analysis (TGA) (Dwivedi et al. 2017; Garcia et al. 2013; Yousif and Al-Zhara 2016).

4.1 Metals or metal oxide nanoparticles

Researchers have taken advantage of biological materials for the production of metallic nanoparticles to build low-cost and environmentally friendly substances. In biological synthesis, a biological source or its extract is used as a source of reductants, either extracellularly or intracellularly, to reduce metal ions in the precursor. The components of biological materials are responsible for the reduction of metal ions and formation of the metallic nanoparticles. Algae, yeast, viruses, bacteria, and plants are a few of the significant biological components used in the creation of metallic nanoparticles (Alqarni et al. 2022; Vijayaraghavan and Ashokkumar 2017). Several green metal and metal oxide nanoparticles prepared using different sources were utilized as corrosion inhibitors (Table 1).

Silver nanoparticles Ag NPs were prepared using the extract of Artemisia annua and Sida acuta leaves that act as a reducing and capping agent. The green Ag NPs when added to the corrosion medium, displayed inhibition properties for mild steel against acidic media, 0.5 M of HCl. Electrochemical studies showed that Ag NPs prepared using Artesimiz annua displayed an inhibition efficiency of a maximum of 87.85% while Ag NPs prepared using Sida acuta had an inhibition efficiency of 78.8% (Johnson et al. 2014).

Idrees et al. prepared silver nanoparticles using the leaves and stems of the plant sida acuta as a capping agent. The prepared nanoparticles interacted with the medium and displayed antimicrobial efficiency toward several strains. Additionally, the prepared nanoparticles displayed anticorrosion behavior for mild steel in an acidic medium (0.1 H₂SO₄) reaching a maximum of 86% efficiency (Idrees et al. 2019).

Due to the outstanding antibacterial properties of green silver nanoparticles, Ag NPs were applied as an inhibitor for biocorrosion. Silver nanoparticles were prepared using theLysinibacillus sp. NOSK bacteria. The green biosynthesized silver nanoparticles showed inhibition efficiency toward biocorrosion on copper caused by the halophilic bacterium when added to a marine medium reaching a maximum of 74% (Keskin et al. 2021). Ag NPs were also prepared using the leaves of Azadirachta indica as a reducing and stabilizing agent. The plant-based nanoparticles had an inhibition effect toward biocorrosion on mild steel caused by the bacterium, Bacillus thuringiensis EN2 in the cooling water system. The silver nanoparticles inhibition efficiency was 77% compared to 52% caused by the plant extract alone. The inhibition was explained that the nanoparticles were adsorbed on the mild steel leading to a surface protection (Narenkumar et al. 2018).

Green Ag NPs were prepared using the edible wild Macrolepiota mushroom and tested as biocorrosion inhibitors by adding them to a cooling water tower system. Bacillus thuringiensis EN2, Terribacillus aidingensis EN3, and Bacillus oleronius EN9 are types of bacteria that cause the biocorrosion of mild steel. Using several methods like gravimetric, electrochemical, and surface studies, green Ag NPs were found to possess an efficiency of 59%. The inhibition is attributed to a layer on the mild steel surface which prevented the growth of the biofilm in the cooling water tower system (Preethi et al. 2022).

Another implication of green Ag NPs was by doping the nanoparticles with palm oil, Elaeis guineensis, and leaf extracts to form EG/Ag NPs. The prepared nanocomposite was found to inhibit corrosion for reinforcement steel when added to a saline environment. Several electrochemical methods and surface and structural measurements were applied to test the inhibition efficiency during 365 days of exposure to the corrosion medium. Corrosion inhibition of 94.74% was achieved using 5% of the green mixed-type inhibitor. The inhibition action was attributed to the adsorption of a thin layer of calcium silicate hydrate gel which blocked the corrosion sites (Asaad et al. 2018a). Assad et al. (2018b) used the same nanocomposite in an acidic medium (1 M of HCl). Using gravimetric and electrochemical measurements, the addition of 10% (v/v) of the mixed-type inhibitor (EG/Ag NPs) to the corroding medium was found to lead to inhibition with an efficiency of 94.1% due to the formation of a protective layer of the adsorbed on the mild steel surface.

Silver nanoparticles were also synthesized using Allium cepa peel ethanol extracts (Et-ACPE). The green Ag NPs surface was rich in electrons due to the presence of oxygen atoms provided by the phytocompounds. The prepared nanoparticles showed corrosion inhibition of X80 steel corrosion when added to an acidic medium (1 M HCl). Analysis of the corrosion resulting products and surface characteristics of the steel demonstrates effective surface protection by Et-Ag NPs adsorption, which is primarily made possible by the existence of oxygen and unsaturated alkyl sites leading to inhibition of up to 90%. In comparison to the crude extract, Et-Ag NPs were found more effective and thermally anticorrosion addition for industrial cleaning and pickling operation (Figure 1) (Ituen et al. 2021c).

Figure 1:

Comparison between the Allium cepa crude extract inhibition efficiency (red line) and the inhibition efficiency of green nanocomposite prepared using Allium cepa (black line) at (a) 30 °C and (b) 60 °C (Ituen et al. 2021c).

Silver nanoparticles Ag NPs were also greenly synthesized and mediated using tangerine peel extract (TPE) as a reducing agent. The formed nanocomposite (TPE-Ag NPs) was applied as a novel material as a biocide and an inhibitor of bio and acid corrosion of pipeline (X80) steel in an oilfield medium. At 30 °C and 60 °C, the effectiveness of 1.0 g/L TPE-Ag NPs in preventing corrosion was 93.9% and 90.3%, respectively, when added to the medium. Adsorption of the nanoparticles via carbonyl, alkene, ether, and hydroxyl group sites results in corrosion prevention by increasing charge transfer and surface resistance at the steel-electrolyte interface. Compared to the crude extract, the nanoparticles showed more stability with temperate and increased inhibition efficiency, and they may be used in oilfields (Ituen et al. 2020a).

Silver nanoparticles were prepped and mediated using the aqueous extract of red onion peels. The nanocomposite possessed corrosion inhibition efficiency for pipeline (X80) steel in an acidic medium (1 M HCl), Ag NPs showed strong anticorrosion activity reaching a maximum of 86% at 60 °C. The inhibition was attributed to the nanocomposite being adsorbed using its oxygen and nitrone site on the steel surface. Surface morphology measurements revealed a decrease in pitting corrosion by 70% (Ituen et al. 2021b).

Iron oxide nanoparticles (NPs) were prepared using the extract of the plant Swertia chirata and found to be a mixed-type corrosion inhibitor for stainless steel in Ringer’s solution, a medium similar to body fluids. The NPs were 10–20 nm in size and had spherical form. Weight loss, electrochemical techniques, and surface investigation determined corrosion inhibition of 78.37% at a concentration of 100 ppm of the nanoparticles added to the medium (Sharma et al. 2021).

Green Cr₂O₃ NPs were synthesized from Cannabis sativa. Cr₂O₃ NPs with a diameter of 85–90 nm worked as a corrosion inhibition of mild steel in acidic (0.5 M HCl and HNO₃) and basic media (0.5 M NH3). Using several corrosion inhibition measurements, the nanoparticles were found to obtain more inhibition efficiency when added to basic media compared to acidic media. The nanoparticles’ strongest inhibitory performance in an acidic media was found to be 80% at 1.0 g/L concentration, and in a basic (NH₃) medium to be 89% (Sharma and Sharma 2021).

Nickel oxide nanoparticles (NiO NPs) were created employing an ultrasound-assisted method and using the extract of Delonix elata leaves that acted as a capping and reducing agent. The synthesized NiO NPs were prepared into slurry that was used as a coating for the plates. In the presence of different media (3.5% NaCl, 6 M KOH, 1 M HCl, and 1 M H₂SO₄), NiO NPs displayed anticorrosion behavior for Zn and Mg plates. NiO NPs had the strongest inhibition efficiency in an acidic medium, that is, 1 M H₂SO₄ with 88.6% for Zn plates and 79.5% for Mg plates (Sudha et al. 2021).

Nickel nanoparticles were prepared using alcoholic extracts of the plant A. cepa to create nanoscale nickel. The green nanoparticles are employed by adding them to the corroding medium to concurrently control the rates of X80 steel corrosion and hydrogen gas production in a 1 M HCl solution. The effectiveness of inhibition is temperature-dependent. The inhibition is due to the nanoparticles being physically adsorbed on the surface of X80 steel via several functional groups. The inhibitor functioned as a mixed-type corrosion inhibitor with a pronounced effectiveness in inhibiting hydrogen evolution (the cathodic reaction). Based on surface morphology, the roughness of the surface was decreased by 76% compared to when the inhibitor was used. The nanoparticles showed strong corrosion inhibition at a concentration of 100 ppm of the nanoparticles gave higher anticorrosion activity at all temperatures compared to 1000 ppm of the plant extract (Ituen et al. 2020b).

Green nickel nanoparticles (Ni NPs) were mediated utilizing the aqueous extract of Citrus reticulata peels (CRE) to prepare the green nanocomposite (CRE-Ni NPs). The nanocomposite CRE-Ni NPs had a size of 40–55 nm and was effectively adsorbed on the surface of X80 steel which led to preventing corrosion in an acidic medium (I M HCl). The nanocomposite had an inhibition efficiency of 87.3% at 30 °C and 80.6% at 60 °C when added to the corrosion medium. The prepared nanoparticles showed antibacterial activity leading to biocorrosion inhibition. CRE-Ni NPs are more thermally stable, more effective, and provide greater surface protection at high temperatures when compared to crude extract (Ituen et al. 2021a).

Using extracts of flower petals, such as rose and lotus petals, as a source for phytochemicals was utilized to prepare manganese oxide (MnO NPs) nanoparticles using the ultrasonic wave-aided green synthesis method. The produced nano-manganese oxides were utilized to improve mild steel’s corrosion resistance in an acid media (1 M HCl). Using electrochemical measurements, the corrosion inhibition of MnO NPs prepared using rose petals and lotus petals were 72.63% and 51.5%, respectively (Khadar et al. 2021).

Green zinc oxide nanoparticles (ZnO NPs) were also applied as corrosion inhibitors. The green NPs were synthesized using an extract of Myrrh as a capping agent (Al-Dahiri et al. 2020). ZnO NPs were greenly prepared also using the combustion method and the leaf extract of Lantana camara (Surendra et al. 2022). The prepared green ZnO-NPs displayed anticorrosion activity on steel and mild steel when added to an acidic medium (1.0 M HCl). Using electrochemical techniques, the corrosion inhibition efficiency was found to have a maximum of 92%. Polarization studies found this inhibitor to work as a cathodic inhibitor type and the inhibition mechanism depended on the protective layer formation.

Additionally, ZnO NPs were synthesized using the extract of the neem plant. The green ZnO nanoparticles displayed anticorrosion activity when prepared as slurry and coated over zinc plates in saltwater medium (3.5% NaCl). Electrochemical measurements determined 50% inhibition efficacy when ZnO NPs are utilized as corrosion inhibitors compared to the zinc plates without coating (Ramamoorthy et al. 2022).

Copper oxide nanoparticles were prepared using Moringa oleifera leaves extract. The green CuO NPs worked as corrosion inhibitors by coating them on the mild steel surface using the spin coating method in a saline medium (3.5% NaCl). Electrochemical measurements showed that the green CuO NPs displayed an inhibition efficiency of 56% (Surendhiran et al. 2021).

Another example of nanoparticles that have been utilized in corrosion inhibition is green gold nanoparticles. Odusote et al. prepared green Au NPs using crude oil enzyme of the fungi Aspergillus niger L3 and Trichoderma longibrachiatum. Using gravimetric and electrochemical methods, the inhibition efficiency for the green Au NPs for three substrates aluminum, mild steel, and stainless steel in acidic media was determined. The inhibitor when tested on the three substrates, aluminum, mild steel, and stainless steel when added to 1 M of HCl had inhibition efficiency of 88%, 98%, and 96%, respectively. Additionally, potentiodynamic polarization found that AuNPs inhibited corrosion by altering the mechanism of anodic reaction adsorption on the metal samples’ surfaces protecting them from corrosion (Odusote et al. 2021).

Using the ethanolic olive leaf extracts (OLE), titanium nanoparticles were prepared. The nanocomposite particles were between 70 and 74 nm in size. The nanoparticles showed anticorrosion activity for mild steel by utilizing them as an additive to 1 M HCl at 30–60 °C. While OLE crude extract worked as a corrosion inhibitor for mild steel in acid, the addition of Ti nanoparticles improved the performance of the inhibitor. The crude extract and the nanocomposite had respective Nano’s inhibitory efficiencies were 83.5% and 94.3% at 30 °C but fell to 51.7 and 85.4 percent at 60 °C. The nanocomposite worked as a mixed-type inhibitor and prevented corrosion due to adsorption on the steel surface (Essien et al. 2018).

A mixture of two or more metals or metal oxide nanoparticles can be synthesized by green means. The nanocomposite cantinas different species that could potentially lead to better adsorption on the substrate surface and hence better corrosion inhibition. There is limited research in preparing such nanocomposites using green methods. The basic method of synthesis is preparing a mixture of the desired metal salt solutions followed by adding the reducing and capping agent which is usually a green substance (plant extracts, algae extract, bacteria … etc.). CoO/Co₃O₄ nanocomposite was synthesized using egg white. CoO/Co₃O₄ NPs performed as corrosion inhibitors for low-carbon steel in an acidic medium (1 M HCl). The inhibition is due to the physicochemical, spontaneous, and exothermic adsorption of CoO/Co₃O₄ NPs on the carbon steel surface. According to electrochemical measurements, CoO/Co₃O₄ NPs functioned as a mixed inhibitor with an effectiveness of 93% at 80 ppm of the inhibitor that is added to the corroding media (Al-Senani and Al-Saeedi 2022).

4.2 Functionalized metallic nanocomposites

Additional functionalization can be performed on green metals or metal oxide nanoparticles to enhance their properties. When metal or metal oxide NPs are greenly prepared, they can be further mixed with different green substances leading to the formation of surface-functionalized metallic nanocomposites. There are several methods, most commonly ultrasonication, to functionalize metal nanoparticles and increase their corrosion inhibition properties (Alghamdi et al. 2022; Virkutyte and Varma 2011). Various functionalized green metals and metal oxide nanoparticles have been prepared and used as corrosion inhibitors (Table 1).

After the green synthesis of zirconium oxide nanoparticles (ZrO₂) using Eucalyptus globulus stem, they were functionalized with glycine (Gly) to form the nanocomposite ZrO₂-Gly NC. Using the gravimetric method and electrochemical measurements, Aslam et al. explored the inhibitory impact of different ZrO₂-Gly NC concentrations on the corrosion of mild steel in 1 M HCl. Inhibition efficacy increased as concentration and temperature increased, peaking at 500 ppm at 70 °C and about 81.01%, before declining at 80 °C and showing 73.5% inhibition efficiency. ZrO₂-Gly NC functions as a mixed-type inhibitor, mostly blocking cathodic sites, according to polarization measurements. Using surface morphology measurements, inhibition was attributed to a protective layer of the nanocomposite inhibitor on the mild steel surface (Aslam et al. 2022).

A nanocomposite composed of cysteine and green silver/gold (Ag/Au) nanocomposite was used as a corrosion inhibitor for mild steel in 1 M of HCl. The Ag/Au nanocomposite was prepared using pomegranate fruit and then functionalized with cysteine (Cys). The nanocomposite had 96% of inhibition efficiency at 303 K and 300 ppm (Figure 2). Cys/Ag–Au NC acted as a mixed corrosion inhibitor (Basik et al. 2020).

Figure 2:

SEM images at 303 K of (a) polished mild steel, (b) at a medium of 1 M HCl, and (c) at 1 M HCl solution with 300 ppm of the green inhibitor Cys/Ag–Au nanocomposite (Basik et al. 2020).

4.3 Green polymer nanocomposites

In recent years, there is a significant academic and industrial interest in polymer-based nanocomposite systems incorporating inorganic nanofiller particles. Due to the synergistic interaction between the polymers and filler particles, the polymer nanocomposites’ physical, chemical, and mechanical properties are superior to those of the polymer alone (Solomon and Umoren 2016).

The main drawbacks of polymers are their lack of solubility in aqueous solution and their desorption at high temperatures, which stand in the way of their promising inhibitory effect. Different modifications have been employed by adding specific compounds to the polymer matrix that can improve critical qualities like heat resistance, solubility, mechanical strength, biocompatibility, self-healing, etc. Green polymer nanocomposite is an example of these modifications that enhanced both corrosion inhibition and polymer properties (Tiu and Advincula 2015).

There are two main types of polymers, synthetic and natural. The main difference between the two is their source. While the source of synthetic polymers is chemical synthesis, the source of the natural polymer as the name implies is nature. Both types were incorporated with green nanoparticles to create greener corrosion inhibitors (Silva et al. 2021; Verma and Quraishi 2022). While natural polymer is attracting more interest due to being more environment friendly, there is considerable work in both types which will be discussed here. Both types of polymer nanocomposites have been explored and tested as corrosion inhibitors (Table 1).

4.3.2 Green synthetic polymer nanocomposites

The application of natural polymers, specifically carbohydrate polymers, in corrosion resistance provided a path to employ materials that are stable, environmentally friendly, biodegradable, economical, and renewable (Verma and Quraishi 2022).

The carbohydrate polymer, chitosan, that was naturally extracted from the shell of shrimps and crabs which is dually a waste, was used along with green silver nanoparticles to synthesize chitosan-Ag NPs nanocomposite (Figure 3). The prepared Ag NPs were prepared using chitosan as a reducing and capping agent and their size ranged between 3 and 6 nm. The prepared nanocomposite exhibited corrosion inhibition of mild steel in industrial chilling water with an efficiency of 97%–98% acting as a mixed-type inhibitor. WLM and electrochemical methods were used to determine the corrosion inhibition of the nanocomposite when added to the corrosion medium. Adsorption measurements revealed that the nanocomposite achieved anticorrosion behavior due to spontaneous physisorption. Biological studies concluded that the nanocomposite expressed an antibacterial activity (Fetouh et al. 2020).

Figure 3:

Synthesis of the natural polymer nanocomposite chitosan-Ag NPs (Fetouh et al. 2020). Reproduced with permission from Elsevier, license number 5445480677314.

Umoren et al. (2022) used chitosan (CHT) also along with green copper oxide CuO prepared using olive oil (CHT-CuO NC). The nanocomposite worked as a corrosion inhibitor for X60 carbon steel when added in 5% wt to the HCl medium. Using the gravimetric and electrochemical methods, the nanocomposite was found to possess an inhibition efficiency of 90.35%. The inhibition is due to active site blocking method by the formation of film adsorbed on the carbon steel which inhibited corrosion (Umoren et al. 2022).

Green silver nanoparticles prepared using natural honey as a reducing agent was incorporated with Carboxymethyl cellulose CMC forming then nanocomposite CMC/AgNPs. The nanocomposite was used as a corrosion inhibitor on St37 steel in an acidic medium (5% H₂SO₄). Gravimetric and electrochemical measurements supported by surface evaluation found that the CMC/AgNPs nanocomposite performs better than the CMC alone. CMC/AgNPs nanocomposite had an inhibitory effectiveness maximum of 96.37% when added to the corrosion medium. Both anodic and cathodic reactions are shown to be delayed by CMC/Ag NPs showing a mixed type inhibitor behavior (Solomon et al. 2017).

Cellulose is an example of a natural carbohydrate polymer. The polymer was modified to enhance inhibition, leading to the production of primary aminated modified cellulose (PAC). Subsequently, nanocomposites of PAC and several metal oxide nanoparticles were synthesized. PAC/Fe₃O₄ NPs, PAC/CuO NPs, and PAC/NiO NPs exhibited anti-corrosion behavior when tested on C-steel by adding the inhibitor to 1 M of HCl with mixed inhibitor characteristics. With 250 ppm of PAC, PAC/CuO NP, PAC/Fe₃O₄ NPs, and PAC/NiO NPs, the corrosion inhibition was 88.1, 93.2, 96.1, and 98.6%, respectively (Figure 4) (Gouda and El-Lateef 2021).

Figure 4:

An electrochemical study using the Tafel polarization method of carbon steel in an acidic medium using (a) different concentrations of the PAC polymer and (b) 250 mg/L of different nanocomposite inhibitors (Gouda and El-Lateef 2021).

A green nanocomposite of silver and gum Arabic, which is another natural polymer, was also prepared using honey as a reducing agent. The prepared nanocomposite was used as a corrosion inhibitor for steel in acidic media (15% HCl and 15% H₂SO₄). Gravimetric measurements and electrochemical methods concluded that the nanocomposite worked with different mechanisms when added to the two media. The nanocomposite was recognized as a mixed-type inhibitor when added to 15% H₂SO₄ and it was found to be an anodic inhibitor in the medium of 15% of HCl. Both the ionic and neutral forms of the nanocomposite are adsorbed in the presence of composite causing the observed inhibition. AgNPs are found as Ag°, Ag₂O, and AgO on the surface of the steel (Solomon et al. 2018).

Another application of biopolymers was using cellulose and niacin. The nanocomposite showed inhibition efficiency toward copper when the inhibitor was added to the corrosion medium of saline solutions (3.5% NaCl). The inhibition efficiency was tested using potential polarization, EIS techniques, SEM, and EDX. Among many forms of the composite, ethyl cellulose-niacin (NEC) campsite had the best performance. NEC acted as a mixed corrosion inhibitor with a 94.6% efficiency (Hasanin and Al Kiey 2020).

4.4 Nonmetallic nanoparticles inhibitors

Some nonmetallic nanoparticles were applied as green corrosion inhibition (Table 1). Silica is one of the leading nonmetallic nano-green inhibitors. Silica (Si), or silicon dioxide (SiO₂), has the ability to form a protective monolayer on the surface of the metal that protects the metal from corrosion. Silica also has superior mechanical properties that put it on the lead of nonmetallic nanoparticles that are used for corrosion inhibition. However, the synthesis of silica by conventional method is hazardous which put the need for a green approach. Several researchers applied greener methods in preparing silica and silica nanocomposites to act as corrosion inhibitors for different media (Govardhane and Shende 2021).

Silica was synthesized from rice husk ash and used to inhibit corrosion for carbon steel in distilled water medium. The rice husk was burned to form white ash. Then, silica was extracted from white ash by treatment with several bases and acids. The nano-silica precipitate was washed and dried for 24 h at 50 °C. The silica nanoparticle had a size ranging between 10 and 20 nm and when added to the corrosion medium showed corrosion inhibition efficiency of 99% at 20 ppm (Awizar et al. 2013). Vijayalakshmi et al. (2015) used silica prepared using rice husk also as a corrosion inhibitor for mild steel in a saltwater medium (3.5% NaCl). The prepared silica nanoparticles were coated on steel using dip coating. The silica coating showed considerable inhibition compared to the uncoated sample with an efficiency of 42% (Vijayalakshmi et al. 2015). Othman et al. (2016) found that the efficiency of silica nanoparticles prepared from rice husk increases with increasing concentration where they reached an efficiency of 85% at 1500 ppm (Othman et al. 2016).

Silica nanopowder prepared using rice husk was also applied for the same metal but the inhibitor was added to an acidic medium (0.5 M HCl). The nanosilica inhibitor had an average size of 10–20 nm and was investigated using gravimetric, electrochemical, and surface measurements. The inhibitor effectiveness was found to reach a maximum of 99% at 175 ppm and it prevented pitting on the steel surface (Asra Awizar et al. 2013).

Sulfur nanoparticles inhibitor is another nonmetallic nanoparticle that has been applied in literature. Sulfur nanoparticles were also utilized as a green corrosion inhibitor for aluminum (Al) in an acidic medium (0.05 N HCl). Green sulfur nanoparticles (SNP) were prepared using Alhagi plant extract. The size of the sulfur nanoparticle was 82.39 nm. The green sulfur nanoparticles had higher inhibition at low temperatures reaching a maximum of 87.15% when added to the corrosion medium (Jasim et al. 2020).

A summary of the green nanoparticles used in literature and discussed in this review is shown in Table 1.