Comparison fabrication, identification and avoidance of corrosion potential of M-CuO NPs/S-CuO NPs to suppress corrosion on steel in an acidic solution

The utilization of plants to produce metal oxide nanoparticles has recently received a lot of attention due to its ease of usage and environmental friendliness. Therefore, the aim of this study was to synthesize CuO NPs using two distinct methods, including an aqueous extract of moringa oleifera leaves (M-CuO NPs) and a synthetic approach (S-CuO NPs). Nanoparticles produced were evaluated by SEM, EDX, TEM and XRD, to consider the creation of CuO NPs and to determine the morphological, elemental constitution and size the samples. The average particles size 14.95 and 35.73 nm for S-CuO and M-CuO NPs, respectively, is having potentials for application as inhibitor in corrosion of MS. The inhibition and adsorption properties of prepared M-CuO NPs and S-CuO NPs on MS in 1 M HCl were investigated using the wt reduction assessment. The maximal IE of prepared M-CuO NPs and S-CuO NPs was 95.06% and 92.10%, respectively, at 1000 ppm. The IE % improves with greater concentration of the prepared M-CuO NPs and S-CuO NPs. According to the findings, M-CuO NPs are the most effective green potential inhibitor for MS in acidic conditions. It is demonstrated that the Langmuir isotherms are obeyed by the produced CuO NPs and MS substrate.


Introduction
Industries use Mild Steel (MS) for several purposes, but because it corrodes quickly, an inhibitor is needed. The easiest method to prevent MS from corroding in an acidic medium is to use inhibitors (Mallika 2019; Gopi et al. 2010;Li et al. 2017Li et al. , 2006. Many artificial materials pose serious risks to both people and the environment (Allaoui et al. 2020(Allaoui et al. , 2017Singh et al. 2012a, b). Because of the potentially dangerous consequences associated with the evacuation of these chemicals, several effective and ecologically friendly inhibitors have been developed. The majority of the time, the goods are natural, non-toxic, biodegradable, and widely accessible. The most crucial components of the plant utilized as a corrosion inhibitor are the seeds, fruits, leaves, and flowers Al-Bonayan 2015).
Extracts are often inexpensive, ecologically friendly compounds for metal corrosion inhibitors in a variety of mediums. The term "eco-friendly inhibitor" refers to chemicals that are easily accessible, derived from renewable resources, naturally biocompatible, and acceptable to the environment. These qualities led researchers to test the effectiveness of some common plant-based compounds and their byproducts as metal inhibitors in various situations (Idu et al. 2016;Acharya et al. 2013). Many studies and research efforts are made in this regard to find new green inhibitor alternatives because many extracts of common plants have been discovered to be helpful (Beenakumari 2011;Abiola et al. 2010;Ji et al. 2015;Vishalakshi et al. 2016a, b;Vishalakshi et al. 2016a, b;Adejumo et al. 2012;Gopi et al. 2015;Zakeri et al. 2022). Some consideration has been given to the use of moringa oleifera as a possible addition for reducing mild steel corrosion in an acidic environment (Jalajaa et al. 2019;Nnanna et al. 2015). One of these significant plants, moringa oleifera (Moringaceae), which is found in many tropical and subtropical nations, is well-known for its medicinal 1 3 applications and high nutritional value. The numerous parts of this plant contain a large amount of minerals and considered as suitable source of proteins, vitamins, amino acids, and various phenolic compounds (Akalezi et al. 2020). Additionally, it offers a diverse range of medical benefits, including antioxidant, anti-cancer, antibacterial, and antifungal properties. Anti-properties have demonstrated the ability to both receive pain and to repair wounds. Also, the usage of root barks as analgesics, alexetics, antihelminthics, and remedies for heart issues (Allaoui et al. 2020;Farooq et al. 2007). moringa oleifera has been demonstrated to provide a versatile of medical benefits. Its anti-nociceptive effects and ability to heal wounds have also been proven (Adejumo et al. 2012). According to phytochemical screening results, the leaves include phenolics, tannins, flavonoids, and steroids, all of which are crucial for preventing metal corrosion (Odusote et al. 2016). The aim of the present work is to synthesize copper oxide nanoparticles (CuO NPs) using aqueous extract of moringa oleifera leaves (M-CuO NPs) and by using chemical synthetic method (S-CuO NPs). In addition, weight reduction assessment was used to compare the effectiveness of their two methods for preventing MS corrosion in acidic media. Transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), and X-ray diffractometry (XRD) were used to assess two compounds. Also, the effectiveness of the layer's adsorption and efficiency inhibition were examined.

Fabrication of leaf moringa oleifera extract
The moringa oleifera fresh leaves were from Egyptian supermarkets. To get rid of any remaining debris, the gathered leaves were cleaned with deionized water and allowed to air dry. Moringa oleifera leaves weighing 10 g were cut up into small, wilted pieces and placed in a 250 mL beaker with 100 mL of deionized H 2 O. As shown in Fig. 1, the mixture was refluxed at 80 to 95 °C for 15 min, allowed to cool at ambient temp., and then filtered using regular filter paper. The filtered extract is utilized to biosynthesize CuO NPs and is kept in a refrigerator at 4 °C.

Green synthesis of the moringa oleifera-copper oxide (M-CuO) NPs
In deionized water, 0.06 M copper acetate monohydrate was produced as an aqueous solution. Moringa oleifera leaf extract was combined in a 250 mL beaker with 0.06 M aqueous copper acetate in a 1:1 ratio while being constantly stirred at 500 rpm using a magnetic stirrer (25 min). After some time, the solution's hue changes to a dark yellow. The mixture was spun at room temperature for five minutes at a speed of 6000 rpm using a cooling centrifuge machine, and the residue was collected after the liquid supernatant was discarded. The obtained moringa nanocrystalline M-CuO NPs powder was carefully collected and packaged for characterization after one hour at 500 °C in a muffle furnace, where it was discovered to be a fine, dark black hue. (Khanderao et al. 2020).

Preparation of S-CuO by chemical synthetic method
By employing copper acetate as a precursor, CuO nanoparticles were created using the precipitation method. The first precursor was dissolved to a concentration of 0.3 M in 100 mL of deionized water. 2 M NaOH solution was gradually added while being vigorously stirred until pH reached 14. The pH was raised to 7 by periodically washing blue precipitates with 100% ethanol and deionized water. The washed precipitates were then dried for 16 h at 80 °C. After one hour at 500 °C in a muffle furnace, the produced moringa nanocrystalline M-CuO NPs were carefully collected and packaged for characterization, where it was found to be a fine, dark black color.

Characterization and analysis of S-CuO and M-CuO NPs
Copper oxide nanoparticle powdered materials were analyzed after preparation by green and chemical methods using XRD, SEM, TEM and EDX.

Electrode preparation
The working electrode was made from mild steel (MS) having composition (wt%) of C 0.09, Si 0.09, Mn 0.46, P 0.03 and the remains Fe. In the weight reduction experiments, MS was mechanically cut from commercially available steel into coupons of 4.0 × 1.5 × 0.0457 cm dimensions, polished with silicon carbide paper from 600 to 1200 grades to a metallic shine. The electrodes were then rinsed with distilled H 2 O, degreased in ethanol, rinsed again with distilled H 2 O and finally dried before being weighed. To check reproducibility of results, triplicate samples were immersed in a glass beaker containing 25 mL of the corrosive solution free and containing different concentrations of inhibitors.

Corrosive solution and inhibitors
By diluting reagent grade 37% concentrated acid with bidistilled water at 25 °C, the corrosive 1 M HCl was created.
The tests were carried out in 1 M HCl both with and without various concentrations (200-1000 ppm) of M-CuO NPs/S-CuO NPs.

Corrosion test by weight loss assessment
Samples were first measured in an electronic balance for weight. At temperature range (20-60 °C), 1 M HCl with various concentrations was added to weighed samples. The samples were removed from solution, dried in desiccators, and reweighed using an accurate when the temperature was adjusted and the specified immersion time had passed. The weight difference in each circumstance was interpreted as the weight loss. According to Eq. (1), the rates of corrosion were computed .
where C R is the corrosion rate mils per year penetration (mpy), ΔW (g), weighs differently before and after being submerged in fluid, A, is the MS specimens' area (cm 2 ), t, is the exposure period (h) and d, is density of MS in g/cm 3 . Using Eq. (2), the surface coverage degree (θ) is computed.
where W i and W o , are the weight losses of MS in inhibited and uninhibited solutions, respectively. The Inhibitor Efficiency IE% is computed based on Eq. (3): SEM and EDS were used to analyze the surface morphology of the corrosion product on MS after 150 h of immersion in 1 M HCl with varied concentrations of M-CuO NPs or/and S-CuO NPs inhibited and uninhibited. Energy-dispersive X-ray analysis (EDX) is a technique used for the measurement of nanoparticles by SEM. In this technique, the nanoparticles are analyzed by activation using an EDS X-ray spectrophotometer, which is generally present in modern SEM. The individual separated nanoparticles are deposited on a suitable substrate that does not interfere in the characterization of nanoparticles. This method has found (1) 1 3 some limitations about accurate dimensional and elemental characterization.

X-ray Diffraction instrument (XRD)
The crystallinity and phase components of samples were examined and used to identify the structure, composition and size of nanoparticle powdered materials. A powder XRD employing X'Pert-PRO diffractometer and fitted with monochromatic Cu-K α1 radiation (λK α1 = 1.5406 Å) from 2θ = 4.01°-90°, V 40 kV, I 40 mA, power of 1.6 KW, and counting time of 0.6 s/step. The generated data were analyzed by Sigma Pro and the resultant peaks and two theta values were compared with the standard CuO NPs values from the Egyptian petroleum research institute. From where β is full width at 0.5 maximum, K is shape factor, which is assigned 0.9 for polymers. λ is wavelength 0.154 nm, L θ is diameter of the crystallite size, and θ is Bragg peak position (Mashrafi et al. 2022). The crystallite size was found as the following in Table 1 for S-CuO and M-CuO NPs indicating its good crystalline in nature.

Scanning electron microscopy (SEM)
The calcined CuO NP morphology was exhibited in highresolution SEM micrographs which are shown in Fig. 3. This image demonstrates that, aside from a small amount of aggregation, the M-CuO and S-CuO NPs have a form that is almost spherical. This study outperforms the literature result in which the monoclinic structure of CuO NPs was created via a chemical synthetic process in a green manner (Alaa El Din et al. 2021). Figure 4 shows the TEM micrograph of CuO NPs. The TEM imaging revealed that the size of the produced nanoparticles was not consistent; rather, it ranged between 10 and 100 nm for M-CuO and S-CuO NPs. The produced NPs showed approximate uniform spherical nanoparticles. The diffraction pattern of mixed oxide nanoparticles at various sizes is shown in Fig. 4c. The corresponding diffraction and bright spot indicate the formation of crystalline M-CuO and S-CuO NPs, which is also consistent with XRD results this result agreement with (Akintelu et al. 2020).

Energy-dispersive X-ray analysis (EDX)
The EDX spectrum is shown in Fig. 5, which demonstrates Cu and O in the sample at high intensity, further supported the synthesis of M-CuO NPs. The EDX spectrum analysis is 62.61% Cu, 32.56% O, and 4.83% C. The carbon atom observed in the spectrum is from the various organic biomolecules present in the plant extract, which acted as (4) L = K Cos     (Nasser et al. 2016). Figure 6 represents the impact of extended immersion on weight loss of MS corrosion in 1 M HCl various of M-CuO and S-CuO NPs, as extracted from Table 2. The expansion of the film and the rearrangement of the inhibitor on the MS surface can be linked to changes in efficiency over time, whereas the desorption of extract components that had been adsorbed on the MS can be linked to changes in efficiency (Singh et al. 2012a, b). Surface coverage and IE% increased as inhibitor concentration was increased, with IE% (95.06) occurring at 1000 ppm of M-CuO NPs. By increasing the concentration of tested materials in solution, improving the effectiveness of inhibition. As a result, a protective film that is more durable and sticky forms on the MS surface (Saraswat et al. 2022). Figure 6 illustrates the change in weight loss (mg/cm 2 ) of MS in 1 M HCl over time with and without of M-CuO and S-CuO NPs. According to the graph, weight loss increased as immersion time increased, but that it was less in the presence of tested solutions than it was in blank solution. The linearity of weight loss with time showed that there was no surface coating that was insoluble during corrosion. Additionally, M-CuO and S-CuO NPs were initially adsorbed into the metal surface and obstruct corrosion (Abbas et al. 2022).

Impact of immersion period
The results showed good trend in IE % using aqueous extract of moringa oleifera leaves (M-CuO NPs).

Equation 5 describes adsorption of M-CuO and S-CuO NPs molecules on MS surfaces according to the Langmuir adsorption isotherm:
where θ is surface coverage in Table 2, C is extract (ppm), and K ads is adsorption constant. Figure 7 shows the linearity relation between C/θ and C, which with R 2 nearly equivalent to 1. It supports the M-CuO and S-CuO NPs ability to adsorb to the MS surface, demonstrating that the components are occupying the MS/electrolyte interface's active spots (Oubaaqa et al. 2022). M-CuO and S-CuO NPs molecules are stated to be adsorbed on the surface in a single layer without interacting with other molecules (Kouache et al. 2022 where R is the gas constant, T is absolute temperature, ΔG • ads is the standard free energy of adsorption and C w is the concentration of water in the solution. The reaction occurs when the free energy is negative Table 3, in accordance with   thermodynamic relations. Additionally, compared to chemical absorption, less heat is generated during the physical absorption process. M-CuO and S-CuO NPs spontaneously adhered to the MS surface (Ayoola et al. 2022).

Chemical kinetics of corrosion inhibition
After adjusting the corroding MS unit of concentration from mg/cm 2 to g/L and then to molar concentrations, the system's kinetics may be suggested. Assuming that a (mol/L) represents the starting concentration of the MS and x (mol/L) represents the final concentration of MS after time (t) in hrs had passed and it had converted to corrosion products. The corrosion concentration of MS at time (t) is therefore (a-x) mol/L. Using the following rate Eq. (7) (El Refay et al. 2022), the given result showed that a relation between log (a-x) or log [MS] on the Y-axis against time on the X-axis yields straight line R 2 values that are just about unit, demonstrating a first-order kinetics (Fig. 8).
where K 1 is rate constant. Equation 8 also shows a relationship between the K 1 and t 0.5 as:  Values of K 1 and t 0.5 were calculated from the slopes of the kinetic plots are documented in Table 4. The data showed that t 0.5 of MS with M-CuO and S-CuO NPs were longer than those obtained for 1 M HCl, implying that M-CuO and S-CuO NPs increased the t 0.5 of MS in 1 M HCl. K of MS in the presence of M-CuO and S-CuO NPs was also shorter than that of 1 M HCl.

Activation energy
The Arrhenius equation, which is expressed by Eq. (9), showed how corrosion rate depends on temperature: where A is extrapolation factor, E * a is activation energy, R is gas constant and T is absolute temperature. The Arrhenius equation is represented graphically in Fig. 9, and Table 5 (9) summarizes the computed activation energies based on the slopes of the plots. The Eyring transition state (Eq. 10) as: where N is Avogadro's No. and h is the Planck's. Figure 10 shows the plots of log C R ∕T versus 1∕T . Straight lines are obtained with a slope of (−ΔH * ∕2.303R) and intercept of log (R∕Nh) + (ΔS * ∕2.303R) which ΔH * and ΔS * were calculated and given in Table 5. Figures 9 and 10 show that as the temperature rose, corrosion rates rose as well, resulting in a drop in protection efficiency. Table 5 shows that the values of E * a and ΔH * were higher when M-CuO and S-CuO NPs were present than when they were not, confirming higher IE %. This may be explained by raising the energy barrier for the reaction; in other words, the adsorption process increased the activation energy and enthalpy of the corrosion process (Nsude et al. 2022). The endothermic nature of the activation process was revealed by the positive values of ΔH * , which showed that it would be difficult and sluggish to dissolve in the presence of the tested substances. ΔS * values were high and negative in both the presence and absence of M-CuO and S-CuO NPs. This reflects the development of a stable, organized inhibitor on MS surface (Muhammad et al. 2022).
Energy-dispersive X-ray spectrometer (EDX) Figure 12 depicts the spectrum of the MS surface in the absence and presence of the inhibitor following immersion in a 1 M HCl. Figure 12a demonstrates the accumulation of corrosion products (oxides) on the MS surface. The intensity of oxygen molecules is higher than that of carbon atoms, indicating that there are more iron oxides present. The EDX spectrum analysis is 75.95% Fe, 19.21% O, 0.46% Mn, and 4.22% C. On the other hand, EDX spectrum of Fig. 12b and c reveals the presence of elements on composition of MS surface from the tested solutions, indicating that the solutions adsorbed on the surface of mild steel and created a corrosion-resistant layer. The elements found in the protective layer in the presence of (M-CuO or S-CuO NPs) inhibitor were C, O 2 and Fe. The presence of C in the EDX profile indicates that (M-CuO or S-CuO NPs) molecules have adsorbed on the MS surface. The carbon atom observed in the spectrum is from the various organic biomolecules present in the plant extract, which acted as reducing and capping agents during the reaction to form the M-CuO NPs.

Conclusions
In this study, the comparative synthesis CuO NPs by two methods using aqueous extract of moringa oleifera leaves (M-CuO NPs) and synthetic method (S-CuO NPs). Nanoparticles produced were evaluated by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), transmission electron microscopy (TEM) and X-ray diffractometry (XRD). The average size 14.95 and 35.73 nm for S-CuO and M-CuO NPs respectively, having potentials for application as inhibitor in corrosion of MS. The comparative inhibitive properties of prepared M-CuO NPs/S-CuO NPs on MS in 1 M HCl were investigated using the Wt reduction assessment. The IE % increases with increasing concentration of prepared M-CuO NPs and S-CuO NPs, with the maximum IE being 95.06% and 92.10%, respectively, at 1000 ppm. M-CuO NPs, a promising inhibitor for MS corrosion, were successfully used as an acid corrosion inhibitor after being isolated from aqueous moringa oleifera leaves. The adsorption shifts to physisorption based on the trend of IE % with temperature change and thermodynamic factors. The adsorption of produced M-CuO NPs and S-CuO NPs on MS by protective coating is inferred from the well-fit Langmuir adsorption isotherms.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Conflict of interest
The authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
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