## 2 Utilization of Various Nano-Modified Coatings for Corrosion Mitigation
### Plant Extract and Natural Source-Based Nanoc coatings
[8] produced nanocrystalline cellulose (CEL) through the acidic hydrolysis of macrocrystalline cellulose (NCC). A morphological representation of NCC revealed a rod-shaped framework with a mean diameter of 10-25 nm, and a thickness and a width of 100-200 nm. Pristine NCC and CEL were discovered to have BET surface areas of 10.41 and 27 m\({}^{2}\)/g, correspondingly. DFT analyses were carried out to examine the degree of  
Figure 9: Illustration for the green production of Cr\({}_{2}\)O\({}_{3}\) NPs utilizing _Abutilon indicum_ (L.) Sweet leaf extract. Reprinted with permission from Ref. [5]. MDPI (2021). interactions between the active centers of NCC and CEL molecules on the substrate of the SS316 alloy. Figure 10 shows the optimal geometries for CEL and NCC's HOMO and LUMO orbitals. As per the FMO concept, HOMO and LUMO energies determined whether an inhibitor molecule was acting as a donor or acceptor at the contact between the inhibitor and the metallic substrate. The compounds with higher EHOMO and reduced ELUMO values were regarded as very potent CIs as a result. In comparison to CEL's maximal EHOMO value of 5.73 eV, the NCC's is 5.45 eV. Figure 10 shows that the HOMO value was found on the hydroxy, pyran, and sulfate moieties of the inhibitor compounds, indicating that the sulfur and oxygen atoms were the preferred spots for electrophilic interactions on the Fe surface. Furthermore, MC simulations were used to identify the associations between the inhibitory molecules and the SS316 alloy substrate as well as to propose a descriptive notion for the operation of adsorption. The broadest appropriate adsorption arrangements for the inhibitory molecules over the SS316 alloy substrate are shown in Figure 11 as a result. These layouts are produced by the adsorption locator unit and proffer basic plane frameworks, indicating an increment in the adsorption and the extremely high surface treatment. Additionally, the NCC (2831.49 kcal mol\({}^{-1}\)) exhibited a more elevated negative value of the absorption coefficient than the [PERSON] (2133.54 kcal mol\({}^{-1}\)), which also supports the notion that NCC was aggressively adsorbed onto the substratum of the SS316  
Figure 10: The optimized molecular configurations, HOMO, and LUMO of both inhibitors using DMol\({}^{3}\) array. Reprinted with permission from Ref. [8]. MDPI (2021). alloy, forming a staunchly adsorbed coating that prevents corrosion. These outcomes are consistent with the findings of the experiments. Furthermore, the adsorption energy values of NCC for the post-geometry optimal phase, i.e., relaxed (130.73 kcal mol\({}^{-1}\)), were also greater than [PERSON] (61.39 kcal mol\({}^{-1}\)) and much more negative than CEL (2962.22 kcal mol\({}^{-1}\)) for the pre-geometry optimal phase, i.e., unrelaxed (2194.93 kcal mol\({}^{-1}\)). Similarly, [PERSON] et al. [83] researched the ability of gum arabic-NPs (GA-NPs) to prevent corrosion in reinforced concrete after 180 days of exposure to a CO\({}_{2}\) atmosphere. Utilizing a variety of accepted methods, the steel reinforcement of concrete was tested both with and without GA-NPs. Figure 12 shows the physical examinations of accelerating carbonated samples following 180 days of CO\({}_{2}\) treatment (both with and without the GA-NP inhibitor). When a 3% GA-NP inhibitor was used, it could be observed that the cube concrete sample (cube) had shallower carbonation depths than the reference sample. Figure 11: The optimal arrangement for adsorption of the CEL and NCC over the Fe (1 1 0) surface attained through the adsorption locator array. Reprinted with permission from Ref. [8]. MDPI (2021). Figure 13 shows the 2D and 3D AFM geometries of the steel reinforcing substrate for treated and untreated concrete using the GA-NP inhibitor. Figure 13a,b shows that the exterior of the reinforced steel was badly eroded and severely damaged, exhibiting a surface roughness of 2.186 m, in the absence of GA-NP inhibitor.