## 2 Utilization of Various Nano-Modified Coatings for Corrosion Mitigation
### Carbon Allotrope-Based Nanoc coatings  
Owing to their inexpensive, non-hazardous, readily accessible key ingredients, and simple manufacturing processes, carbon allotrope-based nanocoatings have attracted significant attention in the past ten years [70; 89]. They are used in pharmacology, detectors, optoelectronics, catalyst supports, and cell imaging. Currently, carbon dots (CDs) are used as sustainable CIs and have shown encouraging outcomes. For instance, dopamine was used by [PERSON] et al. [90] to create unique nitrogen-doped carbon dots (NCDs), which were then tested in a 1 M HCl medium as CIs for Q235 CS. The corrosion behavior of CS in 1 M HCl medium was then investigated using the EIS methodology in both the exclusion and inclusion of NCDs. Figure 17a shows the Nyquist graphs, which are constituted of a depressed and inductive capacitive curve. The semi-diameter of the capacitive curve grows as the intensity of NCDs rises, showing that the inclusion of NCDs successfully hinders the charge transfer operation at the steel-solution functionality. The finding that all Nyquist graphs exhibit the identical depressed form indicates that NCDs do not affect the steel corrosion in HCl media. Instead, this depressed curve is mostly due to corrosion-induced non-uniformity and deposited molecules over the metallic substrate. The impedance modulus ratio at the minimum frequency (10 Hz) in the addition of NCDs is larger than those in blank media in the context of the Bode graphs in Figure 17b, and it increases with rising NCD contents. The maximal impedance modulus measurement at 400 ppm NCDs is 295 \(\Omega\)cm\({}^{2}\), which is 10 times more than the value at zero concentration. Additionally, while NCDs are introduced to the HCl medium, the phase angle peak increases in size and becomes more pronounced as the quantity of NCDs increases. These results demonstrate substantial NCD binding over the steel substrate and enhanced NCD concentration-dependent corrosion inhibition. The surface topography and hardness of the specimens were also examined using LSCM and SEM following 60 h of exposure. The steel substrate was pristine before it was exposed to the corrosive substance, and it only had sanded marks (Figure 18a). The steel had a surface irregularity of around 0.27 \(\upmu\)m (Figure 18b). The steel substrate had been exposed to a 1 M HCl medium for 60 h when it was significantly rusted, turned weak, and then became permeable (Figure 18c). When contrasted to the corrosion-free steel, the substrate roughness of the steel significantly improved to 5.18 \(\upmu\)m (Figure 18d). In contrast, the deterioration of the steel substrate was greatly reduced with the introduction of 400 ppm NCDs, but the refined flaws remained discernible (Figure 18e). The corrosion-damaged steel's surface hardness was just about 1.0 \(\upmu\)m at this time (Figure 18f). These findings show that the adsorbed NCDs successfully prevent acid steel corrosion. By attaching HMS to the GO substrate, [PERSON] et al. [91] created novel dual-functional GO-based (GAS) NMs. The efficacy of GAS-passive covering's corrosion prevention was improved by GAS NMs' superior dispersal and solubility with epoxy resin. Following 40 days of exposure, \(\mid\)Z\(\mid\)1.0.01 Hz of GAS-Coat persisted at 890 \(\Omega\)M cm\({}^{2}\), and Fb barely rose to 0.48. Furthermore, GAS-Coat only offered passive protection against corrosion  
Figure 17: EIS findings of Q235 CS in 1 M HCl medium in the exclusion and inclusion of various dosages of NCDs: (**a**) Nyquist graph and (**b**) Bode graph. Reprinted with permission from Ref. [90]. MDPI (2021). Figure 18: LSCM and SEM scans of Q235 CS substrate following 60 h of subjection to 1 M HCl medium with the exclusion and inclusion of 400 ppm NCDs, (**a**,**b**) refined steel, (**c**,**d**) in blank media, (**e**,**f**) with the inclusion of 400 ppm NCDs. Reprinted with permission from Ref. [90]. MDPI (2021). while the covering had been degraded.