## 3 Effect of Grain Size and Corrosion Inhibition Mechanism  
Noticeably, innate micro-architectural permeability, which exhibits weakness in impeding the permeation of superfluous entities deleterious to a particular surface, typically inhibits the effective utilization of nanocoatings for inhibiting or reducing corrosion, fouling, and scratching from self-healing functionality. The efficacy of nanocoatings against corrosion could be considerably impacted by grain size. It has been shown in numerous trials that the corrosion resistance of nanocoatings with greater grain size showed superior corrosion resistance when the grain size of the NPs was increased while annealing at greater temperatures. This trend might be a result of twins forming after annealing. It was frequently noted that when grain size grew, the density of the twins also grew. Low free energy is a characteristic of twin boundaries that increases resistance to corrosion. Therefore, although the annealed coatings exhibited a higher grain size, improved twin density after annealing improved the uniformity of the passive coating and decreased the sensitivity of the annealed coverings to pitting corrosion. The corrosion investigations of nanocrystalline nickel-tungsten (Ni-W) nanocoatings have also demonstrated that the corrosion rate enhances with grain size in an acidic medium (pH 3), while the corrosion rate reduces with an increment in grain size in an alkaline medium (pH 10) [93; 94]. This indicates that the pH affects the nanocoatings' corrosion behavior in addition to their grain size. The development of passive oxide coating, which relies on W concentration, and the active sites accessible for corrosion, which rely on grain boundary volume, were discovered to be two conflicting elements that affect how these nanocoatings (grain size: 5 to 63 nm)behave in media of varying pH levels. Therefore, the emergence of corrosion in an alkaline medium, where corrosion intensified with the reduction in grain size, was attributed to the increasing grain boundary volume. On the other hand, in an acidic saline condition, the rate of corrosion reduced as the grain size increased. In this instance, W concentration was the dominant factor in corrosion, and coatings with greater W concentrations demonstrated better corrosion resistance as a result of robust oxide layer development. As a result, nanocoatings demonstrated greater corrosion resistance in an alkaline medium than in an acidic medium. Furthermore, the NPs' smaller size is more favorable since it allows them to enter extremely tiny pores, indentation patterns, and capillary regions both in the coating matrices and at the metal substrate. The superior corrosion-mitigation tendency of nanocoatings has been centered on the high surface-to-volume ratio and light characteristics as a result of particle size as a major advancement in nanotechnology [67; 95; 96; 97]. All metallic substrates, either electroplated or precoated, could be covered with nanocoatings using a variety of covering techniques, which might shield the metallic surface against corrosion impacts for multiple decades. Often, the coverings are translucent, and they also improve the surface's aesthetic effect. The most crucial prerequisite for a potential adsorbent is, indeed, a broad specific surface region regarded as adsorption, which is a phenomenon that involves the accumulation of corrosion inhibitor molecules on an adsorbent's surface. The permeability of the adsorbents defines their adsorption capabilities, whether adsorption is driven by long-range linkages (physisorption) or by chemical interactions, including ionic or covalent bonds (chemisorption). Since physical adsorption favors porosity with a size similar to the adsorbent's molecular radius, substances such as nanomaterials that are frequently characterized by small mesopores and micropores are frequently regarded as effective adsorbents. The most common means of immobilization is physical adsorption which is easily initiated at mild temperatures.