*3.3. Surface Topography*

Surface topography analysis of the uncoated and coated SS 316L samples, which were conducted using an AFM device, is shown in Figures 6 and 7 (alternatively, Figure 3a–d). The experimental results have revealed that the nanostructures on the uncoated surface have a range of height between 87.3 to 204 nm (Figure 7a), with almost 47.5% of the structure height being in the range of 116 to 130.5 nm. Moreover, the maximum height of surface (MHS), which was obtained by adding up the surface maximum peak height and maximum valley depth, and root mean square roughness (RMSR) of the uncoated sample, were shown to be 291 nm and 12 nm, correspondingly, as demonstrated in Figure 6. On the other hand, as the deposited layer thickness increased (Figure 7b–d), the structure height on the surface, RMSR, and MHS were seen to reduce, reaching values between 35.9 to 83.7 nm, 6.86 nm, and 120 nm, respectively. Furthermore, the degree of symmetry of the surface heights about the mean plane was also seen to improve with the deposited film thickness, as the obtained skewness (Ssk) values of the measured samples (Table 1) were shown to move closer to the mean plane (i.e., zero) with the increase in fabricated layer thickness. It is worth noting that the sign of Ssk represents the predominance of the comprising surface peaks (Ssk > 0) or valley structures (Ssk < 0). The aforementioned changes in surface conditions can be attributed to the deposited particles occupying the vacant spaces on the surface structure, which consist of valleys, hills, and micro gaps, leading to the height variation on the surface to narrow down [41]. Moreover, the presence of inordinately high peaks or deep valleys was also found on the examined substrates, as indicated by the surface kurtosis (Sku) values, where Sku > 3.00 suggests the existence of a peaks/valleys defect on the surface and Sku < 3.00 illustrates a lack thereof (i.e., insufficient surface information). Such an observation is not surprising as it is commonly present on most surfaces [42]. The average roughness values were found to be 7.87 nm, 7.48 nm, 6.00 nm, and 4.88 nm for the uncoated, 50 nm, 100 nm, and 150 nm coated substrates, correspondingly. These results confirmed the smoothening effect caused by the increase in EB-PVD deposited film thickness on the SS 316L substrate surface. The roughness results can also be used as a general

indication of the corrosion behaviour, as it has been reported by other authors that decreasing the surface roughness of passive alloys tends to reduce the pitting susceptibility and corrosion rate [43,44]. The height parameters values obtained from the AFM analysis of the samples can be seen in Table 3.

**Figure 6.** Root mean square roughness and maximum height of surface variation with deposition thickness on SS 316L substrates.

**Figure 7.** Surface topography analysis of SS films on SS 316L substrates, where (**a**) 2D and 3D rendered AFM topograph and height distribution of the surface of the uncoated SS 316L substrate, and (**b**–**d**) 2D and 3D rendered AFM topograph after 50, 100, and 150 nm SS deposition on substrates and their height distribution.


**Table 3.** Height parameters of the AFM analysis of the uncoated, 50 nm, 100 nm, and 150 nm coated SS substrates.

*3.4. Deionised Water Properties Variation with Temperature*

The deionised waters used were selected to have a pH of 4, 7, and 9 to observe the acidity, neutrality, and alkalinity of the liquid effect on the surfaces wettability. Analyses results of the changes in properties, namely, kinematic viscosity (ν), density (), and pH value of the three liquids, within our temperature range, are shown in Figure 8a. Comparing the and ν characterisation outcomes, of the as-prepared DIW of pH 7, with the available data on pure water in literature [45] has shown a deviation of 0.015% and 3.67%, respectively, thus verifying the measurement approach conducted. Moreover, regardless of the examined DIW pH value, the as-fabricated liquids ν and were seen to have a negligible difference in their values at each investigated point of temperature. For example, at 30 ◦C, the DIW ρpH4, ρpH7 and ρpH9 ad an outcome of 0.99564, 0.99562, and 0.99563 g/cm3, respectively. In contrast, manipulating the temperature was seen to have a notable influence on all three properties of the DIW's (i.e., ν, , and pH value), as demonstrated in Figure 8a. This can be explained by the fact that ν is inversely related to the , and that is a representation of substance mass to its volume, where at a constant volume, the mass is influenced by the bonds distance of the molecules and their forming atoms. Our as-prepared DIW's consist of four types of bonds: (1) Polar covalent bond between a single or a pair of hydrogen atoms and one atom of oxygen, (2) Dative covalent bond between a single atom of H<sup>+</sup> and a H2O molecule, (3) hydrogen bond between the oxygen atom of a H2O molecule and a hydrogen atom of a neighbouring H2O molecule, and (4) Ion–dipole interaction between the H2O molecules and a Cl<sup>−</sup> atom (e.g., DIW of pH 4) or Na+ atom (e.g., DIW of pH 9). The four previous bounds are shown in Figure 8b–d. Based on the obtained data, it is believed that at a point of temperature, the bond distances of the newly introduced dative covalent bond (pH 4) and ion – dipole interaction (pH 9) are very close in distance to the other two initially existing bonds in neutral water, causing this neglectable changes in the liquid mass. On the other hand, raising the temperature weakens all four bonds, because of the increase in molecular vibrations, causing the bonds distance to widen; and hence the liquid mass reduces and becomes more acidic due to the release of H+ and growth in its concentration [46]. The different formed reactions in our as-prepared DIW's, based on the Bronsted–Lowry theory of acids and bases [47], are demonstrated in Equations (2)–(4) as the following:

$$\text{H} \cdot \text{H} \cdot \text{HCl} + \text{H}\_2\text{O} \leftrightharpoons \text{H}\_3\text{O}^+ + \text{Cl}^- \text{ (Reerves reaction)}\tag{2}$$

$$\text{pH 7: } 2\text{H}\_2\text{}^+ + \text{O}\_2\text{}^- \rightarrow 2\text{H}\_2\text{O (Chemical reaction)}\tag{3}$$

$$\text{pH 9: NaOH} + \text{H}\_2\text{O} \rightarrow \text{Na}^+ + \text{OH}^- + \text{H}\_2\text{O} + \text{Heat (Exothermic reaction)}\tag{4}$$

**Figure 8.** Water atoms and molecules bonds, and properties variation with temperature, where (**a**) shows the DIW's kinematic viscosity, density, and pH value changes with temperature, and (**b**–**d**) illustrates the bonds in water of pH 7, 4, and 9, respectively.
