## 4 Discussion: Optical vs. Electromagnetic Methods  
Table 4 summarizes the results obtained with the three techniques. Besides the mean thickness values, for OCT and CRM the minimum and maximum thicknesses are reported to indicate their variability. In the first place, it is necessary to consider the different sizes of the areas analyzed. The EC system employed in this work offers average information from an area of 19.63 mm\({}^{2}\) (\(\diameter\)5 mm), while the optical methods provide information with a microscopic resolution, thus registering variability throughout the film. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & \(\Delta\) (\(\mu\)m) & **Std. dev. (\(\mu\)m)** & \(\frac{-}{z}\) (\(\mu\)m) & **Std. dev. (\(\mu\)m)** & \(h_{z}\) \\ \hline PB-72\({}_{20}\) & 41.8 & 3.2 & 62.3 & 4.8 & 10.6 \\ PB-72\({}_{25}\) & 24.2 & 0.5 & 36.1 & 0.8 & 8.0 \\ PB-72\({}_{10}\) & 5.5 & 1.0 & 8.3 & 0.0 & 3.8 \\ \hline PB-44\({}_{25}\) & 18.7 & 1.2 & 27.7 & 1.7 & 7.0 \\ PB-44\({}_{10}\) & 8.3 & 1.5 & 12.3 & 2.3 & 4.7 \\ PB-44\({}_{45}\) & 5.6 & 0.8 & 8.2 & 1.1 & 3.8 \\ \hline Zapon\({}_{70}\) & 18.2 & 0.5 & 28.1 & 0.8 & 7.1 \\ Zapon\({}_{40}\) & 6.7 & 0.6 & 9.5 & 0.6 & 4.1 \\ Zapon\({}_{20}\) & 3.5 & 0.3 & 5.5 & 0.3 & 3.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Average CRM results from three measures: nominal depth (\(\Delta\)) as measured by the instrument compared to the corrected thickness (\(\overline{z}\)), with the higher error value (\(h_{z}\)). Despite the variability of the thickness values measured with the three techniques, the results are comparable. It is worth noticing that, in addition to the mean values, OCT and CRM data show the minimum and maximum thickness of the layers at a microscopic scale. Spot CRM results depend on the area analyzed; however, performing a depth-profile mapping could solve this issue, offering a profile comparable to the OCT results. The results from the thinner samples (i.e., the ones obtained with lower polymer concentration solutions) showed lower variability for the three techniques due to the homogeneity of the layers. This points to the main disadvantage of the EC: irregularities in the coating are not registered due to the averaging area. Moreover, OCT and CRM offer morphological information about the layer, thus registering the topography of the coating induced by possible irregularities present in the metallic surface; this feature is not registered by EC. Regarding the presence of corrosion products in the metallic surface, such as passive patina layers, the optical methods are not affected by them. OCT registers changes in the refractive index and, consequently, the corrosion patina does not interfere in the estimation of the polymeric layers. Accordingly, CRM is able to differentiate between the polymeric coatings and the metallic patina/surface. The main advantage of CRM is the possibility to simultaneously characterize the corrosion products present and, in some cases, the possible inclusions (i.e., colorants in artificial patinas) in the coating layers [20]. On the contrary, EC results may be influenced by the presence of corrosion products under the coating layer and even by superficial deposits (i.e., dust). It either includes their thickness in the estimation of the polymeric coating or potentially detects a multilayer system without a specific identification, since it exclusively registers conductivity differences. In contrast, CRM, as used in this study, is non-portable and only samples that can fit under the microscope can be studied. Additionally, fluorescence emission by the probed material can interfere with the profile measurements. Ultimately, the measurements are highly time consuming. Both OCT and CRM require corrections for the material refraction index. Table 5 summarizes the main features of each technique. The choice of one or the other must be evaluated according to the object's characteristics.