## 2 Materials and Methods
### Characterization  
Several imaging techniques providing information at macro- and microscopic scales were used in order to evaluate the surface morphology of the mild steel panels. The microscale information was collected with SEM imaging performance using a Hitachi Tabletop Microscope TM3030 Scanning Electron Microscopy equipped with an energy dispersive X-ray spectrophotometer (EDS) system (QUANTAX 70), and with an ultra-high resolution scanning electron microscopy (UHR-SEM) using NOVA NANOSEM 230 (FEI Company). The macroscale information with respect to optical details was acquired using a Samsung Galaxy A7 auto focus triple camera 24 MP + 5 MP + 8 MP. The chemical analysis was conducted via micro-Raman measurements using a Renishaw inVia spectrometer working in backscattering configuration and equipped with a near-infrared diode laser emitting at 532 and 785 nm. The spectra were recorded by focusing the laser beam on the sample surface and adjusting the light power so that 1 mW was provided for a spot of about 1 \(\upmu\)m diameter. The compounds that are created on the subjected mild steel panels were studied via XRD measurements. The crystal structure was identified by powder X-ray diffraction using an X Bruker D8 Advance Twin Twin, employing Cu-K\(\alpha\) radiation (\(\lambda\) = 1.5418 A). The electrochemical characterizations based on linear polarization resistance (LPR) and potentiodynamic polarization (PP) were conducted using a VersaSTAT 3 Potentiostat/Galvanostat/Frequency Response Analyzer instrument (Princeton Applied Research, AMETEK). Concerning the PP measurements, a three-electrode electrochemical cell was used, consisting of a saturated silver chloride electrode [Ag/AgCl,KCl(sat)] as reference, a working electrode (\(\approx\)1.0 cm\({}^{2}\) of exposed area), and a platinum foil as counter electrode. The assessment of the polarization curves on the bare alloy was accomplished after different exposure intervals. Cathodic and anodic branches of polarization curves were recorded separately with the potential scan rate of 1 mVs\({}^{-1}\) in the cathodic and in the anodic directions, starting from the open circuit potential (OCP, \(E_{\text{OC}}\), corrosion potential), at room temperature. Regarding the LPR measurements, the scan rate was 0.1 mVs\({}^{-1}\) and the potential range was \(\pm\) 2.5 mV versus OCP. The \(R_{\text{P}}\) (polarization resistance) is the definition of the ratio of the applied potential to the applied current (\(\Delta E/\Delta I\)). The slope of the potential versus current plot is used for the calculation of the \(R_{\text{P}}\)[37]. The \(R_{\text{P}}\) denotes the degree of the specimen susceptibility to be corroded [38; 39]. The corrosion protection performance of the inhibitors was estimated via electrochemical impedance spectroscopy (EIS). The EIS measurements were conducted using the aforementioned potentiostat instrument with the arrangement of the three-electrode electrochemical cell, in a frequency range from 10 \(\upmu\)Hz to 1 MHz. All spectra were recorded at open circuit potential, applying a 10 mV sinusoidal perturbation (rms signal) at room temperature. During all measurements, the electrochemical cell was placed in a Faraday cage. The Z-view Software (Scribner Associates Incorporated, SAI) was used for the analysis of the obtained spectra utilizing the adequate equivalent electric circuits. Two different types of electrolytic solutions (300 mL) were used: (i) water from the supply system of Athens city (WSS: water of the Athens city supply system after a softening process, Table 1), and (ii) 3.5 wt % NaCl solution prepared in distilled water [ASTM G44-99 (2013)] [40]. Moreover, the corrosion phenomena on a metallic uncoated surface was further evaluated with respect to the influence of corrosion inhibitors. Consecutively, 6 mM electro \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**K** & **Ca** & **Mg** & **Fe** & **Mn** & **Zn** \\ \hline
1.7733 & 0.346 & 0.09755 & 1.36 & 0.005 & 0.0052 \\ \hline \hline \end{tabular}
\end{table}