## 3 Results and Discussion
### Electrochemical Impedance Spectroscopy
From the CPE parameters, the values of the pseudo-capacitances (C) were calculated using the equation below:  
\[\mathrm{C}=\left(\mathrm{R^{1-n}Q}\right)^{1/n} \tag{4}\]  
The fitting parameters for the bronze corrosion in the absence and in the presence of varying concentrations of NAC are listed in Table 2.  
The polarization resistance values, \(\mathrm{R_{P}}\), calculated as \(\mathrm{R_{ct}}+\mathrm{R_{F}}\), are also depicted in Table 2. As previously reported [71], the low-frequency limit of the impedance, \(\mathrm{R_{P}}\), correlates better with the corrosion rate, when a redox process involving the corrosion products occurs at the interface. Accordingly, the \(\mathrm{R_{P}}\) values were further used to estimate the inhibition efficiency, \(\mathrm{n_{limp}}\), using Equation (1). Figure 3: Equivalent electrical circuits used for the computer fitting of the experimental data obtained in the absence (**a**) and presence of the drug (**b**) in the corrosive electrolytes. The addition of NAC to both tested electrolytes resulted in an increase in the charge transfer resistance, \(\rm R_{ct}\) values, and this effect was enhanced as the drug concentration increased up to certain values, i.e., 36 mM in the 3.5% NaCl solution and 6 mM in 0.2 g L\({}^{-1}\) Na\({}_{2}\)SO\({}_{4}\) + 0.2 g L\({}^{-1}\) NaHCO\({}_{3}\) + 0.2 g L\({}^{-1}\) NaNO\({}_{3}\) (pH = 3.4) electrolyte, respectively. These concentrations of NAC in _Fluimucil_ appeared to be the optimal ones in the investigated experimental conditions. A slight decrease in the \(\rm R_{ct}\) values was observed at higher concentrations of NAC in the simulated acid rain solution, but they remained above the one of the uninhibited electrolyte (Table 2). These results indicate that NAC was able to hinder the charge transfer process in both tested electrolytes, most likely through its adsorption on the metal surface. The adsorption of NAC on bronze also changes its interfacial capacitance, according to the Helmholtz model [72]:  
\[\rm C_{dl}=\frac{\varepsilon\cdot\varepsilon_{0}}{d}\cdot A \tag{5}\]  
where \(\varepsilon_{0}\) and \(\varepsilon\) are the vacuum and local dielectric constants, respectively; A is the electrode area; and \(d\) is the thickness of the double-layer. Thus, the pre-existing water molecules from the bronze surface are replaced by the adsorbed drug molecules with lower permittivity, and this reduces the local dielectric constant, while increasing the thickness of the double layer [72]. Furthermore, the adsorption of NAC on the bronze reduced the exposed surface area in contact with the corrosive solutions. These effects accounted for the decrease in the double layer capacitances, \(\rm C_{dl}\), generally observed in the drug-containing electrolytes (Table 2). As expected, the lowest \(\rm C_{dl}\) values were obtained in the presence of the NAC concentrations that also gave the highest \(\rm R_{ct}\) values. The faradaic resistance, \(\rm R_{F}\), increased with NAC concentration up to its optimal values (36 mM in the saline solution and 6 mM in simulated acid rain), while the faradaic capacitance, \(\rm C_{F}\), simultaneously decreased. This behavior allowed the assumption that the adsorption of NAC on the bronze surface inhibited to some extent the development of the corrosion products (most likely \(\rm Cu_{2}O\)), which became less active in the redox processes, thereby providing improved corrosion protection. However, by increasing the NAC