# Corrosion Behavior of Stainless Steel in Seawater in the Presence of Sulfide
## 3 Results and Discussion
### Electrochemical Impedance Spectroscopy Measurements  
EIS is considered one of the least destructive and most informative techniques for characterizing electrochemical reactions at the metal/electrolyte interface and monitoring the interfacial phenomena of corrosion product film formation and modulation. The results of EIS measurements for AISI 316L stainless steel in seawater under different experimental conditions (temperature, presence of S\({}^{2-}\) ions) are shown in Figures 2 and 3. These results were further analyzed by fitting them with a suitable electrical equivalent circuit (EEC). In the Nyquist complex plane, an incomplete capacitive semicircle is observed whose diameter decreases with increasing temperature (Figure 2a) and concentration of sulfide ions (Figure 3a). In the Bode complex plane (Figures 2b and 3b), the capacitive behavior of the studied systems is highlighted in a wide frequency range (at \(f\) < 1 kHz). The pronounced capacitive behavior is determined by a Bode line with a constant slope and phase angle with a broad peak and is the result of two overlapping time constants. This response indicates the formation of a surface film at the AISI 316L/seawater phase boundary. Figure 1: Open circuit potential curves for AISI 316L stainless steel in seawater at different (**a**) temperatures and (**b**) S\({}^{2-}\) concentrations (at 298 K). Figure 2: (**a**) Nyquist and (**b**) Bode diagrams for AISI 316L in seawater at different temperatures. The total impedance of the phase boundary decreases with increasing temperature (Figure 2b) and concentration of sulfide ions (Figure 3b). A similar behavior is shown in the sample of AISI 304L steel, indicating that the protective properties of the surface film are lower under all experimental conditions. The obtained results are in agreement with the data in the literature [14; 21; 40; 41; 42; 43]. When steel is exposed to seawater and neutral solutions containing inorganic chloride, sulfide, and sulfate-reducing bacteria, many authors have obtained similar impedance responses, and in analyzing the results, the equivalent electric circuit (EEC) model with two relaxation time constants is often used to represent a thin corrosion product film formed on the metal surface. Figure 4 shows the EEC model used to fit the EIS data. Figure 3: (**a**) [PERSON] and (**b**) [PERSON] diagrams for AISI 316L in seawater in the presence of \(\mathrm{S}^{2-}\) ions at 298 K.  
In the analysis of EIS results, the deviation of individual EEC elements from ideal behavior is described by the constant phase element (CPE), whose impedance, \(Z_{\text{CPE}}\), is given by equation [44,45]:  
\[Z_{\text{CPE}}=\big{[}Q(j\omega)^{n}\big{]}^{-1} \tag{1}\]  
where \(\omega\) is the angular frequency (\(\omega\) = 2\(\pi\)f), \(j\) = \(\surd\)\(-\)1, while \(Q\) is a combination of properties mainly related to the electrode surface. The CPE exponent \(n\) can take different values from \(-\)1 to +1 and accordingly replaces different elements in the EEC. The most common cases are when CPE represents the capacitance (\(n\) = 1) and the diffusion process (\(n\) = 0.5) [44,45]. In the proposed EEC, \(R_{\text{el}}\), \(R_{\text{f}}\), and \(R_{\text{ct}}\) are the electrolyte resistance (\(\approx\)6 \(\Omega\) cm\({}^{2}\)), surface film resistance, and change transfer resistance, respectively. \(Q_{\text{f}}\) and \(Q_{\text{ct}}\) are the constant phase elements, which represent the capacitance of the surface film and electrochemical double layer, respectively, based on the corresponding parameter \(n\). The calculated EEC parameters for the studied samples in seawater at different temperatures and concentrations of S\({}^{2-}\) ions are shown in Table 4.  
The results obtained show that the experimental conditions (temperature and presence of S\({}^{2-}\) ions) reduce the stability of the surface layer and favor the corrosion of the steel, which is more pronounced in the AISI 304L sample.