# Corrosion Behavior of Stainless Steel in Seawater in the Presence of Sulfide
## 3 Results and Discussion
### Electrochemical Impedance Spectroscopy Measurements
The data in Table 4 confirm that the dynamics of the changes in EEC parameters (\(R_{\text{ct}}\), \(R_{\text{f}}\), \(Q_{\text{ct}}\), and \(Q_{\text{f}}\)) are the same for both steel samples under different experimental conditions. Let us now consider the data for the AISI 316L sample. The values of \(R_{\text{ct}}\) and \(R_{\text{f}}\) are high at 182.85 k\(\Omega\) cm\({}^{2}\) and 113.57 k\(\Omega\) cm\({}^{2}\), respectively, at 288 K, which is due to the formation of a stable and compact surface film. As the temperature increases, the resistances \(R_{\text{ct}}\) and \(R_{\text{f}}\) decrease significantly and are 102.04 k\(\Omega\) cm\({}^{2}\) and 57.34 k\(\Omega\) cm\({}^{2}\) at 308 K, respectively. This indicates that an increase in temperature has a strong effect on the thermodynamic stability of the oxide film. Apparently, the accelerated corrosion  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \(T\) & \(Q_{\text{f}}\times 10^{6}\) & \(n_{1}\) & \(R_{\text{f}}\) & \(Q_{\text{dl}}\times 10^{6}\) & \(n_{2}\) & \(R_{\text{ct}}\) \\ (K) & (\(\Omega^{-1}\) s\({}^{n}\) cm\({}^{-2}\)) & & (k\(\Omega\) cm\({}^{2}\)) & (\(\Omega^{-1}\)s\({}^{n}\) cm\({}^{-2}\)) & & (k\(\Omegarates associated with higher temperatures increasingly perturbed the interface, resulting in a less ordered, more porous, and less protective corrosion product layer [46; 47]. The above changes lead to an increase in the rates of interfacial reactions that favor the corrosion of steel in seawater. This is confirmed by the fact that the thickness of the surface film (which is proportional to \(1/Q_{\rm f}\)) increases slightly with temperature while the thickness of the double layer (\(1/Q_{\rm dl}\)) decreases. The presence of sulfide ions significantly increases the corrosion of steel in seawater (possibly due to the synergistic effect of the aggressive anions S\({}^{2-}\) and Cl\({}^{-}\)). Thus, the \(R_{\rm f}\) value for AISI 316L in a 40 ppm sulfide-containing medium (at 298 K) is \(\approx\)14 k\(\Omega\) cm\({}^{2}\), which is 60% lower than the value in pure seawater, i.e., \(\approx\)90 k\(\Omega\) cm\({}^{2}\). The decrease in \(R_{\rm c}\) value is even more pronounced. \(R_{\rm ct}\) in 40 ppm sulfide-containing seawater is \(\approx\)50 k\(\Omega\) cm\({}^{2}\) compared to \(\approx\)150 k\(\Omega\) cm\({}^{2}\) in pure seawater (so the decrease is more than 70%). On the other hand, \(Q_{\rm dl}\) and \(Q_{\rm f}\) increase dramatically while the corresponding parameters \(n_{1}\) and \(n_{2}\) decrease (i.e., the thickness and compactness of these films decrease). The obtained results are consistent with the data in the literature, where many researchers have studied the effects of sulfide (including inorganic [13; 14; 15; 21; 54; 55; 56; 57]) and biogenic sulfide by SRB [15; 39; 40; 41; 42; 54; 57]) on the corrosion behavior of various stainless steels in seawater under different experimental conditions. EIS measurements show that small amounts of dissolved free sulfide (i.e., H\({}_{2}\)S, Na\({}_{2}\)S, HS\({}^{-}\), and S\({}^{2-}\)) cause a significant decrease in the overall resistance and an increase in the capacity of stainless steels [14; 15; 58; 59; 51; 52; 53; 54; 51; 55; 53]. Moreover, sulfides cause similar behavior in other media (cooling water, hydrocarbons). For example, [PERSON] et al. indicate that the addition of sulfide (up to 9 mM) to simulated cooling water causes a rapid reduction in impedance. The film resistance decreases from 13.01 to 0.30 M\(\Omega\) cm\({}^{2}\) within 1 h [50]. These results are attributed to the detrimental effect of sulfide ions on the integrity and protective properties of the corrosion product film due to the transformation of iron oxides to iron sulfides [15; 54; 55; 56; 57; 58; 59; 40; 41; 48; 51; 54; 53]. In other words, the sulfidation of the passive film leads to a decrease in the \(R_{\rm f}\) value, and the faster the sulfidation process proceeds, the lower the resistance of the passive film.