# Corrosion Behavior of Stainless Steel in Seawater in the Presence of Sulfide
## 3 Results and Discussion
### Polarization Measurements
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \(\mathbf{T}\) & \(\mathbf{E_{\mathrm{corr}}}\) & \(\mathbf{i_{\mathrm{corr}}}\) & \(\mathbf{i_{\mathrm{P}}}\) & \(\mathbf{E_{\mathrm{dp}}}\) & \(\mathbf{E_{\mathrm{dp}}}-\mathbf{E_{\mathrm{corr}}}\) & \(\mathbf{R_{\mathrm{P}}}\) & \(\mathbf{v_{\mathrm{corr}}}\) \\ (**K**) & **(V)** & **(\(\mathbf{\mu}\)A cm\({}^{-2}\))** & **(\(\mathbf{\mu}\)A cm\({}^{-2}\))** & **(\(\mathbf{\
u}\)A cm\({}^{-2}\))** & **(V)** & **(V)** & **(\(\mathbf{k}\)\(\mathbf{\ell}\)A cm\({}^{2}\))** & **(mm y\({}^{-1}\))** \\ \hline \multicolumn{10}{c}{AISI 304L} \\ \hline
288 & \(-0.18\) & 1.89 & 3.27 & 0.40 & 0.58 & 212.79 & 0.0195 \\
298 & \(-0.18\) & 2.56 & 7.84 & 0.34 & 0.52 & 167.15 & 0.0265 \\
308 & \(-0.20\) & 3.36 & 13.94 & 0.24 & 0.44 & 116.05 & 0.0347 \\
10 ppm S\({}^{2-}\) & \(-0.23\) & 3.95 & 14.35 & 0.30 & 0.53 & 101.99 & 0.0408 \\
40 ppm S\({}^{2-}\) & \(-0.28\) & 5.85 & 31.53 & 0.21 & 0.49 & 46.38 & 0.0605 \\ \hline \multicolumn{10}{c}{AISI 316L} \\ \hline
288 & \(-0.11\) & 0.95 & 1.89 & 0.57 & 0.68 & 284.24 & 0.0099 \\
298 & \(-0.12\) & 1.26 & 4.10 & 0.48 & 0.60 & 209.32 & 0.0132 \\
308 & \(-0.13\) & 1.86 & 7.53 & 0.42 & 0.55 & 149.82 & 0.0194 \\
10 ppm S\({}^{2-}\) & \(-0.18\) & 2.24 & 9.21 & 0.34 & 0.52 & 145.84 & 0.0234 \\
40 ppm S\({}^{2-}\) & \(-0.23\) & 3.20 & 19.57 & 0.28 & 0.51 & 62.65 & 0.0334 \\ \hline \end{tabular}
\end{table} 
Table 5: Corrosion parameters for the stainless steels AISI 304L and AISI 316L in seawater at different temperatures and different Na\({}_{2}\)S concentrations (at 298 K). A potentiodynamic polarization curve is usually presented in a semi-logarithmic diagram and consists of cathodic and anodic curves, which are the result of electrochemical reactions in the system. The polarization curves in Figure 6 show clear evidence of an active-passive transition, as expected for stainless steels in seawater environments. Accordingly, three potential regions can be observed on the anodic branch of the polarization curves (potentials more positive than \(E_{\text{corr}}\)): active, passive, and metal depassivation regions. In the active potential region, the steel samples dissolve and release metal ions to the seawater, and the current increases exponentially with increasing potential (i.e., a linear relationship log \(i\)-\(E\) is observed on the PD curve). In seawater, the metal ions come into contact with the OH\({}^{-}\) ions (formed by the ionization of water) and form hydroxides that cover the steel surface, which further slows down the metal dissolution. During dehydration, the metal hydroxide transforms into the corresponding oxide. During further anodic polarization, the rate of dissolution of the metal becomes equal to the rate of oxide film formation, and a passivation current (\(i_{\text{p}}\)) is established. By further increasing the potential, the rate of the dissolution of the metal significantly slows down the process of oxide film formation. Eventually, the entire surface of the metal is covered with a passive film, and a more or less defined \"current plateau\" forms on the PD curve. The \"current plateau\" (the independence of the current from the potential in the potentiodynamic conditions of the experiment) is associated with the growth (thickening) of the passive film by ionic conductivity, which involves a transport process driven by the electric field in the oxide layer [59]. The starting point for the analysis of the PD curves is the corrosion potential (\(E_{\text{corr}}\)) and its corresponding corrosion current density (\(i_{\text{corr}}\)). With increasing temperature, the \(E_{\text{corr}}\) of both steels shifts very slightly into the negative range, while the \(i_{\text{corr}}\) increases (Table 5). Although the influence of the temperature increase (from 288 to 308 K) on the \(E_{\text{corr}}\) shift is almost insignificant (\(\approx\)20 mV), the \(i_{\text{corr}}\) actually increases twice (from 1.89 to 3.36 \(\upmu\)A cm\({}^{-2}\) for AISI 304L and from 0.95 to 1.86 \(\upmu\)A cm\({}^{-2}\) for AISI 316L).