# Corrosion Behavior of Stainless Steel in Seawater in the Presence of Sulfide
## 3 Results and Discussion
### Surface Analysis  
After impedance measurements (at \(E_{\rm OC}\)) and after anodic polarization measurements (which ended at 0.8 V) in seawater at 298 K, the steel surfaces were examined by SEM/EDS analysis, and the results obtained are shown in Figure 7 and Table 6. It should be emphasized that corrosion products were not removed from the surface of the samples after the electrochemical measurements. Figure 7: SEM image after impedance measurements for (**a**) AISI 304L and (**b**) AISI 316L and after anodic polarization measurements for (**c**) AISI 304L and (**d**) AISI 316L in seawater at 298 K.  
Exposure of AISI 304L to seawater results in damage to most of the surface, which is covered with coarse deposits of corrosion products and NaCl (according to EDS analysis). In contrast, under the same conditions, the surface layer of AISI 316L is smooth and even and shows almost no signs of the corrosion process. It should be noted that anodic polarisation produces a rough surface morphology of corrosion products over the damaged AISI 304L steel. On the AISI 316L specimen, there are several small damages, such as pits, covered by a fine layer of corrosion products. EDS analysis of total surface area AISI 304L and AISI 316L steels in seawater under EIS and PD measurements show the presence of Fe, Cr, Mn, Ni, and Si (as the main alloying elements), then the presence of Na, Mg, Cl, K, Ca, and O (as constituents of seawater). The presence of oxygen indicates that the surfaces of both samples are covered with a protective oxide layer. Compared to AISI 304L, a higher O content and an almost negligible percentage of Na and Cl were found on the surface of AISI 316L, while the other elements were consistent with the original composition of the alloy (Table 1). This additionally confirms the fact that the oxide layer on the AISI 316L sample is more resistant to chloride adsorption and ingress and has a better corrosion property. Indeed, the highly perturbed surface of the AISI 304L can be related to the high chloride affinity of the oxide layer. In the literature, the general opinion is that the surface oxide layer on stainless steel is very thin (order of a few nm [66; 67; 68; 69; 70]) and consists mainly of a mixture of iron and chromium oxides/hydroxides, and in some cases, also contains small amounts of molybdenum oxide [68; 69; 70]. Results of electrochemical measurements 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 into iron sulfides [15; 39; 40; 41; 54; 55; 56; 57]. This statement is confirmed by the SEM/EDS analysis performed on both steel samples under different experimental conditions in seawater with 40 ppm S\({}^{2-}\) ions at 298 K. The obtained results are shown in Figures 8 and 9 and Tables 7 and 8 (the layers of corrosion product were not removed from the surface of the sample product). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**AISI 304L**} & \multicolumn{2}{c}{**AISI 316L**} \\ \hline
**Element (wt.\%)** & **EIS** & **PD** & **EIS** & **PD** \\ \hline O & 2.80 & 2.38 & 5.79 & 4.33 \\ Na & 15.08 & 11.34 & - & 1.85 \\ Mg & 1.38 & 1.05 & - & 0.45 \\ Si & 0.42 & 0.34 & 0.04 & 0.44 \\ Cl & 7.14 & 5.04 & 0.07 & 1.10 \\ Cr & 13.77 & 14.78 & 16.36 & 16.42 \\ Mn & 0.99 & 1.15 & 1.11 & 1.00 \\ Fe & 52.67 & 57.50 & 67.91 & 64.91 \\ Ni & 5.75 & 6.02 & 7.43 & 7.25 \\ Mo & - & - & 1.92 & 2.25 \\ K & - & 0.19 & - & - \\ Ca & - & 0.21 & - & - \\ Total & 100.00 & 100.00 & 100.00 & 100.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Average elemental composition of the entire surface for AISI 304L and AISI 316L after impedance (EIS) and potentiodynamic polarization (PD) measurements in seawater at 298 K.  
Figure 8: SEM image after impedance measurements for (**a**) AISI 304L and (**b**) AISI 316L in seawater with 40 ppm S\({}^{2-}\) ions at 298 K with marked parts of EDS analysis (yellow numbers).