# Corrosion Behavior of Stainless Steel in Seawater in the Presence of Sulfide
## 3 Results and Discussion
### Surface Analysis
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**AISI 304L**} & \multicolumn{6}{c}{**AISI 316L**} \\ \hline
**Element (wt.\%)** & **1** & **2** & **3** & **4** & **1** & **2** & **3** & **4** \\ \hline O & 1.90 & 1.06 & 0.87 & 1.88 & 8.73 & 3.65 & 2.76 & 2.71 \\ Na & 13.31 & 15.67 & 2.14 & 2.05 & 20.21 & 20.53 & 1.28 & 4.53 \\ Mg & 1.36 & 0.55 & - & - & 3.67 & 1.29 & 1.17 & - \\ Si & 0.40 & 0.38 & 0.47 & 0.47 & 0.14 & 0.27 & 0.32 & 0.34 \\ S & 0.25 & 0.28 & 0.08 & 0.08 & 3.27 & 0.66 & 0.62 & 0.51 \\ Cl & 5.46 & 5.73 & 0.70 & 0.62 & 18.16 & 9.97 & 0.60 & 1.70 \\ Cr & 15.04 & 14.63 & 17.84 & 17.65 & 8.53 & 12.77 & 17.05 & 16.06 \\ Mn & 0.97 & 1.38 & 1.25 & 1.41 & 0.69 & 1.72 & 1.23 & 1.28 \\ Fe & 56.91 & 54.33 & 68.50 & 68.16 & 30.44 & 42.45 & 64.60 & 61.76 \\ Ni & 5.76 & 5.99 & 8.15 & 7.68 & 4.06 & 5.77 & 8.71 & 8.71 \\ Mo & - & - & - & - & 0.75 & 0.76 & 1.66 & 2.40 \\ K & - & - & - & - & 0.65 & 0.16 & - & - \\ Ca & - & - & - & - & 0.70 & - & - & - \\ Total & 100.00 & 100.00 & 100.00 & 100.00 & 100.00 & 100.00 & 100.00 & 100.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Elemental composition on marked parts of the surface of AISI 304L and AISI 316L steels after impedance measurements in seawater with 40 ppm S\({}^{2-}\) ions at 298 K.  
Figure 9: SEM image after potentiodynamic polarization 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). Compared to pure seawater (Figure 7), the surface of both samples in the presence of sulfide ions exhibits a rougher morphology, with individual steel areas covered with higher quantities of corrosion products. These changes are particularly pronounced after polarization measurements on a sample of AISI 304L steel. EDS analysis of the different parts of the AISI 304L and AISI 316L surface after EIS and PD measurements (positions 1-6) shows significant changes in the elemental composition, especially in Fe, Cr, and other basic elements of the alloy, the content of which has decreased significantly due to the dissolution. In parallel with these changes, a decrease in O content and an increase in S content were observed. The changes on the surface are again greatest for the AISI 304L sample after the measurements of PD. To determine the condition of the surface under the corrosion products after the potentiodynamic polarization measurements, the layers of corrosion products were removed from the steel specimens with ultrasonic cleaning in deionized water. The occurrence of pitting corrosion (density, depth, and width of pits) on the surface of the samples was confirmed with optical microscopy and a non-contact 3D profilometer. The light microscopy results are shown in Figures 10 and 11. The pits on the surfaces of both types of stainless steel can be clearly seen in these figures. It should be noted that the AISI 304L steel has larger pits at the same seawater temperature, confirming that the AISI 304L stainless steel is more susceptible to pitting corrosion compared to the AISI 316L steel. The introduction of sulfide ions into seawater leads to a change in the shape and size of the pits and their size, which become larger in AISI 304L stainless steel (Figure 11a), while in AISI 316L, the number of pits on the alloy surface increases (Figure 11b). More accurate information about the surface topography was obtained by profilometric measurements, as shown in Figure 12. The technique of non-contact 3D profilometer analysis provides an ideal, user-friendly way to maximize surface examination when pitting analysis is required, along with the advantages of combined 2D and 3D capability [71].