# Corrosion Behavior of Stainless Steel in Seawater in the Presence of Sulfide
## 3 Results and Discussion
### Polarization Measurements
When 40 ppm of sulfide is added, \(i_{\text{p}}\) increases to \(\approx\)20 \(\upmu\)A cm\({}^{-2}\), which is about five times higher than the \(i_{\text{p}}\) value in the system free of sulfide. When testing 316L stainless steel in simulated cooling water containing various sulfide concentrations, a significant increase in \(i_{\text{p}}\) was also observed (an increase in sulfide ion concentration up to 9 mM increases \(i_{\text{p}}\) twofold, from \(\approx\)13 to \(\approx\)24 \(\upmu\)A cm\({}^{-2}\)) [50]. Namely, in a sense, \(i_{\text{p}}\) reflects the rate of penetration of corroden through the passive film. Its increase indicates the deterioration of the protective performance of the passive film and illustrates that the sulfide has changed the performance of the passive film. [PERSON] [62] suggested that sulfide can weaken the binding energy of the metal-metal bonds on the surface, leading to an increase in the anodic dissolution rate of the metal. That is, the presence of sulfide lowers the activation energy of the metal dissolution reaction and accelerates the anodic dissolution process of the metal. Moreover, as mentioned earlier, the addition of sulfide leads to a significant reduction in corrosion potential. Further polarization leads to the pitting or depassivation potential (\(E_{\text{dp}}\)), which becomes more negative with increasing temperature and seawater pollution (i.e., the presence of S\({}^{2-}\) ions) in the steel specimens studied (Table 5). At the \(E_{\text{dp}}\), degradation of the passive layer and local metal dissolution occurs, which is accompanied by a sudden increase in the anodic current. Since the \(E_{\text{corr}}\) value of the steels studied depends on the experimental conditions (Table 5), the range of passive potential is more precisely determined by the potential difference, given as \(\Delta E=E_{\text{dp}}-E_{\text{corr}}\), which decreases for both samples with increasing temperature and sulfide ion concentration. In agreement with the literature, the potential difference \(E_{\text{dp}}-E_{\text{corr}}\) (denoted as \(\Delta E\), representing the range of passive potentials) is indicative of the pitting initiation rate; the higher it is, the slower the pitting initiation rate [63; 64]. Table 5 contains the values of the corrosion rate \(v_{\rm corr}\) (expressed in millimeters per year; mm/y) calculated using the following equation:  
\[v_{\rm corr}=\frac{i_{\rm corr}M}{\rho Fz} \tag{4}\]  
where \(M\) is the molar mass of the corroding species, \(\rho\) is the density of the corroding species, \(F\) is the Faraday constant, and \(z\) is the electron number. In calculating the corrosion rate, the data for \(\rho\), \(M\), and \(z\) were determined based on the percentage of the main elements in AISI 304L (Fe and Cr) and AISI 316L (Fe, Cr, Mo) steel [65]. The corrosion rate of both steels increased with increasing temperature and concentration of sulfide ions, with the AISI 316L steel exhibiting a lower corrosion rate. The literature data for \(v_{\rm corr}\) vary considerably. For example, [PERSON] et al. recorded as much as ten times higher values for \(v_{\rm corr}\) (\(\approx\)0.72 mm y\({}^{-1}\)) in chloride solutions saturated with H\({}_{2}\)S at 298 K [53], which is probably a consequence of a higher concentration of S\({}^{2-}\) ions, while [PERSON] et al. found as much as ten times lower values (\(\approx\)0.005 mm y\({}^{-1}\)) [60] (which is a consequence of the much lower concentration of Cl\({}^{-}\) ions compared to the conditions in this work; seawater).