# Corrosion Behavior of Stainless Steel in Seawater in the Presence of Sulfide
## 2 Materials and Methods  
Stainless steel rods (AISI 304L and AISI 316L) with a diameter of 6 mm and a length of 150 mm were purchased from commercial sources (Ronsco, China) with the chemical composition listed in Table 1.  
The preparation of the electrodes for the electrochemical measurements from the steel rods has been described previously [26]. Before each measurement, the working surface of the electrode was ground with abrasive paper to a grit size of 2500 using a Metkon Forcipol 1V grinding and polishing machine and then polished with a diamond polishing suspension Metkon Diapat M (particle size 1 \(\upmu\)m). The final processing step before immersion in the electrolyte was a 5-min ultrasonic cleaning in ethanol and deionized water.  
The seawater for this study was collected at \(\dot{Z}\)njan beach in the city of Split, Croatia (coordinates: 43.5018\({}^{\circ}\) N, 16.4763\({}^{\circ}\) E). The physical parameters of the seawater used as a test environment are listed in Table 2, and the chemical composition of the seawater is listed in Table 3.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Sample** & **Cr** & **Ni** & **Mn** & **Mo** & **Si** & **Cu** & **C** & **Fe** \\ \hline AISI 304L & 16.76 & 8.66 & 2.23 & 0.11 & 0.52 & 0.38 & 0.03 & 71.58 \\ AISI 316L & 16.47 & 10.54 & 1.16 & 2.53 & 0.47 & 0.27 & 0.002 & 68.55 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of investigated stainless steels (in wt.%).  
To prepare seawater solutions with a sulfide content of 10 and 40 ppm, the mass of Na\({}_{2}\)S salt p.a. purity was calculated, weighed on an analytical balance, and, finally, dissolved in a given volume of seawater in a volumetric flask.  
A three-electrode double-walled glass cell containing the working electrode, Ag/AgCl as the reference electrode, and a Pt-sheet counter-electrode was used for the electrochemical measurement. The Huber Kiss K6 cooling bath thermostat was connected to a cell to maintain the desired operating temperatures (288, 298, and 308 K).  
An EG&G Princeton Applied Research Model 273A potentiostat-galvanostat was used for the electrochemical measurements along with a PAR M 5210 lock-in amplifier (Princeton, NJ, USA) for the electrochemical impedance measurements (EIS).  
The open circuit potential (\(E_{\text{OC}}\)) was recorded every 20 s after the working electrode was immersed in the electrolyte for 60 min. EIS measurements were performed at the \(E_{\text{OC}}\), with the voltage perturbation amplitude of 10 mV in the frequency range from 50 kHz to 30 mHz with 5 points per decade. Linear polarization resistance (LPR) measurements were recorded in a potential range of \(\pm\)20 mV versus \(E_{\text{OC}}\) with a scan rate of 0.2 mV s\({}^{-1}\). Potentiodynamic polarization measurements (PD) were carried out with a scan rate of 1 mV s\({}^{-1}\) starting from \(-\)250 mV versus \(E_{\text{OC}}\) up to 800 mV. All electrochemical measurements were performed in triplicate to ensure the reproducibility of results.  
The optical microscope MXFMS-BD (Ningbo Sunny Instruments Co., Ningbo, China) with 100\(\times\) magnification was used to examine the corroded steel surfaces after the potentiodynamic polarization measurements, and the depth of pitting corrosion was examined with a Profilm3D 3D optical profilometer (KLA Corporation, Milpitas, CA, USA). Prior to these surface examinations, the electrode surfaces were immersed in deionized water, cleaned in an ultrasonic bath for 5 min, and then dried in a laboratory dryer at a temperature of 323 K.  
The morphology of the corrosion products and the elemental composition in some characteristic areas of the sample surfaces were determined using a field emission scanning electron microscope (FEG SEM) Thermo Scientific Quattro S (FEG SEM, Hillsboro, OR, USA) connected to an EDS SDD Ultim(r)Max detector, Oxford Instruments, for semi-quantitative analysis. For SEM/EDS analysis, the electrode surfaces were dried without ultrasonic cleaning.