# Effect of Corrosion Characteristics on Long-Term Aging of Austenitic 304 Steel
## 2 Experimental Details  
The test materials are austenitic AISI 304 stainless steel. The composition of the test specimens is shown in Table 1. It was analyzed by the inductively coupled plasma mass spectrometry (ICP-MS). In order to observe the microstructure of the aging specimens for a long time, the specimens were prepared by cutting them into 10 mm \(\times\) 10 mm in size from the aging specimens and mounting them at low temperature using a mounting resin. The observation surface was polished using alumina paste of 1 \(\upmu\)m in turn after mechanical polishing from #220 to #2000. The surface was corroed by chemical etching, and the corrosion solution was immersed in Vilella's reagent (Glycerol 45 mL + Nitric acid 15 mL + Hydrochloric acid 30 mL) solution at room temperature for about 1 min, corroded, and washed clean. The microstructural variation of long-term aging samples was observed by optical microscope (OM, Cal Zeiss: Axio Vert. A1, GER) and scanning electron microscope (SEM, Seron: AIS 2000C, KOR).  
In order to perform precise surface element analysis, an electron probe microanalyzer (EPMA, Jeol: JXA-8230, JAP) was used. Elemental analysis was performed with an acceleration voltage of 15 kV, a beam current of 20 nA, and a beam size of 1 \(\upmu\)m.  
As shown in Figure 1, the test specimens were subjected to austenitizing treatment at 1200 \({}^{\circ}\)C for 6 h and water quenching, followed by artificial aging at 700 \({}^{\circ}\)C for a determined time. The interrupt test was conducted to fabricate the test samples with different aging time, such as 100, 1000, 5000, and 10,000 h, respectively.  
The austenitic AISI 304 steel is typical stainless steel and has a face-centered cubic (FCC) structure. In general, \(\delta\)-ferrite formed during the casting process is present at grain boundaries. X-ray diffraction analysis was performed to identify the crystal structures. Scan intervals from 30 degrees to 100 degrees using an X-ray diffractometer (XRD, Rigaku; X'pert PRO MPD, JAP) were performed in a 20 scan with  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**C** & **Si** & **Mn** & **P** & **S** & **Mo** & **Ni** & **Cr** & **Cu** & **Fe** \\ \hline
0.038 & 0.62 & 1.48 & 0.014 & 0.025 & 0.40 & 8.68 & 18.27 & 0.60 & Bal \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of AISI 304 stainless steel (wt %).  
0.01 degrees and holding time of 1 s. X-rays were set to a current of 30 mA and a tube voltage of 40 kV under CuKa (\(\lambda\) = 1.54060) target conditions.  
A ferrite scope was used to measure the fraction of \(\delta\)-ferrite presented at the austenite grain boundaries. Ferrite scopes are measured according to the magnetic induction method. The magnetic field generated by the coil interacts with the magnetic component of the test specimen, and the change in the magnetic field causes a voltage change proportional to the ferrite component in the secondary coil. This voltage is evaluated to determine the ferrite component [11]. Ten times were measured for each test piece using this ferrite scope (Fischer; Mp-30E, AUS). For the purpose of magnetic force microscopy observation, the size of the specimen was prepared by machining 10 mm \(\times\) 10 mm \(\times\) 5 mm. The surface was ground for 4 h using a vibratory polishing machine.  
The electrochemical corrosion test was employed in a standard three-electrode system (WonATech; ZIVE SP 1, KOR). The working electrode at which the AISI 304 specimen was exposed with 1 cm\({}^{2}\) area. A saturated calomel electrode was used for a reference electrode, and a platinum wire electrode was also used for the auxiliary electrode. All corrosion test was achieved in a 0.75% NaCl solution at room temperature. The experiment was carried out using the SM6 program, and the potential range was specified from \(-0.5\) V\({}_{\text{sec}}\) to 1.5 V\({}_{\text{sec}}\) and was open-circuit. The scan rate was 1 mV/s, and the current (I) range was 10 mA, and the experiment time was about 30 min.