# Effect of Corrosion Characteristics on Long-Term Aging of Austenitic 304 Steel
## 3 Results and Discussion
### Corrosion Properties  
The electrochemical polarization test was conducted and extracted the corrosion potential (E\({}_{\text{corr}}\)) and corrosion current (I\({}_{\text{corr}}\)) for the polarization curves using the Tafel extrapolation method, as shown in Figure 9 and Table 2. The corrosion potential of austenitized specimen exhibited \(-293\) mV, and it is slightly increased for 100 h aging to \(-238\) mV. Additionally, for 1000 h of aging, the corrosion potential shows the minimum value of \(-405\) mV. The aging process promotes the active dissolution of test samples due to the generation of grain boundary carbides, which is the metastable intermetallic M\({}_{23}\)C\({}_{6}\). Next, the corrosion potential increased to \(-308\) mV for 5000 h aging and \(-231\) mV for 10,000 h aging, respectively. Despite the difference in corrosion potential during long-term aging, the corrosion current density consistently increased with aging time. The corrosion current density is a significant value, which is closely related to the corrosion resistance more than the corrosion potential [15]. Because it is directly proportional to the corrosion rate based on [PERSON]'s law, the corrosion current density may be considered to estimate the corrosion behavior of long-term aging samples. The corrosion current density of the austenitized specimen exhibited a minimum value of 0.04 uA/cm\({}^{2}\) \begin{table}
\begin{tabular}{c c c} \hline \hline
**Aging Time** & **Icorr [μ/cm\({}^{2}\)]** & **Ecorr [mV]** \\ \hline
0 h & 0.04 & \(-293\) \\
100 h & 0.04 & \(-238\) \\
1000 h & 0.09 & \(-405\) \\
5000 h & 0.473 & \(-308\) \\
10,000 h & 5.832 & \(-231\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The average value of current density and corrosion potential of long-term aging of AISI 304. Figure 8: MFM micrographs at each long-term aging time; (**a**) 0 h, (**b**) 5000 h, (**c**) 10,000 h.  
Figure 9: Polarization curve of long-term aging of AISI 304.  
and it is the same value with 100 h aging due to the not enough sensitization. Then, for 1000 h aging, it increased to 0.09 uA/cm\({}^{2}\) and up to 5.8 uA/cm\({}^{2}\) for 10,000 h aging. During the polarization test, the oxidation on the surface of the test specimen takes place when the cathodic scan ends and the anodic scan starts. The oxide film starts to form very slowly, and when the surface of metal exposed to the test is completely covered, the corrosion current becomes independent of potential. In general, when the material is lower, it means that it is easier for the material to lose its electrochemical reactivity (i.e., passivate current density, I\({}_{\text{pass}}\)) [16; 17]. Easier passivation was observed until 1000 h aging, then it decreased a lot after 5000 h aging. As for pitting resistance, it appears but not clearly with increasing aging time. The highest pitting resistance was exhibited by the austenitized specimen having the highest pitting potential due to the absence of carbides and non-sensitization. Diffusion along the grain boundary is sufficiently rapid and generates intermetallic phases near grain boundaries. The metastable intermetallic Cr-rich phase, M\({}_{23}\)C\({}_{6}\) carbides, are responsible for Cr depletion in the vicinity of the \(\gamma\)/\(\gamma\) grain boundary interface and inside the ferrite phases [18]. This Cr depletion promotes the sensitization near the grain boundary and increases the current density with long-term aging. This phenomenon is dominant after 5000 h aging due to the formation of Cr rich carbides within gran interior, as previously shown in Figure 5. Therefore, the current density increased abruptly and resulted in a decrease in corrosion resistance. This Cr depletion is directly related to the generation of M\({}_{23}\)C\({}_{6}\) carbides. Therefore, we should also consider the size and distribution of carbides during long-term aging when we evaluate the effect of precipitates on the corrosion resistance.