# Effect of Corrosion Characteristics on Long-Term Aging of Austenitic 304 Steel
## 3 Results and Discussion  
### Microstructure Analysis  
Figure 2 depicts the OM photographs of the surface microstructure at each aging treated sample. As shown in Figure 2a, the initial aging state of the austenitized sample showed that the grains had polygonal shape and the typical annealing twins were observed in most grains. [PERSON] et al. reported that annealing twins exist inside the austenite grains in equiaxed grains [11]. Most of all, the grain boundaries are not clear due to no precipitation along the grain boundary. Figure 2b is an optical micrograph after 100 h of heat treatment, and the grain boundaries start to appear clearly, and the \(\delta\)-ferrite appears as a dark island on the \(\gamma\)/\(\gamma\) interface of the grain boundary. Figure 2c is a micrograph of 1000 h heat treatment sample, the grain boundary becomes clearer due to precipitation along the grain boundary, and the grain shape starts to change into a round shape. Figure 2d is a micrograph of 5000 h of heat treatment sample. In addition to the grain boundary, the precipitates can be observed within the grain interior after 5000 h of heat treatment, and this phenomenon is more prominent at 10,000 h heat treatment sample as shown in Figure 2e. As the heat treatment time increased, many precipitates were formed along with \(\gamma\)/\(\gamma\) interface of the grain boundary and also within a grain interior. Figure 1: Heat treatment history of AISI 304 stainless steel. Figure 3 shows the micrographs of grain boundaries observed by the SEM. In the initial state of the austenitized sample, no precipitates were observed along the grain boundary as already mentioned in Figure 2. However, as the heat treatment time increased, the precipitates generated and coarsened along the grain boundaries, which are known as Cr-rich intermetallic carbides [12]. For 100 h aging sample, the most intermetallic carbides M\({}_{23}\)C\({}_{6}\) generated along grain boundaries. However, in addition to the grain boundary, the precipitation also starts to generate within the grain interior as shown in Figure 3c. This precipitation within the grain interior is dominant after 5000 h of aging time. Figure 4 shows the SEM micrographs of \(\delta\)-ferrite phase along the grain boundary. It can be seen that the decomposition of \(\delta\)-ferrite proceeds rapidly with increasing aging time, and the \(\delta\)-ferrite decomposes into M\({}_{23}\)C\({}_{6}\), \(\sigma\)-phase, and new austenite with increasing aging time as is noticed in micrographs [13]. The transformation of the \(\delta\)-ferrite has been known as the decomposition into two other phases [13]. \[\delta\rightleftarrows\mathrm{M}_{23}\mathrm{C}_{6}+\gamma \tag{1}\]  
\[\delta\rightleftarrows\sigma+\gamma \tag{2}\]  
Figure 5 schematically shows the decomposition model from \(\delta\)-ferrite to austenite and \(\sigma\)-phase, as shown in Figure 4. Since the amount of chromium required for the formation of the \(\sigma\)-phase exceeds the amount of chromium contained in the \(\delta\)-ferrite, additional chromium must be supplied. This chromium is supplied from the \(\delta\)-ferrite zone so that austenite can be transformed simultaneously with the \(\sigma\)-phase. Thus, the new austenite is depleted of chromium compared to \(\delta\)-ferrite. Phase balance also  
Figure 2: Optical micrographs of long-term material degradation; (**a**) 0 h, (**b**) 100 h (**c**) 1000 h, (**d**) 5000 h, and (**e**) 10,000 h.  
requires splitting for a nickel. Both the \(\sigma\)-phase/\(\delta\)-ferrite have low content. Therefore, the nickel content is reduced at the \(\delta\)-ferrite/\(\sigma\)-phase interface for \(\sigma\)-phase formation. As shown in Figure 5, the splitting of nickel and chromium is easily seen by diffusion along with the vacancy of austenite and \(\delta\)-ferrite interfaces. This interfacial diffusion promotes the migration of substituted (nickel, chromium) elements as compared to volume diffusion through the bulk phase at relatively low annealing temperatures.