**4. Conclusions**

In this study, IGC testing was conducted using oxalic acid on thermally sensitized type 304 stainless steel specimens with varied EPR ratios. The susceptibility of GBs to IGC was investigated based on the GB structure, such as their misorientation and Σ value. For low-angle GBs (misorientations ranging from 5◦ to 15◦), GBs with misorientations of less than 10◦ exhibit little IGC susceptibility, while some GBs with misorientations of 10–15◦ exhibit IGC susceptibility for high DOS specimens. In high-angle

GBs (misorientations >15◦), the IGC susceptibility cannot be characterized by the Σ value, but can be characterized by the unit cell area of CSL boundaries, irrespective of the EPR ratio of the steel. The IGC susceptibility increases with increasing unit cell area, and then levels <sup>o</sup>ff. The IGC susceptibility of the GBs increases with increasing EPR ratio. To easily characterize the IGC susceptibility of CSL boundaries, the unit cell area of *A*CSL, which is simply determined based on atom arrangemen<sup>t</sup> at CSL boundaries, is very useful.

**Author Contributions:** Conceptualization, T.F. (Tomoyuki Fujii) and K.T.; validation, T.F. (Tomoyuki Fujii), K.T. and Y.S.; formal analysis, T.F. (Tomoyuki Fujii), K.T. and Y.S.; investigation, T.F. (Tomoyuki Fujii) and T.F. (Takaya Furumoto); resources, T.F. (Tomoyuki Fujii); data curation, T.F. (Tomoyuki Fujii) and T.F. (Takaya Furumoto); writing—original draft preparation, T.F. (Tomoyuki Fujii); visualization, T.F. (Tomoyuki Fujii) and T.F. (Takaya Furumoto); supervision, T.F. (Tomoyuki Fujii); project administration, T.F. (Tomoyuki Fujii); funding acquisition, T.F. (Tomoyuki Fujii). All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by JSPS KAKENHI Grant Number 17H04899.

**Conflicts of Interest:** The authors declare no conflict of interest.
