Structural materials in nuclear reactors are subjected to high temperatures and high radiation fluences, which lead to radiation-induced swelling, embrittlement, precipitation, and segregation. Combined with exposure to corrosive coolant, these effects can lead to significant stress corrosion cracking and ultimate failure. Nanostructured steels and high-entropy alloys (HEAs) have both been shown to have high strength and improved radiation resistance. Additionally, they are predicted to have enhanced corrosion resistance, however, their corrosion behavior needs to be established experimentally. In this study, corrosion behavior of austenitic steels of three different grain sizes (coarse-grained, ultrafine-grained, and nanocrystalline) were investigated and compared to each other as well as to hot-rolled FeNiMnCr10 HEA. Mass loss were measured based on ASTM A262-15. Scanning electron microscopy, energy dispersive X-ray spectroscopy, electron backscatter diffraction and transmission electron microscopy were performed to study the microstructure of the materials after corrosion. Corrosion mechanisms were discussed and compared among different materials.