Characterization of oxide film formed on Type 316 SS in 288 C water under cyclic NWC/HWC conditions

Characteristics of the oxide film formed on 316 SS in 288 C water under cycling the NWC/HWC conditions were examined by SEM, TEM and AES. It was found that the oxide film mainly consists of three structures having different particle sizes; the outer oxide layer with a large particle (Ni-enriched Fe3O4 type structure), the outer oxide layer with an intermediate size particle and grain (α-Fe2O3), and the inner oxide layer with a very fine grain (Cr-enriched Fe3O4 type structure). In addition, it is evident that under cycling water chemistries (NWC and HWC) the existing outer oxide layer is dependent upon the water chemistry while the inner oxide layer remains unaffected. This may result from the concentration gradient of oxygen through the oxide film upon changing water chemistries. It was also observed that a closely packed oxide with a small particle size was formed under the NWC condition, while a less packed oxide with a large particle size was formed under the HWC condition. In addition, a thicker oxide was formed under the NWC environment than the HWC condition