Characterization of the oxide film formed on type 316 stainless steel in 288 C water in cyclic normal and hydrogen water chemistries

Characteristics of the oxide film formed on type 316 (UNS S31600) stainless steel (SS) in 288 C water under cyclic normal water chemistry (NWC) and hydrogen water chemistry (HWC) conditions were examined by scanning electron microscopy (SEM). Transmission electron microscopy (TEM), and Auger electron spectroscopy (AES). It was found that the oxide film mainly consists of two oxide layers: an outer oxide layer with a large particle (nickel-enriched magnetite [Fe3O4]-type structure) and an intermediate or small particle (hematite [α-Fe2O3] structure) and a very fine-grain inner layer with a chromium-enriched Fe3O4-type structure. Under cyclic water chemistries (NWC and HWC) the existing outer oxide layer composition was shown to be dependent upon the water chemistry, while the inner oxide layer remained unaffected. This may result from the concentration gradient of oxygen through the oxide film upon changing water chemistries. It also was observed that a closely packed oxide with a small particle size formed under the NWC condition, while a less-packed oxide with a large particle size formed under the HWC condition. A thicker oxide formed under the NWC environment than under the HWC condition