Analysis of Oxide Film Formed on Type 304 Stainless Steel in 288°C Water Containing Oxygen, Hydrogen, and Hydrogen Peroxide

The oxide film formed on type 304 stainless steel (SS, UNS S30400) in high-temperature, high-purity water containing oxygen (O₂), hydrogen (H₂), and hydrogen peroxide (H₂O₂) was analyzed by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Electrochemical corrosion potential (ECP) measurements also were conducted under various water chemistry conditions. A more rapid shift of ECP to the noble direction was observed when H₂O₂ was present in 288°C water, compared to ECP values measured at the same levels of O₂. This could have resulted from electrochemical and/or chemical changes on the oxide film. AES data indicated a thicker oxide was formed under 200 ppb O₂ or 200 ppb H₂O₂ conditions than under 150 ppb H₂. The oxide film consisted of two layers: the outer oxide layer with different particle sizes and the inner, fine-grained layer. Structures of the outer oxide particles (large or small) formed in 200 ppb O₂, in 200 ppb H₂O₂, and in 150 ppb H₂ were mainly hematite (a-Fe₂O₃), maghemite (g-Fe₂O₃), and magnetite (Fe₃O₄), respectively. The inner, fine oxide layers with an iron chromate (FeCr₂O₄) structure were formed in 200 ppb O₂ and in 150 ppb H₂, while H₂O₂ promoted formation of the Cr-deficient/Ni-enriched magnetite structure (nickel ferrite [NiFe₂O₄]) in the inner oxide layer with the g-Fe₂O₃ structure in the outer oxide particles.