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 (O-2), hydrogen (H-2), 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 degrees C water, compared to ECP values measured at the same levels of O-2. 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 O-2 or 200 ppb H2O2 conditions than under 150 ppb H-2. 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 O-2, in 200 ppb H2O2, and in 150 ppb H-2 were mainly hematite (alpha-Fe2O3), maghemite (gamma-Fe2O3), and magnetite (Fe3O4), respectively. The inner, fine oxide layers with an iron chromate (FeCr2O4) structure were formed in 200 ppb O-2 and in 150 ppb H-2, while H2O2 promoted formation of the Cr- deficient/Ni-enriched magnetite structure (nickel ferrite [NiFe2O4]) in the inner oxide layer with the gamma-Fe2O3 structure in the outer oxide particles.