抄録
The present authors have studied the structures and chemical compositions of oxide films produced on surfaces of 18–8 and 18 Cr stainless steels in 300°C water and vapor. Effects of the oxidation duration, polishing conditions, and the dryness of the vapor on the structures and chemical compositions of the oxides were investigated through electron microscopy, transmission electron diffraction and X-ray microanalysis.
Main results obtained are as follows :
(1) A corundum type oxide produced on a mechanically polished surface of 18–8 stainless steel at the early stage of oxidation in high temperature water (300°C, 1 hr) is identified to be (Cr, Fe)2O3 alone or its coexistence with α-Fe2O3. The corundum type oxide has been changed to a spinel type oxide containing Ni after a prolonged heating time (24 hr).
(2) After heating in high temperature vapor (dryness : 0.18) of 18–8 stainless steel, the corundum and spinel type oxides are found on both mechanically and chemically (or electrolytically) polished surfaces. In the case of the higher dryness (1.0), however, the corundum type oxides are detected on the three differently polished surfaces.
(3) On heating 18 Cr stainless steel in high temperature water and vapor under the same conditions, the oxide films are of the corundum type containing Fe and Cr. This evidence throws a light on the contribution of Ni to the formation of the spine1 type oxides detected in the initial oxide films of 18–8 stainless steel.
