The present authors have studied the structures and chemical compositions of the oxide films produced on the surfaces of 18-8 and 18Cr stainless steels, in 300°C water and vapor under various polishing conditions. Especially, the effects of the oxidation duration, polishing conditions and dryness of the vapor on the structures and chemical compositions of the oxides were investigated through the electron microscope, transmission electron diffraction and the X-ray microanalyzer.
The main results obtained are as follows:
(1) The corundum type oxide produced on the surfaces by mechanical polishing at 300°C for 1 hr is identified to be (Cr, Fe)₂O₃ alone or its coexistence with α-Fe₂O₃. The corundum type oxide has been transferred to the spinel type oxide containing Ni with prolonged heating time (24 hr). This spinel type oxide is presumed to be formed by the solid reaction of NiO with (Cr, Fe)₂O₃ and/or α-Fe₂O₃.
(2) The oxide films formed on the chemically and electrolytically polished 18-8 stainless steels were the spinel type oxides having Ni.
(3) After the heating in high temperature vapor (dryness: 0.18) of 18-8 stainless steel, the corundum type and spinel type oxides were found on both the mechanically and chemically (or electrolytically) polished surfaces. In the case of the higher dryness (1.0), however, the structure difference by the polishing conditions was not observed; i.e., the corundum type oxides were detected on the three differently polished surfaces.
(4) On heating in the high temperature water and vapor of 18Cr stainless steels under the same conditions, the oxide films were of the corundum type containing Fe and Cr. This evidence throws a light on the contribution of Ni to the formation of spinel type oxides detected in the initial oxide films of 18-8 stainless steel.