Abstract
Concentration profiles through the initial oxide films formed on the mechanically or chemically polished surfaces of both fine and coarse grain 18–8 stainless steels in water at 300°C were investigated by means of the in-depth analysis of ion micro mass analyzer.
Regardless of the difference of the grain size of the sample, the oxide films consisting mainly of corundum type oxide containing Fe and Cr were produced on the mechanically polished surfaces. The difference in the concentration profiles of Fe and Cr in the film by the grain size was not observed. Namely, a large amount of Cr was detected in the oxide films formed on both fine and coarse grain samples and the concentration gradient of Cr was extremely sluggish.
On the chemically polished surfaces of both fine and coarse grain samples, spinel type oxides containing Ni besides Fe and Cr were formed. In the outer part of the film Fe and Ni were enriched, while Cr was condensed in the inner part. The enrichment of Fe, Ni and Cr was remarkable in the fine grain sample, and especially an increase of the amount of Cr in the inner part of the film with the oxidation time was clearly observed.
It was made clear that the difference in the structures and concentration profiles of the initial oxide films according to the way of surface polishing and the grain size was due to the variation of the composition of alloy surfaces by the polishing methods employed. That is, the significant impoverishment in Cr was found in the chemically polished surfaces of the alloy, especially in the case of the fine grain sample, while the variation of ratios of Cr to Fe and Ni was hardly recognized in the mechanically polished surfaces of the alloy.
