Abstract
Surface compositions of SUS304 and SUS347 stainless steels at high temperatures were observed in vacuum with Auger electron spectroscopy.
Non-metal elements, such as carbon, nitrogen, phosphorus and sulfur, segregated on the surfaces of stainless steels. These segregated non-metal elements were replaced one by one according to their surface activities, and finally the most surface active element, sulfur, remained on the surface. The order of surface activities of non-metal elements, however, could change if metal elements co-segregated with them. In this experiment, it was found that chromium and niobium co-segregated with nitrogen, whereas iron and nickel did with phosphorus. The order of surface activities of non-metal elements on the surfaces of SUS304 and SUS347 was as follows; carbon<nitrogen (chromium)<phosphorus (iron, nickel)<nitrogen (niobium)<sulfur. The co-segregating metal elements are indicated in the parentheses.
The segregation-isotherm has been derived on the supposition of the interaction between segregants. From this equation, the following two phenomena are deduced; (1) the interaction energy between non-metal element and co-segregating metal element changes the surface activity of non-metal element; (2) when the interaction between metal element and non-metal element becomes strong and the interaction among non-metal elements weak, the co-segregation of metal element with non-metal element is more remarkably observed. This coincided with the following experimental results; (1) the surface activity of nitrogen co-segregating with niobium was larger than that with chromium; (2) non-metal elements with weak interaction, such as nitrogen and phosphorus, co-segregated with metal elements, whereas sulfur, with strong interaction, did not.
