Surface composition of some austenitic stainless steels after different surface treatments

Abstract

Three kinds of stainless steel have been analyzed: AISI 304 ordinary plane sheets, AISI 316 L cold-pressed corrugated sheets, and AISI 316 LN rolled-up and machined plates (the last two materials employed for the vacuum vessel of the Frascati Tokamak FT). Their surfaces were subjected to two different chemical cleaning treatments: one of them based on a detergent, alkaline bath and the other on a brightening, acid bath. Both of these treatments were followed by baking in vacuo to 350°C or more. As a common result outgassing rates below 10−12 Torr l s−1 cm−2 at room temperature were in any case obtained. Nevertheless, the residual content of nonmetallic impurities (mainly oxygen and carbon) found by the depth profile AES analysis significantly varied from sample to sample, depending on the different kind of material (and according to the different chemical process undergone. The adopted brightening bath proved very effective in relieving all the treated surfaces from every kind of impurities, including oxides (oxygen was reduced to the minimum possible amount). The alkaline bath proved more effective in removing carbon, which in any case was reduced to a very small amount. Both the AISI 316 materials used for the vacuum vessel of the FT device showed, after either of the mentioned treatments, fewer nonmetallic impurities than the most common AISI 304 sheets.