Chemical and topographical properties of tritium-resistant stainless steel surfaces formed by chromium diffusion coating

In order to depress the tritium sorbability (ad- and ab-sorbability), the chromium diffusion coating has been attempted on the surfaces of four kinds of stainless steel, Types 316, 304, 316L and 321, by utilizing a nascent atomic chromium which was generated from mixed powders of metallic Cr, NH4Cl and Al2O3 in an Ar atmosphere. An effective surface suppressing the tritium sorption was obtained for every type of stainless steel. In particular, for the titanium-containing Type 321 stainless steel with a high tritium sorbability, the tritium sorption was considerably reduced, although the coated surface was roughened by the chromium deposition in the form of cluster. It was found that the resistance for tritium sorption was due to the formation of the stable double layer covered with passive film of several nm thick for every type of stainless steel. (orig.)