CHANGE IN TRITIUM-SORPTION PROPERTY OF STAINLESS-STEEL BY THERMAL SURFACE OXIDATION

The purpose of this work was to clarify the influence of bakeout on the tritum-sorption property of a stainless steel surface. After oxidation of the surface of stainless steel by heating in air, its tritium-sorption property and chemical structure was examined. The thermal surface oxidation at 623 +/- 150 K was found to have a desirable effect on depressing the sorption of tritium. The stainless steel surface was constituted of an outer thin oxide layer and of an inner oxide layer; the former consisted of Fe(III) with trace amounts of Ni(II) and Cr(IV) and the latter of Fe(III)/Fe(II)/Cr(III) containing a small amount of Ni(o). From these experimental results it was inferred that the surface oxidized at several hundreds degrees Kelvin consisted of Fe(Fe(1-x)Cr(x))2O4 (x = 0-1) of inverse spinel structure covered with thin Fe2O3 layer containing trace amounts of CrO2 and inverse spinel NiFe2O4 and such a stable surface layer blocked effectively the sorption of tritium onto stainless steel.