Influence of Heat Treatments on the Near-Surface Tritium Concentration Profiles

Austenitic stainless steel, type 316, absorbs significant quantities of tritium into the near surface (<1 mu m) on exposure to tritium containing gases at room temperature. The tritium concentrations that develop in the near-surface region (<1 mu m) are shown to persist for several years without changing. The current work shows the effect of heat treatments on the near-surface tritium-concentration profiles in stainless steel. Identical stainless-steel samples were exposed to tritium at room temperature and then were heated to different temperatures between 100 degrees C and 300 degrees C for 2 h under a stagnant argon atmosphere. After this initial preheating, the concentration profiles in the first 10 mu m were measured by using a combination of a ZnCl2 wash and an etching procedure. Tritium was thermally released by heating the samples to 550 degrees C to measure the residual tritium present in the bulk of the sample. The data show two dominant features. First, preheating causes tritium to migrate both out of the sample and deeper than >1 mu m into the bulk. This effect increases with increasing temperature. Second, the depletion of tritium from the near surface does not occur until a temperature of 200 degrees C.