Distribution of Tritium in the Near Surface of Type 316 Stainless Steel

The distribution of tritium in the near surface of Type 316 stainless steel has been measured using a combination of a zinc chloride (ZnCl₂) wash and acid etching with diluted aqua regia. This method improves upon etching measurements reported in the literature: Results show depth resolutions of ~10 nm using the diluted aqua regia. The ZnCl₂ wash results show very high surface concentration (~1.5 × 10¹³ Bq/cm³), which decreases by a factor of 10⁶ after etching to a depth of ~10 μm. Further, the tritium concentrations in the near surface (<10 μm) of unmodified stainless steel samples do not change significantly over the course of 233 days, which indicates a quasi-equilibrium state has been reached. Tritium migration to the surface from the subsurface region was measured by etching a sample and then storing it in air for 2 to 4 days. After storing in air, the surface concentrations increased a thousandfold and rapidly decreased to base levels after etching an additional ~2 μm. These measurements indicate that perturbing the quasi-equilibrium concentration profile results in tritium migration to the surface in order to reestablish the prior equilibrium state.