Effect of surface treatments on near surface composition of 316 nuclear grade stainless steel

The chemical composition of the surface and near surface region plays a significant role in determining the corrosion behavior of stainless steels. During industrial processing, stainless steels are often subjected to sandblasting, acid pickling, or electropolishing either for the removal of oxide scales or for generating a smooth surface. The effect of such pretreatments on the topography and surface composition has been investigated for 316 nuclear grade stainless steel using scanning electron microscopy and Auger electron spectroscopy. Sandblasted specimens have a rough textured surface whereas acid-pickled and electropolished specimens are relatively smooth. Sandblasted surfaces contain as much as ∼25 at. % Si and exhibit a low concentration of Ni (∼5 at. %). Surfaces which were acid pickled in HF+HNO3 solution have a chromium oxide outer layer similar to that observed in cold rolled sheet. Acid pickled surfaces and sandblasted surfaces, when subsequently electropolished (in a mixture of chromic and phosphoric acids) revealed (i) enrichment of chromium in the outer ∼200 Å and (ii) a sharp oxide–bulk interface. These results are utilized to explain the corrosion susceptibility and radioactive buildup of nuclear grade stainless steels with different surface treatments.