Effect of Oxygen Concentration on the Corrosion Behaviour of Coated and Uncoated 316L Stainless Steel in Liquid Lead

The 316L stainless steel, uncoated and coated with two types of EB-PVD thin-film deposits, was tested in liquid lead both under oxygen-saturated conditions (~10−3 wt.%) for exposure times of 1000 and 2000 h and under low-oxygen conditions (~108 wt.%) for 1000 h. The first coating consisted of a ~1 µm NiCrAlY thin film. At the same time, the second was a NiCrAlY/Al2O3 multilayer with a total thickness of ~3 µm, on top of which an additional 100–200 nm metallic Cr layer was deposited. Uncoated specimens tested under oxygen-saturated conditions developed a duplex oxide layer on their surface. SEM-EDS analyses revealed that the inner layer was denser and contained Fe, Cr, and O, whereas the outer layer was more porous and composed mainly of Fe and O. Microscopic examinations indicated that the multilayer-coated specimens exposed to low-oxygen conditions exhibited no signs of material degradation. In contrast, both the uncoated samples and those coated only with a single NiCrAlY layer showed generalised corrosion over the entire surface after exposure to liquid lead at low oxygen concentrations. The austenitic microstructure was degraded to a depth of 100–200 µm. Vickers microhardness indentations performed on the structurally altered regions revealed two distinct corrosion zones with markedly different hardness values.