On the structure of oxide layers of stainless steels formed in simulated light water reactor conditions

The oxide of two types of stainless steels (AISI 304 and 321) formed under conditions typical of boiling water (BWR) and pressurized water (PWR) reactors have been studied by X-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning electron microscopy, energy and wave length dispersive X-ray microanalysis and X-ray diffraction. It was found that the oxide layers of austenitic stainless steel samples studied had a two-layer structure: the outer layer consisted of large (1-3 μm) distinct crystals and the inner uniform layer of small crystals (about 0.5 μm). Divalent and trivalent iron, trivalent and divalent and metallic nickel were found in the layers. The oxide layers formed under PWR-conditions consisted of M₃O₄(M=Fe,Cr,Ni) spinels and those formed under BWR-conditions of M₃O₄ spinels and hematite (α-Fe₂O₃). Enrichment of nickel and depletion of chromium were found in the oxide layers of AISI 304 steel under BWR-conditions and enrichment of iron in the oxide layers of AISI 304 steel under PWR-conditions. Sensitization and plastic deformation had no effect on the structure of the oxide layers. Electrolytically polished exhibited thinner oxide layers than mechanically finished ones.