Oxidation and decontamination mechanisms of stainless steels studied by electron spectroscopy

The oxide layers of austenitic stainless steels (AISI 304 and 321) formed under simulated reactor (O2-enriched BWR and typical PWR) conditions have been studied by scanning electron microscopy (SEM), X-ray photo-electron spectroscopy (XPS) and Auger electron spectroscopy (AES). It was found that the oxide on austenitic stainless steels has a layered structure. The outer layer consists of large, distinct oxide crystals and the inner uniform layer of small oxide crystals. Divalent and trivalent iron, trivalent chromium and divalent and metallic nickel exist in the oxide layers. The oxide layers formed under PWR conditions consist of M3O4(M=Fe,Cr,Ni) spinels and under BWR conditions of M3O4 spinels and haematite (α-Fe2O3). Decontamination of autoclaved specimens was performed chemically with various methods, the main one being a two-step method consisting of an alkaline potassium permanganate (AP) step and an oxalic acid, citric acid and EDTA (CITROX E) step. In addition, electrochemical decontamination in phosphoric acid was employed. Alternating oxidation and decontamination tests (three cycles) have been performed. After various treatments the surfaces were analysed with SEM, XPS and AES. Composition changes in surfaces during decontamination affect the oxide growth in subsequent oxidations. (author)