Microstructural, electrochemical and mechanical characterization of austenitic stainless steels used in high-level radioactive waste storage

An experimental study and a literature review on some metallic alloys used in the high level radioactive waste disposal is accomplished in this work. Types 304, 304L, 316 and 316L stainless steels specimens, not irradiated, are microstructurally characterized by optical, scanning electron microscopy and x-ray diffraction techniques. All those techniques reveal austenitic structures presenting a very small percentage of inclusions (<1%), probably formed by sulfides. Microhardness tests show that type 316 L steel presents the highest relative hardness. The electrochemical studies consisted of anodic potenciodynamics tests and measurements of the repassivation potential in natural aqueous environment and bidestilled water containing 0,01 mol.L-1 of chloride ions and 0,01 mol.L-1 of chloride + 1 mM of peroxide. This last environment try to simulate the effect caused by the radiolysis. Corrosion potential (Ecorr) values become more positive in peroxide environment, showing an increase of 100 to 150 mV. The observed trend for Ecorrvalues can also be noticed for the pitting potential (Epit) values, even so one cannot affirm that increase is significant, being around (450± 50) mV, in all the media studied. The steel that presents the best performance in repassivation of the pits, is the type 304L stool, whose potentials for all studied environment present values in the range of (250 ± 30) mV. (author)