Behavior of Alloy 800 in sodium contaminated by sodium hydroxide

The behavior of various metallic materials in NaOH-contaminated sodium was studied in the temperature range 475-5500C. It was observed that the presence of NaOH in sodium induced a generalized corrosion of ferritic steels while austenitic material were subjected to intergranular penetrations even without stress. In a second part, alloy 800 was specially investigated, since it was chosen as a standard material for Superphenix steam generators. Parametric tests were carried out under tensile stress to study susceptibility to cracking of this alloy. A device for introducing NaOH into sodium at the experiment temperature was specially designed. The results show that, for high concentration of NaOH (8 weight % and more), there is a real risk of stress cracking. So ruptures are observed in samples which show or not welded areas after 20 hours with 16% of NaOH under 130 N.mm-2 stress. On the other hand, for low initial concentrations (about 1%) and with the same stress conditions, no cracking occurs after 100 hours. Finally surface condition of material has an effect on the formation of the oxided surface layer but this parameter has no significant influence on the sample life-time