Effects of pre-oxidation on metal release from precipitation hardened nickel base alloy

Inconel X-750, which is used in the neutron field, is currently considered to be the major source of activated corrosion products, such as ⁵⁸Co and ⁶⁰Co. In order to reduce these corrosion products, Inconel X-750 was pre-oxidized in air, and the effect of pre-oxidation on metal release into high temperature water was examined by autoclave tests. The oxide film formed on the metal surface was characterized and the relationship between the metal release and the characteristics of the oxide film was discussed. The release rates of nickel, chromium and iron from Inconel X-750 into high temperature water were decreased by pre-oxidation. The oxidation effect on inhibition of nickel release is greater for the higher oxidation temperature and the longer time. The nickel release rate for Inconel X-750 pre-oxidized at 973K for 72ks in air decreased to about 1/5 of that for Inconel X-750 autoclaved at 663K. The oxide film effective for metal inhibition was composed of double oxide layers. The outer layer mainly consists of NiFe₂O₄ and NiO, and the inner layer consists mainly of Cr₂O₃. Such double oxide layers were stable in the high temperature water. It is believed that the aging treatment could be effective for inhibiting the metal release, provided that an appropriate atmospheric condition during the treatment is employed. (author)