Corrosion Behavior of Fe, Cr, Ni and their Alloys in Sodium Molybdate Melts

Abstract

A pyrochemical process using alkali molybdate melts has been studied as a candidate reprocessing process for spent FBR oxide fuels, but it was confirmed that SUS316 cladding tubes were corroded in Na{sub 2}MoO{sub 4}-Na{sub 2}Mo{sub 2}O{sub 7} melts in our previous study. Moreover, in the vitrification process of HLLW, the molten phase containing alkali molybdates was generated in a borosilicate glass melt after mixing HLLW, and it is thought that the structural materials such as Ni-Cr alloys suffer from severe corrosion in the molten phase. Therefore, it is necessary to investigate the corrosion resistance of these materials in alkali molybdate melts. In order to understand the corrosion behavior of stainless steel and Ni-Cr alloys in sodium molybdate melts, the corrosion tests of Fe, Cr, Ni, SUS316, 55Ni-45Cr, and Inconel 690 specimens were carried out in Na{sub 2}MoO{sub 4} and Na{sub 2}Mo{sub 2}O{sub 7} melts at 750°C for 2 h. As a result, it was found that the corrosion rate decreased with increasing chromium content in these materials. The Cr{sub 2}O{sub 3} layer was formed on the surface of Cr, 55Ni-45Cr, and Inconel 690 specimens by immersing them in the Na2Mo{sub 2}O{sub 7} melt, and these materials with the Cr{sub 2}O{sub 3} layer showed a lower corrosion rate. We concluded that the corrosion behavior of these materials in the melt was controlled by the Cr{sub 2}O{sub 3} layer. (author)