Nodular Corrosion of Zirconium Alloy N18

Nodular corrosion was found in N18 zirconium alloy processed by rolling deformation and 780 ℃/5 h heat treatment in the superheated steam of 500 ℃/10.3 MPa. The microstructure of N18 zirconium alloy and the oxidation films were observed by SEM and TEM. The results show that the heat treatment carried out in the (α+β) intercritical temperature range promoted the movement of alloying elements such as Nb, Fe and Cr to the boundaries of α-Zr grains and the formation of Zr(NbFeCr)2 and β-Zr phases, thus led to the severe depletion of alloying elements in α-Zr grains and resulted in nodular corrosion in the heat treated N18 zirconium alloy. It is suggested that the heat treatment of N18 alloy should avoid to be performed in the (α+β) intercritical temperature range for long time so as to prevent nodular corrosion.