Corrosion of Vanadium and Vanadium Alloys in Liquid Sodium

The corrosion of vanadium and its binary alloys V-20% Ti, V-10% Al, V-6%Cr, V-5%Nb, V1,5% Zr and V -0,8% Si has been studied at 600° C under experimental conditions representative of the conditions liable to be encountered in a fast neutron reactor. The circuits were fitted with electromagnetic pumps providing a sodium flow of about 2 m/s as well as bypass clean-up systems utilizing cold or hot traps. When sodium purification is performed by hot trap the positive and negative variations of weight are invariably very slight - about 0.1 mg/cm2 month. Neither the structure nor the hardness of the alloys are modified appreciably. A slight surface hardening does however occur in the case of the titanium and zirconium alloys. In the case of cold trap sodium purification the vanadium suffers considerable weight losses (about 7 mg/cm2 month) as a result of the erosion of oxidation products by the flow of sodium. A straight line representing weight losses as a function of time does not pass through the point of origin. It is probable that the metal is initially enriched in oxygen, and that this results in lower weight losses at the start of the exposure period and favours the subsequent formation of oxidation products leading to accelerated metal degradation. In the non-alloyed vanadium very pronounced core hardening occurs. Of the additives investigated titanium and zirconium do not seem very favourable as the weight losses for the relevant alloys are at least equal to those occurring with non-alloyed vanadium. Aluminium and molybdenum additives, which give rise to increases in weight, seem better. However, considerable surface hardening occurs with the V-Al alloy whereas a considerable increase occurs in the core hardness of the V-Mo alloy. On the basis of weight losses it would seem that silicon, niobium and chromium reduce the susceptibility of vanadium to oxidization by a factor of 3; however, these substances also undergo considerable core hardening. Chromium can nevertheless be considered as one of the most valuable additives since it forms a relatively adhesive oxide layer on the corresponding alloys which could result in a long -term slowing up of corrosion kinetics. (author)