Corrosion resistance of several metals in plutonium nitrate solution

Corrosion behavior of several metals exposed in plutonium nitrate solution was studied. Plutonium nitrate solution with the plutonium concentration ranging from 0.01 to 65g/l was used as a corrosive medium. Specimens made of type 304L (304L) stainless steel, type 310Nb (310Nb) stainless steel, titanium(Ti), titanium-5% tantalum alloy (Ti-5Ta) and zirconium(Zr) were used. Corrosion behavior of these metals in plutonium nitrate solution at the boiling point was evaluated through examining electrochemical characteristics and corrosion rates which were obtained by weight loss measurement. Three immersed corrosion tests, each being 96 hours long, were performed under conditions of continuous heating, no recharge of solution, and specific volume of 8 ml/cm2. Anodic and cathodic polarization curves were measured using a potentio-static method. Polarization was started from the corrosion potential of each metal up to 1.1V vs SCE at a sweep rate of 50 mV/min. The surface of the specimens after undergoing immersed corrosion tests were observed through a scanning electron microscope (SEM). From the results of the corrosion tests, it was found that the corrosion rate of stainless steel, namely 304L and 310Nb, was enhanced by co-existant plutonium in the nitric acid solution. The corrosion potential of stainless steel shifted to the noble region in proportion to the increase of plutonium concentration. It is thought that the shifts in corrosion potential of the stainless steel to the noble region caused an increase in anodic current which brought about an incremental increase in corrosion rate. Valve metals, namely Ti, Ti-5Ta and Zr, showed good corrosion resistance regardless of plutonium concentration. The stainless steel specimen surfaces observed by SEM became rough due to grain boundary corrosion in accordance with increases in plutonium concentration, whereas valve metals showed no signs of corrosion. (J.P.N.)