The Relationship Between The Microstructural Character And Corrosion Behavior In The U-Nb System

Uranium metal is extremely sensitive to the environmental factors such as oxygen,hydrogenation or humid air,and hence easily be corroded.Therefore,uranium corrosion and corrosion protection has always been the focus in the application field of nuclear industry.Alloyed uranium with niobium can effectively improve the corrosion behavior.Good corrosion resistance can be obtained for this binary alloy when a sufficient concentration of solute Nb is uniformly distributed throughout the martensite structure,which can be achieved by solution annealing the high-temperature austenite phase followed by cooling,typically by quenching with water,at a rate greater than the critical quenching rate.Nevertheless,this homogeneous microstructure is a Nb-supersaturated metastable phase owing to the significant difference in Nb solid solubility between y-uranium and ?-uranium.According to the equilibrium phase diagram of U-Nb system,thermodynamically,formation of this phase is expected to result in Nb redistribution,eventually yielding the pearlite microstructure composed of a Nb-depleted phase and a Nb-rich phase in the matrix structure during long-term storage or other conditions involving thermal history.In addition,niobium is also easy to form inclusion with the impurity element carbon in the environment during the alloying process.The chemical distribution statues of Nb will undoubtedly affect the corrosion resistance of U-Nb alloys.However,the relationship between the microstructure and the corrosion remain unclear and have not been verified experimentally.In the present work,the corrosion behavior of the pearlite and martensite microstructure of U2Nb and U5.5Nb alloys under electrolyte solution and hydrogen atmosphere was investigated respectively.And the microstructural mechanism of corrosion behaviors in U-Nb alloys was discussed.This study can improve our understanding of the corrosion mechanism of U-Nb alloys and provide some reference for the evaluation and prediction of the performance of alloy parts.The research results obtained in this thesis are as follows:(1)The surface potential of martensitic U-Nb alloy approximates a two-stage linear increase with the increase of niobium content.When the niobium content is less than 1.0wt%,the surface potential of the U-Nb alloy increases linearly with the niobium content in according to EV=-592.5+160·xNb.In this interval,the added niobium will greatly improve the corrosion resistance of the U-Nb alloys.When the niobium content is greater than 1.0wt%,the surface potential increases linearly with the niobium content in according to EV=-419.4+16.1·x.In this interval,the efficiency of improving the corrosion resistance of the U-Nb alloys by increasing the niobium content is one-tenth of the previous interval.The critical additional amount of alloying Nb element in single-phase U-Nb alloy for the effective improvement of corrosion resistance should be greater than 1.0wt%.The two-segment linear curve is a good explanation for the close corrosion rates of martensitic U2Nb and U5.5Nb alloys,which are an order of magitude lower than unalloyed uranium.(2)The main inclusions in martensitic U-Nb alloys are Nb2C and UN.Among them,the Nb2C has a coherent interface relationship with the matrix microstructure,and there is no component fluctuation in the matrix around the Nb2C inclusions.As the carbon content increases,the UN and C further combine to form U(C,N)composite inclusions,which easily aggregated with Nb2C to form inclusion clusters.There is a significant potential uneven distribution in the cluster zones,in which U(C,N)inclusions possess the lowest potential,and corresponded to the pitting initiation location during the electrochemical corrosion of martensitic U2Nb and U5.5Nb.(3)In the weakly acidic Cl-containing aqueous solution,the U5.5Nb pearlite structure with small lamellar spacing exhibits similar passivation characteristics as the martensite structure;the U2Nb pearlite structure with large lamellar spacing exhibits the characteristics of active dissolution similar to that of unalloyed uranium.The electrochemical corrosion of pearlite U-Nb alloy is a typical pitting behavior at the microscopic level.The initial pitting corrosion occurs in the Nb-depleted lamella with the lowest potential in the microstructure.And then,the pits expand into the alloy matrix along with the dissolution of Nb-depleted lamella,which resulting a porous corrosion damage layer.The larger the lamellar spacing is,in essence,the larger the size of a single niobium deficient phase is,the easier the pitting is to expand into the alloy.However,when the lamellar spacing is small,the pitting expansion will be blocked by the Nb-rich lamella.(4)There are two types of preferred hydride growth orientation in U-Nb alloys with pearlite micro structure:(a)The Nb-depleted lamella.The growth rate of hydride in U2Nb alloy possessed large lamellar spacing is close to that of unalloyed uranium.The growth rate of hydride in U5.5Nb alloy possessed small lamellar spacing is about one order of magnitude lower than that of unalloyed uranium.(b)The deformation bands derived from the expansion stress during the generation of hydride.When the comprehensive strength and toughness of the corrosion product are greater than that of the matrix alloy,the deformation bands will appear in the alloy matrix,and then,further growths of hydride will be preferentially along with the deformation bands.This preferential growth will lead to a surface layer corrosion characteristics,such as the hydriding corrosion behavior of pearlite U5.5Nb alloy.On the contrary,corrosion products will be burst by expansion stress and further growths of hydride will be preferentially along with the Nb-depleted lamella,such as the hydriding corrosion behavior of pearlite U2Nb alloy.(5)The corrosion resistance of quenching martensitic U5.5Nb alloy to hydrogenation is much higher than that of non-alloyed uranium and pearlite U-Nb alloys.During the hydriding corrosion at 100?,no obvious correlation was found between the nucleation of the hydride and the microscopic defects in the martensitic microstructure of U5.5Nb alloy such as transformed twins and surface inclusions,but it was more sensitive to the geometrical shape and topography of the surface.In the process of hydride corrosion of martensite U5.5Nb alloy,the expansion stress accompanying the generation of hydride will result in high-density deformation twinning defects in the matrix micro-region at the front of the hydride,which acts as the preferred growth path for the further growth of the hydride.This preferential growth of hydride eventually led to the macro-hydrogenation corrosion process of U5.5Nb martensite showing surface layered corrosion characteristics.