The Research Of The Effect Of Nb And Cu To The Corrosion Resistance Of Zirconium Alloys

Zirconium alloys are widely used as fuel cladding material in PWRs because ofthe low thermal neutron absorption cross-section, good corrosion resistance, and goodmechanical properties. In order to meet the demand for higher burn-up and longerrefueling period in reactor, great efforts have been made to improve the corrosionresistance of zirconium alloys. The water-side corrosion resistance of zirconiumalloys is the main factor that affects the service life of the fuel component, thereforethe water-side corrosion of the zirconium alloy calls for urgent research. Generallyspeaking, most of the zirconium alloys are developed through empirical formulamethod. Although the effects of alloy elements and water chemistry on the corrosionresistance behavior have been widely studied, the deep effects of these two factors aredifficult to understand because of the complicated interaction among various elements.In this paper, high purity zirconium produced by commercial iodide method wasadopted to prepare the zirconium alloys of Zr-xNb (x=0.1%,0.2%,0.1%), Zr-yCu (y=0.05%,0.2%,0.05%), Zr-0.2Nb-zCu (z=0.05%,0.2%,0.05%), Zr-vNb-0.2Cu (v=0.1%,0.1%) and reference remelted with the same process. The corrosion tests wereconducted in static autoclaves with superheated steam at500℃/10.3MPa,deionized water at360℃/18.6MPa and litigated water with at360℃/18.6MParespectively. The microstructure of oxide film formed on the zirconium alloys and theoxide of second phase particles (SPPs) was investigated by high resolution scanningelectron microscopy (HRSEM) and HRTEM. Further, the influence of Nb, Cu on thestructure and corrosion resistance mechanism of the Zr-Nb-Cu alloys wassystemically explored. The rule of the influence of microstructure and corrosionresistance of the Zr-Nb-Cu alloys with different content of Nb, Cu was studied. Theconclusions are summaried as follows:(1) Even in high purity zirconium, impurity element Fe still partly precipitates asZr2Fe SPPs. The Nb element in Zr-0.2Nb alloy can completely dissolve into-Zrwhile some Nb precipitates in the form of Zr (Nb, Fe)2leadings to lower Nb content in-Zr when0.2%Cu was added. If the content of Cu further increases, Nb elementis hard to precipitate as Fe segregates in the Zr2Cu SPPs.(2) The Cu element added in the high purity Zr almost precipitates as Zr2Cu from-Zr. When proper amount of Nb are added in Zr-1.0Cu alloy, the precipitate ofZr2Cu is in smaller size and finer distributed. When β-Zr, stabilized by large amountof Nb, exists in the alloy, Cu element enriches in this second phase rather thanprecipitating as Zr2Cu.(3) Whether adding Nb or Cu, once the total content of alloying elements inZr-Nb-Cu alloy is relatively lower, serious furuncle corrosion appears in500℃/10.3MPa superheated steam. The corrosion resistance Zr-0.2Nb-0.2Cu andZr-0.2Nb-1.0-Cu alloy in500℃/10.3MPa superheated steam is even better thanthat of reference alloy-N18.(4) In500℃/10.3MPa superheated steam, the oxidation behavior of Zr2CuSPPs are different in various alloys. In ones containing Nb element, Cu wasdischarged and distributed in the direction parallel to the O/M interface. Nb elementdelays both the oxidation of Cu and the ionization diffusion process of Cu4O3andconsequently the oxidation of Zr2Cu is postphoned. Proper addition of Nb and Cu isbeneficial to the thick and arrange of the column grain in the oxide film, whichimprove the corrosion resistance.(5) The corrosion behaviors of Zr-Nb-Cu alloys in360℃/18.6Mpa/0.01MLiOH aqueous solution is totally different from that in500℃/10.3MPasuperheated steam. The overall corrosion resistance in former condition is poor andthe corrosion accelerating phenomenon appears too early. It is believed that theexistence of Li+or OH-disturb the growth of ZrO2in the O/M interface.(6) In360℃/18.6MPa/0.01M LiOH aqueous solutions, the small amount ofNb or Cu adding in pure zirconium can delay the corrosion twist. The homogeneouscorrosion rate increases with larger content of Nb, but it can delay the appearance ofthe twist. Increasing Cu content will cause heterogeneous corrosion and deterioratethe corrosion resistance. (7) In360℃/18.6MPa/0.01M LiOH aqueous solutions, the acting mechanismof Nb element is closely related to its high solid solution content in the alloy and thecharacteristics of the delay oxidation. These features can keep the oxide film formedon the O/M interface as integrity. The mechanism of action of Cu element has closerrelations with the fast oxidation characteristics of Zr2Cu. In the process of itsoxidation, large amount of defects, such as micro cracks and pores, will produce andbecome the spread channel of Li+or OH-, thus accelerate the corrosion process.