Effects Of Surface Nanocrystallization On The Corrosion Resistance Of Zircaloy-4

Zirconium alloys are widely used as fuel claddings and internals in water-cooled nuclear reactors, due to its low thermal neutron capture cross-section, reasonable mechanical properties and adequate corrosion resistance in high temperature water. As the first safety barrier in reactors, fuel claddings are attacked by radioactive fission products and high temperature water. Waterside corrosion resistance becomes a key issue to be improved by composition adjustment, structure modification and surface treatment.In this paper, the matrixes and the oxide films of surface nano-structure (SNS)Zircaloy-4 alloy, as well as the coarse-grain structure(CGS) Zircaloy-4 alloy have been analyzed by means of TEM, SEM and XRD. Corrosion experience were operated in autoclave under the condition of 673k/10.3MPa. We draw conclusions as following:1,After high speed shot-peening(HSSP) processing, nano-structure (ns) layer was successfully obtained on the surface of Zircaloy-4.The final structure was characterized by nano-structure grains layer, ultrafine grains structure layer and basal coarse-grains structure. The mean grain sizes of them were several nano-meters, hundreds nano-meters and gradually to several micrometers respectively.2,The grain refinement mechanism during HSSP treatments proposed as follow: the peening loads will generate dislocations and eventually result in plastic deformation by twining in the surface layer of the material. Under repeated peening loads, grain refinement occurs in the random orientation twining grains through necking and closing off of small lengths of twins, as well as the movement of high-density faults . With the increase of peening time, ns layer will be developed and its thickness increases too.3,The TEM show that, after 42 days' corrosion period, the corrosion only occurred on the grain boundary inω-Zr layer of SNS Zicaloy-4,while the CGS Zicaloy-4 corrosion seriously.4,The corrosion power function n of SNS Zicaloy-4 is much smaller than that of CGS Zicaloy-4, which indicates that the SNS Zicaloy-4 is much more protected.5,The transition time of corrosion rate of SNS Zicaloy-4 is about 100 days, however, the CGS Zicaloy-4 occurs at about 42 days, which mean that corrosion process of the former is lag behind than the latter.6,The XRD analysis shows that the value of stress in the oxide film of SNS Zicaloy-4 is higher than that of CGS Zicaloy-4, which is helpful to maintain the tetragonal zirconia, so to low down the corrosion rate. Experimental evidences indicated that the ns Zircaloy-4 has better corrosion resistant property. Not only the grain refinement can influence the corrosion resistance of Zircaloy-4, but also the alloy composition, heat treatment technique, the surface condition, chemistry of reactor coolant, temperature of reactor coolant , irradiation effects and so on. Ether of them changes, the corrosion property is change. Therefore, in order to have a comprehensive knowledge about the corrosion properties of Zircaloy-4, synthetically consideration of these factors are necessary.