GROWTH AND CHARACTERIZATION OF OXIDE LAYERS ON ZIRCONIUM ALLOYS

In the range 265-435 degrees C Zr-2.5Nb corrosion takes place in two s tages, as opposed to the cyclic behaviour of Zry-4. The Zry-4 corrosio n stages are described by a single equation, in terms of the dense oxi de layer thickness that decreases sharply at each transition. Tetragon al zirconia is present in the oxide layers of both alloys. In Zry-4, i ts volume fraction decreases as the oxide grows; it is barely discerni ble in Zr-2.5Nb in films below 1 mu m, to later increase up to the tra nsition. In both alloys, compressive stresses are developed associated with the oxide growth. Their relaxation at the transition correlates with the transformation of ZrO2(t) to ZrO2(m) and with the decrease of the dense oxide layer. In Zr-2.5Nb, oxide ridges form on the beta-Zr phase filaments, at the very onset of film growth. The cyclic behaviou r associated with the periodical breakdown of the dense oxide layer is therefore blurred, although optical microscopy shows that the scale r etains the multilayered structure typical of Zry-4.