PROCESSES OCCURRING DURING BREAKDOWN OF OXIDE FILMS ON ZIRCONIUM ALLOYS

It has been possible to deduce the sequence of events occurring during oxide film breakdown on some zirconium alloys by combining the use of a mercury porosimeter, designed to measure the size distribution of cracks and pores in insulating oxide films, with an electron microscope study of the same oxide films. The investigation has shown that the primary cause of the transition in the oxidation kinetics is the generation of a network of small pores (10-50 Å radius). These pores penetrate to the oxide/metal interface and no impervious barrier layer of oxidr is apparent during post-transition oxidation. The occurrence of large cracks due to temperature cycling, or stresses generated by the film growth is a subsidiary and often minor contributor to the processes participating in oxide breakdown. They are probably the cause of cyclic post-transition oxidation, although not of transition itself. The small pores which lead to transition are thought to be generated by an oxide recrystallisation process. © 1969.