Breakaway detection was performed with acoustic emission on Zy-4 sample oxidized at 700 A degrees C under Ar/O-2 atmosphere. After breakaway, 42 % of hits had an energy value greater than a transition energy set to 0.3 fJ. These events were associated to the cracking process in the oxide layer. The transition energy was ten times smaller than that obtained in the Ti/TiO2 system. The difference was related to the fissuring mode in the oxide scale: the cracks being located at the metal oxide interface in the case of titanium oxide and in the oxide scale for the Zy-4 case. The Zy-4 oxide scale was stratified with numerous small and wavy lateral fissures, which is contrary to the titanium oxide case where the lateral cracks were periodic, straight and continuous through the oxide layer. Acoustic emission signatures of the small decohesions in the oxide layer of Zy-4 appeared to be less energetic than the interfacial debonding occurring in the Ti/TiO2 system.