ROLE OF INTERMETALLICS IN THE ACTIVATION OF AL-MG-ZN ALLOYS

A metallurgical investigation of Al-Zn-Mg alloys (Al90Mg5Zn5 and Al95M g2.5Zn2.5) showed the existence of an aluminium based solid solution a nd an intermetallic phase finely dispersed in the aluminium matrix. Th e study of the alloy structure using SEM, TEM, EDX and AES showed that , in both the alloys, the alloy matrix was an aluminium a-solid soluti on and the intermetallics were of the Al2Mg3Zn3 (tau-phase) type. The preparation of alloys characterised by this morphology allowed the inv estigation of the effect of this intermetallic on the activation capab ility of Al-Mg-Zn alloys when exposed to an aggressive environment con taining chlorides. The intermetallics act as anodic depolarisant sites producing aluminium barrier oxide film breakdown and alloy dissolutio n. (C) 1998 Elsevier Science S.A.