Role of intermetallics in the activation of Al-Mg-Zn alloys

A metallurgical investigation of Al-Zn-Mg alloys (Al90Mg5Zn5 and Al95Mg2.5Zn2.5) showed the existence of an aluminium based solid solution and an intermetallic phase finely dispersed in the aluminium matrix. The 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 solution and the intermetallics were of the Al2Mg3Zn3 (tau-phase) type. The preparation of alloys characterised by this morphology allowed the investigation of the effect of this intermetallic on the activation capability of Al-Mg-Zn alloys when exposed to an aggressive environment containing chlorides. The intermetallics act as anodic depolarisant sites producing aluminium barrier oxide film breakdown and alloy dissolution. (C) 1998 Elsevier Science S.A.