Corrosion behaviour of twin belt cast EN AW 7075 alloy



The corrosion behaviour of the twin belt cast EN AW 7075 alloy is governed by intermetallic phases, namely Al12(Fe,Cr,Mn)3Si, Mg2Si and CuAl2, and by Mg(Zn,Cu,Al)2 precipitates. The former are responsible for pitting activities while the Mg(Zn,Cu,Al)2 precipitates play a key role in intergranular corrosion. The very fine dispersion of Mg(Zn,Cu,Al)2 precipitates in samples aged to peak hardness undergo coarsening, particularly along the grain boundaries, when the hot band samples are overaged. Overageing improves the resistance to intergranular corrosion while the samples in T6 temper suffer heavy attack along grain boundaries. While ageing treatments hardly produce any change in the features of the intermetallic particles, they nevertheless seem to impact the pitting response. This may be accounted for also by the precipitation activities which in turn, change the chemistry of the solid solution matrix. Overageing to the T73 temper implies a higher purity matrix and thus changes the microgalvanic effects when exposed to neutral chloride solutions.