Influence of iron addition on the microstructure and the electrochemical corrosion of Al-Zn-Mg alloys

The effect of Fe addition on the microstructural properties and the corrosi on resistance of Al-Zn-Mg alloys submitted to different heat treatments (ca st, annealed and aged), has been studied in chloride solutions using optica l microscopy (OM), scanning electron microscopy (SEM), transmission electro n microscopy (TEM), energy-dispersive X-ray (EDX), cyclic polarization (CP) and open circuit potential (o.c.p.) measurements. The presence of 0.3% Fe in the alloy limited the growth of the MgZn2 precipitates, both in the anne aled and in the quenched specimens. No effect of Cr on the grain size in th e presence of Fe was found because of the accumulation of Cr in the Fe-rich particles. Fe in the Al-Zn-Mg alloys also made them more susceptible to pi tting. Pitting occurred mainly near the Fe-rich particles both, under o.c.p . conditions in O-2-saturated solutions and during the CP.