Corrosion behaviour of different tempers of AA7075 aluminium alloy

AA7075 is susceptible to localised corrosion like pitting, intergranular and exfoliation corrosion. Its susceptibility is strongly affected by heat treatments. In order to study the initiation of localised corrosion, solution heat treated, peak-strength (T6) and overaged (T76) tempers were characterised using complementary techniques like SEM-EDS, TEM, scanning confocal laser microscopy, scanning Kelvin probe force microscopy and micro-capillary cell. The localised attack initiated at intermetallics, which caused local dissolution of the matrix. At a later stage intergranular corrosion and exfoliation corrosion took place. The SKPFM investigation proved that the initiation of the attack at the intermetallics was caused by strong potential differences between intermetallics and matrix. The investigation with the micro-capillary cell showed that higher potential differences were associated with more negative local breakdown potentials for areas containing a few intermetallics. Besides, the susceptibility to intergranular corrosion was related to the precipitation of MgZn2 particles at the grain boundaries. It is concluded that the corrosion behaviour of AA7075 is dependent on microstructure changes caused by heat treatment.