An X-ray microtomography monitored, electrochemically controlled corrosion experiment with high spatial resolution was performed. A modified electrochemical microcell setup was mounted on the sample holder of the synchrotron beamline. This setup enabled in-situ corrosion tomography at a small-defined volume combined with well-defined electrochemical control. Hence high spatial submicrometer resolution and early detection of single corrosion events were possible. Three heat treatments are investigated. I) 45min at 180°C showing high intergranular corrosion susceptibility. Preferential initiation at intermetallics lying on grain boundaries was found with subsequent penetration into the material. Correlation of intermetallics and propagation path could not be found. II) 16h at 180°C showing susceptibility to pitting. An exfoliation-like attack was found which attacks straight into the bulk with a width of approximate 2 μm. III) 111h at 350°C showing a high amount of large MgSi phase. intermetallics and MgSi are not initiation sites for corrosion propagation even though dissolution can occur.