In Al–Mg–Si alloys, MgSi and Si precipitates are reported to activate corrosion processes. However, their impact is not completely clear because only indirect studies on the influence of Mg and Si on corrosion processes within Al–Mg–Si alloys are available. For this reason, a detailed electrochemical and compositional characterization of MgSi precipitates in Al–Mg–Si during corrosion processes were performed in this study. It was found that in 1 M NaCl solution, MgSi particles are selectively de-alloyed by Mg, the process starting within the first few seconds and ceasing after about 20 min. The open circuit potential of the whole surface is controlled by the active Mg dissolution within these 20 min. After de-alloying, the MgSi remnant (Si-rich) is cathodic-active. The cathodic current density is raised by a factor of 3 due to the presence of 0.6% surface fraction MgSi remnants. With regard to the anodic activity, it is seen that MgSi remnants (hole with Si enrichment) do not change the pitting potential in electrochemical micro- and macro-cell experiments. Fe-containing intermetallics can be inactive for the first minutes of immersion and then become active within about 40 min of exposure even though the mixed carbon, Al-oxide layer has not completely dissolved.