Abstract <p>The effect of different amount of Sn addition on the microstructure and corrosion behaviour of the Al–Mg–Si alloy was investigated by Optical microscopy (OM), scanning electron microscopy (SEM), and electrochemical techniques. The results revealed that the predominant second phases in Al–Mg–Si–Sn alloys include Mg₂Si, Si, Mn₁₂Si₇Al₅, and Mg₂Si_1–<i>x</i>Sn_<i>x</i>. Sn atoms partially substitute for Si atoms and exists in the form of Mg₂(Si,Sn), resulting in an excess of silicon element in the Al–Mg–Si alloy. The Sn addition can effectively affect the corrosion depth of the Al–Mg–Si alloy. With the increase of Sn content, the open-circuit potential (OCP) and the corrosion current density (<i>i</i>_corr) first increases and then decreases. The positive effect of Sn addition is highly dependent on the Sn content. The addition of a small amount of Sn is beneficial for reducing the width of grain boundary PFZ due to the strong binding of Sn atoms to vacancies. An excess of Sn elements can facilitate the precipitation of Si in elemental form, shifting the alloy’s OCP to the negative. The Mg elements in Mg₂(Si,Sn) exhibit a preference for dissolution during corrosion, leading to the accumulation of Si and Sn elements in the Mg₂(Si,Sn) phase and an increase in intergranular corrosion (IGC) sensitivity.</p>