Tin (Sn) forms monoatomic wetting layers on aluminium (Al) interfaces under ultra-high vacuum (UHV) conditions. The wetting layer spreads over the Al vacuum interface or over Al grain boundaries via the emergence of Sn atoms from Sn islands. The islands can be generated by sputter-deposition of a 10 nm thick Sn film on a polycrystalline Al underlayer. If the Al/Sn bi-layer is covered by an Al capping layer, Sn will penetrate the capping layer along the grain boundaries and form a wetting layer on its surface. Al surfaces covered by the Sn wetting layer exposed to oxygen (O) are oxidized significantly slower compared to bare Al surfaces. The shape of the adsorption isotherms suggests that the oxidation process involves the formation of oxygen nuclei.Depositing the Al capping layer in the presence of oxygen leads to a striking effect in the optical appearance and the chemical composition of the capping layer: Light microscopy shows that, in the vicinity of Sn islands, the capping layer has a shiny metallic appearance while the residual areas have a dark colour. By secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM), it could be shown that the dark regions contain significantly more oxygen. This effect can be attributed to the suppression of Al oxidation during the growth of the Al capping layer by the presence of Sn in the vicinity of the Sn islands.