In this work, the role of the Si phase microstructure in the anodizing behavior of Al-Si alloys is studied. Experiments were conducted using an additive manufactured (AM) Al-Si10-Mg specimen and a cast alloy of approximately the same chemical composition. A systematic characterization of the anodic oxide film was conducted using a variety of surface analysis techniques. Clear evidence is provided through X-ray photoelectron spectroscopy analysis for the almost entire oxidation of Si during the anodizing of the additive manufactured specimens, which can explain the lower anodizing efficiency observed in the AM material compared to the cast alloy. Additionally, a more detailed characterization of the oxide layer revealed that a slightly thinner anodic oxide film is formed at the borders of the characteristic melting tracks resulting from the metal additive manufacturing process. Furthermore, the features observed in the voltage–time response during the anodizing of the AM specimens seem to be closely associated with the dimensions of the Al/Si cells present in the plane parallel to the direction of the oxide front growth.