Effect of Mg content on the anodization behavior and optical properties of 5xxx aluminum alloys

Magnesium (Mg), as a principal alloying element in 5xxx series aluminum alloys, significantly influences the growth kinetics and microstructure of anodic oxide films, which in turn affects their optical properties. However, a systematic understanding of the underlying mechanisms remains limited. In this study, six aluminum alloys with Mg contents ranging from 0.00 to 6.00 wt% were systematically investigated to assess the influence of Mg on microstructural evolution, anodic film characteristics, and optical performance. The results revealed that increasing Mg content leads to pronounced grain refinement, with average grain size decreasing from 542 μm to 145 μm. During anodization, the presence of Mg accelerates oxide film growth, increasing film thickness from 11 μm to approximately 27 μm, and induces significant changes in the film's microstructure. Higher Mg levels result in significantly larger pore diameters and porosity, as well as increased interface roughness between the oxide film and substrate. These changes lead to reduced reflectance, particularly in the short-wavelength region (400–550 nm), due to enhanced wavelength-selective light scattering. As Mg content rises, gloss values of the anodized films decrease markedly from 1601 GU to 44.8 GU. Concurrently, CIE-Lab chromaticity measurements reveal that brightness (L∗) diminishes with Mg addition, while hue shifts toward a more neutral and slightly yellow tone. This study clarifies the structural–optical coupling mechanism driven by Mg addition and provides theoretical insight for alloy composition design and surface treatment optimization of aluminum alloys.