Behavior of Second-Phase Particles in Al5052 Alloy during Anodizing in a Sulfuric Acid Solution CSLM Observation

The behavior of second-phase particles during anodizing of Al5052 alloy in 16 wt% sulfuric acid solution was investigated by confocal scanning laser microscopy (CSLM). Two different types of second-phase particles were observed in the Al5052 alloy, Al-Mg particles and Al-Mg-Fe particles. The Al-Mg particles showed a groove-type morphology because of selective dissolution of Mg during anodizing, while Al-Mg-Fe particles showed a protrusion-type morphology in the CSLM height image (top view). The CSLM contrast image of cross sections of the anodic oxide film showed the presence of two different types of imperfections in the anodic oxide film, bright- and dark-type imperfections. The bright-type imperfections were determined to contain metallic iron, indicating that the metallic iron had been transferred from the substrate into the oxide film without oxidation during anodizing; this reflects the incident laser beam strongly to give a bright CSLM contrast image. The dark-type imperfections were explained by a scattering of the laser beam with vacant or irregularly structured regions, resulting from the selective dissolution of Mg.