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The occurrence of preferential grain etching (PGE) during alkaline etching of aluminum extrusion alloys from the 6XXX series is often linked to the presence of certain impurity elements such as zinc, causing an undesired etching appearance. In the presented work, an additional culprit in this context is identified, which has not been investigated yet. A clear relation between PGE and the presence of a subsurface shear layer is identified for extruded Al 6060 alloys containing 0.02 and 0.06 wt% Zn. This shear layer can be distinguished from the bulk of the metal by its difference in crystallographic texture as visualized by electron backscatter diffraction (EBSD). For the Zn enriched alloy, the <111>//ND grains are etched away faster than grains with other orientations, resulting in the grainy appearance typical for PGE. Independent of the Zn content in the alloy, once the shear layer is removed and <111>//ND grains are practically absent on the new surface, the depths variations caused by preferential etching disappear. Instead, the surface of the alloy is attacked uniformly by the caustic etch bath. [ABSTRACT FROM AUTHOR]

The Zinc level in many 6000 Al alloys (Al-Mg-Si-type) was set to a maximum of 0.03 wt% as common industrial practice. Concentrations above 0.03 wt% can modify the alkaline etching mechanism causing the surface to go from a desired smooth look associated with a grain boundary attack (GBA), into an undesired speckled appearance due to preferential grain etching (PGE), visible after anodizing. This significantly limits the ambition of reducing the carbon footprint of Aluminum by increasing the amount of recycled material in the production process. Because Cu has been reported to counteract this negative effect of Zn, the present study is dedicated to contribute to the understanding of this interaction and is focused on Zn and Cu in peak aged AlMgSi alloys. The chemical composition of the various precipitates formed in two Al6063 alloys was studied by means of TEM and EDX, whereby Zn was found in the Q phase (AlMgSiCu) grain boundary precipitates. Two alloys, with different content of Cu and Zn were studied. The Zn/Cu ratio in the bulk was similar for both alloys, but the level of Zn in the Cu containing Q particles in the grain boundaries was different. The increased Zn concentration in Q phase precipitates is believed to decrease the potential of the precipitates with respect to the surrounding matrix. Thus, the Cu/Zn ratio in these alloys is extremely important as it defines the potential differences that in turn cause either GBA or PGE. Image 1 • Zinc accumulates in Q phase (AlMgSiCu) grain boundary precipitates (GBPs). • Increasing the Zn level in the alloy, increases the Zn content in the GBPs. • The interaction of Zn and Cu has and effect on the alkaline etching behavior of Al-Mg-Si alloys. [ABSTRACT FROM AUTHOR]