Influence of impurities in aluminium on surface treatment

The origin of grain orientation dependent textures on a surface treated aluminium Al-0.2% Mg alloy has been probed by atomic force microscopy, transmission electron microscopy and elemental depth profiling by radio frequency glow discharge optical emission spectroscopy. Magnesium cations are observed to act as a tracer species, assisting definition of the residual alumina film thickness, developed by pretreatment. Copper impurities, enriching in the alloy immediately below the alumina film, act as a marker, assisting definition of the alloy/film interface. Conversely, the absence of iron and silicon in the depth profiles, present in the alloy as impurities at similar levels to copper, suggests their role in the development of the texture. Such elements are considered to be present in the alloy as fine segregates; their location at alloy ridges defines the positions of cathodic sites, with the adjacent matrix undergoing dissolution through the residual alumina film during pretreatment. (C) 2002 Elsevier Science Ltd. All rights reserved.