Surface nanotextures on aluminium

Abstract

Chemical polishing and electropolishing of aluminium result in fine cellular textures of nanometre dimensions, which play a role in the electrochemical behaviour of aluminium and its response to further treatment processes such as conversion coating and anodizing. Concerning the latter, the surface texture is highlighted by porous anodic film growth for protection of the substrate. The influence of residual impurities and low additions of alloying elements on the development of the textures is investigated here for superpure aluminium (99.99%) and an Al-0.4wt.%Mn alloy, respectively. Complementary analytical approaches, including atomic force microscopy and transmission electron microscopy, disclose scalloped cellular textures comprising adjacent peaks and troughs, with a dependence on the grain orientation of the aluminium substrate. In addition to texture development, enrichment of copper impurity in the alloy occurs adjacent to the alloy/oxide film interface, as revealed by in-depth elemental profiles generated by r.f. glow discharge optical emission spectroscopy. The absence of similar enrichments of iron and silicon in the alloy suggests that these elements are distributed mainly as fine segregates in the aluminium, unlike copper, which is in solid solution. At low alloying levels, manganese reveals limited enrichment and is also probably present mainly as segregates. The origin of the textures and their dependence on grain orientation are thus considered to be related to the presence and location of relatively high-melting-point elemental impurities in the alloy and their local electrochemical responses relative to the adjacent alloy surface.

Features in aluminium alloy grains and their effects on anodizing and corrosion

“…The optical anisotropic effect produced by the oxide has been attributed to the chemically induced surface features of the oxide film [35]. Chemically induced features are also observed when certain alloys are subjected to chemical polishing and electropolishing [36,37]. Although there have been suggestions linking the optical anisotropy effect and the chemically induced features of the anodic oxide layer to the surface profile of the substrate [30,35,37], there is no clear established link to the knowledge of the authors.…”

Crystallographic dissolution of high purity aluminium

“…Early work considered the origin of the texture, with the influence of oxide film development, cellular segregation of impurities and interfacial electrochemical effects variously introduced (Hirsch 1956; Welsh 1956Welsh -1957Cuff & Grant 1958-1959. More recently, the surface texture has been re-examined, and convincing evidence for an explanation based on segregation of impurity elements has been obtained, even for so-called superpure (99.99 wt%) aluminium, after immersion in acid or alkaline solutions (Caicedo-Martinez et al 2002a;Koroleva et al 2002). For superpure aluminium, with residual iron, silicon and copper impurities, the segregates provide local cathodic sites to support the anodic reaction on the adjacent, filmed aluminium matrix during surface treatment.…”

Tensile stress and plastic deformation in aluminum induced by aqueous corrosion

“…The radius of curvature of the convex ridges between dimples is much smaller, roughly 20e30 nm. These ridge and dimple dimensions are typical of patterns produced by etching or chemical polishing of aluminum [31,32]. Spectral analysis of the SEM images was carried out to determine the characteristic length scales of the dissolutioninduced morphology.…”