Electrochimica Acta 49 (2004) 1127–1140 Influence of substrate microstructure on the growth of anodic oxide layers L.E. Fratila-Apachitei a,∗ , H. Terryn b , P. Skeldon c , G.E. Thompson c , J. Duszczyk a , L. Katgerman a a Department of Materials Science & Technology, Delft University of Technology (TUD), Rotterdamseweg 137, 2628 AL Delft, The Netherlands b Department of Metallurgy, Vrije Universiteit Brussel, Electrochemistry and Materials Science, Pleinlaan 2, 1050 Brussel, Belgium c Institute of Science and Technology, Corrosion and Protection Centre, University of Manchester P.O. Box 88, Manchester M60 1QD, UK Received 31 July 2003; received in revised form 17 October 2003; accepted 26 October 2003 Abstract The effects of permanent mold cast microstructure on the growth of anodic oxide layers on three different aluminum substrates (i.e. Al99.8, AlSi10, and AlSi10Cu3, wt.%) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), and laser scanning confocal microscopy (LSCM). The anodic oxidation was performed galvanostatically in 2.25 M H 2 SO 4 , at 0 ◦ C. The oxide layers developed a microscale topography mainly determined by the morphology of aluminum grains and cells. A low amount of insoluble impurities, uniformly distributed, would contribute to the growth of oxide layers with minimum defects and uniform thickness on the pure aluminum substrate whereas for the binary and ternary systems, a fine cell structure and a modified morphology of Si particles would be favorable. The Al–Fe and Al–Fe–Si particles were occluded in the oxide layers next to Si particles, blocking locally the oxide growth whereas Al 2 Cu particles were preferentially oxidized. In addition, the presence of Si particles in the layer influenced pore morphology by development of deflected pores around the particles. © 2003 Elsevier Ltd. All rights reserved. Keywords: Anodic oxidation; Microstructure; Anodic oxide layers; Morphology; Second phase particles 1. Introduction Anodic oxidation of aluminum continues to find appli- cations in areas such as, membrane technology and micro- electronics for production of ordered nanoscale structures [1–10], as a cost-effective and readily to implement process. In these fields, the research is focused mainly on tailoring the process for a desired pore and cell morphology. In the case of aluminum alloys, however, the process still encounters prob- lems related to the growth of sufficiently thick or hard layers (i.e. hard anodizing process), particularly on highly alloyed substrates, such as 2xxx (Al–Cu–Mg), 7xxx (Al–Zn–Mg) wrought and 3xx.x (Al–Si–Cu), and 4xx.x (Al–Si) cast al- loys [11–14]. The reasons are believed to be related to the presence of second phase particles that affects surface reac- tivity during pretreatment and during anodic oxidation, as- sisting secondary reactions that determine local changes in ∗ Corresponding author. Tel.: +31-15-2789083; fax: +31-15-2786730. E-mail address: e.l.apachitei@tnw.tudelft.nl (L.E. Fratila-Apachitei). surface/oxide composition and morphology [15–19]. There- fore, efforts are increasingly directed on understanding the relations between alloys microstructure and anodic behavior, as a necessary step in closing the substrate–(anodic oxida- tion) process–coating knowledge chain, rather than looking only at the process per se or resultant coating properties. This trend is also observed for other types of surface treat- ments (e.g. chemical and electrochemical polishing) and also in corrosion research, being enabled and supported by the availability of the advanced microscopy and electrochemi- cal measurement techniques, like atomic force microscopy (AFM), laser scanning confocal microscopy (LSCM), scan- ning Kelvin probe microscopy (SKPM), scanning vibrating electrode techniques (SVET), medium-energy ion scatter- ing (MEIS) [19–22]. One of the outputs of these studies is the major role of impurities with high melting point or low solid solubility in aluminum, that segregate at cellular boundaries during casting and determine the development of specific local nanotextures following surface treatments [23,24]. Therefore, this research opens the possibility of un- 0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.10.024