Optimization of anodizing cycles for enhanced performance M. Curioni, a * T. Gionfini, b A. Vicenzo, c P. Skeldon a and G. E. Thompson a Anodizing of aluminium alloys is often used to improve appearance, corrosion resistance or adhesion with organic coatings, with properties of the oxides tailored by controlling the anodizing conditions. Some electrolytes can be used in relatively wide ranges of concentration, temperature and potential or current, while others display a narrower operational range. Optimization of the anodizing process is a non-trivial task, involving the control of voltage and electrolyte nature, concentra- tion and temperature. In this work, potentiodynamic anodizing is proposed as a tool to characterize rapidly the behaviour of electrolyte/alloy combinations over a wide range of potential. It is shown that each electrolyte/alloy displays a fingerprint response, carrying information on the potential/current intervals suitable for porous oxide growth, on the oxidation behaviour of the second phase material on the alloy surface and on the maximum applicable potential or current. Copyright © 2013 John Wiley & Sons, Ltd. Keywords: anodizing; aluminium; corrosion resistance; porous anodic oxide Introduction The generation of porous anodic oxide on aluminium in non- neutral electrolytes is the result of overlapping phenomena including ionic migration, mechanical displacement and chemical/field-assisted dissolution. [1,2] The electrolyte pH is the main parameter that determines whether a barrier or a porous type film is generated, although, for a given electrolyte compo- sition and pH, temperature and current can also play a role. Regardless, pore nucleation, if any, starts after a pseudo- barrier-type film is grown during the early stages of anodizing. [3] For acidic electrolytes, generating porous oxides, the type and concentration of anion, the potential and the temperature deter- mine the growth rate and the potential intervals where a porous oxide can be obtained without the onset of burning. The growth mechanism depends on the nature of the electrolyte: acids such as sulphuric, oxalic and phosphoric generate well-arranged pore with smooth walls; chromic acid and (hot) borax electrolytes tend to generate less regular pores that display walls with a feathered morphology. [4,5] The applied potential determines the oxide geometry and, in particular, the barrier layer thickness and the pore diameter. [1] For a given electrolyte concentration and temperature, the potential determines the anodizing current and, consequently, the growth rate. Increase in electrolyte concentration and/or temperature generally increases the growth rate, but also the chemical attack at the pore mouths. Due to the interdependence of the previously mentioned effects, prediction of the anodizing behaviour and optimization of practical anodizing cycles are difficult tasks and require extensive experimental effort. In this work, potentiodynamic anodizing is proposed as a practical tool to disclose the anodizing behaviour of a given electrolyte/alloy combination, providing information on suitable anodizing potential ranges, oxidation behaviour of intermetallic particles and appropriate selection of concentration/temperature/potential to obtain a desired oxide morphology at the required growth rate. Experimental Specimens of high-purity aluminium (99.99% wt.) and AA2024-T3 aluminium alloy were degreased in acetone, etched for 30 s in 10% wt. NaOH and desmutted in 30% vol. HNO 3 for 15 s. After etching and desmutting, the specimens were dried in a cool air stream and stored in a desiccator. Prior to anodizing, the specimens were masked to expose a well-defined area to the anodizing electrolyte. Potentiodynamic anodizing was carried out at room temperature with mild stirring in a three-electrode cell at 1 V min 1 , from the open circuit potential to the burning of the specimen, or to a speci fic potential for transmission electron microscopy (TEM) observation. A saturated calomel electrode was used as reference electrode. The electrolytes were selected to cover a wide spectrum of types and included ammonium pentaborate 0.1 mol l 1 (quasi-neutral), borax 0.13 mol l 1 (alkaline), oxalic acid 0.3 mol l 1 and sulphuric acid 0.4 mol l 1 (strong) and tartaric acid 3 mol l 1 (weak). Prior to TEM observation, the specimens were prepared by ultramicrotomy, as explained in detail elsewhere. [6] * Correspondence to: M. Curioni, Corrosion and Protection Centre, School of Materials, The University of Manchester, M13 9PL, England, UK. E-mail: michele.curioni@manchester.ac.uk a Corrosion and Protection Centre, School of Materials, The University of Manchester, M13 9PL, England, UK b Almeco s.p.a, Via della liberazione 15, 20098 San Giuliano Milanese, Milano, IT, Italy c Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano Via Mancinelli, 7, 20131 Milano, Italy Surf. Interface Anal. 2013, 45, 1485–1489 Copyright © 2013 John Wiley & Sons, Ltd. Special issue article Received: 26 June 2012 Revised: 27 September 2012 Accepted: 18 December 2012 Published online in Wiley Online Library: 14 January 2013 (wileyonlinelibrary.com) DOI 10.1002/sia.5222 1485