Thin Solid Films 410 (2002) 76–85 0040-6090/02/$ - see front matter  2002 Elsevier Science B.V. All rights reserved. PII:S0040-6090 Ž 02 . 00253-5 Study of the formation of chromate conversion coatings on Alclad 2024 aluminum alloy using spectroscopic ellipsometry P. Campestrini *, S. Bohm , T. Schram , H. Terryn , J.H.W. de Wit a, b c c a,b ¨ NIMR, Netherlands Institute for Metals Research, Rotterdamseweg 137, P.O. Box 5008, 2600 GA Delft, The Netherlands a Department of Materials Science, Section Corrosion Technology and Electrochemistry, Faculty of Applied Science, b Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands Department of Metallurgy, Electrochemistry and Materials Science, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussel, Belgium c Received 17 July 2001; received in revised form 2 February 2002; accepted 5 March 2002 Abstract The influence of pH of a chromate bath on the morphological and chemical properties of a chromate conversion coating, formed on Alclad 2024-T3 aluminum alloy, has been investigated by variable angle spectroscopic ellipsometry in the visible and infra-red regions. Other techniques such as glow discharge optical emission spectroscopy, transmission and scanning electron microscopy, atomic force microscopy and Auger electron spectroscopy have been used in order to confirm and sustain the results obtained with this technique. The combination of different analytical methods showed a decrease in thickness together with changes in the morphology and chemical composition of the chromate film when the pH is increased from 1.2 to 2.4. Although the complexity of the chromate system and the pronounced roughness of commercial rolled aluminum limit the accuracy of spectroscopic ellipsometry, it is demonstrated that this technique can be usefully applied to the study of thin chromate films formed on industrially relevant aluminum products.  2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminium corrosion; Ellipsometry; Surface composition 1. Introduction Although chromate conversion coatings are under ecological pressure due to the presence of toxic w1x hexavalent chromium compounds, they are still largely used as pretreatment for aluminum alloys, especially in aircraft application. This is mainly due to the excellent corrosion behavior of these conversion coatings, which is known for more than 40 years. Yet the mechanisms of nucleation, growth, and corrosion protection of chro- mate films are not completely clear. To better understand the phenomena that take place during treatment and their dependency with the conversion bath parameters (temperature, pH, time, etc.) a detailed surface charac- terization is required. A number of studies involving transmission and scanning electron microscopy (TEM and SEM) w2–4x, X-ray photoelectron spectroscopy (XPS) w4–6x, Auger electron spectroscopy (AES) w5,7x, *Corresponding author. Tel.: q31-15-278-1849; fax: q31-15-278- 2591. E-mail address: P.Campestrini@tnw.tudelft.nl (P. Campestrini). X-ray absorption near edge spectroscopy (XANES) w8– 10x have been used to study film formation. In recent years, at the Vrije Universiteit in Brussel the applicabil- ity of spectroscopic ellipsometry (SE) as a non-destruc- tive alternative method in the study of both thin conversion coatings w11–15x and anodic oxide films w16,17x on aluminum has been established. It has been shown the possibility to determine the thickness and morphology of the film using a suitable optical model and also the chemical composition using infrared spec- troscopic ellipsometry (IR-SE). Indeed, SE measures the variation in polarization of the radiation due to interaction with the system under study. The measured values are expressed as the two ellipsometric angles (C and D), which are related to the ratio of the Fresnel reflection coefficients, in the function of the wavelength (l). Fitting the spectra of these two parameters in the visible range using a proper optical model enables to determine the thickness and the optical constants of the film. Besides, the C spectra in the infrared region provide information concerning the chemical composi-