Studies on Chromium-free Conversion coatings on Aluminum

Abstract

The development of a chromium-free conversion coating on aluminum has been studied using transmission electron microscopy (TEM), Auger Electron (AES) and Secondary ion mass spectroscopy (SIMS) techniques. Within the limits of the resolution of the TEM, the coating is uniformly clear and featureless. It is composed essentially of Zirconium Oxide over a diffused layer of Zirconium and Aluminum Oxides. @JASEM Conversion coatings are formed in-situ by electrochemical reactions with the metal surface. This results in modification of the metal surface, such that the coating formed is an integral part of the parent metal. Such coatings find application as a good base for paint adhesion, where surface features, both micro and macro, may be important in the improvement of paint adhesion on aluminum (Thompson et al 2000). Although, chromate and chromate-phosphate conversion coatings have been in use for decades and they both provide improvement in corrosion resistance and paint adhesion on aluminum. Recent developments have made it possible for the use of non-chromium conversion coatings where effluent problems and toxicity of Cr 6+ species is encountered. Establishments, such as in the canning industries, prefer the use of non-chromium conversion coatings, which is free of any chromium species. In addition, such coating must be uniformly clear, colorless in appearance, and show some improvement in adhesion to subsequently applied polymer coatings. One such non-chromium formulation incorporates a zirconium salt, boric acid and a source of fluoride in the coating solution (Oki, 2006). This type of coating is a relatively recent development, and not much has been published about its morphology, composition and other characteristics. However (Smith et al 1993) have experimented on cerium as a possible substitute for chromium. Results obtained were encouraging. In view of these shortcomings, analytical and transmission electron microscopy techniques have been employed to study the non-chromium coating on aluminum. MATERIALS AND METHODS Materials: Aluminum specimens, 99% purity, of dimensions 30 x 10 x 1mm were electro polished in a perchloric acid/ ethanol mixture for 5 minutes, rinsed in distilled water before drying in a cool air stream. The electro polished specimens were subjected to varying periods of immersion at 298k in a conversion coating bath containing 0.5g/1 Zr (NO 3) 4 , 2g/1NaF, 5g/1 H 3 BO 3 about 1m1 HNO 3 for pH adjustment and tartaric acid. Methods: Prior to transmission electron microscopy examination, ultramictromed sections of the aluminum substrate and the coatings were obtained with LKB III 8800 ultra microtome. The thin samples thus prepared were examined in a Philips 301 transmission electron microscope, with further analysis of the coatings in a Philips EM400T transmission electron microscope. Information regarding coating composition was obtained using a Vacuum Generator's auger electron spectroscopy, with a 3KeV primary electron beam and current of 200 uA. Depth profile analysis was obtained by alternate analysis and in-situ ion beam etching of the specimens. During secondary ion mass spectrometry runs, the coatings were sputtered, using gallium ions with a beam current of 4nA. Appropriate elemental profiles were obtained by monitoring the intensities of instrumentally induced ions of aluminium, fluorine etc for a period of 30 minutes. RESULTS AND DISCUSSIONS Initial visual observation of specimens treated for various times in the coating bath showed that the specimens remained reflective after 15mins of treatment. Indicating that, any coatings formed within this period are thin and colourless. Also any irregularity and roughening at both the coating solution and metal coating interfaces were minimal. The sectional morphology of the ultramictromed section of the specimen treated for 1 min revealed that the initial thin electro polishing film had been replaced by a coating of about 17nm in thickness. The relatively thin coating has undulating metal/coating and coating solution interfaces. The