CORROSION BEHAVIOR OF FLUOTITANATE PRETREATED AND PAINTED ALUMINUM SHEETS

''Chromium free'' aluminium conversion treatments usually produce a ve ry thin chemical conversion layer; moreover they do not possess the we ll known inhibition properties of chromates. Hence, the corrosion prot ection of the substrate is, in general, less effective than that offer ed by the thicker chromate layers. However, the aluminium/paint adhesi on is noticeably improved by the use of fluotitanate pretreatment bath s. Our previous works also showed that the behaviour of fluotitanate p retreatments can be similar to that of chromatized aluminium surfaces when the aluminium is painted with a protective thermoplastic polyeste r resin. The aim of this work is to study new bath formulations and tr eatment procedures in order to further improve the effectiveness of th is kind of environmentally friendly aluminium pretreatment. Some produ ction parameters such as surface degreasing and pickling, pH and salt concentration of the bath and time of immersion were varied in order t o obtain more efficient conversion layers. Aluminium alloy sheets were pretreated in fluotitanate baths and for comparison, in traditional c hromate baths or they were only degreased and pickled; these samples w ere then studied with and without the application of organic coatings by the use of electrochemical impedance spectroscopy. The painted samp les were coated with epoxy or vinyl resin types. The coatings were kep t sufficiently thin (10-20 mu m) in order to speed up undercoating cor rosion thus highlighting the pretreatment action. By the use of proper testing solutions and of suitable fitting procedures of the impedance data, it was possible to identify the contribution of the pretreatmen t to the corrosion resistance of the painted aluminium alloys. Copyrig ht (C) 1997 Elsevier Science Ltd.