Corrosion protection of Clad AA2024 Anodized in Sulphuric and tartaric-Sulfuric Acide Sealed with Sol-Gel Hybrid Coatings, 10th Symposium on Electrochemical Methods in Corrosion Research

The 2xxx and 7xxx series aluminium alloys are frequently used in the aircraft industry because of their high specific strength and light weight [1]. These alloys include elements, as copper, used to improve their mechanical properties, however forming microscopic galvanic couples which result in lower corrosion resistance. To improve their corrosion resistance, these alloys require different surface treatments, as anodizing, which have traditionally been performed in chromic acid bath. This treatment is normally followed by sealing in aqueous potassium dichromate solution. The anodizing process based on Cr (VI) compounds is widely used to create layers affording excellent corrosion protection [1-5]. Nevertheless, because of the toxicological and environmental problems associated with Cr (VI), a range of developments of Cr (VI)-free surface treatments has emerged in recent years. Researchers have focused on the replacement of chromic anodizing by sulphuric acid anodizing (SAA) [1-5]. The research has increased and several attempts have been made with new additives in the anodizing bath such as molybdate, cerium (IV) or permanganate anions or using alternative acids as boric or tartaric acids. This latter is sometimes used in anodizing bath for pure aluminium due to its ability to form anodic films with self-ordered porosity [2-5]. Due to this, a new anodizing procedure was proposed involving the addition of tartaric acid in dilute sulphuric acid bath, called tartaric-sulphuric acid anodizing (TSAA) [4].
The use of Cr (VI)-free sealing products has also been investigated such as rare earth metal salts, nickel acetate, triethanolamine or simply boiling water [2]. Recently a promising approach for sealing by the sol-gel route was studied by Zemanová and Chovancová [5] the principle of which is to form a thin protective layer on the anodized surface. The thin hybrid sol-gel films have been reported to be environmentally friendly alternatives to replace chromate conversion coatings. They are based on a hybrid organic-inorganic coating formed by hydrolysis and condensation reactions of metal or silicon-alkoxydes precursors, forming thin, dense and chemically inert films on substrates [6,7]. The organic compound provides flexibility, reduces defects and improves compatibility with polymer coatings while the inorganic constituent provides the superior adhesion to the metal surface and improves scratch resistance. The hybrid organic-inorganic sol-gel films are one of the most promising alternatives to pre-treat aluminium alloys because they are able to close the gap between the inorganic substrate and the organic primer, offering effective barrier protection and good adhesion [6,7]. The researchers have developed hybrid organic-inorganic sol-gel coating sintered at low temperatures offering higher resistance and being thicker than conventional chemical conversion coatings [8]. However, the hybrid sol-gel coatings do not present self-healing properties, then, to overcome this drawback, nanoparticles or inhibitive species can be introduced to reinforce the anticorrosion properties of the hybrid matrix [8,9].
This work presents the comparative study of the corrosion resistance of clad AA 2024 T3 anodized either in sulphuric or in tartaric-sulphuric anodizing baths and then sealed with boiling water or hybrid organic-inorganic coatings applied by the sol-gel route. The investigation was carried out by means of electrochemical techniques and scanning electron microscopy (SEM) coupled to an energy dispersive X-ray spectrometer analyzer (EDX).