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Abstract

The emphasis of this doctoral dissertation is on the corrosion protection of aluminum alloys. As a first step in the investigation, the effect of different pre-treatments of aluminum alloys 7075-T6 and 2024-T3 on their surface properties was studied. Mechanical pre-treatments, water-based grinding and non-water based polishing were studied, while the chemical cleaning consisted of a non-commercial cleaner NaOH and of a commercial cleaner SurTec, both followed by nitric acid desmutting. It was found out that the mechanical pre-treatments have a higher effect on aluminum alloy 7075-T6 than on the 2024-T3. Both chemical treatments decrease the corrosion resistance of these aluminum alloys and their surface became hydrophilic.
After pre-treatment, aluminum alloy’s surface is protected against corrosion using different procedures. Traditionally, chromate conversion or phosphate coatings have been used for this purpose; as chromate coatings are nowadays recognized as toxic, alternative procedures are being sought to replace them. In this doctoral dissertation hybrid sol-gel coatings were regarded as a potentially suitable replacement for chromate coatings. Hybrid coatings were based on (3-glycidyloxypropyl) trimethoxysilane (GPTMS) and tetraethoxysilane (TEOS) and applied on aluminum alloy 7075-T6. SiO2 nanoparticles were added to the precursors ҆s solution to achieve barrier property of coating, while Ce(NO3)3 was added to obtain an active corrosion protection. Optimization of sol synthesis was based on ATR-FTIR spectroscopy and UV-vis-NIR spectroscopy and the opening of the epoxy rings and completion of hydrolysis and condensation reactions were confirmed, based on ATR-FTIR results. A high degree of cross-linking between GPTMS and TEOS was achieved, in situ, during the synthesis and the effect of the curing process on coating ҆s properties was confirmed. A double-layer system was applied on aluminum alloy 7075-T6, where the first layer was doped with Ce(NO3)3 and the second was un-doped. It was found out that cerium has a role in the self-healing only if it is "locked" within the first layer of multi-layer coating. Coating characterization was carried out by the immersion test and electrochemical potentiodynamic measurements and electrochemical impedance spectroscopy. The self-healing effect was confirmed by immersion of un-scribed and scribed coated AA7075-T6 in electrolyte NaCl by using the immersion test, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy and scanning electron spectroscopy equipped with energy dispersive X-ray spectroscopy.
A systematic study of mechanical and chemical pre-treatment of aluminum alloys 7075-T6 and 2024-T3 was performed followed by the development of hybrid sol-gel coating which achieved an efficient corrosion protection of aluminum alloy 7075-T6 in chloride containing electrolyte and represents a potential replacement for chromate coatings.