Effect of organic chelates on the performance of hybrid sol–gel coated AA 2024-T3 aluminium alloys

Abstract

a b s t r a c tSol-gels are organic-inorganic polymers formed by hydrolysis/condensation reactions of alkoxide precursors, primarily silanes, which have found applications as electronic, optical and protective coatings. These coatings possess important characteristics such as chemical stability, physical strength and scratch resistance. Further performance improvement is achieved through the incorporation of zirconium and titanium based nanoparticles, also formed through the sol-gel process. However due to the inherent difference in the reactivity of the precursors, the hydrolysis of each precursor must be carried out separately before being combined for final condensation. Zirconium precursors are commonly chelated using acetic acids, prior to hydrolysis, to lower the hydrolysis rate.In this body of work various ligands such as organic acids, acetyl acetone (AcAc) and 2,2 -bipyridine (Bipy) were used to control the zirconium hydrolysis reaction and form nanoparticles within the silane sol matrix.Nanoparticle modified coatings formed from the silane sol on AA 2024-T3 aluminium were characterised spectroscopically, electrochemically and calorimetrically to evaluate the potential effect of the different chelates on the final film properties while neutral salt spray tests were performed to study their anti-corrosion performance. Results indicate that the acid ligand modified coatings provided the best performance followed by AcAc, while Bipy was the poorest. In all cases the zirconium nanoparticle improved the protective properties of the sol-gel coating.

Corrosion protection of AA 2024-T3 aluminium alloys using 3, 4-diaminobenzoic acid chelated zirconium–silane hybrid sol–gels

“…Solution A was slowly added to solution B over 10 min and was characterised by an exothermic reaction indicating the hydrolysis of the remaining propoxy bonds on the zirconium precursor, rather than a physical dispersion, was occurring. Following a further 45 min, water (pH 7) was added to the mixture to give a final molar ratio of 2.5:1:1:5 (MAPTMS: Ligand: TPOZ: H 2 O), as used previously [31,32]. For comparative purposes a MAPTMS sol-gel was prepared by combining the silane precursor, dilute HNO 3 (aq) as catalyst, ethanol as solvent and water for hydrolysis with a molar ratio of 1/0.001/2.5/5 (MAPTMS/HNO 3 /ETOH/ H 2 O).…”

“…Assignments of 29 Si NMR signals for monomeric and oligomeric species obtained from the MAPTMS hydrolysis-condensation [31].…”

Electrochemical and Salt Spray Testing of Hybrid Coatings Based on Si and Zr Deposited on Aluminum and Its Alloys

“…Chemical additives, such as organic acids, acetylaceton and 2,2-bipyridine, have often been used as chelating ligands to slow the hydrolysis and (poly)condensation reactions of nonsilicon metal alkoxides. 19 The effect of organic chelates on the performance of hybrid sol-gel coatings deposited on the alloy AA2024-T3 has been studied. 19 Good corrosion properties were achieved when ZTP was chelated with methacrylic acid (MAA).…”

“…19 The effect of organic chelates on the performance of hybrid sol-gel coatings deposited on the alloy AA2024-T3 has been studied. 19 Good corrosion properties were achieved when ZTP was chelated with methacrylic acid (MAA).…”

“…19 The effect of organic chelates on the performance of hybrid sol-gel coatings deposited on the alloy AA2024-T3 has been studied. 19 Good corrosion properties were achieved when ZTP was chelated with methacrylic acid (MAA).20 Similar behavior was observed when hybrid coatings based on TEOS, MAPTMS, ZTP and MAA were applied to AA7075-T6 alloy. 20,21 The chelating reagents also influence the polymerization of the sol constituents, especially the acrylic group that polymerizes in the presence of UV light.…”