Correlation between the structure and the anticorrosion barrier properties of hybrid sol–gel coatings: application to the protection of AA2024-T3 alloys

Abstract

Hybrid sol-gel materials have been extensively studied as viable alternatives to toxic chromate (VI)-based coatings for the corrosion protection of AA2024-T3 in the aerospace industry, due to the wide range of available chemistries they offer and the tremendous development potential of innovative functional coatings. However, so far, little work has been performed in identifying the effect of the employed chemistries on the structure and anticorrosion properties of the coatings. This work proposes to contribute to a better understanding of the relationship existing between the structure, morphology and anticorrosion properties of hybrid sol-gel coatings deposited on AA2024-T3 aluminium surfaces, the most widely used alloy in the aerospace industry. The sol-gels are prepared employing two hybrid precursors; an organosilane, 3-trimethoxysilylpropylmethacrylate, and a zirconium complex prepared from the chelation of zirconium n-propoxide, and methacrylic acid. The structure of the hybrid sol-gel formulation is modified by altering the concentration of the transition metal complex. The structure and morphology of the coatings are characterised by dynamic light scattering, fourier transform infrared spectroscopy, silicon nuclear magnetic resonance spectroscopy, differential scanning calorimetry, scanning electron microscopy, atomic-force microscopy and the anticorrosion barrier properties characterised by electrochemical impedance spectroscopy and neutral salt-spray. It is found that the transition metal concentration affected the morphology and structure, as well as the anticorrosion performances of the hybrid sol-gel coatings. A direct correlation between the morphology of the coatings and their final anticorrosion barrier properties is demonstrated, and the optimum material amongst this series is determined to be comprised of a concentration of between 20 and 30% of transition metal.

Innovative Formulation Combining Al, Zr and Si Precursors to Obtain Anticorrosion Hybrid Sol-Gel Coating

“…Hybrid precursors are used for their effect on mechanical properties and especially for shaping and flexibility [ 13 ]. To improve the affinity between sol-gel coating and aluminium alloy, an aluminium alkoxide precursor can be used [ 14 ] and for durability and barrier properties, zirconium alkoxide is employed as precursors [ 15 , 16 , 17 , 18 , 19 ].…”

The role of the hydrolysis and zirconium concentration on the structure and anticorrosion performances of a hybrid silicate sol-gel coating

“…The sol-gel synthesis was performed in a three-step process employing MAPTMS (3-methacryloxypropyltrimethoxysilane, Assay 99% in methanol, Aldrich) and ZPO (zirconium (IV) n-propoxide, Assay 70% in propanol, Aldrich) as alkoxide precursors and MAAH (methacrylic acid, C 4 H 6 O 4 , Assay > 98%, Aldrich) as a complexing agent to the ZPO, as described recently [23]. Here, the content of the ZPO complex has been varied along with the hydrolysis degree (HD = H 2 0/M-OR, where M = Si or Zr) to evaluate their impacts on the physicochemical properties of the materials, as summarised in Table 1.…”

“…Here, the content of the ZPO complex has been varied along with the hydrolysis degree (HD = H 2 0/M-OR, where M = Si or Zr) to evaluate their impacts on the physicochemical properties of the materials, as summarised in Table 1. The materials prepared with a 50% hydrolysis degree (B, E, H and K) have been reported in reference Cullen et al [23] but are presented here as comparison with the other materials prepared with other hydrolysis degrees.…”

“…The final stabilisation of the coatings consisted of curing at 120°C for 1 h in order to achieve touch-dry coatings while minimising the effect of the thermal condensation. All characterisation techniques employed here namely DLS, FTIR, 29 Si-NMR, DSC, SEM and EIS have been recently described in detail by us [23].…”

The Role of the Chelating Agent on the Structure and Anticorrosion Performances of an Organosilane—Zirconium Sol-Gel Coatings

“…Recently the authors have investigated the effect of the organosilane/transition metal (TM) ratio [20]. It was found that the concentration of the ZC and the hydrolysis degree has a significant impact on the formation of the siloxane and silicon-ZC oxide species as well as on the overall condensation of the hybrid sol-gel coatings and their anticorrosion performances.…”