Corrosion Protection of AA2024-T3 Alloy by Modified Hybrid Titania-Containing Sol-Gel Coatings

Abstract

It is widely known that AA2024-T3 aluminium alloys are susceptible to localized corrosion. Corrosion protection of AA2024 has historically been via chromate treatments and chromate-containing coatings, which are shortly to be banned due to the hazardous and carcinogenic nature of Cr (VI).Hybrid sol-gel coatings are promising systems to replace chromates for the corrosion protection of AA2024. However, due to the porous nature of sol-gel coatings, they have limited protection properties and need to be used together with functional polymers, inhibitors and barrier pigments. Candidate systems for improving the corrosion resistance of sol-gel coatings include; the co-polymer polyvinyl butyral (PVB) for reducing the porosity of organic coatings, the conductive polymer, polyaniline (PANI) as an efficient corrosion inhibitor and glass flake (GF) as an excellent barrier pigment. In this study, a hybrid titania-containing sol-gel coating, designated the 'standard system', was modified by PVB, PANI and GF to investigate the influence of these pigments for the corrosion protection of AA2024. Electrochemical impedance spectroscopy (EIS) results of these coatings immersed in 3.5% NaCl solution revealed that the modification by PVB in a volume ratio of sol-gel/PVB=4:1 can increase the resistance to electrolyte ingress of the sol-gel layer, whilst the PANI in agglomerate-type particles can produce a stable intermediate layer between the coating and substrate. No obvious improvement was observed by the addition of GF. Differential scanning calorimetry and thermogravimetry (DSC/TG) analysis and water contact angle measurements indicated that GF increase the thermal stability and decrease the hydrophobicity of the standard coating, while PVB/PANI had negative and positive effects on both properties, respectively. Cross-hatch adhesion tests showed high adhesion strength between all the modified coatings and alkaline etched AA2024. Fourier transformed infrared spectroscopy (FTIR) analysis suggested that all these pigments affected the hydrolysis and condensation reactions taking place during the sol-gel synthesis.

Date of Award	31 Dec 2014
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Akid (Supervisor) & Rob Lindsay (Supervisor)