Hybrid sol-gel/polyaniline coating for the corrosion protection of AA2024.

It is widely accepted that Cr(VI)-containing coatings will not be an acceptable component of any future coatings system due to the adverse health & safety aspects of Cr[6+]. Conductive polymers, such as polyaniline and polypyrrole, have been proposed as potential suitable replacements for chromate containing coatings. However, whilst Polyaniline (PANI) has unique electrical and optical properties; is relatively cheap, easy to synthesise and very stable under a wide variety of experimental conditions, it has not been used widely as a coating due to a lack of adhesion to substrates and poor mechanical properties. Sol-gel technology is finding increasing applications, for example, as hydrophobic self-cleaning and decorative colour coatings, formation of low-temperature cure high purity optical components and biomedical applications. The basic advantage of the sol-gel process is its ability to form inorganic structures and hybrid organic and inorganic network structures at relatively low temperatures using conventional coating techniques such as dip-, spin- or spraying. In this study a novel anti-corrosion coating based upon the combination of a silica hybrid sol-gel system and polyaniline is presented. Chemically prepared PANI and a silica based sol were combined and applied to an Al alloy, AA2024-T3 substrate, to form a protective coating. The corrosion performance of these coated samples was evaluated by Electrochemical Impedance Spectroscopy (EIS). EIS test showed the impedance of the PANI/sol-gel coating remained stable for up to 24 months immersion in 3.5%NaCI solution. A Salt Spray Test (SST) study showed that the PANI/sol-gel coating can pass 500 hrs without showing any sign of corrosion nor delamination. The "self healing" property was also investigated using Scanning Vibrating Electrode Technique (SVET) and scratch test. The corrosion properties of the AA were studied in acidic, alkaline and neutral 3.5% NaCI solutions respectively. The PANI/sol-gel coating showed different mechanisms of corrosion protection according to the solution pH. The surface morphology was characterised by Scanning Electron Microscopy (SEM) which revealed that increasing PANI content increased the coating porosity. Mechanical properties of these coatings, notably adhesion, werestudied using Pull Off and Cross Cut techniques. The sol-gel coating exhibited excellent adhesion; however, increasing the PANI content had an adverse effect on coating adhesion.Differential Scanning Calorimetry (DSC) was used to investigate the curing temperature of PANI/sol-gel coating. The technique showed that the coating was completely cured after drying for 5 hrs at 70°C. The chemical characteristics of the coatings were evaluated using X-Ray Photoelectron Spectroscopy (XPS) and Fourier Transformation Infrared (FTIR) Spectroscopy. The corrosion protection of AA2024 using the PANI/sol-gel coating was attributed to the formation of a complex compound containing the (Al-O-N) group that was produced from an interaction between PANI and the Al substrate. Transmission Electron Microscopy (TEM) was used to assess the interfacial interaction of the coating and metal substrate.