Preparation and characterization of anti-corrosion self-healing and hydrophobic coating on aluminum alloy

Aluminum alloy has been widely used as the basic material in many industries because of its low density and high strength to weight ratio. Poor resistance to corrosion, however, limits the variety of practical applications of aluminum alloy. Traditionally, this problem was faced with the use of chromate conversion coatings which result in effective corrosion protection because of the self-healing property. However, the presence of toxic hexavalent chromium compounds makes those coatings very hazardous to the environment. Consequently, it is necessary to develop new environmentally compliant, chromate (VI)-free alternatives. Recently, sol-gel technology provides a new promising approach to prepare protective coatings on aluminum alloy. It has many advantages, such as the simple operation and environmental protection, etc. In this work, an Ormosil coating was developed on aluminum alloy through the sol-gel method using triethoxyoctylsilane (TEOCS) and tetraethylorthosilicate (TEOS) as the precursors. The sol-gel coatings were deposited by dip-coating method on aluminum alloy substrate. The sol-gel coatings doped with different concentrations of cerium salt (Ce(NO₃)₃·6H₂O) were investigated. Surface morphology, wettability, anti-corrosion and self-healing of the sol-gel coatings non-doped and doped cerium salt were characterized by using atomic force microscopy, contact angle measurements, electrochemical impedance spectroscopy and scanning electrochemical microscopy. The hydrophobic of the coatings was evaluated by means of contact angle measurements. The corrosion resistance of the coatings was investigated by means of electrochemical impedance spectroscopy measurements, and the anti-corrosion, self-healing properties were discussed based on equivalent circuit fitting. The corrosion behavior of the damaged sol-gel coatings was studied by scanning electrochemical microscopy. The results indicated that it had a marked effect on the surface morphology, corrosion resistance and hydrophobicity when cerium salt was added to the Ormosil sol-gel coating. The contact angle of the Ormosil sol-gel coating is about 92.0°. Cerium salt doped coatings have a better hydrophobicity due to marked improvement of the surface morphology. This positive effect was more evident when the concentration of the doped cerium salt is 0.005 mol/L in the silane solution. It was found that cerium salt doped coatings were less resistant to corrosion than non-doped coating at initial immersion. However, the coating doped with 0.005 mol/L cerium salt rendered improved protection after longer time immersion because of the inhibitive action of Ce³⁺. It can be released at the defects, hindering the further corrosion reactions at defective sites and showing the self-healing ability of the doped with cerium salt Ormosil sol-gel coating.