Inhibitor-Doped Hybrid Sol-Gel Films as Pre-Treatment for Aluminium Alloys

The most effective pre-treatments used in aeronautical industry are based on strongly toxic and carcinogenic chromates. Use of organic-inorganic hybrid thin films are recently reported among the most promising approaches for the replacement of chromate based pre-treatment methods for high strength structural aluminium parts.
Sol-gel films are actively investigated during the last decade as possible candidates for environmentally-friendly pre-treatments. However, the important drawback in this case is the lack of active corrosion protection and consequently self-healing ability.
In this work a certain number of corrosion inhibitors for AA2024 were used as additives to the hybrid sol-gel formulations in order to confer active corrosion protection without damaging the coating. The 8-hydroxyquinoline, benzotriazole and cerium nitrate were added at different stages of the synthesis process to understand eventual interaction of the inhibitor with components of the sol-gel system.
The Scanning Vibrating Electrode Technique and Electrochemical Impedance Spectroscopy were employed as two main techniques to characterize the corrosion protection performance of the hybrid sol-gel films doped with inhibitors and to understand the mechanisms of corrosion protection.
The results demonstrate that 8-hydroxyquinoline and cerium nitrate do not affect the stability of sol-gel films and confer additional active corrosion protection effect. In contrast to this, benzotriazole leads to deterioration of the corrosion protection properties of hybrid sol-gel films. Different strategies of immobilization of active inhibitors on nanostructured carriers are suggested in order to diminish the negative role of the corrosion inhibitor on stability of hybrid sol-gel films.