Electrochemical and Salt Spray Testing of Hybrid Coatings Based on Si and Zr Deposited on Aluminum and Its Alloys

Hybrid coatings based on Si and Zr were assessed for their protection of aluminum and its alloys 2024-T3 and 7075-T6 against corrosion in 0.5 M NaCl. Coatings were prepared from sol synthesized from mixtures of tetraethyl orthosilicate and 3-methacryloxypropyl trimethoxysilane to which various concentrations of zirconium tetrapropoxide and methacrylic acid were added. Short- and long-term electrochemical behavior was assessed. The degree of protection was dependent on the sol composition, ageing time and curing temperature. A coating containing Zr/Si ratio of 0.41, aged for 48 h and cured at 100 degrees C exhibited the best combination of resistance towards general and localized corrosion. Electrochemical parameters obtained under polarization (corrosion current density of 4.4 nA/cm(2) and stability up to 7 V), and under open circuit conditions (impedance in the few tens of Mohm cm(2) after one week immersion) prove that the protection was high. At other conditions, such as higher Zr/Si ratios, shorter ageing time or curing at room temperature, protection was excellent early after immersion but then lessened rapidly. Coatings prepared under optimal conditions provided a high degree of protection over 500 hours salt spray testing, especially on aluminum metal. These coatings have the potential to provide good protection in chloride environments. (c) The Author(s) 2015. Published by ECS. All rights reserved.