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

Abstract

© The Author(s) 2015. 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°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.