Tailored Sol-Gel Coatings as Environmentally Friendly Pre-Treatments for Corrosion Protection

Abstract

Concerning the development of new alternatives for replacement of the toxic chromate surface treatments, the aim of the present study is to design and produce new electrochemically active sol-gel thin films of different composition in the ZrO2-SiO2 system, to act as first chemically active layer for pre-treatments of metallic painted surfaces. For its evaluation, a new methodology combining accelerated test, and global and localised electrochemical impedance spectroscopy (EIS) is proposed. For the localised measurements, a new mini-cell system (MCS) is introduced. An active protection mechanism at the cathodic sites underneath the sol-gel thin film was observed. This inhibitor effect is associated with a self-sealing mechanism by the hydrolysis of Zr4+-ions chemically incorporated in the sol-gel network. After the accelerated test, these Zr4+-ions tend to form links different from Si-O-Zr (i.e. zirconium hydroxides and/or oxides). Such new insoluble compounds plug the pores and defects in the sol-gel coatings increasing the low frequency impedance during immersion in NaCl aqueous solutions.