Adhesion pretreatment of aluminum by sol–gel processing

A waterborne coating system for aluminum sheets is described with advantageous properties for adhesion pretreatment. The system is based on ethylene glycol-modified silanes and acetic acid-modified titanium or zirconium alkoxides as precursors for sol–gel processing. The precursor mixture was optimized with regard to the influence of the titanium/zirconium, epoxy-substituted silane and water proportion on the shear strength of lap joints. The coatings have sufficient long-term stability of the bonding under corrosive conditions, where the lap shear strength slightly increased during the neutral salt spray test. The latter phenomenon was investigated by infrared reflection absorption spectroscopy and electron energy loss spectroscopy.