SKPFM and SEM study of the deposition mechanism of Zr/Ti based pre-treatment on AA6016 aluminum alloy

The formation of Zr/Ti based pre-treatments is strongly affected by the microstructure of 6xxx alloys for application in the automotive industry. AA6016 was pre-treated using a fluotitanate/fluozirconate acid based model solution at room temperature. In order to study the mechanism of formation of the pre-treatment, the open circuit potential was measured during the layer formation. The effect of the microstructure was investigated at different stages of the deposition by means of SEM–EDS and scanning Kelvin probe force microscope (SKPFM). The electrochemical behaviour of the pre-treated alloys was characterized by means of open circuit potential measurements and potentiodynamic polarization in aggressive solutions containing chlorides. The deposition of the Zr and Ti containing oxide is an electrochemically driven process. The existence of cathodic sites on the alloy surface is the driving force for the formation of the conversion layer. During the initial stages of dipping in the conversion bath the naturally formed oxide film is removed or thinned due to the presence of fluorides in the bath. Successively, the deposition of the conversion layer initiates onto cathodic intermetallics. The film exhibits lateral growth in the region surrounding the intermetallics, progressively covering the entire surface. The formation of the conversion layer progressively reduces the Volta potential difference between intermetallics and matrix. This potential difference is completely eliminated for relatively long immersion times. This is associated to an improvement of the corrosion behaviour of AA6016, as shown by potentiodynamic polarization curves. Besides, the deposited conversion layer improves the adhesion of an acrylic paint on the alloy.