Addition of phosphates or copper nitrate in a fluotitanate conversion coating containing a silane coupling agent for aluminium alloy AA6014

Abstract

Abstract Development of environmentally friendly replacements for chromate conversion coatings is an important research topic in the field of aluminium corrosion protection. A novel, commercially available conversion treatment for aluminium substrates is considered in this work to convert the substrate surface for successive application of organic coatings. The substrate is aluminium alloy AA6014. The conversion treatment is a fluotitanate conversion system containing a silane coupling agent. The treatment was carried out by conventional immersion process at room temperature followed by drying at 100 °C for 10 min. After conversion coating, the substrates were painted with a polyester powder coating. The morphology and chemical composition of the conversion layers were investigated by means of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDXS). Moreover, in-depth composition profiles were acquired by glow discharge optical emission spectroscopy (GDOES) in order to study the chemical composition of the coatings and to evaluate film thickness. The electrochemical behaviour of the substrates after conversion coating and after paint application was investigated by means of open circuit potential measurements, potentiodynamic polarization and electrochemical impedance spectroscopy. Moreover, the painted samples underwent salt spray and cross-cut tests. The fluotitanate conversion coatings containing a silane coupling agent provide a good barrier effect to the substrate due to the filming behaviour of the silane component. The addition of phosphates to the conversion coating strongly improves the barrier properties of the conversion film, while the addition of copper nitrate negatively affects the corrosion resistance provided by the conversion film. The conversion coatings exhibit excellent adhesion promotion for paint application.