Preparation of a self-developed chrome-free colored coating based on a titanium conversion coating on 6063 Al alloy

Abstract

A new conversion coating improved via the combination of a self-developed Mannich base and hexafluorotitanic acid has been studied and compared with chromate conversion coatings and coatings without the Mannich base. The Mannich base was synthesized from polyether amine D230, formaldehyde and catechol easily at 35 °C. The structure of the self-developed Mannich base was determined by Fourier transform infrared spectroscopy (FTIR) and the properties of the conversion coatings were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), a polarization curve, a K₂Cr₂O₇ dropping test and a saltwater immersion test, respectively. The results of SEM, the K₂Cr₂O₇ dropping test and the saltwater immersion test demonstrated that the Ti-Mannich base conversion coating had a better surface morphology and corrosion resistance than those of titanium conversion coatings. The results of FTIR and XPS showed that the main components of the conversion coating were TiO_x, Al₂O₃ and organics. A formation mechanism of the coating was proposed, in which an orange-red chelate compound formed by a chelation reaction of phenolic hydroxyl groups and Ti(IV) could bind to an Al surface later, and the coating appeared yellow because of the chelate. This new conversion coating, which has better performance, is a potential alternative to chromate conversion coatings.