A Citrate-Based Coating System for AZ31 Magnesium Alloys

In light of highly active chemical properties of magnesium, surface modification is always essential to control its corrosion behavior in application.  Among the various modification processes, conversion coating is widely used due to its simplicity.  For the past decades, many conversion coating systems, including chromate, permanganate, phosphate, stannate, and cerium-based system, have been developed.  However, with ever-increasing concerns about environmental issues, a green process without the use of potentially hazardous species is required.  This study thus aims at developing a citrate-based coating system for AZ31 magnesium alloys.

  Prior to coating process, the AZ31 plates were abraded using emery paper up to 1200 grit, rinsed with deionized water, and dried by a stream of air. The conversion solution was applied to the AZ31 using a roll coater, and subsequently heated by hot air of approximately 75 °C for drying.

  The citrate-based coating system employs trisodium citrate as solute that is dissolved in aqueous or glycerin solution.  The coating formed in both solutions exhibits an identical surface morphology.  However, with sulfate ions coexisting with citrate, the coating formed in aqueous solution shows a completely different morphology.  Together with the change in the microstructure, the effect of sulfate ions is also observed in electrochemical analyses, including potentiodynamic polarization and electrochemical impedance spectroscopy (EIS).

  To further enhance the corrosion resistance, a post treatment using a citrate-based polyamide, poly(hexamethylene succinimide-co-citramide) (PSICA), is also proposed in this study.  It was found that the total impedance of the AZ31 in EIS increases from around 5,000 to 10,000 Ωcm² after citrate-based conversion coating, and can be further elevated to near 15,000 Ωcm² with the PSICA post treatment.