Improved corrosion resistance of AZ31B Mg alloy by eco-friendly flash-PEO coatings

Abstract

The aim of this work was the preparation of an environmentally friendly protective coating on the AZ31B alloy using Flash plasma electrolytic oxidation (F-PEO) process. It was developed with different electrolyte compositions, that determine the morphology and properties of the coatings, this being crucial to understand the anti-corrosion properties. The incorporation of carbonate ions to the electrolyte proved to enhance the electrical response of the F-PEO process, resulting in a more efficient process with an energy reduced consumption of 1.1 kW h m⁻²μm⁻¹. Surface and cross-sectional morphology analysis of the coatings revealed the presence of isolated pores structure with small pore size (less than 1 µm) that delays the infiltration of aggressive ions towards the substrate. The characterisation by XRD, EDX and Raman spectroscopy showed the presence of amorphous carbonate and phosphate phases in the FPEO-CO layer, that provide a self-restauration effect through a dissolution/reprecipitation mechanism. The lowest value of the corrosion current density was obtained for FPEO-CO coating, 4.60 × 10⁻⁷ A·cm⁻², together with the highest impedance modulus (f<0.1 Hz), ⁓10⁴ Ω·cm², two orders of magnitude higher than the AZ31B Mg alloy. Furthermore, the corrosion protection properties of FPEO-CO coating were also analysed through an immersion test in 3.5 wt.% NaCl, confirming the excellent response of the coating for long times up to 336 h (2 weeks). The synergy between a more compact coating and the self-repairing ability of carbonate amorphous species plays a critical role in improving the corrosion resistance properties of the AZ31B Mg alloy, offering an eco-friendly alternative to chromate conversion coatings.