A Black Conversion Coating Produced by Hot Corrosion of Magnesium with Deep Eutectic Solvent Membrane

Formation of conversion coatings involves chemical reactions at the solid/liquid interface. A liquid membrane on Mg and its alloys was adopted to fabricate conversion films based on the hot corrosion reactions at the choline chloride-urea deep eutectic solvent (DES) membrane/substrate interface. Colour and corrosion resistance of the conversion films on pure Mg and AZ31B Mg alloys are found to be highly depended on the hot corrosion time. The conversion film possesses a porous surface. The size of crater-like features on the AZ31B is much smaller than that on the pure Mg (similar to 80 tun vs. similar to 0.7 mu m). The conversion film is mainly composed of MgCO3 and MgH2. The AZ31B magnesium alloy after the hot corrosion treatment by DES membrane shows black appearance, and the reflectance is very low (similar to 10%) from visible light to near infrared light (400-2000 nm). Electrochemical polarization measurement and immersion test in NaCl aqueous solution show that the conversion film can improve the corrosion resistance of the substrates obviously.