Hydrothermal synthesis of protective coating on magnesium alloy using de-ionized water

High corrosion rate is the "Achilles' heel" of magnesium and its alloys, which has severely limited their applications. Coating on these kinds of materials is an effective way to overcome this weakness. Protective coatings were successfully synthesized on AZ31 magnesium alloy by the hydrothermal method with de-ionized water as mineralizer in this paper. The structure, morphology, and composition of the coatings were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy-dispersive spectroscopy (EDS), respectively. Potentiodynamic polarization tests and immersion tests for 7 and 14days in 3.5wt.% NaCl aqueous solution at room temperature were conducted to evaluate anti-corrosion abilities of coatings. The influences of hydrothermal temperature and time on the thickness of the coating and corrosion resistance were investigated. The results show that the coatings are uniform and compact, composed of hexagonal magnesium hydroxide (Mg(OH) ₂) and a spot of monoclinic aluminum magnesium hydroxide (Mg2Al(OH) ₇). The thickness of coating varied from 2.2μm to 27.2μm, increasing with the hydrothermal temperature and time. Polarization curves and results of immersion tests of coated and uncoated AZ31 substrates demonstrate that coatings can improve the corrosion resistance effectively and the corrosion resistances are mainly increased with the thicknesses of the coatings and increased with hydrothermal temperature if the thicknesses are very close to each other. The static water contact angles of the coatings are all less than 13.5°, whereas that of the substrate is 40.5°, indicating that the coatings are highly hydrophilic. Tape test further verifies that there is a strong adhesion between the coating and the substrate.