Corrosion resistance of nanostructured magnesium hydroxide coating on magnesium alloy AZ31: influence of EDTA

A hexagonal nanosheet Mg(OH)2 coating was prepared through a one-step hydrothermal method using LiOH solution as mineralizer and then modified by ethylenediaminetetraacetic acid (EDTA) to minimize the rapid corrosion of AZ31 Mg alloy. The performance of the coating was evaluated using electrochemical technique, hydrogen evolution measurements, nanoscratch test, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) patterns and field-emission scanning electron microscopy (FESEM). The results suggested that the corrosion rate of bare AZ31 Mg alloys was significantly reduced by one and two orders of magnitude through the protection from Mg(OH)2 coating and modification with EDTA (i.e., EDTA-Mg(OH)2 coating), respectively. FESEM micrographs indicated that the modification in EDTA elicits to the formation of an EDTA-Mg(OH)2 composite with a thickness as twice as that of as-prepared Mg(OH)2 coating. Nanoscratch tests revealed strong adhesion between the composite or Mg(OH)2 coating and the substrate. The study of formation and corrosion mechanisms of the coatings manifested that Mg(OH)2 was first formed near the intermetallic compound AlMn particles and gradually covered the entire surface, wherein the AlMn particles played an important role in the coating growth process. And it also proved that EDTA accelerated the formation of Mg(OH)2.