Duplex coating combining layered double hydroxide and 8-quinolinol layers on Mg alloy for corrosion protection

The effects of feeding sequence of reactants on the properties of a MgAl layered double hydroxide (LDH) coating on Mg alloy are reported herein for the first time. Based on the X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra, MgAl-LDH coatings were successfully obtained with different feeding sequences. However, characterizations of scanning electron microscopy (SEM) and electrochemical methods show that the surface morphologies and corrosion resistance of the MgAl-LDH coatings are dependent upon the feeding sequence. Furthermore, the MgAl-LDH coatings were modified in an 8-quinolinol (8HQ) solution by a facile soaking measurement. Energy dispersive X-ray spectroscopy (EDS) maps and X-ray photoelectron spectroscopy (XPS) spectra demonstrate the effective adsorption of 8HQ layer on the MgAl-LDH primer. The high-resolution N 1s XPS spectra manifest the existence of chemisorption owing to formation of a complex chelate, (bis (8-hydroxyquinolinato) magnesium, Mg(HQ)(2)), between 8HQ anions and dissolved Mg2+. The modified MgAl-LDH coatings, which are named as 8HQ@MgAl-LDH coatings in the body text, show significant enhancement in corrosion resistance and remains excellent corrosion protection even exposure to a NaCl aqueous solution for one month. The mechanism of corrosion inhibition for the 8HQ@MgAl-LDH coatings is proposed finally, which mainly includes the increase in water contact angles, Cl- and CO32- anion-exchangeability, and redeposition of Mg(HQ)(2) precipitation. These findings of this study provide some valuable insights into the corrosion protection mechanism of the MgAl-LDH coatings and the designment of a highly corrosion resistant coating on Mg alloy and other lightweight alloys. (C) 2018 Elsevier Ltd. All rights reserved.