Formation mechanism of Mg-Al layered double hydroxide-containing magnesium hydroxide films prepared on Ca-added flame-resistant magnesium alloy by steam coating

Corrosion resistant films were prepared on the Ca-added flame-resistant magnesium alloy AZCa612 by steam coating at different treatment time and temperature. The formation mechanism of the films was investigated by using the digital microscope, FE-SEM, XRD, FT-IR, XPS and the potentiodynamic polarization curve measurements in a 5mass% NaCl aqueous solution. XRD, FT-IR and XPS studies indicated that the film was composed mainly of Mg(OH)2 and carbonated-based Mg–Al LDHs. In addition, MgCO3 and AlO(OH) were also incorporated slightly in the film. The formation mechanism of the film on AZCa612 was proposed that amorphous Mg(OH)2 was initially formed by reaction of steam and MgO as a natural oxide film on the substrate. With an increase in the temperature and pressure, the amorphous Mg(OH)2 was crystallized, and further increase in temperature, pressure, and treatment time induced the formation of Mg-Al LDH and AlO(OH) in the film. [Display omitted] •Anticorrosive film was prepared by steam coating treatment on the AZCa612 substrates.•Formation mechanism of the anticorrosive film on the AZCa612 was investigated.•Six types of film on AZCa612 substrates treated for different time were analyzed.•The anticorrosive film formed on the AZCa612 consists of Mg(OH)2 and LDH.•Contacting MgO on the AZCa612 with vapor or liquid water generates Mg(OH)2.