Corrosion resistance of in situ steam LDH coating on AZ31 and AM30 Alloys: Influence of NaOH and Al–Mn phase

Abstract

In situ formation mechanism of steam Mg–Al layered double hydroxide (Mg–Al–CO3-LDH) coatings on AZ31 and AM30 alloys was compared in presence of NaOH aqueous solution. The microstructure and elemental composition of the obtained coatings were analyzed using SEM, EDS, XRD and FTIR. The corrosion resistance of the coated samples was evaluated using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and salt spray test. The results indicated that the addition of NaOH significantly influenced the morphology as well as the thickness of the prepared LDH coating. The effect of different Al–Mn phase contents of AZ31 and AM30 alloy on the growth mechanism of the LDH coatings was discussed. The addition of 0.01 M NaOH promoted the growth of the LDH coating on AZ31 and AM30 alloys. The AM30-NaOH-0.01 sample possessed the most compact and uniform surfaces as well as the maximum thickness. The corrosion current density of the samples was reduced by three orders of magnitude compared to their substrates. It was revealed that the addition of a moderate amount of NaOH in the steam would raise the pH level, which would benefit the dissolution of the aluminum phase and promote the growth of LDH coating.