Effect of treatment time in the Mg(OH)2/Mg-Al LDH composite film formed on Mg alloy AZ31 by steam coating on the corrosion resistance

The corrosion resistant films were fabricated on Mg alloy AZ31 substrates by steam coating method using Al(NO3)3·9H2O aqueous solution as a steam source. The treatment temperature was maintained at 433 K, while the treatment time was varied at 3, 5, 7 and 9 h. X-ray diffraction (XRD) analysis demonstrated that the coated films were composed of a mixed structure of Mg(OH)2 and Mg-Al layered double hydroxide (Mg-Al LDH) phases. As revealed by the scanning electron microscopy (SEM) observation, the surface of coated films had a good uniformity of changing treatment times. The deposition rate increased exponentially with increasing treatment time. Fourier transform infrared (FT-IR) spectra showed that carbonate and nitrate ions were co-existed in the interlayer of Mg-Al LDH. The potentiodynamic polarization curves of the film coated for 7 h exhibited the lowest corrosion current density, which was almost four orders of magnitude lower than that of bare AZ31. The enhanced corrosion resistance was well consistent with the increase of Mg-Al LDH content in the films.