Preparation of corrosion resistant film on magnesium alloy by steam coating

Corrosion resistant films were prepared on magnesium alloy AZ31 by steam coating using NaNO₃ aqueous solution as steam source. The chemical and corrosive resistant properties of the films were estimated using XRD, FT-IR, SEM, and electrochemical method. XRD patterns revealed that the film was composed of crystal Mg(OH)₂ and Mg-Al system layered double hydroxide (LDH), independent of treatment time. The film grew densely and uniformly with an increase in treatment time. Cross sectional SEM images revealed that the deposition rate was 8 μm/h. FT-IR spectra showed that the Mg-Al LDH had carbonate-ions and nitrate-ions in the interlayer. Potentiodynamic polarization curves of the film treated at 433 K for 7 h indicated that the corrosion current density decreased by more than four orders of magnitude as compared to that of the uncoated one. Immersion test in 5 mass% NaCl aqueous solution at 308 K for 168 h revealed that the chemical composition on the surface of the film treated at 433 K for 7 h changed hardly before and after the immersion test. These results indicates that the film coated AZ31 has a superior corrosive resistance.