Effect of particle size and concentration of Al₂O₃ on the corrosion behavior of silane-Al₂O₃/MWCNT double-layer coating: towards protecting the AZ91 magnesium alloy

The current research reports the influence of particle size and concentration of Al₂O₃ additive on protective performance of double-layer coating of silane /multi-walled carbon nanotube (MWCNT) deposited on AZ91 magnesium alloy through sol-gel dip coating. The main objective of Al₂O₃ incorporation to the base silica is to seal different types of defects in the coating and increase the surface hardness, anti-wear, and anticorrosion properties of the coatings which can be beneficial for real-world applications of AZ91 magnesium alloy in industries such as automotive, aircraft, and aerospace. For this purpose, several coatings with different Al₂O₃ contents of various sizes (50 nm, 100 nm, 300 nm, and 1 μm) were deposited on AZ91 magnesium alloy with thicknesses of about 5–10 µm. It was found that the optimal Al₂O₃ contents depend on their particle size; such that 1.5, 2, 3, and 5 wt% of Al₂O₃ were determined as the optimal values for particle sizes of 50, 100, 300, and 1000 nm, respectively. Results revealed that incorporation of an optimal amount of Al₂O₃ into double-layer coating improved the corrosion resistance where, the polarization resistance of the coatings containing the optimal contents of Al₂O₃ with particle sizes of 50 nm, 100 nm, 300 nm, and 1 μm were 360.1, 265.3, 240.8, and 157.2 kΩ.cm², respectively. Moreover, the coating comprising 1.5 wt% Al₂O₃ with a particle size of 50 nm exhibited denser, finer, and lower porosity.