The Effect of Coating Morphology on Anti-Corrosion Behavior of Modified Alumina Coating Over Aluminum Alloy

The nano-alumina coatings over aluminum alloy have been deposited via dip coating method using oxime-modified aluminum(III) isopropoxide as a sol-gel precursor. The influence of important dip coating parameters: withdrawal rate and coating layers on the quality, uniformity and corrosion resistivity were investigated. Taking best withdrawal rate i.e 25 mm min –1 , coating were deposited with 4, 8, 12 and 16 layers. The X-ray diffraction (XRD) pattern of the alumina coating, sintered at 120°C indicates presence of alumina in the amorphous phase. The scanning electron microscopy (SEM) images of coated surfaces suggest appearance of cracks with increasing coating layers. The cross sectional view of the coated samples suggests thickness of coating increases with increasing coating layers. Roughness of the coated surfaces was evaluated using optical profilometric studies. The electrochemical studies, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PD) in 3.5 wt % NaCl aqueous solution support the above SEM results, with increasing coating layers, corrosion resistance decreased because of cracks in the coatings. Lower withdrawal rate 25 mm min –1 with 4 coating layers offered excellent corrosion protection in 3.5 wt % NaCl aqueous solution.