Corrosion behaviour of nano structured sol-gel alumina coated 9Cr–1Mo ferritic steel in chloride bearing environments

A stable boehmite sol was synthesized using Al-isopropoxide as a precursor in an appropriate ratio with water. Afterwards, the 9Cr–1Mo steel specimens were coated with prepared boehmite sol by dip coating technique. AFM analysis of the coated specimens confirmed the presence of nano sized particles (8–12 nm) in the coating. Electrochemical measurements like potentiodynamic and potentiostatic polarisations, electrochemical impedance spectroscopy (EIS) in different concentrations of Cl− ions indicated that the sol-gel alumina coating is able to make an appreciable improvement in the corrosion resistance of the base alloy. It was also observed that the sol-gel coatings can control the pitting attack up to a certain extent in 100 ppm Cl− containing solution. However, the coatings become susceptible towards pitting attack in 200 ppm Cl− bearing solution. The SEM micrographs of the corroded surfaces revealed the occurrence of severe pitting on the uncoated specimens in 100 ppm Cl− solution and also on the coated specimen in 200 ppm Cl− solution.