Development of conversion coating on mild steel prior to sol gel nanostructured Al2O3 coating for enhancement of corrosion resistance

A conversion coating was formulated and applied on mild steel prior to sol–gel Al2O3 coating to improve the corrosion resistance of mild steel. The comparison was made between conversion coated and conversion followed by sol–gel Al2O3 coated mild steel. The modified surfaces, produced by conversion coating and conversion followed by sol–gel Al2O3 coating were characterized by X-ray diffraction, Raman and X-ray photoelectron spectroscopy, SEM, and AFM. The corrosion resistance of the sol–gel coating was evaluated in 3.5 wt.% NaCl solution by open-circuit potential measurement, DC polarization, and electrochemical impedance spectroscopy. The sol–gel coating reduced the corrosion current density of the substrate by 5 orders of magnitude and showed barrier type protection of the substrate up to more than 1.0 VSCE.