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 V-SCE. (C) 2011 Elsevier B.V. All rights reserved.