Protective ability and impedance response of sol-gel reversely transformed ceria conversion coating on aluminium

Abstract

Ceria coating is formed on Al from the ceria sol prepared exclusively by forced hydrolysis of Ce(NO3)(4), in order to test their ability to protect Al from corrosion. The characterization of sol-gel-processed ceria coating and coating/Al assembly brings new issues on Al corrosion and protection caused by unique properties of the sol-gel coating. The corrosion behaviour of bare Al and Al/CeO2 is investigated by the electrochemical impedance spectroscopy (EIS) during the exposure to NaCl aqueous solution. The morphology and composition of the samples are examined by scanning electron microscopy and energy dispersive spectroscopy. EIS data showed greater corrosion resistance of CeO2-coated aluminium in comparison to bare Al at long exposure times. CeO2 coating does not hinder completely the corrosion processes on Al. The Al beneath CeO2 coating is subjected to intrinsic formation of a uniform protective coating/Al interphase, cross-linked by AlO(OH) fibre-like structures. They appear created by corrosive medium from the coating reversely gelled by NaCl solution. As the fibre network is created during exposure, Al becomes better protected than by spontaneous formation of porous passive layer of Al(OH)(3) on bare aluminium. Sol-gel ceria coatings thus improve corrosion resistance of aluminium during prolonged exposure to corrosive medium.