Sol–gel coatings with phosphonate functionalities for surface modification of magnesium alloys

Hybrid organic–inorganic coatings with phosphonate functionalities have been synthesized and evaluated as prospective surface treatments for magnesium materials. These coatings have been processed via a sol–gel route by hydrolysis and condensation of a mixture of diethylphosphonatoethyltriethoxy-silane and tetraethoxy-silane with variable molar ratios. The coatings morphology and the surface chemistry at the coating/substrate interface have been characterized using scanning electron microscopy, X-ray photoelectron spectroscopy, 31P nuclear magnetic resonance spectroscopy, and time-of-flight secondary ion mass spectrometry. The corrosion protection performance of the coatings deposited on magnesium alloy AZ31B has been examined by a group of electrochemical techniques including potentiodynamic polarization and electrochemical impedance spectroscopy. In addition, a scanning Kelvin probe technique has been used to investigate interfacial properties of the coatings. The improved corrosion protection of phosphonate-containing coatings as compared to pure silica sol–gel coatings has been observed and explained by the favorable combination of barrier properties of the organo-silicate matrix with strong chemical bonding of phosphonate groups to the magnesium substrate.