Hybrid silica sol–gel coatings on Austempered Ductile Iron (ADI)

Sol–gel organic–inorganic hybrid coatings are being increasingly used to protect metal surfaces. The presence of non-hydrolysable groups in their structure can lead to the formation of coatings with properties intermediate between glasses and organic polymers. Crack-free, homogeneous and adherent hybrid silica sol–gel coatings around 1 μm thick were obtained by dip-coating on glass and Austempered Ductile Iron (ADI). Homogeneous multilayer coatings up to 4 μm were obtained on glass substrates, but not on ADI substrates since coating cracked after the densification of a second layer. The hardness of the coating on ADI substrates was slightly higher than that of as received ADI. Preliminary electrochemical impedance polarization spectroscopy (EIS), potentiodynamic polarization and polarization resistance tests in NaCl solution have shown an improvement of corrosion resistance of ADI.