Hybrid sol–gel coatings for corrosion protection of hot-dip galvanized steel in alkaline medium

Abstract

Two distinct organic–inorganic hybrid (OIH) matrices were synthesised using the sol–gel method and deposited on hot-dip galvanized steel (HDGS) by dip-coating. These OIHs, called ureasilicates, U(X), and amino-alcoholsilicates, A(X), were obtained using a functionalised siloxane and five oligopolymers with different molecular weights (MWs). Besides pure OIH coatings, HDGS samples were coated with OIHs doped with Cr3+, which was tested as a corrosion inhibitor. The OIH coatings deposited by one and three dip steps were assessed by electrochemical studies (polarisation resistance and macrocell current) in mortar in order to compare their performance. The SEM results were consistent with the data obtained by GD-OES and with the electrochemical analyses. The results indicated that the HDGS samples coated with OIHs show enhanced corrosion resistance in highly alkaline environment when compared to uncoated samples. The coatings involving the use of OIH matrices based on U(X) gels show superior corrosion behaviour when compared to the matrices based on A(X). The lowest and the highest corrosion rates were found for HDGS samples coated by one dip step of U(X) and for three dip steps of A(X) doped with Cr3+, using oligopolymers with MWs of 400 and 900, respectively.