Modification of bis-silane solutions with rare-earth cations for improved corrosion protection of galvanized steel substrates

Chemical formulations based on silane solutions are currently used for the pre-treatment of metallic substrates. These pre-treatments provide corrosion protection of the metallic substrates due to the good barrier properties of the silane films that form on the surface. The corrosion protection of silane-based pre-treatments can be improved by adding dopants to the silane solutions. The dopants must present corrosion inhibition properties and must keep or improve the barrier properties of the silane film without modification of surface functionality and bulk properties.The present work aims at assessing the corrosion behaviour of hot dip galvanised steel substrates pre-treated with bis-[triethoxysilylpropyl]tetrasulfide and bis-1,2[triethoxysilyl]ethane silane solutions doped with cerium nitrate or doped with lanthanum nitrate. The effect of the dopants on the barrier properties of the silane film and its corrosion-inhibition ability is studied via electrochemical impedance spectroscopy. Complementary studies using atomic force microscopy and surface analysis were also performed. The results show that the corrosion resistance of substrates pre-treated with the silane solutions doped with the rare-earth cations increases by more than one order of magnitude when compared with the non-doped pre-treatments. The results also show that the protective behaviour of the pre-treatment is dependent on the dopant and on the silane film. The work discusses the role of the dopant on the anti-corrosion properties of the pre-treated substrates.