Effect of Transition Metal and Different Rare-Earth Inhibitors-Based Sol–gel Coatings on Corrosion Protection of Mild Steel

Autonomous healing coatings based on encapsulation of corrosion inhibitors into nanocontainers is gaining a lot of attention. In the present work, coating formulations based on hybrid organic–inorganic sol–gel coatings impregnated with chrome-free corrosion inhibitors such as Zr 4+ (Z) and various rare-earth elements (RE: Ce 3+ , La 3+ , Gd 3+ , Er 3+ ) were investigated on mild steel substrates. Effects for both the direct addition of corrosion inhibitor (Z/RE) into the matrix sol and the addition of halloysite nanoclay encapsulated corrosion inhibitor (Z/RE-H) were evaluated. To evaluate the coating performance and self-healing action of different corrosion inhibitors, various characterization techniques such as potentiodynamic polarization studies, electrochemical impedance spectroscopy, micro-Raman spectroscopy, salt spray test and scanning electron microscopy-elemental analysis were used. Electrochemical studies and salt spray tests clearly confirm that coatings derived from a direct addition of corrosion inhibitors to the matrix sol result in an excellent performance. The current densities of coatings based on direct addition of Zr 4+ and Ce 3+ were very low, in the order of 10 –10 and 10 –9 A/cm 2 , respectively, when compared to other RE and all Z/RE-H coatings, which exhibited a current density of 10 –6 A/cm 2 . Micro-Raman analysis confirmed the self-healing action of zirconium and cerium-based coatings. Graphical Abstract