Multivariable Analysis of Electrodeposited Silane/Zinc Coating—Improvement of Cathodic Protection of Steel

In order to enhance corrosion protection efficiency of electrodeposited sol–gel zinc/silica coatings, this paper resorted to a central composite design as a tool to improve cathodic protection effectiveness time and electrochemical tests to understand the film protection mechanism. Coatings were produced by electrodeposition of a mixture of tetraethoxysilane (TEOS) and Zn²⁺ onto carbon steel. The amount of silane agent, concentration of zinc nitrate and the applied potential were chosen as the variables to be studied. Open-circuit potentials in NaCl 3.5% (wt.) solution were monitored to verify the duration of cathodic protection, according to ISO 15589-2:201. Microstructures characterization and distribution of deposited zinc were determined by scanning electron microscopy and energy-dispersive x-ray spectroscopy. At last, coating degradation was analyzed by potentiodynamic polarization curves, electrochemical impedance spectroscopy and visual inspection of corrosion products. Results revealed films whose cathodic protection endured more than 600 min of immersion when higher concentrations of Zn²⁺ were combined with lower potentials and lower silane content. Furthermore, visual inspection of immersed specimens showed no corrosion signs for carbon steel samples deposited with zinc and TEOS, even after 20 h, providing a superior protection when compared to the sample deposited solely with zinc.