Ti-Based Conversion Coatings on Cold-Rolled Steel Substrate: The Effect of Practical Parameters and Ti Source on Surface and Electrochemical Properties

Chromating and phosphating processes have caused severe environmental, health, and industrial concerns. Therefore, developing chrome-free conversion coatings has played an important role in overcoming these issues. In this study, two kinds of titanium-based conversion coatings (H₂TiF₆ and TiCl₄) were developed for cold-rolled steel (CRS) substrate. The effects of practical parameters (immersion time, pH, and Ti concentration) on anti-corrosion performance were evaluated using direct current polarization and electrochemical impedance spectroscopy. It was revealed through investigating corrosion current densities and polarization resistance behavior that the source that provides Ti cation in solution strongly affects the required immersion time for gaining the best anti-corrosion performance. Field emission scanning electron microscopy displayed more compactness and uniformity for optimized TiCl₄-based conversion coating. X-ray photoelectron spectroscopy analysis represented that the Ti-based conversion coatings consisted of Fe and Ti oxides and/or hydroxides. Static water contact angle indicated that the surface, after conversion coatings application, became more hydrophobic. Finally, after achieving the optimum conditions, epoxy organic coating was applied on the Ti treated CRS samples. Adhesion strength was measured by pull-off technique, which exhibited higher values for TiCl₄ (3.28 MPa) and H₂TiF₆ (1.82 MPa) treated samples, compared to untreated one (1.6 MPa).