Titanium-phytic acid nano structured conversion coating formation on CRS substrate

Over the last few years, finding replacements for chromate and phosphate conversion coatings has been significantly put emphasis on more environment friendly one with high anti-corrosion performance and adhesion to top layers. In this research, a chemical conversion treatment solution containing hexafluorotitanic acid and phytic acid (PhA) was studied for cold rolled steel (CRS) substrate. The corrosion behavior of the uncoated CRS sample and also the samples coated by Ti-based (Ti), titanium-phytic acid (TiPhA), phytic acid (PhA) and tri-cationic phosphate conversion coatings were studied by electrochemical methods such as electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The highest polarization resistance (Rp = 2670 ohm cm2) was recorded for TiCC sample which is about 8 times higher than that of the bare sample. Field emission scanning electron microscopy (FE-SEM) images showed compact and fine particles uniformly distributed on the TiCC and TiPhACC samples, while PhACC and phosphate coatings revealed crystalline structures. The static water contact angle via sessile drop method indicated the hydrophilic behavior for TiCC, TiPhACC and Tri-Cat phosphate samples. Finally, epoxy organic coating was applied on the conversion coating treated CRS samples. Adhesion strength was measured by pull-off technique before and after 14 days immersion in corrosive solution. Results indicated that the lowest adhesion loss (7%) had occurred for TiPhACC sample. Consequently, although a certain additive such as phytic acid might not modify conversion coating characteristics such as anti-corrosion performance; it could improve adhesion performance in an organic coated system.