Corrosion behavior of chromium-free dacromet coating in seawater

In order to overcome the environmental objection of chromium(VI) in dacromet, a novel chromium-free dacromet was developed. Electrochemical impedance spectroscopy (EIS) and d.c. potential measurements were employed to study the anticorrosion behavior of the sintered ceramic zinc-aluminum coatings in still seawater. Results of X-ray diffraction (XRD) indicate that the main corrosion products are composed of Al₂O₃, ZnCl₂, Zn(OH)₂ and Fe₂O₃. The scanning electron microscope (SEM) and energy dispersive X-ray spectroscope (EDX) analysis were carried out to study the morphological appearance and the local chemical composition of run-out coatings. The results show that the evolution of open circuit potential can be divided into four stages during the lifetime of CFD/steel. The EIS behaviors of Zn-Al coatings immersion in seawater at the first day are similar to the ceramic CrN coating prepared by PVD, while the impedance data of the next days are parallel with multilayer protective coatings. The galvanic protection effect of the coating keeps active only in the first 3 days and then the physical shielding function play a dominant role.