Study Of Phytic Acid-Cerium Composite Coating On Galvanized Steel

A chromium free conversion coating for hot-dip galvanized （HDG） steel by phytic acidsolution was obtained. After post-processing in Ce（NO₃）₃·6H₂O solution, a composite coatingwith better performance was obtained. The microstructure, composition, formation process andcorrosion resistance of phytic acid conversion coating/phytic acid-cerium composite coatingwas studied by SEM, EDS, FTIR, XPS, electrochemical polarization, electrochemicalimpedance spectroscopy （EIS） and neutral salt spray （NSS）.The optimal process parameters of phytic acid conversion coating are as follows: phyticacid10ml/L, pH4, T40℃, t5min. The optimal post-process time for phytic acid-ceriumcomposite coating is30min, and the process parameters are as follows: Ce（NO₃）₃·6H₂O20g/L,H₂O220ml/L, pH3⁴, T40℃.It is shown that the thickness of the single phytic acid conversion coating increase rapidlywith the increase of treatment time, microcracks are visiable in the conversion coating which istreated for5min. After treated for30min, the microcracks will gradually occupy on the wholesurface of the conversion coating and warps will appear, the cracks and warps will lead to thedecrease of the corrosion resistance of the conversion coating. The phytic acid-ceriumcomposite coating grows slower. Thickness of composite coating keep increasing in the first30min, after that white microdot with high Ce content appeared nearby the grain boundary, andits size gradually increasing eventually lead to cracks and warps.The results of EDS and XPS show that the phytic acid conversion coating is composed ofphytic acid salt and Zn（OH）₂/ZnO. Surface of the phytic acid-cerium composite coating iscomposed of Ce（OH）3/Ce₂O₃, Zn（OH）₂/ZnO and a few Ce（OH）₄/CeO₂.The formation process of phytic acid coversion coating consisted of the diffusion of thezinc coating and products of the chelation reactions absorbed on the surface of zinc layer. Nodiffusion of the zinc coating occurred during the formation process of phytic acid-ceriumcomposite coating, so the phytic acid coversion coating obtained in pre-process directly reactswith the passivation solution of the post-process produce the phytic acid-cerium compositecoating.The results of NSS analysis show that the corroded area of the phytic acid-ceriumcomposite coating reaches5%in the5thcycle while the single phytic acid conversion coatingreaches the same in the2thcycle and the HDG within the1stcycle. Therefore, the corrosionresistance of the phytic acid-cerium composite coating is more outstanding than that of thesingle phytic acid conversion coating. The results of electrochemical test show that compared with HDG and phytic acidcoversion coating, the polarization resistance of the phytic acid-cerium composite coatingincrease from0.6kΩ·cm²to12kΩ·cm²to40kΩ·cm², meanwhile the corrosion current densitydecrease from12μA·cm-2to1μA·cm-2to0.3μA·cm-2.The corrosion process of the phytic acid-cerium composite coating is divided into twostages: electrolyte solution diffuse into the conversion coating as the dominate reaction at thefirst stage; the corrosion of zinc become the dominate reaction at the second stage.