The Study Of Molybdate/Phosphate Passive Film Formed On Galvanized Steel

The main task is to prepare molybdate passive film formed on galvanized steel and study the film's anti-corrosion performance.Molybdate passive film was prepared by dipping galvanized steel in molybdate/phosphate solution with certain additive. A series of tests was taken to select best experimental procedure parameter, such as the best addictive, solution's concentration, bath temperature, solution's PH and dipping time. As a result, the best parameters of technological process of preparing molybdate passive film formed on hot-dipping galvanized steel were selected. Moreover, an organic amine addictive was confirmed as the best additiveScanning electron microscopy was used to observe the morphology of the film. Elements of passive film are conformed by using EDS and XPS. Moreover, XPS was also used to study the formation mechanism of the molybdate passive films with the organic amine. Influence of an organic amine addictive on anti-corrosion performance of molybdate passive film formed on hot-dipping galvanized steel was investigated by potentiodynamic polarization and electrochemical impedance measurements.It was found that the concentration increase of the addictive in a certain range was favorable to the formation of the conversion coating, and could improve corrosion performance of the film. When the addictive increased to a certain concentration, the film cracked due to the extremely quick reaction, which reduced the corrosion performance of molybdate passive film.SEM showed that the surface of the film was even. P,Mo,O,Zn and a small amount of Fe were found in the film by the test of EDS and XPS. XPS also showed that P, Mo, Zn existed as five price, six price and two price respectively in the surface of the passive film.Potentiodynamic polarization showed that marked passivating characteristic was found in anodic polarization curve. The oxide thickening processes played an important role on the corrosion protection mechanism. The corrosion potential of the film was higher than that of matrix, and corrosion current of the film was smaller than that of matrix. EIS showed that Nyquist picture was characterized as a complete capacitive impedance curve, corrosion process was controlled by the electrical chemical mechanism.