Preparation and Corrosion Resistance of Silicate Phosphate Composite Passivation Film on Zinc Substrate by Simple Chemical Impregnation Method

A silicate and phosphate composite passive film on zinc was prepared from a composite passivation solution containing zinc dihydrogen phosphate and sodium metasilicate. The morphology, composition, valence and functional groups of the passive film with different passivation times were analyzed through field-emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy, X-ray photoelectron spectroscopy and infrared spectroscopy to understand its evolution process. The passive film exhibits a cyclic film-forming process with a periodic ratio of silicate and phosphate, which is caused by the periodic changes in the alkalization degree of the interface between the electrode and the solution. Such periodic film-forming process promotes the passive film growth and makes the passive film uniform and dense. Electrochemical analysis, immersion tests and copper sulfate spotting tests illuminate that the composite passive film is more corrosion-resistant than the pure silicate passive film. This composite passivation film exhibits a uniform corrosion behavior with gradually expanding micropores due to the effective inhibition of inward diffusion and permeation of chloride ions by the dense film structure of cyclic growth, which avoids the pinhole corrosion and improves the protection of the passivation film.