Chromium-free composite passivator for hot-dip galvanized steel sheet and preparation and application methods of composite passivator

The invention relates to a chromium-free composite passivator for a hot-dip galvanized steel sheet. The passivator is prepared from the following raw materials in percentage by weight: 2.20-2.30% of water-base resin, 2.60-2.65% of main salt compound, 0.10-0.15% of organic silane, 0.07-0.09% of sulfate, 0.11-0.13% of inorganic zircon salt, 0.09-0.12% of inorganic acid and the balance of water. A preparation method of the passivator comprises the following steps: firstly, dissolving the organic silane in water; stirring till the organic silane is fully dissolved and adding the main salt compound; after the main salt compound is dissolved, adding the inorganic acid to measure the pH value; then, adding the sulfate and the inorganic zircon salt in the stirring state; after the sulfate and the inorganic zircon salt are fully dissolved, adding the water-base resin; stirring till the water-base resin is fully dissolved; adjusting the pH value to 3-4 by virtue of ammonium water; and standing for later use. An application method of the passivator comprises the following steps: firstly, degreasing the hot-dip galvanized steel sheet by virtue of an alkaline degreaser during use; then, respectively washing by using tap water and deionized water; and after drying, coating the composite passivator on the surface of the hot-dip galvanized steel sheet. The passivator provided by the invention is free from chromium and non-toxic and harmless. The thickness of a passivation film can reach 8-1.0mg/m , and the corrosion resistance is close to that of a hexavalent chromium passivating liquid.