Study On Chromate-free Passivation Processes Of Hot-dip Galvanizing

In order to develop a chromium-free passivation technology with excellent corrosion resistance for hot-dip galvanizing,using Zn and Zn-5% Al alloy coatings as objects,the influences of passivation factors on the microstructure and properties of hot-dip galvanized passivation layer were studied,and some new types of hot-dip galvanizing passivation processes of silicate passivation,molybdate passivation,titanium salt passivation,cerium passivation and composite passivation were investigsted by using optical microscope,XRD,electrochemical test,drip test,adhesion tester and full immersion test in this paper.Determined the composition of the silicate passivation solution is: 20.0g/L Na₂SiO₃,10mL/L 30%H₂O₂,3g/L thiourea,10mL/L 65%HNO₃.The passivation process conditions:pH value 2,passivation temperature 40?,passivation time 60 s,drying temperature 80?,drying time 4h.The cerium salt passivation process was optimized.The composition of the passivation solution is as follows: 30g/L Cerium nitrate,Ti?SO4?2 3g/L,H₂O₂ 20mL/L.The process conditions of galvanized cerium salt passivation are as follows: pH value 3,passivation temperature 35?,passivation time 90 s,drying temperature 80?,drying time 4hours.The titanium salt passivation process of hot-dip coating was gaven out: Ti?SO4?2 8g/L,H₂O₂ 90mL/L,HNO₃ 5mL/L,additive 8g/L,pH value 1³,passivation temperature 60?passivationtime 15 s,using 0.5 moL/LKMNO4 as a blocking agent,blocking temperature70⁸0?,blocking time 10¹5s?It can significantly improve the corrosion resistance of passive film for using titanium salt to passivate and seal hot-dip galvanized layer.The experimental results of salt water immersion test for the molybdate composite passivation film showed that the inorganic passivation film is of good protective effect to hot-dip galvanizing substrate,and can greatly reduce the corrosion rate of decay,improve the capacity of salt water immersion,achieve the level of chromate passivation.The mechanism of composite passivation film is reducing the corrosion current density of the galvanizing layer by inhibiting cathodic process during the galvanizing layer corrosion,in turn inhibits the entire electrochemical process.The passivation does not affect the phase composition of Zn-5%Al alloy coating.The best corrosion resistance can be obtained when passivation temperature is 30? and passivation time is 50 s.The research and development of chrome-free passivation technology has considerable difficulty and can only be used in the product function,film forming speed,technical stability,control cost and so on.It can replace the traditional chromate Technology.In the above chrome-free passivation process,the silicate passivation process has a compact film,uniformity,good corrosion resistance,low pollution,low cost,fast film formation,etc.,to promote the value of a larger.