Effect of sealing treatment on corrosion resistance of zinc phosphating film prepared under ultrasonic condition
Ultrasonic was introduced in the phosphating process of Q345 steel to improve the surface flatness and compactness of zinc phosphating film.In order to further improve the corrosion resistance of zinc phosphating film prepared under ultrasonic condition,oil sealing agent,cerium nitrate solution and sodium silicate solution were used for sealing treatment respectively.The results show that the sealing effect of sodium silicate solution is better than that of oil sealing agent and cerium nitrate solution.After sealing treatment in sodium silicate solution,the zinc phosphating film prepared under ultrasonic condition is composed of Zn3(PO4)2·4H2O,Zn2Fe(PO4)2·4H2O,ZnSiO3 and Na2SiO3 phases and shows good compactness and corrosion resistance.The time against corrosion of copper sulfate dropping exceeds 150 s,and the corrosion current density is as low as 3.20×10-6 A/cm2,which is nearly an order of magnitude lower than that of the unsealed zinc phosphating film prepared under ultrasonic condition.Corrosion leveling phenomenon occurs during the sealing process of zinc phosphating film in sodium silicate solution,and ZnSiO3 is formed and Na2SiO3 is precipitated which can play the role of filling the pores.Therefore,the compactness of the zinc phosphating film prepared under ultrasonic condition is improved to a greater extent,thus significantly enhancing the corrosion resistance.
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