硅酸钠预处理对镀锌层稀土镧盐转化膜耐蚀性的影响

Effect of silicate pretreatment on corrosion resistance of lanthanum conversion coating on hot-dip galvanized steel substrate

对热镀锌钢板先进行硅酸钠(水玻璃)预处理再镧盐钝化,以进一步提高镧盐转化膜的耐蚀性。用扫描电镜、能谱仪、盐雾腐蚀试验、塔菲尔极化曲线等方法研究了硅酸钠预处理对热镀锌层镧盐转化膜微观形貌和耐蚀性的影响。结果表明,硅酸钠预处理使得镧盐转化膜成膜均匀,膜层增厚且耐蚀性明显提高。通过正交试验得到最佳工艺为:先在室温下于0.5g/L的硅酸钠溶液中预处理1min,随后在70°C下于含La(NO3)3.6H2O9g/L和H2O210mL/L的溶液中钝化处理10min。与未经硅酸盐处理所得镧盐转化膜相比,经硅酸盐处理后,镧盐转化膜极化电阻提高了1倍,腐蚀电流密度减小了1个数量级。

To improve the corrosion resistance of lanthanum conversion coating, the hot-dip galvanized （HDG） steel was pretreated with sodium silicate （water glass） solution before dipping in lanthanum solutions. The effect of sodium silicate pretreatment on the rnicrostructure and corrosion resistance of lanthanum conversion coating was studied by scanning electronic microscopy, energy-dispersive spectroscopy, neutral salt spray test, and Tafel polarization curves. The results showed that the lanthanum conversion coating is uniform after sodium silicate pretreatment, and its thickness and corrosion resistance are improved markedly. The optimal process parameters were obtained by orthogonal test as follows： pretreatment in 0.5 g/L sodium silicate solution at room temperature for 1 min followed by passivation in a solution containing La（NO3）3·6H2O 9 g/L and H2O2 10 mL/L at 70℃ for 10 min. Compared to the lanthanum conversion coating obtained without sodium silicate pretreatment, there is a two-fold increase in polarization resistance and an orderof magnitude decrease in corrosion current density of the conversion coating pretreated by sodium silicate.