镁锂合金表面锰/锡复合转化膜的制备及性能

Preparation and performance of Mn/Sn composite conversion coating on surface of magnesium–lithium alloy

在含有7.5 g/L Na2Sn O3和2.5 g/L KMn O4的处理液中采用化学浸渍法在镁锂合金表面制备了锰锡复合转化膜,研究了处理液p H对转化膜性能的影响。利用扫描电子显微镜(SEM)和能谱仪(EDS)对不同p H转化液获得的转化膜的表面形貌和化学组成进行了观测和分析,通过动电位极化曲线测量、电化学阻抗谱以及酚酞溶液点滴试验分析了处理液不同p H对转化膜耐腐蚀性能的影响。结果表明,所制备的转化膜主要成分为Sn、Mn的氧化物或磷酸盐。当转化液的p H为3.5时,Mn/Sn复合转化膜分布均匀、结构致密,耐腐蚀性能最好。此时镁合金基体与转化膜之间的电阻最大(达3 515?/cm2),腐蚀电流密度最小(为3.427×10-6 A/cm2),酚酞点滴试验变色时间最长(为20.1 s)。

A manganese/tin composite conversion coating was prepared on the surface of magnesium-lithium alloy by chemical immersion process in a bath containing Na2SnO3 7.5 g/L and KMnO4 2.5 g/L. The influence of pH value of the bath on performance of the conversion coating was studied. The surface morphology and chemical composition of the conversion coating obtained at different pH values were observed and analyzed by scanning electron microscopy （SEM） and energy- dispersive spectroscopy （EDS）, respectively. The corrosion resistance of the conversion coatings obtained at different pH values was examined by potentiodynamic polarization curve measurement, electrochemical impedance spectroscopy, and NaC1 dropping corrosion test with phenolphthalein as indicator. The results showed that the conversion coating is mainly composed of Sn and Mn oxides or phosphates. The MrdSn composite conversion coating obtained at pH = 3.5 is welldistributed and compact, and has the largest resistance as 3 515 Ω/cm2 to Mg-Li alloy substrate, minimum corrosion current density of 3.427 × 10-6 A/cm2, and longest time （20.1 s） to endure color change in NaC1 dropping corrosion test, showing the best corrosion resistance.