钐盐含量对AZ31B镁合金表面铁氰化物转化膜耐蚀性的影响

Effect of Samarium Salt Content on Corrosion Resistance of Ferricyanide Conversion Coating on AZ31B Magnesium Alloy

目的改善AZ31B镁合金表面单一铁氰化钾转化膜附着力以及提高单一膜层的耐腐蚀性能。方法选用钐盐对镁合金单一膜层进行处理,着重探讨不同钐盐含量对膜层的影响。利用两步法进行化学浸渍成膜,并且利用扫描电镜(SEM)、X射线衍射仪(XRD)、能谱仪(EDS)和X射线光电子能谱(XPS)对复合膜层表面形貌和组成成分进行表征。采用动电位极化曲线和电化学交流阻抗对复合膜层在3.5%NaCl溶液中的腐蚀行为进行探究。结果经过钐盐处理的镁合金铁氰化钾转化膜表面生成了新的膜层,该复合膜层主要成分为SmFe(CN)6和Sm(OH)3。其不仅具有比单一膜层更优异的耐蚀性能以及膜层与基体的结合力也有所提高,而且形成的复合膜层也较稳定,可以对基体起到更好的保护作用。另外,钐盐含量为5 g/L时处理的膜层最为致密平整,自腐蚀电流密度最低,为2.129×10^(‒9) A/cm^(2),电荷转移电阻和膜层电阻最大,分别为8.164×10^(4)Ω·cm^(2)和1.293×10^(7)Ω·cm^(2),耐蚀性能最好。结论使用钐盐对镁合金表面铁氰化钾膜层进行改性,可以进一步提高单一膜层的耐蚀性,并且最佳钐盐含量为5 g/L。

A single potassium ferricyanide conversion coating on the surface of magnesium alloys can improve the corrosion resistance.On the contrary,there are still defects such as poor bonding between the coating and the substrate.The film layer on the surface was easy to peel off after soaking for a period of time in a corrosive environment,and its corrosion resistance needs to be further improved.In order to solve this problem,the author's research team used samarium salt to treat the potassium ferricyanide film on the surface of magnesium alloy,and optimized its process,focusing on the effect of samarium salt content on the composite film.In this paper,a two-step method was used to form the film.First,a layer of potassium ferricyanide conversion film was prepared on the surface of the pretreated sample with a size of 20 mm×20 mm×1 mm by chemical dipping method,and then the sample was placed on the same method.In the samarium nitrate solution,the secondary film formation treatment was performed to obtain the final composite film layer,which was finally dried by hot air and then placed in a blast drying oven for heat preservation and drying,and then taken out for testing.In this paper,the scanning electron microscopy(SEM),X-ray diffractometer(XRD),energy dispersive spectrometer(EDS)and X-ray photoelectron spectroscopy(XPS)were used to analyze and characterize the surface morphology and composition of the composite film;The polarization curve(Tafel)and electrochemical impedance(EIS)were used to explore the corrosion behavior of the composite film at different stages in 3.5wt.%NaCl solution;the coating thickness gauge was applied to measure the thickness of the film formed under different samarium salt content treatment;used 3%copper sulfate solution for spot drop test,observed that the solution changes from blue to colorless and produces small black spots on the sample time;the metallographic microscope and scanner was used to observe the surface morphology of different samples for long-term soaking experiments,and then to explore the film adhesion and stability of the composite film.The results showed that a new film layer was formed on the surface of the magnesium alloy potassium ferricyanide conversion film treated with samarium salt,and the main components of the composite film layer were SmFe(CN)6 and Sm(OH)3.It had better corrosion resistance than a single film layer,and the bonding force between the film layer and the substrate was also improved.The composite film layer was more stable and could play a better protective effect on the substrate.In addition,when the content of samarium salt was 5 g/L,the film was the most compact and smooth,the self-corrosion current density was the lowest,which was 2.129×10^(‒9) A/cm^(2),and the charge transfer resistance and film resistance were the largest,which were 8.164×10^(4)Ω·cm^(2) and 1.293×10^(7)Ω·cm^(2),the corrosion resistance of the film was the best.All in all,the use of samarium salt to modify the potassium ferricyanide film layer on the surface of magnesium alloy can not only further improve the adhesion between the film layer and the substrate,and improved the corrosion resistance of magnesium alloy,but also the surface of the formed composite film layer was more uniform,dense and stable.Its optimum samarium salt content was 5 g/L.