成膜时间对AZ31B镁合金镨盐转化膜形貌和耐蚀性的影响

Effect of Film Formation Time on the Morphology and Corrosion Resistance of Praseodymium Salt Conversion Film of AZ31B Magnesium Alloy

目的探究新型稀土盐镨盐转化膜的腐蚀防护特性。方法采用化学转化法,在镁合金基体进行表面改性,制备出一层致密镨盐转化膜。试验重点探讨不同成膜时间制备转化膜的表面性能,在模拟海水(3.5%NaCl溶液)条件下,通过电化学测试(Tafel极化曲线和电化学阻抗谱)、浸泡试验、点滴试验分别评价膜层的耐腐蚀特征,通过扫描电子显微镜(SEM)、傅里叶变换红外光谱仪(FTIR)、X射线能谱分析(EDS)和X射线光电子能谱(XPS)对微观形貌、结构成分进行测试分析,通过显微硬度计和摩擦磨损试验机,从力学角度分析膜层的耐磨性和表面硬度。结果进行对比试验后发现,成膜时间过短或过长都不利于膜层的形成。Tafel曲线显示,成膜时间为30 min时,自腐蚀电流密度为1.740×10^(-9)A/cm^(2),相比裸镁试样下降了4个数量级;自腐蚀电位为-0.681 V,相比裸镁试样正移了900 mV;电化学交流阻抗谱显示,成膜30 min时,膜层的容抗弧曲率半径最大,EIS拟合后电荷转移电阻也最大,为45650Ω·cm^(2)。浸泡试验和点滴试验结果表明,成膜30 min时,膜层具有较少的点蚀区域和裂纹缺陷,点滴时间为82.14 s,膜厚可达26.2μm,膜层较为牢固,试验结果与电化学测试参数具有一致性。组成分析则说明转化膜主要含有C、H、O、Mg、Pr等元素,膜层主要物质为镨的氢氧化物和镨的部分氧化物。稀土金属镨的加入一定程度提高了膜层的力学性能,提高了膜层的耐磨性和硬度。结论由组成分析结果可推测镁合金表面镨盐转化膜的成膜机理,符合阴极成膜机制,Pr(NO_(3))_(3)·6H_(2)O作为成膜主盐可以有效改善AZ31B镁合金的表面性能,当成膜30 min时,镁合金镨盐转化膜可以有效阻止腐蚀介质的入侵,其耐蚀性和稳定性最佳。

In order to explore the corrosion protection properties of the new rare earth salt praseodymium salt conversion coating,a dense praseodymium salt conversion coating was prepared by chemical conversion method to modify the surface of the magnesium alloy substrate.This experiment focuses on the surface properties of conversion coatings prepared with different film forming times.Under simulated seawater conditions(3.5%NaCl solution),the corrosion resistance characteristics of the films were evaluated by electrochemical tests(Tafel polarization curve and electrochemical impedance spectroscopy),immersion tests,and titration tests.Scanning electron microscopy(SEM),Fourier transform infrared spectroscopy(FTIR),X-ray energy dispersive spectroscopy(EDS)and X-ray photoelectron spectroscopy(XPS)were used to test and analyze the microscopic morphology and structural components.The wear resistance and surface hardness of the film were analyzed from a mechanical point of view by means of a microhardness tester and a friction and wear tester.After comparative experiments,it was found that too short or too long film formation time is not conducive to the formation of film layers.The Tafel curve shows that when the film formation time is 30 min,the self-corrosion current density is 1.740×10^(-9)A/cm^(2),which is 4 orders of magnitude lower than that of the bare magnesium sample;the self-corrosion potential is-0.681 V,compared with the bare magnesium sample.The magnesium sample was shifted by 900 mV.Electrochemical AC impedance spectroscopy showed that the capacitive arc curvature radius of the film was the largest when the film was formed for 30 min,and the charge transfer resistance was also the largest after EIS fitting,which was 45650Ω.cm^(2).The immersion test and saline drip test show that the film has fewer pitting areas and crack defects when the film is formed for 30 minutes,the spotting resistance time is 82.14 s,the film thickness can reach 26.2μm,and the film is relatively firm,the test results are consistent with the electrochemical test parameters;The composition analysis shows that the conversion coating mainly contains C,H,O,Mg,Pr and other elements,and the main materials of the coating layer are praseodymium hydroxide and praseodymium partial oxide;The addition of rare earth metal praseodymium improves the mechanical properties of the film to a certain extent,and improves the wear resistance and hardness of the film.Finally,the film formation mechanism of praseodymium salt conversion film on the surface of magnesium alloy can be inferred from the composition analysis results,which is consistent with the cathode film formation mechanism.Pr(NO_(3))_(3)·6H_(2)O as the main film-forming salt can effectively improve the surface properties of AZ31B magnesium alloy.At 30 min,the magnesium alloy praseodymium salt conversion coating can effectively prevent the invasion of corrosive medium,and its corrosion resistance and stability are the best.