【作者】 吴磊；

【导师】 罗伟；

【作者基本信息】 浙江大学 ， 材料加工工程， 2006， 硕士

【摘要】 第三代稀土永磁合金钕铁硼材料由于具有优异的磁性能、良好的机械加工特性和相对低廉的价格,在许多领域得到广泛的应用。但NdFeB合金的化学稳定性差,在使用环境中容易发生氧化,而且在湿热条件下会发生严重的电化学腐蚀,大大影响了其应用范围。本文在综述了近年来NdFeB永磁材料腐蚀机理和防护处理主要研究进展的基础上,采用化学镀的方法,在NdFeB磁体表面镀覆镍基合金,以提高磁体的耐腐蚀性能。 采用正交试验法对NdFeB磁体表面化学镀镍磷合金的工艺进行了优化,测量了镀层和基体在3.5wt%NaCl溶液、10vol%盐酸和20wt%NaOH溶液中的腐蚀速度,以及在3.5wt%NaCl溶液中的极化曲线和交流阻抗谱,对比分析了在酸性和碱性条件下所得Ni-P镀层的结构和表面形貌。结果表明,采用EIS谱图及等效电路模型可对镀层和磁体在介质中的电化学参数进行拟合分析,化学镀Ni-P合金能够显著改善NdFeB磁体的耐腐蚀性能,且酸性条件下所获得的镀层为非晶态结构,表面胞状组织呈密集连续分布,耐腐蚀性能更佳。 对化学镀沉积反应活化能的实验测试表明,由于酸性镀液中采用pK值较低的乳酸作为络合剂,反应体系的活化能较低,镀速较高。对酸性条件下化学镀Ni-P合金的沉积动力学进行了初步探讨,并提出了Ni-P合金沉积动力学的经验速率方程式。 研究了NdFeB磁体表面化学镀Ni-Co-P三元合金及其耐腐蚀性能。优化了镀液配方以及施镀工艺,分析了镀液pH值和金属离子配比([Co²⁺]/[Ni²⁺+Co²⁺])对沉积速度和镀层成分的影响,测量了NdFeB基体和不同镀层在3.5wt%NaCl溶液中的极化曲线。结果表明,随镀液pH值增加,沉积速度提高,镀层中CO含量升高,Ni含量和P含量逐渐降低;随镀液中Co²⁺比例增加,沉积速度下降,镀层中Co含量升高,Ni和P含量降低。化学镀Ni-Co-P合金后的NdFeB磁体在3.5wt%NaCl溶液中的腐蚀速度降低了约两个数量级;当镀液金属离子配比[Co²⁺¹]/[No²⁺+Co²⁺]=0.3时,镀层耐腐蚀性能最好,且优于相同施镀条件下所得到的Ni-P镀层。更多还原

【Abstract】 NdFeB alloy as the third-era permanent magnet is widely used in many industries because of its outstanding properties, such as excellent magnetic properties and machinability, and comparatively low price. Unfortunately, it is prone to oxidation in working environments, especially in damp heat atmosphere, which leads to a severe electrochemical erosion. Poor corrosion resistance is the obstruction to broaden applications of NdFeB alloy. In this thesis, reports on corrosion mechanism and anticorrosion treatment of NdFeB permanent magnet are reviewed. On this basis, electroless deposition of Ni-based alloy was employed to NdFeB magnet to enhance its corrosion resistance.Parameters of electroless deposition of Ni-P alloy were optimized by an orthogonal experiment. The structure and morphology of the Ni-P layer on magnets deposited from an acidic bath and from an alkaline bath were studied and compared. The corrosion rate of Ni-P deposited magnets in 3.5wt% NaCl, 10vol% HC1 and 20wt%NaOH solutions were measured. The corrosion behaviors of both types of Ni-P deposits were evaluated by electrochemical potentiodynamic polarization and impedance techniques. The results show that both deposits enhance the corrosion resistance of the substrate. Ni-P deposited from the acidic bath provides a high degree of protection. The difference in the corrosion behavior of the two deposits is attributed to their distinct structure and density.The experimental analysis of activation energy of electroless deposition reaction shows that acidic bath reaction system possesses a low activation energy and a high deposition rate. The reaction kinetics of electroless deposition of Ni-P alloy in acidic bath was discussed. Reaction kinetic parameters were obtained and an empirical equation of deposition rate was derived.Electroless Ni-Co-P ternary alloy deposits were prepared by optimizing the bath composition and operating conditions to improve the corrosion resistance of NdFeB permanent magnets. The effects of pH and metallic ion ratio ([Co~2+]/[Ni~2++Co~2+]) on plating rate and deposit composition were investigated. Polarization curves of NdFeB matrix with and without Ni-Co-P deposits in 3.5wt%NaCl solution were examined. It is found that the plating rate and the cobaltcontent of the deposit increase with increasing pH, while the nickel and phosphorus contents decrease. With increasing metallic ratio, the plating rate decreases and the cobalt content of the deposit increases with a simultaneous decrease in the nickel and phosphorus contents. The polarization curves exhibit that the corrosion rate of NdFeB magnets decreases by two orders of magnitude in 3.5wt%NaCl solution after electroless plating with the Ni-Co-P deposits. A metallic ion ratio at 0.3 in the bath solution achieved the best corrosion resistance.Effects of pre-plating in ultrasonic field on the adhesion between deposit and NdFeB matrix were investigated. The results show that a uniform and compact deposit is prepared by pre-plating with ultrasonic, and the adhesion is improved effectively. It is suggested that ultrasonic pre-plating contributes to the improvement of adhesion through three aspects: activation, cavitation and stirring action.更多还原