Impeding corrosion of sintered NdFeB magnets with titanium nitride coating

Sintered NdFeB magnets have poor corrosion resistance that renders them susceptible to corrosion in industrial and marine environments. This paper evaluates the properties of cathodic arc physical vapour deposited (CAPVD) titanium nitride coating for corrosion protection of sintered NdFeB permanent magnets. The performance of titanium nitride coating has been compared to the electrodeposited nickel–copper–nickel multilayer coating. The rates of coatings degradation in simulated marine environment were estimated with electrochemical impedance spectroscopy (EIS). Cyclic polarization was carried out to assess the pitting potential. The surface chemistry and coating morphologies were studied with scanning electron microscope (SEM). X‐ray diffraction (XRD) was used for qualitative phase analyses of coatings and the substrate. It was figured out that the charge transfer resistance of CAPVD titanium nitride coating increased with exposure time. The negative rate of Rp‐degradation for titanium nitride coating compared to the nickel–copper–nickel multilayer for equivalent exposure time is a unique and valuable result. Polarization results showed that ‘pits re‐passivation’ of titanium nitride coating could be responsible for the extended corrosion protection of the NdFeB substrate. The magnetic properties remained comparable for both types of coatings.     

Corrosion protection of sintered NdFeB magnets by CAPVD Ti2N coating

Sintered NdFeB magnets have poor corrosion resistance and are readily susceptible to corrosion under different environmental conditions. Cathodic arc physical vapour deposited (CAPVD) titanium nitride coating for sintered NdFeB permanent magnets has been investigated in this paper. Tafel extrapolation was employed to study the corrosion behaviour in 3.5% NaCl solution at ambient temperature. The adhesive strength of the coating was estimated with the help of the scratch test. The surface chemistry and coating morphologies were studied with scanning electron microscope (SEM). X-Ray diffraction (XRD) was used for qualitative phase analyses of coatings and substrate. The properties of CAPVD titanium nitride coating were compared with electrodeposited multilayer nickel–copper–nickel coating. It was figured out that the CAPVD titanium nitride coating had better adhesion strength and shifted the free corrosion potential (FCP) of the system towards positive potential, providing protection to the NdFeB substrate. However, the corrosion rate of CAPVD titanium nitride coating was more than the electrodeposited multilayer nickel–copper–nickel coating. The magnetic properties remained comparable.     

Electrochemical corrosion behaviors and protective properties of Ni–Co–TiO2 composite coating prepared on sintered NdFeB magnet

In this paper, a protective Ni–Co–TiO₂ composite coating was prepared on the sintered NdFeB magnet by direct current electrodeposition. The surface morphologies, microstructure, and chemical composition of the composite coating were studied using scanning electron microscope (SEM), X‐ray diffraction (XRD), and energy dispersive spectroscopy (EDS), respectively. The surface morphologies and microstructure analysis showed that the composite coating possessed cauliflower‐like grain colonies, and formed face‐centered cubic (fcc) solid solution. The electrochemical corrosion behaviors of the composite coating in 0.5 mol/L H₂SO₄, 0.6 mol/L NaOH, 0.6 mol/L Na₂SO₄, and neutral 3.5 wt% NaCl solutions were evaluated by potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS), showing good protection for NdFeB magnet. In order to further investigate the protective properties of the composite coating for NdFeB magnet and the practicability of the composite coating, the long‐term immersion test was carried out in neutral 3.5 wt% NaCl solutions using EIS. The results of long‐term corrosion test showed that the Ni–Co–TiO₂ composite coating could provide long‐term protection in neutral 3.5 wt% NaCl solutions for NdFeB magnet.     

钕铁硼磁体表面电沉积纳米氧化硅复合镀锌层的研究

目的增强钕铁硼材料的耐腐蚀性能。方法在钕铁硼磁体材料表面电沉积含有氧化硅、氧化钛纳米粒子的复合锌层。结果在氯化物镀锌溶液中加入10 g/L的氧化硅纳米粒子,可以获得氧化硅质量分数在3%以上的氧化硅纳米粒子复合镀锌层,而加入10 g/L的氧化钛纳米粒子的镀液所获得的复合镀层中,氧化钛的质量分数只有0.3%。结论中性盐雾腐蚀和盐水浸泡腐蚀试验结果表明,含有氧化硅纳米颗粒的复合镀锌层的耐腐蚀性能得到了提高,而含有氧化钛纳米颗粒的复合镀锌层的耐腐蚀性能则没有提高。含有纳米粒子的复合镀液经过1年的放置和间断使用,仍然保持着纳米颗粒均匀分散的稳定性和在镀层中稳定析出氧化硅纳米粒子的特征。     

A comparative study on the corrosion behavior of NdFeB magnets in different electrolyte solutions

Sintered NdFeB magnets possess excellent magnetic properties. However, the corrosion resistance property of NdFeB is very poor due to its multiphase microstructure consisting of matrix phase Nd₂Fe₁₄B, Nd‐rich phase, and B‐rich phase. The corrosion behavior of NdFeB magnets in sodium hydroxide (NaOH), sodium chloride (NaCl), nitric acid (HNO₃), and oxalic acid (H₂C₂O₄) solutions was investigated by immersion and electrochemical tests. HNO₃is the strongest corrosive electrolyte compared with the other three solutions. The increase in HNO₃concentration can accelerate the corrosion of NdFeB magnets. NaCl belongs to medium corrosion electrolyte. A NaCl concentration of 0.5 M shows the severest corrosive feature in comparison with other concentrations of NaCl solution. NdFeB hardly suffers corrosion in NaOH and H₂C₂O₄solutions owing to the formation of passivation films on the surface of magnets. Based on the corrosion behavior of NdFeB in different electrolytes, the possible corrosion mechanisms are discussed.     

烧结NdFeB永磁体在不同酸溶液中的腐蚀行为（英文）EI北大核心CSCD

通过酸洗、浸泡法和电化学腐蚀测试分析了烧结Nd Fe B永磁体在不同浓度的H2SO4溶液、HCl溶液和HNO3溶液中的腐蚀行为。结果表明,烧结Nd Fe B磁体在HNO3溶液中的腐蚀机制为均匀腐蚀,而在H2SO4溶液和HCl溶液中的腐蚀机制为选择性的晶间腐蚀。烧结Nd Fe B磁体在酸溶液中的腐蚀速率随着酸溶液浓度的增加而增加。在浸泡试验和电化学腐蚀试验中,烧结Nd Fe B磁体在HNO3溶液中的腐蚀速率均最小。因此,HNO3溶液更适合作为烧结Nd Fe B磁体的酸洗液。     

Corrosion resistance of environment‐friendly sealing layer for Zn‐coated sintered NdFeB magnet

In this paper, a protective sealed Zn coating (SZC) was prepared on sintered NdFeB magnet by the combination of electrodeposition and sol–gel method. The unsealed Zn coating (UZC) was also studied for a comparison. The surface morphology of UZC and the cross‐section morphology of SZC were investigated using scanning election microscope (SEM). The microstructure of Zn coating and structure of sealing layer were studied by X‐ray diffraction (XRD) and Fourier transform infrared (FT‐IR) spectrum, respectively. The corrosion characteristics of SZC and UZC in neutral 3.5 wt% NaCl solution were evaluated using electrochemical measurements including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test, indicating that the anticorrosive properties of SZC coated specimens increased 20 times compared with that of UZC coated specimens. In order to further investigate the anticorrosive properties of SZC, a long‐term immersion test was carried out in neutral 3.5 wt% NaCl solution using EIS. The results of long‐term corrosion test showed that the SZC could provide long‐term protection in neutral 3.5 wt% NaCl solution for NdFeB magnet.     

低压烧结对NdFeB磁体微观结构与服役稳定性的影响（英文）

研究了NdFeB磁体微观结构和服役稳定性的内在联系。结果表明,低压烧结NdFeB磁体具有更加细小的晶粒尺寸和分布更为均匀的晶间富钕相,有利于磁体获得更小的矫顽力温度系数,从而提高其温度稳定性。对比真空烧结后的磁体,低压烧结磁体的矫顽力温度系数从-0.488%/℃减小至-0.472%/℃。但是富钕相从三角晶界向主相晶间流动形成了完整的网状结构,不利于磁体的耐腐蚀性能。低压烧结磁体在3.5%（质量分数）NaCl溶液中浸泡后腐蚀失重更为严重,表现出更强的腐蚀倾向。     

烧结钕铁硼电镀锌铁合金工艺与耐蚀性能

采用弱酸性氯化物镀液在钕铁硼基体上制备了高耐蚀性的锌铁合金镀层,讨论主要工艺参数对镀层铁含量的影响,优化工艺条件。采用盐雾试验(NSS)、SEM和电化学方法研究镀层的耐蚀性能和耐蚀机理。结果表明,优化工艺条件后合金镀层含铁质量分数为0.92%,钝化后在质量分数3.5%的Na Cl溶液中出白锈时间达到196 h。合金镀层对钕铁硼基体起到阳极保护的作用,镀层结晶致密,填补了钕铁硼基体的固有缺陷,同时又为获得致密的钝化膜创造了条件,减少了镀层表面的缺陷,使镀层整体具有极高的电阻,提高了其耐蚀性能。     

电沉积镀镍层的无铬钝化工艺

采用正交试验法筛选了一种以植酸、钼酸钠和缓蚀添加剂为基本组成的无铬钝化液。通过电化学阻抗谱测试,确定了最佳成膜时间;通过5%CuSO4点滴试验、10%NaCl浸泡试验和极化曲线测试,对电沉积镀镍层无铬钝化后的耐蚀性能进行了研究。结果表明,采用本无铬钝化工艺后,镀镍层耐蚀性能良好。     

Comparative study on corrosion protection properties of electroless Ni‐P‐ZrO2 and Ni‐P coatings on AZ91D magnesium alloy

Electroless Ni‐P‐ZrO₂ and Ni‐P coatings on AZ91D magnesium alloy were prepared, and their corrosion protection properties were compared in this paper. The potentiodynamic curves and electrochemical impedance spectroscopy (EIS) of the coated magnesium alloy in 3.5% NaCl solution showed that the corrosion performance of Ni‐P‐ZrO₂ composite coating was superior to that of Ni‐P coating. The same conclusion was obtained with salt spray and immersion tests. The corrosion morphologies of two kinds of coatings with various immersion time intervals in 3.5% NaCl solution indicated that most corrosion products concentrated on the nodules boundaries of Ni‐P coating and blocked corrosion pit was the main corrosion form. For the Ni‐P‐ZrO₂ coating, tortuous nodules boundaries were not the weak sites of the coating and corrosion initiated from the nickel phosphor alloy around the nanometer powders. Open corrosion pits occurred on the composite coating surface, and the coating was corroded gradually. Thus, the Ni‐P‐ZrO₂ coating exhibited better corrosion protection property to magnesium alloy substrate than Ni‐P coating.     

镀锌层表面钝化膜在5%NaCl溶液中的腐蚀降解过程和半导体行为

采用扫描电化学显微镜(SECM)分析技术、电化学阻抗谱(EIS)和电容-电位测试研究了镀锌层表面钝化膜在质量分数5%NaCl溶液中的腐蚀降解过程和半导体行为。结果表明:浸泡过程中随着水和离子逐渐侵入钝化膜,钝化膜发生着缓慢的腐蚀降解,钝化膜在浸泡初期保持稳定,能够对基体金属起到较好的保护作用;钝化膜表现出n型半导体特征,随着浸泡时间的延长,Mott-Schottky曲线拟合直线的斜率逐渐减小,钝化膜载流子密度逐渐增大,表明钝化膜在浸泡过程中发生缓慢的腐蚀降解。     

Study on Phosphating by Ultrasonic Wave at Room Temperature for Sintered NdFeB Magnet

The corrosion behavior of a NdFeB magnet obtained at room temperature,with and without ultrasonic were investigated.The corrosion resistance was investigated by corrosion immersion tests in 3 wt%neutral NaCl solutions,potentiodynamic polarization tests and electrochemical impedance spectroscopy(EIS).The morphology of the conversion coatings on NdFeB surface were analyzed by scanning electron microscopy(SEM).And the forming mechanism of coatings was studied by potential-time curves.The tests of corrosion resistances show that the combination of phosphating treatment with ultrasonic will further improve the corrosion resistance of the NdFeB.The SEM also confirms that phosphating coating under ultrasonic is more homogeneous and compact.     

镁合金直接化学镀Ni-B镀层的腐蚀电化学行为研究

  研究了镁合金表面化学镀Ni-B合金的电化学行为,采用电化学动电位扫描极化曲线和交流阻抗研究了Ni-B镀层的腐蚀电化学行为,结果表明,Ni-B镀层在3.5%NaCl溶液中具有优良的耐蚀性能.所得Ni-B镀层的自腐蚀电位在－400 mV左右,相对于基体-1460 mV提高了1000 mV,自腐蚀电流密度小于0.7 μA/cm²,相对于基体28.5 μA/cm²降低了近两个数量级,说明Ni-B镀层能够有效地提高AZ91D 镁合金的耐腐蚀性能,使AZ91D镁合金在35%NaCl溶液腐蚀介质中的腐蚀速度明显降低.电化学交流阻抗测试结果符合极化曲线的测量结果,化学镀Ni B镀层后的AZ91D镁合金在3.5％NaCl溶液中的阻抗值相对于基体提高两个数量级,表现为自腐蚀电流降低,阻抗值相应提高.     

NdFeB磁体表面致密化Ni镀层制备及其耐腐蚀性能

采用高能球磨法实现烧结NdFeB磁体表面Ni镀层的致密化,并对致密化后Ni镀层进行膜/基结合力、维氏硬度测试,通过中性盐雾实验和高温PCT实验研究磁体的耐腐蚀性能,采用静态全浸腐蚀实验进一步分析磁体的腐蚀过程。结果显示,球磨处理工艺可以实现磁体表面Ni镀层的致密化,当转速400 rpm,球磨时间为24 h时,Ni-D24/NdFeB磁体的显微硬度由427.95 HV增加至502.67 HV,结合力由16.30 MPa提升至23.85 MPa,具有更好的耐机械损伤性能。镀层的自腐蚀电流密度较Ni/NdFeB磁体降低了1个数量级,耐中性盐雾时间由312 h提升至480 h,具有更好的耐腐蚀性能。     

烧结NdFeB永磁体的腐蚀行为研究

通过静态腐蚀实验,研究了烧结钕铁硼磁体在3.5 wt.% NaCl溶液中浸泡不同时间段的腐蚀特征。采用扫描电镜、拉曼光谱分析各个阶段的腐蚀形貌和腐蚀产物组成,通过动电位极化曲线、电化学阻抗谱研究不同腐蚀阶段磁体表面结构的变化及腐蚀机理,采用等效电路模型模拟各阶段的腐蚀行为。研究结果表明,烧结NdFeB磁体在3.5 wt.% NaCl溶液中依次发生点蚀、选择性晶间腐蚀、全面腐蚀行为,形成的腐蚀产物层结构疏松,只能产生短时减缓腐蚀速率的效果,而富钕相优先溶解会造成主相颗粒脱落,导致磁体粉化、破坏。腐蚀产物类型随腐蚀时间延长而有所变化,主要为铁和钕的氧化物或氢氧化物。     

Corrosion of AZ91D magnesium alloy with a chemical conversion coating and electroless nickel layer

A chemical conversion treatment and an electroless nickel plating were applied to AZ91D alloy to improve its corrosion resistance. By conversion treatment in alkaline stannate solution, the corrosion resistance of the alloy was improved to some extent as verified by immersion test and potentiodynamic polarization test in 3.5 wt.% NaCl solution at pH 7.0. X-ray diffraction patterns of the stannate treated AZ91D alloy showed the presence of MgSnO₃ · H₂O, and SEM images indicated a porous structure, which provided advantage for the adsorption during sensitisation treatment prior to electroless nickel plating. A nickel coating with high phosphorus content was successfully deposited on the chemical conversion coating pre-applied to AZ91D alloy. The presence of the conversion coating between the nickel coating and the substrate reduced the potential difference between them and enhanced the corrosion resistance of the alloy. An obvious passivation occurred for the nickel coating during anodic polarization in 3.5 wt.% NaCl solution.     

用电化学阻抗谱(EIS)研究环氧树脂涂层的防腐蚀性能

测试了环氧树脂/钢体系在3.5%NaCl溶液中的电化学阻抗谱,并结合激光拉曼光谱测量和扫描电镜观察,系统地研究了有机涂层/金属体系性能与失效过程。重点在于涂层内部腐蚀性介质的传输以及涂层/金属界面发生的变化。结果表明:浸泡初期阻抗减小,中期阻抗增大,后期阻抗又减小,这与界面处生成的腐蚀产物膜有关。随着涂层在电解质溶液中浸泡时间的延长,涂层中Cl-浓度增加,破坏了界面处的腐蚀产物膜。     

Al-Zn-Sn-Ga阳极腐蚀过程的电化学阻抗谱

通过测定Al-Zn-Sn-Ga阳极在3.5%NaCl(质量分数)溶液中浸泡不同时间的电化学阻抗谱(EIS),研究该阳极的腐蚀发展过程和腐蚀特征。结果显示:当合金刚被浸入3.5%NaCl溶液时,EIS谱为反应电阻很大的容抗弧,表明此时合金处于钝化态;随着浸泡时间的延长,EIS谱中高频段的容抗弧明显减小且低频段出现感抗弧,合金进入点蚀期;继续延长浸泡时间,EIS谱中除高频段的容抗弧和中、低频段感抗弧外,在低频段出现另一容抗弧,合金处于点蚀扩展期;随着浸泡时间的继续增加,低频段感抗弧消失,EIS谱由两个容抗弧组成,合金达到均匀腐蚀期。因此,合金的腐蚀由钝化态开始,经点蚀期和点蚀扩展期,达到均匀腐蚀期。     

利用EIS研究环氧防锈涂层实验室模拟实验和实海浸泡实验相关性

用电化学阻抗法 (EIS) 研究了环氧防锈涂层在实海浸泡实验及在3.5%NaCl溶液中浸泡实验和盐雾实验两种实验室模拟实验中的腐蚀失效行为,探讨了实海浸泡实验与2种实验室模拟实验的低频阻抗模值|Z |_{0.01 Hz}之间的对应关系。结果表明：3种腐蚀环境对所研究的涂层体系的破坏作用由小到大依次为：3.5%NaCl溶液<实海浸泡<盐雾;相对实海浸泡实验,盐雾实验对环氧涂层腐蚀失效的加速因子约为2.3。