Research ArticleOrganic-inorganic composite passivation and corrosion resistance of zinc coated NdFeB magnets
Introduction
As the third-generation rare earth permanent magnet material, sintered NdFeB has the advantages of high remanence, high coercivity and high magnetic energy product [1], [2], and is widely used in various fields. However, due to its multiphase structure and large potential difference between phases, it is prone to electrochemical corrosion [3], [4], which greatly shortens the service life of the magnet and limits its development. Therefore, how to improve the corrosion resistance of sintered NdFeB magnet has become the focus of this field.
At present, there are two main methods to improve the corrosion resistance of NdFeB, one is alloying method, the other is surface protection. Alloying method refers to adding one or more metals, alloys or compounds to the original components to adjust the alloy composition in the process of preparing NdFeB magnet, so as to improve the magnet phase structure, reduce the potential difference between phases and reduce the corrosion force, so as to protect the magnet [5], [6]. However, the addition of alloying elements will reduce the reaction activity of the grain boundary phase, the remanence and magnetic energy product [7], [8]. Moreover, the alloying method will also increase the production cost, and cannot fundamentally solve the corrosion problem of NdFeB magnet.
Surface protection is mainly to coat one or more protective layers on the surface of NdFeB to isolate the external environment and corrosive medium, and achieve the long-term protection effect. At present, the most commonly used surface protective coating on NdFeB magnets include metal (alloy) coatings [9], organic coatings [10], ceramic coatings [11] and composite coatings [12]. Metal or alloy coatings mainly include Zn, Al, Ni, Cu and Ni-P [13], [14], [15], [16], [17], [18], [19], which are coated on the surface of NdFeB magnet by electroplating, electroless plating, electrophoresis and physical vapor deposition [20], [21], [22], such as electroplating Zn and vacuum evaporating Al. Cao et al. [23] prepared a bright and uniform Zn coating on NdFeB from the plating baths containing alumina-silica sols by electroplating, which greatly enhanced the corrosion resistance of NdFeB. Ding [17] electrodeposited a layer of Al-Mn coating on the surface of NdFeB magnet with MnCl2-AlCl3-EMIC ionic liquid. Al-Mn coating can protect NdFeB matrix as sacrificial anode without affecting magnetic properties, which greatly prolongs the service life of NdFeB magnet in corrosive environment. However, Zn and Al et al. are active metals, and the growing coating is not dense enough with many pores among columnar crystals, the untreated surface will react with water and oxygen in the air, thus affecting the service life of the matrix NdFeB. Therefore, passivation treatment needs to be carried out on the surface of metal coatings.
Passivation means that the ions in the solution react with the metal layers to form an insoluble passivation film. At present, the passivation of the coating surface mainly includes chromate passivation [24], phosphate passivation [25], [26], [27], rare earth passivation [28] and composite passivation [29], [30].
Huang et al. [31] prepared a layer of chromate conversion film with a thickness of about 0.5 µm on the surface of metal coating, showing that this dense and self-healing layer could improve the anti-corrosion performance of specimens by 8 ∼ 10 times. However, hexavalent chromium salt is highly toxic and difficult to eliminate after being absorbed by human body, causing great pollution to human beings and the environment, which is prohibited by many countries[32], [33]. Phosphate passivation is that the metal surface reacts with phosphate to produce a layer of crystalline or amorphous phosphate protective film on its surface. Rare earth salt passivation has become a chromium-free passivation process, which has attracted more and more attention. Rudd A.L.[34] used Ce (NO3)3 as the main film-forming material to prepare cerium salt passivation film on pure magnesium and magnesium alloy. Through the fitting analysis of polarization curves, it is found that the formed passivation film can block the cathode process and anode process of electrochemical reaction at the same time, so the corrosion resistance of pure magnesium and magnesium alloy has been significantly improved.
Considering the uneven film formation and poor adhesion of rare earth passivation film [35], it cannot provide long-term and effective protection for NdFeB magnet. Organosilane has a unique spatial structure [36], which can form a dense "molecular bridge" with good adhesion at the interface of organic and inorganic materials. This film has strong corrosion resistance and strong adhesion. Using the characteristics of organosilane and the synergistic effect of organosilane and Ce salt to improve the performance of passivation film can well solve the problems of Ce salt passivation film. Therefore, composite passivation is of great significance in the field of protection of NdFeB magnets.
Composite passivation refers to the preparation of compound passivation solution with organic and inorganic inhibitors in a certain proportion, which can effectively improve the corrosion resistance of metals after treatment, and is often more effective than a single passivation solution. At present, composite passivation mainly includes the inorganic substances (such as ZrO2, SiO2, CeO2 and Al2O3), organic silane [37], organic resin [38] and organic acid [39]. Li C [40] prepared a corrosion resistant superhydrophobic surface containing cerium salt and organic silane on the surface of galvanized steel by one-step solution immersion method, providing excellent chemical stability for the Zn coating.
In this study, a composite passivation layer is formed by adding different concentrations of organosilane solution into the Ce salt passivation solution to further increase the corrosion resistance of Zn coated NdFeB. At the same time, the optimal scheme was selected for salt spray experiment to explore the corrosion process by XPS and electrochemical analysis.
Section snippets
Preparation of electroplating Zn coating
The unmagnetized NdFeB magnet (Brand: 38SH, 63.8 % Fe, 26 % Nd, 6.5 % Pr, 2.5 % Dy, 1.2 % B) with a size of 12 mm × 12 mm × 2 mm was plated in a mixed solution containing 65 g/L ZnCl2, 215 g/L KCl and 37 g/L H3BO3 for 50 min with a current density of 1 A/dm2. The obtained Zn plated specimens were named Zn/NdFeB.
Composite passivation of Zn/NdFeB
Zn/NdFeB was acidified in 0.6 mass% HNO3 solution for 10 s, and then ultrasonically cleaned with ethanol for 3 min. Organosilane hydrolysis solution was prepared according to the ratio
Characterizations
Fig. 1 shows scanning electron microscopy (SEM) surface morphologies of Zn/NdFeB and Zn(CS)/NdFeB specimens obtained with different passivation solution concentrations, and the energy dispersion spectrum (EDS) elemental analysis is illustrated in the inset. As shown in Fig. 1(a), the surface of the unpassivated specimen is relatively flat, which is a dendritic structure with low density and many pores. After being passivated, the surface is covered with a passivation layer and the dendritic
Conclusions
In this work, an organic-inorganic composite passivation layer is successfully prepared on the surface of Zn/NdFeB by liquid phase method. Results show that the addition of organosilane can greatly enhance the anti-corrosion performance of Zn coating on sintered NdFeB magnets. Among them, the Jcorr of the optimized Zn(CS-1)/NdFeB specimens is 8.58 × 10–5 A/cm2, which is one order of magnitude lower than that of unpassivated specimens. Ecorr positively shifts from − 1.31 V to − 1.22 V. The
CRediT authorship contribution statement
Liangsong Duan: Conceptualization, Methodology, Validation. Jing Chen: Data curation, Writing – original draft. Pengjie Zhang: Visualization, Investigation. Guangqing Xu: Supervision, Project administration. Jun Lv: Funding acquisition, Validation. Dongmei Wang: Writing – review & editing. Wangqiang Shen: Resources, Writing – review & editing. Yucheng Wu: Project administration, Visualization.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the Key Project of BGRIMM Technology Group Co. Ltd (20190898000002), Hefei Municipal Natural Science Foundation (2021026), the Key Research and development Project of Anhui Province (202004a05020048, 202004a05020051) and the Fundamental Research Funds for the Central Universities (PA2020GDJQ0026).
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