Protection of NdFeB magnets by corrosion resistance phytic acid conversion film

Accepted Manuscript Title: Protection of NdFeB magnets by corrosion resistance phytic acid conversion film Author: Haiyang Nan Liqun Zhu Huicong Liu Weiping Li PII: DOI: Reference:

S0169-4332(15)01734-1 http://dx.doi.org/doi:10.1016/j.apsusc.2015.07.167 APSUSC 30894

To appear in:

APSUSC

Received date: Revised date: Accepted date:

4-2-2015 7-7-2015 23-7-2015

Please cite this article as: H. Nan, L. Zhu, H. Liu, W. Li, Protection of NdFeB magnets by corrosion resistance phytic acid conversion film, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.07.167 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Protection of NdFeB magnets by corrosion resistance phytic acid conversion film

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Haiyang Nan, Liqun Zhu, Huicong Liu*, Weiping Li

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Key Laboratory of Aerospace Advanced Materials and Performance (Ministry of Education),

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School of Material Science & Engineering, Beihang University, Beijing, 100191, China

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* Corresponding authors.

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E-mail addresses: [email protected]

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Tel.: +86 010 82317113; fax: +86 010 82317133.

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Abstract:

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Phytic acid conversion film was prepared on NdFeB magnets by dipping the NdFeB

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into phytic acid solution. The morphology, composition, structure and corrosion

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resistance of the film were systematically investigated. The results showed that the

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phytic acid film was effective in improving the corrosion resistance of NdFeB

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magnets. XRD, TEM and FT-IR analyses revealed that the film was amorphous and

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had a strong peak of phosphate radical (PO43-). The formation mechanism of the film

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was also explored by XPS and the potential of zero charge (Epzc) measurement at the

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solution-metal interface.

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Keyword: NdFeB; phytic acid; corrosion resistance;

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Phytic acid conversion film was prepared on NdFeB magnets. Potential of zero charge was used to explore the state of solution-metal interface. The formation mechanism was discussed by XPS measurement. The film is effective in protecting NdFeB magnets.

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1. Introduction As the third generation of permanent magnet materials, NdFeB permanent

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magnets exhibit excellent magnetic properties[1] and are widely used in advanced

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technologies such as hard disk drive, electrical automobile, and magnetic resonance

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imaging. However, NdFeB magnets are porous and composed of matrix Nd2Fe14B

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phase and intergranular Nd-rich phase. the electrochemical potential of Nd-rich phase

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is much more negative than that of Nd2Fe14B phase. Due to this specific

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microstructure, NdFeB magnets are prone to intergranular corrosion in corrosive

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environments, resulting in pulverization failure and significant magnetic deterioration.

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This seriously limited their further applications[2-6]. In order to improve the

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corrosion resistance, numerous attempts have been employed, such as the addition of

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alloying elements and application of surface coatings. The addition of alloying

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elements[7-11] such as Cu, Zn, Ga, Si, can improve the corrosion resistance of

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NdFeB magnets, but usually deteriorate their magnetic properties. Electroplating or

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phosphating conversion coatings are widely employed, but often accompanied by

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environmental problems[12-17]. Therefore, it is very significant to develop

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environmentally friendly and low cost coatings for protecting NdFeB magnets from

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corrosion.

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In recent years, various phytic acid conversion films have been developed on Al

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or Mg alloys due to their low treatment cost and eco-friendliness[18-24]. Phytic acid

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exists naturally in all plant seeds, most of roots and tubers, and is used as food

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additive[25]. It consists of 24 oxygen atoms, 12 hydroxyl groups and 6 phosphate

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carboxyl groups[24, 26] and its structure is shown in Fig. 1. This peculiar structure

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gives phytic acid a powerful capability of chelating with many metal ions, which can

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deposit on the surface of the metal substrate and thus improve the corrosion resistance.

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Thereby, phytic acid has potential to form corrosion resistance conversion film on

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NdFeB magnets.

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In this paper, a corrosion resistance phytic acid conversion film was prepared on

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NdFeB magnets. The morphology, structure and composition were investigated. The

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formation mechanism of the film was also discussed.

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2. Experimental 2

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2.1 Samples preparations

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The powder-sintered NdFeB magnets of grade 42 H (Nd15.1Tb0.6Fe77.4B6.9) were

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used in the investigation. Samples (size Ф10 mm × 3 mm) were polished with SiC

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abrasive papers (of grades in the range 600-2000), ultrasonically cleaned in alcohol

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and then dried in air. Chemical purity grade phytic acid (PA, 70 wt.% in water), analytical purity grade

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sodium hydroxide and deionized water were used for solution preparation. The

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treatment solutions contained various concentrations of phytic acid (0.2 mM, 1 mM, 5

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mM and 25 mM) , and the pH value was adjusted to 4 by sodium hydroxide.

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Treatment solutions of pH value near 4 were appropriate according to the previous

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studies of PA conversion films[21, 22] [27] .

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The phytic acid conversion films were formed by dipping the NdFeB magnets

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samples in the treatment solutions for 1 h at 30 ℃, then the samples were washed

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with deionized water for 2 min and dried thoroughly in air.

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2.2 Characterization

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A three-electrode system was employed for the electrochemical tests on a

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CHI660e electrochemical workstation (Chenhua, China). The NdFeB magnets

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samples were used as the working electrode with the exposed area of 0.785 cm2, while

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the saturated calomel electrode (SCE) and platinum electrode were used as reference

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electrode and counter electrode, respectively. The polarization measurements were

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carried out in the potential range from -0.95 V to -0.45 V (vs. SCE) with a scanning

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rate of 2 mV/s. The linear Tafel segments of the cathodic curves were extrapolated to

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the corrosion potential (Ecorr) to obtain the corrosion current density (icorr)[28, 29]. The

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polarization measurements were tested in 3.5 wt.% NaCl solution at 30 ℃ and carried

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out at least three times to ensure reproducible results.

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The surface morphology and energy dispersive spectroscopy (EDS) of NdFeB

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magnets with and without phytic acid conversion film were characterized by scanning

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electron microscopy (FESEM, Apollo 300). Phase analysis was examined by x-ray

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diffraction (XRD, Rigaku, Japan) with Cu Kα radiation over an angle range of 20°-

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90° (2θ values). Transmission electron microscopy (TEM) was carried out by FEI

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Tecnai G2 F20 operated at 200 kV. XPS spectra of the sample surfaces were recorded 3

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using an ESCALAB 250 Xi system with an Al Kα anode. The XPS analysis was

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conducted after the surfaces were etched by argon-ion-beam. All energy values were

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corrected according to the C 1s spectral line at 284.6 eV. The impedance-potential measurements were used to obtained the potential of

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zero charge (Epzc) through the plot of double layer capacitance (Cdl) versus applied

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electrode potential (E). The measurements were carried out in the potential range

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from -0.7 V to -0.4 V (vs. SCE) with a interval of 10 mV. The test frequency was 1

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Hz with an amplitude of 5 mV. The impedance-potential measurements were tested in

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1 mM PA solution at 30 ℃ and carried out at least three times to ensure reproducible

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results. Infrared absorption spectra were measured with a Fourier transform infrared

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spectroscopy (FTIR, Nicolet 6700,USA) in the spectral range 400-4000 cm−1.

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3. Results and discussion

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3.1 Polarization measurements

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The polarization curves of the bare NdFeB and NdFeB with phytic acid (PA)

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films (prepared from different concentrations of PA solutions) are shown in Fig. 2.

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The corresponding parameters are listed in Table 1. Compared with the bare NdFeB,

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the corrosion current densities of NdFeB with PA films were much lower, indicating

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that the PA films were effective in protecting the NdFeB magnets from corrosion.

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With the increasing concentration from 0.2 mM up to 1 mM, the current densities

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decreased by more than one order of magnitude with respect to that of the bare

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NdFeB. The current densities increased when the preparation concentration was

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higher than 5 mM. And the value of cathodic Tafel slope had similar tendency. The

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NdFeB with 1 mM PA film had the highest cathodic Tafel slope. This might be due to

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the formation of PA film on the NdFeB magnets. It changed the surface state of

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NdFeB and provide good corrosion resistance. The results showed the concentrations

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of PA solution had significant impact on the corrosion resistance of PA films. This

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impact was further studied by SEM and EDS.

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3.2 SEM and EDS characterization

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SEM and EDS were employed to characterize the morphology and composition of

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bare NdFeB and PA film. The corresponding images and spectra are shown in Fig. 3

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and Fig. 4. As illustrated in Fig. 3, the morphology of NdFeB with PA films prepared 4

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from 0.2 and 25 mM PA solution were similar with that of bare NdFeB. The

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morphology of NdFeB with PA films prepared from 1 and 5 mM PA solution were

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coarser. As shown in Fig. 4, EDS spectra of NdFeB with PA films contained extra

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peaks of P and O elements. The signal of P suggested the existence of PA in the film.

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The peaks of P and O elements of PA films prepared from 1 and 5 mM PA solution

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were relatively stronger than those of PA films prepared from 0.2 and 25 mM PA

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solution. This indicted more PA molecules were involved in the formation of PA film

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in 1 and 5 mM PA solution. The PA films prepared from 1 and 5 mM PA solution

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also had better corrosion resistance. This results were consistent with that of

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polarization measurements.

To accurately determine the thickness of the PA film, a Cu coating was

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electrodeposited on the NdFeB with PA film. The cross section image is shown in Fig.

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5. As shown in Fig. 5, the PA film was very thin and difficult to be observed between

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NdFeB and Cu coating. The formation of PA film was due to the dissolution of metal

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ions of NdFeB in PA solution. While the PA molecules chelated with these ions and

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deposited on the NdFeB. However, the formed PA film would block the dissolution of

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metal ions. And the thickness of PA film might not further increase. This also

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demonstrated the formed PA film was compact. Due to the low concentration, the PA

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film prepared from 0.2 mM PA solution for 1 h might not integral. In 25 mM PA

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solution, the PA molecules could fully adsorbed on the surface NdFeB as corrosion

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inhibitor because of the relatively high concentration. The fully adsorbed PA

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molecules could also block the dissolution of metal ions and suppress the formation

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of PA film. This explained the relatively lower corrosion resistance and of PA films

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prepared from 0.2 and 25 mM PA solution. And the content of O and P elements were

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also lower.

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3.3 XRD and TEM analyses

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In order to investigate the structure of the PA film, x-ray diffraction (XRD) was

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utilized. the XRD patterns of bare NdFeB and NdFeB with PA film prepared from 1

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mM PA solution are shown in Fig. 6. As seen in Fig. 6, the diffraction peaks of (004),

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(105), (006) and (008) were characteristic peaks of NdFeB magnets (PDF 36-1296,

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PDF 07-0090)[12, 30, 31]. Compared with the bare NdFeB, there was no new

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diffraction peak appeared in the pattern of NdFeB with PA film, indicating that the 5

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PA film formed on the NdFeB magnets was amorphous[21, 32]. Transmission

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electron microscopy (TEM) was used to further studied the structure of the PA film.

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The TEM images and diffraction patterns of bare NdFeB and NdFeB with PA film

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prepared from 1 mM PA solution are shown in Fig. 7. The tetragonal structure of

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NdFeB magnets was clearly shown in Fig. 7 (a,b). While the disordered structure and

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blurred ring pattern indicated the PA film was amorphous. PA film were composed of

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metal ions and PA molecules of low molecular weight. This mixture are prone to form

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amorphous structure. Some PA films formed on Al alloy were also confirmed as

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amorphous structure[21].

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3.4 XPS analysis

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The chemical state changes of the Nd, Fe and B elements are important in

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exploring the formation mechanism of PA film on NdFeB magnets. In order to

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determine the chemical states of the elements in bare NdFeB and PA film, XPS

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analysis was used and the high resolution spectra of P 2p, Nd 3d, Fe 2p and B 1s are

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presented in Fig. 8. Compared with bare NdFeB, there was a peak of P 2p in the PA

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film (Fig. 8a), implying the existence of PA. This result was in accordance with that

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of EDS. In the PA film, the bonding energy of Nd 3d (984.7 eV), Fe 2p (708.1 eV)

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and B 1s (192.4 eV) were in good agreement with the oxidation state of Nd3+, Fe2+

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and B3+[32-36]. In the bare NdFeB, the bonding energy of Nd 3d (981.6 eV), Fe 2p

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(706.7 eV) and B 1s (187.7 eV) were in good agreement with the metal state of Nd,

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Fe and B[35-39]. The results showed that the Nd, Fe, B elements changed from metal

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state to oxidation state in the formation process of PA film.

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3.5 Potential of zero charge (Epzc) and formation of PA film

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The surface charge state of NdFeB magnets in PA solution is important in

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studying the formation process of PA film. It can be determined by the comparison of

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open circuit potential (Eocp) with the potential of zero charge (Epzc). Epzc is of

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fundamental importance in surface science, which is a concept relating to the

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phenomenon of adsorption, and describing the condition when the electric charge

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density on a surface is zero[40]. It can be obtained by impedance-potential

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measurements through the plot of double layer capacitance (Cdl) versus applied

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electrode potential (E)[41-44]. Fig. 9 displays the dependence of Cdl on the applied

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potential of NdFeB magnets in 1 mM PA solution at 30 ℃. The values of Eocp and 6

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Epzc were -0.59 V and -0.49 V, respectively. The value of ψ (ψ = Eocp - Epzc) was -0.1

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V, indicating that the surface of NdFeB magnets was negatively charged.

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In PA solution, the PA may exist in anion form in equilibrium with the

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corresponding molecular form due to deprotonation (Eq. 1)[22, 45, 46].

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H i Phy (12−i )− ⇔ H + + H i −1Phy (12−i +1)−

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When the NdFeB magnets were immersed in the PA solution, the elements of Nd, Fe

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and B dissolved in the solution as multivalent cations, such as Nd3+, Fe2+ and B3+ (Eq.

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2-4). It also resulted in the negative charge on the NdFeB surface. The negative

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charge would suppress the further generation of Nd3+, Fe2+ and B3+.

(1)

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Nd → Nd 3+ + 3e −

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Fe → Fe 2+ + 2e −

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B → B3+ + 3e −

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The negatively charged NdFeB surface could absorb the cations ( Nd3+, Fe2+, B3+, H+

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and Na+) in the solution via electrostatic interaction[40] and form a electrical double

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layer. The absorb cations would attract the anionic PA in PA solution. And the

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anionic PA tended to chelate with these cations, which could precipitate and form the

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PA film (Eq. 5-7)[45]. H+ also gained the electrons released from the negatively

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charged NdFeB surface and formed hydrogen in the process (Eq. 8). The reduce of H+

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would promote the deprotonation of PA and furtherly enhance the chelation and

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precipitation. And it also reduced the negative charge on the NdFeB surface, which

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would be conducive to the generation of Nd3+, Fe2+ and B3+. These reactions would

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finally be suppressed when a compact PA film was produced on the NdFeB surface.

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xNd 3+ + H i Phy (12−i ) − → Nd x (H i Phy) ↓

(5)

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xFe 2+ + H i Phy (12−i )− → Fe x (H i Phy) ↓

(6)

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xB3+ + H i Phy (12−i )− → B x (H i Phy) ↓

(7)

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2H + + 2e − → H 2 ↑

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3.6 FT-IR analyses

(2) (3) (4)

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FTIR spectrum was performed for the PA solution and PA film prepared from 1

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mM PA solution. As seen in Fig. 10, The characteristic peaks of PA were centered at

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1077 cm-1, 1638 cm-1 and 3440 cm-1, assigning to phosphate radical (PO43-),

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phosphate hydrogen radical (HPO42-) and hydroxyl (OH-), respectively[19, 21, 22, 26].

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For the PA film, the characteristic peaks of PO43- and HPO42- were centered at 1102

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cm-1 and 1635 cm-1, respectively. Compared with the spectrum of PA solution, the

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peaks of PO43- and HPO42- were strengthened and weakened, respectively. As shown

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in the Eq. 1 and Eq. 5-7, the PA molecules were prone to react with the metal ions and

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dissociate H+ in the formation process of PA film. Thereby, this would increase the proportion of PO43- and decrease the proportion of HPO42-.

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4. Conclusions

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PA film on NdFeB magnets was successfully prepared from 1 mM PA solution.

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The corrosion current value decreased by more than one order of magnitude,

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suggesting that PA film was effective in improving the corrosion resistance of NdFeB

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magnets. As shown in the XRD, TEM and FT-IR analyses, the film was amorphous

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and had a strong peak of phosphate radical (PO43-). The film contained elements of C,

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O, P, Nd, Fe and B. And the elements of Nd, Fe and B existed in the film as oxidation

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state. In PA solution, the surface of NdFeB magnets was negatively charged and

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absorbed the cations in the solution via electrostatic interaction. Thus the anionic PA

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chelated with these cations on the NdFeB surface, which could precipitate and form

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the PA film.

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References

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[23] R.K. Gupta, K. Mensah-Darkwa, J. Sankar, D. Kumar, Enhanced corrosion resistance of phytic acid coated magnesium by stearic acid treatment, Transactions of Nonferrous Metals Society of China, 23 (2013) 1237-1244. [24] R.K. Gupta, K. Mensah-Darkwa, D. Kumar, Effect of Post Heat Treatment on Corrosion Resistance of Phytic Acid Conversion Coated Magnesium, Journal of Materials Science & Technology, 29 (2013) 180-186. [25] Y. Li, J.B. He, M. Zhang, X.L. He, Corrosion inhibition effect of sodium phytate on brass in NaOH media. Potential-resolved formation of soluble corrosion products, Corrosion Science, 74 (2013) 116-122. [26] R.F. Zhang, H.W. Shi, Z.L. Liu, S.F. Zhang, Y.Q. Zhang, S.B. Guo, Property of anodic coatings obtained in an organic, environmental friendly electrolyte on aluminum alloy 2024-T3, Applied Surface Science, 289 (2014) 326-331. [27] Y.Q. Chen, G.J. Wan, J. Wang, S. Zhao, Y.C. Zhao, N. Huang, Covalent immobilization of phytic acid on Mg by alkaline pre-treatment: Corrosion and degradation behavior in phosphate buffered saline, Corrosion Science, 75 (2013) 280-286. [28] C.J. Zou, X.L. Yan, Y.B. Qin, M. Wang, Y. Liu, Inhibiting evaluation of betaCyclodextrin-modified acrylamide polymer on alloy steel in sulfuric solution, Corrosion Science, 85 (2014) 445-454. [29] N. Dinodi, A.N. Shetty, Alkyl carboxylates as efficient and green inhibitors of magnesium alloy ZE41 corrosion in aqueous salt solution, Corrosion Science, 85 (2014) 411-427. [30] W.Q. Liu, C. Sun, M. Yue, H. Sun, D.T. Zhang, J.X. Zhang, X.F. Yi, J.W. Chen, Improvement of coercivity and corrosion resistance of Nd-Fe-B sintered magnets by doping aluminium nano-particles, Journal of Rare Earths, 31 (2013) 65-68. [31] H. Yang, S. Mao, Z. Song, The effect of absorbed hydrogen on the corrosion behavior of sintered NdFeB magnet, Materials and Corrosion-Werkstoffe Und Korrosion, 63 (2012) 292-296. [32] J.L. Li, S.D. Mao, K.F. Sun, X.M. Li, Z.L. Song, AlN/Al dual protective coatings on NdFeB by DC magnetron sputtering, Journal of Magnetism and Magnetic Materials, 321 (2009) 3799-3803. [33] E.I. Neacsu, V. Constantin, K. Yanushkevish, A. Galyas, O. Demidenko, J. Calderon-Moreno, A.M. Popescu, Obtaining, structural, magnetic and corrosive properties of Nd-Fe-B alloy thin films on glass, Applied Surface Science, 314 (2014) 30-39. [34] X. Xue, G.Q. Tan, W.L. Liu, H.J. Ren, Nd doping effect on Bi1xNdxFe0.97Co0.03O3 thin films: Microstructural, electrical, optical and enhanced multiferroic properties, Materials Chemistry and Physics, 146 (2014) 183-191. [35] V. Bilovol, S. Ferrari, D. Derewnicka, F.D. Saccone, XANES and XPS study of electronic structure of Ti-enriched Nd-Fe-B ribbons, Materials Chemistry and Physics, 146 (2014) 269-276. [36] C.L. Li, Y.T. Ma, Y. Li, F.H. Wang, Corrosion mechanism of Mo/Nd16Fe71B13/Mo film in a simulated marine atmosphere, Corrosion Science, 53 (2011) 2549-2557. [37] M. Matsumiya, Y. Kikuchi, T. Yamada, S. Kawakami, Extraction of rare earth ions by tri-n-butylphosphate/phosphonium ionic liquids and the feasibility of recovery by direct electrodepositiori, Sep. Purif. Technol., 130 (2014) 91-101. [38] M. Matsumiya, K. Ishioka, T. Yamada, M. Ishii, S. Kawakami, Recovery of rare earth metals from voice coil motors using bis(trifluoromethylsulfonyl)amide melts by wet separation and electrodeposition, Int. J. Miner. Process., 126 (2014) 62-69.

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[39] K. Ishioka, M. Matsumiya, M. Ishii, S. Kawakami, Development of energy-saving recycling process for rare earth metals from voice coil motor by wet separation and electrodeposition using metallic-TFSA melts, Hydrometallurgy, 144 (2014) 186-194. [40] A.O. Yuce, B.D. Mert, G. Kardas, B. Yazici, Electrochemical and quantum chemical studies of 2-amino-4-methyl-thiazole as corrosion inhibitor for mild steel in HCl solution, Corrosion Science, 83 (2014) 310-316. [41] B. Xu, Y. Liu, X. Yin, W. Yang, Y. Chen, Experimental and theoretical study of corrosion inhibition of 3-pyridinecarbozalde thiosemicarbazone for mild steel in hydrochloric acid, Corrosion Science, 74 (2013) 206-213. [42] G. Moretti, F. Guidi, F. Fabris, Corrosion inhibition of the mild steel in 0.5M HCl by 2-butyl-hexahydropyrrolo[1,2-b][1,2]oxazole, Corrosion Science, 76 (2013) 206218. [43] W. Chen, H.Q. Luo, N.B. Li, Inhibition effects of 2,5-dimercapto-1,3,4thiadiazole on the corrosion of mild steel in sulphuric acid solution, Corrosion Science, 53 (2011) 3356-3365. [44] R. Solmaz, Investigation of corrosion inhibition mechanism and stability of Vitamin B1 on mild steel in 0.5 M HCl solution, Corrosion Science, 81 (2014) 75-84. [45] F. Crea, C. De Stefano, D. Milea, S. Sammartano, Formation and stability of phytate complexes in solution, Coord. Chem. Rev., 252 (2008) 1108-1120. [46] Q.C. Chen, B.W. Li, Separation of phytic acid and other related inositol phosphates by high-performance ion chromatography and its applications, J. Chromatogr. A, 1018 (2003) 41-52.

an

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d

Table 1 The electrochemical parameters of the bare NdFeB and NdFeB with PA coatings (prepared from different concentrations of PA solutions) in 3.5 wt.% NaCl at 30 ℃. Concentration

Ecorr versus SCE

(mM)

(mV)

Bare NdFeB

Ac ce pt e

25 26

-bc (V dec-1)

icorr (μA cm-2)

-803

2.71

23.6

0.2

-778

4.86

2.2

1

-812

8.12

1.2

5

-804

7.35

1.7

25

-767

3.89

3.6

29 30 31 32 33 11

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Fig. 1. The chemical structure of phytic acid.

2 3

Fig. 2. The polarization curves of the bare NdFeB and NdFeB with PA films (prepared from different concentrations of PA solutions) in 3.5 wt.% NaCl at 30 ℃.

4 5

Fig. 3. SEM images of the bare NdFeB magnets (a) and NdFeB with PA films prepared from different concentrations of PA solutions: (b)0.2 mM; (c)1 mM; (d) 5 mM; (e) 25 mM.

6 7

Fig. 4. EDS spectra of the bare NdFeB magnets (a) and NdFeB with PA films prepared from different concentrations of PA solutions: (b)0.2 mM; (c)1 mM; (d) 5 mM; (e) 25 mM.

8 9

Fig. 5. The cross section image of bare NdFeB and NdFeB with PA film prepared from 1 mM PA solution.

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1

Fig. 6. The XRD patterns of bare NdFeB and NdFeB with PA film prepared from 1 mM PA solution.

12 13

Fig. 7. The TEM images and diffraction patterns of bare NdFeB (a, b) and NdFeB with PA film (c, d) prepared from 1 mM PA solution.

14 15

Fig. 8. High resolution XPS spectra of P (a), Nd (b), Fe (c) and B (d) in bare NdFeB and NdFeB with PA film prepared from 1 mM PA solution.

16 17

Fig. 9. The plots of Cdl vs. applied electrode potential for NdFeB magnets in 1 mM PA solution at 30 ℃.

18

Fig. 10. FT-IR spectra of the PA solution and PA film prepared from 1 mM PA solution.

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