Effects of Y 3+ on properties of electroless NiYP coatings

Material and specimen preparation

The sintered Nd–Fe–B magnets with dimensions of 12 × 12 × 4 mm served as substrates of the Ni–Y–P coating. Before electroless deposition, the substrates were mechanically grinded using SiC paper up to a grit of 1200, cleansed with alkaline solution containing 15–20 g L^− 1 NaOH, 20–30g L^− 1 Na₃PO₄, 10–18 g L^− 1 Na₃CO₃ and 6 g L^− 1 Na₂SiO₃ in ultrasonic field for 5 min at 80°C and rinsed with deionised water. Then, the substrates were immersed in 10%HNO₃ solution for 10 s and rinsed again with deionised water.

The basic bath contained nickel sulphate, sodium hypophosphite, trisodium citrate, sodium acetate and thiourea for the preparation of NiYP deposits (Table 1). The initial pH value of the basic bath was adjusted by diluting lactic acid to 5.0 ± 0.1. The bath temperature was kept in a constant temperature water bath at 85 ± 2°C under atmospheric environment. The substrates were immersed into electroless plating solution for 1 h. Y³⁺ ranging from 2 to 8 mmol L^− 1 was added into the base solution in the form of YCl₃·6H₂O.

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Table 1

Bath composition and deposition conditions

Composition	Condition
NiSO4·6H2O	18 g L− 1	pH = 5.0 ± 0.1
NaH2PO2·H2O	23 g L− 1	T = 85 ± 2°C
CH3COONa	30 g L− 1
Na3C6H5O7·2H2O	21 g L− 1
Y3+	2–8 mmol L− 1
CN2H4S	Defined amount

The deposition rate was calculated by the deposition time and thickness. The total thickness of the coatings was estimated based on the SEM images of the cross-section. Three readings were taken for each coatings, and the values were then averaged.

Microstructure analysis

The morphologies of the electroless NiYP coatings were investigated using SEM (JSM-6390A). During SEM observations, EDAX was employed to examine the phase composition of the coatings. Four readings were taken for each coatings, and the values were then averaged.

Electrochemical measurements

The polarisation curves of the electroless NiYP coatings in 3.5%NaCl solution were tested to evaluate the anticorrosion property. Saturated calomel electrode was used as a reference and Pt electrode as the assistant electrode. The exposed surface area of the studied sample was 1.0 cm². The scanning speed was 10 mV s^− 1.

Effect of Y³⁺ concentration on deposition rate of coating

When concentrations of Y³⁺ in bath solution were 0, 2, 4, 6 and 8 mmol L^− 1, the average thicknesses of the coatings measured by analysis of the cross-section were 38.25, 36.02, 31.89, 21.67 and 18.53 μm, and the standard deviations of the film thickness were 0.2902, 0.4055, 0.2214, 0.3027 and 0.2773 μm respectively. The dependence of the deposition rate on the concentration of Y³⁺ is illustrated in Fig. 1. It could be seen that the deposition rate decreased with the increase in the concentration of Y³⁺. The depositing rate was achieved at 36 μm h^− 1 with the Y³⁺concentration of 2 mmol L^− 1 in the deposition solution.

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1

Dependence of deposition rate on concentration of Y³⁺

The oxidation–reduction reactions of electroless Ni–P deposition in acidic bath could be broadly expressed as follows ¹⁶ Yttrium was the surface active element with the larger atomic radius (0.227 nm), the smaller ionic radius (0.09 nm) and the peculiar 4d¹5s² electronic configuration. When yttrium was added into the bath solution, yttrium tended to combine with complexing ions in the bath to form the coordination compounds containing yttrium element. ²¹ Under the action of yttrium coordination compound with sexadentate, four old d orbits being degenerated in the central ion yttrium were divided into the e_g group and the t_2g group. The e_g group concluded two orbits in higher energy level than before. The t_2g group took in three orbits in lower energy level compared to the original state. Compared with the Ni element with the 3d⁸4s² electronic configuration, the activations of yttrium coordination ions were more powerful.^22,23 With unique electron configuration and unique behaviour, rare earth yttrium tended to be preferentially adsorbed on the surface defects of sintered Nd–Fe–B magnets (dislocations, grain boundaries and vacancies). ²⁴ The tendency of the adsorbed yttrium was the loss of electrons and formed positive ions easily due to the 4d and 5s electrons of yttrium atom. In addition, because the electronegativity of Ni or P was higher than that of yttrium, the phosphorus atom absorbed unpaired electron in orbits of atoms of nickel and yttrium, and a bond of M–P was formed. ²³ In other words, electrons transferred from Ni and Y to P. The adsorbed yttrium on the surface of the substrate changed the interface structure of the electrode and hindered the electron transfer between hypophosphite ions and nickel and hydrogen atoms. ¹⁸ Therefore, the deposition and diffusion of the nickel ions were impeded. Moreover, the number of adsorbed rare earth yttrium on the surface of the substrate increased, which shielded the catalytic centre of self-catalytic reaction in the bath solution, slowed down the speed of self-catalytic reaction and led to the decrease in the deposition rate of NiYP alloy coatings.

Surface morphology and elemental analysis

The properties of electroless NiYP deposits were ascribed to their microstructural characteristics. Figure 2 showed the typical SEM images of the coatings with different concentrations of Y³⁺ significantly affecting the surface morphology of electroless NiYP alloy coatings. Hemispherical clusters formed in all the NiYP coatings. When concentrations of Y³⁺ were fewer than 8 mmol L^− 1, the surface of NiYP alloy coatings were uniform, and average size of hemispherical clusters decreased with the increase in Y³⁺ concentrations in the solution. When concentrations of Y³⁺ in the bath approached 8 mmol L^− 1, more surface defects and hemispherical clusters in large size appeared. These clusters confirmed the presence of nickel or nickel and trace amounts of yttrium in the coatings. When concentrations of Y³⁺ were up to 6 mmol L^− 1, the clusters contained 86.83% nickel, 12.65% phosphorus and 0.52% yttrium, as determined by EDAX. Moreover, the concentration of these clusters was different from the bulk concentration. Because the absorbed nickel or yttrium became the active centre and a lot of crystal nuclei of the clusters formed, the coatings with large cluster density were retained up to the yttrium concentration of 6 mmol L^− 1 in bath solution. ²⁵ When further increasing Y³⁺ to 8 mmol L^− 1, the size of the active centre decreased and the charged yttrium cations might form neutral molecule with , preventing its oxidation.^25,26 It was not enough to form large numbers of clusters of crystal nucleus. This might be the reason for the low cluster density for 8 mmol L^− 1. Therefore, the addition of the rare earth yttrium in small amounts to the solution significantly modified the surface of the coatings. However, yttrium ions of high concentration could inhibit the oxidation reaction of hypophosphite, resulting in non-uniform deposition of alloy and the low quality of the alloy coatings.

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2

Surface morphologies of electroless deposited NiYP coatings at different concentrations of Y³⁺: a 0 mmol L^− 1; b 2 mmol L^− 1; c 4 mmol L^− 1; d 6 mmol L^− 1; e 8 mmol L^− 1

When rare earth yttrium concentrations in the optimised bath and conditions were fixed at 2, 4, 6 and 8 mmol L^− 1 respectively, the NiYP alloy coatings contained the corresponding rare earth yttrium of 1.15, 3.36, 3.93 and 4.15 wt-%, as determined by EDAX. The composition of the NiYP alloy coatings measured by EDAX with different concentrations of Y³⁺ in the baths is shown in Table 2. From Table 2, it was evident that composite codeposition of nickel–yttrium–phosphorus and the high phosphorus deposit appeared. In the case of NiYP ternary system, Y³⁺, Ni²⁺ and H⁺ competed for the electrons released during the oxidation of causing the yttrium element contents to increase with the increase in the concentration of Y³⁺ in the bath. The deposition of the rare earth yttrium element in the NiYP alloy coatings resulted in the reduction of nickel element. When the concentration of Y³⁺ was < 8 mmol L^− 1, nickel element content decreased with the increasing concentration of Y³⁺ in the bath. When the concentration of Y³⁺ in the bath was 6 mmol L^− 1, nickel element content in the coatings dropped down to 82.25 wt-%. For yttrium ion concentrations of 6 and 8 mmol L^− 1 in the solution, the content of nickel in alloy coatings showed little difference. According to the electrochemical deposition theory, was adsorbed to the substrate surface, and free electron was a result of the oxidisation. ¹⁷ After accepting free electrons, nickel and yttrium ions in the solution were reduced and preferentially nucleated on the substrate surface in the electroless process. However, excess adsorbed yttrium ions hindered the adsorption and deposition of nickel ions on the substrate surface, resulting in the decrease in nickel element content and deposition rate. When the concentration of Y³⁺ was < 6mmol L^− 1, phosphorus element increased with the increasing Y³⁺ concentration in the solution. The yttrium ions promoted the transfer of free electrons produced by the oxidisation of and codeposited along with Ni and P by secondary reactions. Once the concentration of Y³⁺ was more than 6 mmol L^− 1, the P content of the NiYP alloy coatings was almost 13.55wt-%, indicating that yttrium ion concentration had a little effect on the phosphorus content.

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Table 2

Effect of Y³⁺ concentration on element content of electroless NiYP coatings*

Element		a	b	c	d	e
Mass/%	P	12.17	12.90	13.96	13.82	13.55
	Ni	87.83	85.95	82.68	82.25	82.30
	Y	0	1.15	3.36	3.93	4.15
Error/%	P	0.20	0.18	0.20	0.19	0.19
	Ni	1.38	1.31	1.34	1.33	1.41
	Y	0	0.53	0.54	0.52	0.58

*

a: 0 mmol L^− 1; b: 2 mmol L^− 1; c: 4 mmol L^− 1; d: 6 mmol L^− 1; e: 8 mmol L^− 1.

Corrosion resistance of coatings

Figure 3 shows the potentiodynamic polarisation curves of the NiYP coatings prepared from the baths with different concentrations of Y³⁺. The values of E_corr and I_corr for coated NdFeB magnets by fitting the potentiodynamic polarisation curves with Tafel epitaxial method are listed in Table 3. As the specimens were immersed in 3.5%NaCl solution, the self-corrosion current density i_corr of the bare NdFeB was 703 μA cm^− 2, and the self-corrosion potential E_corr was − 792 to − 906 mV.^27,28 In comparison with the bare NdFB substrate, it appeared that the self-corrosion potential E_corr moved positively for the magnet coated with NiYP alloy film. Accordingly, the corrosion current greatly decreased about one to two orders of magnitude. It indicated that the electrochemical stability of the magnet coated with NiYP alloy film was enhanced, resulting in better corrosion resistance. The self-corrosion potential of NiYP alloy coatings on the magnet first increased and then decreased with the increasing concentration of Y³⁺ in the bath. Therefore, the self-corrosion current density first decreased significantly and then increased with increasing concentration of Y³⁺. When the concentration of Y³⁺ in the solution was 6 mmol L^− 1, the maximum of self-corrosion potential and the minimum of the self-corrosion current density were obtained, and their values were − 513 mV and 1.15 μA cm^− 2 respectively. This indicated that NiYP coatings with 6 mmol L^− 1 concentration of Y³⁺ in baths had the best protective ability for NdFeB magnets compared with others. The main reason for the electrochemical corrosion resistance of NiYP alloy coatings on NdFeB magnets was their fewer defect and uniform surface (Fig. 2). Furthermore, the improvements of corrosion resistance could be also resulted from acceleration of the phosphorus rich film formation in the interface between coatings and the bath solution (Table 2), due to the fact that P content in coatings increased by the addition of yttrium ions in the baths. ²⁹ In addition, because P content had higher stability and rare earth passivated the surface of Ni–P film, corrosion resistance of the coatings (with Y³⁺ concentration of 6mmol L^− 1 in bath solution), which had the more P and Y contents, was better.^23,26 When the concentration of Y³⁺ in the solution approached 8 mmol L^− 1, the self-corrosion current density of the coatings was up to 7.24 μA cm^− 2, indicating that the corrosion resistance of the coatings decreased. The significant decrease in corrosion resistance was closely relevant to reduced nickel deposition and deposition rate because of excessively high yttrium ions adsorbed on the surface of the specimen.

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3

Potentiodynamic polarisation curves of NiYP coatings prepared from baths with different concentrations of Y³⁺

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Table 3

E_corr and I_corr of electroless NiYP coatings

Concentration of Y3+/mmol L− 1	Self-corrosion potential Ecorr/mV(SCE)	Self-corrosion current density Icorr /μ A cm− 2
2	− 672	28.81
4	− 572	17.37
6	− 513	1.15
8	− 593	7.24