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Corrosion protection of sintered NdFeB magnets by CAPVD Ti2N coating
Abstract
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.
... So, new developments are always on cards. Currently, the cathodic arc physical vapour deposited transition metal nitride coating has been reported to cater the corrosion protection of sintered neo magnets [19] . In CAPVD process the plasma assisted high energy cathodic arc ejects the metal vapours from the solid metal at ambient temperature in an evacuated chamber. ...
... Automatic microprocessor controlled feeding system was used to introduce nitrogen into the chamber with controlled partial pressure. Keeping other parameters constant as reported earlier [19] , three coating cycles of 40 min each were run to deposit the multilayer ceramic coating in an attempt to deposit a thick and dense coating with low density of permeable defects. ...
... Electrodeposition of nickel and copper was carried out with the Watt's solution and copper sulphate bath respectively. The bath composition and coating parameters have already been reported [19] . A nickel strike layer was deposited followed by the copper interlayer and again nickel layer above all. ...
Sintered NdFeB magnets have complex microstructure that makes them susceptible to corrosion in active environments. The current paper evaluated the anticorrosion characteristics of multilayer titanium nitride ceramic coating applied through cathodic arc physical vapour deposition (CAPVD) for protection of sintered NdFeB permanent magnets. The performance of ceramic coating was compared to the electrodeposited nickel coating having a copper interlayer. Electrochemical impedance spectroscopy (EIS) and cyclic polarization in simulated marine environment were employed to determine the rates of coatings degradation and passivation behaviour respectively. The coating morphologies and surface chemistry were studied with scanning electron microscope (SEM). X-ray diffraction (XRD) was used for identification of component phases in the coatings and the substrate. The results showed that the polarization resistance of ceramic coating increased with the exposure time. The rate of degradation of Rp for the ceramic coating had an extraordinary negative slope followed by a stable duration, before declining towards the coating failure. In comparison the nickel coating with copper interlayer degraded sharply. The vapour deposited ceramic coating was found to have permeable defects that tended to “re-passivate” during exposure providing prolonged corrosion protection to the NdFeB substrate. The magnetic properties were unaffected and remained at par with the nickel coating having copper interlayer.
... aluminium, titanium, nickel) are deposited by electroplating [10], DC magnetron sputtering [11], vacuum evaporation [12] and HVOF spraying [13]. Ceramic coating such as titanium nitride [14], zirconium oxide (ZrO 2 ) [15] and aluminium oxide (Al 2 O 3 ) are deposited by phase vapour deposition (PVD) techniques, sputtering techniques [16] and wet chemistry techniques [17]. Among wet depositions, the sol-gel process is a cheap, easily exploitable, wet deposition process [18], which allows to obtain ceramic thin films at low temperatures with high purity and homogeneity [19] even to cover big components with complicated geometries, whereas the majority of the above cited PVD techniques usually require high deposition temperatures or the use of complex vacuum devices. ...
Sol-gel alumina coatings have been deposited on neodymium-iron-boron magnets for corrosion protection. The coatings present a compact amorphous structure due to the low thermal treatment (500 °C) with a thickness ranging from 0.8 to 2 microns, a Vickers hardness of 10 GPa and good adhesion to the substrate, measured by scratch test, with a failure mode of tensile cracking and chipping. The coatings are effective in protecting the NdFeB magnets from corrosion with reduction of current density (i.e. 2.07 × 10⁻⁶ A/cm² for 2dip sample) of one order of magnitude with respect to the neat sample (1.15 × 10⁻⁵ A/cm²).
Graphical abstract
A phosphating passivation is conducted on the surface of Al coated NdFeB magnets by a gaseous process. The surface morphology, structure and elemental compositions of the samples are characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The anti-corrosion performances of the phosphating passivated Al/NdFeB magnets are analyzed by neutral salt spray test, potentiodynamic polarization curve and electrochemical impedance spectroscopy measurements. The corrosion process and corrosion mechanism of the samples are studied by analyzing the coating morphologies, potentiodynamic polarization curves and surface elemental compositions during the immersion test in 3.5 mass% NaCl solution for different time. The surface passivation of Al coated NdFeB magnets can be realized by gaseous phosphating technology, which can effectively improve the corrosion resistance of Al coating. The optimized sample obtained with phosphating temperature at 300 °C achieves the improvement of NSS time from 96 h to 288 h. Gaseous phosphating can effectively seal the pores in vacuum evaporated Al coatings, and form a phosphating passivation film on the coating surface, which contribute to the enhancement of anti-corrosion performances of Al coated NdFeB magnets.
Amorphous SiO2 coatings were prepared on sintered NdFeB magnets by micro-arc oxidation (MAO) in silicate solution. The surface and cross-sectional morphologies, element and phase composition, corrosion resistance and magnetic properties of the coatings were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), potentiodynamic polarization test and physical properties measurements system (PPMS). The results showed that the surface morphologies of the coatings exhibited the “coral reef” like structure, different from the typical MAO porous structure. With increasing the voltages, the thickness of the coatings increased from 12.72 μm to 19.90 μm, the content of Si element increased, while the contents of Fe, Nd and P elements decreased. The coatings were mainly composed of amorphous SiO2 and a few amorphous Fe2O3 and Nd2O3. The amorphous SiO2 coatings presented excellent thermal shock resistance, while the thermal shock resistance decreased with increasing the voltages. The corrosion resistance of the coatings increased with increasing the voltages, and it could be enhanced by one order of magnitude compared to the uncoated NdFeB magnets. The MAO coatings slightly decreased the magnetic properties of the NdFeB samples in different degrees.
Zinc coated NdFeB specimens were prepared with different pretreating technologies, such as polishing, pickling (50 s), sandblasting and combined technology of sandblasting and pickling (5 s). Morphologies of the NdFeB substrates pretreated with different technologies were observed with a scanning electron microscope equipped with an energy dispersive spectrometer and an atomic force microscope. The tensile test was performed to measure the adhesive strength between Zn coating and NdFeB substrate. The self-corrosion behavior of the NdFeB specimen was characterized by potentiodynamic polarization curve. The anticorrosion properties of Zn coated NdFeB specimens were characterized by neutral salt spray tests. The pretreating technologies possess obvious impact on the adhesive strength and anticorrosion property of Zn coated NdFeB specimens. Combined pretreating technology of sandblasting and pickling (5 s) achieves the highest adhesive strength (25.56 MPa) and excellent anticorrosion property (average corrosion current density of 21 μA/cm2) in the four pretreating technologies. The impacting mechanisms of the pretreating technology on the adhesive strength and anticorrosion properties are deeply discussed.
The anticorrosion characteristics of Ni/Ti2N composite coating applied through electrodeposition and cathodic arc physical vapour deposition (CAPVD) on sintered NdFeB permanent magnets were studied in simulated industrial environment (0.5% Na2SO4). The corrosion mechanism of bare and composite coating was evaluated by Tafel Polarization scans and degradation behaviour was estimated by AC impedance spectroscopy. The coating morphology, surface chemistry and identification of component phases were analyzed by scanning electron microscope and X-ray diffraction method respectively. It was deduced that the corrosion reactions of composite coated magnet were controlled by anodic polarization as compared to bare magnet where the degenerative tendency toward corrosion was rapid. The early coated magnet protection was deteriorated by kinetically controlled corrosion reactions as predicted from impedance spectroscopy. The magnetic properties remained un-affected for both composite coated and bare NdFeB magnets.
An Al coating was achieved on NdFeB substrate via plasma assisted physical vapor deposition method. The coatings were treated with trivalent chromium passivation and were characterized by a scanning electron microscope equipped with an energy dispersive spectrometer. The adhesive strength between the coatings and NdFeB substrates was evaluated by tensile test. The coatings achieve much higher adhesive strength (25.7MPa) than that of Al coating without plasma assistance. The anticorrosive property of the coatings was studied by electrochemical impedance spectroscopy, neutral salt spray and potentiodynamic polarization curve tests. The corrosion current density of the passivated Al coated NdFeB specimens is 0.2205μA·cm-2, which is two orders of magnitude lower than that of bare NdFeB substrates. Also, the NSS and EIS tests confirm the excellent anticorrosive properties of the Al coated NdFeB specimens with plasma assisted physical vapor deposition method.
In this paper, the nanometer titania particles enhanced acrylic resin composite coatings were prepared on sintered NdFeB permanent magnets by cathodic electrophoretic deposition. The microstructure, microhardness and wettability of the composite coatings were investigated. The corrosion resistance of the composite coatings was analyzed by H2SO4 solution immersion test, neutral salt spray test and electrochemical corrosion test, respectively. The results showed that the particles embedded in the acrylic resin matrix increased with increasing the concentration of the titania. At the same time, the microhardness of the composite coatings increased, while the contact angles decreased. Moreover, the particles presented uniformly dispersed distribution over the matrix as the concentration of the titania lower than 20 g/l. The weight loss of the composite coated NdFeB samples immersed in H2SO4 solution was much lower than that of the uncoated NdFeB, and the NSS time of the 20T-coated sample could reach 15 day. The corrosion current density of 20T-coated sample was nearly four orders of magnitude lower than that of the uncoated NdFeB as immersed in 3.5% NaCl for 8 h, and further increasing the immersion time from 24 h to 72 h, the corrosion current density increased, about two orders of magnitude lower than that of the uncoated NdFeB.
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.
A scratch test is used to measure the adhesion between corrosion product film (CPF) and carbon steel in deionized water under different pH and temperatures. The fuzzy feature model is modified to minimize errors between calculation and scanning electron microscopy (SEM) results. Findings illustrate that the maximum error decreases to 11%. The adhesion between CPF and substrate increases with rising temperature and pH of deionized water. The Poisson's ratio and Young's modulus of CPF are in the ranges [0.34 and 0.45] and [0.08 and 0.11 GPa], respectively, as temperature and pH change.
Al–Mn coatings were electrodeposited on sintered NdFeB permanent magnet in MnCl2–AlCl3–1-ethyl-3-methylim-idazolium chloride (MnCl2–AlCl3–EMIC) ionic liquid at room temperature. The coatings were characterized by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The adhesion strength of the coating on NdFeB substrate was evaluated by thermal shock and scratch test. The hardness and corrosion behavior of Al–Mn coating were measured by a Knoop microhardness tester, immersion test and neutral salt spray test respectively. The results showed that the amorphous structure of the deposits was obtained at the current density of 6 mA/cm2, while higher current densities resulted in a mixed structure of amorphous and crystalline. The Al–Mn coating showed excellent adhesion strength on NdFeB substrate with the thermal shock test over 30 cycles and Lc > 80 N. The hardness of Al–Mn coating was up to 5.4 GPa. The amorphous Al–Mn coating showed an anodic sacrificial protection with a low corrosion rate for NdFeB. Meanwhile, the magnetic properties measured by an AMT-4 magnetic measurement device showed that Al–Mn coating did not deteriorate the magnetic property of NdFeB.
Di-titanium nitride coatings were synthesized on commercial AISI 304 stainless steel using a Cathodic Arc Plasma Deposition
(CAPD) technique. The effect of substrate bias voltage on the formation of coatings was studied keeping other parameters constant.
The study revealed that 120 volts of substrate bias voltage is the most suitable for formation of Ti2N on the substrate. The thickness of the Ti2N coating was 0.70 to 0.80 μm for all the experiments. The hardness of the thin Ti2N coatings, determined by using Johnson and Hogmark relation was 2000 Hv. The structure of the coatings was studied by XRD.
The surface roughness and texture was also studied which are important parameters for the given films. The case studies for
high energy magnets and fertilizer industry are also discussed to show the worthiness of the work for industrial applications.
Sintered NdFeB magnets have complex microstructure that makes them susceptible to corrosion in humid or moist environments. The paper presents the anticorrosion characteristics of a novel Ni/Ti(2)N composite coating applied through electrodeposition and cathodic arc physical vapour deposition (CAPVD) to sintered NdFeB permanent magnets. The performance of composite coating was evaluated in simulated marine environment with the help of dc polarization techniques. The rate of coating degradation was also determined by employing ac electrochemical impedance spectroscopy (EIS). The coating morphology and surface chemistry were studied with scanning electron microscope (SEM). X-ray diffraction (XRD) was used for identification of component phases in the coating-substrate system. The results showed that the composite coating provided an adequately improved corrosion protection to the sintered NdFeB magnets in the simulated marine environment compared to the earlier reported ceramic and metallic coatings. The composite coating did not damage the magnetic properties of coating-substrate system that remained at par with the ceramic and nickel coating having copper interlayer.
Corrosion behavior of sintered NdFeB magnets in nitric acid and oxalic acid was investigated. The experiment results indicated that NdFeB is strongly corroded in nitric acid solution, and is passivated in oxalic acid solution. Based on the corrosion behavior, the possible corrosion mechanisms are discussed.
The paper presents the anticorrosion characteristics of a novel Ni/Ti2N composite coating applied through electrodeposition and cathodic arc physical vapour deposition for protection of sintered NdFeB permanent magnets that are susceptible to corrosion in active environments. The composite coating was evaluated in simulated marine and industrial environments with the help of dc polarisation techniques. The rate of coating degradation was determined through ac electrochemical impedance spectroscopy. The coating morphology and surface chemistry were studied with scanning electron microscope. X?ray diffraction was used for the identification of component phases in the coating?substrate system. The results showed that the corrosion current density iCorr of the composite coated NdFeB magnets came out to be 0?8575 and 0?3011??A?cm?2 that is about 10 and 4% of the ceramic coated NdFeB magnets in the marine and industrial environments respectively. The magnetic properties remained unaffected and were as good as for the uncoated, NiCuNi and ceramic coated NdFeB magnets.
This article has been retracted due to a substantial overlap with a previously published article in another journal.
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.
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.
Titanium nitride coatings have been deposited using cathodic arc physical vapor deposition technique on the commercially available steels. Effect of nitrogen partial pressure was studied keeping bias voltage, arc current and deposition time constant. Phases formed during coating were identified by x-ray diffraction. Microhardness was also determined using Vickers microhardness tester. The study revealed that only Ti2N was formed as a function of nitrogen partial pressure. The enhancement in hardness was found to be dependent on substrate material. The deposited coatings exhibited variation in colors (from light golden yellow to greenish golden yellow) which were attributed to the nitrogen partial pressure and the substrate material.
A new compound composed of Nd, Fe, and a small quantity of B (about 1 wt. %) has been found, which has a tetragonal structure with lattice constants a=0.880 nm and c=1.221 nm. This phase, which has the approximate composition, 12 at. % Nd, 6 at. % B and balance Fe, possesses remarkable magnetic properties. From the approach to saturation an anisotroy constant of about 3.5 MJ/m<sup>3</sup> can be calculated, while saturation magnetization amounts to 1.35 T. The magnetization versus temperature curve shows a Curie temperature of 585 K, which is much higher than those of the Fe and light rare earth binary compounds. Based on the new compound, sintered permanent magnets have been developed which have a record high energy product. Permanent magnet properties and physical properties of a typical specimen which has the composition Nd 1 5 B 8 Fe 7 7 are as follows: B r =1.23 T, H cB =880 kA/m, H cI =960 kA/m, (BH) max =290 kJ/m<sup>3</sup>, temperature coefficient of B r =-1260 ppm/K, density=7.4 Mg/m<sup>3</sup>, specific resistivity=1.4 μΩm, Vickers hardness=600, flexual strength=250 MPa.
Golden yellow cerium conversion film was obtained on magnesium alloys surface by immersion method and the preparation parameters were established. The influence of different process parameters on the surface morphology and performance of the conversion film were analyzed by means of SEM and electrochemical method. Formation dynamics about cerium conversion film on magnesium alloy in solution containing cerium salt and the anti-corrosion behavior of the conversion film in 3.5% NaCl solution were studied by electrochemical method respectively. The results shows that the conversion film is more compact at room temperature when concentration of cerium sulfate is 10 g·L−1 in the solution; the open circuit potential of the magnesium sample moves up to positive direction about 100 mV, the surface of conversion film becomes even and lustrous, and the adhesion intensity of conversion film increases when adding aluminum nitrate into the solution containing cerium salt. The pH value of the solution and immersion time of the sample in the solution also affect the surface morphology and anti-corrosion property of the conversion film. After covered by rare earths conversion film, the anti-corrosion property of magnesium alloy is obviously improved. Rare earth conversion film has self-repairing capability in corrosion medium.
A significant effect on the intrinsic coercivity (iHc) of sintered magnets was observed while comparing two methods of doping Al into NdFeB alloy at the preparation stage. Adding 0.25% Al by weight via FeB resulted in a 20% increase of i Hc over doping pure Al. It is believed that changes in the sintered microstructure are responsible for the observed effect
The corrosion behaviour of Nd–Fe–B type magnets was characterised by conducting gravimetric and electrochemical studies with synthetic preparations of the three maiN intermetallic phases of these materials. Tests were conducted in deaerated sulphate solutIOns at pH 0·3–9; some accelerated atmospheric corrosion tests were also carried out. In acidified solutions the neodymium rich (Nd4Fe) synthetic phase was the most active with a high corrosion rate, which could be further accelerated by cathodic hydrogenation; the boron rich (NdFe4B4) phase was the most noble and had the lowest corrosion rates. In neutral solutions corrosion of the magnets is inhibited by the protective properties of neodymium oxide-hydroxide layers formed on the surface of the neodymium rich phase. The protective properties of these layers disappeared at pH < 4.
NdFeB magnets are highly susceptible to corrosion in various environments. A nickel/alumina composite coating for NdFeB magnets was investigated in this paper. The microstructures of electrodeposited nickel coating and nickel/alumina composite coating were determined by scanning electron microscope (SEM). The corrosion behavior of nickel coating and nickel/alumina composite coating in 3.5 wt% sodium chloride solution was studied by polarization curves and electrochemical impedance spectroscopy (EIS). The results show that the nickel coating and nickel/alumina composite coating can both provide adequate protection to the NdFeB substrate. Furthermore, the free corrosion potential of nickel/alumina composite coating is more positive and the passivation region is more obvious compared with nickel coating, and the capacitance loop diameter of nickel/alumina composite coating is significantly larger than that of nickel coating. With the increase in immersion time, a flat of passivation film is formed on the surface of nickel/alumina composite coating, resulting in excellent corrosion resistance after 288 h immersion in 3.5% sodium chloride solution according to EIS testing.
It is shown experimentally that an addition of a large amount of
refractory elements, especially vanadium, in combination with a proper
amount of boron is very effective in enhancing the coercivity of
sintered Nd-Fe-B permanent magnets. A magnet consisting of Nd<sub>16
</sub>Fe<sub>72</sub>V<sub>4</sub>B<sub>8</sub> which has a coercivity
as high as 1.38 MA/m consists of three phases, Nd<sub>2</sub>Fe<sub>14
</sub>B, an Nd-rich phase, and V<sub>2</sub>FeB<sub>2</sub>, instead of
Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub> which is contained in
conventional Nd-Fe-B magnets
A few atomic percent addition of Mo considerably improves the intrinsic coercivity (H cJ ) of Nd‐Dy‐Fe‐Co‐B sintered magnets if the Mo content is optimized against B content. It is shown that upgrading of the magnet can be realized by decreasing both Mo and B contents, keeping the optimum Mo‐B relation. Dependence of corrosion resistance of both uncoated and aluminum‐coated magnets on Co and Nd contents are studied and significant improvement of the corrosion resistance is attributed to formation of an Nd‐Co compound, probably Nd 3 Co.
The corrosion mechanism of Nd‐Fe‐B sintered magnets has been investigated. A correlation between the lifetime of epoxy‐coated magnets in a humidity test (80 °C, 90% RH) and the rate of weight loss (ΔW/Δt) of the bare magnets in the pressure cooker test (PCT: 125 °C; 85% RH; 2 atm) has been established. In this system, corrosion is attributed to the selective oxidation of grain boundaries (the Nd‐rich phase). The value of ΔW/Δt is closely related to the concentration of neodymium and it decreases when cobalt or nickel is added. Consistency of corrosion resistance of the surface‐coated Nd‐Fe‐B sintered magnets can be achieved by controlling ΔW/Δt below a certain critical value.
We report the properties of a new class of high‐performance permanent magnets prepared from Nd‐Fe‐B and Pr‐Fe‐B alloys. Magnetic hardening is achieved by rapid solidification. Energy products of these isotropic materials can exceed 14 MGOe with intrinsic coercivities of ∼15 kOe. X‐ray and microstructural analyses indicate that the alloys exhibiting optimum characteristics are comprised of roughly spherical crystallites, strongly suggesting that the coercivity mechanism is of the single‐domain particle type. The crystallites are composed of an equilibrium R‐Fe‐B intermetallic phase having tetragonal symmetry, and the stability of this phase with respect to other rare earths and other metalloids has been investigated.
The magnetic properties of rapidly quenched FeRM alloys where R=La,Y,Pr,Nd,Gd and M=B,Si,Al,Ga,Ge have been investigated over a wide range of chemical compositions. The samples are generally magnetically soft in the as‐quenched state. Magnetic hardening is produced by annealing the samples around 700 °C. The best properties have been obtained in samples containing Pr and Nd together with B and Si. An energy product of 13 MGOe and a coercive field of 15 kOe have been obtained in a Fe 7 6 Pr 1 6 B 5 Si 3 sample. The higher Fe content samples appear to be more promising with a potential energy product of 49 MGOe. Thermomagnetic data show that a structural transformation takes place upon heating the samples to 700 °C. The Curie temperature of the as‐quenched phase is around 160 °C while that of the new phase is around 320 °C. Transmission electron microscope studies show fine precipitates (∼100 Å) dispersed in a matrix of different chemical composition. X‐ray and electron diffraction data indicate that the precipitates have the Fe 2 1 R 3 B tetragonal structure. The high anisotropy of this phase together with its fine size and distribution give rise to the observed high coercive fields.
The corrosion characteristics of RE‐Fe‐B magnets in elevated temperature and/or high humidity and salt‐containing environments are reported. Magnetic properties, magnet integrity, and corrosion product were evaluated. Methods of improving the corrosion resistance of RE‐Fe‐B magnets were also investigated. Epoxy‐type coatings were found to be successful for many applications.
Sintered Nd–Fe–B magnets were coated by pulse nickel plating at different plating conditions. Optimal pulse plating condition was established (average current density=1 A/dm <sup> 2 </sup>, peak current density=6 A/dm <sup> 2 </sup> with T<sub> on </sub>:T<sub> off </sub>=1:2 ). In order to make a comparison, magnets with similar nickel coating thickness plated by dc were also prepared. The corrosion resistance of the coated magnets was evaluated by (i) Normal Salt Spray Test (5% NaCl, 35 ° C ) and (ii) potentiodynamic polarization measurement (3.5% NaCl solution). It was found that the corrosion resistance of the pulse nickel plated magnet was significantly improved as compared with that of the conventional dc plated ones, with negligible deterioration in magnetism. The microstructure of the coating was examined by optical microscopy and scanning electron microscopy. It was found that the porosity was much lower, and the grains much finer in the pulse-plated layer as compared with the dc plated ones. © 1998 American Institute of Physics.
The current paper presents the corrosion characteristics of two permanent magnetic alloys, Nd–Fe–B and SmCo5 in the acidic environments. The results revealed that the corrosion rate is significantly high for both the magnets. It is approximately double for SmCo5 alloy than the Nd–Fe–B in the acidic environments. The results also revealed that both the alloys are unable to form protective oxide scales on their surfaces upon immersion in acidic environments. Though, the corrosion rate of Nd–Fe–B alloy is low when compared to SmCo5 alloy, the corrosion rate is considerably high for use in the systems exposed to acidic environments. AC impedance results confirm the above findings. The studies also showed that the degradation of Nd–Fe–B alloy take place due to grain boundary corrosion while SmCo5 alloy due to pitting corrosion in the acidic environments. Finally, the necessity of surface engineering techniques in combating corrosion very effectively without affecting the magnetic properties and thereby enhancing their life in acidic environments has been highlighted.
In this investigation the effect of surface treatments on the corrosion resistance of a commercial NdFeB sintered magnet has been investigated. A solution of 10 g L− 1 NaH2PO4, acidified to pH 3.8 has been used for phosphating this magnet. The corrosion resistance of the phosphated magnet was investigated in a 0.10 mol L− 1 Na2SO4 solution by electrochemical impedance spectroscopy and cyclic voltammetry with rotating disc electrode. The obtained results reveal that the resistance decreases with exposure time due to the development of pores and/or defects in the conversion coating exposing the substrate to corrosive attack. The effect of tungstate incorporation into the phosphate conversion coating resulting from a phosphating treatment prior to immersion in the tungstate solution was evaluated. The proposed treatment consists of re-immersing the phosphated samples in a 0.1 mol L− 1 Na2WO4 solution during 72 h at the open circuit potential (OCP). Under these conditions, the corrosion resistance of the magnet was improved and this was attributed to the formation of a protective layer due to the adsorption of tungstate anions at the metallic substrate exposed in the coating, decreasing metal dissolution.
- Bala