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Journal of Spectroscopy
NdFeB sintered magnets have been widely applied in various industries because of their remarkable magnetic properties [ 2 ]. However, these types of magnets could not be used for some commercial applications due to their very low resistance against corrosion in various ambient environments [ 3 — 5 ]. This very low resistance against corrosion of NdFeB sintered magnets may be attributed to the existence of complex multiple phases in their microstructure.
The most important ferromagnetic phase present in NdFeB sintered magnets is the compound Nd2Fe14B matrix phase, -phase. Fidler [ 7 ] indicated that it is possible for a single magnet to have different types of the Nd-rich phase, where the main Fe: Nd stoichiometric ratios may be 1: However, the Nd-rich phase has a poor corrosion resistance; its electrochemical activity is highest.
The anodic dissolution of the Nd-rich phase occurs at more active potentials than the corrosion potential of the NdFeB magnet [ 8 ]. The dissolution of the Nd-rich phase can provoke the disintegration of the magnet [ 9 ].
Influence of the Chemical Composition in the Electrochemical Response of Permanent Magnets
Corrosion performance of permanent magnets is carried out by immersion in a neutral salt solution or a neutral salt spray test. It has been shown that, in a neutral NaCl solution, the corrosion rates of the uncoupled single phases are different, for the reason that the Nd-rich phase has the lowest corrosion resistance and the matrix phase the highest one [ 5 ]. The electrochemical potentials of the present phases lead to galvanic corrosion, resulting in preferential corrosion by grain boundary.
This is because the area fraction of the anode Nd-rich phase and B-rich phase is much smaller than that of the matrix phase cathodethereby accelerating corrosion of the anode [ 1011 ].
It is said that Nd can absorb H2 evolved by the corrosion process, and this can lead to an expansion of the Nd-rich phase causing the embrittlement of the permanent magnet and finally its pulverization [ 91213 ].
Extensive research has been carried out to improve the performance of the permanent magnets, for example, increasing the Curie temperature by modification of the chemical composition substituting Fe by other elements and increasing the corrosion resistance by either applying protective coatings or adding alloying elements to the substrate [ 14 ]. However, this is always accompanied by a sacrifice of the magnetic properties, and the protective coatings are not perfect barriers.
For this reason the NdFeB magnets have a poor corrosion resistance in moist environments. In general, the Nd-rich phase catalyzes the formation of H2 from H2O and then H2 forms a hydride with the Nd-rich phase causing its disintegration.
Therefore, if an uncoated NdFeB magnet is used, it will be corroded within weeks. This is the reason why the magnets are coated with Ni or Ni-Cu-Ni layer [ 15 ].
The aim of the present work is intended to study the corrosion resistance of commercial permanent magnets with different chemical composition in 3. Experimental Procedure 50 NdFeB magnets obtained from computer hard disks of various brands were analyzed and then classified according to elemental chemical composition.
The compositions selected for testing are shown in Table 1. In general the chemical composition of NdFeB magnets varies depending on its final application. For example, permanent magnets for high temperature environments contain Dy or Ho, or Pr can partially replace Nd. Because Nd and Pr differ by only one atomic number, during the extraction process Nd and Pr are not separated completely [ 15 ].
Chemical compositions of the magnets tested in 3. The magnets were then cold mounted in epoxy resin. Wet grinding technique was employed to grind the samples with grit to grit SiC paper. All the samples before exposure to the test corrosion were degreased with ethanol in an ultrasonic bath, dried in air, and stored prior to testing.
Electrochemical measurements were carried out in a Gamry Interface Potentiostat controlled by a personal computer. All the experiments were performed in a standard three-electrode cell arrangement with the magnets as working electrode, together with a saturated calomel electrode SCE as the reference electrode and a high-purity graphite rod as the auxiliary electrode.
Corrosion performance was determined in 3. Open-circuit potentialcurrent densityand Tafel slopes were determined by means of potentiodynamic polarization curves. Before starting the test the system was kept stabilizing for a period of 0. The measurements were recorded every one hour for 24 hours. The value of the polarization resistance was deduced from the equationwhere is the step of the potential applied at the corrosion potential and is the resulting current.
After determining the polarization resistance, the corrosion current densities can be calculated by applying the Stern-Geary equationwhere the values of and were previously obtained see 1 and 2: Electrochemical impedance spectroscopy EIS measurements were made at frequencies from 0. After the corrosion tests, the surface morphology of the corroded sintered magnets was analyzed with a Hitachi SU scanning electron microscopy SEM.
Results and Discussion 3. Potentiodynamic Polarization Curves Figure 1 shows characteristic potentiodynamic current density-potential curves polarization curves of the Nd magnets when immersed in 3. In the anodic region, the 35Nd magnet shows an active dissolution process in the entire potential range. It is observed that increasing the Nd content 40Nd magnet displaced the polarization curve to more active potential with an increase in value and favoring the apparent formation of a passive zone.
Furthermore, the addition of Co 40Nd3Co magnet causes a shift of the polarization curve to nobler potentials, with a decrease in value and the apparent disappearance of passive region. Potentiodynamic polarization curves of the Nd magnets tested in 3. The different phases present in the permanent magnets are Nd-rich phase grain boundaryB-rich phaseand Nd2Fe14B matrix phase. Part of the hydrogen ions is evolved as H2, causing the breaking of the corrosion products layer.
On the other hand, the atomic hydrogen can be absorbed by the matrix phase, resulting in the lattice expansion and therefore decreasing its magnetic properties [ 1718 ]. It is known that, in aqueous NaCl solution, the dissolution reaction of the Nd-rich phase takes place according to the following anodic reaction [ 1920 ]: On the other hand, the cathodic reaction includes two processes, the H2-evolution reaction and the O2-consuming reaction.
The decomposition of the neodymium hydroxide can occur in two stages as follows [ 19 ]: Figure 2 shows the polarization curves for the NdPr magnets measured in 3. The magnets tested in this case have the same Nd content but differ in Pr content. The scientific words for these interactions is: Materials attracted by magnets: Paramagnetic Materials repelled by magnets: Diamagnetic Materials which spontaneously form magnetized domains: Ferromagnetic Materials in which neighboring spins like to align opposite to each other: Antiferromagnetic Only ferromagnets are useful for making permanent magnets.
Paramagnetic and diamagnetic forces tend to be very weak, except for the diamagnetism of superconductors, which is strong enough to levitate them. This alloy seems to be not very magnetic. Older pure nickel nickels stick to magnets. Some steels steel is mostly made of iron are more magnetic than others. Try comparing stainless steel with other kinds of steel you might find around the house. Two suggestions of things that can affect your experiments: The rest of the material will be just as magnetic as it ever was, just minus the rusty layer.
You may not notice any effect of the rust if most of the material is intact.
soft question - Does magnetism affect corrosion? - Physics Stack Exchange
Be sure that the shapes of your uncorroded and corroded objects are the same when you compare their magnetic attraction or you could have other effects confusing the results. Non-magnetic rust spots Q: The body of my car has orange rust spots on it. My magnetic screwdriver is attracted to the carbut not to the rusty spots. Rust a collection of some iron oxides: If the sheet metal on your car has really rusted through, there will be almost no magnetic force between it and the magnetized screwdriver.
It sounds like those spots should be patched, since that's not just a thin surface layer of rust.