Corrosion of Neodymium Magnets in Polyligand Solutions
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CORROSION OF NEODYMIUM MAGNETS IN POLYLIGAND SOLUTIONS A. O. Maizelis1, 2 and B. I. Bairachnyi1 We present the results of investigation of the corrosion behavior of rare-earth neodymium magnets doped with cobalt and dysprosium in media that contain ammonium and pyrophosphate ions and belong to the class of complex electrolytes intended for electrodeposition. We use the methods of cyclic and linear voltammetry, chronoamperometry, chronopotentiometry, and the weight method. In polyligand ammonium-pyrophosphate solutions, the corrosion potential of a magnet occupies an intermediate position between the values typical of pyrophosphate and ammonium solutions and the corrosion current is lower. The weight losses of doped magnets in polyligand solutions without currents decrease as a result of alkalization. It is shown that, in the presence of currents, the weight losses are minimal near the stationary potential and increase both in the case of anodic dissolution of the magnet and in the presence of cathodic degradation caused by hydrogen release. Under the conditions of periodic variations of the potential of the magnet near its stationary value realized in the course of formation of the multilayer coating, the weight losses become much lower and equal to 0.42 mg/(cm 2 ⋅ h) . The potential of a doped
magnet immersed in a polyligand electrolyte for the deposition of (Cu–Ni)/(Ni–Cu) multilayer coatings corresponds to the potential range (– 0.8–(– 0.7 V)) of deposition of compact copper layers. The current density of contact exchange almost attains its maximum value of 0.95 mA/cm 2 for the first minute and almost completely decays after 40 min.
Keywords: NdFeB magnets, ammonium-pyrophosphate electrolyte, contact exchange.
Magnets of the NdFeB system are extensively used in the aviation, electronics, metrology, medical tools, etc. They are important components of various devices, such as electric motors, computers, CD players, microwave ovens, and cars (electric and hybrid cars). These magnets are also used in various sensors and units in aerodynamics, in magnetic-resonance devices, in biomedicine, in the acoustic and household electronics, in the production equipment, and in the scientific research. As one of the main problems encountered in using rareearth neodymium magnets, we can mention their relatively low corrosion resistance and brittleness. The process of formation of oxide films on the surfaces of magnets worsens their magnetic properties, which restricts their applicability in electronics. The low corrosion resistance of NdFeB magnets is explained by the following factors: the presence of numerous phases in their structure formed by the Nd 2 Fe14 B matrix phase, the Nd1 + εFe 4 B4 phase, and the phase enriched with neodymium; a large difference between the volumes of the Nd 2 Fe14 B phase and the Nd-enriched phase; high porosity of magnets; the absence of passive films on the surfaces of magnets; a trend to oxidation caused by a high content of electrically negative neodymium, and susceptibility of rare-earth ele
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