| NdFeB permanent magnets which serve as important metal functional devices are widely used in various fields, such as electronics, information, communication, energy, transportation, medical apparatus, aviation and so on. They play more and more important roles in our high-tech and daily life. Due to the high chemical activity of Nd and easily oxidized, poor corrosion resistance of NdFeB magnets greatly limit the application. Constant hot-humid(HH), neutral salt spray(NSS) and pressure cooker test(PCT) are usually used to evaluate corrosion resistance of NdFeB magnets. Corrosion behavior and mechanism of different alloy composition of NdFeB magnets in the above corrosive climates were discussed. The effects of alloy composition and different corrosive environments on magnetic flux loss were also discussed. Electrochemical corrosion behavior and mechanism were investigated via dynamic potential polarization curve and electrochemical impedance spectroscopy.The corrosion velocity rate in PCT climate was much slower than that in NSS and HH climates through the NSS, HH and PCT tests. The thickness of liquid film of magnet surface in PCT climate was much bigger than those in NSS and HH climates because of its saturated humidity(100%RH). It was probably because that high-pressure steam led to an oxygen-deficient supply, and the steam/liquid film on the surface of magnets would hinder the diffusion of oxygen. The atmospheric corrosion happened electrochemical reaction under the thin liquid film. The corrosion velocity rate under full immersion state(PCT) was much slower than those under thin liquid membrane state(NSS and HH).The corrosion process of NdFeB magnets in NSS climate included two steps, and corrosion reaction dynamic velocity equation was given. In NSS climate, magnet surface formed pitting corrosion at first, and then the corrosion areas expanded increasingly due to the synergy effect of chloride ion which could adsorb and diffuse into the matrix phases. In the beginning, magnet surface formed oxidation film when formation velocity of corrosion products on the magnet surface was faster than dissolution velocity of them, corrosion process displayed weight gain slightly. Then corrosion proceeded further, corrosion products film spalled and dissolved into the solution because of internal stress resulted from volume expansion. When dissolution velocity of corrosion products film was faster than formation velocity, corrosion process displayed weight loss fast. The corrosion behavior of NdFeB magnets prepared by sintering and hot deforming were compared. Effect of microstructures on corrosion resistance of NdFeB magnets had been discussed. Hot deformed magnets exhibited much better corrosion resistance than sintered ones in the neutral salt spray climate. It was because that hot deformed magnets had platelet-shaped nanograins microstructure, in which grain boundary Nd-rich phases were much fine, uniform and disperse. The different microstructure between sintered and hot deformed magnets caused the different corrosion behavior.The corrosion behavior and mechanism of sintered NdFeB magnets under different electrochemical conditions were investigated by means of electrochemical technology including polarization curve, electrochemical impedance spectroscopy. The corrosion dynamic models of sintered NdFeB magnets in different solutions were established. In alkaline solution, NdFeB magnets exhibited passive phenomenon evidently. The uniform and compact hydroxide passivation film formed on the magnets surface could protect the matrix of NdFeB magnets away from attack. In piped water, corrosion product film could not prevent corrosion reaction occuring, but could slow corrosion reaction velocity to a certain extent. In NaCl solution, a large member of Cl ions would not only destroy corrosion product film, but also accelerate active dissolution of anode areas. In HCl solution, sintered NdFeB magnets occured activation dissolution easily. Therefore, corrosion rate in acid solution was obviously higher than those in other solutions.The corrosion behavior and mechanism of different alloy composition of NdFeB magnets in different corrosion climates and electrochemical electrolytes was investigated. It showed that corrosion resistance of NdFeB magnets could be improved obviously by Dy substitution for Nd partially and minor Co addition. It is very significant for NdFeB manufacturers to improve corrosion resistance of magnet substrate through alloy composition design. |
