Hydrogen induced corrosion mechanism in NdFeB magnets

Abstract

An investigation was conducted comparing the corrosion behavior of NdFeB magnets in flowing hydrogen and in the heat and humidity of an autoclave. The results show that corrosion is both macroscopically and microscopically similar in both environments. In both cases, the corrosion progressed most rapidly in those areas where the magnetic orientation of the Nd2Fe14B matrix grains was perpendicular to the outer surface. A corrosion mechanism involving the reaction of hydrogen—either as a pure gas or as a by-product of the decomposition of water vapor—with the neodymium-rich grain boundary phase is proposed.

Processing and Protection of Rare Earth Permanent Magnet Particulate for Bonded Magnet Applications

“…Rapid solidification processing has proven to be a reliable processing route for the production of isotropic MRE 2 (Fe+Co) 14 B+Zr+TiC permanent magnet material. The hard magnetic properties are highly dependent on intrinsic and extrinsic factors brought about by alloy design and processing considerations.…”

“…Without providing a protective surface coating, i.e. epoxy or metallic, or alloying addition to a RE element, exposure to moisture was found to cause the reactions listed as equations 3 and 4 [14]. However, this benefit was derived at the cost of reduced intrinsic coercivity.…”

Effect of Microstructure on the Corrosion Resistance of Nd-Fe-B Permanent Magnets

“…The reaction resultant of H then diffuses into the grain boundaries, reacts with the Nd-rich phases to form NdH 3 . The NdH 3 causes a volume expansion of the grain boundary phase, so creates a local stresses which lead to the spalling of the matrix Nd 2 Fe 14 B grains [2,3]. Because the corrosion potential is different between the matrix Nd 2 Fe 14 B phase and the boundary Nd-rich phase, the corrosion microbattery forms, and therefore the electro-chemical corrosion occurs for Nd-Fe-B magnets in a high humidity atmosphere when water (such as the little liquid droplets) exists on the surface of the magnet [4].…”

Study of sintered Nd-Fe-B magnet with high performance of Hcj (kOe) + (BH)max (MGOe) > 75

“…This anisotropic corrosion phenomena was also observed by others. 45,46 So, the weight losses of unit surface area from PCT were calculated in two ways: weight loss divided either by total surface area or only by area of two pole surfaces. The absolute value of weight losses for both samples are ≤3.2 mg/cm 2 based on the total surface, and ≤6.2 mg/cm 2 based on the two pole surface.…”