Corrosion behavior of NdFeB permanent magnet in two neutral environments

Abstract

The NdFeB uncoated magnets have been magnetically characterized by demagnetization curves and their corrosion behavior have been evaluated (nonmagnetized state) by accelerated, immersion and electrochemical corrosion tests. The hygrothermal corrosion resistance was evaluated by: high accelerated stress test (HAST) and bulk corrosion test (BCT). The immersion tests were performed in ultrapure water with or without addition of chloride ions. The electrochemical tests, open circuit potential (OPC) and electrochemical impedance spectroscopy (EIS) were carried out in two neutral solutions (NaCl and Na2SO4), and on the two distinct surfaces of cubic magnets, i.e. parallel and perpendicular (pole) to the crystallographic c-axis of aligned ferromagnetic grains. Both accelerated tests presented weight loss below 1.0 mg/cm2 (BCT grade A). The chloride ions additions in immersion tests caused differential aeration corrosion. The EIS tests carried out in NaCl solution were more aggressive than in Na2SO4 solution, and indicated a different corrosion behavior between the parallel and pole faces evidenced by the response of the Nyquist diagrams.