Low temperature Al2O3 sol-gel coatings on neodymium iron boron sintered magnets: study of adhesion and corrosion performance

Sol-gel alumina coatings have been deposited on neodymium-iron-boron magnets for corrosion protection. The coatings present a compact amorphous structure due to the low thermal treatment (500 ?) with a thickness ranging from 0.8 to 2 microns, a Vickers hardness of 10 GPa and good adhesion to the substrate, measured by scratch test, with a failure mode of tensile cracking and chipping. The coatings are effective in protecting the NdFeB magnets from corrosion with reduction of current density (i.e. 2.07 x 10(-6) A/cm(2) for 2dip sample) of one order of magnitude with respect to the neat sample (1.15 x 10(-5) A/cm(2)).