Corrosion protection of Nd-Fe-B type permanent magnets by zinc phosphate surface conversion coatings

The use of protective zinc phosphate top coatings to protect Nd-Fe-B type p ermanent magnets against corrosion is discussed. The progress of the phosph atisation process has been tested by simultaneous measurement of pH near su rface, corrosion potential, substrate mass loss and phosphate coating mass gain. The corrosion behaviour of the magnet in phosphate solution was analy zed by the anodic polarization technique and the general resistance of the magnet to corrosion was evaluated by the Akimov drop test. It is shown that immersion of the magnet in the acidified (pH = 2) phosphating solution con taining Zn(II), nitrate and fluoride ions resulted in the formation of well adhered, thin and tight hopeite layers, causing prolonged corrosion resist ance of the magnet. (C) 2001 Elsevier Science Ltd. All rights reserved.