A zinc coating method for Nd-Fe-B magnets

A newly developed Low Pressure Pack Sublimation (LPPS) process has been use d to coat fully dense Nd-Fe-B sintered magnets with zinc. The process was b ased on a form of sherardizing where the component to be coated is placed i n a rotating chamber at 390 degrees C in a mixture of sand and zinc dust. H owever, during the LPPS process, a moderate vacuum is applied and the chamb er is not rotated. LPPS produced an adherent surface coating, which, when p laced in a corrosive environment, displayed superior performance in terms o f weight loss and reduction in magnetic properties compared to those exhibi ted by the commercially electroplated and uncoated samples. The severe corr osion conditions were imposed using an autoclave (100 degrees C, 1bar press ure and saturated humidity). The composition and characteristics of the sur face layers were examined using optical analysis, XRD and Scanning Electron Microscopy. Under the conditions employed in these studies, the LPPS proce ss produced a small reduction in remanence and coercivity (approximate to 1 0% and approximate to 5% respectively) by changing the surface conditions o f the magnets. The reduction in properties was found to be related to coati ng thickness and temperature effects. By the use of LPPS a cheap and effect ive barrier to corrosion has been produced. (C) 2000 Elsevier Science S.A. All rights reserved.