CORROSION OF RAPIDLY SOLIDIFIED NEODYMIUM-IRON-BORON (ND-FE-B) PERMANENT-MAGNETS AND PROTECTION VIA SACRIFICIAL ZINC COATINGS

Rapidly solidified Nd-Fe-B alloys were found to corrode actively at op en circuit in aerated Na2SO4 and NaCl at 30 degrees C and 80 degrees C . Estimated penetration rates (greater than 27 mil year(-1)) indicate the need for corrosion control methods. The use of sacrificial zinc co atings in controlling the corrosion of Nd-Fe-B alloys may have previou sly been overlooked because of two potential limitations: incomplete p rotection and hydrogen damage. The electrochemical activity of Nd may prevent zinc from providing complete sacrificial protection to Nd-Fe-B at breaks in the coating, and cracking could occur when hydrogen prod uction is galvanically stimulated on Nd-Fe-B at breaks in the coating. This study focuses on the evaluation of the protection conferred to N d-Fe-B when a Zn/Nd-Fe-B galvanic couple is formed due to coating pene tration. Quantitative solution analysis was used to demonstrate that t he dissolution of Nd, Fe and B is essentially prevented by the galvani c coupling of Nd-Fe-B to zinc in NaCl at 25 degrees C. Galvanic coupli ng to zinc also reduces the rate of environmentally assisted cracking of Nd-Fe-B. Sacrificial zinc coatings appear to be a viable, cost-effe ctive corrosion control method for Nd-Fe-B, and the limitations descri bed above should not preclude their use in this application.