Improvement of humidity resistance of magnesium powder using dry particle coating

Dry particle coating is used to enhance the moisture resistance of ground magnesium powder (primary size 75 Am) by coating its surface with carnuba wax (primary size 15 Am). Coating was done using magnetically assisted impact coating (MAIC), and two high-speed impaction-coating devices, the hybridizer and mechanofusion. The uncoated and coated samples are characterized by scanning electron microscopy (SEM), humidity tests, and X-ray diffraction. SEM images indicate that in MAIC the coated wax is mainly observed in and around the cracks, whereas in the hybridizer and mechanofusion the wax was softened and spread more evenly over the magnesium surface. The results of 150-h humidity tests as well as extended 400-h tests show a significant improvement in the moisture resistance of ground magnesium powder after coating in all three devices. Extended 400-h tests show that in some cases, the wax-coated samples perform similar to atomized magnesium. In particular, the mechanofusion-coated product showed moisture resistance comparable to atomized magnesium, with an amount of wax as little as 2%. These results are also verified by XRD analysis to measure the amount of hydroxide formation. Thus the wax coating increases humidity resistance by delaying hydroxide formation, hence increasing the shelf life of the coated ground magnesium. D 2004 Elsevier B.V. All rights reserved.