• Stjohn, David
• Hirmke, J.

Abstract

A new surface coating technique, namely packed powder diffusion coating (PPDC), for AZ91E magnesium alloy is reported. This new technique uses a powder mixture of aluminium and zinc as diffusion source and produces uniform and thick coatings at temperatures below 420 degrees C. Experimental results showed that zinc in the powder mixture significantly promotes the formation of intermetallic layers on the surface of the magnesium alloy at process temperatures between 350 degrees C and 413 degrees C, which is more than 50 degrees C lower than the previously reported processes. Depending on the temperature and the Zn-content in the powder. X-ray diffraction analysis identified three intermetallic phases and Mg(Al, Zn) solid solution that consist of the surface alloyed layer. The intermetallic compounds are tau-Mg(32)(Al,Zn)(49), phi-Al(5)Mg(11)Zn(4) and beta-Mg(17)(Al,Zn)(12). The hardness of the over 500 mu m thick surface alloyed layers is up to four times higher than the substrate. Both the beta-Mg(17)(Al,Zn)(12) phase and the tau-Mg(32)(Al,Zn)(49) phase show one to two order magnitude higher corrosion resistance than the alpha-phase (solid solution) and the phi-Al(5)Mg(11)Zn(4) phase in 5% NaCl solution. A process parameter window for the layer thickness as well as a schematic model for the formation of the layer is proposed. The PPDC process is a promising technique that provides effective protection of AZ91E alloy from both wear and corrosion. (C) 2011 Elsevier B.V. All rights reserved.

Topics

• impedance spectroscopy
• surface
• compound
• corrosion
• phase
• x-ray diffraction
• Magnesium
• magnesium alloy
• Magnesium
• aluminium
• zinc
• hardness
• intermetallic