Influence of microstructure on the corrosion of diecast AZ91D

The corrosion of die cast AZ91D was studied and related to its microstructu re. For comparison and to more fully understand the behaviour of die cast A Z91D, corrosion studies and microstructural examinations were also carried out using slowly solidified high purity AZ91, Mg-2%Al, Mg-9%Al, low purity magnesium and high purity magnesium. Corrosion was studied in 1N NaCl at pH 11 by (1) observing the corrosion morphology, (2) measuring electrochemica l polarisation curves and (3) simultaneously measuring both the hydrogen ev olution rate and the magnesium dissolution rate. The skin of die cast AZ91D showed better corrosion resistance than the interior. This is attributed t o a combination of(1) a higher volume fraction of the beta phase, (2) a mor e continuous beta phase distribution around finer alpha grains, and (3) low er porosity in the skin layer than in the interior of the die casting. This study showed that the casting method can influence the corrosion performan ce by its influence on the alloy microstructure. (C) 1999 Elsevier Science Ltd. All rights reserved.