The EIS technique was used to analyze the electrochemical reaction behavior of Alloy AZ91 in H3PO4/KOH buffered K2SO4 solution at pH 7. The corrosion resistance of Alloy AZ91 was directly related with the stability of Al2O3 center dot xH(2)O rich part of the composite oxide/hydroxide layer on the alloy surface. The break down of the oxide layer was estimated to occur mainly on the matrix solid solution phase in Alloy AZ91. The mf capacitive loop arose from the relaxation of mass transport in the solid oxide phase in the presence of Al2O3 center dot xH(2)O rich part and from Mg+ ion concentration within the broken area in the absence of Al2O3 center dot xH(2)O rich part in the composite oxide structure on the alloy surface. The If inductive loop had tendency of disappear when the dissolution rate of the alloy decreased as a result of the formation of the protective oxide layer. (c) 2006 Elsevier Ltd. All rights reserved.