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For better understanding of the roles of beta phase in the corrosion processes of AZ91D alloy, corrosion behavior of a cast (alpha + beta phase) and a homogenized (beta phase) AZ91D alloy was investigated in NaCl aqueous solution by gas collection and electrochemical measurements. According to the hydrogen evolution properties during corrosion, two different types of corrosion tend might be differentiated. For type I, hydrogen diffused into alloy, which evoked the decreasing of hydrogen evolution rate (HER) and the weakening of negative difference effect (NDE). For type II, in the corrosion process, hydrogen in the alloys diffused into the product film (P-type semiconductor), which suggested that hydrogen entering the film would be ionized. The generated electrons during ionization decreased the concentration of vacancies in the valence band of the product films, indicating an improvement of corrosion resistance of AZ91D alloy. (c) 2005 Elsevier Ltd. All rights reserved.