Corrosion behaviour of AZ21, AZ501 and AZ91 in sodium chloride

The corrosion behaviour of AZ21, AZ501 and AZ91 was studied in 1 N NaCl at pH 11 by measuring electrochemical polarization curves, electrochemical AC impedance spectroscopy (EIS) and simultaneously measuring the hydrogen evol ution rate and the: magnesium dissolution rate. The corrosion rates increas ed in the following order: AZ501 < AZ21 < AZ91. The: corrosion behaviour wa s related to alloy microstructure as revealed by optical and electron micro scopy. The beta phase was very stable in the test solution and was an effec tive cathode. The beta phase served two roles, as a barrier and as a galvan ic cathode. If the beta phase is present in the alpha matrix as intergranul ar precipitates with a small volume fraction, then the beta phase mainly se rves as a galvanic cathode, and accelerates the corrosion of the alpha matr ix. If the beta Fraction is high, then the beta phase may mainly act as an anodic barrier to inhibit the overall corrosion of the alloy. The compositi on and compositional distribution in the alpha phase is also crucial to the overall corrosion performance of dual phase alloys. Increasing the aluminu m concentration in the alpha phase increases the anodic dissolution rate an d also increases the cathodic hydrogen evolution rate. Increasing the zinc concentration in the alpha phase may have the opposite effect. (C) 1998 Els evier Science Ltd. All rights reserved.