Abstract
The corrosion resistance of die cast and freely solidified or electromagnetically stirred thixocast AZ91D alloy has been studied using electrochemical noise technique and electrochemical impedance spectroscopy in a 0.05 M NaCl solution. Specimens polished at different depths were immersed in the solution in order to assess the influence of the microstructure on corrosion kinetics and morphology. At depths between 10 and 50 µm (skin), all specimens showed general non-uniform corrosion with the lowest corrosion resistance. Between 100 and 200 µm (interior skin), the observed corrosion was accompanied by superficial undefined pits due to metastable pitting. The corrosion form in the interior skins gave the best corrosion resistance. Stable pitting corrosion was observed beyond 400 µm deep on the bulk specimens. The skin of all thixocast specimens prepared from both types of billets showed a more corroded surface than that of die cast. On the other hand, the interior skin as well as the bulk of thixocast specimens showed better corrosion performance than that of the die cast specimens.
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