Evaluation of microstructural eects on corrosion behaviour of AZ91D magnesium alloy Rajan Ambat*, Naing Naing Aung, W. Zhou School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 3 September 1999; accepted 13 November 1999 Abstract The eect of microconstituents on the corrosion and electrochemical behaviour of AZ91D alloy prepared by die-casting and ingot casting route has been investigated in 3.5% NaCl solution at pH 7.25. The experimental techniques used include constant immersion technique, in-situ corrosion monitoring, and potentiodynamic polarisation experiments. Surface examination and analytical studies were carried out using optical and scanning electron microscopy, EDX and XRD. The corrosion behaviour of microconstituents namely primary a, eutectic a and b phases was signi®cantly dierent. Coring of aluminum showed in¯uence on corrosion behaviour more signi®cantly in ingot material. Areas with aluminium concentration less than about 8% were found to be prone to corrosion attack compared with either those with higher amount of aluminium or b phase. Die-cast material with smaller grain size and ®ne b phase oered marginally lower corrosion rate and better passivation compared with the ingot. In die-cast and ingot, hydrogen evolution took place preferentially on b phase. XRD pattern of non-corroded and corroded surface revealed the removal of b phase from alloy surface during corrosion. The corrosion products for ingot consisted of Mg(OH) 2 with small amounts b phase, magnesium-aluminium oxide and MgH 2 while for die-cast, the product showed a highly amorphous structure. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Magnesium alloys; AZ91D; Ingot; Die-cast; Microstructure; Corrosion 0010-938X/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S0010-938X(99)00143-2 Corrosion Science 42 (2000) 1433±1455 * Corresponding author. Tel.: +65-790-4963; fax: +65-791-1857. E-mail address: marajan@ntu.edu.sg (R. Ambat).