The cathodic behaviour of Al–Mn precipitates during atmospheric and saline aqueous corrosion of a sand-cast AM50 alloy Mohsen Danaie a,⇑ , Robert Matthew Asmussen b , Pellumb Jakupi b , David W. Shoesmith b , Gianluigi A. Botton a a Department of Materials Science and Engineering, Brockhouse Institute for Materials Research and Canadian Centre for Electron Microscopy, McMaster University, Hamilton, Ontario, Canada b Department of Chemistry and Surface Science Western, Western University, London, Ontario, Canada article info Article history: Received 18 December 2013 Accepted 14 February 2014 Available online 22 February 2014 Keywords: A. Magnesium A. Alloy B. TEM B. STEM abstract The behaviour of Al–Mn precipitates during atmospheric and aqueous corrosion of an AM50 Mg alloy was investigated using site-specific analytical electron microscopy. After air-exposure, localized attack was observed close to Al–Mn precipitates, with the top layer of the intermetallic enriched in Al and O. During immersed corrosion, these precipitates developed protruding domes of corrosion products, with crystal- line Mg(OH) 2 on top and an inner layer of crystalline MgO. After prolonged immersion, these precipitates showed evidence of preferential Al dissolution, ultimately developing a fragmented interlayer of Mn 3 O 4 . This phase transformation is linked to the enhanced hydrogen evolution rates adjacent to these precipitates. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Magnesium alloys with high strength-to-weight ratio, good castability and machinability are desirable candidate materials for automotive and aerospace applications. The major hindrance to their wider application is their susceptibility to corrosion, espe- cially in chloride-containing aqueous environments [1]. The poros- ity, poor adhesion, and a narrow pH range of stability of the corrosion product (generally Mg(OH) 2 ), combined with the high electrochemical anodic activity of pure Mg results in high corro- sion rates [2]. Since Mg is so anodically reactive, its corrosion can be exacerbated when the generally a-Mg matrix contains more no- ble secondary phases to which it can galvanically couple. Such phases are commonly incorporated in the matrix to enhance the mechanical properties of the alloy [3–5]. The presence of impuri- ties such as Ni, Cu, and Fe can also enhance the anodic reactivity and the influence of microgalvanic coupling upon it [5,6]. The role of alloying elements is critical in optimizing the corro- sion properties of Mg alloys. Addition of Mn has long been known to enhance the corrosion resistance of cast Mg alloys by capturing the residual Fe during casting [7,8]. A secondary advantage of add- ing Mn is grain refinement [9–11]. In the absence of Fe, numerous intermetallic phases are possible in the Al–Mn system [12]. In the Mg-rich Mg–Al–Mn (0 6 Mn (wt.%) 6 3 and 0 6 Al (wt.%) 6 15) system it has been demonstrated, both experimentally [13] and theoretically [14], that in the melt temperature range (700– 750 °C) the equilibrium phases are b-Mn (Cubic) and Al 8 Mn 5 (rhombohedral). Since their atomic radii are almost equal, Fe can replace Mn in the latter phase to form the substitutional solid solu- tion phase, Al 8 (Mn,Fe) 5 [9]. Electrochemical measurements have shown that the Al–Mn intermetallics are cathodic with respect to the a-Mg matrix [3,15–17] (but less so than the Fe impurities that Mn is meant to capture). The eutectic intermetallic b-Mg 17 Al 12 phase has been shown to be also weakly cathodic with respect to the a-Mg matrix [18–22], but can form a passive layer which results in an overall improvement in corrosion resistance provided that this phase is present in a continuous and uniform network [17,19,23]. The a-Mg grains themselves will have different potentials depending on the amount of Al in solid solution, with more Al resulting in slightly more noble potentials [16,24–27]. According to Mathieu et al. [16], the corrosion potential of a-Mg increases linearly, from 1.55 V SCE for pure Mg to 1.40 V SCE for a-Mg containing 9 at.% Al. The lowered corrosion rates of Al-rich a-Mg grains has been linked to accumulation of Al 3+ entities on the surface [28–30], percolation of amorphous Al 2 O 3 within the MgO/Mg(OH) 2 corrosion layer [31], and most recently shown by the present authors [32,33], to the development of a metallic Al-rich layer at the metal/corrosion layer interface. http://dx.doi.org/10.1016/j.corsci.2014.02.030 0010-938X/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 905 525 9140x24862; fax: +1 905 521 2773. E-mail addresses: danaiem@mcmaster.ca (M. Danaie), gbotton@mcmaster.ca (G.A. Botton). Corrosion Science 83 (2014) 299–309 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci