The stability of aluminum-manganese intermetallic phases under the microgalvanic coupling conditions anticipated in magnesium alloys R. M. Asmussen, W. J. Binns, R. Partovi-Nia, P. Jakupi and D. W. Shoesmith* The electrochemical behaviour of two Al-Mn materials (Al- 5.5 at % Mn and Al- 13.5 at % Mn) has been studied in 0.275 M NaCl and 0.138 M MgCl 2 solutions to simulate the cathodic environment of Al-Mn particles during the corrosion of a Mg alloy. Upon polarization in NaCl solution to a potential in the range expected on a corroding Mg alloy, the Al-5.5 at % Mn alloy proved unstable undergoing de-alloying (loss of Al) and delamination of layers of the Al(OH) 3 formed. This leads to a steady increase H 2 O reduction current. When polarized in MgCl 2 solution the surface was partially protected from de-alloying and the current for H 2 O reduction suppressed by the deposition of Mg(OH) 2 . The Al-13.5 at % Mn alloy was considerably more stable when cathodically polarized. This increased stability was attributed to the higher density of Mn-enriched areas in the alloy surface. This simulation of the microgalvanic cathodic behaviour of Al- Mn intermetallic particles confirms that the appearance of corrosion product domes on the Al-Mn intermetallic particles during the corrosion of Mg alloys as an indication of their cathodic behaviour and that Al-Mn intermetallic particles are ef ficient, yet unstable cathodes. 1 Introduction Lightweight Mg alloys have promising applications in the automotive and aerospace fields due to their high strength to weight ratio [1], but experience rapid corrosion when coupled to a more noble material [2]. Impurities found in Mg alloys such as Fe and Cu [3–5] are detrimental to the corrosion properties of the alloy. However, their distribution can be limited through the addition of Mn which can scavenge these elements in the melt leading to improved corrosion performance [6–8]. However, the formation of Al-Mn intermetallic particles can render the a-Mg matrix susceptible to microgalvanic corrosion. It is generally accepted that such particles, especially when contaminated with Fe [9], can act as cathodes in a range of Mg alloys [10]. Microgalvanic coupling of Al 2 Mn particles in the AZ31, AZ80 and AZ91D alloys was shown to cause localized corrosion in the vicinity of the particles [11] in 3.5 wt % NaCl. In salt fog experiments AlMn particles ranging in composition from Al 19 Mn 4 (Al 4.75 Mn) to Al 8 Mn 5 (Al 1.6 Mn) only influenced the early stages of alloy corrosion but no explanation for this short term activity was noted [12]. Studies using custom synthesized AlMn specimens with a high Mn content (Al 1.5 Mn) showed only very weak galvanic activity when coupled to the AZ91 alloy [13]. However, these last experiments were conducted in low ionic strength solutions containing only millimolar concentrations of Na 2 SO 4 and NaCl in which the range of microgalvanic couples would be limited. On corroded Mg alloys, some Al-Mn intermetallic particles have been observed to collect a dome of deposited corrosion product [11,14] while others remain exposed, a trait also observed on supposed cathodic sites on other Mg alloys [15–17]. Based on scanning electrochemical microscopy (SECM) measurements, it was suggested that deposition of corrosion products occurred on cathodically active sites [18,19] with microscopic evidence of corrosion product accumulation [19] at these sites. Recently, we suggested that these corrosion product deposits on Al-Mn intermetallic particles (Al 8 Mn 5 (Al 1.6 Mn)) in the AM50 alloy are a result of their cathodic behaviour when microgalvanically- coupled to the a-Mg matrix 20 , with the cathodic reaction being the reduction of H 2 O to H 2 [8]. As H 2 production proceeds, the local pH at the cathode surface increases leading to two observable features: (i) deposition of Mg(OH) 2 to create the corrosion product dome; and (ii) delamination of the surface of the intermetallic due to the loss by dissolution of Al [20]. These results suggest that the activity of these intermetallic phases may R. M. Asmussen, W. J. Binns, R. Partovi-Nia, P. Jakupi, D. W. Shoesmith Department of Chemistry and Surface Science Western, Western University, 1151 Richmond St, London, Ontario, Canada E-mail: dwshoesm@uwo.ca Materials and Corrosion 2016, 67, No. 1 DOI: 10.1002/maco.201508349 39 www.matcorr.com wileyonlinelibrary.com © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim