Journal of The Electrochemical Society, 161 (10) C501-C508 (2014) C501 0013-4651/2014/161(10)/C501/8/$31.00 © The Electrochemical Society Microstructural Effects on Corrosion of AM50 Magnesium Alloys R. Matthew Asmussen, W. Jeffrey Binns, Pellumb Jakupi, and David Shoesmith *, z Department of Chemistry and Surface Science Western, Western University, London, Ontario, Canada The influence of microstructure and aluminum distribution on the corrosion of three different casts of the magnesium AM50 alloy (sand, graphite, die) was studied in 1.6 wt% NaCl solution. The microstructure of the alloys and the distribution of individual elements were characterized using scanning electron microscopy (SEM) and energy dispersive X-ray analyzes. Differences in the morphology and distribution of corrosion damage were determined using SEM and confocal scanning laser microscopy. Weight change measurements and electrochemical impedance spectroscopy showed that the corrosion resistance improved in the order sand cast < graphite cast ≪ die cast. This increased resistance was shown to be attributable to the increasing tightness of the α-Mg/β-phase/Al-containing eutectic microstructural network, which led to an improved protection of the surface by Al-enriched eutectic and a decrease in the probability of initiating a major damage site on an α-Mg region with low Al content. © 2014 The Electrochemical Society. [DOI: 10.1149/2.0781410jes] All rights reserved. Manuscript submitted May 13, 2014; revised manuscript received July 14, 2014. Published July 26, 2014. Mg alloys are attractive materials for automotive applications due to their high strength-to-weight ratio. However, one major deficiency is their inadequate corrosion resistance when exposed to aqueous and humid environments such as those experienced in automotive applications. 1–5 As the most electrochemically active structural mate- rial, Mg and its alloys are susceptible to galvanic corrosion when in electrical contact with a second metal/alloy, 6–8 as well as microgal- vanic corrosion between the secondary microstructures and the α-Mg phases, both of which can rapidly accelerate corrosion. 9 Al is commonly employed to improve the corrosion resistance of Mg alloys either as a direct coating 10 or through alloying, 11–13 and the corrosion resistance of Mg-Al alloys is commonly expressed in terms of their general Al content. 14,15 However, the alloying elements in Mg are generally segregated into distinct regions making their distribu- tion throughout the microstructure a critical feature in determining corrosion resistance. Consequently, the effects on corrosion of many microstructural features have been studied. These features include grainsize 16 β-phase distribution, 17,18 β-phase morphology 17 and the interactions between the secondary microstructures. 19 In this study, the influence of the surface and sub-surface chemistry associated with the microstructural features on different castings of the AM50 alloy have been studied. The commercial AM50 alloy, used for its exceptional castability, 20,21 contains a Mg-based α-phase, a β-phase (Mg 17 Al 12 ) surrounded by Al-enriched eutectic α-phase, and Al-Mn inter- metallics. Micro-galvanic couples between the β-phase and/or inter- metallics and the α-phase matrix can accelerate the corrosion of the latter in aqueous and atmospheric environments. 22 In a previous study we investigated the microscale corrosion processes occurring on a sand-cast AM50 surface in chloride solution through repeated micro- scopic analyzes of corroded areas. From this work it was reported that; (1) increasing the Al content of a grain reduced its corrosion rate; (2) an Al-enrichment developed at the alloy surface in the eu- tectic regions of the material during the corrosion process providing protection; and (3) the distribution of Al was important in controlling the corrosion process as regions deficient in Al were susceptible to major corrosion damage. The aim of this study is to utilize the mi- croscale approach developed 23 to quantify the extent and distribution of corrosion damage as an effect of microstructure size and distri- bution using sand-, graphite- and die-cast AM50 alloys. Two of the castings selected (sand-cast and graphite-cast) solidify close to equi- librium and generate larger microstructural features compared with die casting. By comparing three castings of the same alloy, any dif- ferences in elemental composition are minimalized, the main variable becoming the microstructure resulting from the casting procedure, in particular the size and distribution of the secondary phases and the Al content. ∗ Electrochemical Society Fellow. z E-mail: dwshoesm@uwo.ca Experimental Sample preparation.— AM50 alloys were supplied by General Motors (Canada). The as-received AM50 sand- and graphite-cast rods were machined into 1 × 1 × 0.7 cm electrodes. For the die-cast alloy square electrodes (1 × 1 × 0.2 cm) were machined from a 0.2 cm thick plate. The compositions of the alloys, determined using ICP- AES, comply with ASTM standard B275 and are listed in Table I. One side of the sample (1 cm 2 ) was tapped to connect to a threaded rod to allow electrical connection to external circuitry. The 1 cm 2 side of the sample to be examined was pre-treated as previously reported. 23 The samples were ground successively up to 4000 grit SiC. The ground alloy surface was then polished on a Struers DP-Dur cloth saturated in 3 μm Struers DP-Suspension A for 5 to 10 min with an ethanol/propanol mixture used in place of water as a lubricant. The final stage consisted of polishing, for 2–3 min, on a Struers OP-Chem cloth using an equal volume mixture of Struers OP-S Suspension and ethylene glycol, as an abrasive. The polished sample was rinsed and sonicated in anhydrous ethanol for 2 min and air dried and stored in a desiccator. The grinding and polishing procedure, which penetrated to a depth of > 10 μm, removed the outer casting skin from the die-cast alloy. This avoids the possibility of contaminants from the die casting process. For all electrochemical measurements, Struers EpoFix epoxy was used to mount the electrodes, leaving only the polished 1 cm 2 alloy surface exposed to electrolyte. For immersion experiments to assess surface damage only, the samples were not mounted in epoxy to facilitate subsequent analytical procedures. Instrumentation.— Electron micrographs were obtained in back- scattered and secondary electron modes using either a LEO 440, Hi- tachi 3400-N Variable Pressure Scanning Electron Microscope or LEO 1540 XB SEM/FIB. X-ray energy dispersive spectroscopy (XEDS) maps were obtained using Quartz One software. Depth profiles on un-corroded and corroded surfaces were mea- sured with confocal laser scanning microscopy (CLSM) by detecting the reflected light intensity from a Zeiss 510 confocal, HeNe 633 nm laser. The polished sample surface was placed downward, suspended by a stage, facing the inverted objective. Light intensities were nor- malized into a depth profile by considering the number of steps (slices) through the focal plane, in the z-direction, required to reach the deep- est region on the sample surface. The differences in light intensities were then converted to a distance. Optical micrographs were col- lected using a Zeiss Lunar V12 microscope equipped with an Axio 1.1 camera. Intermittent immersion experiments.— Prior to a corrosion exper- iment, the polished surface was analyzed by SEM/XEDS and CLSM. The co-ordinates of an area of interest (AOI) were recorded (relative to a surface edge) so that the same area (275 μm × 450 μm) could subsequently be located after each of a series of immersions. Immer- sions were performed in a naturally aerated 1.6 wt% NaCl (reagent ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.100.60.73 Downloaded on 2014-09-10 to IP