Electrochimica Acta 56 (2011) 1737–1745 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Numerical modeling of micro-galvanic corrosion Kiran B. Deshpande ∗,1 General Motors R&D, India Science Lab, Creator Building, ITPL, Bangalore 560066, India article info Article history: Received 15 July 2010 Received in revised form 10 September 2010 Accepted 14 September 2010 Available online 22 September 2010 Keywords: Magnesium Modeling studies SVET Corrosion abstract A novel numerical model capturing the evolution of cross-sectional microstructure to investigate its effect on corrosion behavior of magnesium (Mg) alloys is presented in this work using a moving mesh technique in COMSOL Multi-Physics ® . The model is capable of tracking the moving boundary of the corroding phase ( phase). The effect of  phase fraction and its distribution on the corrosion behavior is addressed here assuming uniform Al content in the  phase. The effects of  phase fraction on corrosion behavior estimated from the model are validated with SVET experiments. A novel formulation using a level set function is used to study the effect of two representative microstructure configurations along the depth of the alloy: (a) continuous  phase network around the  phase, and (b) discrete  phase. The Mg alloy with a continuous  phase network is found to display accelerated corrosion in the initial stage of exposure due to increasing  phase fraction. However, corrosion tends to be halted after the  phase is preferentially dissolved and the continuous  phase network is exposed to electrolyte due to  phase enrichment. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Lightweighting is one of the key thrust areas in the automo- bile industry today. It has been reported in a National Academy of Science report on CAFÉ standards that a 10% reduction in weight results in 6.6–8% increase in fuel efficiency. Magnesium (Mg), being four times lighter than steel and one and half times lighter than aluminum (Al), provides an attractive high strength to weight ratio for automotive applications. However, the use of Mg in automobile is limited to less than 1%, mainly due to its poor corrosion resis- tance amongst other inhibiting factors. Mg is electrochemically the most active structural material so it corrodes preferentially when coupled with either Al alloys or steels. Mg is also prone to micro- galvanic corrosion which occurs due to galvanic activity amongst its primary constituents namely, primary , eutectic  and  phases. Mg alloy, AZ91, has been widely studied to investigate micro- galvanic corrosion [1–10]. The corrosion behavior of Mg alloys is strongly influenced by the Al content within the  phase, the  phase fraction and its distribution around the  phase. The effect of grain size on the corrosion behavior has also been debated in the literature. Thus, microstructure, which can be controlled by pro- cessing conditions such as cooling rate and heat treatment, plays an important role in determining the corrosion behavior of Mg alloys. ∗ Tel.: +91 80 4198 4560; fax: +91 80 4115 8562. E-mail address: kiran.deshpande@gm.com. 1 ISE member. Song et al. [1] investigated the influence of microstructure on the corrosion of AZ91 using high purity AZ91 alloy processed with a slower cooling rate and die cast AZ91 processed with a faster cooling rate. They found that the corrosion resistance of die-cast AZ91, even in the detrimental presence of iron impurities, is higher than that of high purity AZ91, which is attributed to widely dif- ferent microstructures of the above two alloys. The die cast AZ91 has alloy solidified under relatively fast cooling rates consists of smaller grain size, higher  phase fraction and very finely dis- tributed  phase around the  phase. Song et al. [1] reported a dual role of the  phase for different microstructure configurations: (a)  phase acting as a barrier to corrosion with a fine-grain microstruc- ture, higher  phase fraction and continuous  phase around the  phase, and (b)  phase acting as a cathode and accelerating cor- rosion with a microstructure consisting of coarser grain size and discretely distributed  phase. Ambat et al. [2] evaluated microstructural effects on corro- sion using die cast AZ91 and ingot AZ91 alloys. Die cast AZ91 showed a higher corrosion resistance and better passivation than ingot AZ91 alloy, and this was attributed to smaller grain size and finer  phase. In die cast AZ91 alloy, the inter- phase dis- tance was found to be small due to a very fine grain structure and  phase network was found to be continuous or well con- nected. Ambat et al. [2] showed a cellular type  phase network and corrosion sites within the grains, by performing a constant immersion test for die cast AZ91. Ingot AZ91 alloy showed under- mining of the  phase from the surface which was established by X-Ray Diffraction (XRD) studies on the sample surface and 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.09.044