Electrochimica Acta 55 (2010) 1880–1887 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Electrochemical behavior of Mg and some Mg alloys in aqueous solutions of different pH Waheed A. Badawy a,∗ , Nadia H. Hilal b , Mohmmed El-Rabiee b , H. Nady b a Chemistry Department, Faculty of Science, University of Cairo, 12 613 Giza, Egypt b Chemistry Department, Faculty of Science, Fayoum University, Fayoum, Egypt article info Article history: Received 29 July 2009 Received in revised form 30 October 2009 Accepted 30 October 2009 Available online 10 November 2009 Keywords: Alloys Corrosion Impedance Magnesium Polarization abstract The electrochemical behavior of Mg, Mg–Al–Zn and Mg–Al–Zn–Mn alloys were investigated in aqueous acidic, neutral and basic solutions. Conventional electrochemical techniques such as open-circuit poten- tial measurements, polarization methods and electrochemical impedance spectroscopy (EIS) were used. The results have shown that the rate of corrosion in acidic solution is relatively high compared to that in neutral or basic solutions. The presence of Al, Zn and Mn as alloying elements decreases the rate of corrosion of the alloy. The activation energy of the corrosion process occurring at the surface of Mg or Mg alloys in aqueous solutions is less than 40 kJ mol -1 . This value indicates a one electron transfer electrode as a rate controlling process. The impedance data were fitted to equivalent circuit models that explain the different electrochemical processes occurring at the electrode/electrolyte interface. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Magnesium alloys containing 2–10% Al with minor additions of Zn and Mn are widely used for different technological applica- tions. These alloys are comparatively cheap and possess attractive characteristics, especially machinability and corrosion resistance at temperatures above 100 ◦ C [1]. The alloys have high stiffness to weight ratio, ease of workability, high damping capacity, cast- ability, weld-ability and recyclability [2]. Because of these excellent properties they are mostly used in aerospace and automotive industries. The alloy design development, surface treatment meth- ods and understanding of corrosion mechanisms have extended the potential applications of the Mg alloys [3]. The corrosion resis- tance of the alloy depends on its composition, microstructure and ambient medium. It is sensitive to the chloride ion concentration and the pH of the environments [4,5]. In neutral and basic solutions the corrosion resistance is relatively high because of the formation of a partially protective Mg(OH) 2 layer on the alloy surface [4]. The presence of chloride ions promotes the dissolution of the protective layer and leads to increased rates of corrosion [4,6]. The corrosion of Mg alloys in non-oxidizing neutral or basic chlo- ride solutions at free-corrosion potential initiates as irregular pits. The mechanism of the corrosion process is quite different from ∗ Corresponding author. Tel.: +20 2 35676558; fax: +20 2 35685799. E-mail addresses: wbadawy50@hotmail.com, wbadawy@cu.edu.eg (W.A. Badawy). the auto-catalytic pitting experienced by stainless steels and the local increase of pH resulting from the formation of Mg(OH) 2 has no effect on the pitting process [7,8]. The film is not very stable and its free-corrosion potential is more positive than the pitting potential for both single  phase alloys as well as for two phase ( + ) alloys [9–11]. The corrosion of Mg is a localized corrosion, which starts at irregular pits that spread laterally and cover the whole surface [11,12]. In two phase Mg alloys the corrosion mech- anism is influenced by microstructures. Fine, uniformly dispersed, cathodic phases are the most detrimental to the corrosion resis- tance of Mg-base alloys [13]. The casting method influences the corrosion performance through control of the microstructures [14]. The presence of alloying elements with Mg not only modifies its mechanical properties but also improves its corrosion resistance [15]. In this paper it is aimed at the investigation of the corrosion and passivation behaviors of Mg and two different Mg alloys in aqueous solutions covering the acidic, neutral and basic ranges. The effect of temperature on the corrosion rate and the determination of the molar activation energy of the corrosion process have been carried out. Equivalent circuit models for the electrode/electrolyte interface were suggested. 2. Experimental The working electrodes were made from massive rods of Mg (99.9% + 0.03% Cu, 0.03% Ni and 0.04% Fe), Mg–Al–Zn (92% Mg + 7% Al and 1% Zn) and Mg–Al–Zn–Mn (89.2% Mg + 10% Al, 0.5% Zn and 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.10.083