Magnesium corrosion electrochemistry

Corrosion electrochemistry of magnesium (Mg) and its alloys 27 Direct hydration... 

Magnesium exhibits a very strange electrochemical phenomenon known as the negative-difference effect (NDE). Electrochemistry classifies corrosion reactions as either anodic or cathodic processes. Normally, the anodic reaction rate increases and the cathodic reaction rate decreases with increasing applied potential or current density. Therefore, for most metals like iron, steels, and zinc etc, an anodic increase of the applied potential causes an increase of the anodic dissolution rate and a simultaneous decrease in the cathodic rate of hydrogen evolution. On magnesium, however, the hydrogen evolution behavior is quite different from that on iron and steels. On first examination such behavior seems contrary to the very basics of electrochemical theory. 

This chapter presents electrochemical reactions and corrosion processes of Mg and its alloys. First, an analysis of the thermodynamics of magnesium and possible electrochemical reactions associated with Mg are presented. After that an illustration of the nature of surface films formed on Mg and its alloys follows. To comprehensively understand the corrosion of Mg and its alloys, the anodic and cathodic processes are analyzed separately. Having understood the electrochemistry of Mg and its alloys, the corrosion characteristics and behavior of Mg and its alloys are discussed, including self-corrosion reaction, hydrogen evolution, the alkalization effect, corrosion potential, macro-galvanic corrosion, the micro-galvanic effect, impurity tolerance, influence of the chemical composition of the matrix phase, role of the secondary and other phases, localized corrosion and overall corrosivity of alloys. 

LM Peng, JW Chang, XW Guo, A Atrens, WJ Ding, YH Peng, Influence of heat treatment and microstructure on the corrosion of magnesium alloy Mg-10Gd-3Y-0.4Zr, Journal of Applied Electrochemistry, 2009, 39, 913. 

Hence, when reviewing important aspects of non-aqueous magnesium electrochemistry, it is important to consider the reversibility of Mg deposition processes, to map possible corrosion process of Mg electrodes, and to determine the anodic stability of electrolyte solutions in which Mg electrodes behave reversibly. In the following sections, we will briefly review conventional non-aqueous electrolyte solutions and the passivation of active metals in non-aqueous solutions, after which we will describe systematically the behavior of Mg electrodes in various types of conventional and non-conventional non-aqueous electrolyte solutions. Finally, we will review in brief another important aspect of non-aqueous magnesium electrochemistry, which is the electrochemical intercalation of Mg ions into inorganic hosts. 

J.K. Chang, S.Y. Chen, W.T. Tsai, M.J. Deng and I.W. Sun, Electrodeposition of aluminum on magnesium alloy in aluminum chloride (AICI3) - l-ethyl-3-methylimidazolium chloride (EMIC) ionic liquid and its corrosion behavior . Electrochemistry Communications, 9, (2007), 1602-1606. 

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