Hydrogen evolution reaction , magnesium

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. 

When the potential of the magnesium electrode is made more positive, the rate of Mg+ ion formation increases, and with it that of reaction (16.5). Therefore, the rate of hydrogen evolution increases instead of falling off, with increasing anodic polarization of the magnesium (see Section 13.7). This phenomenon has become known as the negative difference effect. 

Mixtures of magnesium (or aluminium) powder with water can be caused to explode powerfully by initiation with a boosted detonator [1], The use of organic coatings on magnesium or aluminium powder in pyrotechnic compositions prevents reaction with atmospheric moisture and problems resulting from hydrogen evolution [2],... 

Cathodic inhibitors either selectively precipitate on cathodic areas or slow the cathodic reaction by increasing hydrogen overvoltage. Other cathodic inhibitors utilize alkalinity increase at cathodic sites to precipitate insoluble compounds on the metal surface. Hydrogen evolution causes the metal-concrete interface to become more alkaline and ions such as calcium or magnesium precipitate as oxides to form a protective layer. 

Hence, in dilute sodium chloride, about half the magnesium corroding anodically appears as Mg(OH)2 and half as MgCl2, accompanied by hydrogen evolution in about the amount expected according to this reaction [15]. Additional lesser side reactions may also take place at the same time. Accordingly, the observed yield of a magnesium anode is only about one-half the lOOOA-h/lb calculated on the basis of Mg + formation. 

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. 

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