Corrosion electrochemistry hydrogen evolution

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. 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

The electric double layer (edl) is fundamental for electrochemistry because the rate and mechanism of the various electrochemical reactions (hydrogen evolution, corrosion and corrosion inhibition by surfactants, metal deposition and dissolution, and so forth) depend on the... 

Hydrogen evolution has been studied extensively in electrochemistry due to its importance in different flelds such as energy storage, fuel cells, and last but not least corrosion. It is a relatively complicated reaction involving many consecutive reaction steps. Apart from the transport of H" ions to the electrode and H2 from the electrode to the bulk electrolyte, there are still three re-... 

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. 

The introduction of lead-calcium alloys started in the United States in the 1930s (37) and was specified for standby batteries in the Bell telephone service in 1951 (38). Corrosion of the lead-calcium alloy does not affect the electrochemistry of the battery, because calcium is not precipitated at the negative electrode but remains as Ca ion in the electrolyte. As a consequence, the hydrogen evolution rate is low and remains practically unaltered during the whole service life of the battery. 

Stefan Christov performed and also promoted profound scientific studies in the field of quantum electrochemistry, physical chemistry, theory of chemical reactions, and solid-state physics [36 3]. His contributions related to hydrogen evolution reactions, corrosion phenomena, and electron transfer reactions are well known to those who work in these particular scientific fields. The same is valid also for the so-called Christov s characteristic temperature related to the transition rate of over- and under-barrier tunneling reactions. 

Cathodic protection Cathodic protection was employed before electrochemistry had been developed. Htrmphrey Davy used cathodic protection on British naval ships in 1824. The principles of cathodic protection may be explained by considering the corrosion of atypical metal in an acid environment. Electrochemical reactions include the dissolution of the metal and the evolution of hydrogen gas ... 

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