Corrosion electrochemistry cathodic process

This chapter has discussed the mechanism of what happens at the steel surface. The chemical reactions, formation of oxides, pitting, stray currents, bacterial corrosion, anodes, cathodes and reference electrode potentials (half cells) have been reviewed. A more detailed account of the electrochemistry of corrosion and corrosion of steel in concrete is given in Appendix B. Chapter 3 will discuss the processes that lead to the corrosion and the consequences in terms of damage to structures. We will then move on to the measurement of the problem and how to deal with it. 

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. 

The processes of cathodic protection can be scientifically explained far more concisely than many other protective systems. Corrosion of metals in aqueous solutions or in the soil is principally an electrolytic process controlled by an electric tension, i.e., the potential of a metal in an electrolytic solution. According to the laws of electrochemistry, the reaction tendency and the rate of reaction will decrease with reducing potential. Although these relationships have been known for more than a century and although cathodic protection has been practiced in isolated cases for a long time, it required an extended period for its technical application on a wider scale. This may have been because cathodic protection used to appear curious and strange, and the electrical engineering requirements hindered its practical application. The practice of cathodic protection is indeed more complex than its theoretical base. 

It is important to emphasise that the above theoretical approach for the electrochemical decomposition of PFC has been developed for the whole electrochemical system without separating it into the cathode and anode sub-systems. Remembering modem theoretical electrochemistry, we must admit that such approach is not common. Really, the partial cathode and anode processes used to be studied separately at different electrodes (except for corrosion studies). It is believed that the adequate pattern can be obtained for the whole system by mechanical joining the separate mechanisms together. Is it valid every time and everywhere The answer is no. We should consider it only as practically useful simplification and remember that there are situations where it is no longer true. 

So, any reaction that favors the consumption of Na20 or SO3 will lead to the dissociation of Na2S04 and vice versa. Molten Na2S04 is an ionic conductor the hot corrosion mechanism should generally be electrochemistry [78]. In other words, hot corrosion itself is an electrochemical process that includes anodic oxidation, cathodic reduction and ion diffusioa As for the hot corrosion of Ti3AlC2, the anodic oxidation process mainly consists of the anodic dissolution of Ti and Al ... 

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