Micro-galvanic cathodes

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. 

Galvanic corrosion can occur in a polycrystaUine alloys, such as pearMc steels, due to differences in microstmctural phases. This leads to galvanic-phase coupling or galvanic microceUs between ferrite (a-Fe) and cementite FesC) since each phase has different electrode potentials and atomic stmcture. Therefore, distinct localized anodic and cathodic microstmctural areas develop due to microstmctural inhomogeneities, which act as micro-electrochemical cells in the presence of a corrosive medium (electrolyte). This is an electrochemical action known as galvanic corrosion, which is mainly a metallic surface deterioration. 

The galvanic corrosion can also occur within an alloy if micro-anodes and cathodes are present in the alloy. Mg alloys are not uniform in terms of their composition, microstructure and even crystalline orientation. These differences can result in various electrochemical activities within a Mg alloy and thereby generate galvanic couples on a micro-scale. 

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