Basics of Metal Dissolution

Model of metal surface with step, kink, and terrace sites after Stranski and Kossel. (From Stranski, I.N., Z. Physik. Chem., 136, 259, 1928 Kossel, W., Nachr. Ges. Wiss. GSttingen. Math. Physik. K.L, 135,1927.) 

Free energy diagram for metal dissolution. (From Strehblow, H.-H., Phenomenological and electrochemical fundamentals of corrosion, in Corrosion and Environmental Degradation, M. Schiitze, editor, Vol. 1, Wiley-VCH, Weinheim, Germany, 2000, pp. 1-66.) 

Many studies have been performed on the dissolution on iron. This reaction is catalyzed by OH ions. Kinetic studies have shown that the electrochemical reaction order for OH ions is 1.5. These findings started a discussion about two mechanisms the Bockris and the Heusler mechanism [63,64]. According to Heusler, it starts with the decomposition of water resulting in the adsorption of OH as there are no OH ions, which could adsorb directly in acidic media. 

This fast reaction is followed by two single-electron steps to form the complex FeOH+, which is finally transferred into solution where it is decomposed. 

FeOHad + OHad + Fe FeOH (aq)+FeOHad + 2e slow (1.147c) 

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