Corrosion process adsorption, corroding metals

ELECTROCATALYSIS IN CORROSION PROCESSES 22.12.1 Adsorption on Corroding Metals... 

Halide ions, according to the adsorption theory of passivity, tend to break down passivity by competing with the passivator for adsorption sites on the metal surface. Should a halide ion find a vacant site and closely approach the surface, hydration and dissolution of metal ions are favored, and the anodic reaction can proceed with low activation energy, in contrast to the high activation energy required when a passivator is adsorbed. The anode reaction, if it persists, is confined to localized areas where the competitive process first succeeds, because surrounding metal immediately becomes cathode of an electrolytic cell, and is protected by flow of current from further anode activity, a process called cathodic protection. This attack at specific sites leads to corrosion pitting typical of metals otherwise passive that are actually corroded by their environment. 

In the previous section, we considered one of the most basic gas-solid kinetic processes the simple adsorption or desorption of atoms to/from a surface under the assumption that the rate is limited by the impingement of atoms from the gas phase to the surface. In this section, we consider a more complex situation in which a gas species actively etches or corrodes a solid surface via a chemical reaction process, thereby continuously removing material from the surface over time. Consider, for example, the corrosion of a Ti metal surface with HCl acid vapor ... 

Study of the inhibition mechanism of molecules requires systematic investigation of the adsorption/desorption processes of inhibitor molecules on the metal/electrolyte interface. The adsorption phenomena of inhibitors on corroding metals are fairly complex and dependent on the surface feature, such as the composition and structure of the oxide-hydroxide layer, the local pH gradient near the interface, and interaction between the inhibitor molecules and components of the oxide layer. The study of adsorption/desorption processes of corrosion inhibitors on a noble metal surface is of great importance for fundamental aspects. Therefore knowledge of the adsorption properties of inhibitor molecules on a well-defined surface structure might be beneficial, and may contribute to a better understanding of the kinetics and mechanisms of inhibition processes on constructional materials of industrial importance. 

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