Aqueous layer water-metal interaction

Brief consideration is now given to the solvent structure at metal/aqueous electrolyte interfaces.Several molecular models have been proposed which treat a single layer of water molecules at the metal surface. Within the layer, the individual water molecules (or clusters of molecules) are allowed to have certain orientations. In the earliest and simplest molecular model, an inner-layer water molecule is oriented as a result of its dipole interaction with the charge on the metal electrode. Orientation is limited to either of the two positions in which the molecular dipole is perpendicular to the electrode surface. More realistic treatments have since been described which variously... 

Atomospheric corrosion is the result of interaction between a material—mostly a metal—and its atmospheric environment. When exposed to atmospheres at room temperature with virtually no humidity present, most metals spontaneously form a solid oxide film. If the oxide is stable, the growth rate ceases and the oxide reaches a maximum thickness of 1 to 5 nm (1 nm = 1(T m). Atmospheric corrosion frequently occurs in the presence of a thin aqueous layer that forms on the oxidized metal under ambient exposure conditions the layer may vary from monomolecular thickness to clearly visible water films. Hence, atmospheric corrosion can be said to fall into two categories damp atmospheric corrosion, which requires the presence of water vapor and traces of pollutants, and wet atmospheric corrosion, which requires rain or other forms of bulk water together with pollutants [3]. 

The atmospheric corrosion of Mg alloys is a complex process which results from the interaction between a metal and its atmospheric environment. A prerequisite for atmospheric corrosion is the presence of a water layer on the surface. The thickness of the water layer varies considerably with the climatic conditions and may range from monomolecular thickness to clearly visible water hlms. The formation of an aqueous layer occurs in humid air by adsorption on the hydroxylated oxide present on most metal surfaces exposed to ambient conditions. The thickness of the reversible adsorbed water him varies with the relative humidity (RH). Table 7.1 shows the approximate number of water monolayers on a metal surface at 25 and steady state conditions (1). Thicker aqueous hlms can also form in the atmospheric environment by condensation, precipitation or water absorption by hygroscopic substances on the surface. 

The experimental data bearing on the question of the effect of different metals and different crystal orientations on the properties of the metal-electrolyte interface have been discussed by Hamelin et al.27 The results of capacitance measurements for seven sp metals (Ag, Au, Cu, Zn, Pb, Sn, and Bi) in aqueous electrolytes are reviewed. The potential of zero charge is derived from the maximum of the capacitance. Subtracting the diffuse-layer capacitance, one derives the inner-layer capacitance, which, when plotted against surface charge, shows a maximum close to qM = 0. This maximum, which is almost independent of crystal orientation, is explained in terms of the reorientation of water molecules adjacent to the metal surface. Interaction of different faces of metal with water, ions, and organic molecules inside the outer Helmholtz plane are discussed, as well as adsorption. 

Spohr describes in detail the use of computer simulations in modeling the metal/ electrolyte interface, which is currently one of the main routes towards a microscopic understanding of the properties of aqueous solutions near a charged surface. After an extensive discussion of the relevant interaction potentials, results for the metal/water interface and for electrolytes containing non-specifically and specifically adsorbing ions, are presented. Ion density profiles and hydration numbers as a function of distance from the electrode surface reveal amazing details about the double layer structure. In turn, the influence of these phenomena on electrode kinetics is briefly addressed for simple interfacial reactions. 

---