Metal surface inhomogeneities

Pits occur as small areas of localized corrosion and vary in size, frequency of occurrence, and depth. Rapid penetration of the metal may occur, leading to metal perforation. Pits are often initiated because of inhomogeneity of the metal surface, deposits on the surface, or breaks in a passive film. The intensity of attack is related to the ratio of cathode area to anode ai ea (pit site), as well as the effect of the environment. Halide ions such as chlorides often stimulate pitting corrosion. Once a pit starts, a concentration-cell is developed since the base of the pit is less accessible to oxygen. 

Recently, a constant-phase element has been found607 to be present at pc-Pb/KF + HaO interfaces by impedance measurements. The Pb electrode was cathodically reduced before use. The assumption has been made that the CPE is due to the inhomogeneity of the metal surface. Frequency-... 

Structural surface inhomogeneity influences the anodic dissolution process in the case of metals with appreciable activation polarization. As a rule, segments with perturbed structure dissolve more rapidly than ordered segments. In a number of cases this causes crystallites to break away from the electrode surface and form metal sludge. 

The inner and outer potential differ by the surface potential Xa — (fa — ipa- This is caused by an inhomogeneous charge distribution at the surface. At a metal surface the positive charge resides on the ions which sit at particular lattice sites, while the electronic density decays over a distance of about 1 A from its bulk value to zero (see Fig. 2.1). The resulting dipole potential is of the order of a few volts and is thus by no means negligible. Smaller surface potentials exist at the surfaces of polar liquids such as water, whose molecules have a dipole moment. Intermolecular interactions often lead to a small net orientation of the dipoles at the liquid surface, which gives rise to a corresponding dipole potential. 

A real surface of a solid metal is inhomogeneous, and nucleation for the growing clusters is favored at certain active sites. To simplify the mathematics we consider an electrode with unit surface area. If there are Mo active sites, the number M(t) of growing nuclei is given by first-order kinetics ... 

H. S. Taylor has laid great stress on this inhomogeneity of catalytic surfaces. He suggests that the atoms constituting a metallic surface can exist in different degrees of saturation, varying from that which would be characteristic of a perfect plane crystal face down to that of a single atom attached at one point only. This would lead one to suppose that adsorption occurs not uniformly over the surface but predominantly on certain active points of the surface. We shall have evidence in favour of this view in a later section. 

Atomic processes that constitute the electrodeposition process, Eq. (6.93), can be seen by presenting the structure of the initial, M"+(solution), and the final state, Mn+(lattice). Since metal ions in the aqueous solution are hydrated the structure of the initial state in Eq. (6.93) is represented by [M(H20)J"+. The structure of the final state is the M adion (adatom) at the kink site (Fig. 6.13), since it is generally assumed that atoms (ions) are attached to the crystal via a kink site (3). Thus, the final step of the overall reaction, Eq. (6.93), is the incorporation of M"+ adion into the kink site. Because of surface inhomogeneity the transition from the initial state [M(H20)J"+(solution) to the final state Mn+(kink)... 

Other effects such as surface inhomogeneities and surface reconstruction can also be considered. In the case of oxidation of H2, electrosorbed anions or cations can act as inhibitors. During the oxidation of metals the formation of surface oxides or salts plays the most important role. For instance, the following model describes... 

Changes of environment over small distances on metal surfaces can lead to specific types of corrosion some of which, once propagated, may be self-sustaining. The nature of the metal surface itself can lead to the production of different environments along its surface for example, an intercrystalline crack or inhomogeneity in the surface will lead to preferential cathodic or anodic reaction which, in turn, will cause changes in the chemistry of the local solute/solvent. These "micro environments and types of corrosion are closely related and some common occurrences are described below. 

Solids also have a snrface tension (althongh this cannot be determined by simple methods). Fignre C2-4 shows snrface tensions of some liquids and solids nnder their own vaponr pressure. The values differ only slightly from what they would be under vacuum. There are two groups small liquid molecules and non-polar solids with low snrface tensions (less than 0.1 N m ) and ionic and metallic surfaces with high tensions. Surface tensions of solids are not as well defined as those of liquids. Solid surfaces are usually inhomogeneous, and they can be rough. As a result, their surface tension varies with position. 

A second problem with surface inhomogeneities in metals is the depletion of a more chemically active phase in contact with a less chemically active phase. When two metallic phases are in electrical contact, the more chemically active phase of the alloy corrodes much more rapidly than normal, even when it is exposed to even a mildly corrosive environment. If a coin has two phases exposed at the surface, such as copper and lead, then the more active metal, namely lead, will corrode preferentially. However, corrosion stops locally as soon as a given lead particle is consumed. The copper matrix then prevents further corrosion of lead because the copper phase must first corrode before additional lead is exposed at the surface. Hence,... 

It should be mentioned, however, that surface inhomogeneities of different dimensionality (cf. Section 2.1) significantly influence the kinetics of metal electrodeposition and the time-dependent surface morphology. Therefore, an exact analysis of corresponding EIS spectra is rather difficult. The necessary presumptions of stationarity and linearity for EIS measurements and quantitative interpretation of EIS data are often violated. The lack of direct local information on surface dynamics strongly hinders a quantitative analysis of the impedance behavior of time-dependent systems. Such considerations have been mainly disregarded in previous EIS data interpretations. In future, a combination of EIS measurements with in situ local probe... 

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