Metal dissolution mechanism

Knowledge about the metal-dissolution mechanism is very important for... 

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

Film is removed and underlying metal surface is mechanically damaged which contributes to overall metal loss i.e. erosion corrosion rate is equal to bare metal dissolution rate plus possibly synergistic effect of mechanical damage. 

The explicit aims of boiler and feed-water treatment are to minimise corrosion, deposit formation, and carryover of boiler water solutes in steam. Corrosion control is sought primarily by adjustment of the pH and dissolved oxygen concentrations. Thus, the cathodic half-cell reactions of the two common corrosion processes are hindered. The pH is brought to a compromise value, usually just above 9 (at 25°C), so that the tendency for metal dissolution is at a practical minimum for both steel and copper alloys. Similarly, by the removal of dissolved oxygen, by a combination of mechanical and chemical means, the scope for the reduction of oxygen to hydroxyl is severely constrained. 

In addition to the basic corrosion mechanism of attack by acetic acid, it is well established that differential oxygen concentration cells are set up along metals embedded in wood. The gap between a nail and the wood into which it is embedded resembles the ideal crevice or deep, narrow pit. It is expected, therefore, that the cathodic reaction (oxygen reduction) should take place on the exposed head and that metal dissolution should occur on the shank in the wood. 

As shown in Fig. 33, the decreasing mechanism of this fluctuation is summarized as follows At a place on the electrode surface where metal dissolution happens to occur, the surface concentration of the metal ions simultaneously increases. Then the dissolved part continues to grow. Consequently, as the concentration gradient of the diffusion layer takes a negative value, the electrochemical potential component contributed by the concentration gradient increases. Here it should be noted that the electrochemical potential is composed of two components one comes from the concentration gradient and the other from the surface concentration. Then from the reaction equilibrium at the electrode surface, the electrochemical potential must be kept constant, so that the surface concentration component acts to compensate for the increment of the concen-... 

A nonuniform distribution of the reactions may arise when the metal s surface is inhomogeneous, particularly when it contains inclusions of other metals. In many cases (e.g., zinc with iron inclusions), the polarization of hydrogen evolution is much lower at the inclusions than at the base metal hence, hydrogen evolution at the inclusions will be faster (Fig. 22.3). Accordingly, the rate of the coupled anodic reaction (dissolution of the base metal) will also be faster. The electrode s OCP will become more positive under these conditions. At such surfaces, the cathodic reaction is concentrated at the inclusions, while the anodic reaction occurs at the base metal. This mechanism is reminiscent of the operation of shorted galvanic couples with spatially separated reactions Metal dissolves from one electrode hydrogen evolves at the other. Hence, such inclusions have been named local cells or microcells. 

The first two pathways (a) and (b) show, respectively, the influence of H+ and of surface complex forming ligands on the non-reductive dissolution. These pathways were discussed in Chapter 5. Reductive dissolution mechanisms are illustrated in pathways (c) - (e) (Fig. 9.3). Reductants adsorbed to the hydrous oxide surface can readily exchange electrons with an Fe(III) surface center. Those reductants, such as ascorbate, that form inner-sphere surface complexes are especially efficient. The electron transfer leads to an oxidized reactant (often a radical) and a surface Fe(II) atom. The Fe(II)-0 bond in the surface of the crystalline lattice is more labile than the Fe(III)-0 bond and thus, the reduced metal center is more easily detached from the surface than the original oxidized metal center (see Eqs. 9.4a - 9.4c). 

The same two-step mechanism of metal dissolution has also been delivered for the anodic dissolution of nickel in acid solutions [Sato-Okamoto, 1964]. The mechanistic concepts for iron dissolution other than the two-step mechanism have also been presented in the literature [Heusler, 1958], 6ind the mechanism of metal dissolution is still a subject of research [Plonski, 1996]. 

CMP processes for oxide planarization (ILD and STI) rely on slurry chemistry to hydrolyze and soften the Si02 surface. Mechanical abrasion then controls the actual material removal. Thus, the key process output control variables (i.e., removal rate and nonuniformity) are strong functions of the mechanical properties of the system, namely, the down force and the relative velocity between the pad and the wafer. Metal CMP processes such as copper CMP rely more on chemical oxidation and dissolution of the metal than mechanical abrasion to remove the metal overburden. Consequently, careful control of the chemistry of the CMP process is more important for these CMP processes than it is for oxide CMP. Thus, CMP tools and processes optimized for ILD may not be optimal for metal CMP and vice versa. 

A dissolution mechanism for zinc, cadmium and mercury in their molten halides has been proposed on the basis of experimental and literature data.949 Dissolution occurs at the metal-salt phase boundary. Adsorbed M24- cations are reduced to M+ ions which then migrate into the salt phase where Mi4" dimers form. The stability of the M2+ ions was found to increase in the order Zn < Cd Hg (the latter is so stable that Hg+ is undetectable in solution). 

The basic mechanism for the instability of ultrapure metals was suggested by Wagner and Traud in a classic paper in 1938.1 The essence of their view is that for corrosion to occur, there need not exist spatially separated electron-sink and -source areas on the corroding metal. Hence, impurities or other heterogeneities on the surface are not essential for the occurrence of corrosion. The necessary and sufficient condition for corrosion is that the metal dissolution reaction and some electronation reaction proceed simultaneously at the metal/environment interface. For these two processes to take place simultaneously, it is necessary and sufficient that the corrosion potential be more positive than the equilibrium potential of the M, + + ne M reaction and more negative than the equilibrium potential of the electronation (cathodic) reaction A + ne — D involving electron acceptors contained in the electrolyte (Fig. 12.8). 

Consider a system consisting of a metal corroding in an electrolyte. The corrosion process involves a metal-dissolution deelectronation (anodic) reaction at electron-sink areas on the metal and an electronation (cathodic) reaction at electron-source areas. (This picture is applicable to a metal s corroding by a Wagner-Traud mechanism provided one imagines the sink and source areas shrunk to atomic-sized dimensions and considers the situation at one instant of time.)... 

What has been presented above is a very elementary account of corrosion under super-ideal conditions. In a few cases, it does give a fairly good agreement with the observed rates of corrosion. Yet, in real systems, corrosion is nearly always too complex a phenomenon for the above simple treatment to be directly applicable. The simple version would be valid if there were no oxide films, if there were a negligible IR drop in the solution, if the corrosion potential dhigh-field approximations [cf. Eq. (12.28)] could be applied, and if the transfer coefficients of the metal-dissolution and electronation reactions were [cf. Eq. (12.25)]. However, the point of an introductory treatment is not to treat the details and the complex realities, but to present the idealized essence about an electrochemical mechanism that has substantial effects in the everyday world. 

It has been sufficiently emphasized that the instability of metal surfaces arises from an electrodic mechanism, an electronation reaction teams up with the metal-dissolution reaction to keep numerous micro corrosion cells running. 

Similar electropolishing experiments were carried out using different grades of stainless steel (410, 302, 304, 316 or 347) and it was found that the mechanism of metal dissolution and the oxidation potentials for the metals were very similar. The slight exception was the 410 series steel (which has no Ni, unlike the 300 series steels which have 8-14%). The 410 steel required a more positive oxidation potential to break down the oxide in the ionic liquid whereas once the oxide was removed the... 

Figure 6 A schematic view of active metal dissolution via a breakdown and repair mechanism of the surface films.

This is the case for magnesium and calcium electrodes whose cations are bivalent. The surface films formed on such metals in a wide variety of polar aprotic systems cannot transport the bivalent cations. Such electrodes are blocked for the metal deposition [28-30], However, anodic processes may occur via the breakdown and repair mechanism. Due to the positive electric field, which is the driving force for the anodic processes, the film may be broken and cracked, allowing metal dissolution. Continuous metal dissolution creates an unstable situation in the metal-film and metal-solution interfaces and prevents the formation of stable passivating films. Thus, once the surface films are broken and a continuous electrical field is applied, continuous metal dissolution may take place at a relatively low overpotential (compared with the high overpotential required for the initial breakdown of the surface films). Typical examples are calcium dissolution processes in several polar aprotic systems [31]. 

Metal corrosion is a superposition of metal dissolution or the formation of solid corrosion products and a compensating cathodic reaction. Both processes have their own thermodynamic data and kinetics including a possible transport control. Furthermore, metals are generally not chemically and physically homogeneous so that localized corrosion phenomena, local elements, mechanical stress, surface layers, etc. may play a decisive role. Therefore, one approach is the detailed analysis of all contributing reactions and their mechanisms, which however does not always give a conclusive answer for an existing corrosion in practice. 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

Considerable progress has been made during the past decade toward a better insight into the basic concepts and mechanism involved in metallic dissolution and corrosion. More emphasis has been placed on the "fundamental particles (metallic ions, electrons, and electron acceptors) and on the use of current-voltage characteristics. The wide recognition of dissolution and corrosion as electrode processes, and the idea of a polyelectrode exhibiting a mixed potential, have augmented the use of electrochemical techniques in the study and interpretation of corrosion phenomena. There is even some evidence that the phenomenon of passivity may soon be clarified. 

---