Surface Film Formation

The assessment for nonalloyed ferrous materials (e.g., mild steel, cast iron) can also be applied generally to hot-dipped galvanized steel. Surface films of corrosion products act favorably in limiting corrosion of the zinc. This strongly retards the development of anodic areas. Surface film formation can also be assessed from the sum of rating numbers [3, 14]. 

The anodes are generally not of pure metals but of alloys. Certain alloying elements serve to give a fine-grained structure, leading to a relatively uniform metal loss from the surface. Others serve to reduce the self-corrosion and raise the current yield. Finally, alloying elements can prevent or reduce the tendency to surface film formation or passivation. Such activating additions are necessary with aluminum. 

Eq. (2-6), but also adversely affects surface film formation. Figure 17-2 shows the relation between protection current density and steaming velocity. Factor Fj relates to undisturbed film formation. The influence of flow is not very great in this case. Factor F2 represents the real case where surface films are damaged by abrasion [15]. The protection current density can rise to about 0.4 A m at uncoated areas. 

In addition, with high solid content of the cooling water and at high flow velocities, severe corrosive conditions exist which continuously destroy surface films. Cathodic protection alone is not sufficient. Additional measures must be undertaken to promote the formation of a surface film. This is possible with iron anodes because the anodically produced hydrated iron oxide promotes surface film formation on copper. 

The first application of the quartz crystal microbalance in electrochemistry came with the work of Bruckenstein and Shay (1985) who proved that the Sauerbrey equation could still be applied to a quartz wafer one side of which was covered with electrolyte. Although they were able to establish that an electrolyte layer several hundred angstroms thick moved essentially with the quartz surface, they also showed that the thickness of this layer remained constant with potential so any change in frequency could be attributed to surface film formation. The authors showed that it was possible to take simultaneous measurements of the in situ frequency change accompanying electrolysis at a working electrode (comprising one of the electrical contacts to the crystal) as a function of the applied potential or current. They coined the acronym EQCM (electrochemical quartz crystal microbalance) for the technique. 

This surface film formation is being proposed to protect corrosion of some reactive metals [112d],... 

Laser Raman spectroscopy has played a major role in the study of electrochemical systems (see Section 3.4). The technique provides molecular-specific information on the structure of the solid-solution interfaces in situ and is particularly suited for spectroelectrochemical studies of corrosion and surface film formation. Metals such as Pb, Ag, Fe, Ni, Co, Cu, Cr, Ti, Au and Sn, stainless steel and other alloys in various solutions have been studied by the technique. 

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

To conclude this section, it should be emphasized that the steady state vol-tammograms described above are quite different from the first scan of the cyclic voltammetry of these systems. During the first polarization of these electrodes to low potentials, pronounced reduction processes of solution components are observed. As a result of these processes, a stable precipitate forms on the electrodes as insoluble films, and hence the above steady state voltammetric behavior reflects electrochemistry which is surface film controlled. The outer, solution side of these films is probably porous, leading to the high interfacial capacity which is reflected by the relatively high non-Faradaic currents which characterize these voltammograms. The next section describes in detail the initial voltammetric behavior of these systems and the surface film formation on the electrodes. 

During the past 14 years, intensive work has been devoted to the study of surface film formation on noble metals in nonaqueous Li salt solutions using surface sensitive techniques. The major goal of this work had been to analyze the surface species formed at different potentials on nonactive electrode surfaces as a function of solution composition. The most successful tool has been ex situ and in situ FTIR spectroscopy [3,4,6,7,16,17], However, other techniques such as Raman [32,33], XPS [12], and EDAX have also been applied. These systems have also been studied in parallel by impedance spectroscopy [34] and EQCM [35], These measurements are complementary to the voltammetric studies de-... 

The overall picture for the surface film formation in alkyl carbonate solutions obtained from the intensive surface studies (to which Figures 11-15 relate) can be summarized as follows ... 

In the case of methyl acetate, there is no surface film formation, since the reduction product of MA, which is lithium acetate, is soluble in the parent solvent. 

Surface film formation on noble metal electrodes at reduction potentials was studied extensively with solutions of DME, THF, 2Me-THF, and DN. Basically, these solvents are much less reactive at low potentials than are alkyl carbonates and esters. However, in contrast to ethereal solutions of TBA+ whose electrochemical window is limited cathodically by the TBA+ reduction at around OV (Li/Li+), in Li+ solutions, ether reduction processes that form Li alkoxides occur at potentials below 0.5 V (Li/Li+) [4], It should be emphasized that the onset potential for surface film formation on noble metals in ethereal solutions is as high as in... 

In the case of LiC104, there is some spectroscopic evidence of anion reduction below 1.5 V [39], The stable surface species which may precipitate due to the Cl()4 reduction is, among others, Li20. We have no spectroscopic evidence for precipitation of stable LiC10x (x = 1-3) or for LiCl onto noble metals at potentials above those of Li bulk deposition. In any event, the above salt anion reduction processes do not dominate the overall surface film formation on nonactive electrodes at low potentials in most aprotic solvents. Thus, both anions can be considered as only moderately reactive. The onset potential for the reduction of the anions from the third group is about 2 V (Li/Li+). This is clearly demonstrated in Figures 18 and 19, which show FTIR spectra measured in situ from... 

Similar to the behavior of nonactive metal electrodes described above, when carbon electrodes are polarized to low potentials in nonaqueous systems, all solution components may be reduced (including solvent, cation, anion, and atmospheric contaminants). When the cations are tetraalkyl ammonium ions, these reduction processes may form products of considerable stability that dissolve in the solution. In the case of alkali cations, solution reduction processes may produce insoluble salts that precipitate on the carbon and form surface films. Surface film formation on both carbons and nonactive metal electrodes in nonaqueous solutions containing metal salts other than lithium has not been investigated yet. However, for the case of lithium salts in nonaqueous solvents, the surface chemistry developed on carbonaceous electrodes was rigorously investigated because of the implications for their use as anodes in lithium ion batteries. We speculate that similar surface chemistry may be developed on carbons (as well as on nonactive metals) in nonaqueous systems at low potentials in the presence of Na+, K+, or Mg2+, as in the case of Li salt solutions. The surface chemistry developed on graphite electrodes was extensively studied in the following systems ... 

In conclusion, the above surface film formation processes are expected to occur with all types of the carbons mentioned (including doped diamonds and fullerenes) in nonaqueous solvents containing metal ion salts. Hence, when carbon electrodes are utilized for electroanalysis or electrosynthesis in such solutions at low potentials, they should be considered as modified electrodes covered with surface films that are, at least partially, electronic insulators (but may be ion conductors). 

Due to the practical implication related to the field of nonaqueous batteries, the solutions studied by this combination of methods were organic carbonates and ethereal-based solutions of Li salts (e.g., for EIS, see Refs. 34 and 95 for AFM, Refs. 96 and 97 and for EQCM, Refs. 35 and 98). Some representative results, which demonstrate the application of these methods to the study of important interfacial phenomena, are described briefly. As an example, we review a study of surface film formation on nonactive metal electrodes (Ni, Cu, Au) in PC with Li salt solutions. 

The above spectra reflect surface film formation which becomes more pronounced as the electrode potential is lower, and its slow dissolution as the electrodes are held at open circuit (no continuous driving force for surface film forma-... 

The brief description above demonstrates how novel techniques such as EQCM and AFM, in conjunction with a more conventional electroanalytical tool (EIS) can be applied for the study of surface film formation on electrodes, which is a fundamental phenomenon in many important nonaqueous systems. 

Basic issues such as surface reactions, surface film formation, passivation, ionic and electronic transport phenomena through surface films, problems in uniformity of deposition and dissolution processes, correlation between surface chemistry, morphology, and electrochemical properties are common to all active metal electrodes in nonaqueous solutions and are dealt with thoroughly in this chapter. It is believed that many conclusions related to Li, Mg, Ca, and A1 electrodes can be extended to other active metal electrodes as well. 

This situation, together with the dissolution-deposition cycles of surface species occurring at steady state, as described above, leads to a porous structure of the outer part (solution side) of the surface films. Hence, we expect that surface films in active metals in solutions should comprise a compact part, close to the metal side of the surface films, and a porous layer at the solution side [21,22], The various situations described above for surface film formation on active metal electrodes in solutions are illustrated in Figures 1-4. 

Figure 3 A schematic view of formation of multilayer surface films on active metals exposed fresh to solution phase. Stage I Fresh surface-nonselective reactions Stage II Initial layer is formed, more selective surface film formation continues Stage III Formation of multilayer surface films Stage IV Highly selective surface reactions at specific points partial dissolution of surface species Stage V Further reduction of the surface species close to the active metal, deposition-dissolution of surface species at steady state the surface film is comprised of a multilayer inner compact part and an outer porous part.

Irish and Odziemkowski [90,91] developed a method to directly study the time constants of surface film formation on fresh Li surfaces. 

The surface film formation processes are very fast, and their properties change rapidly while the films are built up. Hence, it is almost impossi-... 

The first Li insertion capacity is very pronounced, but part of it appears irreversible. Most of this irreversibility is not connected with surface film formation but with irreversible insertion of lithium. The origin of this irreversibility is not yet understood. 

A metal CMP process involves an electrochemical alteration of the metal surface and a mechanical removal of the modified film. More specifically, an oxidizer reacts with the metal surface to raise the oxidation state of the material, which may result in either the dissolution of the metal or the formation of a surface film that is more porous and can be removed more easily by the mechanical component of the process. The oxidizer, therefore, is one of the most important components of the CMP slurry. Electrochemical properties of the oxidizer and the metal involved can offer insights in terms of reaction tendency and products. For example, relative redox potentials and chemical composition of the modified surface film under thermodynamically equilibrium condition can be illustrated by a relevant Pourbaix diagram [1]. Because a CMP process rarely reaches a thermodynamically equilibrium state, many kinetic factors control the relative rates of the surface film formation and its removal. It is important to find the right balance between the formation of a modified film with the right property and the removal of such a film at the appropriate rate. 

Thus, surface film formation and metal dissolution occur via an oxidation reaction with a balancing reduction reaction to sink the electrons generated. 

The region of a metal s stability is important for several reasons. First, different films tend to form on the metal in different regions of stability. CujO, CuO or Cu(OH)2 (hydrated CuO) form on copper at noble potentials and at neutral to high pH, while no film will form at low pH and/or more active potentials in the Cu or Cu regions of Figures 4.31 or 4.32. Because the surface film is, at least to some degree, the surface being polished and removed, surface film formation affects such important parameters as film quality, polish rate, planarization, and the post polish corrosion resistance. 

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