Inhibition metal deposition

Molecules of the additive adsorbed on the surface prevent or inhibit metal deposition. To a first approximation it can be said that the rate of metal deposition is simply proportional to the fraction of the surface that is not covered by the additive. A more detailed analysis shows that adsorption on part of the surface could also have an effect on the rate of metal deposition on the bare sites, but this refinement need not concern us here. As a rule, the molar concentration of the additive in solution is very low compared to that of the metal ion being plated. Consequently, the rate of adsorption of the additive is controlled by mass-transport limitation, while the rate of metal deposition is mostly activation controlled, with possibly some mass transport limitation involved, depending on the ratio of j/ji, where j is the partial current density for deposition of the metal. This helps to produce a smooth surface for the same reason that a rough surface is formed in the absence of a suitable additive. On protruding parts on the surface the rate of mass transport is higher than on flat or... 

Cyclohexylamine is miscible with water, with which it forms an azeotrope (55.8% H2O) at 96.4°C, making it especially suitable for low pressure steam systems in which it acts as a protective film-former in addition to being a neutralizing amine. Nearly two-thirds of 1989 U.S. production of 5000 —6000 t/yr cyclohexylamine serviced this appHcation (69). Carbon dioxide corrosion is inhibited by deposition of nonwettable film on metal (70). In high pressure systems CHA is chemically more stable than morpholine [110-91-8] (71). A primary amine, CHA does not directiy generate nitrosamine upon nitrite exposure as does morpholine. CHA is used for corrosion inhibitor radiator alcohol solutions, also in paper- and metal-coating industries for moisture and oxidation protection. 

When electrodeposition is inhibited the metal becomes harder, less ductile and increases in tensile strength. Metals deposited from acidic solutions of... 

Reaction overpotential. Both overpotentials mentioned above are normally of higher importance than the reaction overpotential. It may happen sometimes, however, that other phenomena, which occur in the electrolyte or during electrode processes, such as adsorption and desorption, are the speed-limiting factors. Crystallization overpotential. This exists as a result of the inhibited intercalation of metal ions into their lattice. This process is of fundamental importance when secondary batteries are charged, especially during metal deposition on the negative side. 

The major types of interferences in ASV procedures are overlapping stripping peaks caused by a similarity in the oxidation potentials (e.g., of the Pb, Tl, Cd, Sn or Bi, Cu, Sb groups), the presence of surface-active organic compounds that adsorb on tlie mercury electrode and inhibit the metal deposition, and the formation of intermetallic compounds (e.g., Cu-Zn) which affects the peak size and position. Knowledge of these interferences can allow prevention through adequate attention to key operations. 

Experience shows that in the deposition of a number of metals (mercury, silver, lead, cadmium, and others), the rate of the initial reaction is high, and the associated polarization is low (not over 20 mV). For other metals (particularly of the iron group), high values of polarization are found. The strong inhibition of cathodic metal deposition that is found in the presence of a number of organic substances (and which was described in Section 14.3) is also observed at mercury electrodes (i.e., it can be also associated with the initial step of the process). 

In contrast to a mixture of redox couples that rapidly reach thermodynamic equilibrium because of fast reaction kinetics, e.g., a mixture of Fe2+/Fe3+ and Ce3+/ Ce4+, due to the slow kinetics of the electroless reaction, the two (sometimes more) couples in a standard electroless solution are not in equilibrium. Nonequilibrium systems of the latter kind were known in the past as polyelectrode systems [18, 19]. Electroless solutions are by their nature thermodyamically prone to reaction between the metal ions and reductant, which is facilitated by a heterogeneous catalyst. In properly formulated electroless solutions, metal ions are complexed, a buffer maintains solution pH, and solution stabilizers, which are normally catalytic poisons, are often employed. The latter adsorb on extraneous catalytically active sites, whether particles in solution, or sites on mechanical components of the deposition system/ container, to inhibit deposition reactions. With proper maintenance, electroless solutions may operate for periods of months at elevated temperatures, and exhibit minimal extraneous metal deposition. 

MgO is a basic metal oxide and has the same crystal structure as NiO. As a result, the combination of MgO and NiO results in a solid-solution catalyst with a basic surface (171,172), and both characteristics are helpful in inhibiting carbon deposition (171,172,239). The basic surface increases C02 adsorption, which reduces or inhibits carbon-deposition (Section ALB). The NiO-MgO solid solution can control the nickel particle sizes in the catalyst. This control occurs because in the solid solution NiO has strong interactions with MgO and, as indicated by TPR data (26), the former oxide can no longer be easily reduced. Consequently, only a small amount of NiO is expected to be reduced, and thus small nickel particles are formed on the surface of the solid solution, smaller than the size necessary for coke formation. Indeed, the nickel particles on a reduced 16.7 wt% NiO/MgO solid-solution catalyst were too small to be observed by TEM (171). Furthermore, two additional important qualities stimulated the selection of MgO as a support its high thermal stability and low cost (250,251). 

Like NiO, CoO and FeO are characterized by the same crystal structure as MgO and have comparable lattice parameters, and, hence, can form CoO/MgO and FeO/MgO solid solutions. Therefore, it was expected that CoO/MgO and FeO/MgO would inhibit carbon deposition and metal sintering, just as Ni/MgO does, resulting in high stability (171). 

It should be understood that even for micro surface features the potential is uniform and the ohmic resistance through the bath to peaks and valleys is about the same. Also, electrode potential against SCE will be uniform. What is different is that over micro patterns the boundary of the diffusion layer does not quite follow the pattern contour (Fig. 12.3). Rather, it thus lies farther from depth or vias than from bump peaks. The effective thickness, 8N, of the diffusion layer shows greater variations. This variation of 8N over a micro profile therefore produces a variation in the amount of concentration polarization locally. Since the potential is virtually uniform, differences in the local rate of metal deposition result if it is controlled by the diffusion rate of either the depositing ions or the inhibiting addition (leveling) agents. 

Figure 12.3. Schematic cross section showing microroughness of a cathode and the attending diffusion layer with leveling agent accumulated at peaks (P). Metal deposition is thus inhibited at peaks but not at valleys (V). Filling the latter results in smoother surfaces. (From Ref. 4, with permission from Wiley.)...

Exploration of the scope of NPS in electrochemical science and engineering has so far been rather limited. The estimation of confidence intervals of population mean and median, permutation-based approaches and elementary explorations of trends and association involving metal deposition, corrosion inhibition, transition time in electrolytic metal deposition processes, current efficiency, etc.[8] provides a general framework for basic applications. Two-by-two contingency tables [9], and the analysis of variance via the NPS approach [10] illustrate two specific areas of potential interest to electrochemical process analysts. 

As seen in reaction (6.5.3) photogenerated holes are consumed, making electron-hole separation more effective as needed for efficient water splitting. The evolution of CO2 and O2 from reaction (6.5.6) can promote desorption of oxygen from the photocatalyst surface, inhibiting the formation of H2O through the backward reaction of H2 and O2. The desorbed CO2 dissolves in aqueous suspension, and is then converted to HCOs to complete a cycle. The mechanism is still not fully understood, with the addition of the same amount of different carbonates, see Table 6.2, showing very different results [99]. Moreover, the amount of metal deposited in the host semiconductor is also a critical factor that determines the catalytic efficiency, see Fig. 6.7. 

In this series of papers we will report on the use of TOFSIMS for the characterization of films of a number of commonly used silanes deposited on various substrates, mainly metals. The background of this interest in metals is the possible application of silanes as corrosion-inhibiting metal pretreatments. 

The deactivation of catalysts concerns the decrease in concentration of active sites on the catalyst Nj. This should not be confused with the reversible inhibition of the active sites by competitive adsorption, which is treated above. The deactivation can have various causes, such as sintering, irreversible adsorption and fouling (for example coking or metal depositions in petrochemical conversions). It is generally attempted to express the deactivation in a time-dependent expression in order to be able to predict the catalyst s life time. An important reason for deactivation in industry is coking, which may arise from a side path of the main catalytic reaction or from a precursor that adsorbs strongly on the active sites, but which cannot be related to a measurable gas phase concentration. For example for the reaction A B the site balance contains also the concentration of blocked sites C. A deactivation function is now defined by cq 24, which is used in the rate expression. 

Silicon electrodes can also be stabilized by depositing a layer of compounds of oxides and silicides such as Ru02, molybdenum dinitrogen complexes, manganese oxide, indium-tin oxide, " aluminum oxide and aluminosilicate. In contrast to the noble metals, deposition of islands of Pb and Cd tends to inhibit hydrogen evolution in H2SO4 solution. ... 

Metal islands or nanoparticles deposited on a semiconductor surface undergo Fermi level equilibration following charging with photogenerated electrons. The effect of Fermi level equilibration is predominantly seen when the metal deposits consist of small islands or small particles. Unlike the ohmic contact observed in bulk metals, the nanoparticles retain the charge before transferring them to the redox species. During extended UV-photolysis, electron-capture by metal islands of Ag, Au and Cu becomes inhibited as their Fermi level shifts close to the conduction band of the semiconductor. Pt on the other hand acts as an electron sink and fails to achieve Fermi level equilibration. 

The model based on metal-hydroxide ions ([MOH]+) was further developed by Grande and Talbot [71]. Sasaki and Talbot [72] demonstrated the extendibility of this model to the electrodeposition of Co—Fe and Ni—Co alloys. They found that there is a slight inhibition of the more positive metal deposition and a promotion (acceleration) of the less positive metal deposition for all binary iron-group alloys. 

Golodnitsky et al. found that inhibition of the more positive metal deposition by the less noble one does not depend on the anion composition of the electrolyte [73]. 

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