Additives cathodic partial reaction

These three passive systems are important in the technique of anodic protection (see Chapter 21). The kinetics of the cathodic partial reaction and therefore curves of type I, II or III depend on the material and the particular medium. Case III can be achieved by alloying additions of cathodically acting elements such as Pt, Pd, Ag, and Cu. In principle, this is a case of galvanic anodic protection by cathodic constituents of the microstructure [50]. 

Eq. (16). The error increases with increasing /corrAA us it does for the idealized case, but the relation is much more complex than a direct proportionality. The errors caused by the neglect of mass transport and the errors caused by the linearization of the polarization equation, see Section IV.6(i), are not simply additive, since the linearization error itself is a funcion of the extent of the mass-transport control. For the sake of simplicity, only the extreme cases of /corrA/ = 0 and 1 are shown for most cases, and only for one positive and for one negative polarization case are intermediate values also shown as examples. The error is much larger at negative polarization values than at positive polarization, because, as mentioned earlier, only the cathodic partial reaction is... 

Finally, inhibition may also be (partially) caused by the presence of ohmic potential drops (e.g. because of the formation of poorly conducting films) between anodic and cathodic surface areas. Flere the rates of the partial reactions are additionally reduced due to the opposite negative and positive potential shifts in the anodic and... 

Kinetics, The major factors determining the rate of the partial cathodic reaction are concentrations of metal ions and ligands, pH of the solution, and type and concentration of additives. These factors determine the kinetics of partial cathodic reaction in a general way, as given by the fundamental electrochemical kinetic equations discussed in Chapter 6. 

Additives can be consumed at the cathode by incorporation into the deposit and/or by electrochemical reaction at the cathode or anode. Consumption of coumarin in the deposition of nickel from a Watts-type solution has been studied extensively. Thus, in this section we discuss the consumption of coumarin, which is used as a leveler and partial brightener. In a series of papers (33, 36), Rogers and Taylor, described the effects of coumarin on the electrodeposition of nickel. They found that the coumarin concentration decreases linearly with time at —960 mV (versus SCE and 485 to 223 rpm at a rotating-disk electrode, for plating times of 8 to 75 min. A rotating-disk electrode was used to achieve a uniform and known rate of transport of additive to the cathode. Rogers and Taylor found that the rate of coumarin consumption is a function of coumarin bulk concentration. Figure 10.16 shows that the rate of consumption... 

Notice that, because of the strong dependence of the kinetics on electrode potential, the determination of the electrochemical reaction order requires that the partial cathodic or anodic current densities are measured at constant potential in addition to the activities of the other species remaining constant. 

In cathodic addition reactions solvated electrons, radical anions or anions are generated at the cathode and added to activated or unactivated double or triple bonds. This broad spectrum of reactions is partially treated a) in section 8.2 when group conversion generates a reactive intermediate which undergoes addition reactions 1 S5a,b and b) in section 12.2 when olefins are coupled via addition of a cathodically generated radical anion to an activated double bond. 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

Methylene chloride has been used for both anodic and cathodic reactions. The major advantages of using methylene chloride seem to be that certain cation radicals are more stable in methylene chloride than in the solvents usually employed in electrochemistry [425.426]. A disadvantage is that chloride ions produced at the cathode may diffuse to the anode compartment and interfere with the anodic reactions. This can partially be avoided by the addition of small amounts of acetic acid to the catholyte whereby the cathodic reaction becomes an evolution of hydrogen rather than formation of chloride ions. 

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