Copper cathodic partial reaction

Interaction Between Partial Reactions. The original mixed-p)otential theory assumes that the two partial reactions are independent of each other (1). In some cases this is a valid assumption, as was shown earlier in this chapter. However, it was shown later that the partial reactions are not always independent of each other. For example, Schoenberg (13) has shown that the methylene glycol anion (the formaldehyde in an alkaline solution), the reducing agent in electroless copper deposition, enters the first coordination sphere of the copper tartrate complex and thus influences the rate of the cathodic partial reaction. Ohno and Haruyama (37) showed the presence of interference in partial reactions for electroless deposition of Cu, Co, and Ni in terms of current-potential curves. 

Thus, from the kinetic aspects, the cathodic partial reaction is an electrochemical reaction [Eq. (8.14)], which is preceded by a chemical reaction [Eq. (8.13)]. Paunovic (31) studied the first step in the cathodic partial reaction of electroless copper deposition by chronopotentiometry and potential sweep (potentiodynamic)... 

In the equilibrium state, the anodic and cathodic partial reactions of an electrochemical reaction have equal rates. The system is in a dynamic equilibrium state, and no net reaction occurs. For example, when a copper sheet is immersed in copper sulfate solution, in the equilibrium state the anodic dissolution rate of copper from sheet to solution equals the cathodic deposition rate from the solution to the surface of the sheet. Theoretically, one can calculate the equilibrium state of an electrochemical reaction from thermodynamic values. This is the standard electrode potential, E°, or equilibrium potential of the electrochemical reaction. The standard electrode potential corresponds to a determined standard state of 0.1 MPa, 25 °C, activity of reactive species of 1 or ideal solution of 1.0 mol L-1, and equilibrium potential of any other state. 

The copper thus corrodes without any external current. The open circuit potential of a mixed electrode undergoing corrosion, is called the corrosion potential (in the literature it is sometimes also called the free corrosion potential). The corrosion potential has a value that lies in between the equilibrium potentials of the partial electrode reactions. In contrast to the equilibrium potential, which is a thermodynamic quantity, the corrosion potential is determined by kinetics its value depends on the rates of both the anodic and the cathodic partial reactions present. 

The second example is electroless deposition of copper from solutions containing dissolved oxygen (49,53). In this case the interfering reaction is the reduction of the oxygen, and the cathodic partial current density is the sum of two components ... 

Corrosion inhibitor - corrosion inhibitors are chemicals which are added to the electrolyte or a gas phase (gas phase inhibitors) which slow down the - kinetics of the corrosion process. Both partial reactions of the corrosion process may be inhibited, the anodic metal dissolution and/or the cathodic reduction of a redox-system [i]. In many cases organic chemicals or compounds after their reaction in solution are adsorbed at the metal surface and block the reactive centers. They may also form layers with metal cations, thus growing a protective film at the surface like anodic oxide films in case of passivity. Benzo-triazole is an example for the inhibition of copper cor-... 

These rate laws demonstrate that Cu(II) is not involved in the anodic partial reaction, whereas both HCHO and OH are involved in the cathodic copper deposition reaction. It is interesting to note that HCHO acts as an inhibitor of the partial cathodic reaction. The above rate laws suggest that the mechanisms are more complex than indicated by simple combination of two independent partial reactions. 

The experimental evidence described above clearly shows that the cathodic and anodic partial reactions interact with each other, and that such interactions are an essential part of the mechanism of electroless copper deposition. It should be noted that the interdependence of partial reactions has also been demonstrated by Bindra et al. [127], based on their results of kinetic and mechanistic analysis. 

In [56], microprobe studies on the distribution of the elements in the surface layers of the pipes in the experiments [55] demonstrated areas of chloride and sulphate concentration at the metal/surface layer phase boundary. This gives the surface layers semi-conductor properties, leading to partial separation of regions at vhich anodic and cathodic part reactions proceed. As the concentration of copper ions increases, the cathodic part current is increased, so that pitting corrosion is intensified at the anodic regions. 

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

The dechlorination can also proceed on a suspension of powdered iron that has been partially coated with copper by cementation [43]. In this case no external current is needed, since the reaction proceeds similarly to corrosion, where the free iron surface acts as the local anode and dissolves to give iron(II) ions, whereas the copper-coated surface represents the local cathode, at which the given chloroderivative is reduced to Cl ions. The reduction rate is lower in this case, since the potential of the local cathodes is less negative than in... 

The simplest way of performing an analytical electrolysis is to maintain the applied cell potential at a constant value. In practice, electrolysis at a constant cell potential is limited to the separation of easily reduced cations from those that arc more difficult to reduce than hydrogen ion or nitrate ion. The reason for this limitation is illustrated in Figure 24-1, which shows the changes of current. IR drop, and cathode potential Ec during electrolysis in the cell for the determination of copper(ll). The cell consists of two platinum electrodes, each with a surface area of 150 cm, immersed in 200 mL of a solution that is 0.0220 M in copper(II) ion and 1.00 M in hydrogen ion. The cell resistance is 0.50 fl. When a suitable potential difference is applied between the two electrodes, copper is deposited on the cathode, and oxygen is evolved at a partial pressure of 1.00 atm at the anode. The overall cell reaction is... 

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