Plating overpotential

It has been shown [16] that the optimum plating overpotential and current density are determined by the upper limit of the validity of the Tafel equation for... 

It was shown in section The effect of the deposition current density on the electrode surface coarseness that the optimum plating overpotential is determined by the upper limit of validity of the Tafel equation for the deposition process. In this case, as can be seen from Fig. 2.23, the optimum deposition overpotentials for Cd deposition are about 40 and 530 mV in the absence and in the presence of adsorption of additive, respectively. 

Electroplating passive alloys Another application of strike baths reverses the case illustrated in the previous example. The strike is used to promote a small amount of cathode corrosion. When the passivation potential of a substrate lies below the cathode potential of a plating bath, deposition occurs onto the passive oxide film, and the coating is non-adherent. Stainless steel plated with nickel in normal baths retains its passive film and the coating is easily peeled off. A special strike bath is used with a low concentration of nickel and a high current density, so that diffusion polarisation (transport overpotential) depresses the potential into the active region. The bath has a much lower pH than normal. The low pH raises the substrate passivation potential E pa, which theoretically follows a relation... 

Hayden et al., 2009]. Figure 16.8 (Plate 16.1) shows the effect of the equivalent thickness of deposited Pt on titania on the activity of the ORR carried out in a 0.5 M HCIO4 electrolyte at 20 °C. In this case, the activity was assessed by determining the potential (vs. RHE) at which the specific current density reaches 0.01 mA cm A lower potential indicates a higher overpotential required to achieve this rate of reaction, and hence corresponds to a reduction in activity. The shaded region of the plot, below about 1 nm equivalent thickness, corresponds to coverages over which distinct particles are... 

Significant advances have been made in this decade in electrochemical H2 separation, mostly through the use of solid polymer electrolytes. Since the overpotentials for H2 reduction and oxidation are extremely low at properly constructed gas diffusion electrodes, very high current densities are achievable at low total polarization. Sedlak [13] plated thin layer of Pt directly on Nafion proton conductors 0.1-0.2cm in thickness, and obtained nearly 1200 mA/cm2 at less than 0.3 V. The... 

Fig. 11. The development of local cathodic overpotentials at a segmented horizontal plate electrode in forced laminar convection. [From Hickman (H3).]...

Mercury, lead, cadmium and graphite are commonly used cathode materials showing large overpotentials for hydrogen evolution in aqueous solution. Liquid mercury exhibits a clean surface and is very convenient for small-scale laboratory use. Sheet lead has to be degreased and the surface can be activated in an electrochemical oxidation, reduction cycle [3, 22], Cadmium surfaces are conveniently prepared by plating from aqueous cadmium(ii) solutions on a steel cathode. 

Chronopotentiometry, galvanostatic transients, 1411 as analytical technique, 1411 activation overpotential, 1411 Clavilier, and single crystals, 1095 Cluster formation energy of, 1304 and Frumkin isotherm, 1197 Cobalt-nickel plating, 1375 Cold combustion, definition, 1041 Cole-Cole plot, impedance, 1129, 1135 Colloidal particles, 880, 882 and differential capacity, 880 Complex impedance, 1135 Computer simulation, 1160 of adsorption processes, 965 and overall reaction, 1259 and rate determining step, 1260... 

In other words, it now includes the term t] = Ed — E which represents the difference between the two expressions, Eqs. (11.1) and (11.2), above. The factor av+ represents, again, the activity value of the cation being deposited (i.e., cation in the film or layer of the bath at the cathode face). Thus tj is the overpotential (deposition factor). It is the extra potential needed to maintain the deposition going at a given desired rate suitable to the nature and properties of the cathode film. In practice, then, calculating the metal deposition potential by the above means that the practitioner must know the values of av+ and 17 for a fixed plating condition, including bath parameters, such as current density and temperature, as well as ionic parameters, such as concentration, valence, and mobility. 

Many plating protocols advocate the use of a flash step where a significantly higher overpotential is applied to ensure that the entire substrate is covered with metal before the potential is reduced to the plating potential. This has been shown to be effective in ionic liquid and significantly improves the corrosion resistance of the coatings [7]. 

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