Titanium cathode, current-potential

It is clearly desirable that the deposit has an even thickness over the whole of the surface to be electroplated. This requires the potential to be the same at all points over the surface of the cathode and this is impossible to attain when the object to be plated has a complex shape. To some extent the evenness of the deposit can be improved by introducing auxiliary anodes (usually platinized titanium electrodes where the reaction is oxygen evolution) at various positions in the electrolyte, the objective being to increase the cathode current density at points where it would otherwise be very low, i.e. at points on the cathode furthest from the normal anodes (e.g. in holes or recesses in the object being plated). The problem with this approach, however, is that a totally new cell geometry is necessary for each new plating job and in any case its success is limited. Hence, in general, we are dependent... 

Figure 6.32 represents expression (6.51) for an n-type semiconductor electrode in dimensionless form on a logarithmic scale. The calculated curve can be compared to the experimental data shown in Figure 6.33. They were obtained with the Fe(CN)g /Fe(CN)5 system on titanium oxide electrodes (an n-type semiconductor) having different concentrations of charge carriers. The measured anodic and cathodic current density-potential curves resemble those theoretically predicted. In general, charge transfer reactions at semiconducting oxide films do not exactly follow the simple theory outlined above, because other, more complex phenomena can also play a role.

On titanium alloys, however, the metal still has an active-passive transition in the crevice environment and the surface potential must be low enough for the active corrosion to occur. Thus, IR drop is required to stabilize the active dissolution [9,74], particularly when large cathodic currents are available. 

Some metals and alloys have low rates of film dissolution (low /p) even in solutions of very low pH, e.g. chromium and its alloys, and titanium. In these cases the value of /p is quite low, and although it increases as the temperature increases, a maximum is reached when the solution boils. The maximum current is below and breakdown does not occur. However, in certain alloys, e.g. Cr-Fe alloys, the protective film may change in composition on increasing the anode potential to give oxides that are more soluble at low pH and are therefore more susceptible to temperature increases. This occurs in the presence of cathode reactants such as chromic acid which allow polarisation of the anode. 

The addition of a small percentage of a noble metal to a base metal such as stainless steel or titanium can provide sites of low overvoltage for the cathodic reduction of dissolved oxygen or hydrogen ions. This permits larger currents and hence more positive potentials to be obtained at the anodic region, and promotes passivation under some circumstances . This effect has been demonstrated for stainless steels but has not been adopted in practice, since under other conditions the noble metal addition accelerates corrosion . 

A mercury cathode finds widespread application for separations by constant current electrolysis. The most important use is the separation of the alkali and alkaline-earth metals, Al, Be, Mg, Ta, V, Zr, W, U, and the lanthanides from such elements as Fe, Cr, Ni, Co, Zn, Mo, Cd, Cu, Sn, Bi, Ag, Ge, Pd, Pt, Au, Rh, Ir, and Tl, which can, under suitable conditions, be deposited on a mercury cathode. The method is therefore of particular value for the determination of Al, etc., in steels and alloys it is also applied in the separation of iron from such elements as titanium, vanadium, and uranium. In an uncontrolled constant-current electrolysis in an acid medium the cathode potential is limited by the potential at which hydrogen ion is reduced the overpotential of hydrogen on mercury is high (about 0.8 volt), and consequently more metals are deposited from an acid solution at a mercury cathode than with a platinum cathode.10... 

A 120-cm2 piece of Nafion 390 was-installed in an electrochemical cell using a Ti02/Ru02 coated titanium anode and a mild steel cathode. The current density of the cell was varied from 0.1 to 0.5 amps cm 2 over a period of several days the fluxes of the various components were measured at each current density. Figure 2 shows the variation of current efficiency with current density at constant external concentrations of sodium chloride and sodium hydroxide (that is, constant chemical potentials). It is evident that the flux of sodium ions increases relative to that of hydroxyl ions, with increasing current density. Figure 3 shows a dramatic change in the ion fluX o/h -... 

The approximate anodic polarization curves for iron, nickel, chromium, and titanium in 1 N H2SO4 are shown in Fig. 5.42. The cathodic reactions are for the environments shown and are representative of curves obtained on platinum. Since they may be displaced significantly when the reactions occur on the other metal surfaces, particularly the shift of the oxygen curves to lower potentials and current densities, the following discussion is qualitative. The conclusions drawn, however, are consistent with observations on the actual metal/environment systems. 

In deaerated 1 N H2SO4 (pH = 0.56), hydrogen-ion reduction is the cathodic reaction with the cathodic polarization curve intersecting the iron, nickel, and chromium curves in the active potential region. Hence, active corrosion occurs with hydrogen evolution, and the corrosion rates would be estimated by the intersections of the curves. The curves predict that the titanium will be passivated. However, the position ofthe cathodic hydrogen curve relative to the anodic curves for titanium and chromium indicates that if the exchange current density for the hydro-... 

The sequence of reactions involved in the overall reduction of nitric acid is complex, but direct measurements confirm that the acid has a high oxidation/reduction potential, -940 mV (SHE), a high exchange current density, and a high limiting diffusion current density (Ref 38). The cathodic polarization curves for dilute and concentrated nitric acid in Fig. 5.42 show these thermodynamic and kinetic properties. Their position relative to the anodic curves indicate that all four metals should be passivated by concentrated nitric acid, and this is observed. In fact, iron appears almost inert in concentrated nitric acid with a corrosion rate of about 25 pm/year (1 mpy) (Ref 8). Slight dilution causes a violent iron reaction with corrosion rates >25 x 1()6 pm/year (106 mpy). Nickel also corrodes rapidly in the dilute acid. In contrast, both chromium and titanium are easily passivated in dilute nitric acid and corrode with low corrosion rates. 

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